Solar Energy Materials & Solar Cells ] (]]]]) ]]]–]]]

Contents lists available at ScienceDirect

Solar Energy Materials & Solar Cells

0927-02

doi:10.1

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journal homepage: www.elsevier.com/locate/solmat

ITO-free inverted polymer solar cells using a GZO cathode modified by ZnO

Soo-Ghang Ihn a, Kyung-Sik Shin b, Mi-Jin Jin b, Xavier Bulliard a, Sungyoung Yun a, Yeong Suk Choi a,Yungi Kim a, Jong-Hwan Park c, Myungsun Sim c, Min Kim c, Kilwon Cho c, Tae Sang Kim a,Dukhyun Choi a,1, Jae-Young Choi a, Woong Choi a,nn, Sang-Woo Kim b,d,n

a Samsung Advanced Institute of Technology, Samsung Electronics, Yongin 446-712, Republic of Koreab School of Advanced Materials Science and Engineering, Sungkyunkwan University, Suwon 440-746, Republic of Koreac Department of Chemical Engineering, Pohang University of Science and Technology, Pohang 790-784, Republic of Koread SKKU Advanced Institute of Nanotechnology (SAINT) and Center for Human Interface Nanotechnology (HINT), Sungkyunkwan University, Suwon 440-746, Republic of Korea

a r t i c l e i n f o

Article history:

Received 8 September 2010

Received in revised form

3 January 2011

Accepted 11 January 2011

Keywords:

Inverted polymer solar cell

Ga-doped ZnO

Transparent conducting oxide

ITO-free

Power conversion efficiency

48/$ - see front matter & 2011 Elsevier B.V. A

016/j.solmat.2011.01.011

esponding author at: School of Advanced Ma

ring, Sungkyunkwan University, Suwon 4

2 31 290 7352; fax: +82 31 290 7381.

responding author. Tel.: +82 31 280 6766; fa

ail addresses: woong1.choi@samsung.com (W

@skku.edu (S.-W. Kim).

esent address: Department of Mechanical En

ity, Republic of Korea.

e cite this article as: S.-G. Ihn, et al

a b s t r a c t

A gallium-doped ZnO (GZO) layer was investigated and compared with a conventional indium-tin-

oxide (ITO) layer for use as a cathode in an inverted polymer solar cell based on poly(3-hexylthiophene)

(P3HT):[6,6]-phenyl-C61 butyric acid methyl ester (PCBM) bulk heterojunctions (BHJ). By modifying the

GZO cathode with a ZnO thin layer, a high power conversion efficiency (3.4%) comparable to that of an

inverted solar cell employing the same P3HT:PCBM BHJ photoactive layer with a conventional ITO/ZnO

cathode was achieved. This result indicates that GZO is a transparent electrode material that can

potentially be used to replace high-cost ITO.

& 2011 Elsevier B.V. All rights reserved.

1. Introduction

Organic photovoltaic technology based on bulk heterojunction(BHJ) photoactive layers composed of conjugated polymers andfullerene derivatives has been attracting a lot of attention due tothe unique applications of this technology, which are facilitated bythe characteristics of the organic solar cell modules such assemitransparency, light weight, a variety of colors, and freedom ofproduct design [1]. A simple wet-solution process and the potentialfor low-cost, high-throughput manufacturing based on roll-to-rollprocessing using low-cost, large-area flexible plastic substrates areadditional advantages of this technology [2]. In BHJ polymer solarcells (PSCs), charge carriers (electrons and holes) are dissociatedfrom photoexcited electron–hole pairs at the polymer/fullereneinterfaces under illumination and are then collected at electrodes. Atransparent conducting indium-tin-oxide (ITO)-coated substrate witha poly(ethylenedioxy) thiophene:poly(styrene) sulfonate (PEDOT:PSS)

ll rights reserved.

terials Science and

40-746, Republic of Korea.

x: +82 31 280 9473.

. Choi),

gineering, Kyung Hee

., Sol. Energy Mater. Sol. Ce

buffer layer is widely employed as the hole-collecting anode inPSCs, while a low work function metal, such as aluminum, with alithium fluoride buffer layer is utilized as an electron-collectingcathode. However, the use of ITO is a barrier to low-cost manu-facturing because indium is scarce and therefore expensive.Various efforts have been made to produce alternative transpar-ent electrode materials to replace ITO in solar cells. For example,ITO-free PSCs employing classical metal grid electrodes have beenstudied and manufactured using roll-to-roll production; however,the performances of these ITO-free PSCs were somewhat inferiorto those of PSCs containing ITO because of poor light transmissionin the ITO-free PSCs [3–5]. Carbon-based nanostructured materi-als such as carbon nanotubes (CNTs) and graphene have also beeninvestigated as potential alternatives to ITO. However, the sheetresistance of CNT network film is higher than that of ITO for acomparable level of transparency [6–10]. In addition, increasingthe thickness of the film to reduce the sheet resistance signifi-cantly degrades its transparency. Graphene composite films alsoperform poorly compared to ITO films and show the same trade-off between transparency and sheet resistance [11–13].

Transparent conducting oxide (TCO) based on ZnO is one of themost important alternative transparent electrode materialsbecause it exhibits high chemical/thermal stability, and its con-stituent elements are much more abundant than are those of ITO.ZnO films doped with impurities, such as aluminum (Al) [14],gallium (Ga) [15], indium (In) [16], boron [17], silicon [18],

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S.-G. Ihn et al. / Solar Energy Materials & Solar Cells ] (]]]]) ]]]–]]]2

titanium [19], and zirconium [20], exhibit not only higher con-ductivities but also better stabilities compared to those ofundoped ZnO films. Al-, Ga-, and In-doped ZnO (AZO, GZO, andIZO) films are transparent to most of the solar spectrum used forphotovoltaic solar cells, and their sheet resistances are compar-able to that of ITO [14–16]. Among these materials, GZO ischeaper than IZO and more stable than AZO (more resistant tooxidation). Furthermore, the covalent bond length of Ga–O(1.92 A) is similar to that of Zn–O (1.97 A). Because of this smalldifference, only a small deformation of the ZnO lattice occurs,even at high concentrations of Ga, resulting in higher conductiv-ity [21]. In this study, we report the fabrication and characteriza-tion of ITO-free highly efficient inverted polymer solar cellsusing an alternative transparent GZO cathode modified with aZnO layer.

Fig. 1. Schematic diagram of the device structure: (a) cross-sectional view and

various cathodes, (b) ITO, (c) ITO/ZnO, (d) GZO, and (e) GZO/ZnO.

Fig. 2. Optical transmittance spectra corresponding to the various cathodes

on glass.

Table 1Integral calculated from each optical transmittance spectrum (Fig. 2) and the sheet

resistance of each cathode (ITO or ZnO).

2. Experiment

GZO was formed using an ion-beam-assisted depositionmethod. Commercially available ITO-coated glass substrates wereprepared, and a thin 50 nm layer of ZnO was formed on both theITO and GZO at room temperature using the radio-frequencysputtering method. The ZnO layer was deposited in an Ar(45 sccm)/O2 (5 sccm) gas mixture at an RF input power of200 W at room temperature using a sintered ZnO target. Then, aphotoactive layer was formed from a poly(3-hexylthiophene)(P3HT):[6,6]-phenyl-C61 butyric acid methyl ester fullerene deri-vative (PCBM) blend with a 1:1 weight ratio in 1,2-dichloroben-zene (DCB) through spin-coating at 1000 rpm for 30 s. The samplewas then dried in a covered glass petri dish [22]. Before thethermal deposition of the MoO3 buffer and the Au anode, thesample was thermally annealed at 120 1C for 5 min. The MoO3

and Au layers were of 3 and 80 nm thickness, respectively.The current density–voltage (J–V) characteristics of the unen-

capsulated solar cells were measured using an Oriel xenon lampwith an air-mass (AM) 1.5 filter under nitrogen gas. The intensityof the simulated light (100 mW/cm2) was calibrated using a KG-5filtered standard Si cell traced to the National Renewable EnergyLaboratory.

The topologies of the four different substrates and the polymerblend active layers on their surfaces were characterized withatomic force microscopy (Digital Instruments Multimode Nano-scope III) in tapping mode with a silicon cantilever.

Cathode

Integral of

transmittance

from 300 to

680 nm (a.u.)

Integral of

transmittance

from 450 to

600 nm (a.u.)

Sheet

resistance

(O/sq)

Thickness

(nm)

ITO 29,783 (100%) 12,436 (100%) 11.9 150

ITO/ZnO 26,186 (87.9%) 11,607 (93.3%) – 150/50

GZO 25,588 (85.9%) 11,948 (96.1%) 24.7 500

GZO/ZnO 24,043 (80.7%) 11,578 (93.1%) – 500/50

3. Results and discussion

The inverted solar cell structure employed in this study isshown in Fig. 1(a). It consisted of a glass substrate, a transparentcathode, a P3HT:PCBM BHJ photoactive layer, and a thermally-deposited MoO3/Au anode. We compared four different transpar-ent cathodes with this device structure: ITO, ITO/ZnO, GZO, andGZO/ZnO, as shown in Fig. 1(b), (c), (d), and (e), respectively.The optical transmittance spectra of these electrodes on glasssubstrates are shown in Fig. 2. The integrals calculated from thespectra were compared for two different wavelength ranges, andthe results of this comparison are presented in Table 1. The500 nm GZO was chosen for use in this study because of thetrade-off between sheet resistance and optical transmittance. Toachieve a low sheet resistance comparable to that of ITO, a thickerGZO was needed. However, in the thicker GZO, the opticaltransmittance was degraded. The sheet resistances of the ITOlayer and the selected 500 nm GZO were 11.9 and 24.7 O/sq,respectively, as shown in Table 1. The optical transmittance of theGZO layer was lower than that of the ITO layer (85.9% of ITO) for

Please cite this article as: S.-G. Ihn, et al., Sol. Energy Mater. Sol. Ce

the wavelength range from 300 to 800 nm. However, consideringthe main absorption wavelength range of the P3HT (450–600 nm), the values are comparable (96.1% of ITO). Transmit-tances of ITO/ZnO and GZO/ZnO in this range were 93.3% and93.1% of the value of ITO, respectively.

The J–V characteristic curves under illumination and theincident photon-to-current conversion efficiencies (IPCE) of thedevices are shown in Figs. 3 and 4. A summary of the efficiencyparameters estimated from the J–V curves under one-sun illumi-nation (Fig. 3) and from the IPCE spectra (Fig. 4) are presentedin Table 2. The shunt and series resistances (Rsh and Rs) were

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calculated from the inverse slope of the J–V curves at 0 and 1.5 V,respectively [23]. There were slight differences in the short circuitcurrents (JSC) but significant differences in the open circuitvoltages (VOC) and fill factors (FF) between the two groupsof devices: those employing a single-layered cathode (ITO orGZO) and those employing a multi-layered cathode (ITO/ZnO orGZO/ZnO).

The devices fabricated on the single-layer transparent cath-odes exhibited low VOC and FF values; these parameters deter-mine power conversion efficiency (PCE). The PCE of the devicewith the GZO cathode was 1.85%, much lower than the reportedPCE of well-developed P3HT:PCBM BHJ solar cells [22]. Theinverted solar cell using a GZO cathode described by Owen

Fig. 3. Comparison of the device performances with different cathodes: J–V

characteristic curves measured under AM 1.5 solar illumination.

Fig. 4. Comparison of device performances with different cathodes: IPCEs for the

inverted solar cells on various cathodes.

Table 2Efficiency parameters estimated from the J–V curves (Fig. 3) and from the IPCE spectra

Cathode Efficiency (%) JSC (mA/cm2) VOC (V) FF (%)

ITO 1.74 10.9 0.40 39.8ITO/ZnO 3.54 12.05 0.59 49.8GZO 1.85 10.9 0.41 41.4GZO/ZnO 3.40 11.64 0.58 50.4

Please cite this article as: S.-G. Ihn, et al., Sol. Energy Mater. Sol. Ce

et al. [15] also exhibited a low PCE (1.4%), even though thatresearch group succeeded in improving the device performancecompared to that of the normal cell structure (GZO/PEDOT:PSS/P3HT:PCBM/Al, 0.35% of PCE) by considering the work functiondifference between GZO and ITO. The poor performances of theircell and ours are possibly due to the substitution of GZO for ITO.However, because our inverted ITO cell also exhibited a low PCE

(1.74%), we cannot simply attribute the poor performance to GZO.The overall performances of the inverted cells improved,

especially in terms of the VOC and FF values, when multi-layeredcathodes such as GZO/ZnO and ITO/ZnO were employed. How-ever, energy loss (in VOC) at the cathode was significant in thedevices with an ITO or GZO cathode, and the FFs of the devicesusing these cathodes were quite low, in accordance with therelatively low Rsh and high Rs values. These results indicate thatthe rectifying properties in the devices employing a ZnO layer as acathode modifier were superior to those in the devices using onlyITO or GZO. The internal built-in field formed by the workfunction difference between the two counter-electrodes sand-wiching the BHJ photoactive layer, called symmetry breaking, isrequired to force transport the dissociated electrons and holestoward the cathode and anode. Despite the internal built-in field,the probability of the recombination at the electrodes resulting ina reduction in VOC remains high due to the unique structure of theBHJ. At the electrodes, not only can the PCBM phase form anohmic contact, but the polymer may also form an ohmic contactwith the metallic ITO or GZO [24]. Therefore, symmetry breakingthrough the selective transport of charged carriers within aninterlayer is introduced between the BHJ photoactive layer andthe electrode. Selective contact of each electrode is essential inthe BHJ system to suppress recombination. Because the ZnO layeracts here as both an electron-collecting and a hole-blocking layer,recombination between the electrons collected in the ZnO layerand the holes residing in the active layer can be significantlysuppressed. Performance improvement, especially in VOC and FF,has been achieved by incorporating hole-blocking interlayersbetween the active layers and cathodes [25–29].

GZO is also likely to have a symmetry breaking functionbecause of its similar band structure to that of ZnO. GZO,however, is a highly doped degenerate semiconductor that actsmore like a metal than a semiconductor, similar to ITO. In thisstudy, we found that the ZnO layer improved the performances ofdevices with both ITO and GZO. To identify more clearly thereason for this performance improvement, the topology of eachsubstrate and the morphologies of the active layers spin-coatedonto the substrates were investigated because the morphology ofan active layer significantly affects device performance. Fig. 5shows 2�2 mm2 atomic force microscopy (AFM) images of thefour substrates and the active layers. The topology of ITO(Fig. 5(a)) is different from that of GZO (Fig. 5(c)). Brighter hill-like domains and darker valley-like domains with dark small dotswere observable in the topography of ITO, even though these dotswere not very clear. Domains were observed more clearly in thetopography of ZnO on ITO (Fig. 5(b)). It appears that the ZnO thinfilm was not conformally deposited onto ITO. In contrast, thetopography of ZnO on GZO was very similar to that of GZO,

(Fig. 4).

Rsh (O cm2) Rs (O cm2) JSC (cal.) (mA/cm2) Error (%)

167 11.3 8.98 21.4

269 3.6 10.35 16.4

222 25.5 9.02 20.8

306 2.6 10.32 12.8

lls (2011), doi:10.1016/j.solmat.2011.01.011

Fig. 5. AFM images (2 mm�2 mm) of the four different substrates and the active

layers thereon. The topologies of the substrates are shown in the upper panels and

those of the active layers coated thereon are shown in the lower panels: (a) ITO,

(b) ZnO on ITO, (c) GZO, and (d) ZnO on GZO.

S.-G. Ihn et al. / Solar Energy Materials & Solar Cells ] (]]]]) ]]]–]]]4

indicating conformal deposition of ZnO on GZO (Fig. 5(d)). Thetopology of ZnO on ITO was significantly different from that of ZnOon GZO, although the same material, ZnO, was simultaneouslydeposited in the same chamber under the same deposition condi-tions. This difference can be explained by the material compatibilitybetween ZnO and GZO, as mentioned in the Introduction. Despitethe different topologies of the substrates, there were no remarkabledifferences in the AFM topology images of the photoactive over-layers coated onto the substrates. Furthermore, although there wasno significant difference between the topologies of GZO and ZnO orbetween the topologies of the active layers thereon, the respectivesolar cells exhibited significant performance differences. This indi-cates that the underlayers, namely ITO, GZO, and ZnO, did not affect

Please cite this article as: S.-G. Ihn, et al., Sol. Energy Mater. Sol. Ce

the morphology of the active layer and that the morphology of theactive layer was not the cause of the performance improvement;rather, symmetry breaking by the hole-blocking ZnO was respon-sible for the performance improvement. The counterselective layerfor electron blocking and hole transporting was created with a thinMoO3 layer [30,31] in all of the devices investigated in this study.

When comparing devices using ITO/ZnO to those using GZO/ZnO,we observed only subtle differences in Rs and JSC; these devicestherefore exhibited similar PCEs. The device with a GZO/ZnOcathode demonstrated a lower Rs (2.6 O cm2) than did the devicewith a ITO/ZnO cathode (3.6 O cm2). We ascribed the lower Rs to thebetter GZO/ZnO interface resulting from material compatibilitybetween GZO and ZnO, including their similar work functions (4.4and 4.5 eV, respectively). These similar characteristics possiblycaused a lower contact resistance between the materials. The JSC

values of the ITO/ZnO-based cell (12.05 mA/cm2) and the GZO/ZnO-based cell (11.64 mA/cm2) were significantly higher than the bestpreviously reported value for P3HT:PCBM cells [13]. Consideringthat non-isolated universal bottom electrodes were used and thatthe J–V measurements were performed without a mask, we rea-soned that the measured JSC may have been overestimated [32]. Thishypothesis was confirmed by the mismatch between the measuredJSC and the calculated JSC from the EQE spectrum, as presentedin Table 2. The errors were in the range of 12.8–21.4%, larger thanthose reported for the P3HT:PCBM cells (around 10% with 0.11 cm2

of active area) [23]. We initially attributed these larger errors to thesmaller active area (0.0575 cm2).

It has been reported that the increased PCE of smaller cells canbe explained by the reduced electrical resistive loss, the enhancedoptical effects, and the finite additional fraction of photogener-ated carriers in the vicinity [32,33]. In particular, we observedthat cells using an ITO or GZO cathode exhibited larger errors(21.4% and 20.8%, respectively) than did cells using multi-layeredcathodes. We attributed these significant errors to the higherconductivities of ITO and GZO compared to that of ZnO; thishigher conductivity caused the layers to act as lateral chargecarrier transport paths or additional cathodes, enlarging theeffective photoactive area and volume. This result is consistentwith a previous study that reported that more highly conductivePEDOT:PSS induced overestimation of the current density in anon-isolated device configuration under a flood of illumina-tion [34]. However, despite the errors, the dependence of themeasured JSC on the electrode type was the same as that of thecalculated JSC. Despite the relatively insignificant overestimationof JSC, higher power conversion efficiencies were achieved fromthe ITO/ZnO-based inverted solar cell (3.54%) and the GZO/ZnO-based cell (3.4%) due to their higher VOC and FF values. Thesehigher values were caused by the selective contact provided bythe ZnO cathode-modifying layer at the cathode/polymer inter-face. This high performance, despite the inferior properties of ourGZO layer (24.7 O/sq of sheet resistance and 80% average trans-parency in the visible spectrum) compared to those previouslyreported (14.3 O/sq of sheet resistance and 90% average transpar-ency in the visible spectrum) [15], indicates that symmetrybreaking is more important than is the quality of GZO in BHJpolymer solar cells. The PCE of our inverted-type P3HT:PCBM BHJsolar cell was quite high for a low-cost GZO cathode. To the bestof our knowledge, this is one of the highest PCE reported forinverted polymer solar cells using a ZnO cathode modifier and forpolymer solar cells using a GZO electrode [35].

4. Conclusion

In summary, we synthesized and characterized an invertedP3HT:PCBM solar cell based on an alternative transparent GZO

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electrode modified with a ZnO layer. Despite the relatively highsheet resistance and low optical transparency of the GZO, a highPCE (3.4%) comparable to that of an inverted solar cell based on aconventional ITO cathode (3.54%) was observed. Our resultsindicate that GZO is a promising low-cost transparent electrodethat can be substituted for high-cost ITO, especially as a cathodein inverted-type polymer solar cells that employ a cathodemodifier with appropriate material compatibility, such as ZnO.

Acknowledgments

S.-G.I. and K.-S.S. contributed equally to this work. Thisresearch was supported by Basic Science Research Programthrough the National Research Foundation of Korea (NRF) fundedby the Ministry of Education, Science and Technology (2010-0015035 and 2009-0077682) and by the New and RenewableEnergy of the Korea Institute of Energy Technology Evaluation andPlanning (KETEP) grant funded by the Korea government Ministryof Knowledge Economy (no. 2009T100100614), and also bySamsung Research Fund, Sungkyunkwan University (S-2010-0675-000).

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