Research Article Characterization of the Organic Thin Film ...Research Article Characterization of...

Research ArticleCharacterization of the Organic Thin FilmSolar Cells with Active Layers of PTB7PC71BM Prepared byUsing Solvent Mixtures with Different Additives

Masakazu Ito1 Kumar Palanisamy1 Abhirami Kumar1 Vijay Srinivasan Murugesan1

Paik-Kyun Shin2 Norio Tsuda1 Jun Yamada1 and Shizuyasu Ochiai1

1 Department of Electrical Engineering Aichi Institute of Technology Toyota Aichi 470-0392 Japan2Department of Electrical Engineering Inha University 100 Inha-ro Nam-gu Incheon 402-751 Republic of Korea

Correspondence should be addressed to Shizuyasu Ochiai ochiaiaitechacjp

Received 11 April 2014 Revised 21 May 2014 Accepted 22 May 2014 Published 10 June 2014

Academic Editor Mohammad Yusri Hassan

Copyright copy 2014 Masakazu Ito et al This is an open access article distributed under the Creative Commons Attribution Licensewhich permits unrestricted use distribution and reproduction in any medium provided the original work is properly cited

Organic thin film solar cells (OTFSCs) were fabricated with blended active layers of poly[[48-bis[(2-ethylhexyl)oxy]benzo[12-b45-b1015840]dithiophene-26-diyl][3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[34-b]thiophenediyl]] (PTB7)[66]-phenyl-C

71-butyric

(PC71BM) The performances of active layers are prepared in chlorobenzene (CB) with different additives of 1-chloronaphthalene

(CN) and 18-Diiodooctane (DIO) by a wet process with spin coating technique The effects of different solvent additives onphotovoltaic parameters such as fill factor short circuit current density and power conversion efficiency of active layers are reportedThe absorption and surface morphology of the active layers are investigated using UV-visible spectroscopy and atomic forcemicroscopy respectivelyThe results indicate that structural andmorphological changes were induced by the additives with solventThe current density-voltage (J-V) characteristics of photovoltaic cells were measured under the illumination of simulated solar lightwith 100mWcm2 (AM 15G) by an Oriel 1000W solar simulator The OTFSCs of PTB7PC

71BM prepared with organic solvent

additives of DIO+CN show more improved PCE of 496 by spin coating method

1 Introduction

Recently a great paradigm shift has occurred in the genera-tion and consumption of energyThe so-called ldquocleanrdquo energyis consequently an utmost critical issue for all the nationsand entire civilized societies In the 34th G8 Toyako summitit has been decided to cut the carbon emission up to 50by the year 2050 [1] and realization of the ldquolow-carbon-societyrdquo has been worked towards on global level Moreoverthe recent ldquoGreat East Japan Earthquakerdquo has aroused agreat attention to severe riskiness of nuclear power so thatrecommencement of electricity generation by the nuclearpower and construction of newnuclear power plant are ratherreluctant Consequently a large scale electricity generation bynewand renewable energy source has been raised as anurgentissue [2] It has been reported [3] that electricity generationin Japan in November 2012 was comprised of 27 from

nuclear power 906 from fossil fuel 62 from hydraulicpower and only 06 from new energy source respectivelyand the portion of renewable energy was still in the minutelevel of 10 All of those kinds of situations have caused acritical status in Japan in terms of supply and demand ofenergy Therefore the Japanese government has devised atarget in the general energy planning aiming for elevatingthe proportion of ldquozero-emission-power sourcerdquo up to 70bythe year 2030 so that introduction of the new and renewableenergy source should be accelerated as fast as possible [4]

Solar light power generation could have a promisingpossibility for contributing to elevate the self-sufficiencyof energy needs by domestic energy resource in Japan Ithas been planned therefore that the mass of introductionfor solar light power generation should be raised by tentimes by the year 2020 and forty times by the year 2030Additionally an innovative solar light power generation is

Hindawi Publishing CorporationInternational Journal of PhotoenergyVolume 2014 Article ID 694541 8 pageshttpdxdoiorg1011552014694541

2 International Journal of Photoenergy

Cl

S

S S

SO

O

I

OO

CN

PTB7

(a)

II

DIO

O

PC71BM

OCH3

(b)

Figure 1 Molecular structures of PTB7 PC71BM CN and DIO

aimed by establishing a sophisticated technology of solarcell through usage of new materials and device structureso that power generation efficiency of over 40 and powergeneration cost of 7 YenkWh should be achieved after theyear 2030 [5]

First-generation solar cells based on Si have demerits ofhigh cost and limitation of installing location due to heavyweight Second-generation solar cells based on inorganiccompounds (amorphouspoly crystalline Si and CIGS) havebeen introduced in commercial market Although second-generation solar cells show still relatively inferior powerconversion efficiency they are advantageous from a cost per-spective It is acutely expected for third-generation solar cellsthat they would have improved power conversion efficiencyof over 40 and a merit of low cost comparable to those ofthe second-generation solar cells Generally organic thin filmsolar cells (OTFSCs) are acknowledged that they are lighterthan any other counterparts and suitable for low-cost fabrica-tion [6ndash9] OTFSC materials have been focused towards theenergy-consumption and environment friendly viewpointRecently power conversion efficiency of the OTFSCs hasbeen improved up to 10-11 [8ndash10] Moreover they could beformed to any shape due to flexible characteristic so that theycould be optimized for portable andor wearable application[5] Similar to the case of liquid crystal displays (LCDs)[11] light emitting diodes (LEDs) inorganic solar cells andorganic light emitting diodes (OLEDs) are based on similarmaterials and structures to those of OTFSCs Recent rapidcommercialization of the OLEDs is mainly concentratedon mobile display applications [12] but it is expected that

the market for OLEDs could be expanded in the near futureto large-size TVs and lighting applications The success ofthe OLEDs has motivated vigorous developments of OTFSCsbased on devices and related materials [13]

In this present work we report the performance ofPTB7PC

71BM based OTFSCs prepared in chlorobenzene

(CB) solvent with different additives The active layers ofPTB7PC

71BM were prepared by wet processes with spin

coating technique Organic solvent additives were variedto prepare the organic active layer and the effect of thesolvent additives was investigated in relation to surfacetopology of PTB7 PC

71BM and PTB7PC

71BM thin films

The performance of the resulting OTFSC devices andsurface topology of the active layers were comparativelydiscussed

2 Experimental Procedure

Active layers of organic thin film solar cell device wereprepared using Poly[[48-bis[(2-ethylhexyl)oxy]benzo[12-b45-b1015840]dithiophene-26-diyl][3-fluoro-2-[(2-ethylhexyl)ca-rbonyl]thieno[34-b]thiophenediyl]] (PTB7) as organicelectron donor material and [66]-Phenyl C

71butyric

acid methyl ester (PC71BM) used as electron accepter

material Figure 1 shows the molecular structure of thePTB7 PC

71BM 1-chloronaphthalene (CN) and 18-

Diiodooctane (DIO) PC71BM is an organic material

chemically modified to be dissolved in an organic solventTo enhance the hole transport characteristic a thin film of

International Journal of Photoenergy 3

ITO

HOMO

PTB7

PCBMAl

LUMO

PEDOTPSS

minus48 eVminus50 eV


minus331 eV


(a)

Al

Active layer

PEDOTPSS

ITO

Glass substrate

h

minus +

(b)

Figure 2 Schematics of the (a) energy band structure of the OTFSC with hole transfer layer of PEDOTPSS and blended active layer ofPTB7PC

71BM and (b) cross section view of OTFSC device

poly(34-ethylenedioxythiophene)poly(styrenesulfonate)[PEDOTPSS] was prepared by using a commerciallyavailable material of PH1000 (Heraeus Clevios) whichserves as a hole transfer layer interfaced with the anode andelectron donor layer of PTB7 While many organic materialscould only be dissolved in organic solvents PEDOTPSScould be dissolved in water and it could be easily used inwet process to prepare thin films Moreover PEDOTPSSwould serve for planarization of the unevenness on theITO substrate surface as well as to enhance the interfacecharacteristic of anode substrate and thin film

As for the process to prepare the thin film componentsof OTFSC device a spin coating technique was adopted forcompatibility to large-area-sized device fabrication Firstlyindium tin oxide (ITO) coated glass substrates were cleanedultrasonically in a sequential step by using a neutral deter-gent distilled water acetone and ethanol for 10min respec-tively The cleaned substrates were exposed to UV in vacuumfor 30min aiming for a hydrophilic state of the treatedITO substrates Then hole transfer layer of PEDOTPSSthin film was prepared on the substrate by spin coatingtechnique rotation speed of 5000 rpm and rotating timeof 30 s The PEDOTPSS thin film was then annealed for15min at a temperature of 140∘C Composite organic activelayers of PTB7PC

71BM with a relative ratio of 1 15 were

deposited also by spin coating technique where differentorganic solvents were used to prepare the precursor solutions(1) 2mLof pristine chlorobenzene (CB) (2) 2mLofCB addedwith an additive of DIO (3 v) (3) 2mL of CB added with anadditive of CN (3 v) and (4) 2mL of CB added with DIO(3 v) and CN (3 v) The PTB7PC

71BM (9mg 135mg)

was dissolved in each organic solvent mixture and the spincoating process was carried out with a rotation speed of800 rpm and rotation time of 60 sThe thickness of the activelayer was sim86 nm as measured using a Dektak II profilome-ter Aluminum (Al) electrode (sim100 nm) for the organic thinfilm solar cell device was then prepared by a thermal vacuumevaporation through a shadow mask in a high vacuum of

13 times 10minus4 Pa and the effective area of the resulting device was9mm2

The organic thin film solar cell device presented inthis report is based on an electron donor layer of PTB7with deep highest occupied molecular orbital (HOMO) leveland low band gap interfaced with a fullerene derivativeof PC

71BM as a representative electron acceptor Figure 2

shows a schematic of energy band diagram and devicestructure of the OTFSC Performance of the OTFSC devicewas investigated by measuring the current density-voltage(J-V) characteristics in a solar simulator illuminated by alight source of Xn lamp combined with an air mass filter100mWcm2 AM 15 A shadow mask made of a thick blacksheet was placed in front of the active device area in orderto avoid the overestimation of the current density and powerconversion efficiency All measurements were carried out inair without any encapsulation

As shown in Figure 2 PTB7 has the lowest unoccupiedmolecular orbital (LUMO) level of minus331 eV and a HOMOlevel of minus515 eV It is generally acknowledged that P3HThas a LUMO level of minus29 eV and HOMO level of minus50 eV[14 15] It could be expected that PTB7would reveal relativelynarrow band gap and excellent light absorption character-istic In addition open circuit voltage of the OTFSC withPTB7PC

71BM would be similar to that of the OTFSC with

P3HTPCBM [16]

3 Results and Discussions

31 Current Density- (J-) Voltage (V) Characteristics Thecur-rent density-voltage (J-V) characteristics of OTFSC deviceswere measured under the illumination of simulated solarlight with 100mWcm2 (AM 15G) by an Oriel 1000 W solarsimulator J-V characteristics of the four different OTFSCdevices based on blended active layer of PTB7PC

71BM are

presented in Figure 3 Photovoltaic parameters of the fourdifferent OTFSC devices along with series resistance (119877

119904)


00 02 04 06 08

0

Voltage (V)

wo additive

minus12

minus10

minus8

minus6

minus4

minus2

With DIO (3 v)With CN (3 v)With DIO (3 v) + with CN (3 v)

Curr

ent d

ensit

y (m

Ac

m2)

Figure 3 J-V characteristics of the four different organic thin filmsolar cell devices based on blended active layer of PTB7PC

71BM

prepared with different organic solvent mixture

and shunt resistance (119877sh) are given in Table 1 short circuitcurrent density (119869sc) open circuit voltage (119881oc) fill factor (FF)and power conversion efficiency (PCE 120578)

The OTFSC of PTB7PC71BM prepared with pristine

organic solvent of CB reveals a PCEof 338which is distinc-tively lower than those of the OTFSC of PTB7PC

71BM pre-

pared with organic solvent of CB with additives [6 7 17 18]On the other hand the OTFSC of PTB7PC

71BM prepared

with organic solvent of CB with additive of DIO (3 v) showshigh PCE of 489 It may be attributed to the formationof interpenetrating network with better morphology by theaddition of DIO which decrease the series resistance andenhance the charge carrier transport properties as well asincrease of equivalent parallel resistance due to suppression ofcharge carrier recombination in the active layer The OTFSCof PTB7PC

71BM prepared with organic solvent of CB with

additive of CN (3 v) shows a moderately improved PCE of386 with low FF of 0443 An additive of DIO with highboiling point (bp) of 169∘C could influence the formationof nanoscale morphology of molecules decrease of 119877

119904 and

increase of shunt resistance (119877sh) due to suppression of hole-electron recombination However relatively higher bp of theCN (263∘C) than that of the DIO might have resulted in amoderate formation of the nanomorphology of molecules Inthe device prepared with CB+CN the 119877

119904is increased and 119877sh

is decreased The decrease in 119877sh is attributed to the leakagecurrent induced by pinholes and traps in the active layermorphology and the increase in 119877

119904is due to recombination

lossTherefore FF of the device is reduced as consequences ofboth119877sh and119877119904TheOTFSC of PTB7PC

71BMprepared with

organic solvent additives of DIO+CN shows more improvedPCE of 496 which implies that the effect of the additive ofDIO is dominant for enhancement of the device performancethrough better intermixing between PTB7 and PC

71BM

300 400 500 600 700 800 90000

01

02

03

04

05

06

Abso

rban

ce (a

u)

Wavelength (nm)

wo additiveWith DIO (3 v)With CN (3 v)With DIO (3 v) + with CN (3 v)

Figure 4 UVVis spectra of four different organic active layers ofPTB7PC

71BM prepared with different organic solvent mixture

The increase in FF (0556) originates from the reduction in119877

119904by the additives of DIO+CN Because of the smaller 119877

119904

and higher 119877sh the FF of the OTFSC prepared with DIO+CNadditives was higher than that of the OTFSC prepared withDIO and OTFSC prepared with CN additives

32 Absorption and Atomic Force Microscopy Studies Ab-sorption characteristic of four different organic active layersof PTB7PC

71BM was investigated by UVVis spectropho-

tometer and depicted in Figure 4 It can be seen that thereis no shift of the absorption peak for the samples preparedwith organic additives from that of the sample prepared withpristine CB It indicates that the thin films prepared withadditives have amorphous phase Topology of the organicactive layers was investigated by atomic force microscopy(AFM) The thin films of PTB7 were prepared by using thethree varieties of organic solvent mixture (1) PTB7 withpristine CB (2) PTB7 with CB and DIO (3 v) and (3) PTB7with CB and CN (3 v) Figure 5 shows the AFM imagesand roughness parameters obtained from the AFM analysisare given in Table 2 peak-to-valley value and room-mean-square value All the three samples show a minute formationof micrograins (Figure 5) and root-mean-square roughnessof one-nanometer level It indicates again that the thin filmsdo not show any crystalline nature

Effects of the organic solvent on the surface morphologyof the PC

71BM thin films were also investigated by AFM

Figure 6 shows the AFM images of the PC71BM thin films

prepared by using pristine organic solvent of CB CB withadditive of DIO (3 v) and CB with additive of CN (3 v)respectively Surface roughness parameters obtained fromthe AFM analysis are presented in Table 3 peak-to-valleyvalue and root-mean-square value All the PC

71BM thin films

(Table 3) reveal distinctively smaller parameters of surface


Table 1 Photovoltaic parameters of the four different organic thin film solar cell devices based on blended active layer of PTB7PC71BMprepared with different organic solvent mixture

PTB7PC71BMin organic solvent mixture

Jsc[mAcm2

]

119881oc[V] FF PCE(120578)

[] 119877

119904[Ω] 119877sh [Ω]

CB without additive 946 072 0496 338 167 3163CB with DIO (3 v) 1278 074 0516 489 74 3304CB with CN (3 v) 1211 072 0443 386 133 2256CB with DIO (3 v)+CN (3 v) 1190 075 0556 496 63 5035

0 05 1510 20

000 984(nm)

(120583m)

05

15

0

10

20(120583

m)

(a)

0 05 1510 20

000 983(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 988(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)

Figure 5 AFM images of the PTB7 thin films prepared with (a) pristine CB (b) CB and additive of DIO (3 v) and (c) CB and additive ofCN (3 v)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)

Figure 6 AFM images of the PC71BM thin films (a) prepared with pristine CB (b) prepared with CB and additive of DIO (3 v) and (c)

prepared with CB and additive of CN (3 v)


0 05 1510 20

000 1433(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 1024(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 284(nm)

(120583m)

05

15

0

10

20(120583

m)

(c)

0 05 1510 20

05

15

0

10

20

000 939(nm)

(120583m

)

(120583m)

(d)

Figure 7 AFM images of the PTB7PC71BM thin films (a) prepared with pristine CB (b) prepared with CB and DIO (3 v) (c) prepared

with CB and CN (3 v) and (d) prepared with CB and DIO (3 v)+CN (3 v)

Table 2 Surface roughness parameters of the PTB7 thin filmsobtained from AFM analysis

Layer Peak-to-valley[nm]

Root-mean-squareroughness [nm]

PTB7 with pristine CB 70 07PTB7 with CB and DIO (3 v) 127 15PTB7 with CB and CN (3 v) 93 06

roughness which is quite different from the cases of PTB7(Table 2) It can be observed (Figure 6) that PC

71BM reveals

excellent solubility in all the organic solvent mixtures ofpristine CB CB with additive of DIO (3 v) and CB withadditive of CN (3 v) [19]

Finally the effect of the organic solvent on the sur-face morphology of the resulting thin films of blended

Table 3 Surface roughness parameters of the PC71BM thin filmsobtained from AFM analysis



PC71BM with pristine CB 34 04PC71BM with CB and DIO(3 v) 33 03

PC71BM with CB and CN(3 v) 38 03

PTB7PC71BM was investigated by AFM which could play

a decisive role in the performance of organic thin filmsolar cell Figure 7 shows the surface topology of threedifferent active layers of blended PTB7PC

71BM [14] Surface

roughness parameters of the PTB7PC71BM prepared in CB


Table 4 Surface roughness parameters of the PTB7PC71BM activelayer obtained from AFM analysis



PTB7PC71BM with pristineCB 134 118

PTB7PC71BM with CB andDIO (3 v) 107 053

PTB7PC71BM with CB andCN (3 v) 118 072

PTB7PC71BM with CB andDIO (3 v) + CN (3 v) 74 064

with different solvent additives are presented in Table 4 ThePTB7PC

71BM thin film prepared by using pristine organic

solvent of CB shows a large micrograin of 1 120583m (Figure 7(a))It implies for the case that molecules of PTB7 and PC

71BM

might have insufficient mutual solubility during the thinfilm formation process and a mutual interaction betweenthe molecules might have an influence on such kind of thinfilm formation [20] On the contrary the PTB7PC

71BM

thin film prepared with a mixture of CB and DIO (3 v)reveals conspicuously smaller micrograins on the surface(Figure 7(b)) An interpenetrated blending through excellentpercolation of micrograins might have been achieved forthe molecules of PTB7 and PC

71BM through the effect of

organic solvent mixture of CB and DIO which could becontributed to the distinctively enhanced PCE (489) ofthe resulting OTFSC (Table 1) [6 17] The PTB7PC

71BM

thin film prepared with a mixture of CB and CN (3 v)(Figure 7(c)) shows smaller micrograins similar to the caseof PTB7PC

71BM thin film prepared with a mixture of CB

and DIO (3 v) (Figure 7(b)) but it reveals an inferiorconnectivity between the micrograins It might be a reasonwhy theOTFSCpreparedwith amixture ofCB andCN (3 v)reveals a moderate PCE of 386 which is distinctively lowerthan that of the OTFSC with PTB7PC

71BM prepared with a

mixture of CB and DIO although it is rather higher than theOTFSC with PTB7PC

71BM prepared with pristine CB The

OTFSC of PTB7PC71BM was prepared with organic solvent

(CB) with additives of CN + DIO (3 v) and the additionof DIO might have resulted in a moderate formation of thenanomorphology of molecules (Figure 7(d)) which impliesthat the PCE of the OTFSC further improved to 496 Theeffect of the additive of DIO is dominant for enhancementof the device performance so that it is similar to that ofthe OTFSC of PTB7PC


mixture of CB and DIO

4 Conclusions

TheOTFSCdeviceswere fabricatedwith blended active layersof PTB7PC

71BM with a relative ratio of 1 15 The active

layers of PTB7PC71BM were prepared by a spin coating

process with four different kinds of organic solvents (1)pristine CB (2) CB with additive of DIO (3 v) (3) CB withadditive of CN (3 v) and (4) CB with additives of DIO

(3 v)+CN (3 v) Solubility of the PTB7 and PC71BM in the

organic solventmixtureswas investigated by surface topologyof the resulting thin films through AFM images PC

71BM

revealed excellent solubility in the organic solvent of pristineCB as well as in mixtures solvents of CBDIO CBCNand CBDIO+CN For the case of pristine CB the resultingcomposite thin film of PTB7PC

71BM revealed relatively

large micrograins of 1120583m on the surface which indicates aninferior mutual solubility and insufficient interpenetration ofPTB7 and PC

71BM in the organic solvent of CB Conspicu-

ously smaller micrograins of one-tenth level were observedfor the PTB7PC

71BMthin filmpreparedwith organic solvent

mixture of CB and DIO (3 v) which is possibly caused byimproved percolation between PTB7 and PC

71BM due to the

effect of DIO Consequently PCE of 489 was achieved forthe resultingOTFSCbased on the interpenetrated active layerof PTB7PC

71BM prepared with organic solvent mixture of

CB and DIO Enhanced transport of charge carriers mighthave been achieved through formation of nanomorphologyin the blended active layer of PTB7PC

71BM due to the

effect of DIO added in organic solvent of CB Althoughthe PTB7PC

71BM prepared with organic solvent mixture

of CB and CN (3 v) showed improved nanomorphologycompared to the sample prepared with pristine CB sufficientconnectivity of themicrograinsmight not be achieved whichcould be resulted in suppression of charge-carrier transportThe moderate increase of PCE up to 386 might have beencaused by the effect of organic solvent mixture of CB withCN However the OTFSC of PTB7PC

71BM prepared with

organic solvent additives of DIO + CN shows formation ofthe intermixed nanomorphology which implies that the PCEof the OTFSC further improved to 496 by spin coatingmethod

Conflict of Interests

The authors declare that there is no conflict of interestsregarding the publication of this paper

Authorsrsquo Contribution

Masakazu Ito and Palanisamy Kumar contributed equally tothis work

Acknowledgments

This research was partly supported by the MEXT-SupportedProgram for the Strategic Research Foundation at PrivateUniversities (2010-2014) and the joint research between AichiInstitute of Technology and NDS

References

[1] Japan Ministry of Foreign Affairs (MOFA) 34th G8 Summitoverview November 2008

[2] K Koyama ldquoLatest IPCC report points to global warming andrelevant human influencerdquo The Institute of Energy EconomicsJapan vol 146 pp 1ndash3 2013


[3] H Matsubara ldquoTracking sustainable energy zones in japanstatus of renewable energy supply in municipalitiesrdquo Japan forSustainability Newsletter no 126 2013

[4] A Poh A Ling S Kokichi andMMasao ldquoThe Japanese smartgrid initiatives investments and collaborations (IJACSA)rdquoInternational Journal of Advanced Computer Science and Appli-cations vol 3 no 7 pp 1ndash11 2012

[5] S Koma ldquoSolar PV R amp D roadmap and activities in Japanrdquo inProceedings of the 6th NEDO-ADEME Workshop New Energyand Industrial Technology Development Organization (NEDOrsquo11) October 2011

[6] Y Liang Z Xu J Xia et al ldquoFor the bright future-bulk hetero-junction polymer solar cells with power conversion efficiency of74rdquo Advance Energy Material vol 22 no 20 pp E135ndashE1382010

[7] C Gu Y Chen Z Zhang et al ldquoAchieving high efficiency ofPTB7-based polymer solar cells via integrated optimization ofboth anode and cathode interlayersrdquo Advance Energy Material2014

[8] J Youa L Doua Z Hongc G Li and Y Yanga ldquoRecent trendsin polymer tandem solar cells researchrdquo Progress in PolymerScience vol 38 no 12 pp 1909ndash1928 2013

[9] O Adebanjo P P Maharjan P Adhikary M Wang S Yangand Q Qiao ldquoTriple junction polymer solar cellsrdquo Energy ampEnvironmental Science vol 6 pp 3150ndash3170 2013

[10] R Tagliaferro D Gentilini S Mastroianni et al ldquoIntegratedtandem dye solar cellsrdquo RSC Advances vol 3 pp 20273ndash202802013

[11] M V Srinivasan P Kannan and A Roy ldquoPhoto and electricallyswitchable behavior of azobenzene containing pendant bent-core liquid crystalline polymersrdquo Journal of Polymer Science APolymer Chemistry vol 51 no 4 pp 936ndash946 2013

[12] K R Sarma ldquoRecent advances in AM OLED technologies forapplication to aerospace and military systemsrdquo in Head- andHelmet-Mounted Displays XVII and Display Technologies andApplications for Defense Security and Avionics VI vol 8383 ofProceedings of SPIE p 83830 Baltimore Md USA 2012

[13] F Zhu ldquoSemitransparent organic solar cellsrdquo Frontiers of Opto-electronics vol 7 no 1 pp 20ndash27 2014

[14] V Svrcek T Yamanari D Mariotti K Matsubara and MKondo ldquoEnhancement of hybrid solar cell performance by poly-thieno [34-b]thiophenebenzodithiophene and microplasma-induced surface engineering of silicon nanocrystalsrdquo AppliedPhysics Letters vol 100 no 22 Article ID 223904 2012

[15] U R Kortshagen C-Y Liu and Z C Holman ldquoHybrid solarcells from P3HT and silicon nanocrystalsrdquo Nano Letters vol 9no 1 pp 449ndash452 2009

[16] B Walker A B Tamayo X-D Dang et al ldquoNanoscalephase separation and high photovoltaic efficiency in solution-processed small-molecule bulk heterojunction solar cellsrdquoAdvanced Functional Materials vol 19 no 19 pp 3063ndash30692009

[17] P Kumar S Kannappan S Ochiai and P-K Shin ldquoHigh-performance organic solar cells based on a low-bandgap poly-thienothiophene-benzodithiophene polymer and fullerenecomposite prepared by using the airbrush spray-coatingtechniquerdquo Journal of the Korean Physical Society vol 62 no 8pp 1169ndash1175 2013

[18] S Ochiai S Imamura S Kannappan K Palanisamy and P-K Shin ldquoCharacteristics and the effect of additives on thenanomorphology of PTB7PC

71BM composite filmsrdquo Current

Applied Physics vol 13 pp S58ndashS63 2013

[19] Z He C Zhong S Su M Xu H Wu and Y Cao ldquoEnhancedpower-conversion efficiency in polymer solar cells using aninverted device structurerdquo Nature Photonics vol 6 no 9 pp591ndash595 2012

[20] W A Hammed R Yahya A L Bola and H N M E MahmudldquoRecent approaches to controlling the nanoscalemorphology ofpolymer-based bulk-heterojunction solar cellrdquo Energies vol 6no 11 pp 5847ndash5868 2013

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Hindawi Publishing Corporationhttpwwwhindawicom Volume 2014

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CatalystsJournal of


Cl

S

S S

SO

O

I

OO

CN

PTB7

(a)

II

DIO

O

PC71BM

OCH3

(b)

Figure 1 Molecular structures of PTB7 PC71BM CN and DIO

aimed by establishing a sophisticated technology of solarcell through usage of new materials and device structureso that power generation efficiency of over 40 and powergeneration cost of 7 YenkWh should be achieved after theyear 2030 [5]

First-generation solar cells based on Si have demerits ofhigh cost and limitation of installing location due to heavyweight Second-generation solar cells based on inorganiccompounds (amorphouspoly crystalline Si and CIGS) havebeen introduced in commercial market Although second-generation solar cells show still relatively inferior powerconversion efficiency they are advantageous from a cost per-spective It is acutely expected for third-generation solar cellsthat they would have improved power conversion efficiencyof over 40 and a merit of low cost comparable to those ofthe second-generation solar cells Generally organic thin filmsolar cells (OTFSCs) are acknowledged that they are lighterthan any other counterparts and suitable for low-cost fabrica-tion [6ndash9] OTFSC materials have been focused towards theenergy-consumption and environment friendly viewpointRecently power conversion efficiency of the OTFSCs hasbeen improved up to 10-11 [8ndash10] Moreover they could beformed to any shape due to flexible characteristic so that theycould be optimized for portable andor wearable application[5] Similar to the case of liquid crystal displays (LCDs)[11] light emitting diodes (LEDs) inorganic solar cells andorganic light emitting diodes (OLEDs) are based on similarmaterials and structures to those of OTFSCs Recent rapidcommercialization of the OLEDs is mainly concentratedon mobile display applications [12] but it is expected that

the market for OLEDs could be expanded in the near futureto large-size TVs and lighting applications The success ofthe OLEDs has motivated vigorous developments of OTFSCsbased on devices and related materials [13]

In this present work we report the performance ofPTB7PC

71BM based OTFSCs prepared in chlorobenzene

(CB) solvent with different additives The active layers ofPTB7PC

71BM were prepared by wet processes with spin

coating technique Organic solvent additives were variedto prepare the organic active layer and the effect of thesolvent additives was investigated in relation to surfacetopology of PTB7 PC

71BM and PTB7PC

71BM thin films

The performance of the resulting OTFSC devices andsurface topology of the active layers were comparativelydiscussed

2 Experimental Procedure

Active layers of organic thin film solar cell device wereprepared using Poly[[48-bis[(2-ethylhexyl)oxy]benzo[12-b45-b1015840]dithiophene-26-diyl][3-fluoro-2-[(2-ethylhexyl)ca-rbonyl]thieno[34-b]thiophenediyl]] (PTB7) as organicelectron donor material and [66]-Phenyl C

71butyric

acid methyl ester (PC71BM) used as electron accepter

material Figure 1 shows the molecular structure of thePTB7 PC

71BM 1-chloronaphthalene (CN) and 18-

Diiodooctane (DIO) PC71BM is an organic material

chemically modified to be dissolved in an organic solventTo enhance the hole transport characteristic a thin film of


ITO

HOMO

PTB7

PCBMAl

LUMO

PEDOTPSS



minus331 eV


(a)

Al

Active layer

PEDOTPSS

ITO

Glass substrate

h

minus +

(b)







71BM (9mg 135mg)








P3HTPCBM [16]



71BM are


119904)


00 02 04 06 08

0

Voltage (V)

wo additive

minus12

minus10

minus8

minus6

minus4

minus2


Curr

ent d

ensit

y (m

Ac

m2)


71BM





71BM pre-


71BM prepared




119904 and






71BMprepared with


71BM

300 400 500 600 700 800 90000

01

02

03

04

05

06

Abso

rban

ce (a

u)

Wavelength (nm)






119904









71BM thin films





Jsc[mAcm2

]

119881oc[V] FF PCE(120578)

[] 119877

119904[Ω] 119877sh [Ω]


0 05 1510 20

000 984(nm)

(120583m)

05

15

0

10

20(120583

m)

(a)

0 05 1510 20

000 983(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 988(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)


0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)




0 05 1510 20

000 1433(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 1024(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 284(nm)

(120583m)

05

15

0

10

20(120583

m)

(c)

0 05 1510 20

05

15

0

10

20

000 939(nm)

(120583m

)

(120583m)

(d)








71BM reveals










71BM [14] Surface













71BM


71BM




71BM











4 Conclusions







71BM








71BM due to the




71BM due to the




71BM prepared with






Acknowledgments


References

































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


ITO

HOMO

PTB7

PCBMAl

LUMO

PEDOTPSS



minus331 eV


(a)

Al

Active layer

PEDOTPSS

ITO

Glass substrate

h

minus +

(b)







71BM (9mg 135mg)








P3HTPCBM [16]



71BM are


119904)


00 02 04 06 08

0

Voltage (V)

wo additive

minus12

minus10

minus8

minus6

minus4

minus2


Curr

ent d

ensit

y (m

Ac

m2)


71BM





71BM pre-


71BM prepared




119904 and






71BMprepared with


71BM

300 400 500 600 700 800 90000

01

02

03

04

05

06

Abso

rban

ce (a

u)

Wavelength (nm)






119904









71BM thin films





Jsc[mAcm2

]

119881oc[V] FF PCE(120578)

[] 119877

119904[Ω] 119877sh [Ω]


0 05 1510 20

000 984(nm)

(120583m)

05

15

0

10

20(120583

m)

(a)

0 05 1510 20

000 983(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 988(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)


0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)




0 05 1510 20

000 1433(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 1024(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 284(nm)

(120583m)

05

15

0

10

20(120583

m)

(c)

0 05 1510 20

05

15

0

10

20

000 939(nm)

(120583m

)

(120583m)

(d)








71BM reveals










71BM [14] Surface













71BM


71BM




71BM











4 Conclusions







71BM








71BM due to the




71BM due to the




71BM prepared with






Acknowledgments


References

































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


00 02 04 06 08

0

Voltage (V)

wo additive

minus12

minus10

minus8

minus6

minus4

minus2


Curr

ent d

ensit

y (m

Ac

m2)


71BM





71BM pre-


71BM prepared




119904 and






71BMprepared with


71BM

300 400 500 600 700 800 90000

01

02

03

04

05

06

Abso

rban

ce (a

u)

Wavelength (nm)






119904









71BM thin films





Jsc[mAcm2

]

119881oc[V] FF PCE(120578)

[] 119877

119904[Ω] 119877sh [Ω]


0 05 1510 20

000 984(nm)

(120583m)

05

15

0

10

20(120583

m)

(a)

0 05 1510 20

000 983(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 988(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)


0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)




0 05 1510 20

000 1433(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 1024(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 284(nm)

(120583m)

05

15

0

10

20(120583

m)

(c)

0 05 1510 20

05

15

0

10

20

000 939(nm)

(120583m

)

(120583m)

(d)








71BM reveals










71BM [14] Surface













71BM


71BM




71BM











4 Conclusions







71BM








71BM due to the




71BM due to the




71BM prepared with






Acknowledgments


References

































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of




Jsc[mAcm2

]

119881oc[V] FF PCE(120578)

[] 119877

119904[Ω] 119877sh [Ω]


0 05 1510 20

000 984(nm)

(120583m)

05

15

0

10

20(120583

m)

(a)

0 05 1510 20

000 983(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 988(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)


0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 463(nm)

(120583m)

05

15

0

10

20

(120583m

)

(c)




0 05 1510 20

000 1433(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 1024(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 284(nm)

(120583m)

05

15

0

10

20(120583

m)

(c)

0 05 1510 20

05

15

0

10

20

000 939(nm)

(120583m

)

(120583m)

(d)








71BM reveals










71BM [14] Surface













71BM


71BM




71BM











4 Conclusions







71BM








71BM due to the




71BM due to the




71BM prepared with






Acknowledgments


References

































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of


0 05 1510 20

000 1433(nm)

(120583m)

05

15

0

10

20

(120583m

)

(a)

0 05 1510 20

000 1024(nm)

(120583m)

05

15

0

10

20

(120583m

)

(b)

0 05 1510 20

000 284(nm)

(120583m)

05

15

0

10

20(120583

m)

(c)

0 05 1510 20

05

15

0

10

20

000 939(nm)

(120583m

)

(120583m)

(d)








71BM reveals










71BM [14] Surface













71BM


71BM




71BM











4 Conclusions







71BM








71BM due to the




71BM due to the




71BM prepared with






Acknowledgments


References

































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of












71BM


71BM




71BM











4 Conclusions







71BM








71BM due to the




71BM due to the




71BM prepared with






Acknowledgments


References

































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of































Journal of

Chemistry


Advances in

Physical Chemistry



Journal of

Volume 2014














Journal of

Spectroscopy



Journal of


Quantum Chemistry






CatalystsJournal of

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