Electronic and Optoelectronic Studies of

Organic Semiconductors

By

Dil Nawaz Khan

Supervised By

Prof. Dr. Muhammad Hassan Sayyad

A dissertation submitted to GIK Institute, in partial fulfilment of

Doctor of Philosophy in Engineering Sciences (Applied Physics)

(2014)

Faculty of Engineering Sciences

Ghulam Ishaq Khan Institute of Engineering Sciences & Technology

Topi, District Swabi, Khyber Pukhtoonkhwa, Pakistan

ii

iii

Dedication

This dissertation is dedicated to:

my latE ParENtS,

Who always wanted me to acquire higher Education.

my wIfE ShaKIla Nawaz,

For her continued support and encouragement during my research.

&

my KIdS EShNa, maham aNd muhammad Ibadullah KhaN,

Their innocent smiles always gave me the courage to complete this

work.

iv

Acknowledgements

I thank Almighty Allah for giving me ability, courage, determination and guidance in

conducting this research work, despite all difficulties.

My tenure in the Faculty of Engineering Sciences, GIK Institute since 2007 as

graduate student has been enriched by the help of many people. Here I would like to take

this opportunity to thank the people who have in no small way made the research in this

dissertation possible.

First and foremost, I would like to gratefully and sincerely thank my supervisor

Prof. Dr. Muhammad Hassan Sayyad for his patience, understanding, guidance,

encouragement and advice during my graduate studies at the GIK Institute. His

mentorship was paramount in providing a well rounded experience consistent my long-

term career goals. He encouraged me not only to grow as an experimentalist and a

Physicist but also as an instructor and an independent thinker. He always gave me the

courage to work and decide individuality and independently.

I would like to express my deep gratitude and respect to Prof. Dr. Jameel Un Nabi

(Dean FES) and Prof. Dr. S. Ikram A. Tirmizi (Ex. Dean FES) for their continued support

and encouragement throughout my stay at the GIK Institute. I would like to thank

Jehangir Bashar (Rector, GIKI) and Prof. Dr. Fazal A. Khalid (Pro-Rector

Academics) for their extended support and co-operation.

It is an honor for me to acknowledge Prof. Dr. Dmitrii F. Perepichka, McGill

University, Montreal Canada for giving me the chance to learn under his supervision by

granting access to his labs and equipment. This dissertation would not have been possible

without his guidance and encouragement.

I am also thankful to all faculty members of FES and other staff for their

cooperation during my stay in GIK Institute.

v

I would like to show my special gratitude to Fazal Wahab for lending his precious

time and extended co-operation in the lab and hostel.

I am forever indebted to my wife Shakila Nawaz for her understanding,

continuous support, encouragement, and endless patience during our whole life and

especially in the past 6 years of my studies. I am deeply grateful to my kids Eshna

Nawaz, Maham Nawaz and Muhammad Ibadullah Khan. They have lost a lot due to my

busy life during my research. I would like to thank my family members especially to my

brothers Akhtar Nawaz Khan and Arshad Nawaz Khan for their cooperation, support and

advice throughout my studies.

I am also thankful to Dr. Muhammad Yaseen, Dr. Munawar Ali Munawar, Dr. Mukhtar

Ali, Matthew Morantz and Dr. Q. Shuai (Steven) for synthesising the organic materials

which were used in this research work.

I am indebted to many of my colleagues and friends Dr. Muhammad Saleem, Dr.

Mutabar Shah, Dr. Zubair Ahmad, Dr. Shahid Mehmood Khan, Dr. Fakhra Aziz, Mr.

Muhammad Tahir and Mr. Aamir Ellahi for helping me to complete this dissertation. It

is a pleasure to thank Dr. Afshin Dadvand, Dr. Andrey Moiseev, Dr. Iryna Perepichka,

Mohini Ramkaran and Béatrice Lego who made my research work possible at the

Chemistry Department, Otto Maas Building, McGill. I am also grateful to my apartment

fellows in Canada Yensy Oritz and David for facilitating me during my stay in Montreal.

I also want to acknowledge Prof. Ayub Awan, Prof. Munsif Khan, Prof. Ghulam

Rasool and Prof. Akhtar Pervaz, Prof. Muhammad Sajjad Khan (Principal GPGC

Abbottabad) for their encouragements and support during my graduate study.

The last but not least, I would like to acknowledge Higher Education Commission

of Pakistan (HEC) for providing me financial support through the 5000 Indigenous PhD

Fellowship Program and International Research Support Initiative program (IRSIP), and

Higher Education Department, Khyber Pakhtunkhwa for granting me study leave for this

period.

Dil Nawaz Khan

vi

Declaration

This dissertation is my original work except where specific acknowledgement is

given. This dissertation has not been submitted in whole or in part to any other

University. Certain aspects of this dissertation have been published or submitted for

publications as follows:

Reviewed Journal papers:

1. Dil Nawaz Khan, Muhammad Hassan Sayyad, Fazal Wahab, Muhammad Tahir,

Muhammad Yaseen, Munawar Ali Munawar, Mukhtar Ali “Temperature

dependant electrical properties of formyl-TIPPCu(II)/p-Si heterojunction diode”

Modern Physics Letters B 28 (2014) 1450100 DOI:10.1142/S0217984914501000.

2. Dil Nawaz Khan, Muhammad Hassan Sayyad, Muhammad Tahir, Fazal Wahab

Muhammad Yaseen, Mukhtar Ali, Munawar Ali Munawar “The sensing of

Humidity by surface type Ag/Formyl-TIPPCu(II)/Ag sensor for Environmental

Monitoring” Surface Review and Letters 21 (2014) 1450048 DOI :

10.1142/S0218625X14500486.

3. Dil Nawaz Khan, Dmitrii F. Perepichka, Matthew Morantz, Qi Shuai

Muhammad Hassan Sayyad “Study of Anthracenediimide Derivatives for n-

channel Organic thin film transistors” (Draft in preparation)

Conference papers

1. Dil Nawaz Khan, Muhammad Hassan Sayyad, Muhammad Yaseen, Munawar

Ali Munawar, Mukhtar Ali “Application of Formyl-TIPPCu (II) for Temperature

and Light Sensing” World Academy of Science, Engineering and Technology

51(2011) 210-212.

Dil Nawaz Khan

vii

Abstract

Organic semiconductors have made inroad into many area of devices which was

formally dominated by inorganic semiconductors because of their wide variety of

electronic and optoelectronic properties. They being low cost, light weight and low

temperature processing materials provide opportunities to fabricate the variety of devices,

such as, solar cells, field effect transistors, lasers, light emitting diodes, sensors, photo

detectors, smart windows, large area displays, e-paper, etc. The material manipulation,

low cost fabrication techniques and the emerging ideas are bringing about much

improved performances in the organic electronic devices. Most of the earlier studies have

been reported on the p-type organic semiconductors and little is known about n-types. In

the development of future organic electronic industry, all organic complementary circuits

are not possible without the availability of both p- and n-type organic semiconductors and

data is required on the junction properties and mobility studies of these materials. Plenty

of data is available on the junction diodes of p-type organic semiconductors but little is

known on the n-type organic semiconductors based junction devices and mobility

investigations. In this dissertation, the n-type organic semiconducting materials formyl-

TIPPCu(II), N,N´-di-n-heptyl-2,3:6,7-anthracenetetracarboxydiimide (ADCI7) and N,N´-

di-n-octyl 2,3,6,7 anthracenetetracarboxydiimide (ADCI8) have been investigated as

active organic materials for their potential application in organic electronic devices.

Using organic semiconductor formyl-TIPPCu(II), junction diode, temperature, light and

humidity sensors have been fabricated, while ADCI7 and ADCI8 have been used for the

fabrication of n-channel organic thin film transistors.

To investigate junction properties of formyl-TIPPCu(II) organic semiconductor,

fabrication of Ag/formyl-TIPPCu(II)/p-Si heterojunction diode was undertaken and it

was made successfully. Its temperature dependent electrical properties are reported. The

values of series resistance, ideality factor, zero bias barrier height are observed strongly

dependent on temperature. The series resistance and ideality factor decease while the zero

bias barrier height increases with the rise in temperature.

viii

The surface type Ag/formyl-TIPPCu(II)/Ag humidity sensors has been fabricated

to study the effects of changing relative humidity on the electrical parameters and their

frequency dependant responses. The values of capacitance and resistance of the sensors

were measured at different humidity levels at frequencies of 1 kHz, 10 kHz and 100 kHz.

An increase in capacitance and decrease in resistance were observed during the rise of

relative humidity from 45 to 95% RH. The hysteresis response of these humidity sensors

was also studied at the frequency of 1 kHz.

Effects of temperature and light are studied on the capacitance and resistance of

the Au/formyl-TIPPCu(II)/Au device. The relative capacitance of the fabricated sensor

increased by 4.3 times by rising temperature from 27 to 1870C, while under illumination

up to 25000 lx, the capacitance of the Au/formyl-TIPPCu(II)/Au photo capacitive sensor

increased by 13.2 times as compared to dark conditions.

ADCI7 and ADCI8 were used to fabricate n-channel organic thin film transistors

(OTFTs) on oxidized silicon wafers. To get the high performance of the devices and to

avoid the trapping of charge carriers, the dielectric surface were modified by developing

the buffer layer of PMMA or by self assembly monolayer (SAM) of HMDS. The OTFTs

exhibited high charge mobility of the order of 10-2 cm2V-1S-1 (ADCI7) and 10-3 cm2V-1S-

1(ADCI8) with the on/off ratio of the order of 104 showing the appreciable enhancement

in the field effect properties of these materials as compared to the previously reported

researches for the same family of materials. ADCI7 is introduced as new compound for

high mobility n-channel OTFTs.

ix

Table of Contents

Dedication .......................................................................................................................... iii

Acknowledgements ............................................................................................................ iv

Declaration ......................................................................................................................... vi

Abstract ............................................................................................................................. vii

List of Tables .................................................................................................................... xv

List of abbreviations ........................................................................................................ xvi

Chapter 1 ............................................................................................................................ 1

1.1 Introduction .......................................................................................................... 1

1.2 Motivation ............................................................................................................ 3

1.3 Objectives ............................................................................................................. 5

1.4 Outline of Dissertation ......................................................................................... 6

1.5 References ............................................................................................................ 8

Chapter 2 .......................................................................................................................... 11

Theory and Literature Review .......................................................................................... 11

2.1 Chemistry of Organic Semiconductors ................................................................... 12

2.2 Mechanism of Charge transport in organic materials ........................................ 13

2.2.1 Organic Thin Film Transistors ................................................................... 14

2.2.2 Factors limiting the mobility in OFETs ...................................................... 16

2.3 Operation of the organic transistors ........................................................................ 17

2.3.1 Threshold voltage........................................................................................ 19

2.3.2 On/Off ratio ................................................................................................. 19

2.3.3 Contact resistances ...................................................................................... 20

2.4 References .......................................................................................................... 21

Chapter 3 ........................................................................................................................... 23

Materials and Experimental Procedures ........................................................................... 23

3.1 Organic Materials.................................................................................................... 23

3.1.1 formyl-TIPPCu(II) ........................................................................................... 23

3.1.2 N,N´-di-n-octyl-2,3:6,7-anthracenetetracarboxydiimide (ADCI8) ............. 23

3.1.3 N,N´-di-n-heptyl-2,3:6,7-anthracenetetracarboxydiimide (ADCI7)................ 24

3.1.4 Poly(methyl methacrylate) (PMMA) ............................................................... 24

x

3.1.5 Hexamethyldisilazane (HMDS) ....................................................................... 25

3.2 Fabrication Techniques ........................................................................................... 26

3.2.1 Substrate Cleaning ........................................................................................... 26

3.2.2 Thin Films Depositions .................................................................................... 29

3.3 Atomic Force Microscopy .................................................................................. 33

3.4 Device Characterization Techniques .................................................................. 35

3.4.1 Electrical Characterization .......................................................................... 35

3.4.2 Current- Voltage (I-V) Characterization ..................................................... 36

3.5 Humidity dependent Characterization .................................................................... 37

3.6 Light and Temperature Dependence Measurements ............................................... 38

3.7 References .......................................................................................................... 39

Chapter 4 .......................................................................................................................... 39

Temperature Dependant Electrical Properties of formyl-TIPPCu(II)/p-Si Heterojunction Diode ................................................................................................................................. 40

4.1 Introduction ........................................................................................................ 40

4.2 Device Fabrication ............................................................................................. 41

4.3 Results and discussion ........................................................................................ 42

4.4 Conclusions ........................................................................................................ 49

4.5 References .......................................................................................................... 49

Chapter 5 ........................................................................................................................... 52

The sensing of Humidity by surface type Ag/formyl-TIPPCu(II)/Ag sensor for Environmental Monitoring................................................................................................ 52

5.1 Introduction ........................................................................................................ 52

5.2 Experimental ...................................................................................................... 54

5.3 Results and discussion ........................................................................................ 55

5.3.1 Atomic Force Microscopy .......................................................................... 55

5.3.2 Capacitance Humidity Characteristics ........................................................ 56

5.3.3 Capacitance Frequency Characteristics ...................................................... 57

5.3.4 Resistance Humidity Characteristics ............................................................... 59

5.4 Conclusion ...................................................................................................... 60

5.5 References .......................................................................................................... 61

Chapter 6 ........................................................................................................................... 64

Application of formyl-TIPPCu(II) for temperature and light sensing .............................. 64

6.1 Introduction ............................................................................................................. 64

6.2 Experimental ...................................................................................................... 65

xi

6.3 Results and Discussion ....................................................................................... 65

6.4 Conclusions ........................................................................................................ 68

6.5 References .......................................................................................................... 69

Chapter 7 ........................................................................................................................... 71

Study of Anthracenediimide Derivatives for n-channel Organic thin film transistors ..... 71

7.1 Introduction ............................................................................................................. 71

7.2 Device Fabrication & Characterization .................................................................. 72

7.3 Thin-film X-ray diffraction ..................................................................................... 73

7.4 Thin film morphology ............................................................................................. 74

7.5 Thin-film transistor device characterization ........................................................... 76

7.6 Conclusion .............................................................................................................. 79

7.7 References .......................................................................................................... 79

Summary ....................................................................................................................... 82

Future Work .................................................................................................................. 83

xii

List of Figures Fig. 1. 1: A broad range of electronic products for daily life use ....................................... 1

Fig. 1. 2: Bardeen and Walter first point contact transistor [7]. ......................................... 2

Fig. 2. 1: Comparison of conductivity of organic semiconductors. .................................. 11

Fig. 2. 2: Scheme of the orbitals of two sp2 hybridized carbon atoms ............................. 13

Fig. 2. 3: Schematic structures of bottom-gate OFETs: top-contact (left) and bottom-

contact (right) .................................................................................................................... 15

Fig. 2. 4 Schematic drawings of top-gate OTFTs: top-gate/top-contact (left) and top-

gate/bottom-contact (right) ............................................................................................... 16

Fig. 2. 5 Transfer (a) and output (b) characteristics of PDIF-CN2 TFT[12] .................... 18

Fig.3. 1 Chemical structure of formyl-TIPPCu(II). .......................................................... 23

Fig.3. 2 The Molecular structure of ADCI8 ..................................................................... 24

Fig.3. 3 The synthesis procedure and chemical structure of ADCI7 ................................ 24

Fig.3. 4 Molecular structure of PMMA ............................................................................ 25

Fig.3. 5 Chemical structure of HMDS. ............................................................................. 25

Fig.3. 6 Chemical Hood for cleaning substrates ............................................................... 28

Fig.3. 7 Plasma cleaner PDC-32G .................................................................................... 28

Fig.3. 8 Inner view of PLASMIONIQUE EVD-400H thermal evaporating system ........ 30

Fig.3. 9 Sample Hoder in PLASMIONIQUE EVD-400H thermal evaporating system

(design by me)................................................................................................................... 30

Fig.3. 10 Edward Auto 306 vacuum thermal evaporator with thickness Monitor ............ 31

Fig.3. 11 PLASMIONIQUE EVD-400H thermal evaporating system ............................. 32

Fig.3. 12 Schematic diagram of spin coating technique. .................................................. 33

Fig.3. 13 Spin coater Model WS-400B-6NPP/LITE ........................................................ 33

Fig.3. 14 Schematic diagram of AFM. ............................................................................. 34

Fig.3. 15 MM Multimode 8 AFM setup ........................................................................... 35

Fig.3. 16 Probe Station(KARL SUSS PM5) with keithley-196 System Digital

Multimeter and keithley-228A I-V source ....................................................................... 36

Fig.3. 17 Keithley 4200-SCS for characterization of OTFTs ........................................... 36

xiii

Fig.3. 18 Inner view of Keithley 4200-SCS ..................................................................... 37

Fig.3. 19 Schematic of experimental setup for humidity characterization ....................... 37

Fig.3. 20 Schematic of experimental setup to study the effect of light ............................ 38

Fig.3. 21 Experimental setup to study the effects of temperature .................................... 39

Fig. 4. 1UV-Visible Spectrum of formyl-TIPPCu(II) ...................................................... 41

Fig. 4. 2 Mass Spectrum of Formyl-TIPPCu(II) ............................................................... 42

Fig. 4. 3 Cross-sectional view of Ag/formyl-TIPPCu(II)/p-Si diode. .............................. 42

Fig. 4. 4 I-V characteristics of formyl-TIPPCu(II)/p-Si diode at 299-339K. ................... 43

Fig. 4. 5 Semi-logarithmic (I–V) characteristics of formyl-TIPPCu(II)/p-Si diode at

different temperatures. ...................................................................................................... 44

Fig. 4. 6 Reverse saturation current of formyl-TIPPCu(II)/p-Si diode at different

temperature. ...................................................................................................................... 45

Fig. 4. 7 Arrhenius plot of saturation current vs 1000/T for formyl-TIPPCu(II)/p-Si diode

........................................................................................................................................... 45

Fig. 4. 8 Forward bias semi-logarithmic plots of current-voltage characteristics of formyl-

TIPPCu(II)/p-Si diode indicating the voltage drop ΔV=IRs across the neutral region. ... 46

Fig. 4. 9 Series resistance-Temperature graph of fomyl-TIPPCu(II) diode. .................... 47

Fig. 4. 10 F(V) vs V plot of formyl-TIPPCu(II)/p-Si diode ............................................. 48

Fig. 4. 11 Variation of Zero bias barrier height and ideality factor of formyl-

TIPPCu(II)/p-Si diode with temperature from 299-339K. ............................................... 49

Fig. 5. 1The cross-sectional view of surface type Ag/formyl-TIPPCu(II)/Ag humidity

sensor ................................................................................................................................ 54

Fig.5. 2AFM image of formyl-TIPPCu(II) on glass substrate. ......................................... 55

Fig.5. 33D AFM image of thin film of formyl-TIPPCu(II). ............................................. 56

Fig.5. 4Capacitance-relative humidity relationship of the Ag/formyl-TIPPCu(II)/Ag .... 56

Fig.5. 5Capacitance-frequency relationship of the Ag/formyl-TIPPCu(II)/Ag sensor at

65%, 75%, 85% and 95%RH. ........................................................................................... 58

Fig.5. 6Hysteresis-relative humidity relationship for Ag/formyl-TIPPCu(II)/Ag. ........... 59

Fig.5. 7Resistance-relative humidity relationship of Ag/formyl-TIPPCu(II)/Ag sensor at

1kHz and 10kHz. .............................................................................................................. 60

Fig.6. 1Cross-sectional view of the Au/formyl-TIPPCu(II)/Au organic sensor. .............. 65

xiv

Fig.6. 2Capacitance/resistance temperature relationships for the Au/formyl-

TIPPCu(II)/Au organic sensor. ......................................................................................... 66

Fig.6. 3Hysteresis in the capacitive and resistive measurements of Au/formyl-

TIPPCu(II)/Au organic sensor. ......................................................................................... 67

Fig.6. 4 Capacitance- illumination relationship for the Au/formyl-TIPPCu(II)/Au organic

sensor. ............................................................................................................................... 68

Fig. 7. 1 Schematic diagram of bottom gate/top contact n-channel OTFT of ADCI7 ..... 73

Fig. 7. 2 X-ray diffraction measurements of thin films of ADCI7 and ADCI8 ................ 74

Fig. 7. 3 AFM micrographs and the histogram of the height profile of thin films of ADI7

........................................................................................................................................... 75

Fig. 7. 4 AFM micrographs and the histogram of the height profile of thin films of

ADCI8 ............................................................................................................................... 75

Fig. 7. 5 Output characteristics of ADCI8 ........................................................................ 77

Fig. 7. 6 Transfer characteristics of ADCI8 ...................................................................... 78

Fig. 7. 7 Output characteristics of ADCI7 ........................................................................ 78

Fig. 7. 8 Transfer characteristics of ADCI7 ...................................................................... 79

xv

List of Tables

Table 7.1 Comparison of electrical properties of ADCI7 and ADCI8 81

xvi

List of abbreviations

ADCI7 N,N´-di-n-heptyl-2,3:6,7-anthracenetetracarboxydiimide

ADCI8 N,N´-di-n-octyl 2,3:6,7 anthracenetetracarboxydiimide

CP Conjugated Polymer

CuTIPP Cu(II) 5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin

DI De-ionized water

DOS Density of states

EL Electroluminescence

FET Field effect transistor

FTM Film Thickness Monitor

HMDS Hexamethyldisilazane

HOMO Highest occupied molecular orbital

ID Identification

ITO Indium tin oxide

I-V Current-Voltage

LCDs Liquid crystal displays

LCP Liquid crystal polymer

LED Light-emitting-diode

LEP Light-emitting- polymer

LUMO Lowest unoccupied molecular orbital

OFETs Organic field-effect transistors

OLEDs Organic light-emitting diodes

OP-TFTs Organic polymer thin film transistors

xvii

OSCs Organic semiconductors

OTFTs Organic thin film transistors

PL Photoluminescence

PMMA Poly(methyl methacrylate)

RFT Radio frequency tags

RH Relative humidity

rpm Revolution per minute

SAM Self Assembly Monolayer

SCLC Space-Charge-Limited Current

TCLC Trapped Charge Limited Current

TIPP 5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin

TOF Time-of-flight

TPP 5,10,15,20-tetraphenylporphyrin

TSC Thermally Stimulated Current

TSCLC Trapping model with space charge limited current

TSL Thermally stimulated luminescence

UPS Ultraviolet photoemission spectroscopy

VETP Vinyl-ethynyl-trimethyl-piperidole

XPS X-ray photoelectron spectroscopy

XRD X-ray diffraction

ZnTPP 5,10,15,20-tetraphenylporphyrin

ZnTIPP Zn(II) 5,10,15,20-tetrakis(4′-isopropylphenyl) porphyrin

1

Chapter 1

1.1 Introduction

In our daily life, we are enjoying with well-developed electronics. Nowadays, no

one can feel alone, colourful mobile phones and laptop keeping connection with family

and friends. Twenty first century brought technological advances in electronic industry

which have not only improved people’s lives in countless ways but also spurred

economic growth. Electronics products have become essential for our lives. Fig. 1.1

depicts a number of application domains of electronic technology in daily life.

Fig. 1. 1: A broad range of electronic products for daily life use

The stream of inventions in electronics was started in early 20th century. Before

this era, there was a little or almost no electronics in the day to day life of a common

man. The explanation of thunder and lighting by Franklin in 18th century is considered as

the beginning of electronics. Franklin’s idea was really a big mystery of that time; he

introduced the idea of charge flow and its consequences in the materials [1]. The idea of

resistance and conductance was introduced by George Simon Ohm which is helpful in

categorization of the materials [2].

2

The invention of thermionic diode by Ambrose Fleming signified the birth of

electronic devices. It was initially used for rectification purposes and then for the

detection of weak signal produced by wireless telegraph [3]. Lee de Forest developed

amplifier vacuum tube triode by introducing a grid between the cathode and anode. Grid

acts as modulator of the electrons so it either stops them or makes them faster causing

current amplification. The triode played important role in the early growth of

communication systems by enabling long distance telephony and amplified radio

technology [4].

The electronic devices which were in daily use in early 20th century were based

on vacuum diodes and triode which were facing problems like high power consumption,

low reliability and needed cooling arrangements. To overcome these problems, the

scientists tried alternatives of vacuum tubes; they turned their researches on solid state

devices. The tremendous development of PN junction diode by Russell Ohl opened new

era of research in the field of solid state electronic devices [5]. The discovery of transistor

effect by three American Physicists i.e. John Bardeen, William Shockley and Walter

Brattain opened the road of real electronics [6]. The photograph of first contact transistor

is shown in Fig. 1.2.

Fig. 1. 2: Bardeen and Walter first point contact transistor [7].

3

Historically, the study of conductivity in organic materials resulted the discovery

of photoconduction of solid anthracene in 1906, but then no appreciable research was

observed in next four decades. The discovery of conducting polymers in 1976 by Heeger,

Macdiarmid and Shirakawa opened the new ways of research in the field of organic

electronics [8, 9]. Inorganic semiconductors with wide range of applications in electronic

devices have some limitations due to their rigid crystalline nature. They need highly pure

materials and high temperature processing techniques so they are much expensive. To

overcome these limitations, the organic semiconductors have provided the alternative

opportunities to fabricate simple, inexpensive light weight, and flexible electronic

devices.

1.2 Motivation

The motivation of using organic semiconductors for the fabrication of electronic

devices arose from their structural flexibility and tunable electronics properties, cost

effectiveness, compatibility with roll to roll processing, easy and low temperature

fabrications, printable, flexible and large area applications due to their mechanically

flexibility and active matrix display backplanes [10, 11, 12], thus they have appealed for

a broad range of devices including junction diodes, sensors, transistors, light emitting

diodes, solar cells, memories, etc. The tunable charge mobility is another importance

property of organic semiconductors for the fabrication of more efficient organic field

effect transistors, solar cells and light emitting diodes [13, 14, 15, 16].

Most of the organic semiconductors can be processed easily at room temperature

in contrast to traditional silicon materials which need very high temperature of about

10000C. This can reduce the mismatches due to thermal expansion and allows OSCs

processing for larger area applications. The compatibility of OSCs with flexible

substrates due to their mechanical properties make them suitable for electronic paper,

smart windows, portable sensors, flat panel displays and RFID tags [17, 18]. Generally

the fabrication techniques for organic semiconductor devices are less complex and do not

require sophisticated photolithographic methods and high vacuum deposition processes

which make them less expensive as compare to traditional electronic technologies.

4

In earlier researches, the organic materials were not considered as reliable because

of their poor conductivity and fragility. These limitations of the organic semiconductors

can be significantly improved by their chemical and structural modification. The

performance of organic devices mostly depends on the surface morphology of the thin

films of molecular or polymeric semiconductors. Molecular orientation and packing can

be controlled by using post deposition processing [19].

By adding different functional groups, a variety of OSCs can synthesised and

tailored to tune their electronic and optoelectronic properties for different specific

applications. Currently the researchers are capable to control the material properties at

molecular level. The electronic properties of OSCs can also be enhanced by either their

doping or by making their composites with nanomaterials. Nanocomposite based organic

junction shows lower value of turn on voltage, ideality factor and reverse saturation

current while rise in charge carrier concentrations [20, 21].

Organic electronic is not only consumer electronics but it is also environmental

friendly technology which provides the best opportunity for renewable energy conversion

from the sun which is the most plentiful energy source. Earth receives solar energy at the

rate of 120 peta watts (1peta= 1015 watts), so the energy falling from the sun on earth in

one hour is sufficient for more than 20 years for the worldwide demands. The high

efficiency photovoltaic systems are mostly based on traditional inorganic materials, they

help to overcome the energy crises but they are not only much expensive but they may

also be harmful for the environment due the poisonous nature of the materials [22]. On

the other hand, the organic photovoltaics are low cost, lightweight, flexible and

environmental friendly [23].

Organic Field Effect transistors (OFETs) have gained great interest as a key

element for all organic electronic applications of the organic circuits; they are used as

switching pixels on/off, for adjusting the brightness level in displays and also in organic

sensors and in RFID tags. OFETs have other interesting applications in biosensors,

vapour sensors (e-nose) and intelligent textile [24].

The sensitivity of OSCs at ambient conditions is another important property

which has been used for the fabrication of various types of sensors. The OSCs are used as

5

active materials for detecting the effects of light, temperature, humidity, pressure and

gasses. They show high chemical stability and low hysteresis.

1.3 Objectives

The organic junction diode is the elementary building block of most of the

electronic and optoelectronic devices like light emitting diodes, solar cells, and OTFTs.

The study of junctions of organic-organic or organic-inorganic materials not only gives

enough knowledge about the interfaces of these materials but also it provides the

opportunities to enhance the performance of organic devices.

The main objective of this dissertation is to study the electronic and optoelectronic

properties of different organic semiconductors in order to find their suitability for the

fabrication of the organic electronic devices. Studies are carried out on the metallo

porphyrin 2-formyl-5,10,15,20-tetrakis(4΄-isopropylpheny)prophyrinatocopper(II) or

formyl-TIPPCu(II), N,N´-di-n-heptyl-2,3:6,7-anthracenetetracarboxydiimide (ADCI7)

and N,N´-di-n-octyl-2,3:6,7-anthracenetetracarboxydiimide (ADCI8).

In phase-1, the copper based porphyrin formyl-TIPPCu(II) is chosen to study its

junction with inorganic semiconductor p-type silicon. As the macrocyclic organic

compounds porphyrins with largely conjugated structure have been extensively studied

over the past decades because of their wide range of applications in different electronic

devices. Porphyrins being tetrapyrrole derivatives show their charge transfer and light

harvesting functions in the photosynthesis. They show high chemical stability and their

structures can easily be synthetically manipulated [25, 26]. The study of effects of

temperature on electrical properties of junction diode is also one of the important

purposes of this research.

Another focus of this research is to explore the sensing abilities of formyl-

TIPPCu(II) to develop inexpensive, reliable and accurate light, temperature and humidity

sensors for environmental monitoring. The morphological study of this organic material

shows the enough porosity for capturing water droplets at different humidity levels and

proves its suitability for detecting humidity.

Keeping in mind the importance of organic complementary circuits, the important

aim of this research is to study the n-type organic semiconductors to fabricate n-channel

6

organic thin film transistors. Mostly organic semiconductors are p-type; naturally n-type

OSCs are very rare. Miller et al has reported the linear acenedicarboximide cores as

electrochemically n-dopable. Then in another research work, ADI8 based organic thin

film transistor was reported as air stable [27, 28]. Therefore, the most important objective

of this research is to investigate the n-channel OTFT of N,N´-di-n-octyl-2,3:6,7-

anthracenetetracarboxydiimide (ADCI8) and then to study the stable field effect

properties of newly synthesized N,N´-di-n-heptyl-2,3:6,7-anthracenetetracarboxydiimide

(ADCI7).

1.4 Outline of Dissertation

This dissertation consists of eight chapters, the details are as under:

Chapter 2

The chapter consists of literature review, theory and basic properties of

conjugated organic materials and their devices. The brief description of organic thin film

transistors are also the part of this chapter.

.

Chapter 3

This chapter focuses on the molecular structure and important properties of

organic materials (used in this research) i.e formyl-TIPPCu(II), ADCI7 and ADCI8. It

also contains the details of devices fabrication and characterization techniques.

Chapter 4

Temperature dependant electrical properties of formyl-TIPPCu(II)/p-Si

heterojunction diode are discussed in this chapter. The discussion consists of the

fabrication and characterization of formyl-TIPPCu(II)/p-Si diode and the effect of

temperature on important diode parameters like series resistance, reverse saturation

current, ideality factor and barrier height.

7

Chapter 5

The sensing of Humidity by surface type Ag/Formyl-TIPPCu(II)/Ag sensor for

environmental monitoring is described in chapter 5. It contains the details of fabrication

and characterization of formyl-TIPPCu(II) based organic humidity sensors. The

capacitive and resistive response of the fabricated sensors along with the possible

hysteresis has been explained.

Chapter 6

This chapter constitutes the investigation of light and temperature sensors

employing formyl-TIPPCu(II) as active sensing material. It contains the procedures of

fabrication and characterization of surface type organic sensors

Chapter 7

Chapter 7 deals with the investigation of ADCI7 and ADCI8 based n-channel

organic thin film transistors. It encompasses the fabrication and characterization of the

OTFTs and also contains the study of the surface morphologies of thin films of ADCI7

and ADCI8.

Summary and Future work

At the end of the thesis, summary of this research work have been reported along

with the suggested future work.

8

1.5 References

[1] P. Benjamin, A History of Electricity, NY: Wiley, 1898.

[2] R. A. Millikan and E. S. Bishop, Elements of Electricity, American Technical

Society, 1917.

[3] J. A. Fleming, "Instrument for converting Alternating electric currents into continous

current". US Patent 803,684, 7 Nov 1905.

[4] L. d. Forest, "Space Telegraphy". US Patent 879532 A, 18 Feb 1908.

[5] R. S. Ohl, "Light-sensitive electric device". US Patent 2,402,662, 25 June 1946.

[6] J. Bardeen and W. Brattain, "Three-Electrode Circuit Element Utilizing

Semiconductor Materials". US Patent 2,524,035, 3 Oct 1950.

[7] "1947 - Invention of the Point-Contact Transistor," Computer History Museum,

[Online]. Available: http://www.computerhistory.org/semiconductor/timeline/1947-

invention.html.

[8] H. Shirakawa, E. J. Louis, A. G. MacDiarmid, C. K. Chiang and A. J. Heeger,

"Synthesis of electrically conducting organic polymers: Halogen derivatives of

polyacetylene," J. Chem. Soc. Chem. Commun, pp. 578-580, 1977.

[9] C. K. Chiang, C. R. Fincher, Y. W. Park, A. J. Heeger, H. Shirakawa, E. J. Louis, S.

C. Gau and A. G. MacDiarmid, "Electrical conductivity in doped polyacetylene,"

Phys. Rev. Lett, vol. 39, pp. 1098-1101, 1977.

[10] A. C. Arias, J. D. MacKenzie, I. McCulloch, J. Rivnay and A. Salleo, "Materials and

Applications for Large Area Electronics: Solution-Based Approaches," Chem. Rev,

vol. 110, pp. 3-24, 2010.

[11] L. M. Yang and R. Pushpa, "Tuning electronic and optical properties of a new class

of covalent organic frameworks," J. Mater. Chem. C, vol. 2, pp. 2404-2416, 2014.

[12] O. Ostroverkhova, Handbook of organic materials for optical and (opto)electronic

devices, Cambridge, U.K: Woodhead Publishing, 2013.

[13] Y. Zhang, F. Zu, S. T. Lee, L. Liao, N. Zhao and B. Sun, "Heterojunction with

9

Organic Thin Layers on Silicon for Record Efficiency Hybrid Solar Cells,"

Advanced Energy Materials, vol. 4, p. DOI: 10.1002/aenm.201300923, 2013.

[14] D. Elkington, N. Cooling, W. Belcher, P. C. Dastoor and X. Zhou, "Organic Thin-

Film Transistor (OTFT)-Based Sensors," Electronics, vol. 3, pp. 234-254, 2014.

[15] Y. Seino, H. Sasabe, Y. J. Pu and J. Kido, "High-Performance Blue Phosphorescent

OLEDs Using Energy Transfer from Exciplex," Advanced Materials, Vols. 1612-

1616, p. 26, 2014.

[16] N. Kaur, M. Singh, D. Pathak, T. Wagner and J. M. Nunzi, "Organic materials for

photovoltaic applications: Review and mechanism," Synthetic Metals, vol. 190, pp.

20-26, 2014.

[17] T. Uchida, M. Shibasaki, T. Matsuzaki and Y. Nagata, "Glare-Tunable Transparent

Electrochemical Smart Window Coupled with Transparent Organic Light-Emitting

Diode," Applied Physics Express, vol. 6, p. doi:10.7567/APEX.6.041604, 2013.

[18] P. Wiltzius and M. Hill, "Organic electronics and e-paper," in Device Research

Conference, 2000. Conference Digest. 58th DRC, Denver, CO, USA, 2000.

[19] A. M. Hiszpanski and Y. L. Loo, "Directing the film structure of organic

semiconductors via post-deposition processing for transistor and solar cell

applications," Energy Environ. Sci, vol. 7, pp. 592-608, 2014.

[20] A. J. Heeger, J. R. Schriefer and W. P. Su, "Solitons in Conducting Polymers," Rev.

Mod. Phys, vol. 40, p. 3439, 1988.

[21] D. N. Khan, M. H. Sayyad, F. Wahab and M. Tahir, "Electrical Characteristics

Enhancement in a Polymer Nanocomposite based Organic/Inorganic Diode," in

ICECCO 2012, Ankara, Turkey, 2012.

[22] D. Elkington, N. Cooling, W. Belcher, P. C. Dastoor and X. Zhou, "Organic Thin-

Film Transistor (OTFT)-Based Sensors," Electronics, vol. 3, pp. 234-254, 2014.

[23] Y. Chu and P. Meisen, "Review and Comparison of Different Solar Energy

Technologies," Global Energy Network Institute (GENI), 2011.

[24] A. Gupta and S. Sharma, "Organic solar cell – A Renewable Energy Resource,"

International Journal of Environmental Science: Development and Monitoring, vol.

10

4, pp. 16-18, 2013.

[25] Y. He, T. Ye and E. Borguet, "Porphyrin self-assembly at electrochemical interfaces:

Role of potential modulated surface mobility," J. Am. Chem. Soc, vol. 124, pp.

11964-11970, 2002.

[26] R. Takahashi and Y. Kobuke, "Hexameric macroring of gable-porphyrins as a light-

harvesting antenna mimic," J. Am. Chem. Soc, vol. 125, pp. 2372-2373, 2003.

[27] S. F. Rak, T. H. Jozefiak and L. L. Miller, "Electrochemistry and near-infrared

spectra of several imide or quinone groups," J. Org. Chem, vol. 55, pp. 4794-4801,

1999.

[28] Z. Wang, C. Kim, A. Facchetti and T. J. Marks, "Anthracenedicarboximides as Air-

Stable N-Channel Semiconductors for Thin-Film Transistors with Remarkable

Current On-Off Ratios," J. Am. Chem. Soc, vol. 129, pp. 13362-13363, 2007.

11

Chapter 2

Theory and Literature Review

The conjugated organic materials exhibit semiconducting properties and their

conductivity lies between insulators and conductors ranging from 10-9 to 103 Ω-1cm-1

(Fig. 2.1). But one of the superiority of organic semiconductors is the enhancement in

their conductivity. By different synthetic procedures the conductivity of these materials

can be tuned according to the requirements of different applications. The band gaps of

organic semiconductors lies between 1.2 to 3.5 eV in comparison to traditional elemental

semiconductors silicon with 1.2 eV and germanium 0.76 eV [1].

Fig. 2. 1: Comparison of conductivity of organic semiconductors.

The invention of transistor was the enormous progress toward the modern

electronics which basically started the domination of inorganic semiconductors over the

metals and vacuum tube technologies. The end of the 20th century brought omnipresence

of semiconductor electronics in the daily life uses. The emerging of organic

semiconductors in the beginning of the 21th century leaded the real revolution in the

electronics which made possible the fabrication of novel devices such as flexible solar

cells, e-paper, smart window, low cost printed circuits and large area flexible displays. In

contrast to inorganic semiconductors, the organic semiconductors are inexpensive, light

Conductivity Scm-1

10-12 10-8 10-4 100 104 108

Copper

Iron

Germanium Silicon

Glass Diamond

Insulators Semiconductors Conductors

Organic Semiconductors

12

weight, soluble and flexible. Their electronic and optoelectronic properties can be

modified.

The organic semiconductors are further divided into two main classes, polymers

and small molecules or oligomers. Both have the same chemical properties but differ only

in their physical properties. Jacob Berzelius introduced the polymers in 1832 as long

chain carbon molecules. These molecules have higher melting points due to stronger

intermolecular forces, while the small molecules have low melting points [2, 3].

2.1 Chemistry of Organic Semiconductors

The carbon atoms are the basic building block of Organic semiconductors. The

atomic number of carbon is 6 and its electronic configuration is 1s22s22p2. In chemical

bonding configuration, it forms sp, sp2 and sp3 hybridized orbitals which are the mixing

of atomic orbitals.

The organic semiconductors are π-conjugated materials and are formed by sp2

hybridization which forms three hybrids, each containing one electron having orientation

of an angle of 1200 to one another. The unhybridized pz orbital which is perpendicular to

the plane of sp2 hybridized orbital contains the remaining electron. The overlapping of

two sp2 orbitals results in the formation of s-bond which basically provides the structure

stability to the organic materials, while the π bond appears due to the pz orbitals and is

responsible for the conductivity of the materials (Fig. 2.2). The π electrons are

delocalized and have the ability to move freely within the chain. The appearance of

delocalized π-orbitals results as the lowest unoccupied molecular orbital (LUMO) and the

highest occupied molecular orbital (HOMO). The smaller energetic difference between

these orbitals provides the materials as semiconducting properties.

13

Fig. 2. 2: Scheme of the orbitals of two sp2 hybridized carbon atoms

2.2 Mechanism of Charge transport in organic

materials

The HOMO-LUMO energy gap of organic semiconductors lies between 1-4 eV

which is an indication of insulating behavior of the organic materials [4]. But the

opportunities of the generation and conduction of charge carriers can be produced by

using different methods including the creation of electron-hole pair by optical excitation,

chemical/electrostatic doping or the injection of charge carriers by metallic electrodes.

The organic semiconductors can be found in different states i.e. amorphous,

polycrystalline or single crystal, so the charge transport in these materials can have

different mechanisms. Organic semiconductors can be described on the basis of the

energy band theory of inorganic semiconductors. According to this theory, the charge

transport mechanism in organic materials is considered to be the same as in the extended

states of the valence and conduction bands of single crystal inorganic semiconductors.

The molecules in organic materials are weakly bonded by van der Waals and dipole-

dipole attractions, which causes the lesser intermolecular orbital overlap that results a

narrower band width and a short mean free path in contrast to the inorganic

semiconductors. According to band theory, the mobility of the materials is influenced by

π-bond

π-bond

σ-bond

pz- orbital pz- orbital

Plane of the

sp2 orbitals

14

the scattering of phonons, impurities and the temperature. The charge mobility is

described by the relation [5]

� ∝ ��(2.1)

Where T represents the absolute temperature and n depends on the scattering

mechanism. In naphthalene single crystals at ambient temperature, the hole FET mobility

was observed about 20-35 cm2/Vs while for same material mobility at cryo-temperatures

of several Kelvin was found of about 102 cm2/Vs by Time-Of-Flight (TOF) experiments

[6, 7]. A phonon-assisted hopping mechanism of charge transport in the amorphous and

polycrystalline materials is due to thermally activated charge mobility. In this

mechanism, the variation in the mobility with temperature is resulted by the thermally

activated hopping between localized sites. Then the activation energy E can be

described with the following [8]

�~�� exp ���

��� exp(2��) (2.2)

where E is the energy difference between two hopping sites, VP is the lattice frequency, T

is the absolute temperature and k is Boltzmann’s constant. The probability of the

localized hopping electron transferring to the next site at an energy E above the initial

one is represented by VP exp(-E/KT). The overlap of the wave-functions between adjacent

hopping sites is described by the term exp R2 where α is the decay rate of the wave-

function and R is the separation between sites.

The tight inter-molecular π-stacking in the packed molecular films results the

higher charge carrier mobilities. Then the charge transport in organic semiconductors

mostly depends on the transfer integral and the reorganization energy which relate with

the packing of organic semiconductors.

.

2.2.1 Organic Thin Film Transistors

Organic thin film transistor (OTFT) is three terminal device which is the

fundamental unit of organic electronic and optoelectronic devices. Without a bulk contact

it resembles with the Si MOSFET. All three types of materials i.e. conductor, insulator

and semiconductor are simultaneously used in organic transistors. It consists of thin films

15

of organic semiconductor, dielectric and three electrodes i.e. source, drain and gate. The

source and drain play the role of flow of charges and are directly connected with the

semiconducting thin film while the gate is used for the modulation of voltage or current.

OFETs are mainly prepared in one of following four different:

1. Bottom gate/top contact

2. Bottom gate/bottom contact

3. top-gate/bottom-contact

4. top-gate/top-contact

The first two configurations are depicted in Fig. 2.3. In first structure, the gate and

dielectric are deposited first on the substrate then both drain and source are deposited on

the top of the thin film of OSCs while in the second configuration, the drain and source

are directly developed on the gate and dielectric then OSCs film covers the whole device

[9].

Fig. 2. 3: Schematic structures of bottom-gate OFETs: top-contact (left) and

bottom-contact (right)

The top-contact transistors exhibit better performance with higher field effect

mobility, which may be attributed due to direct contact of source and drain electrodes

with the semiconducting thin film. But one important drawback of this structure is

possible damage of semiconducting film because the electrodes are usually deposited by

thermal evaporation through the shadow mask instead of lithographic processes.

In bottom-contact OFETs, the crystalline semiconducting material can influence the

charge transport and the prior existence of the metal electrodes can affect the growth of

organic film on the dielectric.

Gate

Source Drain

Gate Dielectric

Substrate

+++++++++++++++

Gate

Source Drain

Gate Dielectric

Substrate

+++++++++++++++Organic semiconductor Organic semiconductor

16

In other two OFET structures, the gate is developed on the top. In the top-

gate/bottom-contact geometry, the drain and source are patterned on the top of the

substrate, followed by the deposition of semiconductor thin film and then the gate

dielectric and finally the gate electrode are developed. In the top-gate/top-contact

structure the organic film is first deposited on the substrate, and then the source and drain

electrodes, the gate dielectric and finally gate electrode are deposited. The degradation in

the structure by ambient atmosphere can be prevented by using a polymeric dielectric.

Fig. 2. 4 Schematic drawings of top-gate OTFTs: top-gate/top-contact (left) and top-

gate/bottom-contact (right)

2.2.2 Factors limiting the mobility in OFETs

The charge mobility is an important parameter of the organic semiconductors

which can be found by different methods like SCLC, Time of flight and by OFETs. The

finding of mobility by OFETs is the best one. The value of mobility depends on the

organic material’s nature. The single crystal materials have higher mobility, but the

growth of single crystal of organic semiconductor is very difficult task. The

polycrystalline organic material posses the smaller value of mobility as compared to

single crystal because of different types of defects in the material like grain boundaries,

dislocation, stacking etc. These defects trap the charge carriers causing the decrease in

the mobility of the organic semiconductors [10].

Gate

Source Drain

Gate Dielectric

Substrate

+++++++++++++++Organic semiconductor

Substrate

Organic semiconductor

Source Drain+++++++++++++++

Gate

Gate Dielectric

17

2.3 Operation of the organic transistors

The organic semiconductors have the low charge carrier density so the gate field

not affects the bulk semiconductor, the charge transport can take place only in thin layer

close to the interface of the dielectric. OFETs can only function in accumulation mode.

When the gate and drain of a p-type OFET are negatively biased and VDS˂VG, a linear

current regime can be observed. By making VDS more negative, the linear rise in the

current IDS can be observed which is basically caused by the decrease in the density of

accumulated charges from the source to the drain (Figure 2.5). When the potential of

drain is more negative than the potential of the gate, the current tends to saturate and a

zone of depletion appears close to the drain (pinching of the channel).

The relation between drain source current and voltage i.e. (IDS & VDS) at fixed gate

voltage (VG) represents the output characteristic while the transfer characteristics give the

relation between IDS and VG at fixed VDS. The same information by the OFET

measurements is obtained when sufficient numbers of drain voltages (gate voltages) are

scanned as transfer (output) characteristics. The linear and saturation regime can be

obtained by transfer characteristics at two fixed VDS: one value as much smaller than the

maximum scanning gate voltage and the other VDS value is set larger than the maximum

scanning VG. Various important parameters like the field effect mobility (μFET), the on/off

ratio and the threshold voltage can be extracted from the transfer characteristics of the

OFET. The model developed for Metal Oxide Semiconductor Field Effect Transistor

(MOSFET) can be used for the characterization of the OFET. According to the standard

MOSFET model, the n-channel device is described by the standard current equation [11].

In the linear region (VDS< VG-Vth):

��� =����

���� − ��� −

���

�� ��� (2.3)

But for saturation operational region (VDS≥VG-Vth):

��� =����

��(�� − ���)

� (2.4)

Both equations show that μFET is a proportional factor that relates the IDS to the

VDS, VG, the length L and width W of channel, threshold voltage Vth. and the gate

capacitance per unit area Ci.

18

The measured values of the μFET in the saturation and linear regions of operation

have much difference, so they are referred as μFET,sat and μFET,lin . In this research work

μFET is measured in the saturation region.

Fig. 2. 5 Transfer (a) and output (b) characteristics of PDIF-CN2 TFT[12]

19

The mobility in OFETs is field-dependent and it may be limited by the existing

traps and by metal/organic contact (Schottky barrier). Therefore, to get the high value of

mobility remedies should be followed to minimize the effect of these factors. By

increasing the bias magnitude, OFETs can be operated away from the injection-limited

regime. This can be achieved by slightly increasing the values of IDS with VDS [13].

2.3.1 Threshold voltage

The gate voltage at which the conduction channel starts to appear is called the

threshold voltage. The charge transport mechanism in OFET is different from MISFETs

i.e. Metal Insulator Semiconductor Field Effect Transistors which are silicon based. It

operates in accumulation region; the conduction channel is not isolated from the substrate

due to absence of depletion layer [14]. Ideally, Vth should be zero for an organic transistor

operating in accumulation mode, but the real Vth is not equal to zero due to various

factors [15]. The appearance of non-ohmic injection barrier at the interface of metal and

semiconductor causes the misalignment of the energy levels of the metal contacts with

the LUMO of n-type materials and HOMO of p-type.

The value of threshold voltage (Vth) can be found by either the measurements by

plotting √IDS versus VG for saturation region or by extrapolating the linear region of the

IDS (VGS) curve to IDS = 0.

2.3.2 On/Off ratio

The ratio between the currents in the OFET in on and off states is referred as the

on/off ratio. The channel in organic transistors is not isolated from the other portion of

semiconductor, Then the total current is the sum of an ohmic current IDS, the conduction

current IDS in the channel (as both currents are parallel). The total current is:

I��,����� = I��,���� + I��,�������andtheratiois���,�������

���,����=

���

���� (1.5)

This ratio exhibits the ability of a transistor to use as a switch from the on state to the off

state. The transistors with on/off ratio of the order of 106 can be used in logic circuits

[16]. This ratio can be increased by different methods. The modification of transistor

20

geometry causes the appreciable rise in the value of the on/off ratio. The ratio of channel

width and length (W/L) has direct influence on the value of the drain-source current IDS

in the saturation region. Thus either by increasing the channel width or decreasing its

length, the value of the on/off ratio can be increased [17]. Another effective way to

increase the on/off ratio is to minimize the off-current. Which can be reduced either by

the purification of organic semiconductors or reducing leaks and noise interferences by

patterning active organic layer.

2.3.3 Contact resistances

The device performance is also influenced by the channel and contact resistances.

The total resistance of the OFET device (Rtotal) is the net value of the contact resistance

(Rc) and the channel resistance (Rch) i.e.

Rtotal =Rc + Rch while Rc= Rsource+ Rdrain (1.6)

The contact resistance which depends on the nature of electrodes i.e. the metal’s

work function and on the geometry of the transistor, is actually the sum of source and

drain resistances. The increase in gate voltage results in the rise in charge carrier density

in the channel which minimizes the contact resistance. In the bottom contact

configuration the contact resistance is high as compared to the top contact structure and it

depends on the drain voltage. The differences exist due to different reasons. In top

contact geometry, the injection surface is larger while in bottom contact the non-uniform

or disturbed growth of organic material by triple interfaces causes the increase in

resistance. Generally the faster response time for OFETs can be achieved by decreasing

the channel length and increasing µ. The interface modification by using Self-Assembled

Monolayer (SAM) can also reduce the contact resistance.

21

2.4 References

[1] D. Yon, H. Wang and B. Du, Introduction to Organic Semiconductr Heterojunctions,

Singapore: John Wiley & sons, 2010.

[2] J. Berzelius, Jahres-Bericht über die Fortschritte der physischen Wissenschaften,

Eilfter Jahrgang, 1832.

[3] R. Farchioni and G. Grosso, Organic Electronic Materials, Berlin: Springer, 2001.

[4] M. Pope and C. E. Swenberg, Electronic Processes in Organic Crystals and

Polymers, 2nd ed, New York: Oxford University Press, 1999.

[5] J. G. Laquindanum, H. E. Katz, A. J. Lovinger and A. Dodabalapur,

"Benzodithiophene rings as semiconductor building blocks," Adv Mater, vol. 9, pp.

36-39, 1997.

[6] E. Menard, V. Podzorov, S. H. Hur, A. Gaur, M. E. Gershenson and J. A. Rogers,

"High-performance n- and p-type single-crystal organic transistors with free-space

gate dielectrics," Adv Mater, vol. 16, pp. 2097-2101, 2004.

[7] W. Warta and K. Norbert, "Hot holes in naphthalene: High, electric-field-dependent

mobilities," Phys. Rev. B, vol. 32, pp. 1172-1182, 1985.

[8] Y. Olivier, V. Lemaur, J. L. Bredas and J. Cornil, "Charge hopping in organic

semiconductors: Influence of molecular parameters on macroscopic mobilities in

model one-dimensional stacks," J. Phys. Chem. A, vol. 110, pp. 6356-6364, 2006.

[9] Y. Y. Lin, D. J. Gundlach and T. N. Jackson, "Contact dependence of alpha-

sexithienyl thin film transistor characteristics," Electrical, Optical, and Magnetic

Properties of Organic Solid State Materials, vol. 413, pp. 413-418, 1996.

[10] H. E. Katz, Z. N. Bao and S. L. Gilat, "Synthetic chemistry for ultrapure,

processable, and high-mobility organic transistor semiconductors," Accounts Chem

Res, vol. 34, pp. 359-369, 2001.

[11] J. Soeda, T. Uemura, Y. Mizuno, A. Nakao, Y. Nakazawa, A. Facchetti and J.

Takeya, "High Electron Mobility in Air for N,N′

1H,1HPerfluorobutyldicyanoperylene Carboxydi-imide SolutionCrystallized Thin-

Film Transistors on Hydrophobic Surfaces," Adv. Mater, vol. 23, p. 3681–3685,

22

2011.

[12] C. D. Dimitrakopoulos and P. R. L. Malenfant, "Organic thin film transistors for

large area electronics," Adv Mater, vol. 14, pp. 99-117, 2002.

[13] A. Dodabalapur, L. A. Wang, D. Fine and D. Basu, "Electric-field-dependent charge

transport in organic thin-film transistors," J Appl Phys, vol. 101, p. 054515, 2007.

[14] G. Horowitz, R. Hajlaoui, H. Bouchriha, R. Bourguiga and M. Hajlaoui, "The

concept of "threshold voltage" in organic field-effect transistors," Adv Mater, vol.

10, pp. 923-927, 1998.

[15] G. Horowitz, P. Lang, M. Mottaghi and H. Aubin, "Extracting parameters from the

current-voltage characteristics of field-effect transistors," Adv Funct Mater, vol. 14,

pp. 1069-1074, 2004.

[16] J. Wang, H. Wang, J. Zhang, X. Yan and D. Yan, "Organic thin-film transistors with

improved characteristics using lutetium bisphthalocyanine as a buffer layer," J. Appl.

Phys, vol. 97, p. 026106, 2005.

[17] T. P. Saragi, T. Fuhrmann-Lieker and J. Salbeck, "High ON/OFF ratio and stability

of amorphous organic field-effect transistors based on spiro-linked compounds,"

Synthetic Met, vol. 148, pp. 267-270, 2005.

23

Chapter 3

Materials and Experimental Procedures

3.1 Organic Materials

In this research the organic semiconducting materials (i) 2-formyl-5,10,15,20-tetrakis(4΄-

isopropylpheny)prophyrinatocopper(II) or formyl-TIPPCu(II), (ii) N,N´-di-n-heptyl-2,3:6,7-

anthracenetetracarboxydiimide (ADCI7) and (iii) N,N´-di-n-octyl 2,3:6,7 anthracenetetracarboxydiimide

(ADCI8) have been used for the fabrication of devices.

3.1.1 formyl-TIPPCu(II)

formyl-TIPPCu(II) with full chemical name 2-formyl-5,10,15,20-tetrakis(4΄-

isopropylpheny)prophyrinatocopper(II) is carbon based metalloporphyrin. Its molecular structure is shown

in Fig. 3.1.1. The organic semiconductor formyl-TIPPCu(II) was synthesised by Yaseen et al [1].

Fig.3. 1 Chemical structure of formyl-TIPPCu(II).

3.1.2 N,N´-di-n-octyl-2,3:6,7-anthracenetetracarboxydiimide (ADCI8)

The ADCI8 is n-type organic material which belongs to the family of

anthracenetetracarboxydiimide and has an optical energy band gap of 2.95 eV. This material is very

24

suitable for fabrication of organic thin film transistors. Its optical and photoluminescence wavelengths in

Dichloromethane (DCM) are 419 and 423 nm. The molecular structure of ADCI8 is depicted in Fig. 3.2.

Fig.3. 2 The Molecular structure of ADCI8

3.1.3 N,N´-di-n-heptyl-2,3:6,7-anthracenetetracarboxydiimide (ADCI7)

The ADCI7 is also n-type organic semiconductor belonging to the family of

anthracenetetracarboxydiimide. Its chemical formula is given in Fig. 3.3. This material has been first time

used for the fabrication of n-channel OTFTs.

Fig.3. 3 The synthesis procedure and chemical structure of ADCI7

3.1.4 Poly(methyl methacrylate) (PMMA)

The synthetic polymer Poly(methyl methacrylate) (PMMA) with molecular formula (C5O2H8)n is

used as buffer layer on dielectric SiO2 to enhance the air stability of n-channel OTFTs [2]. The Molecular

O

O

O

+ H2N R1. CHCl3, RT

2. NaOAc, Ac2O, reflux

N

O

O

R

1, R = n-C7H152, R = n-C8H17

Br2, hv

CCl4

Br

Br

Br

Br

Br

BrBr

Br

1 or 2, NaI

N, N-dimethylacetamideNN

O

O

O

O

RR

4 (ADI7), R = n-C7H15

5 (ADI8), R = n-C8H17

3

structure of PMMA is given in Fig. 3.4. In this research, the commercially avail

forms a stable film because of having high melting point

Fig.3.

3.1.5 Hexamethyldisilazane (HMDS)

The organosilicon compound Hexamethyldisilazane (HMDS) with chemical formula

[(CH3)3Si]2NH is low density compound

monolayer for the fabrication of stable n

commercially available HMDS is used in this research.

Fig.3.

structure of PMMA is given in Fig. 3.4. In this research, the commercially available PMMA w

stable film because of having high melting point [3].

Fig.3. 4 Molecular structure of PMMA

3.1.5 Hexamethyldisilazane (HMDS)

The organosilicon compound Hexamethyldisilazane (HMDS) with chemical formula

NH is low density compound [4]. Being adhesion promoter it is used as self assembly

monolayer for the fabrication of stable n-channel OTFTs. Fig. 3.5 shows its chemical structure. The

available HMDS is used in this research.

Fig.3. 5 Chemical structure of HMDS.

25

able PMMA was used. It

The organosilicon compound Hexamethyldisilazane (HMDS) with chemical formula

. Being adhesion promoter it is used as self assembly

channel OTFTs. Fig. 3.5 shows its chemical structure. The

26

3.2 Fabrication Techniques

This section is composed of the general fabrication methods which have been employed in this

research work to fabricate the junction diodes, organic sensors and organic thin film transistors. These

include the pre-deposition preparation of substrates (glass and silicon wafer) and the material deposition

techniques.

3.2.1 Substrate Cleaning

The performance of organic devices depends on uniform growth of organic thin films on the

substrates. The adhesion and growth of organic material on the substrate is directly influenced by the

topography and cleanliness of the substrate structure. So the removal of contamination from the substrate

structure before any deposition is very necessary.

The procedures for cleaning the substrates depend on the nature of substrate. In this research work,

the silicon and glass substrates have been used for the fabrication of junction diodes, n-channel OTFTs and

organic sensors, respectively. Their cleaning procedures are illustrated here.

3.2.1.1 Glass substrates cleaning

For the detection of light, temperature and humidity, the surface type organic sensors have been

fabricated in this research work. The organic films were deposited on the thin glass slides which are

commercially available for medical purposes. Prior to the deposition, all substrates were cleaned by

following these steps

Detergent- to remove large surface contamination or possible oil droplets

Sonication-to remove surface particles- using distilled water in ultrasonic bath

Drying – in dust free environment

Plasma cleaning – to remove organic contaminations in thermal evaporator

3.2.1.2 Silicon Substrate Cleaning

In this dissertation, two different types of organic devices have been fabricated by using

silicon substrates. The p-type silicon has been used to form its junction diode with organic material, while

SiO2 covered n-type silicon wafers were used as gate electrode to fabricate the n-channel OTFTs. The

procedure of cleaning of silicon substrates is slightly different for both cases.

27

3.2.1.2.1 Cleaning of silicon substrate for junction diodes

To fabricate the formyl-TIPPCu(II)/p-Si, the silicon substrates were cleaned by adopting following

procedures.

Sonication- to remove large surface particles- using acetone in ultrasonic bath

Drying- by using nitrogen gas

Plasma cleaning- to remove the organic particles in plasma cleaner

3.2.1.2.2 Cleaning of silicon substrate for OTFTs

The n-type organic materials are very sensitive and react with water droplets to form hydroxyl ions

which become the traps for electrons. To minimize this effect, the silicon surface is cleaned to avoid

any contaminations. Therefore, the substrates were cleaned by using following steps:

Sonication- using acetone in ultrasonic bath- to remove

surface particles

Sonication- using 2-propanol in ultrasonic both- to remove

surface particles

Piranha cleaning- solution of 96% H2SO4 and 32% H2O2 in 2:1 volume ratio

to remove all possible contaminations

Sonication- in deionized water

Drying- in nitrogen gas

Plasma cleaning

The substrates were cleaned by using acetone, 2-propanol and Piranha in chemical hood.

Fig. 3.6 shows the Hood for cleaning substrates.

28

Fig.3. 6 Chemical Hood for cleaning substrates

In this research work, for OTFTs the substrates were plasma cleaned in Plasma cleaner PDC-32G (Fig.

3.7).

Fig.3. 7 Plasma cleaner PDC-32G

29

3.2.2 Thin Films Depositions

In this research, thin films of the materials were grown on the substrates by using two different

techniques.

1. Vacuum Thermal Evaporation

2. Spin coating

3.2.2.1 Vacuum thermal deposition technique

Vacuum thermal sublimation is one of the most popular techniques for the growth of thin layers of

different material in the range of angstroms to microns. Historically it is a very old technique which can

be traced to the formation of thin metallic film by Michael Faraday by exploding the metallic wire in

vacuum.

This technique consists of three major steps.

1. Generation of vapor

The cloud of vapours is created by heating the solid material (source) in

Vacuum chamber

2. Vapor Transpotation

The vapour are transported from source to target (Substrate)

3. Film growth

The vapor which strikes the substrate sticks with its surface causing the film growth

The organic semiconductors specially the small molecules are deposited by using this technique.

The organic material/metal is loaded in resistively heated crucible/boat. The substrates are placed in sample

holder whose temperature can also be controlled. Both are sealed in vacuum chamber having inside

pressures of ~10-6 mbar. The deposition rate and the thickness of the film can be controlled by the quartz

crystal and oscillator (crystal sensor) monitor. The evaporation can be adjusted to the desired value by

changing the orientation of shutter. The inner view of the vacuum evaporator is shown in Fig. 3.8, with the

sample holder (Fig. 3.9).

30

Fig.3. 8 Inner view of PLASMIONIQUE EVD-400H thermal evaporating system

Fig.3. 9 Sample Hoder in PLASMIONIQUE EVD-400H thermal evaporating system.

In this research work, for the fabrication of junction diodes, organic sensors and OTFTs, the thin

films of metal electrodes and organic materials were formed by thermal evaporation techniques. Both

31

metals (Gold & Silver) and organic material formyl-TIPPCu(II) were deposited on the substrates by using

thermal evaporator AUTO 306 having FTM5 film as thickness monitor in the Faculty of Material and

Manufacturing, GIKI (Fig. 3.10) while the n-channel OTFTs of ADCI7 and ADCI8 were fabricated by

depositing the thin film of organic materials and metals (Gold) on the n-Si substrates by PLASMIONIQUE

EVD-400H thermal evaporating system (Fig. 3.11).

Fig.3. 10 Edward Auto 306 vacuum thermal evaporator with thickness Monitor

32

Fig.3. 11 PLASMIONIQUE EVD-400H thermal evaporating system

3.2.2.2 Spin Coating

Spin coating has been used for many years for developing thin films, and the technique is

still effective for solution processable organic materials. In this technique a liquid or a suspension is

dispensed by a pipette onto the center of a substrate which rotates with uniform angular speed. The

thickness of the material is controlled by spinning speed. The spinning time, surface tension,

viscosity and volatility of a solvent and the amount of solid content present in suspension also affect

the thickness of the film. The schematic diagram of spin coating is shown in Fig. 3.12.

33

Fig.3. 12 Schematic diagram of spin coating technique.

In this research, the thin films of PMMA as buffer layers were deposited on the top of SiO2

dielectric layers for preventing the absorption of water droplets and to increase the mobility of n-

channel OTFTs. The films were grown at the spinning speed of 2500 rpm using spin coater Model

WS-400B-6NPP/LITE which is shown in Fig. 3.13.

Fig.3. 13 Spin coater Model WS-400B-6NPP/LITE

3.3 Atomic Force Microscopy

The performance of organic devices is directly influenced by the nature of organic films. The

charge mobility in OTFTs depends on the thin films growth. The surface morphologies of organic thin

films of OTFTs can easily be monitored by Atomic Force Microscopy. By using this tool, the irregularities

and any type of degradation in the thin layers can be studied.

34

A cantilever with a sharp tip of radius of curvature (~10-50 nm) is used to scan the surface

of sample. Then the Van der Waal interaction (~ 1nN) emerges between the neutral atoms of the cantilever

and surface which deflects the cantilever and the laser beam from the laser diode. A photodetector records

these deflections. The schematic diagram of AFM is shown in Fig. 3.14.

Fig.3. 14 Schematic diagram of AFM.

AFM can function in the following three different modes:

1. Static or contact mode

2. Semicontact or tapping (Intermittent contact) mode

3. Non contact mode

In this research work, surface morphologies of the thin films of the organic materials which were

grown for the fabrication of organic sensors and OTFTs were investigated in tapping mode (NSC 35

Micromasch cantilevers) by using MM Multimode AFM setup (Fig. 3.15). For the surface morphologies of

the thin films for OTFTs, ScanAsyst-Air (Bruker) silicon nitride levers with a nominal spring constant of 0.4

N/m, nominal resonant frequency of 70 kHz and tip radius < 5nm has been used.

Cantilever

Photodetector Laser

35

Fig.3. 15 MM Multimode 8 AFM setup

3.4 Device Characterization Techniques

3.4.1 Electrical Characterization

The current-voltage studies of the junction diode and OTFTs were carried out using the facilities

available at the Faculty of Engineering Sciences, GIK Institute and SCS at Perepichka lab McGill

University, Canada, respectively. These systems are briefly described here.

36

3.4.2 Current- Voltage (I-V) Characterization

The current-voltage measurements of the junction diode were carried out on Probe Station (KARL

SUSS PM5) at temperature range of 229-339K, equipped with keithley-196 System Digital Multimeter and

keithley-228A I-V source. Fig. 3.16 depicts the I-V characterization set up.

Fig.3. 16 Probe Station(KARL SUSS PM5) with keithley-196 System Digital Multimeter and

keithley-228A I-V source

While the electrical measurements of n-channel OTFTs were obtained by Keithley 4200-SCS in

vacuum at ambient atmosphere (Fig. 3.17). The inner view of the apparatus is shown in Fig. 3.18.

Fig.3. 17 Keithley 4200-SCS for characterization of OTFTs

37

Fig.3. 18 Inner view of Keithley 4200-SCS

3.5 Humidity dependent Characterization

Humidity dependent measurements i.e. capacitance and resistance of the fabricated humidity

sensors were investigated in self-assembled humidity setup connected with LCR and digital humidity

meter CEM DT-8860. The schematic diagram of the setup is shown in Fig. 3.19. By flowing the wet and

dry nitrogen in the chamber, the humidity level was changed.

Fig.3. 19 Schematic of experimental setup for humidity characterization

95.7

801.2

Dry N2

Distilled

water

Humidity

meter

LCR

meter

Humidity

Chamber

Sensor

Holder

Ag/formyl-TIPPCu(II) /Ag

Sensor

Exhaust

check valve

38

3.6 Light and Temperature Dependence Measurements

By changing the distance between fabricated sensor and light source the light dependent

measurements were carried out by using CEM DT-1300 light meter. The experimental setup is depicted in

Fig. 3.20

Fig.3. 20 Schematic of experimental setup to study the effect of light

The Temprotic Corporation’s thermo-chuck “Alpha” series probe station (KARL SUSS PM5) model

TP0315A-TS-2 of USA has been used to study the effect of changing temperature from 27 to 1870C on the

organic temperature sensor. Fig. 3.21 represents the experimental setup to study the effects of temperature.

Movable light source

Light sensor CEM DT-1300 light

meter

39

Fig.3. 21 Experimental setup to study the effects of temperature

3.7 References

[1] [2]

M. Yaseen, Mukhtar Ali, Muhammad NajeebUllah, Munawar Ali

Munawar, Irshad Khokhar,

J. Heterocyclic Chem., vol. 46, pp. 251-255, 2009.

"Poly(methyl methacrylate)," Wikipedia, [Online]. Available:

http://en.wikipedia.org/wiki/Poly(methyl_methacrylate). [Accessed 07 May 2014].

[3] W. F. Smith and J. Hashemi, Foundations of Materials Science and Engineering (4th ed.), McGraw-

Hill, 2006.

[4] "Bis(trimethylsilyl)amine," Wikipedia, [Online]. Available:

http://en.wikipedia.org/wiki/Bis(trimethylsilyl)amine. [Accessed 07 May 2014].

40

Chapter 4

Temperature Dependant Electrical Properties of formyl-

TIPPCu(II)/p-Si Heterojunction Diode

4.1 Introduction

Organic electronic is a commercially emerging field which constantly introducing more efficient

and stable commercial appliances like the active matrix display, complementary circuits, RFID tags and

sensors by using varieties of organic materials including small molecule and polymers by exploring their

unique electronic and optoelectronic properties [1-3]. It enables the designing of more eco-friendly and

affordable devices than silicon-based electronics. The organic semiconductors promise more accessible,

innovative and sustainable electronic technologies. The tunebility in the properties of organic

semiconductors either by alteration in chemical structure or by improving their surface morphology make

them special for fabricating a variety of devices with high efficiency and stability. The electrical properties

of conjugated organic materials can be studied by making their junction with inorganic semiconductors [4,

5]. The I-V measurements of organic devices are unable to provide much more information about the

device characteristics at room temperature [6], so the current-voltage characteristics at wide range of

provide useful information about electronic parameters of the devices. Among the small molecules,

porphyrins are more suitable for electronic conduction because of having large π-electron frameworks.

Porphyrin and metalloporphyrin being field responsive materials have been used for fabrication of

electronic and optoelectronic devices [7, 8]. Thin films of porphyrin which are formed by thermal

evaporation or solution methods are of crystalline natures which enhance the different features of organic

devices.

In this research work, the temperature dependent electrical properties of copper based porphyrin i.e.

2-formyl-5,10,15,20-tetrakis(4΄-isopropylpheny)prophyrinatocopper(II) or formyl-TIPPCu(II) have been

investigated by fabricating its junction with p-type silicon to explore more information about this material.

41

4.2 Device Fabrication

The temperature dependent behavior of copper based porphyrin i.e. 2-formyl-5,10,15,20-tetrakis(4΄-

isopropylpheny)prophyrinatocopper(II) or formyl-TIPPCu(II) has been studied by fabricating its junction

diodes with p-type silicon substrates. The synthesis procedure of formyl-TIPPCu(II) has already been

reported elsewhere [10].

Due to presence of carbonyl group in formyl-TIPPCu(II), its FTIR spectrum shows a strong

absorption at 1670 cm-1. In chloroform, its UV-Visible spectrum exhibits a B band or Soret absorption at

430 nm along with 550 and 595 nm as two Q-band absorptions (Fig. 4.1). At 872, the mass spectrum of

formyl-TIPPCu(II) depicts a molecular ion peak of 3% intensity with a base peak at 349 (Fig. 4.2).

Fig. 4. 1UV-Visible Spectrum of formyl-TIPPCu(II)

0

1

2

3

400 420 440 460 480 500 520 540 560 580 600 620 640

Abs

orba

nce

Wavelength (nm)

UV-Visible Spectrum of formyl-TIPPCu(II)

42

Fig. 4. 2 Mass Spectrum of Formyl-TIPPCu(II)

For the fabrication of formyl-TIPPCu(II)/p-Si junction diodes, the p-type silicon substrates with

orientation (100) were cleaned with acetone in ultrasonic bath for 15 minutes. After drying these substrates

by nitrogen gas, these were plasma cleaned for 10 minutes to remove different contaminations. The film of

formyl-TIPPCu(II) with thickness 140 nm was thermally grown on the substrate in the Auto 306 vacuum

coater with diffusion pumping system (Edward). The chamber pressure was kept as 5.5 × 10-5 mbar. The

thickness of formyl-TIPPCu(II) was measured by an FTM5 crystal controlled thickness monitor. The cross-

sectional view of the fabricated Ag/formyl-TIPPCu(II)/p-Si junction diode is shown in Fig. 4.3. The cross-

sectional view of the fabricated Ag/formyl-TIPPCu(II)/p-Si junction diode is depicted in Fig. 4.3. The

temperature dependant I-V measurements were carried out on thermo chunk ‘Alpha’ series probe station

(Karl Suss PM5) with Keithley SMU system 237.

Fig. 4. 3 Cross-sectional view of Ag/formyl-TIPPCu(II)/p-Si diode.

4.3 Results and discussion

The I-V characteristics of formyl-TIPPCu(II)/p-Si junction at temperatures (299-339K with 10 K intervals)

are presented in Fig. 4.4.

p-Silicon

formyl-

TIPPCu(II)

Ag

43

-8 -6 -4 -2 0 2 4 6 8

0.0

5.0x10-7

1.0x10-6

1.5x10-6

2.0x10-6

2.5x10-6

Cu

rren

t (A

)

Voltage (V)

299 309 319 329 339

Fig. 4. 4 I-V characteristics of formyl-TIPPCu(II)/p-Si diode at 299-339K.

The I-V curves exhibit weak voltage dependence for reverse current but these show exponential

response at low forward voltage and then deviate at high voltage which may be caused by the domination

of series resistance and interfacial layers. These graphs show the rectifying nature which is clearly caused

by the formation of organic/inorganic diodes. These diodes have much smaller value of turn on voltage ~

0.5V which are suitable for different applications. The current increases with the increase in temperature

which suggests a negative resistance coefficient of the diode. At the low applied voltage, the thermionic

emission (TE) theory can be used to analyse the I-V characteristics at low applied voltage [11, 12], the

current and voltage are related by the equation by using this theory (for q(V-IRs) > kT).

0 exp 1sq V IR

I InkT

(4.1)

Where voltage across the junction is represented by V, the Boltzmann constant by k, the electronic

charge by q, the temperature in Kelvin by T, the forward current by I and the diode quality factor by n

which describes the disagreement between experiment and simple theory at forward bias of the diode, and

the voltage drop across the parasitic series resistance (Rs) is IRs, the reverse saturation current (I0) can be

measured by semilog forward I-V plot (Fig. 4.5) at zero applied voltage, which can be represented as:

* 20 exp bq

I AA TkT

(4.2)

44

Wherethe effective diode area is represented by A, the effective Richardson constant by A*which

has the values 32 A/cm2K2 for p-Si [13], Φb is the zero bias barrier height..

-8 -6 -4 -2 0 2 4 6 8-21

-20

-19

-18

-17

-16

-15

-14

-13

ln I (

A)

Voltage (V)

299 309 319 329 339

Fig. 4. 5 Semi-logarithmic (I–V) characteristics of formyl-TIPPCu(II)/p-Si diode at different

temperatures.

The decreasing response of the reverse saturation current with decrease in temperature can be seen

in Fig. 4.6. The low value of reverse saturation current i.e. 1.19 × 10-8 A at 299K indicates the good quality

of fabricated junction diode. The changing values of saturation current at different temperature can also be

used to find the activation energy (Ea) of carrier conduction by using Arrhenius plot of logI0 verses 1000/T

with the help of following relation

0 exp aEI

kT

(4.3)

The value of Ea from this plot (Fig. 4.7) is determined as 0.59V.

45

300 310 320 330 3401.1x10

-8

1.2x10-8

1.3x10-8

1.3x10-8

1.3x10-8

1.4x10-8

1.4x10-8

1.5x10-8

1.6x10-8

1.6x10-8

I 0(A

)

Temperature (K)

Fig. 4. 6 Reverse saturation current of formyl-TIPPCu(II)/p-Si diode at different temperature.

2.9 3.0 3.1 3.2 3.3 3.4

-18.25

-18.20

-18.15

-18.10

-18.05

-18.00

-17.95

log

I 0 (

A)

1000/T (K-1)

Fig. 4. 7 Arrhenius plot of saturation current vs 1000/T for formyl-TIPPCu(II)/p-Si diode

The slope of the linear region of the semi-log I-V curve is used to extract the ideality factor of the

device at different temperatures (Fig. 4.5) by applying the expression:

46

ln

q dVn

kT d I

(4.4)

The values of ideality factor are greater than unity which may be possibly caused by different

factors such as inhomogeneities of film thickness, series resistance, interface states, secondary mechanisms

and non uniformity distribution of interfacial charges [14, 15]. It is also observed from the experimental

curves that the rise in temperature causes a decrease in the values of ideality factor. The series resistance

which plays the parasitic role in the diode can also be found by semi-logrithmic plot of forward I-V

characteristics. For high forward voltage, I-V curves deviate from linearity indicating the presence of series

resistance (Fig. 4.8). The voltage drop across the neutral region in semi-log curve ΔV=IRs is obtained by the

horizontal displacement between the extrapolated linear part and the actual curve, can be used for finding

the values of series resistance (Rs) at different temperatures [16].

0 1 2 3 4 5 6 7 8-18.0

-17.5

-17.0

-16.5

-16.0

-15.5

-15.0

-14.5

-14.0

-13.5

-13.0

ln I (

A)

Voltage (V)

299 309 319 329 339

Fig. 4. 8 Forward bias semi-logarithmic plots of current-voltage characteristics of formyl-

TIPPCu(II)/p-Si diode indicating the voltage drop ΔV=IRs across the neutral region.

The variation of series resistance of formyl-TIPPCu(II)/p-Si junction diode with change in

temperature is depicted in Fig 4.9. The rise in temperature results the decrease in the values of Rs which

actually causes the decrease in diode’s ideality factor. The values of zero bias barrier height at different

temperatures have been calculated by using well known Norde method for extraction of electronic

parameters [17]. According to this method the Norde function F(V) is calculated by following relation

47

2ln

I VV kTF V

q AA T

(4.5)

Where γ is the first integer which is greater than n. The plot of F(V) vs Voltage is given in Fig. 4.10.

At the minimum point of F(V), the values of barrier height at different temperature are calculated from the

equation:

00( )b

V kTF V

q (4.6)

300 310 320 330 340

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

5.5

Se

ries R

esis

tan

ce (

)

Temperature (K)

Fig. 4. 9 Series resistance-Temperature graph of fomyl-TIPPCu(II) diode.

48

0 1 2 3 4 5 6 7 80.72

0.74

0.76

0.78

0.80

0.82

0.84

0.86

0.88

F(V

) (V

olt

)

V (Volt)

299K 309K 319K 329K 339K

Fig. 4. 10 F(V) vs V plot of formyl-TIPPCu(II)/p-Si diode

Fig. 4.11 shows that, the rising temperature of diode, decrease the ideality factor while increases the

barrier height. The non ideal behaviour and variation in barrier height of formyl-TIPPCu(II)/p-Si junction

diode at different temperatures indicate the presence of barrier inhomogeneity and/or other mechanism of

electron transport. At lower temperature, the flow of current is mostly through the region of lower barrier

height. The significantly larger value of ideality factor at room temperature may be caused by different

reasons. The higher values of ideality factors are attributed due to secondary mechanisms, oxide layer

between organic material and silicon substrate [18].

49

300 310 320 330 34018

20

22

24

26

28

30

32

34

36

38

40

Ba

rrie

r H

eig

h

b0 (

eV

)

Temperature (K)

0.225

0.230

0.235

0.240

0.245

0.250

Idea

lity fa

cto

r (n)

Fig. 4. 11 Variation of Zero bias barrier height and ideality factor of formyl-TIPPCu(II)/p-Si diode

with temperature from 299-339K.

4.4 Conclusions

The thin films of formyl-TIPPCu(II) were grown on the p-type silicon substrates by thermal

evaporation technique to fabricate formyl-TIPPCu(II)/p-Si heterojunction diodes. The variation in

electrical characteristics of the fabricated device has been investigated in the temperature range of 299-339

K. The important parameters of diode like series resistance, ideality factor, reverse saturation current and

zero bias barrier height were extracted from I-V data. The parameters are strongly temperature dependent.

4.5 References

[1] Y. Karzazi, "Organic Light Emitting Diodes: Devices and applications," J. Mater. Environ. Sci, vol.

5(1), pp. 1-12, 2014.

[2] H. Klauk, U. Zschieschang, J. Pflaum and M. Halik, "Ultralow-power organic complementary

circuits," Nature, vol. 445, pp. 745-748, 2007.

[3] F. Aziz, M. H. Sayyad, K. Sulaiman, B. H. Majlis, K. S. Karimov, Z. Ahmad and G. Sugandi,

50

"Influence of humidity conditions on the capacitive and resistive response of an Al/VOPc/Pt co-planar

humidity sensor," Measurement Science and Technology, vol. 23(1), p. 014001, 2012.

[4] M. Tahir, M. H. Sayyad, F. Wahab, D. N. Khan and F. Aziz, "The electrical characterization of

Ag/PTCDA/PEDOT:PSS/p-Si Schottky diode by current–voltage characteristics," Physica B, vol. 415,

pp. 77-81, 2013.

[5] D. N. Khan and M. H. Sayyad, "Extraction of Electronic Parameters ofPEDOT:PSS-PVA/n-Si

Heterojunction Diode," in Second International Conference on Computer Research and Development,

Kaula Lampur, 2010.

[6] F. E. Cimilli, M. Sağlam, H. Efeoğlu and A. Türüt, "Temperature-dependent current–voltage

characteristics of the Au/n-InP diodes with inhomogeneous Schottky barrier height," Physica B, vol.

404, no. 8-11, pp. 1558-1562, 2009.

[7] L. L. Li and E. W. G. Diau, "Porphyrin-sensitized solar cells," Chemical Society Reviews, vol. 42, pp.

291-304, 2013.

[8] H. X. Wei, Z. X. Bo, S. J. Yong, Z. J. Wen, Z. Y. Di and S. Zongbao, "Colorimetric Artificial Nose for

Monitoring Traditional Solid State Fermentation Process of Vinegar Pei," J Food Processing &

Beverages, vol. 1, pp. 1-6, 2013.

[9] D. N. Khan, M. H. Sayyad, M. Yaseen, M. A. Munawar and M. Ali, "Application of Formyl-TIPPCu

(II) for Temperature and Light Sensing," World Academy of Science, Engineering and Technology,

vol. 51, pp. 210-212, 2011.

[10] M. Yaseen, M. Ali, M. NajeebUllah, M. A. Munawar and I. Khokhar, "Microwave-Assisted Synthesis,

Metallation, and Duff Formylation of Porphyrins," J. Heterocyclic Chem., vol. 46, pp. 251-255, 2009.

[11] E. H. Rhoderick and R. H. Williams, Metal Semiconductor Contacts, 2nd ed., Oxford: Clarendon

Press, 1988.

[12] S. M. Sze, Physics of Semiconductor Devices, 2nd ed., New York: Wiley, 1981.

[13] S. M. Sze, Physics of Semiconductor Devices, 2nd ed, John Wiley & sons, 2004.

[14] C. T. Sah, R. N. Noyce and W. Shockley, "Carrier generation and recombination in p−n-junctions and

p−n-junction characteristics," Proc. IRE, vol. 45, p. 1228, 1957.

[15] S. Gildenblat, S. Cohen and G. Sh., Metal-Semiconducteur Contacts and Devices, VLSI Electronics,

New York: Academic, 1986.

[16] M. N. Borah, S. Chaliha, P. C. Sarmah and A. Rahman, "Studies on current-voltage characteristics of

51

ITO/(n)CdSe-Al heterojunctions," J. Optoelectron. Adv. Mater., vol. 10, pp. 2793-2799, 2008.

[17] H. Norde, "A modified forward I‐V plot for Schottky diodes with high series resistance," J. appl.

Phys., vol. 50, p. 5052, 1979.

[18] M. Soylu, I. S. Yahia, F. Yakuphanoglu and W. A. Farooq, "Modification of electrical properties of

Al/p-Si Schottky barrier device based on 2'-7'-dichlorofluorescein," J. Appl. Phys., vol. 110, pp.

074514-9, 2011.

52

Chapter 5

The sensing of Humidity by surface type Ag/formyl-

TIPPCu(II)/Ag sensor for Environmental Monitoring

5.1 Introduction

Water which is present everywhere in the form of liquid, vapours or ice is essential for all forms of

life. It can be adsorbed on all surfaces as moisture and present in air as humidity. Its existence in air is

important for all living species. The relative humidity is one of the important factors which determine the

level of comfort. The sensing of humidity and moisture have great economic importance for prediction of

floods, preserving and processing of foodstuffs, for plants protection, for the maintenance of the optimum

conditions in manufacturing processes specially in electronic industries and for weather telemetry

applications [1]. It is important tool to control the climate indoors and to predict the climate outdoors for

monitoring environmental changes.

Effective humidity sensors have certain significant factors such as long term physical and chemical

stability, accuracy and repeatability, fast response time and cost effectiveness [2]. It is detected indirectly

from its effects on the materials. The sensing mechanism for humidity is either resistive or capacitive. In

the capacitive sensor, the change in relative humidity is detected by the variation in capacitance which is

caused by the change in dielectric constant due to water adsorption of sensing material while in the

resistive sensor, the variation occurs in the resistance of the material [3]. The capacitive technique is

considered as the most favorable for miniaturised humidity sensors [4]. There are varieties of materials

which can be used as active humidity sensing elements including ceramic, inorganic and organic

semiconductors [5, 6]. The organic materials exhibit excellent sensing behaviour due to their porous nature

as compared to inorganic which shows poor performance with respect to sensitivity, stability and

selectivity. The electrical properties like conductivity and dielectric constant of organic semiconductors

changes with the change in humidity and temperature. The Conventional humidity sensors are mostly based

on ceramic and inorganic semiconductors which have certain drawbacks. The ceramic based humidity

sensors show week humidity detective response and degradation in their sensitivity at high humidity due to

53

their pores widening which is caused by the diffusion of adsorbed water [7, 8]. They also need the periodic

regeneration of heat cleaning to wash out moistures to recover their humidity sensitive properties [9].

While the organic semiconductor based humidity sensors work efficiently in whole humidity levels. Porous

Si is also favourable semiconducting humidity sensing material but it gets contamination by its

environment due to reactivity [10]. The nonlinearilty, lack of reproducibility and temperature sensitivity are

others main limitations of porous Si based sensors [11]. The conventional humidity sensors are not only

much expensive than organic counterparts but also need high temperature fabrication procedures. While the

fabrication processes of organic sensors are not only simple but they also show low hysteresis. Due to

flexible nature of the organic materials, these sensors can be used for large area applications. Organic

sensors also need no heat cleaning process [12].

Organic materials are basically classified as polymers and small molecules. Organic Polymers have

attracted tremendous attention as active sensing materials due to their high sensitivity to humidity due to

their porous nature [13]. Conducting polymers are another important class of the organic materials which

have been used to monitor high humidity levels. Mostly the organic semiconductors are used in capacitive

type humidity sensors [14, 15].

Porphyrins which are basically macrocyclic compounds have high chemical and thermal stability.

They exhibit emission, absorption, charge transfer and complexing properties due to their characteristic

ring structure [16]. Porphyrins exhibit aromaticity because of having 22 π electrons with 18 π electrons in

direct conjugation [17]. For achieving certain specific properties, the metalloporphyrins are formed by

placing the metal ions into the central hole of the porphyrin ring. The copper based porphyrin i.e. formyl-

TIPPCu(II) or 2-formyl-5,10,15,20-tetrakis(4΄-isopropylpheny)prophyrinatocopper(II) has been synthesised

by fitting copper ions into the central porphyrin ring. Its synthesis procedure has already been described

elsewhere [18]. The formyl-TIPPCu(II) has already been investigated for the sensing for temperature and

light [19].

Humidity is an important parameter which directly influence the environment so in this research

work formyl-TIPPCu(II) has been first time used as active sensing material for detecting humidity. The

purpose of this research is to explore the efficient sensing organic materials which can be used to fabricate

the humidity sensors for environmental monitoring in domestic or industrial processes. To avoid the

damages in the active layer and shortening in the device, the surface type sensor is fabricated instead of

sandwich configuration.

54

5.2 Experimental

To detect the variation in Relative humidity, the organic semiconductor formyl-TIPPCu(II) was used as

sensing material to fabricate its surface type sensors with silver electrodes. The title compoundformyl-

TIPPCu(II) was synthesized by the method described by Yaseen et al[20] by placing the copper in central

vacant site of porphyrin and was purified by column chromatography.

The commercially available glass microscope slides were used as substrates. To avoid the different type

of contamination, the substrates were cleaned for 15 minutes in distilled water in ultrasonic bath, after

drying in dust free environment, the substrates were plasma cleaned for 8 minutes in thermal evaporator.

The silver electrodes with thickness of 100 nm were deposited on the substrates by Edward Auto 306

thermal evaporator with the deposition rate of 0.1nm/s by keeping inside pressure of the chamber at 10-5

mbar maintaining the uniformity in the metallic films. The gap of 40 μm was produced between the

electrodes by masking technique. The thin films of formyl-TIPPCu(II) (thickness ~ 140nm) were grown on

the electrodes in same thermal evaporator under same deposition rate and chamber pressure. The schematic

diagram of the fabricated device is shown in Fig. 5.1.

Fig. 5. 1The cross-sectional view of surface type Ag/formyl-TIPPCu(II)/Ag humidity sensor

The surface morphology of the vacuum sublimated thin film of formyl-TIPPCu(II) was studied by

Atomic Force Microscopy ((NSC 35 Micromasch cantilevers) in the standard tapping mode (Fig. 5.2). The

capacitive and resistive measurements of the fabricated sensors were carried out by placing them in a

sealed chamber using digital hygrometer and LCR meter at the frequencies of 1kHz and 10kHz and

100kHz. The level of humidity in the chamber was controlled by injecting the Nitrogen gas after passing

through the water vessel. The measurements were carried out at room temperature (250C).

formyl-TIPPCu(II)

Ag

Ag

Glass

55

Fig.5. 2AFM image of formyl-TIPPCu(II) on glass substrate.

5.3 Results and discussion

5.3.1 Atomic Force Microscopy

In the capacitive type humidity sensor, the dielectric constant of the active sensing material changes due to

adsorption of water which basically depends on the porosity of the material structure [21]. The voids which

appear during the growth of thin film provide rooms for tiny water droplets. The 3D AFM image (Fig. 5.3)

of the thin film of formyl-TIPPCu(II) shows that there are enough adsorption sites for water molecules due

to roughness of the film.

56

Fig.5. 33D AFM image of thin film of formyl-TIPPCu(II).

5.3.2 Capacitance Humidity Characteristics

The capacitance - relative humidity relationship of Ag/formyl-TIPPCu(II)/Ag humidity sensor at different

frequencies of 1kHz, 10kHZ and 100kHz, is shown in Fig. 5.4.

40 50 60 70 80 90 100

0

200

400

600

800

1000

1kHz 10kHz 100kHz

Ca

pa

cit

an

ce

(p

F)

RH%

Fig.5. 4Capacitance-relative humidity relationship of the Ag/formyl-TIPPCu(II)/Ag

57

The experimental results describe the increase in capacitance with the rise of relative humidity of the

device from 45% to 95% RH. There is sharp increase in capacitance of the sensor from 65% to 95% RH of

about 9.4 times at 1kHz, 4.6 times at 10kHz and 2.1 time at 100kHz of AC signal frequency of applied

voltage. The dielectric constant of the organic material increases due to adsorption of enough water

molecules which results in the rise of the capacitance of the sensor. Normally the dielectric constant of

organic materials is low i.e. lying between 4 to 8 [22, 23] but of water is high of about 80 [24]. So with the

adsorption of water the dielectric constant of the sensing material also increases. The adsorption of the

water depends on the porosity and the attractive forces between the surface of active organic material and

water molecules. AFM image shows that there is high degree of porosity in the formyl-TIPPCu(II)

structure. The increase in relative humidity results in the rise of adsorption of water molecules in the pores

of the film which increase the dielectric permittivity of the organic film. The dielectric permittivity of the

film is related with capacitance by following expression [25].

0

n

wS

d

C C

(5.1)

Where εd and εw are the dielectric permittivities of the thin film of formyl-TIPPCu(II) at dry and wet

conditions and C0, CS are corresponding values of capacitance and ‘n’ is the morphological factor.

Polarization of the organic material under the influence of external electric field is another factor for

the variation of capacitance [26]. The organic semiconductors like formyl-TIPPCu(II) acts as dielectric

having no free electrons. With the application of external electric field the bounded charges slightly

displace from their position causing the formation of dipoles in the organic material [27]. In humidity

sensor there are additional dipole formation (H+, OH-) in adsorbed water molecules which results the rise in

capacitance of sensor.

5.3.3 Capacitance Frequency Characteristics

Fig. 5.5. exhibits the capacitance –frequency relationships for four distinct humidity levels of 65%, 75%,

85% and 95%RH. There is no significant variation in the capacitance at high frequency of 100kHz as

compared to lower frequency of 1kHz of AC signal. At 95%RH, the capacitance of the sensor at 1kHz

frequency is 48.5 times greater than at 100kHz of the AC signal of the applied voltage, proving the fact of

unsuitability of organic semiconductors for high frequency electronic applications [28]. Organic

58

semiconductors have low mobility so they have insufficient time response of applied AC signals at higher

frequencies. At low frequency, the net polarization of material increases due to easier hoping of electrons

which results the increase in the capacitance of sensor. While at higher frequencies, the relaxation of charge

carrier is not as rapid as the time variation of the electric field which causes the decease in the capacitance

of the sensor [29, 30].

0 20 40 60 80 100

0

200

400

600

800

Cap

acit

an

ce(p

F)

Frequency (kHz)

%95RH %85RH %75RH %65RH

Fig.5. 5Capacitance-frequency relationship of the Ag/formyl-TIPPCu(II)/Ag sensor at 65%, 75%,

85% and 95%RH.

During the adsorption and desorption of water molecules, one of the important challenges faced by

humidity sensors is hysteresis which cannot be avoided completely because the rate of adsorption and

desorption is never same. The electrical parameters like capacitance and resistance of humidity sensors are

measured due to high porosity and moderate polarity of the organic thin film. The water molecules and

organic material are bounded together with hydrogen bonding and Van der Waal force [31, 32]. Desorption

of water starts when ambient air is drying down causing the destruction of physical bonds which results the

hysteresis at high level of humidity. Porosity of the film is another cause of hysteresis as it supports the

adsorption of water vapours but make difficult the rapid desorption [33]. The hysteresis property of the

humidity sensor of forlmyl-TIPPCu(II) was also studied at higher frequency of 1kHz and it was found

59

about 7% (Fig. 5.6). M. H. Sayyad et al [34] reported 9% hysteresis in capacitive measurements of surface

type Ag/ZnTIPP/Ag (Zn based porphyrin) humidity sensor at 1 kHz, Zubair et al [35] found 12%

repeatability in organic inorganic composite based humidity (Ag/Cu2O-PEPC-NiPC/Ag) sensor.

40 50 60 70 80 90 100

0

200

400

600

800C

ap

ac

ita

nce

(p

F)

RH%

Fig.5. 6Hysteresis-relative humidity relationship for Ag/formyl-TIPPCu(II)/Ag.

5.3.4 Resistance Humidity Characteristics

Resistance – relative humidity curves of Ag/formyl-TIPPCu(II)/Ag humidity sensor at 1kHz and

10kHz are shown in Fig. 5.7. The resistance of the fabricated sensor decreases with increase in relative

humidity from 45% to 95%RH. There is abrupt decrease in the resistance of the sensor from 45% to

80%RH. With the adsorption of water, the conductivity of the film increases which causes the decrease in

resistance with rise of humidity level. The electrical response of the sensor is due to proton hoping between

the chemisorbed hydroxyl groups at lower RH values [100]. The sensitivity of the humidity sensor is found

by the relation [101]

0

%

RR

SRH

(5.2)

60

Where R0 is starting resistance, ∆R is the change in resistance ∆R=Rmax-R0, and ∆(%RH) is change

in relative humidity. The sensitivity of formyl-TIPPCu(II) is found 0.0196%RH.

40 50 60 70 80 90 100

0

1

2

3

4

5

6 1kHz 10kHz

Re

sis

tan

ce

(M

RH%

Fig.5. 7Resistance-relative humidity relationship of Ag/formyl-TIPPCu(II)/Ag sensor at 1kHz and

10kHz.

The long term stability of the fabricated sensor at room temperature (250C) has been experimentally

observed in the laboratory. The good stability of the sensor has been confirmed after 200 cycles at low/high

humidity (45%RH/95%RH) at a temperature of 250C. The reproducibility of the sensing material formyl-

TIPPCu(II) based sensors have been checked by characterizing a number of devices of same structure and

dimension. The repeatability of the sensor is determined by hysteresis which depends on the temperature,

exposure time and total span of humidity cycles. Ag/formyl-TIPPCu(II)/Ag humidity sensor shows an

acceptable values of hysteresis which proves the repeatability of the device.

5.4 Conclusion

In this study, the humidity sensing capabilities of formly-TIPPCu(II) has been investigated by

fabricating surface type Ag/formyl-TIPPCu(II)/Ag humidity sensor. The variations in capacitance and

resistance of the sensor as function of relative humidity (45%-95%RH) have been observed. The fabricated

61

sensor exhibited the adequate sensorial properties and good stability. An acceptable value of hysteresis (~

7%RH) has been found. The film of formyl-TIPPCu(II) showed sensitivity of about 0.0196%RH.

5.5 References

[1] J. Fraden, Handbook of Modern Sensors Physics, Designs, and Applications, New York: Springer,

2010.

[2] C. Y. Lee and G. B. Lee, "Humidity sensors: A review," Sens. Lett, vol. 3, pp. 1-15, 2005.

[3] Y. Sakai, Y. Sadaoka and M. Matsuguchi, "Humidity sensors based on polymer thin films," Sensors

& Actuators B, vol. 85, pp. 35-36, 1996.

[4] Z. M. Rittersma, "Recent achievements in minitiarised humidity sensors-a review of transduction

techniques," Sensors and Actuators A, vol. 96, pp. 196-210, 2002.

[5] B. Ertug, T. Boyraz and O. Addemir, "Investigation of the Electrical Conductivity and Humidity

Sensitivity Characterristics of BaTiO3 Ceramics with PMMA Additive," Archives of Metallurgy and

Materials , vol. 57, no. 2, pp. DOI: 10.2478/v10172-012-0043-2, 2012.

[6] F. Aziz, M. H. Sayyad, K. Sulaiman, B. Majlis, K. S. Karimov, Z. Ahmad and G. Sugandi,

"Influence of humidity conditions on the capacitive and resistive response of an Al/VOPc/Pt co-

planar humidity sensor," Meas. Sci. Technol, Vols. 23, 014001, 2012.

[7] S. Chakraborty, K. Nemoto, K. Hara and P. T. Lai, "Moisture sensitive field effect transistors using

SiO2/Si3N4/Al2O3 gate structure," Smart Materials and Structures, vol. 8, p. 274, 1999.

[8] R. K. Nahar, "Study of the performance degradation of thin film aluminum oxide sensor at high

humidity," Sens Actuators B, vol. 63, pp. 49-54, 2000.

[9] E. Traversa, "Ceramic sensors for humidity detection: the state of the art and future development,"

Sensors & Actuators B, vol. 23, pp. 135-156, 1995.

[10] J. N. Chazalviel and F. Ozanam, Surface modification of porous silicon”, in Properties of Porous

Silicon, INSPEC, London, UK: L. Canham, ed., 1997.

[11] H. Saha, "Porous Silicon Sensors- Elusive and Erudite," International Journal on Smart Sensing and

Intelligent Systems, vol. 1, pp. 34-56, 2008.

62

[12] T. Zhang, X. Tian, B. Xu, W. Dong, L. Sun, S. Xiang and D. Gao, "A polymer sensitive to humidity

and its electrical properties analysis," Journal of Materials Science Letters, vol. 19, pp. 1419-1422,

2000.

[13] T. Fei, H. Zhao, K. Jiang, X. Zhou and T. Zhang, J. Appl. Polym. Sci. vol. 130, pp. 2056-2061,

2013.

[14] B. Adhikari and S. Majumdar, "Polymers in sensor applications," Progress in Polymer Science, vol.

29, pp. 699-766, 2004.

[15] Z. Ahmad, Q. Zafar, K. Sulaiman, R. Akram and K. S. Karimov, "A Humidity Sensing Organic-

Inorganic Composite for Environmental Monitoring," Sensors, vol. 13, pp. 3615-3624, 2013.

[16] A. Rest, J. D. Coyle, R. R. Hill and D. R. Roberts(Eds.), Light, Chemical Change and Life: A

Source Book of Photochemistry, Walton Hall: The Open University Press, 1982.

[17] M. Gouterman, The Porphyrins Vol.111, Part A, Physical Chemistry, New York: D. Dolphin (Ed.)

Academic, 1978.

[18] M. Yaseen, M. Ali, M. Najeebullah, M. A. Munawar and I. Khokhar, "Microwave-Assisted

Synthesis, Metallation and Duff Formylation of Porphyrins," J. Heterocyclic Chem, vol. 46, pp. 251-

255, 2009.

[19] D. N. Khan, M. H. Sayyad, M. Yaseen, M. A. Munawar and M. Ali, "Application of Formyl-

TIPPCu (II) for Temperature and Light Sensing," World Academy of Science, Engineering and

Technology, vol. 51, pp. 210-212, 2011.

[20] I. Sakai, Y. Sadaoka and M. Matsaguchi, "Humidity sensors based on polymer thin films," Sens.

Actuators B, Vols. 35-36, pp. 85-89, 1996.

[21] F. Gutmann and L. E. Lyons, Organic Semiconductors, Part A, Florida, USA: Krieger Robert E.

Publishing Company, Malabar, 1981.

[22] V. A. Kargin, Organic Semiconductors, Moscow: Nauka, 1968.

[23] M. Bjorkqvist, J. Salonen, J. Paski and E. Laine, "Characterization of thermally carbonized porous

silicon humidity sensor," Sens. Actuators A, vol. 112, pp. 244-247, 2004.

[24] J. G. Korvink, L. Chandran, T. Boltshauser and H. Baltes, "Accurate 3D Capacitance evaluation in

Integrated Capacitance Humidity Sensors," Sens. Mater, vol. 4, pp. 323-335, 1993.

63

[25] M. A. Omar, Elementary Solid State Physics: Principles and Applications, MA, USA: Addison-

Wesley:Reading, 1975.

[26] P. Lorrain and D. R. Corson, Electromagnetism: Principles and Applications, New York: Freeman,

1997.

[27] K. J. Baeg, A. Facchetti and Y. Y. Noh, "Effects of gate dielectrics and their solvent on

characteristics of printed n-channel polymer field-effect transistors," J. Mater. Chem, vol. 22, p.

21138, 2012.

[28] P. Bergo, W. M. Pontuschka, J. M. Prison, C. C. Motta and J. R. Martinelli, "Dielectric properties of

barium phosphate glasses doped with transition metal oxides," J Non-Cryst Solids, vol. 348, pp. 84-

89, 2004.

[29] R. S. Kumar and K. Hariharan, "AC conductivity and electrical relaxation studies on 10CuI–60AgI–

30V," Mat Chem Phys, vol. 60(1), p. 28, 1999.

[30] Y. Diamant, G. Marom and L. A. Broutman, "The effects of network structure on moisture

absorption of epoxy resins," J. Appl. Polym. Sci, vol. 26, pp. 3015-3025, 1981.

[31] P. S. Anderson, "Mechanism for the behavior of hydroactive materials used in humidity sensors," J.

Atoms. Oceanic Technol, vol. 12, pp. 662-667, 1995.

[32] A. Sun, Z. Li, T. Wei, Y. Li and P. Cui, "Highly sensitive humidity sensor at low humidity based on

the quaternized polypyrrole composite film," Sensors and Actuators B, vol. 142, pp. 197-203, 2009.

[33] M. H. Sayyad, M. Saleem, K. S. Karimov, M. Yaseen, M. Ali, K. Y. Cheong, F. Ahmad and M.

Noor, "Synthesis of Zn(II) 5,10,15,20-tetrakis(4-isopropylphenyl) porphyrin and its use as a thin

film sensor," Appl Phys A, vol. 98, pp. 103-109, 2010.

[34] N. Guermat, A. Bellel, S. Sahli, Y. Segui and P. Raynaud, "Electrical and structural characterisation

of plasma-polymerized TEOS thin films as humidity sensors," M. J. Condensed Matter, vol. 12, pp.

208-211, 2010.

[35] Q. Y. Tang and Y. C. Chan, "Fast-response Polyimide/multiwall Carbon Nanotube Composite

Films," in 12th Electronic Packaging Technology Conference, Singapore, 2010.

64

Chapter 6

Application of formyl-TIPPCu(II) for temperature

and light sensing

6.1 Introduction

For the utilization of renewable energy sources, the assessment of environmental conditions is an

important factor which can be done by monitoring temperature, humidity barometric pressure, wind speed

and direction, rain fall and air pollution [1, 2]. The organic semiconductors based sensors can be used for

sensing application of these environmental parameters because organic materials are very sensitive to

temperature [3, 4], radiations [5], humidity [6, 7] and various gases [8, 9].

Temperature is the most important parameter of the environment. It has direct influence on the

physical, chemical, biological, mechanical and electronic systems. The efficient working of these systems

is possible only within a certain range of the temperatures. To optimise and control these systems under

temperature limits, the regular monitoring of temperature needs the temperature sensors. These sensors also

help to minimize the possible damages in electronic circuits due to rise or fall in temperature. The

reliability of the electronic devices can be enhanced by proper monitoring the temperature. The electrical

properties of the organic semiconductors are directly affected by the change in temperature, so these

materials can be used as active temperature sensing materials [10-12].

Porphyrin is the heterocyclic organic compound which is found in nature in the form of chlorophyll,

which is thought to be the best photo acceptor in nature. The free base porphyrin can be changed into

metalloporphyrin by inserting the metals into its core [13]. Due to its attractive properties, porphyrin has

been used for the fabrication of different organic devices such as junction diodes [14], sensors [7, 15] and

solar cells [16].

In this research work, copper based porphyrin i.e. 2-formyl-5,10,15,20-tetrakis(4΄-

isopropylpheny)prophyrinatocopper(II) or formyl-TIPPCu(II) has been used to fabricate the organic sensor

for temperature and light sensing application.

65

6.2 Experimental

Thin film of organic semiconductor formyl-TIPPCu(II) is used as a sensing element for the

fabrication of Au/formyl-TIPPCu(II)/Au organic sensor. Microscope glass slides were used as substrates.

The substrates were cleaned ultrasonically in acetone for 25 minutes followed by thorough rinsing with

distilled water. After drying, the substrates were plasma cleaned for 5 minutes. The gold electrodes of 100

nm thickness were thermally deposited on the substrates by keeping 40 μm gaps between them by using the

mask. Then the thin film of formyl-TIPPCu(II) of thickness 140 nm was thermally sublimed on the

substrates. For these thermal depositions, the Auto 306 vacuum coater with diffusion pumping system

(Edward) was used under a chamber pressure of 5.5 × 10-5 mbar. The thickness of formyl-TIPPCu(II) and

gold films were measured by an FTM5 crystal controlled thickness monitor. The cross-sectional view of

the fabricated Au/formyl-TIPPCu(II)/Au sensor is shown in Fig. 6.1.

The electrical capacitance and resistance were measured by using DVM 890L and Kiethley 196

digital multimeters. The temperature dependence measurements of the device were made using Karl Suss

PM5 probe station with a thermo chuk ‘Alpha’ series system, model TP 0315A-TS-2 of Temprotic

Corporation, USA. The fabricated sensor was illuminated by a tungsten filament lamp at room temperature,

and the illumination measurements were made by CEM DT-1300 light meter.

6.3 Results and Discussion

Fig. 6.2 shows the capacitance-resistance versus temperature plots of Au/formyl-TIPPCu(II)/Au

surface type sensor. The measurements were made under the dark conditions at 40% RH. It has been

observed from these curves that capacitance increases by 4.3 times while resistance decreases by 4.4 times

Glass Substrate

Au Au

formyl-TIPPCu (II)

Fig.6. 1Cross-sectional view of the Au/formyl-

TIPPCu(II)/Au organic sensor.

66

with an increase in temperature from 27 to 1870C. The polarization and conductance of sensing film cause

the change in the magnitude of capacitance and resistance of the device [17]. With increasing temperature,

the molecular thermal movement becomes stronger, which increases the polarization and causing the

enhancement in the capacitance of the fabricated sensor. While the resistivity of the sensing material

decreases with the increase in temperature, according to the relation.

0 expT

E

kT

(6.1)

Where ρT is the resistivity at absolute temperature T, ρ0 is pre exponential factor, k is the Boltzmann’s

constant and E is the activation energy of conduction.

20 40 60 80 100 120 140 160 180 200

0.01

0.02

0.03

0.04

0.05

0.06

Temperature [0C]

CT/C

0

0.02

0.04

0.06

0.08

0.10

0.12

RT/R

0

Fig.6. 2Capacitance/resistance temperature relationships for the Au/formyl-TIPPCu(II)/Au organic

sensor.

It can be observed from Fig. 6.3 that the capacitive and resistive measurements have the hysteresis

of 3.2 and 5.1%, which may be caused by polarization of the material.

67

20 40 60 80 100 120 140 160 180 200

0.01

0.02

0.03

0.04

0.05

0.06

Temperature [0C]

CT/C

0

0.02

0.04

0.06

0.08

0.10

0.12

RT/R

0

Fig.6. 3Hysteresis in the capacitive and resistive measurements of Au/formyl-TIPPCu(II)/Au organic

sensor.

Fig. 6.4 shows the relationship between relative capacitance and illumination for Au/formyl-

TIPPCu(II)/Au surface type photocapacitive sensor. Where Cd and Cph are capacitances under dark

conditions and illumination respectively. This plot shows that the photocapacitance of fabricated sensor

increases about 13.2 times with the increase in illumination from 0 to 25000 lx. The concentration of

charge carriers, i.e. electrons, holes, ions and dipoles increases with increasing illumination, which causes

the increase in polarizability in the film which results in the increase in the capacitance of the sensor [18,

19]. The total polarizability (α) can be written as

dip i e t (6.2)

Where αdip, αi, αe and αt are polarizability under illumination due to dipoles, ions, electrons and the transfer

of charge carriers, respectively.

The change in capacitance of the device may be due to electronic and ionic polarizability. The

electronic polarizability arises due to the relative displacement of orbital electrons, while the ionic

polarizability is due to charge-transfer complexes in the formyl-TIPPCu(II). The capacitance is actually

depending on the relative permittivity (εr) of the material who depends on the polarizability according to

Clausius-Mossotti equation [18].

0

1

2 3r

r

N

(6.3)

68

Where N is the concentration of charge carriers, εr is the relative permittivity and ε0 is the permittivity of

free space.

0 5000 10000 15000 20000 250000

2

4

6

8

10

12

14

CP

h/C

d

Illumination (lux)

Fig.6. 4 Capacitance- illumination relationship for the Au/formyl-TIPPCu(II)/Au organic sensor.

6.4 Conclusions

The organic semiconductor formyl-TIPPCu(II) has been successfully used for the fabrication of

surface type Au/formyl-TIPPCu(II)/Au organic sensor. The changes in capacitance and resistance of the

sensor with temperature and light have been observed. The capacitance has increased by 4.3 times while

resistance of the fabricated sensor decreased by 4.4 times by rising temperature from 27 to 1870C. An

acceptable hysteresis for capacitive and resistive measurements was found. The rise of 13.2 times in

capacitance due to illumination was also observed. The association of photocapacitive response of the

organic sensor was assumed with polarization due to the transfer of photo – generated electrons and holes.

69

6.5 References

[1] A. Joyce, J. Adamson, B. Huntley, T. Parr, R. Baxter, "Standardization of temperature observed by

automatic weather stations," Environmental Monitoring and Assessment, vol. 68, pp. 127-136,

2001.

[2] W. Qihao, Y. Shihong, "Urban air pollution patterns, land use and thermal landscape: an

examination of the linkage using GIS," Environmental Monitoring and Assessment, vol. 117, pp.

463-489, 2006.

[3] S. A. Moiz, M. M. Ahmed, Khasan S. Karimov, "Effects of Temperature and Humidity on

Electrical Properties of Organic Semiconductor Orange Dye Films Deposited from Solution,"

Japanese Journal of Applied Physics, vol. 44, pp. 1199-1203, 2005.

[4] M. M. Ahmed, Kh.S. Karimov, S.A. Moiz, "Photoelectric behavior of n-GaAs/orange dye, vinyl-

ethynyl-trimethyl-piperidole/conductive glass sensor," Thin Solid Films, vol. 516, pp. 7822-7827,

2008.

[5] P. Bhattacharya, Semiconductor Optoelectronic Devices. New Jersey: Prentice Hall International,

1994.

[6] F. Gutman, L. E. Lyons, Organic semiconductors, Part A. Malabar, Florida, U.S.A: Krieger Robert

E. Publishing Company, 1981.

[7] M. Saleem, M. H. Sayyad, Khasan S Karimov, Muhammad Yaseen Mukhtar Ali, "Synthesis and

application of Ni(II) 5,10,15,20-tetrakis(4΄-isopropylphenyl)porphyrin in a surface-type

multifunctional sensor," Phys. Scr, vol. 82, pp. 015703 (6pp), 2010.

[8] H. Bai, Gaoquan Shi, "Gas Sensors Based on Conducting Polymers," Sensors, vol. 7, pp. 267-307,

2007.

[9] I. Muzikante, Vicente Parra, Rorijs Dobulans, Egils Fonavs, Janis Latvels, Marcel Bouvet, "A

Novel Gas Sensor Transducer Based on Phthalocyanine Heterojunction Devices," Sensors, vol. 7,

pp. 2984-2996, 2007.

[10] Y. Kurazumi, T. Tsuchikawa, J. Ishii, K. Fukagawa, Y. Yamato and N. Matsubara, "Radiative

and convective heat transfer coefficients of the human body in natural convection," Building

and Environment, vol. 43, pp. 2142-2153, 2008.

[11] B. B. J. Basu and N. Vasantharajan, "Temperature dependence of the luminescence lifetime of

a europium complex immobilized in different polymer matrices," Journal of Luminescence,

70

vol. 128, pp. 1701-1708, 2008.

[12] D. N. Khan, M. H. Sayyad, F. Wahab, M. Tahir, M. Yaseen, M. A. Munawar and

M. Ali, "Temperature dependant electrical properties of formyl-TIPPCu(II)/p-Si

heterojunction diode," Modern Physics Letters B, vol. 28(2014),

p. DOI:10.1142/S0217984914501000, .

[13] X. Zhou, "Synthesis and Characterization of Novel Discotic Liquid Crystal Porphyrins for Organic

Photovoltaics," vol. PhD. Kent, Ohio, USA: Kent State University, 2009.

[14] T. J. Savenije, Ellen Moons, Gerrit K. Boschloo, and Albert Goossens, Tjeerd J. Schaafsma,

"Photogeneration and transport of charge carriers in a porphyrin p/n heterojunction," Physical

Review B, vol. 55, pp. 9585-9592, 1997.

[15] M. Saleem, M. H. Sayyad, K. S. Karimov, M. Yaseen, Mukhtar Ali, "Cu(II) 5,10,15,20-tetrakis (4΄-

isopropylphenyl) porphyrin based surface type resistive - capacitive multifunctional sensor,"

Sensors and Actuators B, vol. 137, pp. 442-446, 2009.

[16] T. Oku, Akihiro Takeda, Akihiko Nagata, Tatsuya Noma, Atsushi Suzuki, Kenji Kikuchi,

"Fabrication and Characterization of Fullerene-Based Bulk Heterojunction Solar Cells with

Porphyrin, CuInS2, Diamond and Exciton-Diffusion Blocking Layer," Energies, vol. 3, pp. 671-

685, 2010.

[17] J. Wang, Wang X-H, Wang X-D, "Study on dielectric properties of humidity sensing nanometer

materials," Sensors Actuators B, vol. 108, pp. 445-449, 2005.

[18] F. Gutman, L.E. Lyon, Organic Semiconductors: Krieger Robert E. Publishing Company, Malabar,

Florida, USA, 1981.

[19] M. Iwamoto, Manaka. T, "Organic films and control of current–voltage characteristics by the

surface polarization," presented at Proc Int. Symp. on Super-Functionality Organic Devices (IPAP

Conf. Series vol 6), 2005.

[20] M. A. Omar, Elementary Solid State Physics: Principles and Applications: Pearson Education,

Singapore, 2002.

71

Chapter 7

Study of Anthracenediimide Derivatives for n-

channel Organic thin film transistors

7.1 Introduction

The high order mechanical flexibility of organic semiconductors enables their use

for flexible, foldable and rollable electronic devices which seems difficult with

conventional inorganic materials [1]. One of special feature of organic materials is the

tailorability in their structure and other properties for their use for different novel

application like smart windows, e-paper and flexible electronic and optoelectronic

devices. The organic molecules have week van der Waal interactions, so they can be

processed at low temperature which provide the opportunities to develop electronic

devices with less cost as compared to traditional inorganic devices.

Over the last few years, organic thin film transistors have received considerable

attention due to their potential applications in low cost and flexible integrated circuits

such as wearable textile, active matrix displays, RFID tags and organic complementary

circuits [2, 3]. OTFTs also play important role for the exploration of important

parameters of the organic materials like charge mobility, on/off current and threshold

voltage. The organic materials which are used for the fabrication of OTFTs, either be p or

n or ambipolar properties. The p-channel OTFTs have exhibited high mobility and

stability but the importance of n-type OTFTs could not be ignored due to their use in

greater power efficient complementary circuits.

The logic gates with low power dissipation and improved noise margin are the

basic requirements of the next generation all organic integrating complementary circuits

which can be achieved by assembling hole and electron transporting (p and n channels)

organic thin film transistors [4, 5].

72

There are certain factors which have crucial role on the performance of the

OTFTs such as nature of organic material (chemical structure and intermolecular

interaction), electrodes and gate dielectric, device configuration, the morphology of

active layer, energy alignments between organic material and electrodes and between the

gate and organic semiconductors [6].

Organic thin film transistor is surface sensitive device, its performance depends

on the nature of interfacial layers between organic and the dielectric layers [7, 8]. The

electron trapping by moisture or hydroxy group at the interface of semiconductor and

dielectric is the main hurdle for getting good performance of n-channel OTFTs. The

electron mobility in these OTFTs can be enhanced by either modification in chemical

structure of organic semiconductors or by dielectric surface modification. The density of

trap sites can be reduced by either developing self assembly monolayer (SAM) or by

depositing a buffer layer of polymer on the dielectric surface.

Small molecules based OTFTs show high performance due to their purity and

crystalline nature. A variety of small molecules like rubrene, pentacene, phtalolocyanine

etc. have been used for the fabrication of OTFTs [9, 10]. The air stable n-channel OTFTs

of ADCI8 have been reported with 2.0 × 10-5 cm2/Vs [11], so in this research work, the n-

channel OTFTs of ADCI8 and newly synthesised ADCI7 have been fabricated to get the

high value of mobility.

7.2 Device Fabrication & Characterization

Organic thin film transistors (OTFTs) of N,N´-di-n-heptyl-2,3:6,7-

anthracenetetracarboxydiimide (ADCI7) and N,N´-di-n-octyl 2,3:6,7

anthracenetetracarboxydiimide (ADCI8) were fabricated in a bottom gate/top contacts

configuration (with symmetric Au drain and source electrodes) and thin films were grown

at room temperature on heavily n-doped silicon wafer (ρ≈0.01 Ohm cm) covered with

thermally grown SiO2 layer (thickness ~ 200 nm, Ci=1×10-8F/cm2). Prior to deposition of

thin films, the substrates were cleaned to eliminate contaminates from the surface. All

samples were sonicated in acetone and 2-propanol for 5-10 minutes to remove

physisorbed impurities and any other tiny dust specs. After that, the samples were

73

cleaned in freshly prepared piranha solution for 15 minutes. Then substrates were

thoroughly rinsed by sonication in deionized water and dried in a steam of nitrogen gas.

Thin film of ADCI7 and ADCI8 were grown by vacuum deposition (10-6 to 10-7 mbar,

deposition rate 0.2-0.4 Å/sec) on SiO2/Si covered with a thin layer of PMMA and HMDS

treated. Thin films of PMMA in Toluene solution were deposited on SiO2 by spin coating

technique and then annealed for one hour at temperature of 1500C. Au drain and source

electrodes were uniformly deposited (at 10-6 mbar, deposition rate 0.1-0.3 Å/sec) to

prevent possible damages to organic film. The schematic diagram of bottom gate/top

contact OTFT of ADCI7 is shown in Fig. 7.1. The characterization of the fabricated

OFETs was performed in vacuum and ambient atmosphere in Keithley 4200-SCS.

Fig. 7. 1 Schematic diagram of bottom gate/top contact n-channel OTFT of ADCI7

7.3 Thin-film X-ray diffraction

X-ray diffraction measurements of thin films of ADCI7 and ADCI8 revealed that

patterns of both films showed sharp peaks and estimated d spacing is 2.19 and 2.37 nm

respectively (Fig.7.2).

n-Si SiO2

ADCI7 Au Au

Source Drain

74

0 20 40

0

10000

20000

30000

ADCI7

X-r

ay inte

nsity (

a.u

)

2(deg)

ADCI8

C7=2.19nm

C8=2.37nm

Fig. 7. 2 X-ray diffraction measurements of thin films of ADCI7 and ADCI8

7.4 Thin film morphology

The surface morphology of early growth stage of vacuum sublimed films of ADI7

and ADI8 was studies by Atomic force microscopy (AFM) in the scanAsyst mode. Fig.

7.3 and 7.4 show the AFM micrographs and the histogram of the height profile of thin

films of ADI7 and ADI8 deposited on PMMA treated SiO2/Si at the deposition rate of 0.1

Å/s while the substrates were kept at room temperature. The typical three dimensional

island (Volmer-Weber) growth was observed from AFM analysis of early growth films.

The formation of 3D islands illustrates the stronger molecule-molecule interactions than

molecule-surface interactions.

75

0 2 4

0.0

0.2

0.4

0.6

0.8

1.0

Cou

nts

(a

.u)

Height (nm)

Fig. 7. 3 AFM micrographs and the histogram of the height profile of thin films of

ADI7

2 4 6 8 10

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Counts

(a.u

)

Height (nm)

Fig. 7. 4 AFM micrographs and the histogram of the height profile of thin films of

ADCI8

The early stages growth of ADI7 film consists of sub-monolayers with average

height of 2-3 nm and average size of 0.004 µm2. By increasing the thickness of the film,

the grains seem to continue growing in one dimension with no substantial lateral growth

and cover the entire surface with number of grain boundaries.

Topographical AFM image of ADI8 also show one dimensional grain growing trend with

number of grain boundaries is shown in Fig. 7.4. The average height sub-monolayers is

2.40 nm and average size of .0036 µm2.

76

7.5 Thin-film transistor device characterization

OTFTs of ADI7 and ADI8 were fabricated by depositing thin films (50 nm) of

these organic materials on the PMMA/HMDS treated SiO2/Si substrates. PMMA/HMDS

were used on SiO2 to minimize the effect of electrons trapping by silanol groups. To

explore the type of majority charge carriers, characterization of the devices were

evaluated under negative and positive gate bias in vacuum.

The performance of the OTFTs was measured in vacuum at room temperature.

Fig. 7.5 and 7.7 show the output characteristics of ADCI8 and ADCI7 based n-channel

OTFTs which basically represents the source to drain current (IDS) vs voltage (VDS)

characteristics at different values of gate bias (VGS). The increase in the channel current

IDS with the rise in voltage VDS at positive gate voltage VGS shows the formation of n-

channel OTFTs. At low drain voltage, these characteristics show the ohmic behavior but

at higher drain voltage, the pinching off the IDS - VDS characteristics is observed with the

existence of the current saturation which confirms that these n-channel OTFTs also

follow the standard FET theory [12]. The IDS - VDS curves can be used to find the charge

mobility for (VDS≥VDS-Vth) by using relation

��� =����

��(�� − ���)

� (7.1)

Where W/L is the ratio of width and length of the channel, Ci is its capacitance,

Vth is the threshold voltage and µ is charge mobility.

The values of charge mobility for the PMMA surface modifies OTFTs of ADCI8

and ADCI7 were found 0.11×10-2 and 0.41×10-2 cm2V-1S-1, while for HMDS, 0.25×10-3

and 0.15×10-3 cm2V-1S-1, respectively. These values are much greater than the

previously reported mobility of ADCI8 [11].

Fig. 7.6 and 7.8 depict the transfer (IDS –VGS) characteristics of ADCI8 and

ADCI7 OTFTs. These curves show that the on/off ratio of these OTFTs is of the order of

104. The threshold voltage for each transistor has also been measured by transfer

characteristics. Table 7.1 shows the values of mobility, on/off ratio and threshold voltage

of OTFTs of ADCI8 and ADCI7.

77

The OTFTs treated with PMMA show high mobility as compare to HMDS treated

dielectric because the PMMA has the good hydrophobicity which prevents the moistures

in penetrating to dielectric layer. The PMMA helps in the molecular ordering of ADCI7

and ADCI8 on the SiO2 dielectric layer and minimize the grain boundaries which results

the reduction in electron trapping and increasing the mobility of the OTFTs [13].

Table 7.1

Compound Deposition temp (0C)

µ (cm2V-1S-1) Ion/Ioff Vth (V)

ADCI8 900C 2.0×10-5 104 45 [11] ADCI8 (PMMA)

230C (rt) 1.07 × 10-3 104 55

ADCI8 (HMDS)

230C (rt) 2.49 × 10-4 104 43

ADCI7 (PMMA)

230C (rt) 4.11 × 10-3 104 65

ADCI7 (HMDS)

230C (rt) 1.45 × 10-4 104 58

0 20 40 60 80 100 120 140

0.00

0.02

0.04

0.06

0.08

0.10

90V

105V

I SD(

A)

VSD

(V)

120V

Fig. 7. 5 Output characteristics of ADCI8

78

0 20 40 60 80 100 120

0.0

0.1

0.3

0.8

2.1

5.6

15.2

41.4

112.5

305.9

VG(V)

I DS(n

A)

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

(IDS )

1/2(

A)1

/2

Fig. 7. 6 Transfer characteristics of ADCI8

0 20 40 60 80 100 120 140

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

90V

105V

I SD

( A

)

VSD

(V)

120V

Fig. 7. 7 Output characteristics of ADCI7

79

0 20 40 60 80 100 120

0.0

0.3

2.1

15.2

112.5

831.5

VG (V)

I DS(n

A)

0.0

0.2

0.4

0.6

0.8

1.0

(IDS )

1/2(

A)1

/2

Fig. 7. 8 Transfer characteristics of ADCI7

7.6 Conclusion

The n-channel organic thin film transistors of ADCI7 and ADCI8 were fabricated

on dielectric layers of SiO2. To minimize the trapping of electron the dielectric surface

were modified by buffer layer of PMMA and self assembly monolayer of HMDS. The

high charge mobility of ADCI8 was found as compare to already reported value. The

newly synthesised organic material of same family ADCI7 has also shown high mobility.

Both OTFTs exhibited the on/off ratio of order of 104.

7.7 References

[1] H. Klauk, "Organic thin-film transistors," Chem.Soc.Rev, vol. 39, p. 2643–2666,

2010.

[2] H. Sirringhaus, N. Tessler and R. H. Friend, "Integrated optoelectronic devices

80

based on conjugated polymers," Science, vol. 280, p. 1741–1744, 1998.

[3] A. N. Flora, W. Yiliang and S. O. Beng, Organic Thin Film Transistor Integration: A

Hybrid Approach, John Wiley & Sons, 2011.

[4] B. Crone, A. Dodabalapur, Y. Y. Lin, R. W. Fillas, Z. Bao, A. LaDuca, R.

Sarpeshkar, H. E. Katz and W. Li, "Large-scale complementary integrated circuits

based on organic transistors," Nature, vol. 403, pp. 521-523, 2000.

[5] D. J. Gundlach, K. P. Pernstich, G. Wilckens, M. Grüter, S. Haas and B. Batlogg,

"High mobility n-channel organic thin-film transistors and complementary

inverters," J. Appl. Phys, vol. 98, p. 064502, 2005.

[6] C. Wang, H. Dong, W. Hu, Y. Liu and D. Zhu, "Semiconductingπ-Conjugated

Systems in Field-Effect Transistors: A Material Odyssey of Organic Electronics,"

Chem. Rev, vol. 112, pp. 2208-2267, 2012.

[7] T. Muck, V. Wagner, U. Bass, M. Leufgen, J. Geurts and L. W. Molenkamp, "In situ

electrical characterization of DH4T field-effect transistors," Synthetic Metals, vol.

146, pp. 317-320, 2004.

[8] R. Ruiz, A. Papadimitratos, A. C. Maye and G. G. Malliaras, "Thickness dependence

of mobility in pentacene transistors," Adv. Mater, vol. 17, p. 1795, 2005.

[9] D. Braga, M. Campione, A. Borghesi and G. Horowitz, "Organic Metal-

Semiconductor Field-Effect Transistor (OMESFET) Fabricated on a Rubrene Single

Crystal," Advanced Materials, vol. 22, pp. 424-428, 2010.

[10] S. Lee, G. Jo, S. J. kang, G. Wang, M. Choe, W. Park, D. Y. Kim, Y. H. Kahun and

T. Lee, "Enhanced Charge Injection in Pentacene Field-Effect Transistors with

Graphene Electrodes," Adanced Materials, vol. 23, pp. 100-105, 2011.

[11] Z. Wang, C. Kim, A. Facchetti and T. J. Marks, "Anthracenedicarboximides as Air-

Stable N-Channel Semiconductors for Thin-Film Transistors with Remarkable

Current On-Off Ratios," J. Am. Chem. Soc, vol. 129, pp. 13362-13363, 2007.

[12] K. Nomura, H. Ohta, A. Takag, T. Kamiya, M. Hirano and H. Hosono, "Room-

temperature fabrication of transparent flexible thin-film transistors using amorphous

81

oxide semiconductors," Nature, vol. 432, pp. 488-492, 2004.

[13] J. Y. Kim, J. M. Kim, T. S. Yoon, H. H. Lee, D. Jeon and Y. S. Kim, "Pentacene

Thin Film Transistors with Various Polymer Gate Insulators," Journal of Electrical

Engineering & Technology, vol. 4, pp. 118-122, 2009.

82

Summary

In this research, the organic semiconducting materials formyl-TIPPCu(II), ADCI7

and ADCI8 have been investigated to explore their electronic and optoelectronic

properties for their applications for developing different organic devices.

The junction diodes and organic sensors of formyl-TIPPCu(II) have been

fabricated by thermal evaporation technique. The electronic parameters of junction diode

of formyl-TIPPCu(II) with p-type silicon were found strongly temperature dependent.

The ideality factor decreases while the barrier height decreases with the rise of

temperature from 299-339K. The parasitic series resistance of the diode which is

basically limiting the current in the device decreases with the increase in temperature.

The formyl-TIPPCu(II) has also been used as active sensing material to detect the

humidity, temperature and light. The surface type Ag/formyl-TIPPCu(II)/Ag humidity

sensors have been fabricated for the detection of relative humidity by measuring the

capacitance and resistance of the sensors from 45% to 95%RH. The remarkable change in

capacitance and resistance has been observed at different humidity levels. An acceptable

7% hysteresis has also observed in these sensors at the frequency of 1kHz of ac signal.

The ability of formyl-TIPPCu(II) for sensing light and temperature has been

studied by fabricating the surface type Au/formyl-TIPPCu(II)/Au sensor. With the rise of

temperature from 27oC to 180oC, the increase in relative capacitance of 4.3 times has

been observed. While the same sensor exhibited 13.2 time rise in relative capacitance by

increasing the illumination from dark to 25000 lx which prove that formyl-TIPPCu(II)

can be used as active sensing material.

Newly synthesised n-type organic semiconductors ADCI7 and ADCI8 have been

used to fabricate n-channel organic thin film transistors. The appreciable increase in the

mobility of ADCI8 has been observed as compared to already reported its value, while

ADCI7 as the compound of same show high mobility and on/off ratio. To get the high

performance of OTFTs, the surface of dielectric has been modified by buffer layer and by

self assembly monolayer of PMMA and HMDS respectively.

83

Future Work

The low cost, light weight and flexible organic complementary circuits is the need of next

generation electronics. The n-channel OTFTs have still limitations which can be minimized by introducing

the air stable n-type organic semiconductors having the electron transporting capability. The atmospheric

humidity also affects the mobility of n-type organic materials which can be reduced by synthesising the

materials which resist these environmental effects or introducing a proper packaging to avoid the

wandering atmospheric water droplets. By improving the fabrication techniques, the performance of the n-

channel OTFTs can also be enhanced.

As the organic single crystal devices show higher performance than the amorphous or

polycrystalline materials, so a comprehensive study is required to develop the single crystal of the organic

materials and to fabricate their devices on nanoscale.

For developing highly efficient organic devices, the need is to know the chemistry of the organic

materials to explore their electronic and optoelectronic properties.

Dil Nawaz Khan