Facile preparation of organometallic perovskite films and high-efficiency solar cells using solid-state chemistry

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1 Nano Research DOI /s Nano Res 1 Facile preparation of organometallic perovskite films and high-efficiency solar cells using solid-state chemistry Lei Chen 1,2,, Feng Tang 2,, Yixin Wang 2,3, Shan Gao 2, Jinhua Cai 2 ( ) and Liwei Chen 2 ( ) Nano Res., Just Accepted Manuscript DOI: /s on December Tsinghua University Press 2014 Just Accepted This is a Just Accepted manuscript, which has been examined by the peer-review process and has been accepted for publication. A Just Accepted manuscript is published online shortly after its acceptance, which is prior to technical editing and formatting and author proofing. Tsinghua University Press (TUP) provides Just Accepted as an optional and free service which allows authors to make their results available to the research community as soon as possible after acceptance. After a manuscript has been technically edited and formatted, it will be removed from the Just Accepted Web site and published as an ASAP article. Please note that technical editing may introduce minor changes to the manuscript text and/or graphics which may affect the content, and all legal disclaimers that apply to the journal pertain. In no event shall TUP be held responsible for errors or consequences arising from the use of any information contained in these Just Accepted manuscripts. To cite this manuscript please use its Digital Object Identifier (DOI ), which is identical for all formats of publication.

2 TABLE OF CONTENTS (TOC) Facile preparation of organometallic perovskite films and high-efficiency solar cells using solid-state chemistry Lei Chen 1,2,, Feng Tang 2,, Yixin Wang 2,3, Shan Gao 2, Jinhua Cai 2 * and Liwei Chen 2 * 1 Shanghai University, China 2 Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, China 3 University of Science and Technology of China, China These authors contributed equally to this work. Page Numbers. The font is ArialMT 16 (automatically inserted by the publisher) A solid-state reaction is developed for the preparation of organometallic perovskite thin films and perovskite solar cells. The method involves facile annealing of precursor films in contact and yields solar cells with the best efficiency reaching 10%. 1

3 Nano Res DOI (automatically inserted by the publisher) Review Article/Research Article Research Article Facile preparation of organometallic perovskite films and high-efficiency solar cells using solid-state chemistry Lei Chen 1,2,, Feng Tang 2,, Yixin Wang 2,3, Shan Gao 2, Jinhua Cai 2 ( ) and Liwei Chen 2 ( ) 1 Department of Chemistry, Shanghai University, Shanghai , China 2 i-lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu , China 3 Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, Jiangsu , China These authors contributed equally to this work. Received: day month year / Revised: day month year / Accepted: day month year (automatically inserted by the publisher) Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2011 ABSTRACT The power conversion efficiency of organometallic perovskite-based solar cells has skyrocketed in recent years. Intensive efforts have been made to prepare high-quality perovskite films that are tailored to various device configurations. Planar heterojunction devices have achieved record efficiencies; however, the preparation of perovskite films for planar junction devices requires the use of expensive vacuum facilities and/or the fine control of experimental conditions. Here, we demonstrate a facile preparation for perovskite films using solid-state chemistry. Solid-state precursor thin films of CH3NH3I and PbI2 are brought into contact with each other and allowed to react via thermally accelerated diffusion. The resulting perovskite film displays good optical absorption and a smooth morphology. Solar cells based on these films show an average efficiency of 8.7% and a maximum efficiency of 10%. The solid-state synthesis of organometallic perovskite can also be applied to flexible plastic substrates. Using this method on a PET/ITO substrate produces devices with an efficiency of 3.2%. Unlike existing synthetic methods for organometallic perovskite films, the solid-state reaction method does not require the use of orthogonal solvents or careful adjustment of reaction conditions, and thus shows good potential for mass production in the future. KEYWORDS solid-state chemistry, perovskite solar cells, planar heterojunction, flexible substrates Address correspondence to Jinhua Cai, jhcai2013@sinano.ac.cn; Liwei Chen, lwchen2008@sinano.ac.cn 2

4 There has been tremendous development in organometallic perovskite solar cells in recent years [1-5]. The power conversion efficiency of perovskite-based planar heterojunction solar cells has reached 19.3% [6]. Organometallic perovskites, such as CH3NH3PbI3, are excellent photovoltaic materials because of their strong absorption coefficients, long carrier diffusion lengths [7, 8], and appropriate energy gap (~1.5 ev), which is very close to the ideal gap of 1.3 ev from detailed balance theory [9]. The preparation of a high-quality perovskite film is a critical step in fabricating planar junction solar cells because only high-quality films can enable the excellent intrinsic material characteristics of perovskites to be fully exploited in solar cells [10-15]. Researchers in this field have developed many methods to achieve high-quality perovskite films in recent years. These methods fall into two categories, vacuum vapor deposition and solution processes. High-quality perovskite films and high efficiency (15%) solar cells have been obtained using vacuum vapor deposition; however, the deposition process requires the use of expensive vacuum facilities [16]. Methods based on solution processes, such as the two-step method and the vapor-assisted solution process, offer the advantage of low cost over expensive vacuum vapor-deposition, but careful control of experimental conditions and parameters is necessary to ensure that the experimental results were reproduced as consistently as possible [17-19]. In this paper, we develop a solid-state reaction method for the preparation of a high-quality perovskite film. The structure, optical absorption, and surface morphology of the resulting films are characterized using X-ray diffractometry (XRD), UV-vis absorption spectroscopy, photoluminescence (PL) spectra, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The prepared perovskite films are used to fabricate planar heterojunction solar cells with a stacking ITO/PEDOT:PSS/perovskite/PC61BM/Al structure. The current density-voltage (J-V) curves and the external quantum efficiency (EQE) spectra are used to evaluate the photovoltaic performance of the prepared cells. The experimental conditions, including the film thickness (which is adjusted by the thickness of precursor film), the annealing temperature and the time, are optimized to produce the highest photovoltaic performance of the corresponding devices. The highest conversion efficiency attained by our cells is 10%. Figure 1a is a schematic of the primary procedures in our method. A lead iodide (PbI2) film and a CH3NH3I film are deposited on two independent substrates; the CH3NH3I film is then placed face-to-face on top of the PbI2 film. The two contacting films are annealed over a certain period, after which a CH3NH3PbI3 film is obtained by removing the top substrate. CH3NH3I vaporizes easily at 1 atm (140 C is a sufficiently high temperature to drive CH3NH3I molecules into the PbI2 film to form perovskite [15]); therefore, the perovskite film forms by a rapid solid-state chemical reaction. The photographs in Figure 1b show a clear color change after the annealing. The stoichiometric ratio of the reaction is difficult to determine because the mass of both the reactant and product thin films are too small to be accurately weighted. Nevertheless, we believe the PbI2 film is fully converted to perovskite as long as there is enough CH3NH3I on the top substrate based on AFM height measurements (see Figure S1 and S2 for the detailed information). Spectroscopic techniques including XRD, PL, UV-vis absorption and SEM imaging further confirm the completion of the solid-state reaction. Figure 2 shows the evolution of XRD pattern with the annealing time for a PbI2 film on a ITO/PEDOT:PSS substrate after the film is covered by CH3NH3I film at an optimized annealing temperature of 135 C. Five samples are prepared and annealed for a series of timeration of a high-quality perovskite film. The structure, optical 带格式的 : 缩进 : 首行缩进 : 2 字符 3

5 absorption, and surface morphology of the resulting films are characterized using X-ray diffractometry (XRD), UV-vis absorption spectroscopy, photoluminescence (PL) spectra, atomic force microscopy (AFM) and scanning electron microscopy (SEM). The prepared perovskite films are used to fabricate planar heterojunction solar cells with a stacking ITO/PEDOT:PSS/perovskite/PC61BM/Al structure. The current density-voltage (J-V) curves and the external quantum efficiency (EQE) spectra are used to evaluate the photovoltaic performance of the prepared cells. The experimental conditions, including the film thickness (which is adjusted by the thickness of precursor film), the annealing temperature and the time, are optimized to produce the highest photovoltaic performance of the corresponding devices. The highest conversion efficiency attained by our cells is 10%. Figure 1a is a schematic of the primary procedures in our method. A lead iodide (PbI2) film and a CH3NH3I film are deposited on two independent substrates; the CH3NH3I film is then placed face-to-face on top of the PbI2 film. The two contacting films are annealed over a certain period, after which a CH3NH3PbI3 film is obtained by removing the top substrate. CH3NH3I vaporizes easily at 1 atm (140 C is a sufficiently high temperature to drive CH3NH3I molecules into the PbI2 film to form perovskite [15]); therefore, the perovskite film forms by a rapid solid-state chemical reaction. The photographs in Figure 1b show a clear color change after the annealing. The stoichiometric ratio of the reaction is difficult to determine because the mass of both the reactant and product thin films are too small to be accurately weighted. Nevertheless, we believe the PbI2 film is fully converted to perovskite as long as there is enough CH3NH3I on the top substrate based on AFM height measurements (see Figure S1 and S2 for the detailed information). Spectroscopic techniques including XRD, PL, UV-vis absorption and SEM imaging further confirm the completion of the solid-state reaction. Figure 2 shows the evolution of XRD pattern with the annealing time for a PbI2 film on a ITO/PEDOT:PSS substrate after the film is covered by CH3NH3I film at an optimized annealing tems, 0, 5, 15, 20, and 30 min. The film clearly initially consists of the PbI2 带格式的 : 字体颜色 : 自动设置 带格式的 : 字体颜色 : 自动设置 带格式的 : 缩进 : 首行缩进 : 2 字符 Figure 1 (a) Schematic illustration of the fabrication process of perovskite film using solid-state reaction; and (b) corresponding photographs of the real films. 带格式的 : 字体颜色 : 自动设置 4

6 for a series of annealing times. At the initial stage (0 min), the spectrum shows a pronounced absorption edge at 500 nm, agreeing well with the fact that the energy gap of PbI2 is 2.5 ev [21, 22]. The absorbance of the film increases with the reaction time and the characteristic absorption edge of CH3NH3PbI3 at 780 nm appears after 15 min [23]. The absorbance saturates after 20 min. Figure 3b shows the PL spectra of bottom film at different reaction time measured at low temperature (3.8 K) [24]. The 带格式的 : 字体颜色 : 自动设置 Figure 2 Evolution of XRD patterns with time for a PbI2 film on a ITO/PEDOT:PSS substrate, where a CH3NH3I film is placed on the PbI2 film. phase (0 min). After the CH3NH3I film has covered the PbI2 film for only 5 min, peaks appear at 2θ=14.16, 28.50, and 43.19, corresponding to the (110), (220), (310) and (330) reflections of the CH3NH3PbI3 phase (the diffraction peaks at 21.4, 30.4, 35.3 and 50.6 indicate the crystallinity of ITO) [1, 15, 20], thereby indicating the formation of the tetragonal perovskite structure. The gradual disappearance of the signature peak of the PbI2 at indicates the progress of the rapid reaction of CH3NH3I with PbI2. After 20 min of annealing, the PbI2 peaks disappear, after which the XRD pattern remains almost unchanged, indicating the complete conversion of PbI2. Figure S3 shows thexrd pattern of the top thin film (CH3NH3I on glass) before (0 min) and after (30 min) the reaction and the bottom film (perovskite on ITO glass) after 30 min of reaction. The results indicate that excessive CH3NH3I is left on the top substrate. Considering that the top film is removed right after the reaction while the temperature of the bottom thin film is still at about 135 C, any remaining CH3NH3I at the bottom film shall evaporate and may not contaminate the perovskite film. Figure 3a shows the UV-vis absorption spectra Figure 3 (a) UV-vis absorption spectra of perovskite film prepared from PbI2 films on ITO/PEDOT:PSS substrate, as covered with a CH3NH3I film and annealed at 135 C for different time periods; and (b) PL spectra of these films obtained at low temperature (3.8 K) with 405 nm excitation. emission peak from PbI2 at 514 nm disappears after 20 min. This result indicates that the reaction of CH3NH3I and PbI2 is nearly complete after 20 min. The evolution of the absorption and PL spectra is consistent with the XRD patterns. 带格式的 : 字体颜色 : 自动设置带格式的 : 字体颜色 : 自动设置带格式的 : 字体颜色 : 自动设置带格式的 : 字体颜色 : 自动设置 5

7 The SEM image clearly shows the change in the grain structure of the corresponding deposited film. Figure 4 shows that voids of different sizes are scattered among polygonal PbI2 grains at the beginning of the procedure. After the PbI2 film is covered by the CH3NH3I film and annealed at 135 C for 5 min, the voids in the PbI2 film become smaller and fewer. After 20 min, the voids almost disappear, and a uniform film forms. The grain size is almost unchanged for an annealing time of 5-20 min, but the grain size of the film that has been annealed for 30 min is significantly larger than obtained with a 20-min annealing time. Although no significant changes can be observed in the XRD (Figure2) or optical absorption spectrum (Figure 3) by increasing the annealing time from 20 min to 30 min, the SEM result clearly shows that the grains become larger, which should facilitate carrier transport in the film. The evolution of the XRD patterns, the UV-vis absorption spectra, and the SEM images with time shows that 30 min is an appropriate annealing time at a temperature of 135 C. We use the films to prepare solar cells with a stacking ITO/PEDOT:PSS/perovskite/PC61BM/Al structure, which is a classical structure in the field of organic solar cells [25, 26]. After the perovskite film is prepared using the solid state reaction, a layer of PC61BM is deposited by spin-coating a chlorobenzene solution of PC61BM onto the film, followed by vacuum evaporation of an Al layer with a thickness of approximately 100 nm. The active area of the cell is 0.12 cm 2 and is defined as the overlap between the ITO electrode and the Al electrode. For one batch of 30 samples, the mean efficiency of the cells is 8.7%, and the maximum efficiency of the cells is 10.0%. In addition to optimizing the annealing temperature (135 C) and the annealing time (30 min), the perovskite layer thickness is optimized at approximately 300 nm, as estimated from the SEM image (see Figure 4f). This optimized thickness is in good agreement with the reported optimized value (300 nm) [15, 27]. Figure 5 shows the J-V curves and the EQE spectrum for our best performing device, which has a conversion efficiency 10.0%, a short-circuit current (Jsc) of 17.9 ma/cm 2, an open-circuit voltage (Voc) of 0.87 V, and a fill factor (FF) of 64.3%. The EQE spectrum shows that photocurrent generation starts at 780 nm, which is in accordance with the bandgap of CH3NH3PbI3 [23], and reaches peak values of over 80% in the visible spectrum. Integrating the Figure 4 Top view SEM images of a PbI2 film on a ITO/PEDOT:PSS substrate after the film has been covered by a CH3NH3I film for 6

8 (a) 0 min, (b) 5 min, (c) 15 min, (d) 20 min and (e) 30 min at 135 C; (f) cross-sectional SEM image of a PbI2 film on a ITO/PEDOT:PSS substrate after the PbI2 film has been covered with a CH3NH3I film for 20 min at 135 C. Figure 5 (a) The J-V curves of the perovskite solar cells prepared using the solid-state chemical reaction method under AM 1.5G irradiation and in dark. (b) EQE spectrum (black) and the integrated photocurrent (red) under AM 1.5G irradiation of the device. overlap of the EQE spectrum with the AM 1.5G solar photon flux yields a current density of 18.0 ma/cm 2, which is similar to that obtained from J-V measurements. Although the Voc is lower than that obtained for devices prepared using other methods, the efficiency is not lower than the value that has been reported in the literature for the same structure [12, 28-30]. The devices that are prepared by the solid-state reaction method also exhibit good consistency. Figure 6 shows the statistics of the device performance for 30 cells that are prepared using our method. The conversion efficiency of 80% of the samples ranges from 8% to 10%, the Voc of 80% of the samples is above 0.83 V, the Jsc of 80% of the samples is above 16 ma/cm 2, and the fill factors of 80% of the samples are above 0.6. This high consistency should translate into high yields in mass production. Our method can be applied to both rigid and flexible substrates. Figure 7 shows a device on a flexible PET substrate. The conversion efficiency in a cell with 0.09 cm 2 area is 3.2%, which is considerably lower than the conversion efficiency of a cell prepared on an ITO glass substrate. This low efficiency mainly manifests in lower Jsc and fill factor values. The low fill factor results from a low shunt resistance and a high serial resistance. Possible causes for low Jsc values are the low annealing temperature (125 C) and the long Figure 6 Statistics of device performance for 30 cells prepared using our solid-state reaction method: (a) Voc, (b) Jsc, (c) FF, and (d) PCE. 7

9 Figure 7 J-V characteristics for the best-performing flexible device with the ITO/PEDOT:PSS/CH3NH3PbI3/PC61BM/Al stacking configuration on PET substrate. Inset: photograph of the device. annealing time (45 min) that are used to prepare the perovskite films on the PET substrate. Although the current efficiency is as low as 3.2%, the method shows potential for application to mass production. The solid-state chemistry method presented here offers significant advantages over existing methods, such as the vapor-assisted solution process and the inter-layer diffusion method [15, 27]. In the vapor-assisted solution process, CH3NH3I powder is cast around the PbI2 film, such that CH3NH3I is transferred onto the surface of the PbI2 film in the vapor phase, and a perovskite film forms via thermal diffusion [15]. The CH3NH3I powder does not come into direct contact with PbI2; thus, a long reaction time is required in this process. The inter-layer diffusion method is similar to the traditional solid-state diffusion method [27], which is conventionally used to deposit alloy films, e.g., CuInGaSe [31], in the field of solar energy. Two layers of precursor films are sequentially deposited onto a substrate, and the perovskite film forms via thermal diffusion. Limitations of this method are that orthogonal solvents are required for the sequential deposition of the bilayer precursor film,and accurate feed ratio of the two precursors are required to obtain the perovskite film with the intended stoichiometry. In the solid-state chemistry method, the CH3NH3I film and PbI2 film are deposited on two different substrates, and thus orthogonal solvents are not required. The excessive CH3NH3I is removed before the perovskite film is used in successive preparation steps for solar cells, therefore, accurate ratio between CH3NH3I and PbI2 is not necessary. The two precursor films are placed in contact with each other during the thermally assisted reaction, resulting in a rapid reaction. In summary, a solid-state reaction method is developed to prepare perovskite thin films with smooth morphology and good optical absorption property. The corresponding solar cell devices show an average efficiency of 8.7%, a maximum efficiency of 10%, and good consistency among prepared batches. The solid-state synthesis of organometallic perovskite can also be applied to flexible plastic substrates. The advantages of this method, such as fast reaction speed, flexibility in selecting solvent and feeding of precursor films, render it much potential for future large-scale production. Materials and Methods CH3NH3I synthesis CH3NH3I is prepared according to previous reports [3]. 30 ml of methylamine (40% in methanol, Sinopharm Chemical Reagent Co., Ltd) and 32.3 ml of hydroiodic acid (57 wt% in water, Sinopharm Chemical Reagent Co., Ltd) are reacted in a 250-ml round-bottom flask at 0 C for 2 h with stirring. The precipitate is recovered by placing the solution into a rotary evaporator and carefully removing the solvents at 50 C. The white (slightly yellow) raw product of methylammonium iodide (CH3NH3I) is washed with diethyl ether three times. After filtration, the solid is collected and dried at 60 C in a vacuum oven for 24 h. The resulting white solid is used without further purification. Solar cell fabrication ITO-coated glass substrates and pure glass 8

10 substrates are cleaned with detergent, deionized water, ethanol, acetone, and isopropyl alcohol by ultrasonication for 10 min and then dried with clean nitrogen. All of the ITO-coated glass substrates and pure glass substrates are treated with oxygen plasma for 5 min. A buffer layer of PEDOT:PSS-4083 (Clevios PVPAI4083, Heraeus, Germany) with a thickness of 35 nm is spin-coated at 3500 rpm onto the ITO-coated glass substrates under ambient conditions and annealed at 120 C for 30 min in a N2-filled glove box. The PbI2 layer is deposited onto the ITO/PEDOT:PSS substrate by spin-coating a 230-mg/ml solution of PbI2 in N,N-dimethylformamide (DMF) at 1500 rpm for 40 s and annealed at 80 C for 15 min. The CH3NH3I film is spin-coated onto the pure glass substrate using a 200-mg/ml solution of CH3NH3I in DMF at 3000 rpm for 20 s. The CH3NH3I film is placed on the PbI2 layer and baked at 135 C for 30 min in case of glass substrate, but baked at 120 C for 45 min in case of PET substrate. After removing the top substrate, a uniform CH3NH3PbI3 thin film appears on the underlying substrate. The PbI2 films, the CH3NH3I films and the CH3NH3PbI3 films are prepared in a glove box. PC61BM (purchased from Luminescence Technology Corp.) is dissolved in chlorobenzene to form a 20 mg/ml solution, and the solution is spin-coated onto the ITO/PEDOT:PSS/CH3NH3PbI3 substrate at 1200 rpm for 1 min. No heat treatment is applied to the PC61BM layer. A 100-nm Al layer is subsequently evaporated under a pressure of Pa through a shadow mask to define the active area of the devices (0.12 cm 2 ). Perovskite film and device characterization XRD analysis of the crystal structures of the perovskite films is conducted using a Bruker D8 Advance X-ray diffractometer (with Cu Kα radiation at nm). An UV-Vis spectrophotometer (Perkin Elmer Lambda 750) is used to get the UV-Vis absorption spectra of the planar perovskite films. PL spectra are characterized using a home-made system with a light souce (56RCS001/HV, MellesGriot, US), a cryogenic refrigerator (PT403, Cryomech Inc., US) and a detector (Spectra Pro 2500i, Princeton Instrument, US). The surface morphologyis obtained with an FEI Quanta 400 FEG Field Emission Environment SEM and model 5500 AFM/SPM from Agilent Technologies, US. The J-V characteristics of the solar cells are measured in the dark and under 100 mw/cm 2 white light from a Hg-Xe lamp that is filtered by a Newport Air Mass Filter using a Keithley2635A source meter. The EQE measurement is performed using a Merlin radiometer (Newport) with a monochromator-calibrated wavelength controller. A calibrated silicon photodiode is used as a reference device to count the incident photons. All of the measurements are performed under the ambient atmosphere at room temperature. Acknowledgements This work was supported by the National Natural Science Foundationof China (Nos: , and ), andthe National Basic Research Programof China (2010CB934700). L. C. acknowledges the support from Jiangsu Provincial Natural Science Foundation (Grant No. BK ). Electronic Supplementary Material: Supplementary material (further details of the chemically strochiometry, AFM measurements, optical micrograph imaging and XRD measurements) is available in the online version of this article at (automatically inserted by the publisher). References [1] Im, J.-H.; Lee, C.-R.; Lee, J.-W.; Park, S.-W.; Park, N.-G., 6.5% efficient perovskite quantum-dot-sensitized solar cell. Nanoscale 2011, 3, 带格式的 : 字体颜色 : 自动设置 带格式的 : 字体颜色 : 自动设置 9

11 [2] Lee, M. M.; Teuscher, J.; Miyasaka, T.; Murakami, T. N.; Snaith, H. J., Efficient hybrid solar cells based on meso-superstructured organometal halide perovskites. Science 2012, 338, [3] Etgar, L.; Gao, P.; Xue, Z. S.; Peng, Q.; Chandiran, A. K.; Liu, B.; Nazeeruddin, M. K.; Grätzel, M., Mesoscopic CH3NH3PbI3/TiO2 heterojunction solar cells. J. Am. Chem. Soc. 2012, 134, [4] Noh, J. H.; Jeon, N. J.; Choi, Y. C.; Nazeeruddin, M. K.; Grätzel, M.; Seok, S. I., Nanostructured TiO2/CH3NH3PbI3 heterojunction solar cells employing spiro-ometad/co-complex as hole-transporting material. J. Mater. Chem. A 2013, 1, [5] Bai, S.; Wu, Z. W.; Wu, X. J.; Jin, Y. Z.; Zhao, N.; Chen, Z. H.; Mei, Q. Q.; Wang, X.;Ye, Z. Z.; Song, T. et al. High-performance planar heterojunction perovskite solar cells: Preserving long charge carrier diffusion lengths and interfacial engineering. Nano Res. in press, DOI: /s [6] Zhou, H. P.; Chen, Q.; Li, G.; Luo, S.; Song, T.-b.; Duan, H.-S.; Hong, Z. R.; You, J. B.; Liu, Y. S.; Yang, Y., Interface engineering of highly efficient perovskite solar cells. Science 2014, 345, [7] Stranks, S. D.; Eperon, G. E.; Grancini, G.; Menelaou, C.; Alcocer, M. J. P.; Leijtens, T.; Herz, L. M.; Petrozza, A.; Snaith, H. J., Electron-hole diffusion lengths exceeding 1 micrometer in an organometal trihalide perovskite absorber. Science 2013, 342, [8] Xing, G. H.; Mathews, N.; Sun, S. Y.; Lim, S. S.; Lam, Y. M.; Graetzel, M.; Mhaisalkar, S.; Sum, T. C., Long-range balanced electron- and hole-transport lengths in organic-inorganic CH3NH3PbI3. Science 2013, 342, [9] Shockley, W.; Queisser, H. J., Detailed balance limit of efficiency of p-n junction solar cells. J. Appl. Phys. 1961, 32, [10] Docampo, P.; Ball, J. M.; Darwich, M.; Eperon, G. E.; Snaith, H. J., Efficient organometal trihalide perovskite planar-heterojunction solar cells on flexible polymer substrates. Nat. Commun. 2013, 4, [11] Kim, H.-S.; Im, S. H.; Park, N.-G., Organolead halide perovskite: new horizons in solar cell research. J. Phys. Chem. C 2014, 118, [12] Liang, P.-W.; Liao, C.-Y.; Chueh, C.-C.; Zuo, F.; Williams, S. T.; Xin, X.-K.; Lin, J.; Jen, A. K. Y., Additive enhanced crystallization of solution-processed perovskite for highly efficient planar-heterojunction solar cells. Adv. Mater. 2014, 26, [13] Eperon, G. E.; Burlakov, V. M.; Docampo, P.; Goriely, A.; Snaith, H. J., Morphological control for high performance, solution-processed planar heterojunction perovskite solar cells. Adv. Funct. Mater. 2014, 24, [14] Dualeh, A.; Tetreault, N.; Moehl, T.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M., Effect of annealing temperature on film morphology of organic-inorganic hybrid perovskite solid-state solar cells. Adv. Funct. Mater. 2014, 24, [15] Chen, Q.; Zhou, H. P.; Hong, Z. R.; Luo, S.; Duan, H.-S.; Wang, H.-H.; Liu, Y. S.; Li, G.; Yang, Y., Planar heterojunction perovskite solar cells via vapor-assisted solution process. J. Am. Chem. Soc. 2014, 136, [16] Liu, M. Z,; Johnston, M. B.; Snaith, H. J., Efficient planar heterojunction perovskite solar cells by vapour deposition. Nature 2013, 501, [17] Liu, D. Y.; Kelly, T. L., Perovskite solar cells with a planar heterojunction structure prepared using room-temperature solution processing techniques. Nat. Photonics 2014, 8, [18] Burschka, J.; Pellet, N.; Moon, S.-J.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M. K.; Grätzel, M., Sequential deposition as a route to high-performance perovskite-sensitized solar cells. Nature 2013, 499, [19] Jeon, N. J.; Noh, J. H.; Kim, Y. C.; Yang, W. S.; Ryu, S.; Seok, S. I., Solvent engineering for high-performance inorganic-organic hybrid perovskite solar cells. Nat. Mater. 2014, 13, [20] Yu, Y.-Y.; Chiang, R.-S.; Hsu, H.-L.; Yang, C.-C.; Chen, C.-P., Perovskite photovoltaics featuring solution-processable TiO2 as an interfacial electron-transporting layer display to improve performance and stability. Nanoscale 2014, 6, [21] Nikitine, S.; Ringeissen, J.; Schmittb.J; Biellmann, J., 10

12 Etude spectrophotomètriquedes raies du spectre excitonique ordinaire de PbI2 A 4,2 o K. J. Phys. Chem. Solids 1964, 25, [22] Gähwiller, Ch.; Harbeke, G., Excitonic effects in theelectroreflectance of lead iodide. Phys. Rev. 1969, 185, [23] Kim, H.-S.; Lee, C.-R.; Im, J.-H.; Lee, K.-B.; Moehl, T.; Marchioro, A.; Moon, S.-J.; Humphry-Baker, R.; Yum, J.-H.; Moser, J. E. et al. Lead iodide perovskite sensitized all-solid-state submicron thin film mesoscopic solar cell with efficiency exceeding 9%. Sci. Rep. 2012, 2, 591. [24] Ha, S. T.; Liu, X. F.; Zhang, Q.; Giovanni, D.; Sum, T. C.; Xiong, Q. H., Synthesis of organic inorganic lead halide perovskite nanoplatelets: towards high-performance perovskite solar cells and optoelectronic devices. Adv. Opt. Mater. 2014, 2, [25] Dennler, G.; Scharber, M. C.; Brabec, C. J., Polymer-fullerene bulk-heterojunction solar cells. Adv. Mater. 2009, 21, [26] Zhang, Q.; Wan, X. J.; Xing, F.; Huang, L.; Long, G. K.; Yi, N. B.; Ni, W.; Liu, Z. B.; Tian, J. G.; Chen, Y. S., Solution-processable graphene mesh transparent electrodes for organic solar cells. Nano Res. 2013, 6, [27] Xiao, Z. G.; Bi, C.; Shao, Y. C.; Dong, Q. F.; Wang, Q.; Yuan, Y. B.; Wang, C. G.; Gao, Y. L.; Huang, J. S., Efficient, highyield perovskite photovoltaic devices grown by interdiffusion of solution-processed precursor stacking layers. Energy Environ. Sci. 2014, 7, [28] Sun, S. Y.; Salim, T.; Mathews, N.; Duchamp, M.; Boothroyd, C.; Xing, G. C.; Sum, T. C.; Lam, Y. M., The origin of high efficiency in low-temperature solution-processable bilayer organometal halide hybrid solar cells. Energy Environ. Sci. 2014, 7, [29] Kim, H.-B.; Choi, H.; Jeong, J.; Kim, S.; Walker, B.; Song, S.; Kim, J. Y., Mixed solvents for the optimization of morphology in solution-processed, inverted-type perovskite/fullerene hybrid solar cells. Nanoscale 2014, 6, [30] Hsu, H.-L.; Chen, C.-P.; Chang, J.-Y.; Yu, Y.-Y.; Shen, Y.-K., Two-step thermal annealing improves the morphology of spin-coated films for highly efficient perovskite hybrid photovoltaics. Nanoscale 2014, 6, [31] Caballero, R.; Guillen, C., Comparative studies between Cu Ga Se and Cu In Se thin film systems. Thin Solid Films 2002, 403,

13 Electronic Supplementary Material Facile preparation of organometallic perovskite films and high-efficiency solar cells using solid-state chemistry Lei Chen 1,2,, Feng Tang 2,, Yixin Wang 2,3, Shan Gao 2, Jinhua Cai 2 ( ) and Liwei Chen 2 ( ) 1 Department of Chemistry, Shanghai University, Shanghai , China 2 i-lab, Suzhou Institute of Nano-Tech and Nano-Bionics, Chinese Academy of Sciences, Suzhou, Jiangsu , China 3 Nano Science and Technology Institute, University of Science and Technology of China, Suzhou, Jiangsu , China These authors contributed equally to this work. Supporting information to DOI /s12274-****-****-* (automatically inserted by the publisher) Figure S1 AFM topographical image and height profile of (a) PbI2 and (b) perovskite films. Subtracting the thickness of the PEDOT:PSS layer (35 nm), we obtain that the thickness of the perovskite is about 245 nm, and that of the original PbI2 film is about 145 nm. Considering the density of PbI2 (6.16 g/cm 3 ) and CH3NH3PbI3 (4.09 g/cm 3 ) [S1, S2], the film thickness of perovskite should be 290 nm if the 145 nm thick PbI2 film is completely converted to perovskite, assuming film area is not changed. We think the difference is mainly due to the porous state of PbI2 film. Address correspondence to Jinhua Cai, jhcai2013@sinano.ac.cn; Liwei Chen, lwchen2008@sinano.ac.cn 12

14 Figure S2 Optical micrographs of CH3NH3I on the top substrate. The top film spin-coated with 100 mg/ml CH3NH3I solution (a) before and (b) after the reaction; those with 200 mg/ml solution (c) before and (d) after the reaction; those with 300 mg/ml solution (e) before and (f) after the reaction. The spin coating speed is 3000 rpm in all preparation. The amount of remaining CH3NH3I increased with the increase of the CH3NH3I concentration. Figure S3 XRD pattern of the top film (CH3NH3I on glass) before (0 min) and after (30 min) the reaction. For comparison, the bottom film after 30 min of reaction (perovskite on ITO/glass) is also included. The signal intensity of the top film before reaction has been reduced (multiply by a factor of 0.4). 13

15 Supporting References [S1] [S2] Stoumpos, C. C.; Malliakas, C. D.; Kanatzidis, M. G., Semiconducting tin and lead iodide perovskites with organic cations: phas e transitions, high mobilities, and near-infrared photoluminescent properties. Inorg. Chem. 2013, 52,