Electrochimica Acta 180 (2015) 435–441

Morphologically controlled ZnO nanostructures as electron transportmaterials in polymer-based organic solar cells

Kyu-Chae Choia, Eun-Jin Leea, Youn-Kyoung Baeka, Dong-Chan Limb, Yong-Cheol Kangc,Yang-Do Kimd, Ki Hyun Kime, Jae Pil Kime, Young-Kuk Kima,*a Powder/Ceramic Research Department, Korea Institute of Materials Science, Changwon, 641-831, Republic of Koreab Surface Technology Department, Korea Institute of Materials Science, Changwon, 641-831, Republic of KoreacDepartment of Chemistry, Pukyong National University, Pusan, 608-737, Republic of KoreadDepartment of Materials Science and Engineering, Pusan National University, Pusan, 609-735, Republic of Koreae Lighting Sources & Materials Research Center, Korea Photonics Technology Institute, Gwangju, 500-799, Republic of Korea

A R T I C L E I N F O

Article history:Received 3 April 2015Received in revised form 28 August 2015Accepted 28 August 2015Available online 29 August 2015

Keywords:ZnOmorphologysolar cellselectron transport layer

A B S T R A C T

The morphology of ZnO electron transport layers based on ZnO nanoparticles were modified withincorporation of ZnO nanorods via their co-deposition from mixed colloidal solution of nanoparticles andnanorods. In particular, the short circuit current density and the fill factor of the constructed photovoltaicdevice were simultaneously improved by applying mixture of ZnO nanoparticles and nanorods. As aresult, a large improvement of power conversion efficiency up to 9% for the inverted organic solar cellshaving a blend of low band gap polymers and fullerene derivative as an active layer was demonstratedwith the morphologically controlled ZnO electron transport layer.

ã 2015 Elsevier Ltd. All rights reserved.

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1. Introduction

Polymer solar cells (PSCs) based on bulk heterojunction (BHJ)morphology made from a blend of electron donating polymers andelectron accepting fullerene derivatives have attracted muchattention as a promising renewable energy source due to theirpotential for flexibility, low-cost, light weight, and ability of large-area processing [1–6]. The conventional PSCs are prepared bydepositing a top electrode on top of the photoactive layer formedon a transparent electrode. The top electrodes of conventional PSCsare chosen from metal groups with low work function for efficientcharge extraction. However, the conventional PSC architecturelacks stability of cell performance and the inverted geometry usingAg or Au as top electrodes are practically more important. Thephotovoltaic performance of PSCs can be highly improved byinserting interfacial layers between the active layer and theelectrode to make a good ohmic contact and selective carriertransport [7].

* Corresponding author at: Principal Researcher Nanofunctional PowderLaboratory, Division of Powders/Ceramics, Korea Institute of Materials Science,#797Changwondaero, Changwon, Kyungnam, Republic of Korea 642-831.Tel.: +82-55-280-3343; fax: +82-55-280-3392.

E-mail address: voice21@kims.re.kr (Y.-K. Kim).

http://dx.doi.org/10.1016/j.electacta.2015.08.1510013-4686/ã 2015 Elsevier Ltd. All rights reserved.

Various electron-transporting and hole-blocking materialsincluding cesium carbonate (Cs2CO3), titanium oxide (TiOx) andzinc oxide (ZnO) have been applied to improve electron transportand charge extraction of the PSCs [6–9]. Among these, ZnO hasbeen extensively used as an electron transport layer (ETL) due to itshigh electron mobility, optical transmittance in the visible range,efficient hole-blocking capability originated from the largeionization potential, stability against environmental oxygen orhumidity, etc [9–11]. In particular, colloidal solution of ZnOnanoparticles (NPs) is popularly used due to availability of lowtemperature processing, solution-based deposition on flexiblesubstrates. Sometimes, in order to achieve further enhancement ofthe cell performance, the one-dimensional (1D) ZnO nanocrystalssuch as nanorods, nanopillars, nanofibers and nanowires havebeen synthesized by several groups [12–17]. ZnO nanocrystals withrod-like morphology ensure higher carrier diffusion due to themonocrystalline nature avoiding boundary scattering of carriers asobserved in NP-based ZnO layers [18,19]. Takanezawa et al.reported improved cell performance by incorporating verticallyaligned array of hydrothermally grown ZnO NRs as an ETL [20]. Inparticular, the power conversion efficiency (PCE) of the con-structed PSC using the blend of poly(3-hexylthiophene) (P3HT)and 6,6-phenyl-C61-butyric acid methyl ester (PCBM) as an activelayer was improved up to 3.9% mainly due to contribution ofenhanced fill factor (FF) which was ascribed to excellent electrical

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properties of ZnO NR array. Hence it is clear that controlling themorphology of the ETL is quite important for improving the deviceperformance of the photovoltaic device [21,22].

2. Experimentals

2.1. Preparation ZnO Nanopaticles (NPs) and Nanorods (NRs)

The chemical synthesis of ZnO nanopaticals (NPs) and ZnOnanorods (NRs) were performed using the well-known precipita-tion method [23]. In detail, zinc acetate dehydrate (2.95 g) wasdissolved in 125 mL methanol at 62 �C, a solution of potassiumhydroxide (1.48 g) in 65 ml methanol was titrated over 10 min intothe zinc acetate solution under vigorous stirring. After �2 hour, theZnO NPs were precipitated and the solution became turbid. Then,ZnO NPs precipitate was separated by centrifugation and theprecipitate was washed twice using 100 ml methanol. Lastly, theZnO NPs were dispersed in mixed solvent of chlorobenzene (CB)and isoproply alcohol (IPA) with 1:1 volumetric ratio. This solutionwas transparent and was stable for more than 6 month without anyinjected solution. ZnO NRs were prepared using synthesized ZnO-NPs for �2 hour (above). The reaction mixture of ZnO-NPs wasthen concentrated by a factor of ten by stirring at roomtemperature. Then, this concentrated solution was heated for4 hour at 60 �C. Subsequently, the heater and stirrer were removedand the precipitate was wash twice and dispersed in the solventmixture of CB and IPA. The mixed colloidal solution of ZnOnanocrystal was fabricated by mixing ZnO NPs and ZnO NRs blendwith a volume ratio of 1:2 in equal solvent (CB: IPA) under vigorousstirring for 30 min. Through this work, the concentration of thecolloidal solution of ZnO nanoparticles was kept to be 10 mg/ml.

2.2. Device fabrication

Indium tin oxide (ITO)-coated glass substrates were cleaned bysuccessive ultrasonification with acetone, isopropanol (IPA) andDI-water for 30 min, respectively. Then, the ZnO (NPs, NRs, NPs +NRs) layer was spin coated on ITO substrates followed by drying at130 �C for 10 min in air. Following that, an active layer wasdeposited on top of the ZnO layer by spin-coating a solution of the

Fig. 1. (a) A schematic illustration of polymer-based solar cells (PSCs) using ZnO as electrnanocrystals; Electrical and optical properties of the constructed PSCs: (c) current den

P3HT (Rieke Metals) and PCBM (Nano-C) blend with a weight ratioof 1:0.8 in 1, 2-dicholorobenzene (30 mg/ml) at 600 pm for30 seconds in Ar-filled glovebox. The active layers annealed at150 �C for 10 min on the hot plate. Sometimes, a blend of PTB7 (1-Material) and PC71BM (Nano-C) was used as the active layer.Finally, a MoO3 layer (10 nm) and an Ag layer (100 nm) weredeposited with vacuum thermal evaporation. A metal shadowmask was used for the Ag deposition to define the device area of9 mm2. Current-voltage curves were obtained using a Keithley2400 source measurement unit operated by a custom madeLabview software. The solar cells were measured underAM1.5 condition simulated with an intensity of 100 mW/cm2.

2.3. Measurements and characterization

The shape of grown ZnO nanocrystals were checked withtransmission electron microscopy (TEM) on a JEOL 2100F. Themorphology of ZnO ETLs on ITO was characterized with scanningelectron microscopy (SEM) with a JEOL 6700F system. The opticalabsorption spectra and reflectance spectra of ZnO nanocrystalswere obtained with an ultraviolet-to-visible (UV-Vis) spectrome-ter (Cary 5000). The steady state photoluminescence (PL) spectraand Raman spectra were obtained from films on ITO glass using aRaman/PL spectrometer (LabRam-HR, Horiba Jovin-Yvon) excitedby 325 nm He-Cd laser for PL and by 514 nm Ar+ laser for Ramanspectroscopy. Output of Ti:sapphire laser at 900 nm was doubled to450 nm to serve as an excitation source for the time-resolvedphotoluminescence (TRPL) of the ITO/ZnO/P3HT:PCBM and thephotoluminescence was measured with streak camera. Here thebandwidth of the pulsed laser was 50fs and its repetition rate wasreduced from 80 MHz to 40 kHz by a pulse picker. The workfunction was obtained from Ultraviolet photoemission spectros-copy (UPS) measurements which were performed using the He Iphoton line (hn = 21.22 eV) of a He discharge lamp with bias of -7 V.

3. Results and Discussions

Generally, various synthetic routes including hydrothermalgrowth, sputtering, electrolysis, sol-gel, etc. have been applied tomodify the morphology of ZnO nanocrystals [19]. For example,

on transport layers (scale bar: 20 nm); (b) Colloidal solutions containing various ZnOsity, (d) external quantum efficiency and (e) reflectance spectra.

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colloidal ZnO nanorods (NRs) are synthesized from zinc acetatedihydrate (ZnOAc) with addition of strong base dissolved inmethanol and further refluxing at moderate temperature [23].Since, however, ZnO NRs tend to be agglomerated in the colloidalsolution, it is hard to achieve enhanced photovoltaic performancewith ZnO NRs [23]. In order to circumvent those problemsoriginated from the colloidal stability of ZnO NRs, we devisedbinary mixtures of ZnO nanocrystals with different shape as ETLs ofPSCs. Herein, we modified the morphology of ZnO ETLs to achievefurther enhancement of the device performance through applyingcomposite-like film morphology for efficient carrier transport inthe inverted PSCs. In particular, we demonstrated successfulmorphological control of ZnO ETLs for the inverted PSCs byapplying mixed colloidal solution of ZnO nanocrystals havingdifferent shape and dimension.

As illustrated in Fig. 1a, an inverted PSC device structure, ITO /ZnO (10 nm)/Active layer (170 nm) / MoO3 (10 nm) / Ag (100 nm),was used throughout this work. The thickness of ZnO ETLs wastypically 10 nm for ZnO NPs. The thickness of ZnO NR-based ETLvaries with position. Regarding to active layer materials, a blend ofP3HT and PCBM, one of the most studied and well-understoodsystem, was used to evaluate the electrical performance of ZnOETLs. Sometimes, a blend of recently developed low bandgappolymer, poly 4,8-bis(2-ethylhexyl)-oxybenzo1,2-b:4,5-b' dithio-phene-2,6-diyl3-fluoro-2-(2-ethylhexyl) carbonylthieno 3,4-b-thi-ophenediyl (PTB7) and 6,6-phenyl-C71-butyric acid methyl ester(PC71BM) was also used as an active layer. As shown in Fig. 1, theETLs are deposited with pure or mixed colloidal solutions of ZnONPs and ZnO NRs with 5 � 6 nm in diameter and 30 � 50 nm inlength. Binary solvent mixture of chlorobenzene and isopropylalcohol was chosen as processing solvent for ZnO nanocrystals[24]. While a clear and transparent colloidal solution was preparedwith ZnO NPs using binary solvent mixture, only cloudy opalescentsolution was obtained with ZnO NRs. Besides, a colloidal solution ofZnO nanocrystals having similar transparency was prepared byconcomitant dispersion of NPs and NRs in the same solvent. Then,it is clear that dispersion of ZnO nanocrystals is highly dependenton the shape of nanocrystals. Shape-dependent dispersion stabilityof nanocrystals can be found in various examples including goldnanocrystals [25]. Diverse solution stability among the shape-varied nanocrystals has been attributed to change of contact areabetween nanocrystals [25]. That is, the spherical NPs allowing onlyminimal contact area with other convex NPs have much largerstability against the elongated NRs with similar diameters, sincethe NRs have extended lateral surface area for contacting eachother in a side-by-side mode.

Current density (J)-voltage (V) curves of the three representa-tive P3HT:PCBM-based devices having ZnO ETLs prepared withdifferent colloidal solutions are shown in Fig. 1c and their deviceperformances are summarized in Table 1. The photovoltaicperformance of the device is recorded with PCE of 3.56% for theopen circuit voltage (Voc) of 0.61 V, the short circuit current density(Jsc) of 10.26 mA/cm2 and a fill factor (FF) of 56 % when ZnO ETL isprepared with ZnO NPs. It can be seen that when ZnO NRs-based

Table 1Device performances of PSCs based on P3HT:PCBM using various ZnO nanocrystals as eleStandard deviations of device parameters were inserted within the parentheses.

ZnO Jsc[mA/cm2]

Voc

[V]FF[%]

NPs 9.55(0.38) 0.60(0.01) 58(1.66)

NRs 11.19(0.13) 0.53(0.02) 43(0.01)

NPs+NRs 11.06(0.30) 0.60(0.01) 58(0.01)

aMeasured for best samplesbmeasured at �1 V

colloidal solution was used for deposition of ZnO ETL, Jsc of thedevice is effectively improved, but sacrificing the fill factor (FF). Asa result, the PCE is largely decreased down to 2.87% in contrast tothe previous report based on vertically aligned ZnO NRs [20].

Interestingly, both Jsc and FF are simultaneously improved byapplying binary mixture of ZnO NPs and NRs as an ETL. The weightratio of ZnO NPs and ZnO NRs is varied from 1:2 to 2:1 to optimizedevice performances. J-V curves of the devices fabricated by usingthe binary mixture of ZnO nanocrystals are shown in Fig. S1 andtheir cell performances are listed in Table S1 (See the SupportingInformation). The best device performance of 4.12 % was obtainedfrom binary mixture of ZnO NPs and NRs with weight ratio of 2:1.

The corresponding external quantum efficiency (EQE) curves ofthe three devices mentioned above are shown in Fig. 1d. Theintegral current density deduced from EQE curve and the globalreference spectrum yields a high Jsc of 11 mA/cm2which is within 5% in difference compared with measured Jsc of the same device.Comparison of EQE curves reveals that the quantum efficiency isimproved throughout the whole spectral response region of P3HT:PCBM blend after incorporation of ZnO NRs to ZnO ETL. Theenhanced Jsc in the device containing mixed ZnO nanocrystals maybe resulted from reduced bimolecular recombination [26] orimproved optical absorption. Since the optical absorption of ZnOnanocrystals is limited to wavelength <380 nm, the enhanced Jsccan hardly be ascribed to modification of optical absorption by ZnOETLs due to changing morphology or thickness. In particular, thereflectance spectra of the three devices shown in Fig. 1e are nearlycoincided and variation of optical absorption has little contributionto enhanced photocurrents of the devices containing ZnO NRs.

To explore the energy levels of ZnO nanocrystals, we performedultraviolet photoelectron spectroscopy (UPS) as shown in Fig. 2a.Upon UPS measurements, the valence band maximum (Ev) of ZnOnanocrystals is calculated from the cut-off energy (Ecutoff) and thevalence offset (Ev,offset) which is VBM = hn–Ecutoff +Ev,offset; Ev (ZnONPs) = 21.22 eV–19.09 eV + 5.48 eV = 7.61 eV for ZnO NPs and Ev(ZnO NRs) = 21.22 eV–18.83 eV + 5.46 eV = 7.85 eV. Conduction bandminimum (EC) is calculated from combination with abovecalculated Ev and the optical band gap measured from opticalabsorption spectra shown in Fig. S3 (See the Supporting Informa-tions). Interestingly, the optical band gap of ZnO NPs is notconsistent to that of ZnO NRs. The band gap of ZnO NRs agrees wellwith that of bulk ZnO, but ZnO NPs exhibit a larger band gap. Sincethe diameter of ZnO NPs estimated from the TEM image shown inFig. 1a is 4.9�0.7 nm which is comparable to the Bohr radius ofZnO, the extended band gap from quantum confinement isexpected. Then, we attributed difference in optical band gap ofZnO NPs to the quantum size effect [27]. As illustrated in theenergy level alignment presented in Fig. 2b, lower Ec of ZnO NRscompared with the known lowest unoccupied molecular orbital(LUMO) of PCBM supports efficient electron injection from PCBMto ZnO NRs. In addition, the Ev of ZnO NRs is much deeper than thatof ZnO NPs, which implies a larger hole injection barrier for carriertransport from PCBM to ZnO NRs. In brief, the energy levelalignments of ZnO NRs expect easier electron injection and

ctron transport layers. The data were averaged over nine cells of the same samples.

PCE[%]

Rsa

[ohm/cm2]Rsh

a

[ohm/cm2]Leakagea,b

[mA/cm2]

3.43(0.13) 9.2 904 262.61(0.11) 13.5 297 614.01(0.05) 7.8 1706 1

Fig. 2. (a) Ultraviolet photoelectron spectra of ZnO nanostructures and (b) corresponding band alignment for ZnO nanostructures and PCBM; (c) steady-state and (d) time-resolved photoluminescence of ITO/ZnO/P3HT:PCBM.

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efficient rejection of hole transport to ZnO NRs from active layers.To some extent, the efficient photoinduced electron transfer fromactive layers can be reflected with the quenching of photo-luminescence (PL) of the conjugate polymers [28]. Fig. 2c shows

Fig. 3. SEM images of the bare ITO substrate and ZnO layers deposited from NPs, NRs a

the steady state PL of P3HT:PCBM films deposited on ZnOnanaocrystals. The photoactive film deposited on ZnO NRs showsa lower PL intensity than the same film deposited on ZnO NPs.Furthermore, the time-resolved photoluminescence (TRPL) was

nd binary mixture of NPs and NRs. (All of scale bars are corresponding to 200 nm).

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also performed to explore the electron transfer at the active layer.All P3HT:PCBM films demonstrate the efficient charge transfer byshowing a faster PL decay than that of pristine P3HT (t = 810 ps). Inaddition, the active layer deposited on ZnO NRs shows much fasterdecay (tavg = 22 ps) than the same film deposited on ZnO NPs(tavg = 34 ps) does. The PL decay time of the P3HT:PCBM filmdeposited on ZnO NPs is slightly smaller than that of the same filmdeposited on substrates without ETLs found in previous study doneby J. Piris et al. [29]. The highly reduced PL decay time for the filmdeposited on ZnO NRs implies an efficient charge extraction fromthe photoactive layer, although the variation in nanoscalemorphology of the blend film may also contribute to chargetransport. That is, ZnO NRs provide more efficient route to extractelectrons from the photoactive layers compared with ZnO NPs,which accounts for enhanced photocurrents in devices with ZnONRs in ETLs.

The morphologies of the ZnO ETLs deposited from the colloidalsolution of ZnO nanocrystals are checked with scanning electronmicroscopy (SEM). Fig. 3 shows the SEM images of the ZnO ETLsprocessed with different colloidal solution of ZnO nanocrystals. Incontrast to the same film deposited with NPs, significantagglomeration of nanocrystals is observed for the ZnO layerdeposited with NRs. We have pointed out that agglomeration ofZnO NRs is revealed with opalescence of the colloidal solutionowing to shape-dependent instability of nanocrystals. As a result ofsignificant agglomeration in the colloidal solution containing ZnONRs, the NRs-based ZnO layer lacks sufficient surface coveragerevealing uncovered ITO surface. The ZnO ETL is usually required toplay a role of barrier for hole injection to the bottom electrode aswell as an efficient route for electron injection in the invertedgeometry of the polymer solar cells [9]. Then, direct contactbetween the active layer and the electrode usually resulted inrecombination of electron/holes depressing fill factors and opencircuit voltage of the PSCs [30,31]. Mixed colloidal solution of ZnONPs and ZnO NRs provides a uniform surface of the ZnO ETLwithout exposure of bare ITO surface. Then, the significantreduction in fill factors observed from the PSC device havingZnO NRs-based ETL is highly relieved, but without decreasingphotocurrent. As a result, the PCE is highly improved by applicationof mixed colloidal solution of ZnO NPs and ZnO NRs.

It is natural to expect the variation of light trapping propertiesof ZnO ETLs with different morphology of ZnO nanocrystals. Tocheck the light trapping properties, the haze properties of ZnOETLs have been characterized [32]. The haze factor is defined asthe ratio of diffuse transmittance to total optical transmittance.Higher haze factor means larger optical scattering which canimprove usage of light in the solar cells. As shown in Fig. 4b, ZnONRs clearly shows a larger haze factor compared with ZnO NPs. Asmentioned in the previous paragraph, ZnO NRs show highlyagglomerated morphology in deposited films and in the colloidal

Fig. 4. (a) Total transmittance and (b) haze factor o

solution. Then, the relatively larger haze factor of ZnO NRs can beattributed to the light scattering by NRs agglomerates. However,the level of haze is limited to be below a few percent and the lighttrapping properties owing to complex morphology of nanorod-containing ETL only provide a limited contribution to improve-ment of the device performance as shown in the reflectancespectra of devices shown in Fig. 1e. The variation of lightabsorption properties of devices, that is, impact of complexmorphology on the optical absorption of the photovoltaic devicesis trivial.

Large amount of defects including dangling bonds andimpurities are popularly present on the surface of nanocrystals.The defects on the surface of ZnO nanocrystals form intermediatelevels within the band gap and can be detected with photo-luminescence (PL). In particular, the photoluminescence of visiblelight from ZnO nanocrystals can be considered as an evidence ofsurface defects [31,33]. Fig. 5a shows the PL spectra of ZnO filmsdeposited with colloidal solution of ZnO nanocrystals. The PLintensity of ZnO NRs is much smaller than that of ZnO NPs. Inaddition, Raman spectroscopy of ZnO nanocrystals normalizedwith E2(high) mode intensity presented in Fig. 5b shows E1(LO)mode at 580 cm�1 from ZnO NPs. It is known that E1(LO) modeoriginated from second order Raman scattering is highly sensitiveto the presence of lattice defects including oxygen vacancies[34,35]. Then, the reduced Raman intensity of E1(LO) modeindicates reduction of defect concentration. Then, both PL andRaman spectroscopy indicate significant reduction of defectconcentration in ZnO nanocrystals during transformation ofmorphology from NPs to NRs on growth. The small defectconcentration of ZnO NRs can explain the large shunt resistance(Rsh) of the PSC containing mixture of ZnO NPs and NRs as an ETL.Also, the leakage current under reverse biased condition issignificantly minimized by modifying nanostructures of ZnO ETLsas shown in Fig. S4 and summarized in Table 1. However, the poorperformance of the PSC device with ETL based only on ZnO NRscan be attributed to the morphological defects providing directcontact between electrodes and active layers as mentionedearlier.

The universality of morphologically controlled ZnO ETLs for thePSCs was demonstrated with a low band gap polymer-based activelayer. In this study, we used a blend of PTB7:PC71BM as an activelayer with controlled thickness [36]. Fig. 6a presents the currentdensity versus voltage (J-V) characteristics of the PSCs withinverted structures adopting nanostructured ZnO as ETLs. The PCEof 9.02% was achieved with mixtures of ZnO NPs and ZnO NRs as anETL, while the device using ZnO NPs as an ETL showed PCE of 7.63%.As summarized in Table 2, the improved PCE is mainly due tohigher Jsc (17.56 mA/cm2) and larger FF (69%), because Voc remainsnear the same level. The EQE curves of the devices are shown inFig. 6b with the enhanced photocurrent resulted from enhanced

f ZnO ETLs deposited on ITO/glass substrates.

Fig. 5. (a) Photoluminescence and (b) Raman spectra of ZnO nanostructures.

Fig. 6. Electrical and optical properties of the constructed PSCs based on PTB7:PC71BM: (a) current density-voltage curves, (b) external quantum efficiencies.

Table 2Device performance of PSCs based on PTB7:PC71BM using various ZnO nanocrystals as electron transport layers. Standard deviation of photovoltaic parameters were insertedwithin the parentheses

ZnO Jsc[mA/cm2]

Voc

[V]FF[%]

PCE[%]

Rs

[ohm/cm2]Rsh

[ohm/cm2]

NPs 16.37(0.15) 0.72(0.01) 64(1.41) 7.63(0.16) 6.0 787NPs+ NRs 17.56(0.17) 0.73(0.01) 69(0.7) 9.02(0.26) 4.1 1250

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EQE from 65% to 71%. The enhanced EQE is coincided with theimproved Jsc of the device containing mixtures of ZnO NPs and ZnONRs as an ETL. In addition, the minimized series resistance and themaximized shunt resistance of the same device can explain theimproved fill factor. Hence, we demonstrated that the morpholog-ical control of ZnO nanostructures in PSCs with inverted geometrycan successfully enhance the device performance of PSCs based onlow band gap polymers.

4. Conclusions

We have demonstrated a large enhancement of the photovol-taic properties of the inverted PSCs where the morphologicallycontrolled ZnO nanocrystals are used as electron transport layers.The improved device performance is successfully demonstratedfrom the inverted PSCs based on the conventional P3HT:PCBMblend as well as from the inverted PSCs based on the low band gappolymer blend, PTB7:PC71BM. In particular, the Jsc and FF aresimultaneously improved by applying mixture of ZnO NPs and NRs.The energy level alignments between fullerene and ZnO ETL aremodified by morphological control of ZnO nanocrystals to providepaths of efficient electron injection and larger hole injectionbarriers. The large openings of ETLs popularly observed afterdeposition of ZnO NRs are successfully filled with simultaneousdeposition of ZnO NPs and NRs from the mixed colloidal solution of

ZnO NPs and NRs. In addition, the morphological control of ZnOnanocrystals also enhanced the light trapping properties andeffectively reduces defect concentration as shown from PL andRaman spectroscopy. Since ZnO nanocrystals can be deposited atambient condition using solution-based method and morphologi-cal control of ZnO nanocrystals can provide an efficient route forhigh performance PSCs, we anticipate that our findings cancontribute to developing practical application of PSCs includingflexible devices.

ACKNOWLEDGMENT

This work was supported by a grant (Code No.2014-063704)from the Global Frontier Research Program of the Ministry ofScience, ICT & Future Planning of the Republic of Korea and by agrant from the R&D Program for Strategic Core Technology ofMaterials funded by the Ministry of Trade, Industry, Energy of theRepublic of Korea.

Appendix A. Supplementary data

Supplementary data associated with this article can befound, in the online version, at http://dx.doi.org/10.1016/j.electacta.2015.08.151.

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