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Copyright copy 2012 American Scientific PublishersAll rights reservedPrinted in the United States of America

Journal ofNanoscience and Nanotechnology

Vol 12 1234ndash1237 2012

Inverted Organic Solar Cells with ZnO NanowallsPrepared Using Wet Chemical Etching in a KOH Solution

Kyung-Sik Shin1 Hye-Jeong Park1 Brijesh Kumar1 Kwon-Ho Kim1Sang-Hyeob Kim2 and Sang-Woo Kim13lowast

1School of Advanced Materials Science and Engineering Sungkyunkwan University Suwon 440-746 Republic of Korea2Electronics and Telecommunications Research Institute Daejeon 305-700 Republic of Korea

3SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nanotechnology (HINT)Sungkyunkwan University Suwon 440-746 Republic of Korea

We report on the photovoltaic (PV) performances of inverted organic solar cells (IOSCs) that werefabricated from PCBMP3HT polymer with a ZnO thin film and ZnO nanowalls as electron transportand hole block layers ZnO thin film on ITOglass substrate was deposited using a simply aqueoussolution route ZnO nanowall structures were obtained via wet chemical etching of ZnO thin filmsin a KOH solution The power conversion efficiency (PCE) of the IOSC with ZnO nanowalls wassignificantly improved by 44 from 1254 to 1811 compared to that of the IOSC with ZnO thinfilm The short circuit current in IOSCs fabricated with the ZnO nanowalls was increased mainlydue to the increase in the charge transport interface area as a result of enhancement in the PCEThis work suggests a method for fabricating efficient PV devices with a larger charge transport areafor future prospects

Keywords Inverted Organic Solar Cell Electron Transport Layer ZnO Nanowall

1 INTRODUCTION

Organic-based photovoltaic devices (OPVs) are promisingconcepts for the energy industry due to their low-costlight-weight compatibility with flexible plastic substratesand ease of fabrication12 However the short exciton-diffusion length and inefficient exciton dissociation in apolymeric matrix result in low quantum efficiency (QE)that limits the PCE and applications of OPVs To over-come this limitation bulk-heterojunctions (BHJs) consist-ing of a mixture of polymerfullerene materials such aspoly(3-hexylthiophene) (P3HT donor material) and (66)-phenyl C60 butyric acid methyl ester (PCBM acceptormaterial) are typically introduced3 The inferior interfacestability of the photoactive layer and corrosion of the ITOin conventional organic solar cells based on BHJs are aconcernTo improve the interface stability and prevent device

degradation45 IOSCs with semiconductor oxide materialsare being extensively studied These structures increase thedevice lifetime by replacing the low work function metalcathode such as Al and PEDOTPSS with high work func-tion materials and by introduction of inorganic materials

lowastAuthor to whom correspondence should be addressed

for the electron transport path An approach for furtherenhancing the PCE is the introduction of various semi-conductor oxide nanostructures as electron transport layersdue to their excellent electrical and optical properties andlarge area compared to those of thin filmsIn this work the photovoltaic (PV) performances of

organic solar cells with zinc oxide (ZnO) nanowall elec-tron transport layers are reported to enhance the chargetransport due to their increased interface areas The ZnOthin film was grown using a simple aqueous solution routeand the ZnO nanowalls were prepared via a wet chemicalmethod of ZnO thin film in a KOH solution Comparedwith the IOSC with ZnO thin film the PCE of the IOSCfabricated with the ZnO nanowall was increased by 44at a simulated air mass (AM) of 15 global full-sun (15 G100 mWcm2 illumination

2 EXPERIMENTAL DETAILS

The ZnO thin film was deposited via an aqueous solutionroute onto an indium tin oxide (ITO)glass substrate6 Inorder to deposit a thin film a seed layer was first coatedonto the substrate via dip-coating for 10 min into mix-ture solution of 01 mM zinc acetate dehydrate 9999

1234 J Nanosci Nanotechnol 2012 Vol 12 No 2 1533-48802012121234004 doi101166jnn20124612

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Shin et al Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution

(Zn(C2H3O22 dissolved in an ethanol (100 ml) Thedeposited substrate was heated at 150 C for 10 min andthis process was repeated seven times Thereafter the seed-coated substrate was immersed into a solution of 005 Mzinc nitrate hexahydrate 9990 (Zn(NO32 middot 6H2O) and005 M hexamethylenetetramine 9900 (C6H12N4 dis-solved in ethanoldeionized water (vol = 11) at 90 Cfor 15 h The grown ZnO thin film was rinsed with deion-ized water and dried on a hotplate at 100 C The ZnOnanowall was prepared via wet chemical etching of ZnOthin film samples in 02 M KOH for 10 and 15 min at50 C The grown ZnO nanowall was rinsed with deion-ized water and dried on a hotplate at 100 C

For the fabrication of IOSCs a polymer blend ofpoly(3-hexylthiophene) (P3HT)(6 6)-phenyl C61 butyricacid methyl ester (PCBM) (11 vol inchlorobenzene)was spin-coated onto ZnO layers (ZnO thin film and ZnOnanowalls prepared via etching for 10 and 15 min respec-tively) at 2000 rpm for 120 sec Then the samples werekept in a covered glass Petri dish for solvent annealingThe post annealing was performed at 150 C for 10 minMolybdenum oxide (MoO3 as an electron blocking layerand a gold (Au) anode were subsequently deposited viathermal evaporationThe morphologies of the ZnO nanostructures were

determined using field-emission scanning electronmicroscopy (FE-SEM) The crystal structures and opticalproperties of the thin film and nanowalls were investigatedusing X-ray diffraction (XRD) and photoluminescence(PL) at room temperature For the characterizations ofIOSCs current densityndashvoltage (JminusV ) measurementswere performed using a solar simulator under an irradia-tion intensity of air mass AM 15 G (100 mWcm2

3 RESULTS AND DISCUSSION

Figure 1 shows the FE-SEM images of the ZnO thin filmand nanowalls on ITOglass substrates The as-grown ZnOthin film had a rough surface in the form of densely tex-tured nanorod arrays with a hexagonal structure as shownin Figures 1(a) Figures 1(b and c) show the morpholog-ical changes in the ZnO nanowall structures produced byetching in KOH solution for 10 and 15 min respectivelyEtching from the top surface of the ZnO textured nanorodarray started slowly The ZnO nanowall structure with asidewall of approximately 30 nm was formed by the partialdissolution of the (001) surface of the ZnO thin film afteretching for 15 min The ZnO nanowall from the thin filmwas synthesized via the chemical etching reaction depictedin Figure 1(d) Formation of the nanowall structure wasattributed to etching along the polar axis78

ZnO+2OHminus+H2Orarr ZnOH2minus4 (1)

Figure 2 shows the XRD patterns of ZnO thin film andnanowalls on ITOglass substrate The XRD pattern of the

Fig 1 FE-SEM images of the deposited ZnO thin film (a) and the ZnOnanowalls by etching at 50 C for different time (b) 10 min (c) 15 minThe inset in Figure 1(a) is the cross-section of ZnO thin film beforeetching And schematic illustration of ZnO nanowall formation processin KOH solution (d)

ZnO thin film showed the dominant (002) peak with (101)(102) and (103) peaks related to ZnO hexagonal wurtzitecrystal Although there was no significant change in theintensity of the (002) peak of the ZnO nanowall preparedby etching for 10 min as shown in Figure 2(b) signif-icant change was observed in the peak intensity for thenanowalls prepared by etching for 15 min This changein the XRD patterns demonstrates that the ZnO thin filmwas dissolved along the c-axis consistent with the surfacemorphology910

The observed PL spectra from the ZnO thin film andZnO nanowalls on the ITOglass substrate at room tem-perature are shown in Figure 3 In the PL spectra a bandedge emission peak at approximately 380 nm and a strongdeep-level emission band near 550 nm were observedThere was a reduction in the UV emission peak inten-sity and enhancement in the deep-level emission peakintensity of the PL from the ZnO nanowall created via

Fig 2 XRD patterns of ZnO thin film (a) and ZnO nanowalls onITOglass substrate by etching at 50 C for different time (b) 10 min(c) 15 min

J Nanosci Nanotechnol 12 1234ndash1237 2012 1235

Delivered by Ingenta toSung Kyun Kwan University

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Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution Shin et al

Fig 3 (a) PL spectra of ZnO thin film and ZnO nanowalls on ITOglasssubstrate by etching at 50 C for different time (b) 10 min (c) 15 min

etching for 10 min compared to those of the thin filmAn enhancement in the deep level emission peak is dueto surface defects created on the thin film such as oxygenvacancies and zinc interstitials by etching for 10 min1112

and reduction in the UV emission peak was related toa reduction in the radiative recombination rate due tothe presence of these surface defects Further etching for15 min produces the nanowall structure we observed thatintensity of the deep level emission peak as well as UVemission peaks was increased further from nanowall struc-ture Further increase of deep level emission peak is dueto increasing the oxygen defects in longer time etchingOn the other hand due to formation of nanowall sur-face area also increases Consequently increased numberof electronndashhole pair generations and recombinations13

Therefore in the nanowall sample that was prepared by15 min etching intensities of the deep level and UV emis-sion peaks were increased as compared to those of thenanowall structure prepared for 10 minA cross-sectional FE-SEM image of the IOSC struc-

ture with a ZnO nanowall is depicted in Figure 4(a) Thetotal thickness of the inverted IOSC was approximately850 nm The thicknesses of the ZnO nanowalls and theP3HTPCBM blend layer were approximately 270 nm and180 nm respectively Figure 4(b) presents the schematicenergy level diagram of IOSCs and the transport direc-tion of the charge carriers The transport of the solar-generated charge carriers in the IOSC with a ZnO thinfilm and nanowalls was the same The schematic illus-tration in Figure 4(c) shows the IOSC structures with aZnO thin film and nanowalls as an electron transport layerThe IOSC devices had the same structure however theinterface between the active layer and the ZnO nanowallincreased compared to that of the ZnO thin filmThe currentndashdensityndashvoltage (JminusV ) characteristics of

the three devices were measured under simulated AM 15illumination as shown in Figure 5(a) The PCE of theIOSCs with ZnO nanowalls prepared by etching for 10 min(Device B) and 15 min (Device C) were 1476 and

Fig 4 (a) Cross-section FE-SEM image of IOSC with ZnO nanowall(b) schematic energy level diagram of IOSCs in this work and (c)schematic illustration of IOSCs with ZnO thin film and nanowall as anelectron transport layer

1811 respectively while that with the ZnO thin filmwas 1254 Table I summarizes the PV performances ofIOSCs fabricated with a thin film and nanowalls Thesedata demonstrate that the PCE was enhanced by 44from 1254 to 1811 by the introduction of the ZnOnanowall This substantial improvement in the PV perfor-mance can be explained in two ways One reason is theincrease in the charge transport interface area between theZnO nanowalls and active layer for which a larger area isfavorable for the transport of a large number of electroncarriers For the IOSCs with ZnO thin films the chargetransport interface between the thin film and organic mate-rial was smaller and most of the photogenerated elec-trons produced distant from the active layer were not ableto reach the interface and their recombination probabil-ity distant from the interface was large For the IOSCswith ZnO nanowalls upon filling the space between theZnO nanowalls with organic materials the charge trans-port area was greatly increased and most of the photo-generated electrons were able to reach the interface beforetheir recombination resulting in an increased JSC Theincreasing interface area between the active layer and ZnOnanowalls contributed to the low series Rs of devices withZnO nanowalls1415

Another reason for the performance enhancement is thatthe increasing oxygen defects on the nanowall surface dueto etching affect the conductivity The oxygen vacanciesare the main native donor defect acting as conductive sites

Table I Performance details (VOC JSC FF and PCE) of the IOSC

JSC VOC FF PCEDevice (mAcm2) (V) () ()

A 5807 0478 45163 1254B 7052 0484 43241 1476C 8552 0480 44129 1811

1236 J Nanosci Nanotechnol 12 1234ndash1237 2012

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RESEARCH

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Shin et al Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution

Fig 5 JminusV characteristics for IOSCs with the deposited ZnO thin filmand ZnO nanowalls by etching for 10sim15 min (a) under AM 15 Girradiation and (b) under forward bias in a dark

in n-ZnO Therefore increasing the number of oxygendefects lead to decreases in the electrical resistivities of theZnO nanowalls Although the reduction in carrier mobilitywas expected due to scattering from the defects the con-ductivity and large charge transport interface area betweenthe nanowalls and organic material are dominating factorsin the enhancement of the PCEs of IOSCs fabricated withZnO nanowalls16 The fill factor (FF) in ZnO nanowall-based IOSCs was reduced due to the increase in leakagecurrent due to the defectsFigure 5(b) shows JminusV curves for IOSCs fabricated

with the thin film and nanowalls under a forward biasin the dark It can be suggested the current densities ofDevices B and C were remarkably increased compared tothat of Device A consistent with the reduction in Rs dueto the increased conductivity caused by incorporation ofthe nanowalls The experimental results support the expla-nation for the improvement in the PV performance by theincorporation of ZnO nanowalls

4 CONCLUSION

We have demonstrated a 44 enhancement in the PCEfrom IOSCs fabricated with ZnO nanowalls compared tothat of the thin film It can be suggested that the enhancedPCE of the IOSCs with ZnO nanowalls was attributedto the increased charge transport area between the ZnOnanowall and the active polymer blend layer

Acknowledgment This research was supported byBasic Science Research Program through the NationalResearch Foundation of Korea (NRF) funded by theMinistry of Education Science and Technology (MEST)(2010-0015035 and 2009-0077682) and ConvergingResearch Center Program through the NRF funded bythe MEST (2010K001165) and by the New and Renew-able Energy of the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) grant funded by theKorea government Ministry of Knowledge Economy (No2009T100100614)

References and Notes

1 G Li V Shrotriya J Huang Y Yao T Moriarty K Emery andY Yang Nat Mater 4 864 (2005)

2 A K K Kyaw X W Sun C Y Jiang G Q Lo D W Zhao andD L Kwong Appl Phys Lett 93 221107 (2008)

3 L H Hoppe and N S Sariciftci J Mater Chem 16 45 (2006)4 G Li V Shrotriya Y Yao J Huang and Y Yang J Mater Chem

17 3126 (2007)5 M P de Jong L J van Ijzendoom and M J A de Voigt Appl

Phys Lett 77 2255 (2000)6 M-J Jin S-D Lee K-S Shin S-W Jeong D H Yoon D Jeon

I-H Lee and S-W Kim J Nanosci Nanotech 9 7432 (2009)7 L Yu G Zhang S Li Z Xi and D Guo J Cryst Growth 299 184

(2007)8 J Su R Che G She X Duan and W Shi J Nanosci Nanotech

8 6306 (2008)9 X Gan X Li X Gao and W Yu J Alloys Compd 481 397 (2009)10 L Xu Q Liao J Zhang X Ai and D Xu J Phys Chem C

111 4549 (2007)11 Q X Zhao P Klason M Willander H M Zhong W Lu and J H

Yang Appl Phys Lett 87 211912 (2005)12 K Vanheusden C H Seager W L Warren D R Tallant and

J A Voigt Appl Phys Lett 68 403 (1996)13 Y Zhang W Zhang and C Peng Opt Express 16 10696 (2008)14 B-Y Yu A Tsai S-P Tsai K-T Wong Y Yang C-W Chu and

J-J Shyue Nanotechnology 19 255202 (2008)15 J Liu S Wang Z Bian M Shan and C Huang Appl Phys Lett

94 173107 (2009)16 Y Ma G T Du S R Yang Z T Li B J Zhao X T Yang

T P Yang Y T Zhang and D L Liu J Appl Phys 95 6268(2004)

Received 26 August 2010 Accepted 13 December 2010

J Nanosci Nanotechnol 12 1234ndash1237 2012 1237

Delivered by Ingenta toSung Kyun Kwan University

IP 11514516612Wed 25 Jul 2012 111047

RESEARCH

ARTIC

LE

Shin et al Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution

(Zn(C2H3O22 dissolved in an ethanol (100 ml) Thedeposited substrate was heated at 150 C for 10 min andthis process was repeated seven times Thereafter the seed-coated substrate was immersed into a solution of 005 Mzinc nitrate hexahydrate 9990 (Zn(NO32 middot 6H2O) and005 M hexamethylenetetramine 9900 (C6H12N4 dis-solved in ethanoldeionized water (vol = 11) at 90 Cfor 15 h The grown ZnO thin film was rinsed with deion-ized water and dried on a hotplate at 100 C The ZnOnanowall was prepared via wet chemical etching of ZnOthin film samples in 02 M KOH for 10 and 15 min at50 C The grown ZnO nanowall was rinsed with deion-ized water and dried on a hotplate at 100 C

For the fabrication of IOSCs a polymer blend ofpoly(3-hexylthiophene) (P3HT)(6 6)-phenyl C61 butyricacid methyl ester (PCBM) (11 vol inchlorobenzene)was spin-coated onto ZnO layers (ZnO thin film and ZnOnanowalls prepared via etching for 10 and 15 min respec-tively) at 2000 rpm for 120 sec Then the samples werekept in a covered glass Petri dish for solvent annealingThe post annealing was performed at 150 C for 10 minMolybdenum oxide (MoO3 as an electron blocking layerand a gold (Au) anode were subsequently deposited viathermal evaporationThe morphologies of the ZnO nanostructures were

determined using field-emission scanning electronmicroscopy (FE-SEM) The crystal structures and opticalproperties of the thin film and nanowalls were investigatedusing X-ray diffraction (XRD) and photoluminescence(PL) at room temperature For the characterizations ofIOSCs current densityndashvoltage (JminusV ) measurementswere performed using a solar simulator under an irradia-tion intensity of air mass AM 15 G (100 mWcm2

3 RESULTS AND DISCUSSION

Figure 1 shows the FE-SEM images of the ZnO thin filmand nanowalls on ITOglass substrates The as-grown ZnOthin film had a rough surface in the form of densely tex-tured nanorod arrays with a hexagonal structure as shownin Figures 1(a) Figures 1(b and c) show the morpholog-ical changes in the ZnO nanowall structures produced byetching in KOH solution for 10 and 15 min respectivelyEtching from the top surface of the ZnO textured nanorodarray started slowly The ZnO nanowall structure with asidewall of approximately 30 nm was formed by the partialdissolution of the (001) surface of the ZnO thin film afteretching for 15 min The ZnO nanowall from the thin filmwas synthesized via the chemical etching reaction depictedin Figure 1(d) Formation of the nanowall structure wasattributed to etching along the polar axis78

ZnO+2OHminus+H2Orarr ZnOH2minus4 (1)

Figure 2 shows the XRD patterns of ZnO thin film andnanowalls on ITOglass substrate The XRD pattern of the

Fig 1 FE-SEM images of the deposited ZnO thin film (a) and the ZnOnanowalls by etching at 50 C for different time (b) 10 min (c) 15 minThe inset in Figure 1(a) is the cross-section of ZnO thin film beforeetching And schematic illustration of ZnO nanowall formation processin KOH solution (d)

ZnO thin film showed the dominant (002) peak with (101)(102) and (103) peaks related to ZnO hexagonal wurtzitecrystal Although there was no significant change in theintensity of the (002) peak of the ZnO nanowall preparedby etching for 10 min as shown in Figure 2(b) signif-icant change was observed in the peak intensity for thenanowalls prepared by etching for 15 min This changein the XRD patterns demonstrates that the ZnO thin filmwas dissolved along the c-axis consistent with the surfacemorphology910

The observed PL spectra from the ZnO thin film andZnO nanowalls on the ITOglass substrate at room tem-perature are shown in Figure 3 In the PL spectra a bandedge emission peak at approximately 380 nm and a strongdeep-level emission band near 550 nm were observedThere was a reduction in the UV emission peak inten-sity and enhancement in the deep-level emission peakintensity of the PL from the ZnO nanowall created via

Fig 2 XRD patterns of ZnO thin film (a) and ZnO nanowalls onITOglass substrate by etching at 50 C for different time (b) 10 min(c) 15 min

J Nanosci Nanotechnol 12 1234ndash1237 2012 1235

Delivered by Ingenta toSung Kyun Kwan University

IP 11514516612Wed 25 Jul 2012 111047

RESEARCH

ARTIC

LE

Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution Shin et al

Fig 3 (a) PL spectra of ZnO thin film and ZnO nanowalls on ITOglasssubstrate by etching at 50 C for different time (b) 10 min (c) 15 min

etching for 10 min compared to those of the thin filmAn enhancement in the deep level emission peak is dueto surface defects created on the thin film such as oxygenvacancies and zinc interstitials by etching for 10 min1112

and reduction in the UV emission peak was related toa reduction in the radiative recombination rate due tothe presence of these surface defects Further etching for15 min produces the nanowall structure we observed thatintensity of the deep level emission peak as well as UVemission peaks was increased further from nanowall struc-ture Further increase of deep level emission peak is dueto increasing the oxygen defects in longer time etchingOn the other hand due to formation of nanowall sur-face area also increases Consequently increased numberof electronndashhole pair generations and recombinations13

Therefore in the nanowall sample that was prepared by15 min etching intensities of the deep level and UV emis-sion peaks were increased as compared to those of thenanowall structure prepared for 10 minA cross-sectional FE-SEM image of the IOSC struc-

ture with a ZnO nanowall is depicted in Figure 4(a) Thetotal thickness of the inverted IOSC was approximately850 nm The thicknesses of the ZnO nanowalls and theP3HTPCBM blend layer were approximately 270 nm and180 nm respectively Figure 4(b) presents the schematicenergy level diagram of IOSCs and the transport direc-tion of the charge carriers The transport of the solar-generated charge carriers in the IOSC with a ZnO thinfilm and nanowalls was the same The schematic illus-tration in Figure 4(c) shows the IOSC structures with aZnO thin film and nanowalls as an electron transport layerThe IOSC devices had the same structure however theinterface between the active layer and the ZnO nanowallincreased compared to that of the ZnO thin filmThe currentndashdensityndashvoltage (JminusV ) characteristics of

the three devices were measured under simulated AM 15illumination as shown in Figure 5(a) The PCE of theIOSCs with ZnO nanowalls prepared by etching for 10 min(Device B) and 15 min (Device C) were 1476 and

Fig 4 (a) Cross-section FE-SEM image of IOSC with ZnO nanowall(b) schematic energy level diagram of IOSCs in this work and (c)schematic illustration of IOSCs with ZnO thin film and nanowall as anelectron transport layer

1811 respectively while that with the ZnO thin filmwas 1254 Table I summarizes the PV performances ofIOSCs fabricated with a thin film and nanowalls Thesedata demonstrate that the PCE was enhanced by 44from 1254 to 1811 by the introduction of the ZnOnanowall This substantial improvement in the PV perfor-mance can be explained in two ways One reason is theincrease in the charge transport interface area between theZnO nanowalls and active layer for which a larger area isfavorable for the transport of a large number of electroncarriers For the IOSCs with ZnO thin films the chargetransport interface between the thin film and organic mate-rial was smaller and most of the photogenerated elec-trons produced distant from the active layer were not ableto reach the interface and their recombination probabil-ity distant from the interface was large For the IOSCswith ZnO nanowalls upon filling the space between theZnO nanowalls with organic materials the charge trans-port area was greatly increased and most of the photo-generated electrons were able to reach the interface beforetheir recombination resulting in an increased JSC Theincreasing interface area between the active layer and ZnOnanowalls contributed to the low series Rs of devices withZnO nanowalls1415

Another reason for the performance enhancement is thatthe increasing oxygen defects on the nanowall surface dueto etching affect the conductivity The oxygen vacanciesare the main native donor defect acting as conductive sites

Table I Performance details (VOC JSC FF and PCE) of the IOSC

JSC VOC FF PCEDevice (mAcm2) (V) () ()

A 5807 0478 45163 1254B 7052 0484 43241 1476C 8552 0480 44129 1811

1236 J Nanosci Nanotechnol 12 1234ndash1237 2012

Delivered by Ingenta toSung Kyun Kwan University

IP 11514516612Wed 25 Jul 2012 111047

RESEARCH

ARTIC

LE

Shin et al Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution

Fig 5 JminusV characteristics for IOSCs with the deposited ZnO thin filmand ZnO nanowalls by etching for 10sim15 min (a) under AM 15 Girradiation and (b) under forward bias in a dark

in n-ZnO Therefore increasing the number of oxygendefects lead to decreases in the electrical resistivities of theZnO nanowalls Although the reduction in carrier mobilitywas expected due to scattering from the defects the con-ductivity and large charge transport interface area betweenthe nanowalls and organic material are dominating factorsin the enhancement of the PCEs of IOSCs fabricated withZnO nanowalls16 The fill factor (FF) in ZnO nanowall-based IOSCs was reduced due to the increase in leakagecurrent due to the defectsFigure 5(b) shows JminusV curves for IOSCs fabricated

with the thin film and nanowalls under a forward biasin the dark It can be suggested the current densities ofDevices B and C were remarkably increased compared tothat of Device A consistent with the reduction in Rs dueto the increased conductivity caused by incorporation ofthe nanowalls The experimental results support the expla-nation for the improvement in the PV performance by theincorporation of ZnO nanowalls

4 CONCLUSION

We have demonstrated a 44 enhancement in the PCEfrom IOSCs fabricated with ZnO nanowalls compared tothat of the thin film It can be suggested that the enhancedPCE of the IOSCs with ZnO nanowalls was attributedto the increased charge transport area between the ZnOnanowall and the active polymer blend layer

Acknowledgment This research was supported byBasic Science Research Program through the NationalResearch Foundation of Korea (NRF) funded by theMinistry of Education Science and Technology (MEST)(2010-0015035 and 2009-0077682) and ConvergingResearch Center Program through the NRF funded bythe MEST (2010K001165) and by the New and Renew-able Energy of the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) grant funded by theKorea government Ministry of Knowledge Economy (No2009T100100614)

References and Notes

1 G Li V Shrotriya J Huang Y Yao T Moriarty K Emery andY Yang Nat Mater 4 864 (2005)

2 A K K Kyaw X W Sun C Y Jiang G Q Lo D W Zhao andD L Kwong Appl Phys Lett 93 221107 (2008)

3 L H Hoppe and N S Sariciftci J Mater Chem 16 45 (2006)4 G Li V Shrotriya Y Yao J Huang and Y Yang J Mater Chem

17 3126 (2007)5 M P de Jong L J van Ijzendoom and M J A de Voigt Appl

Phys Lett 77 2255 (2000)6 M-J Jin S-D Lee K-S Shin S-W Jeong D H Yoon D Jeon

I-H Lee and S-W Kim J Nanosci Nanotech 9 7432 (2009)7 L Yu G Zhang S Li Z Xi and D Guo J Cryst Growth 299 184

(2007)8 J Su R Che G She X Duan and W Shi J Nanosci Nanotech

8 6306 (2008)9 X Gan X Li X Gao and W Yu J Alloys Compd 481 397 (2009)10 L Xu Q Liao J Zhang X Ai and D Xu J Phys Chem C

111 4549 (2007)11 Q X Zhao P Klason M Willander H M Zhong W Lu and J H

Yang Appl Phys Lett 87 211912 (2005)12 K Vanheusden C H Seager W L Warren D R Tallant and

J A Voigt Appl Phys Lett 68 403 (1996)13 Y Zhang W Zhang and C Peng Opt Express 16 10696 (2008)14 B-Y Yu A Tsai S-P Tsai K-T Wong Y Yang C-W Chu and

J-J Shyue Nanotechnology 19 255202 (2008)15 J Liu S Wang Z Bian M Shan and C Huang Appl Phys Lett

94 173107 (2009)16 Y Ma G T Du S R Yang Z T Li B J Zhao X T Yang

T P Yang Y T Zhang and D L Liu J Appl Phys 95 6268(2004)

Received 26 August 2010 Accepted 13 December 2010

J Nanosci Nanotechnol 12 1234ndash1237 2012 1237

Delivered by Ingenta toSung Kyun Kwan University

IP 11514516612Wed 25 Jul 2012 111047

RESEARCH

ARTIC

LE

Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution Shin et al

Fig 3 (a) PL spectra of ZnO thin film and ZnO nanowalls on ITOglasssubstrate by etching at 50 C for different time (b) 10 min (c) 15 min

etching for 10 min compared to those of the thin filmAn enhancement in the deep level emission peak is dueto surface defects created on the thin film such as oxygenvacancies and zinc interstitials by etching for 10 min1112

and reduction in the UV emission peak was related toa reduction in the radiative recombination rate due tothe presence of these surface defects Further etching for15 min produces the nanowall structure we observed thatintensity of the deep level emission peak as well as UVemission peaks was increased further from nanowall struc-ture Further increase of deep level emission peak is dueto increasing the oxygen defects in longer time etchingOn the other hand due to formation of nanowall sur-face area also increases Consequently increased numberof electronndashhole pair generations and recombinations13

Therefore in the nanowall sample that was prepared by15 min etching intensities of the deep level and UV emis-sion peaks were increased as compared to those of thenanowall structure prepared for 10 minA cross-sectional FE-SEM image of the IOSC struc-

ture with a ZnO nanowall is depicted in Figure 4(a) Thetotal thickness of the inverted IOSC was approximately850 nm The thicknesses of the ZnO nanowalls and theP3HTPCBM blend layer were approximately 270 nm and180 nm respectively Figure 4(b) presents the schematicenergy level diagram of IOSCs and the transport direc-tion of the charge carriers The transport of the solar-generated charge carriers in the IOSC with a ZnO thinfilm and nanowalls was the same The schematic illus-tration in Figure 4(c) shows the IOSC structures with aZnO thin film and nanowalls as an electron transport layerThe IOSC devices had the same structure however theinterface between the active layer and the ZnO nanowallincreased compared to that of the ZnO thin filmThe currentndashdensityndashvoltage (JminusV ) characteristics of

the three devices were measured under simulated AM 15illumination as shown in Figure 5(a) The PCE of theIOSCs with ZnO nanowalls prepared by etching for 10 min(Device B) and 15 min (Device C) were 1476 and

Fig 4 (a) Cross-section FE-SEM image of IOSC with ZnO nanowall(b) schematic energy level diagram of IOSCs in this work and (c)schematic illustration of IOSCs with ZnO thin film and nanowall as anelectron transport layer

1811 respectively while that with the ZnO thin filmwas 1254 Table I summarizes the PV performances ofIOSCs fabricated with a thin film and nanowalls Thesedata demonstrate that the PCE was enhanced by 44from 1254 to 1811 by the introduction of the ZnOnanowall This substantial improvement in the PV perfor-mance can be explained in two ways One reason is theincrease in the charge transport interface area between theZnO nanowalls and active layer for which a larger area isfavorable for the transport of a large number of electroncarriers For the IOSCs with ZnO thin films the chargetransport interface between the thin film and organic mate-rial was smaller and most of the photogenerated elec-trons produced distant from the active layer were not ableto reach the interface and their recombination probabil-ity distant from the interface was large For the IOSCswith ZnO nanowalls upon filling the space between theZnO nanowalls with organic materials the charge trans-port area was greatly increased and most of the photo-generated electrons were able to reach the interface beforetheir recombination resulting in an increased JSC Theincreasing interface area between the active layer and ZnOnanowalls contributed to the low series Rs of devices withZnO nanowalls1415

Another reason for the performance enhancement is thatthe increasing oxygen defects on the nanowall surface dueto etching affect the conductivity The oxygen vacanciesare the main native donor defect acting as conductive sites

Table I Performance details (VOC JSC FF and PCE) of the IOSC

JSC VOC FF PCEDevice (mAcm2) (V) () ()

A 5807 0478 45163 1254B 7052 0484 43241 1476C 8552 0480 44129 1811

1236 J Nanosci Nanotechnol 12 1234ndash1237 2012

Delivered by Ingenta toSung Kyun Kwan University

IP 11514516612Wed 25 Jul 2012 111047

RESEARCH

ARTIC

LE

Shin et al Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution

Fig 5 JminusV characteristics for IOSCs with the deposited ZnO thin filmand ZnO nanowalls by etching for 10sim15 min (a) under AM 15 Girradiation and (b) under forward bias in a dark

in n-ZnO Therefore increasing the number of oxygendefects lead to decreases in the electrical resistivities of theZnO nanowalls Although the reduction in carrier mobilitywas expected due to scattering from the defects the con-ductivity and large charge transport interface area betweenthe nanowalls and organic material are dominating factorsin the enhancement of the PCEs of IOSCs fabricated withZnO nanowalls16 The fill factor (FF) in ZnO nanowall-based IOSCs was reduced due to the increase in leakagecurrent due to the defectsFigure 5(b) shows JminusV curves for IOSCs fabricated

with the thin film and nanowalls under a forward biasin the dark It can be suggested the current densities ofDevices B and C were remarkably increased compared tothat of Device A consistent with the reduction in Rs dueto the increased conductivity caused by incorporation ofthe nanowalls The experimental results support the expla-nation for the improvement in the PV performance by theincorporation of ZnO nanowalls

4 CONCLUSION

We have demonstrated a 44 enhancement in the PCEfrom IOSCs fabricated with ZnO nanowalls compared tothat of the thin film It can be suggested that the enhancedPCE of the IOSCs with ZnO nanowalls was attributedto the increased charge transport area between the ZnOnanowall and the active polymer blend layer

Acknowledgment This research was supported byBasic Science Research Program through the NationalResearch Foundation of Korea (NRF) funded by theMinistry of Education Science and Technology (MEST)(2010-0015035 and 2009-0077682) and ConvergingResearch Center Program through the NRF funded bythe MEST (2010K001165) and by the New and Renew-able Energy of the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) grant funded by theKorea government Ministry of Knowledge Economy (No2009T100100614)

References and Notes

1 G Li V Shrotriya J Huang Y Yao T Moriarty K Emery andY Yang Nat Mater 4 864 (2005)

2 A K K Kyaw X W Sun C Y Jiang G Q Lo D W Zhao andD L Kwong Appl Phys Lett 93 221107 (2008)

3 L H Hoppe and N S Sariciftci J Mater Chem 16 45 (2006)4 G Li V Shrotriya Y Yao J Huang and Y Yang J Mater Chem

17 3126 (2007)5 M P de Jong L J van Ijzendoom and M J A de Voigt Appl

Phys Lett 77 2255 (2000)6 M-J Jin S-D Lee K-S Shin S-W Jeong D H Yoon D Jeon

I-H Lee and S-W Kim J Nanosci Nanotech 9 7432 (2009)7 L Yu G Zhang S Li Z Xi and D Guo J Cryst Growth 299 184

(2007)8 J Su R Che G She X Duan and W Shi J Nanosci Nanotech

8 6306 (2008)9 X Gan X Li X Gao and W Yu J Alloys Compd 481 397 (2009)10 L Xu Q Liao J Zhang X Ai and D Xu J Phys Chem C

111 4549 (2007)11 Q X Zhao P Klason M Willander H M Zhong W Lu and J H

Yang Appl Phys Lett 87 211912 (2005)12 K Vanheusden C H Seager W L Warren D R Tallant and

J A Voigt Appl Phys Lett 68 403 (1996)13 Y Zhang W Zhang and C Peng Opt Express 16 10696 (2008)14 B-Y Yu A Tsai S-P Tsai K-T Wong Y Yang C-W Chu and

J-J Shyue Nanotechnology 19 255202 (2008)15 J Liu S Wang Z Bian M Shan and C Huang Appl Phys Lett

94 173107 (2009)16 Y Ma G T Du S R Yang Z T Li B J Zhao X T Yang

T P Yang Y T Zhang and D L Liu J Appl Phys 95 6268(2004)

Received 26 August 2010 Accepted 13 December 2010

J Nanosci Nanotechnol 12 1234ndash1237 2012 1237

Delivered by Ingenta toSung Kyun Kwan University

IP 11514516612Wed 25 Jul 2012 111047

RESEARCH

ARTIC

LE

Shin et al Inverted Organic Solar Cells with ZnO Nanowalls Prepared Using Wet Chemical Etching in a KOH Solution

Fig 5 JminusV characteristics for IOSCs with the deposited ZnO thin filmand ZnO nanowalls by etching for 10sim15 min (a) under AM 15 Girradiation and (b) under forward bias in a dark

in n-ZnO Therefore increasing the number of oxygendefects lead to decreases in the electrical resistivities of theZnO nanowalls Although the reduction in carrier mobilitywas expected due to scattering from the defects the con-ductivity and large charge transport interface area betweenthe nanowalls and organic material are dominating factorsin the enhancement of the PCEs of IOSCs fabricated withZnO nanowalls16 The fill factor (FF) in ZnO nanowall-based IOSCs was reduced due to the increase in leakagecurrent due to the defectsFigure 5(b) shows JminusV curves for IOSCs fabricated

with the thin film and nanowalls under a forward biasin the dark It can be suggested the current densities ofDevices B and C were remarkably increased compared tothat of Device A consistent with the reduction in Rs dueto the increased conductivity caused by incorporation ofthe nanowalls The experimental results support the expla-nation for the improvement in the PV performance by theincorporation of ZnO nanowalls

4 CONCLUSION

We have demonstrated a 44 enhancement in the PCEfrom IOSCs fabricated with ZnO nanowalls compared tothat of the thin film It can be suggested that the enhancedPCE of the IOSCs with ZnO nanowalls was attributedto the increased charge transport area between the ZnOnanowall and the active polymer blend layer

Acknowledgment This research was supported byBasic Science Research Program through the NationalResearch Foundation of Korea (NRF) funded by theMinistry of Education Science and Technology (MEST)(2010-0015035 and 2009-0077682) and ConvergingResearch Center Program through the NRF funded bythe MEST (2010K001165) and by the New and Renew-able Energy of the Korea Institute of Energy TechnologyEvaluation and Planning (KETEP) grant funded by theKorea government Ministry of Knowledge Economy (No2009T100100614)

References and Notes

1 G Li V Shrotriya J Huang Y Yao T Moriarty K Emery andY Yang Nat Mater 4 864 (2005)

2 A K K Kyaw X W Sun C Y Jiang G Q Lo D W Zhao andD L Kwong Appl Phys Lett 93 221107 (2008)

3 L H Hoppe and N S Sariciftci J Mater Chem 16 45 (2006)4 G Li V Shrotriya Y Yao J Huang and Y Yang J Mater Chem

17 3126 (2007)5 M P de Jong L J van Ijzendoom and M J A de Voigt Appl

Phys Lett 77 2255 (2000)6 M-J Jin S-D Lee K-S Shin S-W Jeong D H Yoon D Jeon

I-H Lee and S-W Kim J Nanosci Nanotech 9 7432 (2009)7 L Yu G Zhang S Li Z Xi and D Guo J Cryst Growth 299 184

(2007)8 J Su R Che G She X Duan and W Shi J Nanosci Nanotech

8 6306 (2008)9 X Gan X Li X Gao and W Yu J Alloys Compd 481 397 (2009)10 L Xu Q Liao J Zhang X Ai and D Xu J Phys Chem C

111 4549 (2007)11 Q X Zhao P Klason M Willander H M Zhong W Lu and J H

Yang Appl Phys Lett 87 211912 (2005)12 K Vanheusden C H Seager W L Warren D R Tallant and

J A Voigt Appl Phys Lett 68 403 (1996)13 Y Zhang W Zhang and C Peng Opt Express 16 10696 (2008)14 B-Y Yu A Tsai S-P Tsai K-T Wong Y Yang C-W Chu and

J-J Shyue Nanotechnology 19 255202 (2008)15 J Liu S Wang Z Bian M Shan and C Huang Appl Phys Lett

94 173107 (2009)16 Y Ma G T Du S R Yang Z T Li B J Zhao X T Yang

T P Yang Y T Zhang and D L Liu J Appl Phys 95 6268(2004)

Received 26 August 2010 Accepted 13 December 2010

J Nanosci Nanotechnol 12 1234ndash1237 2012 1237