Organic Electronics 13 (2012) 2422–2428

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Organic Electronics

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Combined alternative electrodes for semi-transparent and ITO-freesmall molecule organic solar cells

Yong Hyun Kim a,⇑, Christoph Sachse a, Alexander A. Zakhidov b,1, Jan Meiss b,2,Anvar A. Zakhidov c, Lars Müller-Meskamp a,⇑, Karl Leo a,b

a Dresdner Innovationszentrum für Energieeffizienz, Institut für Angewandte Photophysik, Technische Universität Dresden,01062 Dresden und Fraunhofer IPMS, 01109 Dresden, Germanyb Institut für Angewandte Photophysik, Technische Universität Dresden, 01062 Dresden, Germanyc Alan G. MacDiarmid NanoTech Institute, University of Texas at Dallas, Richardson, TX 75083, USA

a r t i c l e i n f o

Article history:Received 26 February 2012Received in revised form 30 May 2012Accepted 19 June 2012Available online 20 July 2012

Keywords:Organic solar cellsTransparent conductive electrodesCarbon nanotubesTransparent solar cellsAg nanowiresPEDOT:PSS

1566-1199/$ - see front matter � 2012 Elsevier B.Vhttp://dx.doi.org/10.1016/j.orgel.2012.06.034

⇑ Corresponding authors. Tel.: +49 351 463 327437065 (Y.H. Kim).

E-mail addresses: yong.hyun.kim@iapp.de (Y.Hmeskamp@iapp.de (L. Müller-Meskamp).

1 Present address: Fraunhofer IPMS, 01109 Dresde2 Present address: Reiner Lemoine Institut gG

Germany.

a b s t r a c t

We investigate various electrode combinations of bottom and top contacts for organic pho-tovoltaic (OPV) cells. Silver (Ag), indium tin oxide (ITO), poly(3,4-ethylenedioxythio-phene):poly(styrenesulfonate) (PEDOT:PSS), and silver nanowires (AgNW) are used asbottom electrodes. As top electrodes, thin silver layers (t-Ag) and free-standing carbonnanotube (f-CNT) sheets are employed. The manufactured zinc phthalocyanine (ZnPc): ful-lerene C60 small molecule bulk heterojunction OPV cells with different kinds of bottomelectrodes show efficiencies of 1.9�2.2% and 1.1�1.5%, when comprised of t-Ag and f-CNT top contacts, respectively. We demonstrate alternative electrodes beyond ITO, silver,and aluminum, which can be readily used for organic photovoltaics technology.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction indium-free transparent conducting oxides (TCO) [3,4],

Organic photovoltaic (OPV) cells have attracted agreat deal of attention as low-cost, flexible, light-weight,and mass producible photovoltaic devices. A rapid in-crease in efficiency of OPV cells up to 10.7% has beenachieved in recent years [1]. Among many advantagesof OPV cells, cost-efficient mass production and semi-transparent device technology are two great benefits ofOPV cells. Since indium tin oxide (ITO) as a transparentelectrode constitutes a very large part of the fabricationcosts of OPV cells [2], seeking alternative electrodes isa key issue for achieving a low-cost technology. Thus,

. All rights reserved.

8; fax: +49 351 463

. Kim), lars.mueller-

n, Germany.mbH, 12459 Berlin,

conducting polymers [5,6], thin metal layers [7,8], silvernanowires (AgNW) [9,10], carbon nanotubes (CNT)[11,12], and graphenes [13] are actively investigated asalternative electrode materials. Especially, poly(3,4-ethyl-enedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS)and AgNW thin-films give high conductivity and trans-mittance as well as good flexibility, showing a compara-ble device performance to ITO based OPV cells [5,14].OPV cells with CNT and graphene electrodes still lag be-hind the ITO based OPV cells in performance. ZnO basedTCOs show stability issues under the commonly usedacidic PEDOT:PSS buffer layer. Nevertheless, all of theseITO alternatives show a great potential, but there are stilllimitations to overcome.

So far, the development of transparent conductive elec-trodes for application in OPV is mainly focused on the useas the bottom electrode. However, the research on themore challenging top electrode deposition technology isalso of high importance and inevitable for the fabricationof semi-transparent devices (types of top contacts: thin

Y.H. Kim et al. / Organic Electronics 13 (2012) 2422–2428 2423

metal films [15–17], TCOs [18,19], AgNWs [20], conductivepolymers [21], CNTs [22]). The deposition of the top elec-trode needs to be carried out without damaging underlyingorganic layers, which is a critical issue for top electrodeapplication in OPV and organic light emitting diodes(OLEDs). For example, ITO deposition by sputter processingon organic devices induces serious damage to underlyingorganic layers, causing undesirable leakage currents in de-vices [18,23].

Thin metal layers prepared by thermal evaporationhave been widely investigated as a top electrode of OPVcells and OLEDs [7,8,15–17]. A high efficiency of 4.9% wasachieved by a tandem structure with thin metal top elec-trodes [17], indicating a promising future of semi-trans-parent OPV cells. Furthermore, it is known thatadditional organic capping layers on the thin metal contactremarkably improve the performance of OPV cells by en-hanced light in-coupling [24]. CNT networks are also re-garded as a feasible material for the top electrode of OPVcells. However, a top contact application for CNTs is rarelyreported because of processing difficulties. In our previousstudy, efficient semi-transparent OPV cells with CNT topelectrodes were successfully fabricated by a simple lami-nation process of free-standing CNT (f-CNT) sheets [22].In addition, these f-CNT sheets were used for bottom elec-trodes of OPV cells and OLEDs [25–27].

To achieve low-cost or semi-transparent OPV cells, thecompatibility and favorable combination between bottomand top contacts should be investigated. However, such astudy about alternative electrode combinations is cur-rently missing. In this work, we demonstrate semi-trans-parent or ITO-free small molecule OPV cells utilizingvarious bottom electrodes with either thin metal layersor laminated CNT top electrodes. The results of the elec-trode combinations can be a practical guidance for the con-sideration of electrodes in semi-transparent and low-costITO-free OPV cells with straightforward extension to flexi-ble substrates.

2. Experimental

Silver (Ag), ITO, PEDOT:PSS, and AgNW are prepared asbottom electrodes. Ag 100 nm is deposited on glass byvacuum thermal evaporation as opaque reference. ITO ispurchased from Thin Film Devices Inc. PEDOT:PSS (CleviosPH1000, Heraeus, Germany) mixed with 6 vol.% ethyleneglycol is spun onto glass (1500 rpm, 30 s) and subse-quently annealed at 120 �C in air as described elsewhere[5,28]. AgNWs are prepared by a dip-coating process inAgNW solutions with an average nanowire length of sev-eral micrometers, and a diameter of about 60 nm. Moredetails about the fabrication methods will be publishedelsewhere soon. The AgNWs are subsequently coated withPEDOT:PSS for flattening of the surface roughness. Thepeak height of AgNWs reduces by half after the PED-OT:PSS coating. Small molecule layers are thermally evap-orated onto the bottom electrodes in a high vacuumchamber (K.J. Lesker, U.K.) at a base pressure of around10�8 mbar using shadow masks. As hole transport layer(HTL), 30 nm thick 10 wt.% NDP9 (Novaled AG) doped

N,N0-((diphenyl-N,N’-bis)9,9,-dimethyl-fluoren-2-yl)-ben-zidine (BF-DPB) is deposited on the bottom electrode.Zinc phthalocyanine (ZnPc) and fullerene C60 are co-evap-orated at the substrate temperature of 104 �C with a ratioof 1:1 as 55 nm thick absorber layer. 10 nm thick C60 isfollowed as an additional absorber and electron transportlayer (ETL). 3 wt.% NDN1 (Novaled AG) doped C60 (10 nm)is evaporated as an electron transport layer. The devicesare denoted as OPVX_Y, where the subscripts of X indicatethe type of bottom electrodes and Y is top electrodes.

Finally, thin silver layers (t-Ag) or free-standing carbonnanotubes (f-CNT) sheets are deposited as transparentelectrodes on top of the devices. 1 nm thick Al and 14 nmthick Ag are thermally evaporated in the vacuum chamberas OPVX_t-Ag top contact. The thin Al layer serves as a sur-factant layer for the morphological improvement of thefollowing Ag layer [29]. The f-CNT sheet is laminated ontop of the organic layers in a nitrogen filled glove box with-out air exposure by manual assembly as a top contact forOPVX_f-CNT [22]. For densification of the f-CNT sheets, thedevices are subsequently immersed into the orthogonal li-quid hydrofluoroether (HFE) for several seconds [30],which does not provoke any degradation of devices [22].The f-CNT sheets were drawn from a CNT forest (Alan G.MacDiarmid NanoTech Institute) prepared by a chemicalvapor deposition as described by Zhang et al. [27]. Thethickness of the densified f-CNT sheets used in this exper-iment is 50–100 nm. The active areas of OPVX_t-Ag andOPVX_f-CNT are 5.8–6.8 mm2 and 1.3–1.5 mm2, respectively,measured using an optical microscope. The stack is de-picted in Fig. 1 (b).

The current–voltage characteristics are measured witha source measurement unit (SMU 236, Keithley Instru-ments) under an AM 1.5G illumination with a sun simula-tor (Steuernagel SC1200) in a nitrogen filled glove box. Alldevices are illuminated from the bottom glass side exceptfor OPVAg_t-Ag and OPVAg_f-CNT. The short circuit currentdensities are normalized to 100 mW/cm2 and are not cor-rected for spectral mismatch. The transmittances of theelectrodes and solar cells are measured by a spectropho-tometer (Perkin–Elmer Lambda 900). The atomic forcemicroscopy (AFM) images are taken in tapping mode(AIST-NT Combiscope).

3. Results and discussion

Small molecule OPV cells with ZnPc: C60 bulk hetero-junction (p-i-n structure) are prepared by vacuum thermalevaporation. Scanning electron microscope (SEM) andatomic force microscopy (AFM) images of the electrodesused in this experiment are shown in Fig. 1(a). The f-CNTsheet shows well-oriented CNT networks with individualtube diameters of 10–30 nm, which cause a high surfaceroughness. The AgNW electrode consists of nanowires withabout 60 nm diameter and good wire-wire junctions,resulting in high conductivity electrodes and an even lar-ger surface roughness, compared to the f-CNT electrodes.PEDOT:PSS has a granular surface with an average grainsize of 30–50 nm and a root-mean-square roughness(RMS) of around 1.6 nm, which is measured in an area of

Fig. 1. (a) SEM and AFM images of the f-CNT sheet, AgNW, PEDOT:PSS, and ITO, (b) p-i-n layer stack of the small molecule OPV cells with various bottomelectrodes and top electrodes. ⁄The AgNW electrode actually is a hybrid electrode and includes an additional layer of PEDOT:PSS, which is spin-coated onthe AgNW film for surface planarization.

2424 Y.H. Kim et al. / Organic Electronics 13 (2012) 2422–2428

1 � 1 lm2. ITO exhibits a polycrystalline structure withgrains of several hundred nanometers in size having a verysmooth surface (RMS: 0.42 nm). In Fig. 1(b), the stack of p-i-n OPV cells on various electrodes is shown.

The transmittances of the conductive electrodes areshown in Fig. 2 and summarized in Table 1 along withthe respective sheet resistance values. The transmittancesof bottom electrodes are in the range of 83–87% at a wave-length of 550 nm. The f-CNT and t-Ag employed as top

electrodes have lower transmittances (41–58%) in compar-ison to the bottom electrodes used.

Fig. 3 shows typical current–voltage (I–V) characteris-tics of OPV cells with different kinds of bottom and topelectrodes. All devices are fabricated at the same time inone run, ensuring comparability. Similar short circuit cur-rent density (JSC) values of around 8.5 mA/cm2 are ob-served for OPVPEDOT:PSS_t-Ag, OPVITO_t-Ag, and OPVAgNW_t-Ag

cells. OPVITO_t-Ag and OPVAgNW_t-Ag cells are comparable in

Fig. 3. I–V characteristics of the series of (a) OPVX_t-Ag and (b) OPVX_f-CNT.⁄The top illuminated OPVAg_t-Ag cell exhibits lower JSC due to lowertransmittance of t-Ag compared to that of other bottom electrodes.

Fig. 2. Transmittances (including the glass substrate) of the conductiveelectrodes. ⁄The t-Ag film contains a thin 1 nm Al layer beneath.

Table 1Transmittances and sheet resistances of the transparent conductive elec-trodes used in this work. ⁄The t-Ag film contains a thin 1 nm Al layerbeneath.

Electrodes Transmittance [%](550 nm)

Sheet resistance [ohm/sq]

ITO 84 30PEDOT:PSS 87 155AgNW 83 10f-CNT 58 360t-Ag 41 5

Y.H. Kim et al. / Organic Electronics 13 (2012) 2422–2428 2425

power conversion efficiencies (PCE) of 2.2% and 2.1%,respectively. Despite the rough surface of the AgNWelectrodes, which can cause severe leakage currents, theOPVAgNW_t-Ag cell shows a higher fill factor (FF) of 50.6%compared to the OPVITO_t-Ag cell (48.9%), which is attrib-uted to the lower sheet resistance of the AgNW electrode.The combination of bottom AgNW and top t-Ag is regardedas a promising combination to achieve a high PCE withboth semi-transparent and ITO-free properties. The OPVPE-

DOT:PSS_t-Ag cell shows a decent efficiency of 1.9% but lowerFF of 42% compared with other devices in OPVX_t-Ag series.This is attributed to the more resistive electrode, which re-sults in higher series resistance of the devices. One way toimprove the PEDOT:PSS based cell to become comparableto the ITO based cell is to remove residual water and excessPSS. The residual water in the polymeric electrode signifi-cantly deteriorates the performance of devices as reportedin our previous study [5]. In addition, the solvent post-treatment process for PEDOT:PSS can improve the deviceperformance and lifetime by removal of excess hygro-scopic PSS [28]. The OPVAg_t-Ag cell exhibits higher FF of52.4% but a lower JSC of 7.5 mA/cm2 compared to othercells in the OPVX_t-Ag series. The lower JSC is attributed toless absorbed photons in the absorber layer because ofthe lower transmittance of metal thin-film. In the case ofOPVAg_t-Ag cell, the light is illuminated from the top t-Agelectrode which has lower transmittance (41%) comparedto other bottom electrodes.

The difference of PCEs according to the illuminationdirection for semi-transparent OPV cells was studied inour previous study. The OPVITO_t-Ag cell illuminated frombottom showed a 1.9 times higher PCE than for the top illu-minated case [22]. When comparing opaque (100 nm Agon top) and semi-transparent (14 nm Ag on top) OPV cellsfabricated with a similar stack based on ZnPc: C60 as in thisexperiment, the JSC and PCE of semi-transparent devicewere around 70% and 67%, respectively, of that of opaquedevice. Open circuit voltages and FFs of both devices werepretty similar. It is expected that the introduction of an or-ganic capping layer for optical in-coupling effect on t-Agcan further improve the efficiency of the device as reportedby Meiss et al., who demonstrated that an organic cappinglayer remarkably improves the efficiency of OPV cells from1.27% to 1.84%, which is attributed to an enhancement oflight absorption [7].

The OPVX_f-CNT series exhibits worse device perfor-mance compared to the series of OPVX_t-Ag cell as shownin Fig. 3(b), which is mainly due to the low conductivityof the f-CNT electrodes. The OPVAg_f-CNT cell shows thehighest PCE of 1.5% among the OPVX_f-CNT series. It is inter-esting to note that no significant electrical short is ob-served in the OPVX_f-CNT series despite the very thin

Fig. 4. Light intensity dependence of the device performance of the OPVcells. Graphs are plotted on log scale. Measured data is normalized at100 mW/cm2 due to disparity of PCE values from different OPV fabrica-tion run in this measurement.

2426 Y.H. Kim et al. / Organic Electronics 13 (2012) 2422–2428

transport layers of 20 nm between the absorber layer andthe rough f-CNT top electrode. It is widely known that asmoothing buffer layer such as PEDOT:PSS is essential formost of devices using rough CNT electrodes to preventelectrical short [11,12,25,26,31,32]. OPV cells with moreconductive and less transparent f-CNT top electrodes thanthe one used in this study show equally low leakage cur-rents, as reported elsewhere [22], indicating that the rough

Table 2Characteristics of OPV cells measured under an AM 1.5G illumination and normal

Device Type VOC [V] JSC [mA/cm2] FF

OPVAg_t-Ag ITO-free 0.51 7.5 52OPVPEDOT:PSS_t-Ag ITO-free, Semi-transparent 0.52 8.5 42OPVITO_t-Ag Semi-transparent 0.52 8.6 48OPVAgNW_t-Ag ITO-free, Semi-transparent 0.49 8.5 50OPVAg_f-CNT ITO-free 0.48 6.8 41OPVPEDOT:PSS_f-CNT ITO-free, Semi-transparent 0.48 6.1 38OPVITO_f-CNT Semi-transparent 0.50 6.3 40

a The value of transmittance of OPVAgNW_t-Ag cell does not contain the diffuse

f-CNT top electrode does not cause significant damage ofunderlying organic layers. The trade-off between conduc-tivity and transmittance of CNT networks affects the JSC

and FF of the devices. Therefore, balancing the electricaland optical properties of CNT networks can further im-prove the performance of OPVX_f-CNT cells.

The light illumination intensity (Isun) dependence ofvarious kinds of OPV cells is shown in Fig. 4. Measured datais normalized due to disparity of PCE values from differentOPV fabrication runs in this measurement. Each deviceshows clearly different light dependence behavior. It isnotable that the PCE of OPVAg_f-CNT cell continuously in-creases with decreasing Isun from 100 to 2 mW/cm2, whichresults in enhancement of 61%. In contrast, most other de-vices show peak PCEs at intermediate Isun and decreasedown again at low Isun. It is well known that JSC is nearlylinear dependent on Isun, showing a power law depen-dence, JSC / Iasun, with a being close to 1. Extracted a values,fitted to the power law relationship, are shown in Table 2.It is observed that various electrode combinations giveremarkably different a. The a is known to be related tothe recombination dynamics and balance of charge trans-port [33–35]. The OPVX_f-CNT cells show large deviation ofa from unity a = 1, which shows unbalanced charge extrac-tion behavior at the electrodes, which is at least partiallycaused by the effect of a large sheet resistance in the topelectrode.

Fig. 5 shows the transmittances of the OPV cells. Theaveraged transmittances of the devices in the visible region(400–800 nm) are in the range of 20–30%. The peak trans-mittances of OPVPEDOT:PSS_t-Ag and OPVITO_t-Ag cells reach upto 40%. Further optimization of device transmittance ispossible by introducing an organic capping layer onto thetop contact [16].

In summary, we have demonstrated the combination ofbottom and top electrodes for semi-transparent or ITO-freesmall molecule OPV cells based on ZnPc: C60 bulk hetero-junction. Promising efficiencies up to 2.2% are shown forthe devices with t-Ag top electrodes. The AgNW and t-Agcombination shows a particularly promising PCE withsemi-transparent and ITO-free properties. The devices withtop f-CNT electrodes show an efficiency of 1.5% with nocritical damage being caused by the lamination processto the organic layers although no smoothing layer is usedunder the rough CNT networks. In addition, a differentbehavior of light intensity dependence of the devices is ob-served according to the various electrode combinations.

ized to 100 mW/cm2.

[%] PCE [%] Transmittance [%] (400–800 nm) a J (�1 V)/JSC

.4 2.0 – 0.94 1.17

.0 1.9 29 0.90 1.28

.9 2.2 28 0.95 1.20

.6 2.1 23a 0.94 1.22

.6 1.5 – 0.76 1.42

.6 1.1 22 0.80 1.55

.2 1.3 24 1.41

transmission.

Fig. 5. The transmittances of semi-transparent OPV cells.

Y.H. Kim et al. / Organic Electronics 13 (2012) 2422–2428 2427

We believe that this study provides a comprehensive over-view on the performance and combination possibilities ofdifferent electrode types and allows to optimize semi-transparent and low-cost ITO-free flexible OPV cells.

Acknowledgements

The authors thank O.R. Hild and C. May at IPMS for sup-porting the DIZEeff, A. Elschner at Heraeus for providingPEDOT samples, and J. Bikova at Alan G. MacDiarmid Nano-Tech Institute for preparing carbon nanotubes. This workwas funded by the European Union (EFRE), the FraunhoferGesellschaft, and the Free State of Saxony as part of theDresdner Innovationszentrum Energieeffizienz. A.Z.acknowledges support of Alexander von HumboldtFoundation.

References

[1] Small molecule organic solar cells by Heliatek/IAPP with a certifiedefficiency of 10.7%, press release, April 2012. <http://www.heliatek.com>.

[2] T.D. Nielsen, C. Cruickshank, S. Foged, J. Thorsen, F.C. Krebs, Business,market and intellectual property analysis of polymer solar cells, Sol.Energy Mater. Sol. Cells 94 (2010) 1553–1571.

[3] G. Murdoch, S. Hinds, E. Sargent, S. Tsang, L. Mordoukhovski, Z. Lu,Aluminum doped zinc oxide for organic photovoltaics, Appl. Phys.Lett. 94 (2009) 213301.

[4] K. Schulze, B. Maennig, K. Leo, Y. Tomita, C. May, J. Hüpkes, E. Brier, E.Reinold, P. Bäuerle, Organic solar cells on indium tin oxide andaluminum doped zinc oxide anodes, Appl. Phys. Lett. 91 (2007)073521.

[5] Y.H. Kim, C. Sachse, M.L. Machala, C. May, L. Müller-Meskamp, K. Leo,Highly conductive PEDOT: PSS electrode with optimized solvent andthermal post treatment for ITO free organic solar cells, Adv. Funct.Mater. 21 (2011) 1076–1081.

[6] F. Zhang, M. Johansson, M.R. Andersson, J.C. Hummelen, O. Inganäs,Polymer photovoltaic cells with conducting polymer anodes, Adv.Mater. 14 (2002) 662–665.

[7] J. Meiss, M. Furno, S. Pfuetzner, K. Leo, M. Riede, Selective absorptionenhancement in organic solar cells using light incoupling layers, J.Appl. Phys. 107 (2010) 053117.

[8] J. Meiss, M. Riede, K. Leo, Towards efficient tin-doped indium oxide(ITO)-free inverted organic solar cells using metal cathodes, Appl.Phys. Lett. 94 (2009) 013303.

[9] M.G. Kang, M.S. Kim, J. Kim, L.J. Guo, Organic solar cells usingnanoimprinted transparent metal electrodes, Adv. Mater. 20 (2008)4408–4413.

[10] J.Y. Lee, S.T. Connor, Y. Cui, P. Peumans, Solution-processed metalnanowire mesh transparent electrodes, Nano Lett. 8 (2008) 689–692.

[11] M.W. Rowell, M.A. Topinka, M.D. McGehee, H.J. Prall, G. Dennler, N.S.Sariciftci, L. Hu, G. Gruner, Organic solar cells with carbon nanotubenetwork electrodes, Appl. Phys. Lett. 88 (2006) 233506.

[12] J. van de Lagemaat, T.M. Barnes, G. Rumbles, S.E. Shaheen, T.J. Coutts,C. Weeks, I. Levitsky, J. Peltola, P. Glatkowski, Organic solar cells withcarbon nanotubes replacing In2O3:Sn as the transparent electrode,Appl. Phys. Lett. 88 (2006) 233503.

[13] L. Gomez De Arco, Y. Zhang, C.W. Schlenker, K. Ryu, M.E. Thompson,C. Zhou, Continuous, highly flexible, and transparent graphene filmsby chemical vapor deposition for organic photovoltaics, ACS Nano 4(2010) 2865–2873.

[14] W. Gaynor, G.F. Burkhard, M.D. McGehee, P. Peumans, Smoothnanowire/polymer composite transparent electrodes, Adv. Mater. 23(2011) 2905–2910.

[15] D. Han, H. Kim, S. Lee, M. Seo, S. Yoo, Realization of efficientsemitransparent organic photovoltaic cells with metallic topelectrodes: utilizing the tunable absorption asymmetry, Opt. Exp.18 (2010) A513–A521.

[16] J. Meiss, K. Leo, M. Riede, C. Uhrich, W.M. Gnehr, S. Sonntag, M.Pfeiffer, Efficient semitransparent small-molecule organic solar cells,Appl. Phys. Lett. 95 (2009) 213306.

[17] J. Meiss, T. Menke, K. Leo, C. Uhrich, W.M. Gnehr, S. Sonntag, M.Pfeiffer, M. Riede, Highly efficient semitransparent tandem organicsolar cells with complementary absorber materials, Appl. Phys. Lett.99 (2011) 043301.

[18] H. Schmidt, H. Flugge, T. Winkler, T. Bulow, T. Riedl, W. Kowalsky,Efficient semitransparent inverted organic solar cells with indiumtin oxide top electrode, Appl. Phys. Lett. 94 (2009) 243302.

[19] A. Bauer, T. Wahl, J. Hanisch, E. Ahlswede, ZnO: Al cathode for highlyefficient, semitransparent 4% organic solar cells utilizing TiOx andaluminum interlayers, Appl. Phys. Lett. 100 (2012) 073307.

[20] J.Y. Lee, S.T. Connor, Y. Cui, P. Peumans, Semitransparent organicphotovoltaic cells with laminated top electrode, Nano Lett. 10 (2010)1276–1279.

[21] Y. Zhou, H. Cheun, S. Choi, W.J. Potscavage Jr., C. Fuentes-Hernandez,B. Kippelen, Indium tin oxide-free and metal-free semitransparentorganic solar cells, Appl. Phys. Lett. 97 (2010) 153304.

[22] Y.H. Kim, L. Müller-Meskamp, A.A. Zakhidov, C. Sachse, J. Meiss, J.Bikova, A. Cook, K. Leo, Semi-transparent small molecule organicsolar cells with laminated free-standing carbon nanotube topelectrodes, Sol. Energy Mater. Sol. Cells 96 (2012) 244–250.

[23] T. Dobbertin, M. Kroeger, D. Heithecker, D. Schneider, D. Metzdorf, H.Neuner, E. Becker, H.H. Johannes, W. Kowalsky, Inverted top-emitting organic light-emitting diodes using sputter-depositedanodes, Appl. Phys. Lett. 82 (2003) 284.

[24] J. Meiss, N. Allinger, M. Riede, K. Leo, Improved light harvesting intin-doped indium oxide (ITO)-free inverted bulk-heterojunctionorganic solar cells using capping layers, Appl. Phys. Lett. 93 (2008)103311.

[25] R. Ulbricht, S.B. Lee, X. Jiang, K. Inoue, M. Zhang, S. Fang, R.H.Baughman, A.A. Zakhidov, Transparent carbon nanotube sheets as 3-D charge collectors in organic solar cells, Sol. Energy Mater. Sol. Cells91 (2007) 416–419.

[26] C.D. Williams, R.O. Robles, M. Zhang, S. Li, R.H. Baughman, A.A.Zakhidov, Multiwalled carbon nanotube sheets as transparentelectrodes in high brightness organic light-emitting diodes, Appl.Phys. Lett. 93 (2008) 183506.

[27] M. Zhang, S. Fang, A.A. Zakhidov, S.B. Lee, A.E. Aliev, C.D. Williams,K.R. Atkinson, R.H. Baughman, Strong, transparent, multifunctional,carbon nanotube sheets, Science 309 (2005) 1215.

[28] Y.H. Kim, C. Sachse, M. Hermenau, K. Fehse, M. Riede, L. Müller-Meskamp, K. Leo, Improved efficiency and lifetime in small moleculeorganic solar cells with optimized conductive polymer electrodes,Appl. Phys. Lett. 99 (2011) 113305.

[29] J. Meiss, M. Riede, K. Leo, Optimizing the morphology of metalmultilayer films for indium tin oxide (ITO)-free inverted organicsolar cells, J. Appl. Phys. 105 (2009) 063108.

[30] A.A. Zakhidov, J.K. Lee, H.H. Fong, J.A. DeFranco, M. Chatzichristidi,P.G. Taylor, C.K. Ober, G.G. Malliaras, Hydrofluoroethers asorthogonal solvents for the chemical processing of organicelectronic materials, Adv. Mater. 20 (2008) 3481–3484.

[31] A. Du Pasquier, H.E. Unalan, A. Kanwal, S. Miller, M. Chhowalla,Conducting and transparent single-wall carbon nanotube electrodesfor polymer-fullerene solar cells, Appl. Phys. Lett. 87 (2005) 203511.

2428 Y.H. Kim et al. / Organic Electronics 13 (2012) 2422–2428

[32] L. Hu, J. Li, J. Liu, G. Grüner, T. Marks, Flexible organic light-emittingdiodes with transparent carbon nanotube electrodes: problems andsolutions, Nanotechnology 21 (2010) 155202.

[33] L.J.A. Koster, V.D. Mihailetchi, H. Xie, P.W.M. Blom, Origin of the lightintensity dependence of the short-circuit current of polymer/fullerene solar cells, Appl. Phys. Lett. 87 (2005) 203502.

[34] V.D. Mihailetchi, H. Xie, B. de Boer, L.J.A. Koster, P.W.M. Blom, Chargetransport and photocurrent generation in poly (3-hexylthiophene):

methanofullerene bulk-heterojunction solar cells, Adv. Funct. Mater.16 (2006) 699–708.

[35] I. Riedel, J. Parisi, V. Dyakonov, L. Lutsen, D. Vanderzande, J.C.Hummelen, Effect of temperature and illumination on the electricalcharacteristics of polymer–fullerene bulk-heterojunction solar cells,Adv. Funct. Mater. 14 (2004) 38–44.