Aluminum Doped Zinc Oxide Thin Film for Organic Photovoltaics

by

Fanjie Wei

A thesis submitted in conformity with the requirements for the degree of Master of Applied Science

Graduate Department of Materials Science and Engineering University of Toronto

© Copyright by Fanjie Wei (2010)

ii

Aluminum Doped Zinc Oxide Thin Film for Organic Photovoltaics

Fanjie Wei

Master of Applied Science

Graduate Department of Material Science and Engineering

University of Toronto

2010

Abstract

Aluminum Doped Zinc Oxide (AZO) produced by radio frequency (RF) magnetron

sputtering is thought to be the prospective replacement of the de facto standard indium tin oxide

(ITO) anode in organic solar cells. In order to achieve a proper resistivity and transmittance of

AZO thin film compared to ITO, a systematic study was done to optimize the sputtering

conditions. In this work, two primary parameters: target-substrate distance and sputtering power,

were optimized, and a optimized film thickness was determined. A poly(3-hexylthiophene)

(P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) bulk-heterojunction organic

solar cell was fabricated based on the optimized parameters and the power conversion efficiency

reached 0.83%. A theoretical analysis is given to explain the optimization process. This work

provides a clear pathway to substitute AZO for ITO in organic solar cells for future mass

production.

iii

Acknowledgments

During the last two years, I am really happy to have the chance to work with so many

exceptionally diligent and brilliant people.

Firstly and foremost, I would like to express my sincere thanks to my supervisor Prof.

Zheng-Hong Lu, for his support throughout my project. Professor Lu not only provides the

direction for me, but also motivates all the individuals in this group.

I would like to thank all the members in Lu’s group: Dr. Dan Grozea, Dr. S.W Tsang, Dr

Stella Tang, Dr Julia Jin, Dr Hui Wang, Mark Greiner, Michael Helander, Zhibin Wang, Dong

Gao, Jacky Qiu. I would like to express my gratitude to all these people for their insightful views

and generous help when I meet difficulties which were usual for me. Especially, I am deeply

thankful to Dr Stella Tang. We worked together on sputtering system for a long time. I want to

say my most gratitude to her for a lot of useful discussions and her generosity to provide help to

me at any time.

I would like to thank my friends Yao Ma and Daniel Wen for all the theoretical

discussions and their help to use the SEM facility.

Finally, I am very grateful to my parents and my younger brother and cousins. If it were

not for their support, I may not overcome the difficulties during the last two years.

iv

Table of Contents

Abstract

Acknowledgements

Table of Contents

List of Tables and Figures

Chapter 1 Introduction………………………………………………………..01

1.1 Organic Solar Cell.……………………………………………………………01

1.1.1 Motivation For Solar Cell…….…...….....……………………………01

1.1.2 Photovoltaics…….…...……………………………………………….05

1.1.3 Organic Solar Cell……......……….………………………………….06

1.2 Motivation for Alumina Doped Zinc Oxide….….…………………………..14

1.2.1 ITO Drawbacks………………………………………………………..14

1.2.2 Alumina Doped Zinc Oxide……………….………………………….16

Chapter 2 Experimental……………………………………………………...20

2.1 Experimental Facilities………...……………………………………………..21

2.1.1 Sputtering Chamber………….….……………...……………………21

2.1.2 Spin Coating and Aluminum Evaporation…………..……………..25

2.1.3 Testing System….…………………………………………………….25

2.2 Experimental Procedures……………...…………………..………………...27

v

Chapter 3 Characterization of AZO…………………………………………29

3.1 Optimization of Target to Substrate Distance…….….……………..…….29

3.2 Drude Theory…………………………………………..……………………..34

3.3 Qualitative Explanation………..………………………….…………………37

3.4 Optimization of Deposition Power…...…..…… …………………………..41

3.5 Optimization of Thickness ….………….…………………………………...46

3.6 Conclusions………………………...………………………………………...47

Chapter 4 Application in OPVs.…………….………………………………48

Chapter 5 Summary………………………………………………………….50

References...….……...…………………………………………………………52

vi

List of Tables

Table 1.1 The parameters of ITO and the minimum requirements on alternative TCOs….…..17

Table 1.2 The resistivity and dopant content for ZnO films doped with various dopands [48]..18

Table 2.1 Target to substrate distances and their positions……………………………………..24

Table 3.1 The sheet resistance and resistivity as a function of distances for different power

levels………………………………………………………………………………....29

Table 3.2 Surface resistance and resistivity as a function of thickness for particular target-

substrate distances and power of 50W ……..………………………………………..32

Table 3.3 Resistivity as a function of substrate-target distance with the film thickness 600nm

and deposition power 100W…………………………………………………….…….33

Table 3.4 The deposition rates in different powers…...……………………………….….…….41

Table 3.5 Surface resistance and resistivity as a function of power at constant film thickness of

500nm and target to substrate distance of 8cm………..…………………………..…..42

Table 3.6 Comparison of ITO and optimized AZO properties…...………………………….…47

Table 4.1 The device performance of optimized AZO based solar cell………...……………...49

vii

List of Figures

Figure 1.1 World oil price in dollars from 1986 to 2010 [2]………………..……………………1

Figure 1.2 The trend in globally averaged temperature near the Earth's surface [3]…………….2

Figure 1.3 The breakdown of current and estimated energy sources from 2000 to 2100 [4]…....3

Figure 1.4 World landmass required to provide 18 TW of solar electricity assuming 8% cell

efficiency. The colors in the map show the local solar irradiance averaged over three

years from 1991 to 1993 (24 hours a day) taking into account the cloud coverage

available from weather satellites. ……………………....……………………………4

Figure 1.5 Historical module prices of solar cells historical …………………………………….4

Figure 1.6 The evolution of best research-cell efficiencies in history…………………………...6

Figure 1.7 Estimated PV module prices and the market proportions for the three generations of

solar cells [12]………………………………………………………………………..7

Figure 1.8 The important milestones in the development of organic solar cells...………………8

Figure 1.9 The competitive position of every solar technology is mainly determined by the

factors efficiency, lifetime and cost.…………………………………………………9

Figure 1.10 (left) OLED mode of operation, (right) organic PV mode of operation. In both cases

an organic material is sandwiched between two electrodes. Typical electrode

materials are shown in the figure. In PVs electrons are collected at the metal

electrode and holes are collected at the ITO electrode. The reverse happens in a

LED: electrons are introduced at the metal electrode (cathode), which recombine

with holes introduced at the ITO electrode (anode). [23]..…………………………10

Figure 1.11 The chemical structures of P3HT and PCBM……...………………………………11

Figure 1.12 The mechanism of organic photovoltaics [10]…………......………………………12

viii

Figure 1.13 General mechanism for photoenergy conversion in excitonic solar cells………….12

Figure 1.14 Energy levels and light harvesting [23]………..…………………………..………13

Figure 1.15 The schematic current-voltage (I-V) curve of a polymer-fullerene BHJ solar

cell[23]………………………………………………………............…….………..13

Figure 1.16 Worldwide Indium production from 2002 to 2006 [33]………….………………..15

Figure 1.17 Unit price of indium in 1998 dollars per ton from 1987 to 2007 [33]……….…….15

Figure 2.1 Schematic of the sputtering chamber.……………………………………………...20

Figure 2.2 The homemade sputtering substrate holder………………………………………..21

Figure 2.3 The sputtering machine used in our lab. On the left is the sputtering chamber from

outsite. On the right is the control panel. Below is a view of the inside of the

machine, three targets are put symetrically in it..……………………….…………23

Figure 2.4 The machines used for spin coating and metal deposition. On the left is the spin

coater, which is stored in a glove box. On the right are the cluster tools used for

metal and LiF vacuum deposition………………………………………………….25

Figure 2.5 On the Left is the simulated AM1.5G solar illuminations, on the right is the testing

facilities used in organic solar cell testing…………………………………………26

Figure 2.6 Schematic of four points probes……………………………………………..……..26

Figure 3.1 Resistivity as a function of target to substrate distance for different power levels...30

Figure 3.2 Transmittance spectrum in the visible range (150W 8cm)…………………………31

Figure 3.3 Resistivity as a function of film thickness for material deposited in 50W….……...32

Figure 3.4 Resistivity as a function of substrate-target distance for a 600nm thick film at

100W…………………………………………………………………………….…33

ix

Figure 3.5 The transmittance spectrum in the visible range for 600nm thick AZO thin films

deposited at different target to substrate distances with power of 100W………….34

Figure 3.6 The reflectance and transmittance spectra of an AZO film produced by pulsed laser

deposition…………………………………………………………………………..36

Figure 3.7 The correlation between the transmittance edge in the IR and the Carrier

concentration…………..……………………………………………………………38

Figure 3.8 The transmittance spectrum in the visible and IR range with different target to

substrate distances for 600nm thick films deposited with 100W power……….….38

Figure 3.9 XPS spectra of 600nm thick AZO thin films deposited with different target to

substrate distances using power 100W ……………………………………………39

Figure 3.10 SEM images of 600nm thick AZO thin films deposited with different target to

substrate distances using 100W power...………………………………………..…40

Figure 3.11 Deposition rate as a function of power…………………………….………………42

Figure 3.12 Resistivity as a function of power for a film thickness of 500nm and target to

substrate distance of 8cm ……….……….………………………………..………43

Figure 3.13 The transmittance spectra in the visible range for 500nm thick films deposited with

different sputtering power levels at a target to substrate distance of 8cm…………43

Figure 3.14 The transmittance spectra in the visible and IR range for 500nm thick films

deposited with different sputtering power at target to substrate distance 8cm..…...44

Figure 3.15 SEM images of 500nm thick AZO thin films deposited with different sputtering

power levels of a target to substrate distance 8cm………………….……….……..45

Figure 3.16 Surface resistance as a function of thickness for sputtering power levels of 200W

and 250W………………………………………………………………………..…46

Figure 4.1 The standard structure of an AZO based organic solar cell………………………..48

x

Figure 4.2 I-V curve of an organic solar cell with an AZO anode deposited with the optimized

parameters…………………………………………………………………..….….48

1

Chapter 1 Introduction

1.1 Organic Solar Cell

1.1.1 Motivation for Solar Cell

Total global energy consumption in 2020 is estimated to be 20TW [1]. More than half of

our energy supply is derived from nonrenewable energy sources such as petroleum, coal or

natural gas, while only a small fraction is derived from renewable energy sources. Two main

factors have contributed to the recent boom in renewable energy production.

Firstly, oil prices are becoming extremely expensive. In July 2008, the price reached

$147.27 per barrel [1]. The enormous price fluctuations of oil prices have highlighted the strong

dependence on oil and have contributed to political and economic instability in many oil-

importing countries. Figure1.1 shows the world oil price in dollars from 1986 to 2008. We can

observe the radical increase in oil price in the last 30 years. On the one hand, the oil price has

been steadily increasing from $20 per barrel in the 1980’s to more than $140 per barrel in recent

years, On the other hand, the oil price also tend to fluctuate as a result of any political or

economical turbulence in the world, which makes the situation worse for many countries’ stable

development.

Figure 1.1 World oil price in dollars from 1986 to 2010. [2]

2

Secondly, the environmental problem is a main concern in the pursuit of renewable

energy. From Figure 1.2, we can see the trend of global temperature over the last 150 years after

the industrial revolution. The temperature has increased 1.4°F since the industrial revolution. In

spite of much debate during the recent Copenhagen Conference, we must admit that the carbon

dioxide ( 2CO ) emission from oil consumption may contribute to future environmental costs that

cannot be ignored. We should not only be concerned about global warming, but also the impact

of other pollutants such as 2SO , CO produced by the burning of oil and coal in thermal power

stations and cars.

Figure 1.2 The trend in globally averaged temperature near the Earth's surface. [3]

Since there are huge demands for renewable energy in the next decades, which renewable

energy source will be dominant in the future?

Figure 1.3 shows the breakdown of current and estimated energy sources from 2000 to

2100. According to recent estimates, in 2100, solar energy will play the most important role in

the global power supply: more than half of the electricity will be generated by photovoltaic (PV)

technology.

3

Figure 1.3 The breakdown of current and estimated energy sources from 2000 to 2100. [4]

There are several reasons why solar energy is expected to make up more than half of our

energy supply. First of all, solar is almost an eternal energy source. Global annual energy

consumption is currently estimated to be 15.8TW, increasing to 20TW in 2020. But the Sun’s

radiation onto the surface of the earth corresponds to 120,000 TW per year, which means each

hour the sun delivers to earth the amount of energy used by humans in an entire year! [5] Figure

1.4 shows the area required to produce 18 TW from PV sources; 6 sites globally, with a total area

of 910,019 km2. Secondly, other kinds of renewable energies have significant drawbacks. For

example, nuclear energy is currently an economic energy source compared to solar energy.

However, the fission products will remain for a million years, which will be harmful to the

environment. Thirdly, the solar energy price is decreasing quickly. Figure 1.5 shows the trend in

per module pricing of solar cells. We can estimate that in 10 years, solar energy can compete

with on-grid fossil fuel, and should even be able to achieve the United States Department of

Energy (DOE) cost goals.

4

Figure 1.4 World landmass required to provide 18 TW of solar electricity assuming 8% cell

efficiency. The colors in the map show the local solar irradiance averaged over

three years from 1991 to 1993 (24 hours a day) taking into account the cloud

coverage available from weather satellites. [4]

Figure 1.5 Historical module prices of solar cells historical. [4]

5

1.1.2 Photovoltaics

A solar cell is a device that can capture sunlight energy and then convert it to electricity

directly by photovoltaic effects. There have been three generations of photovoltaics.

1st Generation: The first generation photovoltaics are based on crystalline silicon

technology, which was first developed more than 50 years ago. It has succeeded in achieving

market penetration. For single crystalline based solar cells, the maximum theoretical efficiency

for this type of device is approximately 30%. [6] The highest practical efficiency for crystalline

silicon solar cell is 29% [7]. However, crystalline silicon solar cells have their drawbacks: the

silicon wafers are fragile; the process is very labor and energy intensive, etc. Hence the broad

application of the first generation solar cell technology is limited.

2nd Generation: The second generation photovoltaics are defined as thin film

technologies. Amorphous silicon (a-Si), copper indium gallium diselenide (CIGS), or cadmium

telluride (CdTe) are used as the semiconductor. The promise of low cost power has not been

realized, and efficiency remains lower than that of 1st generation solar cells. The highest

efficiency of 19.9% for CIGS solar cell has been achieved by The United States National

Renewable Energy Lab (NREL) [8].

3rd Generation: It has been estimated that third generation solar technologies will achieve

higher efficiencies and lower costs than first or second generation technologies. Today, the third

generation approaches being investigated include dye-sensitized solar cells, organic

photovoltaics, tandem cells, etc. The organic solar cell is among the most promising alternatives

in the future due to its enormous merits. Until now, the highest efficiency of 6.5% [9] for organic

photovoltaic is achieved by all solution tandem solar cells. There is still a gap to commercialize

organic solar cells. In order to commercialize it, a minimum efficiency of 15% is required.

Figure 1.6 depicts the evolution of best research-cell efficiencies. The first generation

silicon based solar cell has achieved very high efficiency and accounts for the major commercial

market globally. The second generation solar cell such as CdTe and CIGS thin film solar cells

will emerge and penetrate into the photovoltaics market. For organic solar cells, there is still a

long way to go before it can be commercialized

6

Figure 1.6 The evolution of best research-cell efficiencies in history. [8]

1.1.3 Organic Solar Cell

As we know the silicon based solar cell has been commercialized for a few years, but

there are still unsolved problems in order to completely realize the promise of low cost and high

efficiency. Since there are many stubborn problems such as expensive highly purified silicon

wafer, environmental unfriendly techniques in the manufacturing process in silicon solar cell

industry, we can see many attractive features of organic solar cells compared to the first

generation solar cells, among them are: [10]

● Large area coating by spin coating technology

● Easy integration in different devices

● Potential of flexibility and semi-transparency.

● Significant cost reduction compared to traditional solutions such as refined, highly purified

silicon crystal

7

● The potential to be manufactured in a continuous printing process

● Substantial ecological advantages compared to the environmental unfriendly first generation Si

based solar cell

For all the above features, organic solar cell is a less expensive alternative to inorganic

semiconductors like Si. The superior material properties of polymers (plastics) and the potential

for mass production with cheap processing techniques have made organic photovoltaics

(especially polymer solar cell) the most promising technology in the near future.

Until now, the highest efficiency of organic solar cell is not more than 6.5%, which is

much less than the first and second generation solar cells. However, according to Stephen

Forrest’s calculation, the maximum theoretical efficiency for organic power conversion

efficiency is about 20%. [11] Figure1.7 shows us the estimated PV module prices and the market

proportions for the three generations of solar cells. Once we can achieve efficiency of more than

15% for organic solar cells in the future, considering the much less expensive fabrication costs

for organic solar cells compared with the other types of solar cells, organic solar cells will beat

all the other rivals.

Figure 1.7 Estimated PV module prices and the market proportions for the three generations of

solar cells. [12]

8

Becquerel was the first one who actually observed the photo-electrochemical process

[13,14]. In 1839, Becquerel discovered a photocurrent when platinum electrodes, covered with

silver bromide or silver chloride, were illuminated in aqueous solution. Anthracene was the first

organic material in which photoconductivity was observed by Pochettino in 1906 [15]. The big

breakthrough in organic photovoltaics was achieved by Tang in 1986 when he introduced the

first donor-acceptor (DA) device more than a decade ago [16]. After 1986, large progress has

been realized by scientists. Until now, two main methods have been investigated in the effort to

develop organic PV devices[17]: the donor-acceptor bi-layer[18,19,20], usually by vacuum

deposition of small molecular components, and the bulk hetero-junction (BHJ ) [21,22], which

represents the ideal case as a bi-continuous composite of donor and acceptor phases and mainly

is in the form of polymer and processed in solution. The highest efficiency is about 6.5%, which

is achieved by all solution tandem solar cells. There is still a gap to commercialize organic solar

cells. In order to commercialize it, a minimum efficiency of more than 15% is required. The

important milestones in the development of organic solar cell can be described as Figure 1.8.

Figure1.8 The important milestones in the development of organic solar cells.

9

Figure 1.9 The competitive position of every solar technology is mainly determined by the

factors efficiency, lifetime and costs.

In the energy market, the competitive position of every solar technology is mainly

determined by the factors efficiency, lifetime and costs, as shown in Figure 1.9 [10]. All these

three factors must be judged before any practical research can be done. This figure reveals the

important restrictions of successful commercialization of solar cells. All three aspects must be

fulfilled at the same time. Otherwise they will be limited to a niche market. Other criteria such as

semi-transparency, flexibility are beneficial but not enough to make up for a significant

deficiency in one of the triangle’s corners.

An organic solar cell device is the reverse of an organic light emitting diode (OLED), a

technology which has already been commercialized. In both cases an organic material is

sandwiched between two electrodes. Electrons are collected at the metal electrode and holes are

collected at the indium tin oxide (ITO) electrode. Most organic semiconductors are intrinsic

semiconductors and the primary excitation is a Coulomb bound exciton. Figure 1.10 shows us

the modes of OLED and Organic PVs.

10

Figure 1.10 (left) OLED mode of operation, (right) organic PV mode of operation. In both

cases an organic material is sandwiched between two electrodes. Typical electrode

materials are shown in the figure. In PVs electrons are collected at the metal

electrode and holes are collected at the ITO electrode. The reverse happens in a

LED: electrons are introduced at the metal electrode (cathode), which recombine

with holes introduced at the ITO electrode (anode).

A planar-layered structure is the basic structure of organic solar cells. An organic light-

absorbing layer is sandwiched between anode and cathode. A semi-transparent layer is used as

one electrode where light can be transmitted through it and absorbed by the organic functional

layer. Indium-tin-oxide (ITO) is the most used transparent electrode layer in organic PVs. ITO is

often used as anode. The other electrode---cathode is often aluminum, calcium, magnesium, or

gold. Nowadays, poly(3-hexylthiophene) (P3HT) and fullerene derivative [6,6]-phenyl- butyric

acid methyl ester (PCBM) are the most used organic light-harvesting materials. P3HT serves as

an electron donor and transporter of holes to the cell anode, while PCBM network acts as an

electron acceptor and transporter of electrons to the cell cathode [20, 24, 25, 26, 27, 28]. The

chemical structures of P3HT and PCBM are shown in Figure 1.11. PCBM was first synthesized

by Fred Wudl et al. at the UC Santa Barbara [29].

11

Figure 1.11 The chemical structures of P3HT and PCBM.

The fundamental mechanism of organic solar cells is the reverse of OLEDs. Figure 1.12

and Figure 1.13 illustrate the mechanism of organic photovoltaics. Although there are more than

one mechanism to describe their behavior, the commonly accepted mechanism has four

steps[10]:

1) Absorption of light and generation of excitons

2) Diffusion of the excitons

3) Dissociation of the excitons with generation of charge

4) Charge transport and charge collection

When light is absorbed, an electron is promoted from the highest occupied molecular

orbital (HOMO) to the lowest unoccupied molecular orbital (LUMO) and an exciton is formed.

This process will be followed by the exciton diffusion and dissociation. The electron and hole

then will reach the cathode and anode. The charge separation is achieved by the asymmetrical

ionization energy/work functions of the electrodes.

12

Figure 1.12 The mechanism of organic photovoltaics. [10]

Figure 1.13 General mechanism for photoenergy conversion in excitonic solar cells.

13

By irradiation, an electron is promoted to the LUMO leaving a hole in the HOMO.

Electrons are collected at the Al electrode and holes at the ITO electrode. This process can be

described by Figure1.14, in this figure, stands for work function, means electron affinity,

IP is defined as ionization potential and gE is the optical band gap.

Figure1.14 Energy levels and light harvesting. [23]

Figure1.15 The schematic current-voltage (I-V) curve of a polymer-fullerene BHJ solar cell.

[23]

14

scJ is the short-circuit current density, ocV is the open circuit voltage. mJ and mV are the

current and voltage at the maximum power point. The fill factor (FF) which is defined as the

ratio of area of maximum rectangle fitted in the 4th

quadrant I-V and the product of ocV and scI

and efficiency ( ) are defined as following:

( )( )

( )( )

m m

sc oc

J VFF

J V

( )( )out m m

in in

P J VFF

P P

1.2 Motivation for AZO

1.2.1 ITO Drawbacks

ITO is one of the most widely used transparent conducting oxides (TCOs). It is a solid

solution of indium(III) oxide (accounts for 90% by weight) and tin oxide (for 10% by weight).

ITO is transparent in thin layers. It will become yellow or grey in the bulk. ITO is a degenerately

doped n-type semiconducting oxide, with bandgap of approximately 3.75eV. [30, 31]

Recently, in the organic electronics industry, ITO plays an important role mainly in the

following fields: OLED thin film technology for solid state lighting, flat-panel screens, large-

area PV efforts, liquid crystal displays, etc. ITO can fulfill a compromise between electrical

conductivity and optical transparency. ITO serves as anode in OLED and organic solar cells. The

commonly used deposition methods of ITO include sputtering, electron beam evaporation or

physical vapor deposition.

However, the large use of ITO in the flat panel industry, combined with the shortage of

indium resources globally, has made the price of ITO increase significantly in the past few years.

The cost of indium tin oxide in 2006 reached more than 10 times its price in 2003 due to the

explosion of the flat panel display market [32, 33]. Figure 1.16 depicts the annual indium

consumption and prices. Figure 1.17 shows the price of indium in 1998 dollars per ton from 1987

to 2007.

15

In 2006, the consumption of indium was 500t; it has since doubled in 2002. And the price

in 2006 is 8 times higher than that in 2002. An even bigger problem is that the indium overall

reserves in the world is only 16000 tons based on concentrations of indium in zinc ores [34],

which means we will run out of indium in less than 10 years. Although estimates may vary due

to the unexpected factors, we still can conclude that the demand for indium is expected to grow

radically and hence the indium price will increase greatly in the near future as well.

Figure 1.16 Worldwide Indium production from 2002 to 2006. [33]

Figure 1.17 Unit price of indium in 1998 dollars per ton from 1987 to 2007. [33]

300

350

400

450

500

550

2002 2003 2004 2005 2006Ind

ium

Wo

rld

Co

nsu

mp

tio

n (

t)

Year

16

In addition, ITO also has other eternal drawbacks due to the material properties: such as

lacking of flexibility of ITO layer, lacking of chemical stability, fragility and the toxicity of

indium element. All of the above mentioned problems urge us to explore alternatives to the ITO

anode. Some alternative TCO films have been investigated by scientists, among them are:

1) Transparent polymer thins film such as Poly(3,4-ethylenedioxythiophene):Poly(styrene

sulfonate). The conductivity of PEDOT:PSS can reach 1000 S/cm[35]. In addition, it is

transparent and absorption is less than 10% in the visible spectrum. [36]. However,

PEDOT is unstable and the processing is difficult and costly.

2) Carbon nanotubes (CNTs) can solve the problem of film fragility since CNTs have very

good materials and mechanical properties. In addition, CNTs have good electrical

conductivity compared with traditional metals [37]. However, CNTs are extremely

expensive and are difficult to obtain uniform dispersions of the CNT thin films.

3) Transparent conducting oxides such as Alumina doped Zinc Oxide, Gallium doped Zinc

Oxide, etc are prospective alternatives to ITO.

All three alternative transparent conducting thin films are now under investigation by

scientists, and of course are still not yet fully commercially realized. In this thesis, I will explore

the possibility of transparent conducting metal oxide as an alternative to ITO.

1.2.2 Alumina Doped Zinc Oxide

Comparable properties of transparent conductive oxide thin films and economic

considerations must be offered for the ITO alternatives. The required characteristic of an

alternative TCO to replace ITO are summarized in Table1.1. In order to be comparable with ITO

thin film, the replacement TCO must fulfill the basic criteria as given on the right side of Table

1.1.

A lot of effort has been put into a number of transparent conducting metal oxides.

However, most research focused on p-type metal oxides which served as a buffer layer between

the anode and hole transporting layer. The commonly tried oxides include xNiO [38], 2CuAlO

[39], 2CuGaO [40], 3MoO [41], etc.

17

Table 1.1 The parameters of ITO and the minimum requirements of on alternative TCO.

ITO TCOs

Transparency 90% >80%

Resistivity Low (1x10-4

Ωcm) <1x10-3

Ωcm

Work function 4.7eV Comparable

Thickness 100nm <300nm

Sheet Resistance 12Ω/□ <100 Ω/□

Price Expensive Inexpensive

Toxicity Yes No

Reserves Rare Large

Nowadays, TCOs that have been commercialized and act as the anode are n-type metal

oxides doped with other compounds such as 2SnO or 2 3In O . We can achieve high transparency

as well as proper electrical conductivity by selecting a wide-bandgap oxide through the

introduction of substitutional dopants. Nowadays, zinc oxide based thin films have attracted

people’s interest for its good properties and potential huge applications. The following are the

applications of zinc oxide based derivatives. [42,43,44,45]:

● Organic photovoltaics

● Laser diodes and OLED display

● Transparent thin-film transistors

● Biosensor

● Zinc oxide nanorod sensor

● Diluted magnetic semiconductors

18

Zinc oxide’s function as a highly conductive transparent material was discovered in 1982

by Minami [46]. Later in 1984, he found that although the pure zinc oxide thin film is

conductive, the thermal stability is not good enough. It will become unstable beyond 150°C. In

high temperature more than 150°C, the resistivity may not be stable [47]. Minami added Al in

Zinc Oxide and found that the thin film will become more stable compared with un-doped zinc

oxide thin film [47]. Other elements such as Ga, In, Ti, Ge Hf can also be doped in Zinc Oxide

and hence modify the properties of the thin film.

Table1.2 shows the resistivities, and dopant content for ZnO films doped with various

dopants. According to the table, we can conclude that Al, Ga and B doped zinc oxide has the

highest conductivity. Among them, Al doped zinc oxide attracts our attention for two reasons:

1) Aluminum and zinc are two cheap elements and the reserves are huge

2) Aluminum and zinc are non-toxic materials

Table 1.2 The resistivity and dopant content for ZnO films doped with various dopants. [48]

Doant Doping Content(at.%) Resistivity ( 410 cm )

Al 1.6-3.2 1.3

Ga 1.7-6.1 1.2

B 4.6 2.0

Y 2.2 7.9

In 1.2 8.1

19

Sc 2.5 3.1

Si 8.0 4.8

The potential of mass production of the non-toxic and inexpensive aluminum doped zinc

oxide thin film in the future drives us to explore the compromise between optical properties and

electrical properties for AZO thin film and its application in organic solar cells. Chapter 2 will

introduce the sputtering facility used in the experiments, the homemade substrate holder and the

relevant experimental setup. Chapter 3 will be the main content of this thesis. In this chapter, we

discuss the step by step characterization processes and optimized results. In chapter 4, a real

organic solar cell based on the optimized AZO anode is fabricated and measured. Chapter 5 will

summarize the former work and present the conclusion and the future work.

20

Chapter 2 Experimental

2.1 Experimental Facilities

2.1.1 Sputtering Chamber

Figure 2.1 Schematic of the sputtering chamber.

Sputtering is a process whereby atoms are ejected from a solid target material due to

bombardment of the target by energetic particles. Figure 2.1 shows the schematic of the basic

sputtering system used in our lab. The target is at the bottom, the substrate holder is homemade.

A, B, C and D represent four sputtering distances. Figure 2.2 shows us the homemade sputtering

substrate holder.

21

Figure 2.2 The homemade sputtering substrate holder.

Magnetron sputtering technology was first developed in the 1970’s. [49, 50, 51] It didn’t

take long to commercialize it. Since the 1980’s, a lot of research interest was focused on reactive

sputtering of thin films. Magnetron sputtering was at first used for the deposition of metals and

optical films. It was not until a few years later that magnetron sputtering technique was used to

deposit transparent conductive oxides.

Magnetron sputtering has several advantages compared to other thin-film deposition

methods such as evaporation, chemical vapor deposition (CVD) etc. [52, 53, 54]

● Higher energy input into the growing film

● Low substrate temperatures (room temperature)

● Good thickness uniformity and high density of the films

● High deposition rates (up to 12µm per min)

● Good adhesion on the substrate

● Materials with very different vapour pressures can be sputtered easily

22

● Many kinds of compounds can be deposited directly from metallic target by reactive sputtering

in rare/reactive gas mixtures

● Inexpensive deposition method

● Scalable to large area substrates

Penning reported the principle of magnetron sputtering for the first time in 1936. [55]

During the whole sputtering process, air is evacuated continuously from the chamber by vacuum

turbo pumps. A certain amount of Argon gas is fed continuously. A strong magnetic field is

created by magnet inside the chamber. The target is flushed with cooling water. The uniformity

of the thin film is achieved by rotating the substrate in the sputtering chamber during the

deposition.

There are 4 basic steps of the sputtering process [56]:

1 Plasma forming-----high voltage is applied to the target, the argon gas becomes ionized

and thus a gaseous plasma forms along the magnetic field.

2 Secondary electrons created---- Inside the plasma the argon atoms are collided by free

electrons. The atoms become positively charged ions and secondary electrons are created.

3 Target materials ejected----These ions are continuously accelerated towards the negatively

charged target surface. Argon ions are fired at the target surface and transfer their kinetic energy

into it. By this, the materials of the target material are ejected in the form of neutral particles -

either individual atoms, clusters of atoms or molecules. This atom ejection is called sputtering.

4 Thin film growth---- The ejected target materials fly towards the substrate at different

angles. There, they condense on the surface and deposit as a thin layer.

23

Figure2.3 The sputtering machine used in our lab. On the left is the sputtering chamber from

outsite. On the right is the control panel. Below is a view of the inside of the machine,

three targets are put symetrically in it.

24

One mechanical pump is installed for rough pumping and another cryo pump for fine

pumping. The crystal monitor is installed to detect the deposition rate and the thickness by

correlating deposited metal thickness to the amount of deviation from its original crystal resonant

frequency. The deposition rate and thickness can be changed by adjusting different parameters.

There are mainly four parameters that can be changed to adjust the deposition condition

during the experiment. That is the distance between the target and substrate, the sputtering

power, the flow rate of gas and deposition time.

The substrate holder for the sputtering machine is home-made. There are four positions in

this holder. Position A, B, C and D represent different distances between the sputtering target

and sample holders. The different distances are as shown in Table 2.1.

Table 2.1 Target to substrate distances.

A B C D

8cm 12cm 16cm 19cm

Distance is an important parameter that we will change in our characterization

experiments. The sputtering power is controlled by an RF-sputtering-AE RFX-600. For metal

targets, the power can’t exceed 350W. For dielectric targets and targets attached to a backing

plate, the power should be limited to 300W or less.

The Mass Flow Controllers(MFC) on the control panel is used to control the flow rate of

gases. The flow rate of argon and oxygen are parameters that we can change in the deposition

process. In my experiments, there is no oxygen during the deposition; the flow rate of argon is

kept to be 5sccm.

When all the other parameters are kept unchanged, the film thickness is proportional to

deposition time. We can adjust the film properties by changing the deposition time.

25

2.1.2 Spin Coating and Aluminum Evaporation

Figure 2.4 shows the spin coater used in our experiments and the cluster tools used for

metal deposition. Polymer is spin coated on top of the anode layer. The spin coater’s spin speed

and time can be adjusted. Two sequential steps can be chosen. At the first step, the spin time can

vary from 0 second to 18 second; for the second steps; the spin time can vary from 0 to 60

seconds. The maximum spin speed is 10000rpm.

The LiF buffer layer and Al cathode are deposited in the cluster tool. The cluster tool is

used to deposit small molecular thin films and metal layers.

Figure 2.4 The machines used for spin coating and metal deposition. On the left is the spin

coater, which is stored in a glove box. On the right are the cluster tools used for

metal and LiF vacuum deposition.

2.1.3 Testing System

Device current density versus voltage (J-V) characteristics was measured using a

Keithley 6430 sub-femptoamp meter under 2100 /mW cm simulated AM1.5G solar illumination.

The testing system used in our lab is shown in Figure 2.5.

26

Figure 2.5 On the Left is the simulated AM1.5G solar illuminations, on the right is the testing

facilities used in organic solar cell testing.

During the measurements of resistance and resistivity, we use four points measurement to

determine the resistance. A four point probe is a apparatus for measuring the resistivity of

semiconductor samples. It is achieved by passing a current through two outer probes and

measuring the voltage through the inner probes. Figure 2.6 is the schematic of four points probes.

Figure 2.6 Schematic of four points probe. [56]

27

XPS (X-ray Photoelectron Spectroscopy) is a quantitative spectroscopic technique that

measures the elemental composition, chemical state and electronic state of the elements that exist

within a material.

2.2 Experimental Procedures

In order to achieve a compromise between the electrical resistivity and optical

transparency, I will optimize the different parameters of sputtering and hope to find a proper

condition in which the resistivity and transparency can achieve a balance.

The Aluminum Doped Zinc Oxide target was purchased from Kurt J. Lesker. The AZO

target contains 98% of Zinc Oxide and 2% of Alumina, the target diameter is ''2.00 and the

purity is 99.99%.

Aluminum doped Zinc Oxide was sputtered to a bare glass substrate. Before sputtering,

the substrate was cleaned in de-ionized water, a standard regiment of acetone and methanol,

followed by UV-ozone treatment for 11 minutes in order to increase the work function of AZO

thin film. UV-ozone treatment also can create the most conductive films possibly due to

reduction in the number of oxygen vacancies in the AZO lattice, leading to reduced impurity

scattering and higher electron mobility[graham]. The substrates were attached to the homemade

substrate holder as shown in Figure 2.2.

The base pressure of the sputtering machine was ~10-7

Torr. During sputtering, I kept the

flow rate of argon at 5sccm, and no oxygen was used in the optimization. The power was varied

from 50W to 300W. The target to substrate distances were 8cm, 12cm, 16cm and 19cm. After

sputtering, the AZO thin film was UV-ozone treated for 15 minutes.

Spin coating was conducted in a glove box. P3HT and PCBM were purchased from

American Dye Source Inc. PCBM (13.6mg/mL):P3HT(17mg/mL) solution was prepared in 1,2-

dichlorobenzene in a nitrogen-filled glove box, then stirred in dark for 8 hours at 50℃ . This

solution was spin-coated on the various AZO substrates at 500rpm for 45s. The polymer layer

was dried for 24 hours at room temperature in a glove box. Lithium fluoride and aluminum films

were thermally evaporated in a vacuum chamber with base pressure ~10-7

Torr. Film thicknesses

28

were monitored using a calibrated quartz crystal monitor. The final device structure was as

follows: AZO/ PCBM:P3HT (65 nm)/ LiF (1 nm) )/Al (80 nm).

29

Chapter 3 Characterization of AZO

3.1 Optimization of Target to Substrate Distance

Distance is a very important factor that is a determinant for electrical and optical

properties of sputtered thin film. The distance also plays an important role in the morphology of

the thin film surface. In our characterization experiment, the first parameter we optimized was

distance.

We sputtered a series of samples with different target to substrate distances at power

levels of 50W, 100W and 150W. Table 3.1 is the sheet resistance and resistivity for the

aforementioned samples.

Table 3.1 the sheet resistance and resistivity as a function of distances for different power levels.

Power(W) Distance(cm) Thickness(nm) Sheet

Resistance(Ω/□ )

Resistivity(Ωcm)

50 8 2170 6.7 1.45E-3

50 12 1580 90 1.42E-2

50 16 1015 6330 6.3E-1

50 19 533 49900 2.6

100 8 2170 8.3 1.8E-3

100 12 1580 14.8 2.4E-3

100 16 1015 285 2.9E-2

100 19 533 34900 1.86

150 8 2170 7.74 1.67E-3

150 12 1580 6.4 1.01E-3

150 16 1015 191 1.93E-2

150 19 533 13200 0.7

30

According to Table 3.1, we can conclude that for a specific power, the resistivity of the

thin film is sensitive to the target to substrate distance. By decreasing the distance from 19cm to

8cm, the sheet resistance and resistivity can be decreased dramatically. Thus we can conclude

that shorter target to substrate distance is better.

In order to better understand the correlation between the thin film properties and target-

substrate distance, we plot the curve of resistivity as a function of distance of different power

levels as shown in Figure3.1.

7 8 9 10 11 12 13 14 15 16 17 18 19 20

1E-3

0.01

0.1

1

Re

sis

tivity (

oh

m c

m)

Distance (cm)

50W

100W

150W

Figure 3.1 Resistivity as a function of target to substrate distance for different power levels.

From Figure 3.1, we know that the resistivity decreases in similar fashion for different

power. We also know that the deposition power also plays a role in determining the electrical

properties. From the above curve, we can observe that the resistivity will decrease as power

increase with distances kept the same. Generally speaking, higher power can contribute to

decrease in resistivity.

Optical transmittance is another important property that can determine the performances

of organic solar cells. Below, we compare the optical property for these thin film devices.

31

200 300 400 500 600 700 800 900 1000 1100

0

20

40

60

80

100

Tra

nsm

itta

nce

(%)

Wavelength (nm)

2568nm

1925nm

1160nm

605nm

270nm

Figure 3.2 Transmittance spectrum in the visible range (150W 8cm).

Figure 3.2 shows the transmittance spectrum for AZO thin film prepared at a 150W and

a deposition distance 8cm; this shows that the AZO thin films are fairly transparent. Even when

we increase the film thickness from 270nm to 2568nm, the transmittance will only decrease

slightly, from about 90% to 80% in the visible range.

To further compare the resistivity as a function of thickness, at the same power, a series

of samples with different thickness and target-substrate distances were deposited.

As concluded above, the resistivity is very sensitive to target-substrate distance, thus for

large distances like 16cm and 19cm, the resistivity is too high to have practical use in devices.

We will focus on two distances: 8cm and 12cm. For these two distances, we sputtered a series of

devices with different thickness and compare their resistivities. Table 3.2 shows the surface

resistance and resistivity as a function of thickness for a particular target-substrate distances and

power of 50W. We plot the curve of resistivity as a function of thickness in Figure 3.3.

According to the curve, we know that 8cm is the optimized target substrate distance. And for a

specific distance, during the first few hundreds of film thickness, the resistivity decreased

dramatically, but it will become relatively stable if we continue increase the device thickness.

This suggests that the optimized device is in the circle zone.

32

Table 3.2 Surface resistance and resistivity as a function of thickness for particular target-

substrate distances and power of 50W.

50W 8cm 280nm 539nm 855nm 1800nm 2950nm

S.R(Ω/□) 1200 1150 91 12 4.1

Resistivity

(Ωcm)

3.36E-2 6.19E-2 1.80E-3 2.1E-3 1.2E-3

50W 12cm 184nm 397nm 737nm 1126nm 1869nm

S.R(Ω/□) 350000 25000 804 108 91

Resistivity

(Ωcm)

6.4 1.0 5.9E-2 1.2E-2 1.7E-2

0 1000 2000 3000

1E-3

0.01

0.1

1

10

0 1000 2000 3000

1E-3

0.01

0.1

1

10

Re

sis

tivity (

oh

m c

m)

Thickness (nm)

8cm

12cm

Figure 3.3 Resistivity as a function of film thickness for material deposited in 50W.

From Figure 3.3, we confirm that at the same target-substrate thickness and the same

power, the resistivity will decrease as the film thickness increases. On the other hand, when

thickness and deposition power are kept the same, the target-substrate distance also plays an

important role in decreasing the resistivity.

33

Table 3.3 shows the resistivity as a function of substrate-target distance with the same

thickness and deposition power. We plot the curve in Figure3.4.

Table 3.3 Resistivity as a function of substrate-target distance with the film thickness 600nm

and deposition power 100W.

100W 600nm 8cm 12cm 16cm 19cm

S.R(Ω/□) 600 1150 7800 29900

Resistivity(Ωcm) 3.6E-3 6.9E-3 4.7E-2 1.8E-1

8 10 12 14 16 18 20

0.01

0.1

Re

sis

tivity (

oh

m.c

m)

Distance (cm)

Figure 3.4 Resistivity as a function of substrate-target distance for a 600nm thick film at 100W.

According to above curve, when power and film thickness are kept unchanged, the target

to substrate distance will play an important role in determining the film resistivity. The resistivity

will increase from 3.6E-3Ωcm at the distance 8cm to 1.8E-1Ωcm at the distance 19cm. In

conclusion, the resistivity will decrease dramatically as the target-substrate distance decrease and

other parameter are kept the same. The target-substrate distance is a primary parameter in

34

determining the electrical property of the AZO thin film. As we shorten the distance between

target and substrate, we can achieve a more conductive thin film anode.

Figure3.5 shows the transmittance spectrum in the visible range for 600nm AZO thin

films deposited at 100W at different target to substrate distances. We can conclude that in the

visible spectrum, the transmittance does not change when we change the target-substrate distance.

At all four distances, the transmittances are more than 90% in visible spectrum.

Figure3.5 The transmittance spectrum in the visible range for 600nm thick AZO thin films

deposited at different target to substrate distances with power of 100W.

3.2 Drude Model

It is observed that some metal oxide semiconductor behaves like dielectrics, in the visible

region are fairly transparent, while in the infrared region, these films behave like metals and have

high reflectance[57-62]. Plasma wavelength, which is a characteristic parameter for the metal

oxide thin film, defines the crossover between the two different behaviors.

The plasma wavelength will move to shorter wavelength as the free-electron density in

the film increases.

Reflectance is given by the equation3.1: [63]

400 600 800

-20

-10

0

10

20

30

40

50

60

70

80

90

100

8cm

12cm

16cm

19cm

Tra

nsm

itta

nce

(%

)

Wavelength (nm)

35

2 2 2 2( 1) / ( 1)R n n (3.1)

Where n is the refractive index, is the extinction coefficient.

In our case, the reflectance of AZO thin film at the plasma wavelength is very small (less

than 5%), is close to zero. Hence, the following approximations are valid:

2 2

min ( 1) / ( 1) 0R n n (3.2)

Then, the refractive index n at the plasma wavelength is close to 1.0, and the real part of

the dielectric function is 2

1 1n .0

According to Drude theory: [64,68]

The free-electron concentration is a function of plasma frequency by the following

equation: [65, 66,67,68]

2 2 * 2

0/ ( 1)p e eN e m ( 3.3)

Where is the high-frequency dielectrics function, e is the electron charge, em is the

mass of the electron, *

em is the effective mass of electron in the conduction band. is the Drude

scattering frequency and defined by the following relation:

*/ ee m

is the mobility of the electrons. The value of and *

em are 4 and 0.38 em .

36

Figure3.6 The reflectance and transmittance spectra of an AZO film produced by pulsed laser

deposition.

Figure 3.6 shows us the reflectance and transmittance spectra of AZO film by pulsed

laser deposition. In the visible range, transmittance is high and constant while the reflectance is

low. In the IR range, transmittance decreases while the reflectance starts to increase. From the

above curve, we see that the samples exhibited a minimum in the reflectance spectra. Such

minimum is expected to take place near the plasma frequency. The relation is given by the

following equation:

1/2

min ( / 1)p (3.4)

Hence, if min can be determined from the reflectance minimum, p can be calculated.

By substituting the values p , , 2 and *

em into equation 3.4, carrier density can be calculated

for all samples, which can be compared with the data derived from the Hall mobility.

In our characterization experiments, we will do a quantitative analysis on our

experimental data.

37

3.3 Qualitative Explanation According to the experiments, decreasing the distance between target and substrate will

decrease the resistivity without sacrificing the transmittance in visible range.

It is well known, that conductivity is proportional to carrier concentration and carrier mobility.

1/ N (3.5)

Here the carrier mobility is related to grain size. The bigger the grain size, the higher the

carrier mobility.

Where is the conductivity, is the resistivity. N is the carrier concentration, is the

carrier mobility.

This can be understood as: the larger the grain size, the less grain boundary. The barrier in

the grain boundary is a primary factor that can stop the free carrier from moving in the thin film.

We can conclude that conductivity is proportional to carrier mobility, which is proportional to

grain size. The grain size information can be obtained from SEM image.

On the other hand, the carrier density N is relevant to the transmittance spectra in IR according to

Drude theory. We can calculate the free carrier density N according to the transmittance and

reflectance spectrum in IR.

According to Drude theory, N is proportional to p , which is called Plasma frequency

and p is proportional to min . That means carrier density N is proportional to min .

min stands minimum reflectance spectra near plasma frequency. When transmittance spectra in

IR moves to shorter wavelength, the reflectance spectra also moves to shorter spectra, which

means min will move to shorter wavelength and hence min move to larger frequency[63]. We

can conclude that when min move to larger frequency, carrier concentration will increase in

consequence.

In summarize, the faster the edge of the transmittance spectra decreased in IR (the more

transmittance spectra in IR moves to shorter wavelength), the higher the plasma frequency will

shift and the larger the carrier concentration, and hence the higher the conductivity. We can

conclude relationship as shown in Figure3.7.

38

Figure 3.7 The correlation between the transmittance edge in the IR and the Carrier

concentration.

In the distance optimizing experiment, if we extend the spectrum from visible range to IR

range, we can see the spectrum as in Figure 3.8. In the IR range, the transmittance at shorter

substrate-target distances decreases faster, which is related to the smaller resistivity. We can

explain the phenomena by the aforementioned theory [68,69].

500 1000 1500 2000 25000

50

100

8cm

12cm

16cm

19cm

Tra

nsm

itta

nce

(%)

Wavelength (nm)

Figure 3.8 The transmittance spectrum in the visible and IR range with different target to

substrate distances for 600nm thick films deposited with 100W power.

According to Figure 3.8, we can see that with decreasing target to substrate distance, the

absorption edge of the AZO films moves toward shorter wavelengths.

39

Figure3.9 XPS spectra of 600nm thick AZO thin films deposited with different target to

substrate distances using power 100W.

According to XPS of AZO with different target-substrate distances, we know that the

peak shapes and peak positions don’t differ within experimental error. The C1s peak in 16cm

sample was carbon contamination. And the atomic ratio of Zn:Al is about 95: 5 for all the

samples in error range, which means the oxidizes all had Zn:O ratio of approximately 1:1. We

therefore infer by the Drude Theory and the facts that all the samples have the same ratio of

Zn:O that shorter target to substrate distance resulted in higher free carrier concentration for the

AZO films, which will contribute to the decrease in the resistivity.

40

We also took SEM image of sputtered AZO thin films as shown in Figure3.10:

Figure3.10 SEM images of 600nm thick AZO thin films deposited with different target to

substrate distances using 100W power.

And according to the SEM image, the grain size is bigger for smaller target to substrate

distance. The bigger grain size, thus larger electron mobility, will increase the conductivity for

samples with low substrate-target distance.

41

The phenomena can also be understood by the following theory: the Aluminum ion

doping is achieved by replacing Zn ion by Al. Since Al has one more electron than Zn ion, the Al

is more active than Zn and hence has a better conductivity. While increasing the sputtering

distance, there will be more chances for the Al ion to oxidize into the insulating Alumina with no

free electrons. If we sputter AZO with a shorter target to substrate distance, more free Al ions

can reach the thin film and hence make the AZO thin film more conductive. That is why the

carrier density will decrease as sputtering distance increases.

3.4 Optimization of Deposition Power

Power is another parameter that is critical to improve the property of AZO thin film and

the sputtering efficiency.

Table3.4 The deposition rates for different powers.

The deposition rate increased from 0.2angstrom/s to 3angstrom/s. as the deposition power

increased from 50W to 300W. It means that in higher power, we will deposit in a much more

efficient way, which has important implications in industry production. We plot the curve of the

deposition as a function of power in figure 3.11.

Power 50W 100W 150W 200W 250W 300W

Deposition rate

( / s )

0.2 0.7 1 1.5 2.2 2.8

42

Figure3.11 Deposition rate as a function of power.

In order to compare the influence of power on the resistivity of thin films with the same

thickness and the same substrate-target distance, we deposit a series of AZO thin films with

different power, while the other parameters are kept the same. Table 3.5 shows the surface

resistance and resistivity as a function of power in the same film thickness of 500nm thickness

films deposited at a target to substrate distance of 8cm. We plot the curve of surface resistance

and resistivity as a function of power in Figure 3.12 according to Table 3.5.

Table 3.5 Surface resistance and resistivity as a function of power at constant film thickness of

500nm and target to substrate distance of 8cm.

8cm 500nm 150W 200W 250W 300W

S.R(Ω/□ ) 334 148 76.4 67.1

Resistivity(Ωcm) 9.3E-3 4.1E-3 2.13E-3 1.86E-3

0 100 200 300

0

1

2

3

De

po

sitio

n R

ate

(A

ng

str

om

/s)

Power (W)

43

Figure 3.12 Resistivity as a function of power for a film thickness of 500nm and target to

substrate distance of 8cm.

From the curve of Figure 3.12, we know that with the same thickness and distance, if we

increase the power, it will decrease the resistivity of the AZO thin film. In addition, at higher

power, it will be more efficient to deposit the required thickness. We can conclude that higher

power is better. But if the power is too high, the deposition is not consistent and there is a critical

point at which an increase of deposition power does not improve device performance.

400 600 8000

10

20

30

40

50

60

70

80

90

100

150W

200W

250W

300WTra

nsm

itta

nce (

%)

Wavelength (nm)

Figure3.13 The transmittance spectra in the visible range for 500nm thick films deposited with

different sputtering power levels at a target to substrate distance of 8cm.

150 200 250 300

0.002

0.004

0.006

0.008

0.01

Re

sis

tivity (

oh

m c

m)

Power (W)

44

Figure3.13 shows the transmittance in the visible range. We know from the figure that the

transmittance in the visible spectrum is about 90% for all 4 deposition powers, which is

comparable to ITO.

If we enlarge the range of the curve from visible to IR, we can see the spectrum as in

Figure 3.14.

Figure 3.14 The transmittance spectra in the visible and IR range for 500nm thick films

deposited with different sputtering power at target to substrate distance 8cm.

In the IR range, the transmittance with higher power will decrease faster, which is related

to the smaller resistivity. This can be explained by the Drude Theory [67,68] mentioned above.

From the above spectrum, we can see that in the IR range, with increasing power, the

absorption edge of the AZO films moves toward smaller wavelengths. We therefore infer by the

Drude Theory that higher power resulted in higher free carrier concentration for AZO films,

which decreases the resistivity. But the decrease is not as significant as compared to the effect of

distance.

The SEM images of sputtered AZO thin film sputtered with different power levels can be

compared in Figure 3.15.

1000 2000

0

50

100

T

ran

sm

itta

nce

(%

)

Wavelength(nm)

150W

200W

250W

300W

45

Figure 3.15 SEM images of 500nm thick AZO thin films deposited with different sputtering

power levels of a target to substrate distance 8cm.

According to the SEM images in Figure 3.15, there is no significant difference in grain

size and hence in the electron mobility.

Here we can conclude the following: Increasing the power will decrease the resistivity

without sacrificing the transparency. 200W and 250W are the optimal powers. In power 300W,

the sputtering environment is not stable. It means at the power of 300W, the deposition rate is not

consistent, hence we choose moderate power of 200W and 250W as our optimized powers.

46

3.5 Optimization of thickness

Thickness is also an important parameter that is critical to the properties of thin films.

Generally speaking, for thin film applications, 300nm or less is a proper thickness. In our

experiments, not only a compromise between the optical transmittance and electrical resistivity

needed to pursued, but also the thickness had to be kept below 300 nm.

As shown in Figure3.2, the AZO thin film is fairly transparent from 270nm to 2568nm.

That means with a thickness of about 300nm, the transmittance is in an acceptable range. The

circled area in Figure3.16 is the optimized area, in which the thickness and surface resistance can

achieve a balance

Figure 3.16 Surface resistance as a function of thickness for sputtering power levels of 200W

and 250W.

Figure 3.16 shows us the surface resistance as a function of thickness for the deposition

power of 200W and 250W. In order to have comparable characteristics with ITO, we optimize

the thickness of AZO to the range between 300nm and 400nm. When the thickness is between

300nm and 400nm, the resistivity is about 3E-3 ohm cm, the sheet resistance is about

80ohm/square, and the transmittance is more than 90%.

50 100 150 200 250 300 350 400 450

100

1000

S.R

(ohm

/sqr)

Thickness (nm)

250W

200W

47

3.6 Conclusions

From above analysis, we conclude the optimized parameters are as following:

1) 8cm is the optimized distance.

2) 200W and 250W are the optimized sputtering powers

3) Thickness between 300nm and 400nm is the optimized thickness

Table 3.6 Comparison of ITO and optimized AZO properties.

ITO TCOs

Transparency 90% 90%

Resistivity Low (1x10-4

Ωcm) 1x10-3

Ωcm

Work function 4.7eV 4.7eV

Thickness 100nm 300-400nm

Sheet Resistance 12Ω/□ 80Ω/□

Prices Expensive Inexpensive

Toxicity Yes No

Reserves Rare Large

We can conclude after a systematic investigation, as shown in Table 3.6, an AZO thin

film anode which is not only comparable to ITO in electrical, optical and chemical properties,

but also able to overcome the drawbacks of ITO, such as the expensive price, the toxicity and the

low reserves.

48

Chapter 4 Application in OPVs

Organic photovoltaics are one of the most important applications of transparent AZO thin

films. Since the parameters have been optimized, a device is made to demonstrate progress by

fully replacing an ITO anode with AZO. As we know from the last chapter, during sputtering,

the flow rate of Ar is 5sccm, the pressure is 5mTorr, the optimized Power is 200W, the target to

substrate distance is 8cm and the film thickness is 300nm.

The solar cell structure is as in Figure4.1:

Figure4.1 The standard structure of an AZO based organic solar cell.

Figure 4.2 shows the I-V curve of an organic solar cell with an AZO anode deposited with

the optimized sputtering parameters.

Figure 4.2 I-V curve of an organic solar cell with an AZO anode deposited with the optimized

parameters.

-1 0 1

0

10

Cu

rre

nt

De

nsity (m

A/c

m2

)

Voltage (V)

49

Table 4.1 The device performance of optimized AZO based solar cell.

From Table 4.1, we know that the efficiency of AZO based organic solar cell reached

0.82%, the open circuit voltage is about 0.56V, the short circuit current density is 2.77mA/ cm2

,

the fill factor is 0.53, which are comparable to pure ITO based organic solar cell.

We have achieved the first step for the future replacement of ITO by AZO, fabricated by

R.F magnetron sputtering technology.

VOC

(V)

JSC

(mA/cm2

)

FF ηP

(%)

0.56 2.77 0.53 0.82

50

Chapter 5 Summary

Transparent conducting oxides have attracted increasing attention from researchers

globally for their excellent electrical and optical properties. Among them, aluminum doped zinc

oxide is one of the most promising materials. In my research, AZO has been proved to be a

promising alternative for the commonly used ITO in organic photovoltaics. With step by step

optimization of sputtering parameters, we can determine the sputtering parameters that can

produce an AZO thin film which is not only comparable to ITO in optical and electrical

properties, but also able to overcome the drawbacks of ITO.

We can conclude the transparent conducting AZO optimization as following:

1) Decreasing the distance between substrate and target will aid the production of optimized

AZO

2) Increasing sputtering power will benefit to increase the conductivity of AZO thin film,

but the effect is not as significant as the target to substrate distance

3) Increasing the sputtering power will increase the deposition rate and hence make the

sputtering more efficient

4) The film can reach a balance between optical properties and electrical properties at a film

thickness of about 300nm

The phenomena can be explained by Drude theory. According to this theory, the film

resistivity is related to the free carrier concentration, which can be calculated by the absorption

and transmittance spectrum in IR range. By comparing the edge decreasing of spectrum in IR,

combined with the SEM image, in which the carrier mobility can be calculated, the differences in

resistivity at different parameters can be analyzed.

The optimized AZO thin film was used for the fabrication of P3HT:PCBM based polymer

solar cells. The efficiency reached 0.82%. It is a hopeful step to fully realize the replacement of

ITO by AZO.

However, there are still more studies required in the future.

1) Since the substrate holder is homemade, we can only choose from 4 substrate-target

distances for the optimization. Distance is the primary parameter that can decrease the

resistivity radically. In the future, an even shorter distance substrate holder can be made

to test the limit on the decrease in resistivity due to distance.

51

2) Oxygen is another important gas that can be added to the sputter system to change the

defect structure of AZO. By adjusting the ratio of oxygen, the doping rate of aluminum

ion in zinc oxide can be studied and optimized.

3) The morphology of AZO thin films and its influence on the performance of devices can

be studied by AFM

4) A buffer layer with a higher work function (such as NiOx ) between the AZO and the

polymer can be added in order to optimize PV performances

52

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