Solution-Processed Copper Iodide as an Inexpensive and Effective

Article pubs.acs.org/JPCC

Solution-Processed Copper Iodide as an Inexpensive and Effective Anode Buffer Layer for Polymer Solar Cells Weihai Sun,† Haitao Peng,‡ Yunlong Li,† Weibo Yan,† Zhiwei Liu,† Zuqiang Bian,*,† and Chunhui Huang† †

Beijng National Laboratory for Molecular Science, State Key Laboratory of Rare Earth Materials and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, People’s Republic of China ‡ Department of Energy and Resources Engineering, College of Engineering, Peking University, Beijing 100871, People’s Republic of China S Supporting Information *

ABSTRACT: In this work, a CuI anode buffer layer prepared from a facile solution-processed method was introduced in polymer solar cells (PSCs). The CuI films obtained under different spin-coating speeds and anneal treatments, and the performances of corresponding device fabricated with these CuI films were systematically investigated. The results showed that the devices based on CuI anode buffer layer displayed superior performance than conventional devices using poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) sulfonic acid (PEDOT/PSS). The highest power conversion efficiency of the PSCs based on CuI layer reached 4.15% under the illumination of AM 1.5G, 100 mW/cm2. Furthermore, the pristine CuI layer could be applied in PSCs immediately, which shortened the production time of PSCs.

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INTRODUCTION Polymer solar cells (PSCs) as a potential candidate for solar energy conversion technologies have attracted considerable attention in recent years due to their advantages, such as mechanical flexibility, low-cost, and easy fabrication.1−5 Despite the potential of PSCs, the highest power conversion efficiencies (PCE) of 9−10% obtained in recent research6 are still not satisfactory for commercial applications. To improve the performance of PSCs, a variety of strategies have been proposed, including the design of new light-harvesting materials,7,8 the new process of controlling film morphology,9,10 and the optimization of device structures.11 In addition, the interface between polymer materials and electrodes is crucial for device performance. One of the promising strategies is to insert an interfacial buffer layer12−14 between polymer materials and electrodes, which can improve PSCs by tuning the work function of electrodes,15,16 preventing undesired quenching of excitons in the surface of electrodes17 or diminishing the recombination of photogenerated carriers.18 In conventional PSCs, the most frequently used anode interfacial buffer layer is poly(3,4-ethylenedioxythiophene)/ poly(styrenesulfonate) (PEDOT/PSS) due to its high conductivity, high transparency, and high work function.19,20 Nevertheless, the acidic and hygroscopic nature of the PEDOT/PSS could cause device degradation.21,22 These deficiencies have led to extensive exploration of new materials to replace PEDOT/PSS. Inorganic materials such as NiO,17 MoO3,23,24 V2O5,25 and WO326 have been extensively used as anode buffer layers for their ambient stability and suitable © XXXX American Chemical Society

optical and electrical properties. Meanwhile, comparable or even higher PCE has been achieved by incorporating inorganic materials in PSCs comparing with the devices based on PEDOT/PSS. Due to the insolubility in most solvents, inorganic materials are usually deposited by vacuum techniques,17,27 which are substantially more expensive and more complicated than solution-based approaches. To form these inorganic buffer layers, several solution processing methods were reported by using different inorganic precursors,28 nanoparticles,29 or colloidal particles.30 And yet, high temperature treatment or long annealing time is necessary for post processing of the solution-processed inorganic buffer layer in order to remove solvent and achieve the crystalline phase. These preparation processes of inorganic buffer layer are complicated and incompatible with high throughput and scaleup production. There are only a few studies on the use of pristine or low temperature solution-processed inorganic material films for anode buffer layers31,32 in PSCs. Lately, Zilberberg et al.25 demonstrated an isopropyl alcohol solution processed vanadium oxide layer as anode buffer layer without any postdeposition heat treatment. The devices based on vanadium oxide layer had comparable efficiency and substantially higher stability, comparing with those devices using Special Issue: Michael Grätzel Festschrift Received: December 30, 2013 Revised: April 17, 2014

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deposited on the photoactive layer under a vacuum of 5 × 10−5 Pa. The active area of the device was 5 mm2. Characterization of the Thin Films and the Performance of the Solar Cells. The current−voltage curves of solar cells were measured by Keithley 4200 Semiconductor Characterization System and illuminated by a standard silicon solar cell calibrated Oriel 300 W solar simulator (Thermo Oriel 91160−1000) with AM 1.5G filter at an intensity of 100 mW/ cm2. All the photovoltaic performance measurements were performed in ambient air. The thickness of the thin films was characterized by using a KLA-Tencor α-Step Surface Profiler. SEM images and AFM images for the buffer layers were obtained using Hitachi S-4800 and SPA400 SPM (Seiko Instrument Inc.). The optical transmittance and the work function of PEDOT/PSS and CuI thin film were measured on a UV-3100 spectrophotometer (Shimadzu) and by ultraviolet photoelectron yield spectroscopy (Riken Keiki). The X-ray diffraction (XRD) patterns of the deposited CuI films were recorded on a D/MAX-2000 X-ray diffractometer with monochromated Cu Kα irradiation (λ = 1.5418 Å). Unless otherwise specified, the CuI films for characterization and device fabrication were prepared under the optimal spin-coating speed of 3000 rpm and without heat treatment.

PEDOT/PSS. However, the V2O5 layers were fabricated at ambient air for 1 h to undergo hydrolysis, which could not be applied in PSCs immediately after spin-coating. Therefore, there is an urgent need to find new inorganic interfacial materials amenable to solution-processing of PSCs. Cuprous iodide (CuI) with a wide band gap of 3.1 eV has three crystalline phases of α, β and γ. Among the different crystalline phases, γ-phase CuI behaves as a p-type semiconductor. It acts as an I-VII semiconductor with zinc blende structure. Currently, γ-CuI has been presently applied in the fabrication of light emitting diodes,33,34 organic catalysts,35,36 and vacuum fluorescent displays.37 Meanwhile, to solve the problems of electrolyte leakage due to the incomplete sealing and electrode corrosion caused by the use of liquid electrolyte, CuI also has been employed as the hole conductor in solidstate, dye-sensitized38−40 and perovskite solar cells.41 Recently, Xie et al.42 first introduced CuI by thermal evaporation as anode buffer layer to fabricate P3HT/PCBM blend solar cells. They found that the CuI buffer layer not only provides an Ohmic contact with the active layer, but also induces the selforganization of P3HT chains into a well-ordered structure. Although the PSCs with solution-processed CuI interfacial layer was obtained by spin-coating CuI nanoparticles dispersed in ethanol, the PCE of 2.6% is poorer compared with the 3.1% achieved by the device with the vacuum deposited CuI layer. In this paper, we introduced a solution-processed CuI film as the anode buffer layer of PSCs based on poly(3-hexylthiophene) (P3HT) as donor and [6,6]-phenyl-C60-butyric acid methyl ester (PCBM) as acceptor. The fabrication of anode buffer layer was a simple and time-saving procedure without any post-treatment. The influence factors of the morphologies of CuI layers and the corresponding devices have been optimized and investigated carefully.

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RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction patterns of CuI-3000 thin films (before annealing) and a-CuI-3000 films (after annealing).

EXPERIMENTAL SECTION

Materials. Indium tin oxide (ITO) coated glass substrates with a sheet resistance of 24 Ω/sq and an ITO thickness of 64 nm were purchased from CSG Holding Co., Ltd. CuI (purity >99%), PEDOT/PSS (Clevious P VP AI 4083), P3HT (Regioregular), and PCBM (>99%) were purchased from Sigma-Aldrich, H. C. Stark Company, Rieke Metals, and SigmaAldrich, respectively. All these commercially available materials were used as received without further purification. Device Fabrication. The ITO-coated glass substrates were cleaned ultrasonically in detergent, deionized water, acetone, and isopropyl alcohol sequentially and ultraviolet-ozone treated for 10 min. The PEDOT/PSS aqueous solution was filtered through a 0.45 μm filter and spin-coated at 4000 rpm for 60 s on the cleaned ITO substrates. Subsequently, the PEDOT/PSS film (about 40 nm) was baked at 140 °C for 10 min in the oven. The CuI films were prepared by spin-coating (1000, 3000, 5000 rpm) with copper iodide acetonitrile solution (10 mg/mL) on the ITO electrode (i.e., CuI-1000, CuI-3000, CuI5000, respectively). Annealed CuI layer (a-CuI) was fabricated through baking at 100 °C on a hot plate for 10 min compared with pristine CuI layer. The bulk-heterojunction layer was spincast onto the PEDOT/PSS or CuI layer at a coating speed of 1000 rpm for 60 s from a chlorobenzene solution containing 10 mg/mL of P3HT and 8 mg/mL PCBM (or 20 mg/mL of P3HT and 16 mg/mL PCBM). The substrates were annealed at 150 °C for 10 min. Finally, they were transferred to a vacuum chamber and 0.5 nm of LiF and 70 nm of Al were thermally

Figure 1. XRD patterns of CuI-3000 thin film (a) and a-CuI-3000 film (b).

Both of the CuI films exhibited an intense peak at the (111) reflection, which was assigned to γ-phase with a zinc blende face centered cubic structure.43 Comparing the later to the former, a significant increase in the intensity of the CuI (111) reflection was observed and another weak peak at the (222) plane was found in a-CuI-3000 films, which indicated that the crystallinity of CuI film was improved after the annealing process at 100 °C. The average crystallite size at the CuI (111) plane could be obtained by applying Scherrer’s formula. For annealing-free film, the average diameter of CuI crystallite can be calculated to about 31 nm. The particle size of CuI increased to 43 nm after the thermal treatment. And the average diameter of CuI crystallite was basically not influenced by the spincoating speed. The transmission spectra of PEDOT/PSS and various CuI films on quartz substrates are shown in Figure 2. All of these films were highly transparent in the visible range between 400 and 800 nm. Except for CuI-1000 film, other CuI films exhibited high transparency of more than 95%, which was B

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Figure 2. Optical transmission spectra of PEDOT/PSS films and various CuI films on quartz substrates.

better than the transmittance of 90% obtained by the PEDOT/ PSS film. The transmittance of CuI films reduced slightly after heat treatment, due to the enhancement of scattering caused by the increase of the crystallization and grain size of CuI films, which is in good agreement with the XRD results. An obvious hump at about 408 nm was observed in a-CuI-3000 film, which may be caused by the excitation of electrons from the subbands to the conduction bands of CuI.44 With the increase of the spin-coating speed, CuI films exhibited better transmittance in visible region due to the decrease of CuI thickness. The open-circuit voltage (Voc) of PSCs is related to the difference between the highest occupied molecular orbital (HOMO) of the donor and the lowest unoccupied molecular orbital (LUMO) of the acceptor, only when Ohmic contacts are formed in interface. Introducing the interfacial buffer layers can be an effective way to adjust the work functions of electrodes to achieve Ohmic contact. The ultraviolet photoelectron yield spectroscopy was used to assess for the work function of different anode interfacial layers (see Figure S1 in the Supporting Information). The HOMO energy level of CuI3000 film was found to be 5.1 eV from the photoemission cutoff, which was slightly smaller than the previously reported value.42 Although the measured work function of CuI (5.1 eV) was a bit lower than the work function of PEDOT/PSS (5.3 eV), both of them were higher than the work function of ITO (4.6 eV) and the HOMO of P3HT (5.0∼5.2 eV),7 which improved the alignment of the levels of energies in the interface between the anode and the donor materials and facilitated the hole extraction. Besides, the LUMO level of CuI (2.0 eV) was higher than most of the donor materials. Consequently, it can act as an electron blocking layer compared with the inadequate capability of PEDOT/PSS.45,46 Since the adhesion between the anode buffer layer and active layer is critical for good contact and stability, the water contact angle was measured on the disparate materials to explore the surface properties (see Figure S2 in the Supporting Information). It was found that the water contact angle varied from 14.3° for PEDOT/PSS film to 52.7° for CuI-3000 film, which indicated that the surface of CuI-3000 film was more hydrophobic than the surfaces of PEDOT/PSS film.42 The more hydrophobic CuI could provide a closer contact to the followed organic materials than that of PEDOT/PSS layer. To achieve a better understanding of the effect of interfacial layers on device performance, the morphologies of the different buffer layers were observed by AFM and SEM. The surface topographies of ITO, ITO/PEDOT/PSS, ITO/CuI-3000, and ITO/a-CuI-3000 are shown in Figure 3. Moreover, AFM

Figure 3. AFM topography images of (a) ITO, (b) ITO/PEDOT/ PSS, (c) ITO/CuI-3000, and (d) ITO/a-CuI-3000.

images in Figure S3 illustrated the three-dimensional structure of anode interfacial layers. The root-mean-square (RMS) roughness of bare ITO was 1.2 nm, while the RMS roughness slightly increased to 1.5 nm after being covered with the PEDOT/PSS layer. The flat surface was in contrast with the rough films spin-cast from CuI acetonitrile solution, in which RMS roughness increased sharply to 12.3 nm. Obviously, the CuI particles grew larger and the RMS roughness (19.9 nm) increased after the annealing process at 100 °C for 10 min, which is consistent with the result of XRD. As shown in Figure S3, some of CuI particles on the anode were as high as 100 nm, which means they can poke through the subsequently deposited organic active layer (ca. 100 nm) to reach the cathode. Figure 4 shows the SEM images of the PEDOT/PSS, CuI-1000, CuI-3000, CuI-5000, and a-CuI-3000 deposited on the ITO substrates. As shown, the PEDOT/PSS film exhibited a highly smooth surface and covered the ITO completely. Unlike the PEDOT/PSS film, the morphology of CuI films was dramatically influenced by spin-coating speed. The CuI molecules could form a more continuous film at low speed than that formed at high speed. As shown in Figure S4, CuI particles showed a tendency to form discontinuous CuI islands with the increase of the spin-coating speed. Moreover, there were many cracks in the CuI film after the heat treatment, which could reduce the anode interfacial layer coverage for ITO substrates. J−V characteristics of photovoltaic devices with a structure of ITO/CuI-1000 (noted as Device A), CuI-3000 (noted as Device B), CuI-5000 (noted as Device C), PEDOT/PSS (noted as Device D), and a-CuI-3000 (noted as Device E)/ P3HT/PCBM/LiF (0.5 nm)/Al (70 nm) were measured under dark and simulated AM 1.5 irradiation and are shown in Figure C

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Figure 5. J−V curves of devices with a structure of ITO/(different anode buffer layer: (A) CuI-1000; (B) CuI-3000; (C) CuI-5000; (D) PEDOT/PSS; (E) a-CuI-3000) /P3HT/PCBM/LiF (0.5 nm)/Al (70 nm), under dark (hollow) and simulated AM 1.5 irradiation (solid).

to achieving a high FF. According to Table 1, the devices with CuI layer displayed a lower Rs compared with device based on PEDOT/PSS layer, which resulted in higher FF and PCE. In addition, the J−V curves in the dark showed a typical diode behavior in all of the three devices in Figure 5. We observed that the dark current density in CuI devices was obviously higher than that of in PEDOT/PSS device with the additional voltage increases. It can be explained by the work function of PEDOT/PSS measured as 5.3 eV, which was much higher than the work function of ITO (4.6 eV) and created a high energy barrier between the ITO electrode and PEDOT/PSS. The work function of 5.1 eV for the CuI layer was more beneficial for improving the energy level alignment at the interface between the ITO electrode and active layer, where it can form an Ohmic contact and facilitate the hole transportation and collection. The device structure of the PSCs and the schematic energy diagram of the materials in devices are illustrated in Figure 6. The lowest unoccupied molecular orbital (LUMO) of CuI layer was estimated to be 2.0 eV from the work function (5.1 eV) and band gap (3.1 eV) of CuI, which was lower than that of PCBM and thus easy to block electrons away from ITO electrode. On the contrary, the electron blocking capability of PEDOT/PSS is uncertain since PEDOT/PSS could be considered as an electron collecting electrode.45,46 In addition to the above parameters, the hydrophobic nature of CuI can provide better compatibility with the subsequent hydrophobic organic material like P3HT, which could promote an ordered growth of P3HT layer and reduce the contact resistance between the anode interfacial buffer layer and active layer.47 The devices showed high FF and PCE, but poor Voc, when the anode buffer layer of PEDOT/PSS was replaced by CuI. At the same time, we noted the CuI layer induced a decrease of Rsh due to some leakage paths, which may be one reason48,49 for the decrease of Voc. As shown by the SEM study, the ITO electrode could be entirely covered with PEDOT/PSS, but not by CuI, which formed some leakage paths from bare ITO to the active layer. Furthermore, the leakage paths can also be confirmed by the AFM study. It showed that the small islands, which formed a high density of peaks, were observed in CuI layer, and they could dramatically affect the active layer and thus cause large leakage current and small Rsh. After the CuI film was annealed for 10 min, the Rsh and Voc in Device E became smaller owing to the rougher surface, as shown in Figure 3. Besides, the performance of CuI devices performance was influenced by the morphology of CuI layers obtained at

Figure 4. SEM images of (a) the PEDOT/PSS, (b) CuI-1000, (c) CuI-3000, (d) CuI-5000, and (e) a-CuI-3000 deposited on ITO substrates.

5, and the photovoltaic parameters of the corresponding devices are summarized in Table 1. As shown, the Device D fabricated with PEDOT/PSS layer exhibited an open circuit voltage (Voc) of 0.627 V, a short circuit current (Jsc) of 8.88 mA/cm2, a fill factor (FF) of 50.7%, and a power conversion efficiency (PCE) of 2.82%. In contrast, other devices incorporated with CuI layer exhibited the enhanced PCE than that of devices based on PEDOT/PSS layer. The improvement of device performance was basically attributed to the increase in FF. As a matter of fact, a shunt resistance, Rsh, and a series resistance, Rs, play a significant part in FF, which is one of the most important factors affecting performance of devices. An increase in Rsh, a decrease in Rs, or both is essential D

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Table 1. Photovoltaic Parameters of PSCs with Various Anode Buffer Layers (Device A, CuI-1000; Device B, CuI-3000; Device C, CuI-5000; Device D, PEDOT/PSS; Device E, a-CuI-3000) device A B C D E

Voc (V) 0.622 0.603 0.605 0.627 0.580

± ± ± ± ±

0.007 0.010 0.006 0.006 0.012

Jsc (mA/cm2) 8.67 9.77 9.75 8.88 9.92

± ± ± ± ±

0.39 0.14 0.29 0.31 0.21

FF (%) 52.1 58.1 49.2 50.7 56.7

± ± ± ± ±

1.4 2.6 2.2 2.5 1.6

PCE (%)

best PCE (%)

Rs (Ω cm2)a

Rsh (Ω cm2)a

± ± ± ± ±

2.99 3.60 3.15 2.97 3.50

14.3 7.86 9.12 20.8 8.13

427 453 242 2488 386

2.80 3.41 2.90 2.82 3.27

0.12 0.13 0.10 0.11 0.12

a

The series resistance, Rs, stands for the slope of the J−V curve at J = 0 for the best device and the shunt resistance, Rsh, stands for the slope of the J− V curve at J = Jsc for the best device.

The images of SEM and AFM showed that the surface of CuI film was rough, and the height of particles could be as high as 100 nm. These large particles could poke through the organic active layer to reach the cathode, which could lead to increase the chance of direct shorts and local high fields. Consequently, the carrier recombination and leakage current could occur in devices. In order to reduce the chances of the direct contact between the CuI layer and the Al cathode, the thickness of organic active layer was increased by spin-coating with a chlorobenzene solution containing 20 mg/mL of P3HT and 16 mg/mL PCBM. In these devices, the CuI layers were prepared at a spin-coating speed of 3000 rpm and without thermal treatment. The maximum PCE of 4.15%, along with a Voc of 0.61 V, a Jsc of 11.21 mA/cm2, and a FF of 60.5%, was achieved and the J−V curve under white light illuminations is shown in Figure 7, and also, the photovoltaic parameters of PEDOT/PSS

Figure 6. (a) Device structure of polymer solar cells. (b) Schematic energy diagram of various materials in devices.

different spin-coating speeds. With the increase of the speed of CuI from 1000 to 3000 rpm, PCE of the devices with CuI layers increased from 2.80% for Device A to 3.41% for Device B, and the Rs decreased from 14.3 to 7.86 Ω·cm2. This fact may be caused by the reduced thickness of CuI with the increase of the spin-coating speed. On the other hand, with the further increase of the spin-coating speed to 5000 rpm, PCE and FF of Device C decreased dramatically to 2.90 and 49.2%, respectively. Moreover, the Rsh dropped markedly from 453 to 242 Ω·cm2, whereas the Rs increased from 7.86 to 9.12 Ω· cm2. It was found that the uncovered area of ITO distinctly increased with the increasing spin-coating speed, which led to a lower Voc and smaller Rsh. The PCE data of Device B and Device D fabricated with CuI (at a spin-coating speed of 3000 rpm and without thermal treatment) and PEDOT/PSS, respectively, was subjected to pairwise t test. The results of pairwise t test are shown in Supporting Information, Table S1. As can been seen from the table, the P-value is 0, which is less than 0.05 and means there is a significant difference in PCE between CuI-based devices and PEDOT/PSS-based devices. As the mean of PCE of CuI-based device is higher than that of PEDOT/PSS-based device, CuI is a better anode interfacial layer than PEDOT/PSS.

Figure 7. J−V curve of PSC with the ITO/CuI-3000/P3HT/PCBM (20:16 mg/mL)/LiF (0.5 nm)/Al (70 nm).

and CuI devices with the doubled P3HT/PCBM concentration is displayed in Supporting Information, Table S2. Compared with the data in Table 1, both of the Jsc of PEDOT/PSS and CuI devices were increased due to the more light absorbed from the increased thickness of organic layer. The degree of increase of Jsc was greater in CuI devices owing to the greater interface between the CuI layer and organic layer, where it made a contribution to the separation of charge carrier. Meanwhile, the FF of the PEDOT/PSS device was decreased owing to a larger series resistance of the thicker organic layer. However, the FF of CuI device was remained unchanged since the CuI layer could improve P3HT chain packing and crystallization which may facilitate hole extraction and leads to a decreased interfacial resistance.42 And there will be plenty of room for improving the PCE of devices through controlling the surface morphology of CuI layer, which are actively pursued by us. E

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(12) Yip, H.-L.; Jen, A. K. Y. Recent Advances in Solution-Processed Interfacial Materials for Efficient and Stable Polymer Solar Cells. Energy Environ. Sci. 2012, 5, 5994−6011. (13) Ma, H.; Yip, H.-L.; Huang, F.; Jen, A. K. Y. Interface Engineering for Organic Electronics. Adv. Funct. Mater. 2010, 20, 1371−1388. (14) Po, R.; Carbonera, C.; Bernardi, A.; Camaioni, N. The Role of Buffer Layers in Polymer Solar Cells. Energy Environ. Sci. 2011, 4, 285−310. (15) Kim, J. S.; Park, J. H.; Lee, J. H.; Jo, J.; Kim, D.-Y.; Cho, K. Control of the Electrode Work Function and Active Layer Morphology via Surface Modification of Indium Tin Oxide for High Efficiency Organic Photovoltaics. Appl. Phys. Lett. 2007, 91, 112111. (16) Heo, S. W.; Lee, E. J.; Seong, K. W.; Moon, D. K. Enhanced Stability in Polymer Solar Cells by Controlling the Electrode Work Function Via Modification of Indium Tin Oxide. Sol. Energy Mater. Sol. Cells 2013, 115, 123−128. (17) Irwin, M. D.; Buchholz, D. B.; Hains, A. W.; Chang, R. P. H.; Marks, T. J. P-Type Semiconducting Nickel Oxide as an EfficiencyEnhancing Anode Interfacial Layer in Polymer Bulk-Heterojunction Solar Cells. Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 2783−2787. (18) Ratcliff, E. L.; Zacher, B.; Armstrong, N. R. Selective Interlayers and Contacts in Organic Photovoltaic Cells. J. Phys. Chem. Lett. 2011, 2, 1337−1350. (19) Zhang, F.; Johansson, M.; Andersson, M. R.; Hummelen, J. C.; Inganäs, O. Polymer Photovoltaic Cells with Conducting Polymer Anodes. Adv. Mater. 2002, 14, 662−665. (20) Li, G.; Shrotriya, V.; Huang, J.; Yao, Y.; Moriarty, T.; Emery, K.; Yang, Y. High-Efficiency Solution Processable Polymer Photovoltaic Cells by Self-Organization of Polymer Blends. Nat. Mater. 2005, 4, 864−868. (21) Wong, K. W.; Yip, H. L.; Luo, Y.; Wong, K. Y.; Lau, W. M.; Low, K. H.; Chow, H. F.; Gao, Z. Q.; Yeung, W. L.; Chang, C. C. Blocking Reactions Between Indium-Tin Oxide and Poly(3,4-ethylene dioxythiophene)/Poly(styrene sulphonate) with a Self-Assembly Monolayer. Appl. Phys. Lett. 2002, 80, 2788−2790. (22) Kemerink, M.; Timpanaro, S.; de Kok, M. M.; Meulenkamp, E. A.; Touwslager, F. J. Three-Dimensional Inhomogeneities in PEDOT/ PSS Films. J. Phys. Chem. B 2004, 108, 18820−18825. (23) Wong, K. H.; Ananthanarayanan, K.; Luther, J.; Balaya, P. Origin of Hole Selectivity and the Role of Defects in Low-Temperature Solution-Processed Molybdenum Oxide Interfacial Layer for Organic Solar Cells. J. Phys. Chem. C 2012, 116, 16346−16351. (24) Murase, S.; Yang, Y. Solution Processed MoO3 Interfacial Layer for Organic Photovoltaics Prepared by a Facile Synthesis Method. Adv. Mater. 2012, 24, 2459−2462. (25) Zilberberg, K.; Trost, S.; Schmidt, H.; Riedl, T. Solution Processed Vanadium Pentoxide as Charge Extraction Layer for Organic Solar Cells. Adv. Energy Mater. 2011, 1, 377−381. (26) Tan, Z.; Li, L.; Cui, C.; Ding, Y.; Xu, Q.; Li, S.; Qian, D.; Li, Y. Solution-Processed Tungsten Oxide as an Effective Anode Buffer Layer for High-Performance Polymer Solar Cells. J. Phys. Chem. C 2012, 116, 18626−18632. (27) Irwin, M. D.; Servaites, J. D.; Buchholz, D. B.; Leever, B. J.; Liu, J.; Emery, J. D.; Zhang, M.; Song, J.-H.; Durstock, M. F.; Freeman, A. J.; et al. Structural and Electrical Functionality of Nio Interfacial Films in Bulk Heterojunction Organic Solar Cells. Chem. Mater. 2011, 23, 2218−2226. (28) Girotto, C.; Voroshazi, E.; Cheyns, D.; Heremans, P.; Rand, B. P. Solution-Processed MoO3 Thin Films as a Hole-Injection Layer for Organic Solar Cells. ACS Appl. Mater. Interfaces 2011, 3, 3244−3247. (29) Stubhan, T.; Ameri, T.; Salinas, M.; Krantz, J.; Machui, F.; Halik, M.; Brabec, C. J. High Shunt Resistance in Polymer Solar Cells Comprising a MoO3 Hole Extraction Layer Processed from Nanoparticle Suspension. Appl. Phys. Lett. 2011, 98, 253308. (30) Steirer, K. X.; Chesin, J. P.; Widjonarko, N. E.; Berry, J. J.; Miedaner, A.; Ginley, D. S.; Olson, D. C. Solution Deposited NiO Thin-Films as Hole Transport Layers in Organic Photovoltaics. Org. Electron. 2010, 11, 1414−1418.

In conclusion, we demonstrated a high-efficiency P3HT/ PCBM solar cell incorporating CuI as an anode buffer layer by a simple solution process. The evaluations of CuI layers and PSC performance indicated that the CuI layers could act as an efficient anode interfacial layer, which showed higher performance than the devices based on PEDOT/PSS layers. The enhanced device performance was due to the improvement of the Jsc and FF. Furthermore, higher efficiency could be realized without any heat treatment for CuI layer, which indicated the preparation of CuI layer could be a convenient and time-saving process.

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ASSOCIATED CONTENT

S Supporting Information *

The UPS of bare ITO, PEDOT/PSS, and CuI layers, water contact angle of PEDOT/PSS and CuI layers, AFM images of the three-dimensional structure of ITO and different anode buffer layers, and SEM images of CuI films obtained at different rates are discussed. 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]. Tel.: (+86)-010-62753544. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Basic Research Program (2011CB933303) and the National Natural Science Foudation (NSFC) of China (90922004).

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REFERENCES

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