Solution-Processed and Low-Temperature Annealed CrOx as Anode

Article pubs.acs.org/JPCC

Solution-Processed and Low-Temperature Annealed CrOx as Anode Buffer Layer for Efficient Polymer Solar Cells Xiaohe Tu,† Fuzhi Wang,† Cong Li,† Zhan’ao Tan,*,† and Yongfang Li*,‡ †

State Key Laboratory of Alternate Electrical Power System with Renewable Energy Sources, Beijing Key Laboratory of Energy Security and Clean Utilization, North China Electric Power University, No. 2 Beinong Road, Beijing 102206, China ‡ Beijing National Laboratory for Molecular Sciences, Institute of Chemistry, Chinese Academy of Sciences, No. 21 North Street, Beijing 100190, China S Supporting Information *

ABSTRACT: A simple but efficient method has been first developed for the solution preparation of CrOx as anode buffer layer for polymer photovoltaic cells. The chromium acetylacetonate precursor can be transformed into CrOx upon thermal annealing at ∼60 °C, followed by ultravioletozone treatment. The leakage current of the device with the CrOx anode buffer layer was decreased, and short-circuit current density (Jsc) was significantly increased in comparison with t he device wit h the t radit io nal poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) buffer layer. By analyzing the current density− voltage characteristics of the device, it is found that the CrOx anode buffer layer can simultaneously decrease the series resistance and increase the parallel resistance of the device, in comparison with the device with PEDOT:PSS anode buffer layer. For the cells based on poly(3-hexylthiophene) (P3HT) as donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) as acceptor, the power conversion efficiency (PCE) can be improved from 3.71% (with PEDOT:PSS buffer) to 4.27% via introduction of CrOx buffer layer. The PCE of the device based on P3HT as donor and indene-C60-bisadduct (ICBA) as acceptor with CrOx anode buffer layer was further increased from 6.08 (with PEDOT:PSS buffer) to 6.55%. The results indicate that CrOx is a promising anode buffer layer for efficient and stable polymer solar cells. and more solution-processable metal oxides, such as CuOx,35 MoOx,36−40 ReOx,21 VOx,41,42 NiOx,31−33 SnOx,43 WO3,44 TiOx,45 and RuOx,46 have been prepared for stable PSCs electrode modification. Chromium oxide is one of the transition-metal oxides that displays very different energy levels depending on oxygen stoichiometry.47 In previous studies, thermally evaporated chromium oxide has been employed as an electronic tunneling layer and a protective layer in PSCs.48,49 Sputtered amorphous chromium oxide or nitrogen-doped chromium oxide can be used as anode buffer layer in PSCs, which acts as a holetransporting and electron-blocking layer, thereby improving the performance of the devices.50,51 However, the preparation processes of chromium oxide in the literatures are all based on high-cost vacuum techniques. Here we first report a simple and efficient method to prepare CrOx by spin coating the chromium acetylacetonate (Cr(acac)3) precursor, followed by thermal annealing at low temperature of 60 °C and ultraviolet-ozone (UVO) treatment. UVO treatment, similar to O2 plasma

1. INTRODUCTION Polymer solar cells (PSCs) show great potential for low-cost renewable energy source and have received considerable attention in recent years.1−5 Great progress has been made in PSCs with highest power conversion efficiency (PCE) over 9%.6 The device performance can be improved by developing novel donor and acceptor materials,7−14 design of new architectures,6,15−17 optimizing the morphology of the active layer,18,19 and modifying the interfaces between the active layer and the electrodes.20−25 It has been realized that the photovoltaic performance of the PSCs can be affected significantly by interface modification. The acidic poly(3,4ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) is one of the most widely used materials to modify indium tin oxide (ITO) to help hole collection. However, because of its corrosion to ITO and electrical inhomogeneity, PEDOT:PSS is detrimental to the stability of PSCs.26−29 Metal oxide semiconductors, such as MoO3,30 V2O5,30 NiO,31−33 and WO3,34 were used to replace PEDOT:PSS to achieve both efficient hole collection and better device stability. However, in previous studies, the preparation of these metal oxides mainly depends on the vacuum technology, which is incompatible with the concept of low-cost fabricated PSCs. In recent years, more © 2014 American Chemical Society

Received: November 27, 2013 Revised: April 9, 2014 Published: April 11, 2014 9309

dx.doi.org/10.1021/jp411675t | J. Phys. Chem. C 2014, 118, 9309−9317

The Journal of Physical Chemistry C

Article

transferred to a nitrogen-filled glovebox, and the photosensitive blend layer was prepared by spin coating (800 rpm) the ODCB solution of P3HT:PCBM or P3HT:ICBA (1:1 w/w, polymer concentration of 20 mg/mL for both active layers) on the modified ITO electrode for 30 s and then dried in covered glass dishes for slow growth of the active layer, resulting in a photoactive layer with a thickness of 230 nm. The photoactive layer was then annealed at 150 °C for 10 min. Finally, 10 nm of Ca and 100 nm of Al was thermally deposited on the photoactive layer as cathode under a base pressure of 5 × 10−5 Pa. The active area of the device is ∼4 mm2. The current density−voltage (J−V) characteristics of the PSCs were measured on a computer-controlled Keithley 236 Source Measure Unit (SMU). Device characterization was done in a glovebox under simulated AM1.5G irradiation (100 mW/ cm2) using a xenon-lamp-based solar simulator (Newport). The incident photon to current efficiency (IPCE) measurements were performed under ambient conditions at room temperature on a Stanford Research Systems model SR830 DSP lock-in amplifier coupled to a WDG3 monochromator and 500W xenon lamp. The light intensity at each wavelength was calibrated with a standard single-crystal Si photovoltaic cell. The thickness of the films was determined using an Ambios Technology XP-2 surface profilometer under an ambient atmosphere. X-ray photoelectron spectroscopy (XPS) analyses were performed on an ESCA Lab220i-XL electron spectrometer (VG Scientific) using a monochromatic Al Kα source (300 W). The XPS analysis chamber was operated at a base pressure of ∼3 × 10−9 mbar. The spectra were fitted using the freely available software XPSPEAK4.1. The crystal structure of the CrOx film was evaluated by X-ray diffraction (XRD) experiments with a Siemens D5005 diffractometer using Cu Kα radiation at 40 kV and 20 mA. Ultraviolet photoemission spectroscopy (UPS) was performed using an ESCA Lab250xi electron spectrometer under a pressure of ∼2 × 10−10 Pa. He I (21.22 eV) radiation line from a discharge lamp was used in UPS, and the energy resolution was 0.02 eV.

treatment, is a relatively new method to prepare transitionmetal oxide and metal hydroxide from their corresponding precursor,43,52−54 and we have prepared RuOx46 and ReOx21 through this method in recent reports. Compared with conventional vacuum high-temperature or vacuum sputtering methods, UVO treatment is more simple, practicable, and costeffective. With CrOx as anode buffer layer, the PSCs based on poly(3-hexylthiophene):(6,6)-phenyl-C61-butyric acid methyl ester (P3HT:PCBM) exhibit high PCE of 4.27%, and the devices based on P3HT:ICBA (indene-C60-bisadduct) show PCE of 6.55% under the illumination of AM 1.5G, 100 mW/ cm2.

2. EXPERIMENTAL DETAILS Patterned ITO glass (Rs = 10 Ω/□, CSG Holding) was used as the anode. Cr(acac)3, purchased from Alfa Aesar, was used as the precursor material for CrOx. PEDOT:PSS (Clevios P VP AI 4083), purchased from H. C. Starck, was used as the holetransport material. P3HT, purchased from Rieke Metals, was used as donor material. PCBM (Nano-C) and ICBA (synthesized according to the previously published procedure12) were utilized as acceptor materials. Figure 1 shows the molecular structures of Cr(acac)3, P3HT, PCBM, and ICBA. All commercially available materials were used as received without further purification.

3. RESULTS AND DISCUSSION Figure 2a presents the survey scan spectra of the Cr complexes before and after UVO treatment. The binding energy of the spectra is calibrated by the C 1s peak of the aliphatic carbons at 284.6 eV as reference. Figure 2b−d shows the high-resolution spectra of C 1s, Cr 2p, and O 1s core level, respectively. The background is subtracted from the XPS spectra by using a Shirley-type background subtraction. The C 1s spectra for Cr(acac)3 is deconvoluted into three individual components with binding energies of 284.6 (fwhm ∼1.7 eV), 285.4 (fwhm ∼1.3 eV), and 286.8 eV (fwhm ∼1.7 eV), which correspond to the methyl groups (C−CH3), central methene carbon (H3C− OCH−C−COCH3), and the carbonyl carbon (C2−CO) in the delocalized acetylacetonate ligands, respectively.54−56 The experimentally determined atomic ratio of C 1s (284.6 eV), C 1s (285.4 eV), and C 1s (286.8 eV) is 41:20:39, close to the expected theoretical value of 40:20:40 in the acetylacetonate ligand. The characteristics of C 1s spectra indicate the formation of coordination bond between the Cr3+ and the oxygen in the carbonyl group of acetylacetonate ligand. The O 1s spectrum (Figure 2d) is composed of two peaks. The main peak centered at a binding energy of 531.8 eV with a fwhm of 1.7 eV is attributed to the oxygen in acetylacetonate ligands. The smaller peak with higher binding energy of 533.6 eV

Figure 1. Molecular structures of Cr(acac)3, P3HT, PCBM, and ICBA.

The PSC devices were fabricated with the architecture of glass/ITO/PEDOT:PSS or CrOx/active layer/Ca/Al. PEDOT:PSS or CrOx was used as anode buffer layer. Devices without buffer layer were also fabricated for comparison. P3HT:PCBM or P3HT:ICBA were used as photoactive layer. The ITO substrate was cleaned by ultrasonic treatment in detergent, deionized water, acetone, and isopropanol, sequentially. Then, the precleaned ITO substrate was treated in a UV−ozone chamber for 20 min. For the PEDOT:PSS-modified PSCs, PEDOT:PSS aqueous solution was spin-coated at 2000 rpm for 45 s on the ITO substrate and then baked at 150 °C for 10 min in air. The thickness of PEDOT:PSS was ∼30 nm. For the CrOx-modified PSCs, CrOx film was prepared by spin coating the 1,2-o-dichlorobenzene (ODCB) solution of Cr(acac)3 (0.8 mg/mL) with rotating speed of 4000 rpm. The resulting Cr(acac)3 film thickness was ∼10 nm. After annealing at 60 °C for 10 min, the Cr(acac)3 film was subjected to UVO treatment for 30 min. Then, the modified substrate was 9310



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Figure 2. XPS of Cr(acac)3 films on silicon substrate. (a) Survey scan, (b) C 1s, (c) Cr 2p, and (d) O 1s core-level lines of the films before and after UVO treatment for 30 min.

(fwhm ∼1.1 eV) is assigned to surface hydroxyl in physically adsorbed water.57−59 In the Cr 2p high-resolution XPS spectra (Figure 2c) of the as-prepared Cr(acac)3, a pair of peaks is detected with the binding energy of 577.0 and 586.6 eV, corresponding to the 2p3/2 and 2p1/2 orbitals of Cr(III) ion in Cr(acac)3 chelate, respectively. However, great changes have taken place in the C 1s, O 1s, and Cr 2p characteristic spectra after the Cr(acac)3 sample undergoes 60 °C annealing for 10 min and UVO-treatment for 30 min. The C 1s spectrum of the treated sample is composed of five individual components with binding energies of ∼284.6 (fwhm ∼1.7 eV), 285.5 (fwhm ∼1.4 eV), 286.8 (fwhm ∼1.7 eV), 288.3 (fwhm ∼1.6 eV), and 289.3 eV (fwhm ∼1.7 eV). The peak at 284.6 eV corresponds to the nonfunctional carbon. The peak located at 285.5 eV is assigned to nonfunctional carbon linked to carbonyl or carboxyl carbon. The peak at 286.8 eV represents the carbonyl carbon (C2−C O) in the delocalized acetylacetonate ligands. The two peaks appearing at 288.3 and 289.3 eV are related to the carbon in carbonyl groups and carboxyl groups, which exist in the fragments of acetylacetonate ligands, respectively.55,60,61 The change in the XPS spectra of C 1s core level indicates the detachment of ligands from the central metal ion. For the sample after treatment, the high-resolution spectrum of Cr 2p is deconvoluted into four individual components. The peaks appear at binding energies of 576.2, 577, 577.8, and 579.3 eV in the Cr 2p3/2 orbital region and corresponding 585.9, 586.7, 587.5, and 588.7 eV in the Cr 2p1/2 orbital region, respectively. The intensity of Cr 2p3/2 peak is about twice of that of Cr 2p1/2, as usually expected.62 The first component (576.2 and 585.9 eV) is correlated to Cr(III) oxide (Cr2O3).63,64 The peaks with the binding energy of 577.0 and

586.7 eV correspond to the undecomposed Cr(acac)3. A Cr hydroxide species, Cr(OH)3, is recorded at higher binding energy of 577.7 and 587.5 eV.65 The peaks located at the highest energy of 579.3 and 588.7 eV indicate the existence of Cr(VI) species.62,66 Also, the magnitude of the Cr 2p spin− orbit splitting between 2p1/2 and 2p3/2 peaks observed in the experiment is consistent with the reported values 9.7 to 9.9 eV for Cr(III) compounds and 8.7 to 9.4 eV for Cr(VI) compounds.62,67,68 According to the calculated peak area, the ratio of these four species is 10:21:46:23; that is, the main component of the film surface is Cr hydroxide. Fitting of the O 1s spectra is complicated and requires at least four separate peaks. The first peak at 530.5 eV is connected with oxide ions O2− ions, in agreement with the presence of the chromium oxides. The second peak at 531.4 to 531.8 is characteristic of hydrated metal oxide species,69 which overlaps with that of the O 1s peak in the delocalized acetylacetonate ligands; therefore, the peak at 531.6 eV can be attributed to hydrated Cr(III) oxide, Cr(OH)3, and undecomposed Cr(acac)3.69 The peak at 532.6 eV is due to the O 1s in −CO and −C−OH groups in the fragments of acetylacetonate ligands.70 The final small peak at 533.5 eV is assigned to surface hydroxyl in physically adsorbed water.57−59 Consequently, according to changes in XPS spectra of C 1s, Cr 2p, and O 1s, it can be concluded that most of the Cr(acac)3 precursor was decomposed to Cr2O3 after it was annealed at 60 °C for 10 min in air and then UVOtreated for 30 min. Considering that the UVO treatment was performed in air, it can be inferred that the Cr(OH)3 is the result of hydration of the oxide to form hydroxides. The top of Figure 3a shows the chemical transformation of Cr(acac)3 into Cr2O3 through treatment. 9311



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the following equation: (αhν)2 = A(hν − Eg), where Eg is the optical band gap of the samples and A is a constant.71 The value of the bandgap of the CrOx film is estimated at 4.17 eV (as shown in Figure S2 in the Supporting Information). The electron affinity of the film was calculated to be 1.17 eV. To investigate the hole collection effect of the obtained CrOx anode buffer layer, we fabricated the PSCs with the configuration of ITO/CrOx/photoactive layer/Ca/Al (Figure 3a) and the energy diagram of which is shown in Figure 3c. The control devices without anode buffer layer or using PEDOT:PSS as anode buffer layer were also fabricated to study the effect of buffer layers on device performance. The obtained CrOx film bares an amorphous structure, as confirmed by XRD (Figure S3 in the Supporting Information). Upon application of the CrOx modification layer, the root-meansquare (rms) roughness of the ITO substrate can be reduced from 4.2 to 3.1 nm (Figure 3b). The smoother surface is beneficial to the formation of better contact between the anode and the photoactive layer for reduction in contact resistance and which also helps to decrease the leakage current of the devices.46,47 In addition, the decreased roughness is also conducive to the formation of high-quality upper photoactive layer.48 From the height images of the P3HT:PCBM on bare ITO (Figure 4a) and ITO/CrOx (Figure 4b) substrate, it can

Figure 3. (a) Configuration of the PSCs using CrOx as anode buffer layer. The top shows the transformation of Cr(acac)3 into CrOx by thermal annealing and UVO treating. (b) 5 μm × 5 μm AFM image of bare ITO with a rms roughness of 4.2 nm and CrOx on ITO substrate with a rms roughness of 3.1 nm. (c) Energy diagram of the device.

Figure 4. AFM images of (a,b) P3HT:PCBM and (c,d) P3HT:ICBA on bare ITO and ITO/CrOx substrate, respectively.

be seen that the rms roughness of the P3HT:PCBM film reduces from 18.9 to 13.4 nm upon modification of CrOx layer. Similarly, the rms roughness of P3HT:ICBA film can be decreased from 16.0 (Figure 4c) to 12.5 nm (Figure 4d) upon insertion of CrOx buffer layer. Devices with different buffer layers and active layers were fabricated to investigate the effect of CrOx buffer layer on the photovoltaic performance of PSCs. The device structures are as follows: (A) ITO/P3HT:PCBM/Ca/Al, (B) ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al, (C) ITO/CrOx/ P3HT:PCBM/Ca/Al, (D) ITO/P3HT:ICBA/Ca/Al, (E) ITO/PEDOT:PSS/P3HT:ICBA/Ca/Al, and (F) ITO/CrOx/

UPS measurements were conducted to determine the work function (WF) and ionization potential (IP) of the CrOx (as shown in Figure S1 in the Supporting Information). The WF of the Cr complex film is determined to be 4.94 eV. The high WF can be attributed to the formation of Cr2O3 and Cr(OH)3, where the interface dipole can lead to the increase in WF.52,53 The high WF is beneficial to the formation of ohmic contact between the anode and the photoactive layer. The IP is defined as the energy difference between valence band edge and vacuum level, which is 5.34 eV. The optical band gap is determined by a Tauc-plot analysis of the absorption spectra by 9312



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Figure 5. J−V curves of the PSCs based on P3HT:PCBM (a,c) and P3HT:ICBA (b,d) under the dark condition (a,b) and under the illumination of AM 1.5G, 100 mW/cm2 (c,d). IPCE spectra of the PSCs based on P3HT:PCBM (e) and P3HT:ICBA (f).

Table 1. Device Parameters of the PSCs Based on P3HT:PCBM (A,B,C) or P3HT:ICBA (D,E,F) with Different Buffer Layer in the Dark and under the Illumination of AM1.5G, 100 mW/cm2

a

devices

Voc (V)

Jsc (mA/cm2)

FF (%)

PCE (%)

Rs (Ω cm2)a

Rp (KΩ cm2)a

A: ITO/P3HT:PCBM/Ca/Al B: ITO/PEDOT:PSS/P3HT:PCBM/Ca/Al C: ITO/CrOx/P3HT:PCBM/Ca/Al D: ITO/P3HT:ICBA/Ca/Al E: ITO/PEDOT:PSS/P3HT:ICBA/Ca/Al F: ITO/CrOx/P3HT:ICBA/Ca/Al

0.57 0.62 0.62 0.83 0.85 0.87

8.69 9.24 11.25 9.72 9.52 10.74

56.7 64.8 61.2 56.5 74.9 70.3

2.81 3.71 4.27 4.56 6.06 6.55

4.83 2.00 1.95 5.4 3.7 2.61

12.9 35.6 219.5 9.9 45.1 147.9

Rs and Rp for PSCs in the dark are obtained at around 1 and 0 V, respectively.

P3HT:ICBA/Ca/Al. We compared the typical J−V characteristics of the devices under dark conditions with different buffer layers. For the devices without buffer layer, both P3HT and fullerene derivatives directly contact with the ITO anode; therefore, the electrons can transfer from the fullerene acceptor to the anode, which increases the leakage current. As can be seen from Figure 5a,b, the control devices without buffer layer, A (ITO/P3HT:PCBM/Ca/Al) and D (ITO/P3HT:ICBA/Ca/ Al), show maximum reverse dark current density among each system. Upon modification of PEDOT:PSS anode buffer layer, the current density of the devices under reversed bias is decreased by an order of magnitude, and this value can be

further decreased by using CrOx instead of PEDOT:PSS as anode buffer layer. The decrease in dark J−V characteristics indicates that the leakage current can be suppressed efficiently by using CrOx as an anode buffer layer. To further understand the effect of CrOx buffer layer, the series resistance (Rs) and parallel resistance (Rp) for the devices are calculated from the J−V curves under dark conditions at 0 and 1 V, respectively. From the results listed in Table 1, it is found that for the P3HT:PCBM-based devices, Rs for the CrOx-modified device is only 1.95 Ω cm2, which is comparable to that of PEDOT:PSSmodified device, and much smaller than that (4.83 Ω cm2) of the device without buffer layer. Meanwhile, the Rp for the 9313



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Figure 6. Reflectance spectra of CrOx- and PEDOT:PSS-modified PSCs based on (a) P3HT:PCBM and (b) P3HT:ICBA. The inset in panel a is the optical transmittance spectra of the PEDOT:PSS and the CrOx interfacial layer on ITO-coated glass.

We compared the IPCE spectra of the PSCs, as shown in Figure 5e,f. It is obvious that the intensity of the light response in the visible range exhibits the same variation trend with the Jsc of the PSC base on both P3HT:PCBM and P3HT:ICBA systems. The devices with CrOx anode buffer layer show the highest IPCE, which is consistent with the highest Jsc of the corresponding PSCs. However, it can be observed that the CrOx-modified PSCs show relatively stronger light response at a wavelength range longer than 400 nm. From the optical transmittance spectra (see the inset of Figure 6a), there exists a valley in the wavelength range of 370−530 nm for the ITO/ CrOx substrate, but in contrast, a transmittance peak appears in this wavelength range for the PEDOT:PSS-modified ITO substrate. Theoretically, lower transmittance is not conducive to the light absorption by the photoactive layer and therefore leads to lower IPCE. However, it contradicts the experimental IPCE results. To explicate the reason for this phenomenon, we measured the reflectance spectra for the PSCs with PEDOT:PSS or CrOx as anode buffer layer, as shown in Figure 6a,b. For the PSCs with PEDOT:PSS as anode buffer layer, a strong reflection peak appears in the wavelength range of approximately 400−550 nm. On the contrary, the reflection in this wavelength range is much weaker for the CrOx modified devices. The theoretically generated Jsc can be calculated by the expression72

CrOx-modified device increases significantly, as compared with the devices without buffer layer or with PEDOT:PSS buffer layer. The higher Rp also corresponds to the low leakage current across the PSCs. The decreased Rs and increased Rp are both conducive to the improvement of devices performance. The typical J−V characteristics of the PSCs under the illumination of AM1.5G, 100 mW/cm2, are shown in Figure 5c,d, and the device parameters are listed in Table 1. It is noted that the CrOx-modified PSCs show much better performance than the PEDOT:PSS-modified PSCs, indicating that CrOx can be used as efficient hole-collecting materials for PSCs. For P3HT:PCBM-based PSCs, the control device without buffer layer shows a Jsc of 8.69 mA/cm2, a Voc of 0.57 V, and an FF of 56.7%, resulting in a low PCE of 2.81%. The Jsc and Voc are both relatively low for P3HT:PCBM-based devices. The low Jsc could be ascribed to high Rs of the whole device. According to the literatures, the Voc of the PSCs based on P3HT:PCBM system is ∼0.60 V,49,50 and the highest value can reach as high as 0.66 V.51 Generally speaking, Voc is a function of carrier lifetime, regardless of device structure, with the maximum limit determined by the HOMO−LUMO gap. Therefore, the low Voc of 0.57 V for device A could be attributed to low carrier lifetime due to recombination at the ITO anode when a buffer layer is not used. Upon insertion of PEDOT:PSS as anode buffer layer, the four parameters of Jsc, Voc, FF, and PCE can be increased to 9.24 mA/cm2, 0.62 V, 64.8%, and 3.71%, respectively. The increased Voc compared with device A could be attributed to the insertion of PEDOT:PSS layer, which decreases the recombination at the anode due to the increased carrier lifetime. In addition, the Rs of the device is also decreased by the conductive PEDOT:PSS layer, which is part of the reason for the enhancement of Jsc. Using CrOx instead of PEDOT:PSS as anode buffer layer, the device shows a higher PCE of 4.27% with an increased Jsc of 11.25 mA/cm2, which is mainly due to the increased charge transportation induced by decreased Rs. The Voc is 0.62 V, the same as the PEDOT:PSS -modified device, which indicates that the CrOx layer can decrease the recombination at the anode just like PEDOT:PSS. Similar results were obtained with P3HT:ICBA as photoactive layer. The control devices without buffer layer and with PEDOT:PSS buffer layer show PCE of 4.56 and 6.06%, respectively. The use of CrOx instead of PEDOT:PSS as anode buffer layer can further improve the Jsc and Voc of the device to 10.74 mA/cm2 and 0.87 V, respectively, with the FF of 70.3%, leading to a PCE of 6.55%, higher than the reference device with PEDOT:PSS anode buffer layer.

Jsc =

∫ qF(λ)[1 − r(λ)]IPCE(λ) dλ

(1)

where q is the electron charge, F(λ) is the incident photon flux density at wavelength λ, and r(λ) is the incident light loss of light intensity due to absorption and reflection by the conducting glass support. To visualize the photocurrent efficiency enhancement effect of using CrOx as anode buffer layer instead of PEDOT:PSS, we calculated the enhancement of the photocurrent efficiency ΔJsc/Jsc(PEDOT:PSS) according to the following equation ΔJsc Jsc (PEDOT:PSS)

=

Jsc (CrOx ) − Jsc (PEDOT:PSS) Jsc (PEDOT:PSS)

∫ qF(λ)[1 − r(λ)CrO ]IPCE(λ)CrO dλ − ∫ qF(λ)[1 − r(λ)PEDOT:PSS ]IPCE(λ)PEDOT:PSS dλ) /( ∫ qF(λ)[1 − r(λ)PEDOT:PSS ]IPCE(λ)PEDOT:PSS dλ) =(

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(2)



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The calculated values of ΔJsc/Jsc(PEDOT:PSS) are 17.4 and 10.7% for the P3HT:PCBM- and P3HT:ICBA-based devices, respectively, which are a little lower than the experimentally measured values for both system. The reason is that the enhancement of photocurrent originates not only from the antireflective effect of CrOx but also other factors, such as reduced series resistance.

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4. CONCLUSIONS In conclusion, we demonstrate high-performance PSCs using solution-processed CrOx as an anode interfacial buffer layer. The CrOx layer exhibits high light transmittance and can tune the WF of ITO, which improves the hole collection and hence the performance of PSCs. High PCE of 6.55% was obtained for the PSCs based on P3HT:ICBA with the CrOx anode buffer layer. As CrOx is stable in the air, long-term stabilities can be expected for PSCs with CrOx anode buffer layer. These results demonstrate that the solution-processed CrOx layer is a promising interfacial material for highly efficient and stable solar cells.

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ASSOCIATED CONTENT

S Supporting Information *

UPS spectra of the CrOx film on pretreated ITO substrate, Tauc-plot for CrOx film, and XRD patterns of the Si substrate and Cr(acac)3 film on Si substrate. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*Z.T.: Tel/Fax: +86-10-61772186. E-mail: tanzhanao@ncepu. edu.cn. *Y.L.: Tel/Fax:+86-10-62536989. E-mail: [email protected]. Author Contributions

X.T. and F.W. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the NSFC (nos. 51173040, 91023039, 51303052), the Ministry of Science and Technology of China (863 project, nos. 2011AA050523, 2010DFB63530), SRFDP (20130036110007), Program for New Century Excellent Talents in University (NCET-12-0848), the 111 Project (B12034), Fundamental Research Funds for the Central Universities, China (2014ZD11, 2014MS35, 13ZD11) and Beijing Higher Education Young Elite Program (YETP0713).

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REFERENCES

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Solution-Processed and Low-Temperature Annealed CrOx as Anode

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