Solution-Processed Tungsten Oxide as an Effective Anode Buffer

Article pubs.acs.org/JPCC

Solution-Processed Tungsten Oxide as an Effective Anode Buffer Layer for High-Performance Polymer Solar Cells Zhan’ao Tan,*,† Liangjie Li,† Chaohua Cui,‡ Yuqin Ding,‡ Qi Xu,† Shusheng Li,† Deping Qian,† and Yongfang Li*,‡ †

State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, the New and Renewable Energy of Beijing Key Laboratory, North China Electric Power University, Beijing 102206, China ‡ CAS Key Laboratory of Organic Solids, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China S Supporting Information *

ABSTRACT: WO3 is an effective anode buffer layer to substitute PEDOT:PSS in both organic light-emitting diodes and polymer solar cells (PSCs). However, the vacuum deposition of the WO3 layer is not compatible with low-cost solution-processing technology for the roll-to-roll fabrication of PSCs. Here, we report, for the first time, a solution-processed WO3 (s-WO3) anode buffer layer that was prepared by spin-coating tungsten(VI) isopropoxide solution on an ITO electrode and then thermal annealing at 150 °C for 10 min in air, for the application in PSCs. The s-WO3 layer shows a high hole mobility of 9.4 × 10−3 cm2/V·s and high light transmittance. The photovoltaic performance of the buffer layer was investigated by fabricating the PSCs based on poly(3-hexylthiophene) (P3HT) as a donor and (6,6)-phenyl-C61-butyric acid methyl ester (PC60BM), (6,6)-phenyl-C71-butyric acid methyl ester (PC70BM), indene-C60 bisadduct (IC60BA), or indene-C70 bisadduct (IC70BA) as an acceptor. The PSCs with the s-WO3 anode buffer layer show enhanced photovoltaic performance in comparison with the devices with PEDOT:PSS as the anode buffer layer. The power conversion efficiency of the PSC based on P3HT/IC70BA with the s-WO3 anode buffer layer reached 6.36% under the illumination of AM 1.5G, 100 mW/cm2. The results indicate that s-WO3 is a promising solution-processable anode buffer layer material for highefficiency PSCs and for the fabrication of flexible PSCs.

1. INTRODUCTION

However, the majority of studies on WO3 buffer layers in both OLEDs and PSCs are using high-cost vacuum-based techniques,15−23 such as thermal evaporation, electron-beam evaporation, or hot filament vapor deposition, which is incompatible with low-cost solution-processed PSCs for future roll-to-roll scalable manufacturing. There is no solutionprocessed tungsten oxide reported to date for interfacial buffer layer applications either in PSCs or in OLEDs. Therefore, a solution-processable tungsten oxide modification layer is in great demand for high-performance PSCs and OLEDs. Herein, we prepared the tungsten oxide buffer layer on an ITO electrode by solution processing from tungsten(VI) isopropoxide solution (5% w/v in isopropanol) and then thermal annealing at 150 °C for 10 min in air. The solutionprocessed WO3 (hereafter simplified as s-WO3) layer is highly transparent in the visible range and shows an effective hole collection property. We studied the photovoltaic performance of the s-WO3 anode buffer layer by fabricating the PSCs based on poly(3-hexylthiophene) (P3HT) as an electron donor and four soluble fullerene derivatives, including (6,6)-phenyl-C61butyric acid methyl ester (PC60BM), (6,6)-phenyl-C71-butyric

Polymer solar cells (PSCs) have received considerable attention in recent years, and the power conversion efficiency (PCE) of the PSCs has been improved to over 7% by using new donor/ acceptor photovoltaic materials1−4 and new device structures.5−8 Inserting an interfacial buffer layer between active layers and electrodes is an effective strategy to improve the efficiency of PSCs with a given set of photoactive materials. Such a buffer layer can benefit PSCs by preventing undesired recombination of photogenerated carriers,9 and by tuning the effective workfunction of electrodes.10,11 Poly(styrenesulfonate)-doped poly(ethylenedioxythiophene) (PEDOT:PSS) is the widely used buffer layer on an indium tin oxide (ITO) electrode. However, PEDOT:PSS has a side effect on the stability of PSCs due to its corrosion to ITO and electrical inhomogeneity.12−14 Efforts to replace PEDOT:PSS with metal oxide semiconductors, such as MoO3, V2O5, NiO, and WO3, are currently receiving increased attention.9,15−23 For WO3, it has been employed as hole injection layers in organic light-emitting diodes (OLEDs) to reduce the injection barrier at the polymer/electrode interfaces and lead to higher device performance.16−18 In the case of PSCs, WO3 film can also significantly reduce the series resistance and improve the fill factor in conventional, tandem, and inverted PSCs.19−23 © 2012 American Chemical Society

Received: May 20, 2012 Revised: August 9, 2012 Published: August 17, 2012 18626

dx.doi.org/10.1021/jp304878u | J. Phys. Chem. C 2012, 116, 18626−18632

The Journal of Physical Chemistry C

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Figure 1. (a) Fourier transform infrared (FTIR) spectra of s-WO3. X-ray photoelectron spectroscopy (XPS) of s-WO3: (b) survey scan, (c) W 4f, and (d) O 1s core-level spectra.

acid methyl ester (PC70BM), indene-C60 bisadduct (IC60BA), and indene-C70 bisadduct (IC70BA), as an electron acceptor. The power conversion efficiency (PCE) of the PSC based on P3HT/IC70BA (1:1, w/w) with the s-WO3 anode buffer layer reached 6.36% under the illumination of AM 1.5G, 100 mW/ cm2, which is among the best values in the P3HT-based PSCs.

Scheme 1. Transformation of Tungsten(VI) Isopropoxide into Tungsten Oxide by Thermal Annealing at 150 °C for 10 min in Air

2. RESULTS AND DISCUSSION 2.1. Characterization of the s-WO3 Layer. Chemical components and the molecular structure of the s-WO3 sample were investigated by Fourier transform infrared (FTIR) spectra and X-ray photoelectron spectroscopy (XPS). Figure 1a shows the FTIR transmission spectra of the s-WO3 sample prepared by dropping tungsten(VI) isopropoxide solution on a KBr slice and then thermal annealing at 150 °C for 10 min in air. The IR bands in the range of 1400−400 cm−1 correspond to the fundamental vibrations of WO, W−O, and W−O−W bonds. The IR bands at 642, 817, and 902 cm−1 are assigned to stretching vibrations (ν) of W−O−W,24,25 while the IR band at 971 cm−1 is assigned to ν(WO).24,25 A broad band in the region of 3800−2900 cm−1 and the two peaks located at 1461 and 1631 cm−1 are assigned to ν(OH) and δ(OH) modes of the adsorbed water moisture.24,26 Worthy of attention, there are no characteristic IR peaks at 1124 and 1161 cm−1 for the isopropoxy group in the tungsten(VI) isopropoxide precursor in the whole IR spectra,27,28 indicating that the precursor, tungsten(VI) isopropoxide, has completely transformed into tungsten oxide (s-WO3) by the thermal annealing at 150 °C for 10 min in air, as shown in Scheme 1. Further evidence for the purity and composition of the sWO3 film was obtained by XPS, as shown in Figure 1b−d. The binding energies (BEs) obtained in the XPS analysis are corrected for specimen charge by referencing the C 1s to 284.6 eV. As shown in Figure 1b, the survey scan performed in the 0− 1200 eV BE range shows characteristic peaks of W, O, and C

elements. The fully oxidized WO3 (+6 oxidation state, W6+) has a strong spin−orbit doublet due to W 4f7/2 at 35.85 eV and W 4f5/2 at 38 eV, and a weak peak at ca. 42 eV related to W 5p3/2.29 The XPS are constrained by the W 4f7/2−W 4f5/2 spin− orbit separation of 2.15 eV, and the area ratio of the two peaks of each doublet being 0.75.29 In the XPS spectra of the s-WO3 (see Figure 1c), the peak corresponding to W 4f7/2 and W 4f5/2 are observed at 35.90 and 38.05 eV, respectively, with the W 4f7/2−W 4f5/2 spin−orbit separation of 2.1 eV and intensity ratio of 0.73. The results indicate that the tungsten in the sample is in the W6+ oxidation state and there is no lower valence state caused by oxygen vacancy.30,31 The O 1s XPS spectrum of the sample, as shown in Figure 1d, exhibits one strong peak at ca. 530.9 eV, which can be assigned to the oxygen atoms in stoichiometric WO3.32,33 The XPS results confirm that tungsten(VI) isopropoxide is decomposed into WO3 completely after the thermal annealing at 150 °C for 10 min in air. Figure 2 shows the transmittance spectra of the s-WO3 layer on ITO in comparison with those of the ITO substrate and the PEDOT:PSS layer on ITO. For the PEDOT:PSS-modified electrode, the increase of optical transmittance in the range of 350−450 nm was thought to be attributed to the smoothness of 18627



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agreement with previous studies reported in the literature.34 The s-WO3 layer is highly transparent in the visible range, and it is even better than the PEDOT:PSS layer in the wavelength range of 500−650 nm. The X-ray diffraction (XRD) pattern of the s-WO3 layer (see the inset of Figure 2) indicates that the sWO3 layer possesses an amorphous structure. However, the hole mobility of the s-WO3 layer is quite high, which reaches 9.4 × 10−3 cm2/V·s, measured by the space-charge-limited current (SCLC) method35 (the details of the measurement are described in the Supporting Information), as shown in the inset of Figure 2. The high hole mobility of the s-WO3 layer will benefit the hole collection and transportation in the PSCs. Surface morphologies of the s-WO3 layer and the photoactive layer on it were investigated by tapping-mode atomic force microscopy (AFM) measurements, as shown in Figure 3. The root-mean-square (rms) roughness of the ITO surface (Figure 3a) is 4.8 nm. After being modified with the s-WO3 layer, the surface becomes more flat and the rms roughness is decreased to 2.6 nm (Figure 3b), which is comparable with that (2.1 nm) of the PEDOT:PSS-modified ITO substrate. Obviously, the sWO3 layer also can effectively smooth the ITO surface as that of PEDOT:PSS, which can effectively avoid leakage of the devices. Figure 3c−f shows the AFM images of the surface of the P3HT:fullerene active layer on the s-WO3-modified ITO substrate. The rms for the active layer, P3HT:PC60BM, P3HT:PC70BM, P3HT:IC60BA, and P3HT:IC70BA, is 9.6, 9.3, 9.4, and 9.5 nm, respectively. Interestingly, the photoactive layer has a much larger roughness than that of the substrate, regardless that the substrate was modified with s-WO3. The increase in roughness is attributed to P3HT self-organization and phase separation of the blend. P3HT chains self-organizing into an ordered structure can significantly enhance the hole mobility.36 Meanwhile, the aggregation of fullerene molecules can form separate paths for electron transport.36 The nanoscale phase separation can finally result in the enhancement of Jsc of the PSCs.36 2.2. Photovoltaic Performance of the PSCs. The sandwich-structured PSCs with the structure of glass/ITO/sWO3/P3HT:fullerene/Ca/Al (see Figure 4a) were fabricated

Figure 2. Optical transmittance of the s-WO3 layer on ITO glass, ITO glass, and the PEDOT:PSS layer on ITO. Inset: X-ray diffraction (XRD) patterns of the s-WO3 thin film on a silicon substrate (left) and the current−voltage data from the devices of ITO/s-WO3 (60 nm)/ Au, plotted in the format ln(JL3/V2) vs (V/L)0.5, where J is the current density and L is the thickness of the s-WO3 layer (right) for the measurement of hole mobility of s-WO3.

the ITO thin film surface with the PEDOT:PSS modification layer (see Figure 3), which reduced dispersion and extinction of light caused by a rough surface. Our observation was in good

Figure 3. AFM images of (a) bare ITO without the buffer layer, (b) sWO3 on ITO, and (c) P3HT:PC60BM, (d) P3HT:PC70BM, (e) P3HT:IC60BA, and (f) P3HT:IC70BA photoactive layers on the ITO/ s-WO3 substrate. The scan size is 5 μm × 5 μm.

Figure 4. (a) Device structure of the polymer solar cells. (b) The molecular structures of P3HT, PC60BM, PC70BM, IC60BA, and IC70BA. (c) The schematic energy diagram of the materials involved in the PSCs. 18628



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for the studies of the photovoltaic performance of the s-WO3 anode buffer layer. In the PSCs, P3HT is used as a donor and four fullerene derivatives, PC60BM, PC70BM, IC60BA, and IC70BA, are used as an acceptor. The molecular structures of the donor and acceptor molecules are shown in Figure 4b. A schematic energy diagram of the related materials is illustrated in Figure 4c. The energy level of s-WO3 was determined by electrochemical cyclic voltammetry, as shown in Figure S1 in the Supporting Information. The energy levels for P3HT,37 PC60BM,38 PC70BM,39 IC60BA,38 and IC70BA39 were taken from the literature. When light irradiates on the photoactive layer of the PSCs through the ITO electrode, the active layer will absorb photons to produce excitons, and the excitons will diffuse toward and dissociate at the P3HT/ fullerene interface into electrons in the lowest unoccupied molecular orbital (LUMO) of the fullerene acceptor and holes in the highest occupied molecular orbital (HOMO) of the donor P3HT. The HOMO and the LUMO energy level for sWO3 is 7.0 and 4.4 eV, respectively. The high workfunction of the s-WO3 layer will benefit the hole collection at the anode/ active layer interface and will result in a high built-in electric field that benefits the charge transportation in the device.40 Thus, from the viewpoint of energy levels, the device should show high charge collection efficiencies on the electrodes.40−42 To investigate the effect of the s-WO3 interfacial buffer layer on the photovoltaic performance of the PSCs, we designed eight types of devices with different structures: (A) ITO/ P3HT:PC60BM (240 nm)/Ca (10 nm)/Al (100 nm), (B) ITO/PEDOT (30 nm)/P3HT:PC60BM (240 nm)/Ca (10 nm)/Al (100 nm), (C) ITO/s-WO3 (15 nm)/P3HT:PC60BM (240 nm)/Ca (10 nm)/Al (100 nm), (D) ITO/s-WO3 (15 nm)/P3HT:PC70BM (240 nm)/Ca (10 nm)/Al (100 nm), (E) ITO/P3HT:IC60BA (240 nm)/Ca (10 nm)/Al (100 nm), (F) ITO/PEDOT (30 nm)/P3HT:IC60BA (240 nm)/Ca (10 nm)/ Al (100 nm), (G) ITO/s-WO3 (15 nm)/P3HT:IC60BA (240 nm)/Ca (10 nm)/Al (100 nm), (H) ITO/s-WO3 (15 nm)/ P3HT:IC70BA (240 nm)/Ca (10 nm)/Al (100 nm). Devices A and E are control devices without the interfacial buffer layer; devices B and F are control devices with the PEDOT:PSS interfacial buffer layer. For the current density−voltage (J−V) curves measured in the dark, as shown in Figure 5a, the rectification ratio at ±1.0 V for devices A and E without the buffer layer is 1.9 × 102 and 2.6 × 102, respectively, whereas that of s-WO3-modified devices C and G reaches 1.6 × 104 and 1.2 × 104, respectively, 2 orders higher than that of the devices without the interficial buffer layer. These values are also higher than that of the PEDOT:PSS-modified devices (7.9 × 103 and 5.6 × 103 for device B and device F, respectively). The higher rectification ratio of the s-WO3-modified devices results from the higher injection current at the positive voltage and lower leakage current at the negative voltage.2,41 It should be pointed out that the s-WO3 modification layer in the devices was thermal-annealed at 150 °C for 10 min, and the annealing temperature was optimized in the temperature range of 90−190 °C, as shown in Figure S2 and Table S1 in the Supporting Information. J−V curves of devices A−H under the illumination of AM 1.5G, 100 mW/cm2 are shown in Figure 5b, and the device performance parameters (average of 16 devices) are summarized in Table 1. Device A without the buffer layer shows a PCE of 2.16%, a short-circuit current density (Jsc) of 9.48 mA/cm2, an open-circuit voltage (Voc) of 0.40 V, and a fill factor (FF) of 56.9%. The four parameters, Voc, Jsc, FF, and PCE, of device B

Figure 5. (a) Semilogarithmic J−V curves of the PSCs in the dark. (b) J−V curves of the PSCs under the illumination of AM 1.5G, 100 mW/ cm2. (c) Incident photon-to-converted current efficiency (IPCE) spectra of the PSCs.

with the PEDOT:PSS buffer layer are all increased to 0.59 V, 10.19 mA/cm2, 62.7%, and 3.77%, respectively. In comparison with device B, device C with the s-WO3 buffer layer showed a similar Voc of 0.58 V, a higher Jsc of 10.66 mA/cm2, and a higher FF of 70%, and the PCE increased to 4.33%, which is increased by 15% compared with that of the device with the PEDOT:PSS buffer layer. This trend is also seen in device D based on P3HT:PC70BM and with the s-WO3 buffer layer. The four parameters of Voc, Jsc, FF, and PCE for device D reached 0.60 V, 10.57 mA/cm2, 68.8%, and 4.35%, respectively. To further investigate the general suitability of the s-WO3 buffer layer in the PSCs, we select two more fullerene derivatives IC60BA and IC70BA as acceptors in the PSCs. For device E without the buffer layer, it shows a PCE of 4.41% with a Jsc of 9.66 mA/cm2, a Voc of 0.84 V, and an FF of 55.4%. As expected, device G and device H with the s-WO3 buffer layer show a dramatically enhanced performance. The Voc, Jsc, FF, and PCE for device G with IC60BA as acceptor are 0.84 V, 10.60 mA/cm2, 69%, and 6.14%, respectively, and those for device H with IC70BA as acceptor are 0.84 V, 10.85 mA/cm2, 69.8%, and 6.36%, respectively. These values are among the best in the P3HT-based PSCs. In comparison with device F 18629



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Table 1. Device Parameters of the PSCs with Different Structures under the Illumination of AM 1.5G, 100 mW/cm2

a

device structure

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Rpa (Ω cm2)

Rsa (Ω cm2)

A (ITO/P3HT:PC60BM/Ca/Al) B (ITO/PEDOT/P3HT:PC60BM/Ca/Al) C (ITO/s-WO3/P3HT:PC60BM/Ca/Al) D (ITO/s-WO3/P3HT:PC70BM/Ca/Al) E (ITO/P3HT:IC60BA/Ca/Al) F (ITO/PEDOT/P3HT:IC60BA/Ca/Al) G (ITO/s-WO3/P3HT:IC60BA/Ca/Al) H (ITO/s-WO3/P3HT:IC70BA/Ca/Al)

0.40 0.59 0.58 0.60 0.84 0.85 0.84 0.84

9.48 10.19 10.66 10.57 9.66 9.52 10.60 10.85

56.9 62.7 70.0 68.8 55.4 74.9 69.0 69.8

2.16 3.77 4.33 4.35 4.41 6.06 6.14 6.36

125.7 150.3 160.8 163.3 169.1 189.4 235.3 247.8

12.0 2.5 1.9 2.2 9.7 2.7 2.9 2.9

Parallel resistance (Rp) and series resistance (Rs) for PSCs under illumination are obtained at around 0 and 1 V, respectively.

4. EXPERIMENTAL SECTION 4.1. Materials. Patterned ITO glass with a sheet resistance of 10 Ω/sq was purchased from CSG Holding Co., Ltd. (China). Tungsten(VI) isopropoxide (5% w/v in isopropanol) was purchased from Alfa Aesar. P3HT was purchased from Rieke Metals. PEDOT:PSS (Clevious P VP AI 4083) was purchased from H. C. Stark Company. All of these commercially available materials were used as received without further purification. PC60BM and PC70BM were purchased from Nano-C. IC60BA38 and IC70BA39 were synthesized according to the literature. 4.2. Measurements and Instruments. The transmittance spectra of ITO, ITO/PEDOT:PSS, and ITO/s-WO3 were measured by a Hitachi U-3010 UV−vis spectrophotometer. The PEDOT:PSS film was prepared by spin-coating (2000 rpm) a PEDOT:PSS aqueous solution on the precleaned ITO glass. XPS data were obtained with an ESCA Lab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. The base pressure was about 3 × 10−9 mbar. The binding energies were referenced to the C 1s line at 284.8 eV from adventitious carbon. XRD patterns were obtained with a Siemens D5005 diffractometer using Cu Kα radiation at 40 kV and 20 mA. 4.3. Device Fabrication and Characterization of PSCs. The following eight types (devices A−H) of OSCs are designed: device A, ITO/P3HT:PC60BM/Ca/Al; device B, ITO/PEDOT/P3HT:PC60BM/Ca/Al; device C, ITO/s-WO3/ P3HT:PC60BM/Ca/Al; device D, ITO/s-WO3/ P3HT:PC70BM/Ca/Al; device E, ITO/P3HT:IC60BA/Ca/Al; device F, ITO/PEDOT/P3HT:IC60BA/Ca/Al; device G, ITO/ s-WO 3 /P3HT:IC 60 BA/Ca/Al; device H, ITO/s-WO 3 / P3HT:IC70BA/Ca/Al. The ITO glass was cleaned by sequential ultrasonic treatment in detergent, deionized water, acetone, and isopropanol. The precleaned ITO substrate was treated in an ultraviolet-ozone chamber (Ultraviolet Ozone Cleaner, Jelight Company) for 20 min. The PEDOT:PSS aqueous solution was filtered through a 0.45 μm filter and spin-coated at 2000 rpm for 60 s on the ITO electrode. Subsequently, the PEDOT:PSS film was baked at 150 °C for 10 min in air. The thickness of the PEDOT:PSS layer was around 30 nm. The s-WO3 film was prepared by spin-coating (4000 rpm) tungsten(VI) isopropoxide isopropanol solution (1.5 mg/mL) on the precleaned ITO glass, then baked in air at 150 °C for 10 min to complete the transformation into s-WO3. The optimized annealing temperature for s-WO3 was 150−170 °C, as shown in Figure S2 and Table S1 in the Supporting Information, and we set the annealing temperature of s-WO3 at 150 °C in our experiments. Subsequently, the substrate was transferred to the nitrogenfilled glovebox, and the photosensitive blend layer was prepared by spin-coating (800 rpm) the 1,2-dichlorobenzene solution of

with the PEDOT:PSS buffer layer (the Voc, Jsc, FF, and PCE are 0.85 V, 9.52 mA/cm2, 74.9%, and 6.06%, respectively), the Voc and FF just change a little, while the Jsc increases significantly and the overall PCE is improved. The increased Jsc of the sWO3-modified devices could be ascribed to enhanced charge collection at the s-WO3/P3HT interface and enhanced charge transportation, which is benefitted from the higher workfunction of the s-WO3 anode buffer layer. The increased FF of the s-WO3- and PEDOT:PSS-modified devices could result from the great decrease in series resistance (Rs) (from ca. 10 Ω cm2 for the device without the buffer layer to ca. 3 Ω cm2 for the devices with the anode buffer layer), and the increase in parallel resistance (Rp), as shown in Table 1.43 We compared the incident photon-to-converted current efficiency (IPCE) spectra of the devices A−H, as shown in Figure 5c. The IPCE results agree well with the Jsc of the PSCs mentioned above. The higher IPCE of the devices with the sWO3 buffer layer could also be ascribed to enhanced transmittance of ITO/s-WO3 (as shown in Figure 2), and the increased hole collection efficiency and the charge transportation efficiency, which benefitted from the higher workfunction of the s-WO3 buffer layer. It should be mentioned that, although the transmittance of s-WO3 in the wavelength range of 350−500 nm is lower than that of PEDOT:PSS (see Figure 2), the IPCE of the s-WO3-based device is similar with that of the PEDOT:PSS-based device in the wavelength range (see Figure 5c), which could result from the overall light redistribution of the incident light in the active layer of the devices.

3. CONCLUSIONS We demonstrate efficient polymer solar cells (PSCs) using solution-processed s-WO3 instead of acidic PEDOT:PSS as the anode interfacial buffer layer. The FTIR and XPS studies indicate that the precursor tungsten(VI) isopropoxide is decomposed into WO3 by thermal annealing at 150 °C for 10 min in air. The s-WO3 layer shows high hole mobility and high light transmittance. The PSCs with the s-WO3 anode buffer layer show enhanced performance in comparison with the PEDOT:PSS-modified devices, and the PCE of the PSC based on P3HT:IC70BA with the s-WO3 anode buffer layer reached 6.36% under the illumination of AM 1.5G, 100 mW/ cm2, which is among the best values in the P3HT-based PSCs. In addition, the s-WO3 buffer layer is easily prepared and suitable for the application in flexible devices that benefitted from the solution processability and low thermal annealing temperature (150 °C). This work gives a new option for the selection of the solution-processed anode buffer layer in designing higher efficiency and more stable PSCs. 18630



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P3HT and fullerene derivative (PC60BM, PC70BM, IC60BA, or IC70BA) (1:1 w/w, polymer concentration of 20 mg/mL) on the modified ITO electrode for 30 s. The active layer was dried in covered glass Petri dishes for slow growth and then annealed at 150 °C for 5 min. Finally, the substrate was transferred to a vacuum chamber and 10 nm of Ca and 100 nm of Al were thermally deposited on the photoactive layer under a base pressure of 5 × 10−5 Pa. The active area of the device is ca. 4 mm2. The current density−voltage (J−V) measurement of the devices was conducted on a computer-controlled Keithley 236 Source Measure Unit (SMU). Device characterization was done in a glovebox under simulated AM 1.5G irradiation (100 mW/ cm2) using a xenon-lamp-based solar simulator (from Newport Co., Ltd.). The IPCE was measured using a Stanford Research Systems model SR830 DSP lock-in amplifier coupled with a WDG3 monochromator and a 500 W xenon lamp. The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. All the measurements were performed under an ambient atmosphere at room temperature.

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ASSOCIATED CONTENT

S Supporting Information *

Hole mobility and electrochemical cyclic voltammetric measurements of the s-WO3 film, and the effect of s-WO3 annealing temperature on the photovoltaic performance of the PSCs. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Z.T.), [email protected] (Y.L.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the NSFC (Nos. 21004019, 51173040, 91023039, and 21021091), the Ministry of Science and Technology of China (Nos. 2011AA050523 and 2010DFB63530), and the Chinese Academy of Sciences (No. KGCX2-YW-399 + 9-1). Z.T. acknowledges the financial support from the Beijing NOVA Program (No. 2010B038), SRFDP (No. 20100036120007), and Fundamental Research Funds for the Central Universities, China (10MG32).

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REFERENCES

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Solution-Processed Tungsten Oxide as an Effective Anode Buffer

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