Doping Effect of Electron Transport Layer on Nanoscale PhaseSeparation and Charge Transport in Bulk Heterojunction Solar CellsKyung-Sik Shin,†,∥ Hye-Jeong Park,†,∥ Gyu Cheol Yoon,† Soon-Wook Jeong,‡ Brijesh Kumar,*,†

and Sang-Woo Kim*,†,§

†School of Advanced Materials Science and Engineering, Sungkyunkwan University (SKKU), Suwon 440-746, Republic of Korea.‡School of Advanced Materials and System Engineering, Kumoh National Institute of Technology, Gumi, Gyeongbuk 730-701,Republic of Korea§SKKU Advanced Institute of Nanotechnology (SAINT), Center for Human Interface Nanotechnology (HINT), SungkyunkwanUniversity (SKKU), Suwon 440-746, Republic of Korea.

ABSTRACT: We investigated the influence of the Ga dopingin the ZnO interlayer as an electron transport layer (ZnOETL) on the nanoscale phase separation in the bulkheterjunction (BHJ) layer coated on the ETL as well as themorphological and electrical properties of a low temperaturesol−gel-derived pristine ZnO ETL (P-ETL) and Ga-dopedZnO ETL (G-ETL), which affect the performance of invertedorganic solar cells (IOSCs). X-ray photoelectron spectroscopy(XPS) confirms the successful incorporation of the element Gain the ZnO ETL. The short circuit current densities (JSC) ofIOSCs fabricated using a G-ETL were significantly improvedfrom those of IOSCs fabricated with a P-ETL. The maximum JSC was obtained at 2 at. % Ga doping. The IOSCs fabricated with a2 at. % G-ETL demonstrated power conversion efficiencies of 3.51% (P3HT:PC60BM) and 5.43% (PCDTBT:PC70BM), whichwere higher than the power conversion efficiencies of 2.88% (P3HT:PC60BM) and 4.90% (PCDTBT:PC70BM) of the IOSCsfabricated with a P-ETL under simulated air mass 1.5 global full-sun illumination. The better performance was attributed to theimproved electrical properties of the G-ETL and the enhanced nanoscale phase separation in the BHJ active layer.

1. INTRODUCTION

Organic solar cells (OSCs) based on composites of electron-donating conjugated polymers and electron-accepting fullereneshave been attractive for energy harvesting in flexible devices,mainly due to their low cost potential, easy fabrication withlarge-area printing, and coating technologies on lightweightflexible substrates.1,2 In the conventional structure of OSCs, aphotoactive layer is sandwiched between a transparentconducting oxide anode and a low-work-function metallicca thode . Po ly(3 ,4 -e thy lened ioxyth iophene) :po ly -(styrenesulfonate) (PEDOT:PSS) is often used to form theanode interlayer. However, PEDOT:PSS is an acidic water-based solution, which causes interface instability between theanode interlayer and the photoactive layer and corrosion of theindium tin oxide (ITO) layer. To improve the interface stabilityand prevent device degradation, the inverted OSC (IOSC)structure has been considered, with metal oxides such as ZnOand TiO2 serving as the electron transport layers (ETL) and ahigh-work-function metal as the anode.Among the n-type interlayers used in flexible IOSCs, ZnO

interlayer has been regarded as a promising electron transport/hole blocking layer because of its high electron conductivity andhigh transparency. In addition, the energy level of the ZnOETL fits very well with the energy requirement for electronextraction.3−6 However, additional resistance resulting from the

charge-collecting ZnO ETL leads to high series resistance (RS)in the device and reduction in the short-circuit current density(JSC).

7−10 Hence, the performance of IOSCs may be improvedby reducing the resistivity of the interlayer and optimizing theenergy alignment between the acceptor/ETL interfaces.11,12

The group III elements, such as Al, Ga, and In, have beenused as n-type dopants to fabricate n-type ZnO with goodoptical quality and low resistivity, as they can substitute for theZn ions and occupy the Zn vacancies. Generally, Ga is the mosteffective dopant because it has lower reactivity compared to theother dopants, is resistive to oxidation, and has a small influenceon the ZnO lattice. In addition, the slightly smaller bond lengthof Ga−O compared to Zn−O is an advantage because it canminimize the deformation of the ZnO lattice, even at high Gaconcentrations.13−16 Even in the studies about the synthesis ofZnO doped with In, Al, and Ga for IOSCs and about enhanceddevice performance, the deposition temperature of ZnO thinfilms is very high (>200 °C). A high temperature growth ofZnO on plastic substrates is not suitable for flexible IOSCs.Also, there have been only few systematic studies on therelation among the morphological and electrical properties of

Received: July 16, 2013Revised: October 24, 2013Published: October 25, 2013

Article

pubs.acs.org/JPCC

© 2013 American Chemical Society 24692 dx.doi.org/10.1021/jp407017z | J. Phys. Chem. C 2013, 117, 24692−24699

the doped ZnO ETL, phase separation in the bulkheterjunction (BHJ) layer, and device performance.In the present work, we investigated the influence of the Ga

doping in the ZnO ETL on the nanoscale phase separation ofthe BHJ layer coated on the ETL as well as the morphologicaland electrical properties of the low temperature sol−gel-derivedpristine ZnO ETL (P-ETL) and Ga-doped ZnO ETL (G-ETL),which affect the performance of IOSCs. P- and G-ETLs wereprepared successfully by the low-temperature (150 °C) sol−gelprocess and were employed in IOSCs with an active layerfabricated with poly(3-hexylthiophene) (P3HT):(6,6)-phenylC61 butyric acid methyl ester (PC60BM). We manipulated thedoping concentration to optimize device performance. Wefurther applied P- and G-ETL in IOSCs with an active layer ofpoly [2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCDTBT):6,6-phenyl C71 butyric acid methyl ester (PC70BM), and comparedthe performances of these IOSCs with those of the devicesfabricated with P3HT:PC60BM. We demonstrated that theperformances of the IOSCs with the P3HT:PC60BM BHJ activelayer were enhanced by the optimization of the conductivity ofthe G-ETL and by the nanoscale phase separation in theP3HT:PC60BM BHJ layer.

2. EXPERIMENTAL SECTION

Materials. Zinc acetate dihydrate (Zn(CH3COO)2·H2O,99.9%), Ga nitrate (Ga(NO3)3, 99%), ethanolamine (C2H7NO,99%), and 2-methoxyethanol (C3H8O2, 99.8%, Aldrich) werepurchased from Sigma-Aldrich. P3HT and PCDTBT werepurchased from Rieke Metals Inc. PC60BM and PC70BM werepurchased from Sigma-Aldrich., respectively.Deposition of P-ETL and G-ETL. We prepared Ga-doped

ZnO (GZO) sol by dissolving zinc acetate dihydrate as aprecursor and gallium nitrate (Ga(NO3)3) as dopant in 2-

methoxyethanol (CH3OCH2CH2OH, Aldrich, 99.8%) understirring for 24 h in air. The amount of Ga, defined by 100%(Ga)/(Ga + Zn), was varied over a range of 0−3 at. %. In ourtypical device fabrication process, functional P- and G-ETLswere deposited onto ITO/glass substrates from the sols by spincoating and annealed at 150 °C for 30 min.

Fabrication of IOSCs with P-ETL and G-ETL. The P- andG-ETLs were transferred into a glovebox and the active layerwith a thickness of 150 nm was deposited by spin coating of theblend solution made of P3HT and PC60BM at a weight ratio of1:1 in chlorobenzene (30 mg/mL) onto the ETL. Also, theother blend solution of PCDTBT and PC70BM made at aweight ratio of 1:4 in dichlorobenzene:chlorobenzene of ratio3:1 was deposited of thickness of 90 nm by spin coating ontothe ETL. Finally, a 20-nm thick molybdenum oxide (MoO3)electron blocking layer and 100-nm thick Ag anode weresubsequently deposited using thermal evaporation. Averagevalues of the measured results from over 50 devices arepresented.

Characterization Methods. The thickness of the P- andG-ETLs was measured by an ellipsometer. The topography andcurrent images of P- and G-ETLs were observed by contact-mode atomic force microscopy. We used a conductive tip as thetop contact of the device. We used Pt-coated contact-modeAFM probes for all measurements reported in this work andthe tip−sample contact force was kept to a minimum,consistent with topography tracking that uses a typical valueof ∼10 nN. AFM images of the active layers were obtained intapping mode. Transmission electron microscopy (TEM)images of the active layers were obtained with accelerationvoltage of 80 kV. Ga-doping in the ZnO ETL was confirmed byX-ray photoelectron spectroscopy (XPS) measurements. Theoptical transmissions of the P- and G-ETLs were recorded byan ultraviolet−visible (UV−vis) spectrophotometer. The

Figure 1. (a−d) AFM topography and (e−h) current images (2 × 2 μm2) taken from ZnO ETLs of 0, 1, 2, and 3 at. % Ga contents when thesamples were biased with a −2 V. Current histogram corresponds to each current image, quantifying the change in the average value and distributionof the current.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp407017z | J. Phys. Chem. C 2013, 117, 24692−2469924693

fabricated IOSCs were characterized by measuring J-Vcharacteristics using a solar simulator under simulated airmass (AM) 1.5 global (G) full-sun illumination.

3. RESULTS AND DISCUSSIONThe investigated surface morphologies of P-ETL and G-ETLscoated on ITO/glass are shown in the AFM images of Figure

1a-d. The thickness of the P-ETL and G-ETLs deposited onITO/glass were approximately the same (50.5, 52.4, 49.2, and49.6 nm with 0, 1, 2, and 3 at. % Ga contents, respectively), asmeasured by the ellipsometer. The root-mean-square (RMS)values of the ZnO ETLs were 6.73, 2.62, 2.25, and 2.20 nm

with 0, 1, 2, and 3 at. % Ga contents, respectively, as shown inthe topography. All ZnO ETLs with different Ga contentsshowed different morphologies. The roughness and the grainsize of the grown thin films decreased upon Ga doping (Figure1), which attributed to the fact that Ga increases the number ofnucleus when it is incorporated into the ZnO and the fact thatthe Ga atomic radius is smaller than the Zn atomic radius.4,17,18

It is found that G-ETLs have more hydrophilic surface than P-ETL, which relates to surface roughness.In many studies, the electrical properties of ZnO ETLs ware

investigated by using Hall measurements, but the direction ofthe current in the photovoltaic device is perpendicular to thesubstrate. Therefore, Hall measurement of the ZnO ETLs usedin a photovoltaic device is not a suitable approach. Here, we

Figure 2. AFM topographical images (2 × 2 μm2) of (a−d)P3HT:PC60BM blend films and (e−h) PCDTBT:PC70BM blend filmson ZnO ETLs of 0, 1, 2, and 3 at. % Ga contents. The insets showphase images corresponding to the AFM topography images.

Figure 3. TEM images of (a−d) P3HT:PC60BM blend films and (e−h) PCDTBT:PC70BM blend films on ZnO ETLs of 0, 1, 2, and 3 at. %Ga contents.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp407017z | J. Phys. Chem. C 2013, 117, 24692−2469924694

investigate the changes in the distribution of local electronconductivity of ZnO ETLs with Ga-dopants using conductiveAFM (cAFM). Parts e−h of Figure 1 show the evolution of theelectron current distribution obtained with a −2 V bias on theP-ETL and G-ETL samples, which correspond to thetopographies in Figure 1a-d. The average current values ofthe ZnO ETLs are 15.513, 22.581, 24.574, and 1.309 nA with 0,1, 2, and 3 at. % Ga contents, respectively. The average currentvalues of the ZnO ETLs gradually increase from 0 at. % to 2 at.% Ga contents, indicating enhanced conductivity of the ZnOETL with increased Ga doping into the ZnO lattice. However,the average current value of the G-ETL at 2 at. % decreasedrapidly to 1.309 nA at 3 at. % Ga concentration due to theformation of structural defects in the G-ETL and theirconsequent scattering of carriers at this high doping level.19

Furthermore, the valleys of the P-ETL and the grain boundariesof all P- and G-ETLs yielded relatively high current values andcould act as recombination centers and leakage current paths.These AFM results represent the effect of the Ga-doping in theZnO ETL on the morphology and conductivity of the ZnOETL, and subsequently on the performance of electronics andoptoelectronic devices.The morphology of the P3HT:PC60BM BHJ layer can be

affected by the surface morphology and surface energy of the

underlying layer. Therefore, morphology of the P3HT:PC60BMBHJ layer needs to be investigated to determine the origins ofthe observed modulations of the JSC and power conversionefficiency (PCE) with respect to the surface energy. parts a−dof Figure 2 show the top surface morphologies of theP3HT:PC60BM-based BHJ layers coated on the ZnO ETLsprepared with 0, 1, 2, and 3 at. % Ga contents, respectively.Although all of the samples have the typical island and valleymorphology of the P3HT:PC60BM-based BHJ layer in Figure 2,the surface morphology of the BHJ active layer is clearlydifferent. The surface roughness of the BHJ active layerincreases with increasing Ga contents in the ZnO ETL up to 2at. %, but is low at the higher doping level (0.96, 0.99, 1.23, and1.17 nm of BHJ active layers on the ZnO ETLs prepared with0, 1, 2, and 3 at. % Ga contents, respectively). Such trend in theBHJ surface roughness is observed with increasing hydro-philicity of the ZnO ETL, which is confirmed by the contactangle. The increase of surface roughness with increasing Gacontents are also observed from PCDTBT:PC70BM-based BHJlayers as shown in Figure 2e−h (0.57, 0.57, 0.64, and 0.68 nmof the BHJ active layers on the ZnO ETLs prepared with 0, 1, 2,and 3 at. % Ga contents, respectively.Figure 3 shows the nanoscale BHJ morphologies of the

P3HT:PC60BM active layers on the ZnO ETLs for different Ga

Figure 4. XPS spectra of the ZnO ETLs with different Ga contents. (a) Zn 2p peaks with Ga dopant concentration (0−3 at. %), and (b) Ga 2p peakswith with Ga dopant concentration (0−3 at. %). (c, d) XPS spectra of oxygen 1s peak from the surface of ZnO ETLs of different Ga contents (0−3at. %).

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp407017z | J. Phys. Chem. C 2013, 117, 24692−2469924695

contents. The BHJ network of P3HT:PC60BM active layersshow an obvious contrast difference. For the BHJ layer on theP-ETL, the morphology shows large distributions of P3HT-richdomains and PCBM-rich domains of about 100 nm in widthcorresponding to those of hills and valleys on the P-ETLsurface in the AFM image. Since PC60BM has a higher surfaceenergy than P3HT, PC60BM tends to accumulate at the valleyof the P-ETL to reduce the overall energy. However, as theroughness of the G-ETL gradually reduces with increasing Gacontent, the nanscale phase separation in the BHJ layer on theG-ETL becomes well-developed. This well-developed nano-scale phase separation of BHJ layer provides the optimumefficiency. Although the rough surface of the ETL can generallyincrease the number of carrier pathways by enlarging theinterface area between the ETL and the active layer, it can alsointerrupt the well-developed nanoscale BHJ network. Addi-tionally, TEM measurements of PCDTBT:PC70BM activelayers on the ZnO ETLs for different Ga contents were carriedout to further demonstrate the effect of ETL surfacemorphology on nanoscale BHJ morphology (Figure 3e-h). As

the Ga content in ZnO ETL gradually increases, the nanoscalephase separation of PCDTBT:PC70BM is also observed clearlywith appearance of a fibrillar PCDTBT nanostructure, which isconsistent with the phase images in the Figure 2. Hence, Gadoping of the ZnO ETL provides the ETL with the propersurface roughness to support the development of the nanoscalephase separation in the BHJ layer as well as the enhancement inthe conductivity of the ETL.We confirmed the incorporation of Ga into the ZnO ETL by

XPS prior to device fabrication (Figure 4). In the XPS spectrumof the P- and G-ETLs, the peaks located at 1020.71 ± 0.2 eVand 1043.71 ± 0.2 eV correspond to the electronic states of Zn2p3/2 and Zn 2p1/2, respectively (Figure 4a).20,21 In the XPSspectra of the G-ETLs, the two peaks at 1117.14 ± 0.2 eV and1144.24 ± 0.2 eV correspond to the electronic states of Ga2p3/2 and Ga 2p1/2, respectively (Figure 4b).

20,21 The clear Ga-related peaks in the G-ETL samples indicate the successfuldoping of Ga atoms into the ZnO lattices. No peak related toGa is detected from the P-ETL. The XPS spectra of the G-ETLs reveal that the actual Ga contents in the G-ETLs derivedfrom the 1, 2, and 3 at. % Ga(NO3)3 precursors are 0.18%,0.46%, and 0.68%, respectively.It is well-known that the n-type conductivity of ZnO ETL is

due to the presence of intrinsic defects such as oxygenvacancies and interstitial Zn atoms. The incorporation of Gainto ZnO improves the electrical conductivity of the ZnO ETLby creating one extra carrier from the substitutional doping ofGa3+ at Zn2+ sites. As the ionic radius of Ga3+ (0.62 Å) issmaller than that of Zn2+ (0.74 Å), Ga3+ easily substitutes forZn2+ at a low doping concentration. However, Ga3+ may besegregated in the form of Ga2O3, which acts as a trap for freeelectrons and increases the potential barrier of the grainboundary, at the grain boundaries above the solid solubilitylimit. The typical oxygen 1s peaks from the surface of ZnO canbe consistently fitted by three nearly Gaussian distributions,centered at 530.35 ± 0.3 eV, 531.40 ± 0.3 eV, and 533.05 ± 0.3eV (remarked as O 1s-1, O 1s-2, and O 1s-3 in Figure 4,respectively), respectively. The component with low bindingenergy of O 1s-1 is attributed to the O2− ions in the ZnOlattice.20 The intensity of this peak shows the amount of oxygenatoms in the wurtzite structure of the hexagonal Zn2+ ion array.The medium binding energy of O 1s-2, whose intensity partlyrepresents the variation in the concentration of oxygenvacancies, is related to the O2− ions in the oxygen-deficientregions within the matrix of ZnO.20,22 The high binding energypeak of O 1s-3 may be ascribed to a specific chemisorbedoxygen such as the adsorbed H2O or OH− type species on thesurface of the film.19,21

The relative intensity of the peak attributed to oxygenvacancies decreases rapidly with increasing Ga doping level. Forexample, the ratio of the low binding energy peak of O 1s-1 andthe medium binding energy peak of O 1s-2 increases with theGa doping amount, as shown in Figure 4. This means thatoxygen vacancies relatively decrease in the structure. At higherGa dopant concentrations, more Ga is doped into the ZnOlattice, improving the crystalline quality of ZnO by occupyingthe position of Zn vacancies and suppressing the amount ofoxygen vacancies.All the P- and G-ETLs are highly transparent in the visible

region from 400 to 700 nm, and their transmittance fallssharply below 400 nm due to band gap absorption (Figure 5).P- and G-ETLs show no significant differences in opticaltransmittance in the visible region. The obtained transmittance

Figure 5. (a) Optical transmittance spectra of ZnO ETLs of differentGa contents. Inset shows the BM shift in transmittance of the ZnOETLs near the absorption band edge due to the increase in carrierconcentration. (b) Plot of (αhυ)2 against hυ, used for calculating theband gaps of the ZnO ETLs. (c) Schematics of the band structures ofIOSCs with ZnO ETLs of different Ga contents (0−3 at. %).

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp407017z | J. Phys. Chem. C 2013, 117, 24692−2469924696

spectra of the P- and G-ETLs are slightly shifted to the higherenergy side with increasing Ga-content. The Burstein−Moss(BM) effect is responsible for such a blue shift in G-ETL;23 theFermi energy lies in the conduction band for heavy n-typedoping. Thus, the measured band gap determined from theonset of interband absorption moves toward the higher energyside. The band gap was calculated from the optical absorptiondata by using the common procedures for direct energy bandgap semiconductors. The optical absorption coefficient is given

by αhυ = A(hυ − Eg)1/2, where A is a constant, h is Plank’s

constant, ν is the frequency of the incident photon, and Eg isthe optical band gap of the material.21,24 The (αhυ)2 versus hυplot and the Roth procedure were used to calculate the opticalenergy band gap; the band gap varied from 3.37 to 3.45 eV, forthe G-ETLs. These results indicate that the Fermi level of an n-type semiconductor increases with carrier concentration. Adecrease in the work function of an ETL reduces the energybarrier for the electron transport to the ETL, promoting chargecollection.25 Hence, a G-ETL is better than a P-ETL for chargecollection, as depicted in Figure 5c.Parts a and b of Figure 6 show cross-sectional field-emission

scanning electron microscopy (FE-SEM) images of the IOSCsfabricated using the ETL with different active layers(P3HT:PC60BM and PCDTBT:PC70BM). The thickness ofthe ETL is about 50 nm, and the thicknesses of theP3HT:PC60BM and PCDTBT:PC70BM active layers on theETL/ITO/glass are about 150-nm and 90-nm, respectively.The current density−voltage (J−V) curves of the IOSCs

fabricated with different P- and G-ETLs under illumination andin the dark are given in Figure 6. Table 1 summarizes theextracted device parameters. JSC changes significantly with theGa content in the ZnO ETL. JSC values of the devices fabricatedwith P3HT:PC60BM and ZnO ETLs with 0−3 at. % Ga

Figure 6. Cross-sectional FE-SEM images of the device structures with (a) P3HT:PC60BM and (b) PCDTBT:PC70BM active layers. J−Vcharacteristics of IOSCs fabricated with ZnO ETLs doped with different Ga contents under simulated solar irradiation of AM 1.5 G (100 mW·cm−2);(c and e) P3HT:PC60BM and PCDTBT:PC70BM, respectively. (d and f) J−V characteristics of parts c and e in the dark, respectively. The insetshows the reverse saturated dark current of each IOSC for different Ga contents.

Table 1. Summary of the Device Performance Parameters ofIOSCs Fabricated with ZnO ETLs with Different GaContents

Ga content(at. %)

JSC(mA/cm2)

VOC(V) FF

PCE(%)

P3HT: PC60BM 0 10.28 0.57 0.50 2.881 11.07 0.57 0.50 3.122 11.56 0.57 0.54 3.513 10.84 0.56 0.45 2.75

PCDTBT:PC70BM

0 9.29 0.89 0.59 4.90

1 9.90 0.89 0.59 5.172 9.93 0.88 0.62 5.433 9.68 0.87 0.61 5.14

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp407017z | J. Phys. Chem. C 2013, 117, 24692−2469924697

contents are 10.28, 11.07, 11.56, and 10.84 mA·cm−2,respectively. The maximum JSC is achieved with the G-ETLof 2 at. % Ga content. Also, JSC values of the devices fabricatedwith PCDTBT:PC70BM and G-ETLs with the same Gacontents show a similar trend to the devices fabricated withP3HT:PC60BM. The difference in device performance in termsof open-circuit voltage (VOC) was not significant. The phaseseparation in the BHJ layer and the conductivity of the ETL areimportant factors in exciton dissociation and charge collection,respectively, strongly affecting the JSC of an IOSC. The properphase separation in the BHJ layer and higher conductivity ofthe ETL generally yield a lower series resistance (RS), whichsubsequently enhance the JSC of an IOSC by promotingeffective carrier transport. Furthermore, the reduced RSenhances the fill factor which is determined by RS and shuntresistance, and subsequently enhances the PCE of an IOSC.The dark J−V curve of the IOSC fabricated with a G-ETL of

2 at. % Ga content showed higher forward current densitiesthan those of the IOSCs fabricated respectively with a P-ETLand G-ETLs of 1 at. % Ga and 3 at % Ga contents. This isevidence of lower and optimized RS of these IOSCs (Figure 6dand f). Furthermore, bimolecular recombination can also bereduced through the decrease in RS, which subsequentlyenhances JSC. The estimated RS of IOSCs fabricated with ZnOETLs of 0, 1, 2, and 3 at. % Ga contents, were 1.36, 1.23, 1.06,and 1.39 Ω·cm−2, respectively. The lowest RS value, 1.06 Ω·cm−2 of the IOSC fabricated with the 2 at. % G-ETL isadditional evidence of the optimized phase separation in theBHJ layer, the highest conductivity of this ETL, and theeffective charge collection by reduced energy barrier at thejunction of the active layer and the ETL.The RS initially decreases upon doping because the dopant

acts as an additional charge carrier to increase the conductivity,but it increases again with an increase in dopant concentrationdue to the impurities from the precursor, which were identifiedfrom the cAFM results. For low [Ga]/[Zn] ratios, the decreasein resistivity is due to the increase of the Ga atoms that areincorporated into the ZnO lattice at the Zn sites, with each Gaatom supplying one electron to the conduction band, until themaximum solubility of Ga into the ZnO lattice is reached(minimum value of the resistivity curve). At higher [Ga]/[Zn]rates in the solution, the Ga atoms do not occupy more Znsites, and Ga segregates in the form of an oxide at the grainboundaries or interstices, decreasing the conductivity andmobility and consequently increasing the electrical resistivity.The average current value of the 3 at. % G-ETL is lower thanthat of the P-ETL in Figure 1, but the JSC of device with the 3at. % G-ETL is higher than that with the P-ETL. This can bedue to the increase in the photocurrent density by the effectiveexciton dissociation induced by the enhanced phase separationin the BHJ active layer. Therefore, it can be concluded that thehigher conductivity and increased band gap of the G-ETL of 2at. % Ga content, and the optimized nanoscale phase separationin the BHJ layer are main causes of the significant enhancementin the JSC, resulting in the enhancement of PCE of the IOSC.

4. CONCLUSIONSIn summary, we investigated the influence of the Ga doping inthe ZnO ETL on the nanoscale phase separation in the BHJlayer coated on ETL as well as the morphological and electricalproperties of the low temperature sol−gel-derived P- and G-ETLs, which affect the performance of inverted organic solarcells (IOSCs). Low-temperature sol−gel derived P- and G-

ETLs were prepared with different Ga contents for IOSCs. TheGa doping affected the conductivity as well as the surfacemorphology of the ZnO ETL. These properties affected thenanoscale phase separation in the BHJ active layer coated onthe ETL. Therefore, a significant enhancement in JSC,subsequently in the PCE of an IOSC fabricated with a G-ETL, is attributed to the higher electron conductivity of theoptimized G-ETL, effective phase separation, and reducedenergy barrier between the active layer and the G-ETL. Thiswork suggests that such a low-temperature sol−gel-derived G-ETL can greatly enhance the PCE in flexible IOSCs.

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: kimsw1@skku.edu (S.-W.K.).*E-mail: kumar@skku.edu (B.K.). Tel: +82-31-290-7352, Fax:+82-31-290-7410.Author Contributions∥These authors contributed equally to this work.NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThis research was supported by the Energy InternationalCollaboration Research & Development Program of the KoreaInstitute of Energy Technology Evaluation and Planning(KETEP) funded by the Ministry of Knowledge Economy(MKE) (2011-8520010050), the National Research Founda-tion of Korea (NRF) grant funded by the Ministry of Science,ICT & Future Planning (MSIP) (2009-0083540), and a grant(2011-0032151) from the Center for Advanced Soft Electronicsunder the Global Frontier Research Program of the MSIP.

■ REFERENCES(1) Kim, J. Y.; Lee, K.; Coates, N. E.; Moses, D.; Nguyen, T.-Q.;Dante, M.; Heeger, A. J. Efficient Tandem Polymer Solar CellsFabricated by All-Solution Processing. Science 2007, 317, 222−225.(2) Jorgensen, M.; Norrman, K.; Krebs, F. C. Stability/Degradationof Polymer Solar Cells. Sol. Energy Mater. Sol. Cell. 2008, 92, 686−714.(3) Ma, Z.; Tang, Z.; Wang, E.; Andersson, M. R.; Inganas, O.;Zhang, F. Influences of Surface Roughness of ZnO Electron TransportLayer on the Photovoltaic Performance of Organic Inverted SolarCells. J. Phys. Chem. C 2012, 116, 24462−24468.(4) Shin, K.-S.; Lee, K.-H.; Lee, H. H.; Choi, D.; Kim, S.-W.Enhanced Power Conversion Efficiency of Inverted Organic SolarCells with a Ga-Doped ZnO Nanostructured Thin Film PreparedUsing Aqueous Solution. J. Phys. Chem. C 2010, 114, 15782−15785.(5) Cheun, H.; Fuentes-Hernandez, C.; Zhou, Y.; Potscavage, W. J.,Jr.; Kim, S.-J.; Shim, J.; Dindar, A.; Kippelen, B. Electrical and OpticalProperties of ZnO Processed by Atomic Layer Deposition in InvertedPolymer Solar Cells. J. Phys. Chem. C 2010, 114, 20713−20718.(6) Park, S.; Tark, S. J.; Lee, J. S.; Lim, H.; Kim, D. Effects of IntrinsicZnO Buffer Layer Based on P3HT/PCBM Organic Solar Cells withAl-Doped ZnO Electrode. Sol. Energy Mater. Sol. Cell. 2009, 93, 1020−1023.(7) Krebs, F. C.; Gevorgyan, S. A.; Alstrup, J. a Roll-to-Roll Processto Flexible Polymer Solar Cells:Model Studies, Manufacture andOperational Stability Studies. J. Mater. Chem. 2009, 19, 5442−5451.(8) Chen, L.-M.; Hong, Z.; Li, G.; Yang, Y. Recent Progress inPolymer Solar Cells: Manipulation of Polymer:Fullerene Morphologyand the Formation of Efficient Inverted Polymer Solar Cells. Adv.Mater. 2009, 21, 1434−1449.(9) Xu, Z.; Chen, L. M.; Yang, G.; Huang, C. H.; Hou, J.; Wu, Y.; Li,G.; Hsu, C. H.; Yang, Y. Vertical Phase Separation in Poly(3-Hexylthiophene):Fullerene Derivative Blends and its Advantage for

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp407017z | J. Phys. Chem. C 2013, 117, 24692−2469924698

Inverted Structure Solar Cells. Adv. Funct. Mater. 2009, 19, 1227−1234.(10) Wang, J.-C.; Weng, W.-T.; Tsai, M.-Y.; Lee, M.-K.; Horng, S.-F.;Perng, T.-P.; Kei, C.-C.; Yu, C.-C.; Meng, H.-F. Highly efficientflexible inverted organic solar cells using atomic layer deposited ZnOas electron selective layer. J. Mater. Chem. 2010, 20, 862−866.(11) Sekine, N.; Chou, C.-H.; Kwan, W. L.; Yang, Y. ZnO Nano-Ridge Structure and its Application in Inverted Polymer Solar Cell.Org. Electron. 2009, 10, 1473−1477.(12) Steim, R.; Kogler, F. R.; Brabec, C. J. Interface Materials forOrganic Solar Cells. J. Mater. Chem. 2010, 20, 2499−2512.(13) Yuan, G.-D.; Zhang, W.-J.; Jie, J.-S.; Fan, X.; Tang, J.-X.; Shafiq,I.; Ye, Z.-Z.; Lee, C.-S.; Lee, S.-T. Tunable N-Type Conductivity andTransport Properties of Ga-Doped ZnO Nanowire Arrays. Adv. Mater.2008, 20, 168−173.(14) Joseph, M.; Tabata, H.; Kawai, T. P-Type Electrical Conductionin ZnO Thin Films by Ga and N Codoping. Jap. J. Appl. Phys. 1999,38, L1205−L1207.(15) Ko, H. J.; Chen, Y. F.; Hong, S. K.; Wenisch, H.; Yao, T.; Look,D. C. Ga-Doped ZnO Films Grown on GaN Templates By Plasma-Assisted Molecular Beam Epitaxy. Appl. Phys. Lett. 2000, 77, 3761.(16) Zhou, M.; Zhu, H.; Jiao, Y.; Rao, Y.; Hark, S.; Liu, Y.; Peng, L.;Li, Q. Optical and Electrical Properties of Ga-Doped ZnO NanowireArrays on Conducting Substrates. J. Phys. Chem. C 2009, 113, 8945−8947.(17) Park, W. J.; Shin, H. S.; Ahn, B. D.; Kim, G. H.; Lee, S. M.; Kim,K. H.; Kim, H. Investigation on Doping Dependency of Solution-Processed Ga-Doped ZnO Thin Film Transistor. J. Appl. Phys. Lett.2008, 93, 083508.(18) Khranovskyy, V.; Grossner, U.; Nilsen, O.; Lazorenko, V.;Lashkarev, G. V.; Svensson, B. G.; Yakimova, R. Structural andmorphological properties of ZnO:Ga thin films. Thin Solid Films 2006,515, 472−476.(19) Kvit, A. V.; Yankovich, A. B.; Avrutin, V.; Liu, H.; Izyumskaya,N.; Ozgur1, U.; Morkoc, H.; Voyles, P. M. Impurity Distribution andMicrostructure of Ga-Doped ZnO Films Grown by Molecular BeamEpitaxy. J. Appl. Phys. 2012, 112, 123527.(20) Wang, H.; Baek, S.; Song, J.; Lee, J.; Lim, S. Microstructural andOptical Characteristics of Solution-Grown Ga-Doped ZnO NanorodArrays. Nanotechnology 2008, 19, 075607.(21) Ahn, B. D.; Oh, S. H.; Lee, C. H.; Kim, G. H.; Kim, H. J.; Lee, S.Y. Influence Of Thermal Annealing Ambient On Ga-Doped Zno ThinFilms. J. Cryst. Growth 2007, 309, 128−133.(22) Islam, M. N.; Ghosh, T. B.; Chopra, K. L.; Acharya, H. N. XPSand X-Ray Diffraction Studies of Aluminum-Doped Zinc OxideTransparent Conducting Films. Thin Solid Films 1996, 280, 20−25.(23) Ahn, C. H.; Han, W. S.; Kong, B. H.; Cho, H. K. Ga-DopedZnO Nanorod Arrays Grown by Thermal Evaporation and TheirElectrical Behavior. Nanotechnology 2009, 20, 015601.(24) Donga, B.-Z.; Fang, G.-J.; Wang, J.-F.; Guan, W.-J.; Zhao, X.-Z.Effect of Thickness on Structural, Electrical, and Optical Properties ofZnO:Al Films Deposited by Pulsed Laser Deposition. J. Appl. Phys.2007, 101, 033713.(25) Kyaw, A. K. K.; Sun, X.; Zhao, D. W.; Tan, S. T.; Divayana, Y.;Demir, H. V. Improved Inverted Organic Solar Cells with a Sol−GelDerived Indium-Doped Zinc Oxide Buffer Layer. IEEE J. Sel. Top.Quantum Electron. 2010, 16, 1700−1706.

The Journal of Physical Chemistry C Article

dx.doi.org/10.1021/jp407017z | J. Phys. Chem. C 2013, 117, 24692−2469924699