communications Solution-Processed Cu 2 O and CuO as Hole Transport Materials for Efficient Perovskite Solar Cells

communications www.materialsviews.com Perovskite Solar Cells Solution-Processed Cu 2 O and CuO as Hole Transport Materials for Efficient Perovskite Solar Cells Chuantian Zuo and Liming Ding * Perovskite solar cells provide a new choice for the photovoltaic industry due to their unique advantages such as ease of fabrication, low cost, and high efficiency. [1 6] Perovskite solar cells with a power conversion efficiency (PCE) of around 20% have been made via interface engineering, [6] composition engineering, [7] and hot-casting techniques. [8] Besides rapidly increasing PCE, a switchable photovoltaic effect in perovskite devices [9] and an outstanding charge carrier diffusion length in CH 3 NH 3 PbI 3 single crystal were recently reported, [10] attracting great interest in perovskite materials and devices. Large-area fabrication techniques such as printing and doctor-blade coating were developed for mass production of perovskite solar cells. [11,12] Besides high efficiency, cost and stability should be concerned for the commercialization of solar cells. High cost and low stability of commonly used hole transport material (HTM) 2,2,7,7 -tetrakis( N,N-di-p- methoxyphenylamine)-9,9 -spirobifluorene (spiro-meotad) and other organic HTMs limit the commercialization of perovskite solar cells. [13] Low-cost copper iodide (CuI) [14] and copper thiocyanate (CuSCN) [13] were used as HTMs to replace spiro-meotad and gave PCEs of 6.0% and 12.8%, respectively. Poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) is a common organic HTM, [8,15] and its acidity causes device degradation. Inorganic HTMs such as NiO [16] x and Cu-doped NiO x [17] were developed to replace PEDOT:PSS. A 15.4% PCE and enhanced device stability were achieved for the latter cells. [17] But the preparation of Cu:NiOx needs high temperature (550 C), thus increasing the fabrication cost and limiting industrial applications. Cuprous oxide (Cu 2 O) and copper oxide (CuO) are well-known p-type semiconductors. [18 21] Low-lying valence bands of Cu 2 O and CuO match well with CH 3 NH 3 PbI 3 and minimize the energy loss when being used as HTMs in perovskite solar cells. [20,21] Traditional methods for making Cu 2 O film are electrodeposition, thermal oxidation, sputtering, and metal-organic chemical vapor deposition. [19] These methods are complicated and costly, requiring advanced equipment. New approaches should be developed to reduce fabrication cost. Here, we report a facile low-temperature method to prepare Cu 2 O and CuO films and use them as HTMs in C. Zuo, Prof. L. Ding Laboratory of Nanosystem and Hierarchical Fabrication National Center for Nanoscience and Technology Beijing 100190, China E-mail: ding@nanoctr.cn DOI: 10.1002/smll.201501330 perovskite solar cells. Cu 2 O film was prepared via in situ conversion of CuI film in aqueous NaOH solution. CuO film was made by heating Cu 2 O film in air. Compared with the cells using PEDOT:PSS, perovskite solar cells using Cu 2 O and CuO as HTMs show significantly enhanced open-circuit voltage ( V oc ), short-circuit current ( J sc ), and PCE. PCEs of 13.35% and 12.16% were achieved for Cu 2 O and CuO cells, respectively, which are close to the highest PCE of perovskite solar cells using inorganic HTMs. Furthermore, Cu 2 O cells present improved stability. The preparation processes for Cu 2 O and CuO films are shown in Figure 1a. CuI film was prepared by spin coating its acetonitrile solution. [22] Being immersed in aqueous NaOH solution, CuI reacts with NaOH and produces Cu 2 O in situ, with the color changing from gray to yellow (Figure S1, Supporting Information). The Cu 2 O film was then rinsed with water, dried with nitrogen and heated at 100 C for 10 min. The thickness of the Cu 2 O film can be adjusted by changing the concentration of CuI solution. The thickness adjustment for Cu 2 O and CuI films is shown in Table S1 (Supporting Information). Repeating the spin coating and immersing steps of the CuI film can double the thickness of the Cu 2 O film. Cu 2 O can react with O 2 and form CuO after being heated at 250 C in air (Figure 1 a). CuO film can also be obtained by heating CuI film in air, while high temperature (>300 C) is required. UV vis absorption spectra for CuI, Cu 2 O, and CuO films are shown in Figure 2a. CuI, Cu 2 O, and CuO films show absorption onsets at 416, 600, and 840 nm, respectively. The absorption spectra of the films are consistent with the reported results. [18] The X-ray diffraction (XRD) peak at 25.9 for CuI film is assigned to the (111) plane of CuI crystal (Figure 2 b). The diffraction peak for CuI disappears after CuI reacting with NaOH. The emerging peaks at 29.9, 36.8, and 42.7 can be assigned to the (110), (111), and (200) planes of Cu 2 O. After being heated at 250 C for 30 min, the diffraction peaks of CuO show up. XRD patterns for Cu 2 O and CuO are consistent with the reported results. [23,24] UV vis spectra and XRD patterns for CH 3 NH 3 PbI 3 films on PEDOT, Cu 2 O, and CuO were studied (Figure 2c,d). CH 3 NH 3 PbI 3 fi lms on PEDOT, Cu 2O, and CuO were prepared under the same conditions. The thicknesses of the CH 3 NH 3 PbI 3 films were 200 nm. CH 3 NH 3 PbI 3 films on Cu 2 O showed stronger absorption than those on PEDOT and CuO. The enhanced absorption favors charge carrier generation in CH 3 NH 3 PbI3 film. XRD patterns for CH 3 NH 3 PbI 3 fi lms on PEDOT, Cu 2 O, and CuO coated glass substrates are almost the same (Figure 2 d). The diffraction intensity of CH 3 NH 3 PbI 3 fi lm on Cu 2 O is much stronger than 5528 www.small-journal.com 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2015, 11, No. 41, 5528 5532

www.materialsviews.com Figure 1. a) Preparation process for Cu 2 O and CuO films; b) device structure; and c) energy level diagram. [20,21,25 ] that of CH 3 NH 3 PbI 3 films on PEDOT and CuO. The intensity ratio for 14.5 diffraction peaks for CH 3 NH3 PbI 3 films on PEDOT, Cu 2 O, and CuO is 1:2.4:1.9, suggesting higher crystallinity for CH 3 NH 3 PbI 3 films on Cu 2 O and CuO. High crystallinity for perovskite films can facilitate charge carrier transport and enhance device performance. [10] Photoluminescence (PL) spectra for CH 3 NH 3 PbI 3 fi lms on glass, PEDOT, Cu 2 O, and CuO films were measured to compare charge carrier extraction capability of PEDOT, Cu 2 O, and CuO. As shown in Figure 3, Cu2 O and CuO can effectively quench the fluorescence of CH 3 NH 3 PbI3 as PEDOT, demonstrating that they can function as HTMs in perovskite solar cells instead of PEDOT. The morphology for CuI, Cu 2 O, CuO, and CH 3 NH3 PbI 3 films was studied by atomic force microscopy (AFM) ( Figure 4 ). After reacting with NaOH and forming Cu 2 O, CuI crystals become smaller (20 30 nm) (Figure 4 b). CuO nanocrystals are smaller than Cu 2 O nanocrystals (Figure 4 c). Surfaces for Cu 2 O and CuO films are more uniform than that of indium tin oxide (ITO) substrates (Figure S2a, Supporting Information), with a root-mean-square (RMS) roughness of 2.81 and 3.32 nm, respectively. Cu 2 O film prepared by using 1 mg ml 1 CuI solution cannot cover the surface of ITO glass well (Figure S2b, Supporting Information), and the film prepared by using 5 mg ml 1 CuI solution (Figure S2c, Supporting Information) shows similar morphology to that of the Figure 2. a) Absorption spectra for CuI, Cu 2 O, and CuO films; b) XRD patterns for CuI, Cu 2 O, and CuO films; c) absorption spectra for CH 3 NH 3 PbI 3 films on PEDOT:PSS, Cu 2 O and CuO; and d) XRD patterns for CH 3 NH 3 PbI 3 fi lms on PEDOT:PSS, Cu 2 O, and CuO. small 2015, 11, No. 41, 5528 5532 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com 5529

communications www.materialsviews.com Table 1. Performance data for perovskite solar cells using different HTMs. HTM V oc [V]J sc [ma cm 2 ] FF [%] PCE a) [%] None 0.98 11.48 69.72 7.84 (6.97 ± 0.89) PEDOT:PSS 0.95 14.82 78.39 11.04 (10.70 ± 0.28) Cu 2 O 1.07 16.52 75.51 13.35 (12.69 ± 0.32) CuO 1.06 15.82 72.54 12.16 (11.61 ± 0.47) a) The average PCE and standard deviations in the brackets were obtained from eight cells. Figure 3. PL spectra for CH 3 NH 3 PbI 3 films on glass, PEDOT:PSS, Cu 2 O, and CuO (excitation at 530 nm). film prepared using 10 mg ml 1 CuI solution (Figure 4 b). The uniform surfaces for Cu 2 O and CuO films favor the formation of a high-quality CH 3 NH 3 PbI 3 layer. AFM images for CH 3 NH 3 PbI3 films are shown in Figure 4 d f. CH 3 NH 3 PbI 3 films on PEDOT, Cu 2 O, and CuO show RMS roughnesses of 8.95, 8.02, and 9.18 nm, respectively. Similar morphologies were observed for CH 3 NH 3 PbI 3 films on Cu 2 O prepared using 10 mg ml 1 CuI and 5 mg ml 1 CuI solutions, respectively (Figures 4 e and S2e, Supporting Information). Perovskite solar cells with a structure of ITO/HTM/ CH 3 NH 3 PbI3 /PC 61 BM/Ca/Al were fabricated (Figure 1 b) and the thicknesses for Cu 2 O and CuO films were optimized by adjusting the concentration of CuI solution (Tables S2 and S3, Supporting Information). The thicknesses for Cu 2 O and CuO films were difficult to measure when less than 10 nm (CuI concentration below 10 mg ml 1 ), so the thicknesses for Cu 2 O and CuO films were represented by the concentration of CuI solution. Decreasing concentration of CuI solution can decrease thicknesses for Cu 2 O and CuO films, leading to lower series resistances ( R s ) and higher fill factors (FF) (Tables S2 and S3, Supporting Information). When using 1 mg ml 1 CuI solution, R s increased and got close to R s for a bare ITO glass-based device, which might be due to the surface of the ITO glass not being well covered with HTM (Figure S2b, Supporting Information). The heating temperature for Cu 2 O film was also optimized (Table S4, Supporting Information). Perovskite solar cells fabricated using Cu 2 O film dried at room temperature gave a PCE of 8.62%. The PCE was significantly enhanced by heating Cu 2 O at 100 C or above. Low performance for the cells using unheated Cu 2 O films might be due to residual water in the films. The performance data for solar cells using different HTMs are listed in Table 1. J V curves and external quantum efficiency (EQE) spectra are shown in Figure 5. Compared Figure 4. AFM height images for a) CuI, b) Cu 2 O, and c) CuO films and for CH 3 NH 3 PbI 3 fi lms on d) PEDOT:PSS, e) Cu 2 O, and f) CuO. 5530 www.small-journal.com 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim small 2015, 11, No. 41, 5528 5532

www.materialsviews.com which leads to reduced trap states. Trap states on the surface and at grain boundaries of perovskite films could cause the discrepancy, if any. [27 ] The efficiency stability for a Cu 2 O cell was tested (Figure S5b, Supporting Information). The facile and low-temperature preparation of Cu 2 O and the high performance for Cu 2 O cells make Cu 2 O more promising than PEDOT:PSS and CuO in practical applications. The stability for Cu 2 O cells was further studied since stability is a crucial issue in the solar cell industry. Solar cells with a structure of ITO/HTM/CH 3 NH 3 PbI 3 /PC 61 BM/Al were studied. A Ca layer was not used to enhance device stability. The solar cells were tested in air and stored in a N 2 glovebox. The average PCE for PEDOT cells decreased from 10.11% to 6.79% after being stored for 70 d (Figure S6, Supporting Information). The average PCE for Cu 2 O cells decreased from 11.02% to 9.96%. The PCE for Cu 2 O cells remains above 90% of the initial value, while the PCE for PEDOT cells remains 67%. The hygroscopic and acidic properties for PEDOT:PSS account for the lower stability for PEDOT cells. [17 ] In summary, we develop a novel and facile method to prepare Cu 2 O and CuO films and use them as HTMs in perovskite solar cells. Cu 2 O and CuO cells show significantly enhanced V oc, J sc and PCE compared with PEDOT cells. A PCE of 13.35% was achieved for Cu 2 O cells, which is among the top PCEs for perovskite solar cells using inorganic HTMs. Cu 2 O cells also show nice stability, with a PCE remaining above 90% of the initial value after being stored for 70 d. The good stability and high PCE for Cu 2 O cells make Cu 2 O a promising material for further application in perovskite solar cells. Figure 5. a) J V curves and b) EQE spectra for perovskite solar cells using PEDOT:PSS, Cu 2 O and CuO as HTMs, respectively. with the cells using PEDOT, Cu 2 O, and CuO as HTMs, the cells using bare ITO showed much lower J sc, which resulted from weak hole extraction. Perovskite solar cells using PEDOT produced a V oc of 0.95 V, a J sc of 14.82 ma cm 2, a FF of 78.39%, and a PCE of 11.04%. Cu 2 O cells showed significantly enhanced V oc (1.07 V) and J sc (16.52 ma cm 2 ), leading to higher PCE (13.35%). Enhanced V oc (1.06 V), J sc (15.82 ma cm 2 ), and PCE (12.16%) were also obtained in CuO cells. Enhanced V oc results from the deep-lying valence bands for Cu 2 O and CuO (Figure 1 c). Enhanced J sc results from higher crystallinity of CH 3 NH 3 PbI 3, which favors light harvesting and charge carrier transport. Higher transmittance for Cu 2 O and CuO also benefits J sc (Figure S3, Supporting Information). The integrated photocurrents for the cells using PEDOT, Cu 2 O, and CuO are 14.12, 16.41, and 14.89 ma cm 2, respectively, which are consistent with J sc values obtained from J V measurements. Cu 2 O cells presented better performance than CuO cells, which might be due to higher crystallinity and better morphology of the CH 3 NH 3 PbI 3 layer on Cu 2 O. J V curves under forward and reverse bias scans were measured to study the discrepancy (Figure S4, Supporting Information). J V curves measured under forward and reverse bias scans are nearly the same. This might be due to the uniform surface of the CH 3 NH 3 PbI 3 fi lm and high crystallinity of CH 3 NH 3 PbI 3 realized by using our method, [25,26 ] Supporting Information Supporting Information is available from the Wiley Online Library or from the author. Acknowledgements This work was supported by the National Natural Science Foundation of China (U1401244 and 21374025). [1] A. Kojima, K. Teshima, Y. Shirai, T. Miyasaka, J. Am. Chem. Soc. 2009, 131, 6050. [2] J.-H. Im, C.-R. Lee, J.-W. Lee, S.-W. Park, N.-G. Park, Nanoscale 2011, 3, 4088. [3] M. M. Lee, J. Teuscher, T. Miyasaka, T. N. Murakami, H. J. Snaith, Science 2012, 338, 643. [4] J. Burschka, N. Pellet, S.-J. Moon, R. Humphry-Baker, P. Gao, M. K. Nazeeruddin, M. Grätzel, Nature 2013, 499, 316. [5] M. Liu, M. B. Johnston, H. J. Snaith, Nature 2013, 501, 395. [6] H. Zhou, Q. Chen, G. Li, S. Luo, T.-B. Song, H.-S. Duan, Z. Hong, J. You, Y. Liu, Y. Yang, Science 2014, 345, 542. [7] N. J. Jeon, J. H. Noh, W. S. Yang, Y. C. Kim, S. Ryu, J. Seo, S. I. Seok, Nature 2015, 517, 476. small 2015, 11, No. 41, 5528 5532 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim www.small-journal.com 5531

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Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2015. Supporting Information for Small, DOI: 10.1002/smll.201501330 Solution-Processed Cu 2 O and CuO as Hole Transport Materials for Efficient Perovskite Solar Cells Chuantian Zuo and Liming Ding *

Copyright WILEY-VCH Verlag GmbH & Co. KGaA, 69469 Weinheim, Germany, 2013. Supporting Information Solution processed Cu 2 O and CuO as hole transport materials for efficient perovskite solar cells Chuantian Zuo and Liming Ding* C. Zuo, Prof. L. Ding Laboratory of Nanosystem and Hierarchical Fabrication, National Center for Nanoscience and Technology, Beijing 100190, China. E-mail: ding@nanoctr.cn Preparation of perovskite precursor Perovskite precursor solution was prepared using a modified recipe. [1] CH 3 NH 3 I and PbI 2 (equimolar) were dissolved in DMF with a concentration of 0.6 M. The solution was stirred at 60 C for 12 h inside a N 2 glovebox. After cooling to room temperature, 20 mg/ml NH 4 Cl was added into precursor solution and stirred for 1 h. Materials characterization Absorption spectra of the films were recorded on a Shimadzu UV-1800 spectrophotometer. Photoluminescence spectra were measured on a fluorescence spectrophotometer (LS55, Perkin Elmer). X-ray diffraction (XRD) was performed on a RIGAKU D/MAX-TTRIII (CBO) with Cu-Kα radiation. Atomic force microscope (AFM) was performed on a Dimension 3100 microscope (Veeco) (tapping mode). Film thicknesses were measured with an Alpha-Step profilometer (KLA Tencor D-120). Solar cells fabrication and measurements Patterned ITO glass with a sheet resistance of 15 Ω sq -1 was cleaned by ultrasonics in detergent, deionized water, acetone, isopropanol sequentially and then treated with UV-ozone for 10 min. For the preparation of PEDOT:PSS films, a 30 nm thick PEDOT:PSS layer was formed on ITO glass by spin coating an aqueous dispersion (PEDOT:PSS, Clevios TM P VP AI 4083) onto ITO glass (4000 rpm for 30 s). PEDOT substrates were dried at 150 C for 10 min, 1

and then transferred into a N 2 glovebox. For Cu 2 O films, the solution of CuI in acetonitrile (1~40 mg/ml) was spin coated onto ITO glass (2000 rpm for 30 s). The substrate was immersed in 10 mg/ml aqueous NaOH solution for 5 s, rinsed with distilled water and dried by N 2. Then the substrate was transferred into glovebox and heated at 100 C for 10 min. CuO film was prepared by heating Cu 2 O film at 250 C for 30 min in air. CH 3 NH 3 PbI 3 layer was prepared by spin coating precursor solution onto HTL (3000 rpm for 60 s). Then the substrate was heated at 100 C for 30 s. PC 61 BM solution (20 mg/ml in chlorobenzene) was spin coated onto CH 3 NH 3 PbI 3 layer at 1500 rpm for 30 s. Finally Ca (10 nm) and Al (100 nm) were deposited onto PC 61 BM layer through a shadow mask (10-4 Pa). The effective area for the cells is 4 mm 2. J-V curves were measured using a computerized Keithley 2420 SourceMeter. Device characterization was performed in air using a solar simulator (Newport 91159A, AM 1.5G, 100 mw/cm 2 ). The illumination intensity was determined using a monocrystalline silicon cell (Oriel 91150, 2 2 cm 2 ) calibrated by NREL. EQE spectra were measured using a QE-R3011 system (Enli Technology). 2

Figure S1 Photographs for bare substrate and CuI, Cu 2 O and CuO coated substrates. Table S1 Effect of CuI concentration on thickness of CuI and Cu 2 O films. CuI concentration [mg/ml] 40 30 20 10 CuI thickness [nm] 47 37 25 15 Cu 2 O thickness [nm] 35 23 16 10 3

Figure S2 AFM height images for bare ITO (a), Cu 2 O films on ITO prepared by using 1 mg/ml CuI (b) and 5 mg/ml CuI (c) solutions, CH 3 NH 3 PbI 3 films on Cu 2 O prepared by using 1 mg/ml CuI (d) and 5 mg/ml CuI (e) solutions. 4

Table S2 Effect of Cu 2 O film thickness on performance of Cu 2 O cells. The thickness of Cu 2 O film was adjusted by CuI concentration. CuI concentration [mg/ml] V oc [V] J sc [ma/cm 2 ] FF [%] PCE [%] R s [Ω cm 2 ] 20 1.03 13.55 63.93 8.92 6.92 10 1.04 15.16 67.49 10.64 6.56 5 1.07 14.89 75.90 12.09 5.24 2 1.06 15.28 76.20 12.34 5.20 1 1.05 13.40 74.72 10.51 6.16 without CuI 0.99 12.12 62.81 7.54 10.44 Table S3 Effect of CuO film thickness on performance of CuO cells. The thickness of CuO film was adjusted by CuI concentration. CuI concentration [mg/ml] V oc [V] J sc [ma/cm 2 ] FF [%] PCE [%] R s [Ω cm 2 ] 10 1.01 9.56 47.91 4.63 17.24 5 1.00 13.09 68.99 9.03 6.80 2 1.05 15.03 75.38 11.90 4.68 1 1.03 15.46 71.35 11.36 5.52 Table S4 Effect of heating temperature of Cu 2 O film on performance of solar cells. Temperature ( C) V oc [V] J sc [ma/cm 2 ] FF [%] PCE [%] r.t. a 0.97 16.03 55.41 8.62 100 1.05 16.32 76.41 13.09 150 1.06 16.13 71.90 12.29 200 1.06 16.59 68.62 12.07 a Room temperature. 5

Figure S3 Transmittance for 30 nm PEDOT:PSS, 10 nm Cu 2 O and 10 nm thick CuO films. 6

Figure S4 J V curves for perovskite solar cells using PEDOT:PSS (a), Cu 2 O (b) and CuO (c) as HTMs under forward (from short circuit to open circuit) and reverse (from open circuit to short circuit) scans with a scan rate of 0.13 V/s. 7

Figure S5 (a) J-V curves for a Cu2O cell under forward and reverse scans; (b) the test on efficiency stability. 8

Figure S6 Performance data change for perovskite solar cells using PEDOT:PSS and Cu 2 O as HTMs and being stored in N 2 glovebox. The average value and standard deviation were obtained from 8 cells. References [1] C. Zuo, L. Ding, Nanoscale, 2014, 6, 9935. 9