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1 Supporting Information Efficient Light Harvester Layer Prepared by Solid/Mist Interface Reaction for Perovskite Solar Cells Xiang Xia, a Hongcui Li, a Wenyi Wu, b Yanhua Li, a Dehou Fei, a Chunxiao Gao b, * and Xizhe Liu a, * a Jilin Provincial Key Laboratory of Applied Atomic and Molecular Spectroscopy, Institute of Atomic and Molecular Physics, Jilin University, Changchun, , China b State Key Laboratory for Superhard Materials, Jilin University, Changchun, , China AUTHOR INFORMATION Corresponding Author * (L. X.) liu_xizhe@jlu.edu.cn * (G. C.) gaocx@jlu.edu.cn S-1

2 1. Experimental Section Preparation of methylammonium iodine (MAI): MAI was synthesized by reacting ml CH 3 NH 2 (33 wt% in ethanol) and ml HI (47 wt% in water) in a round-bottom flask at 0 o C for 4 h with stirring. Then, crude MAI power was obtained from the mixed solution by evaporation. Finally, the crude MAI power was dissolved in ethanol, recrystallized from diethyl ether, and dried at 60 o C in a vacuum oven for 12 h to get white MAI power. Fabrication of perovskite solar cells: Firstly, fluorine doped tin oxide (FTO) glass was etched and cleaned. Then, compact TiO 2 film (c-tio 2, ~30 nm in thickness) was deposited by spray pyrolysis with 50 mmol/l Ti(OPr) 2 (AcAc) 2 in ethanol solution at 450 o C. Mesoporous TiO 2 film (mp-tio 2 ) was deposited on the c-tio 2 film by spin-coating with ethanol diluted TiO 2 paste (18NRT, Dyesol) solution at 5000 rpm for 30 sec. The mass ratio of TiO 2 paste to ethanol is 1:6. Then, the sample was dried and sintered at 80 o C for 30 min, 125 o C for 30 min, 325 o C for 30 min, and 500 o C for 30 min. After further treatment with 40 mm TiCl 4 at 70 o C for 30 min, the sample was rinsed with water and ethanol, and sintered at 450 o C for 30 min. Then, PbI 2 film was prepared on the sample by spin-coating with 462 mg/ml PbI 2 in N,N-Dimethylformamide solution at 4000 rpm for 30 sec. After drying and preheating at the desired reaction temperature for 5 min on a hot plate, 10 mg/ml MAI in isopropanol solution was nebulized and sprayed on the top of the PbI 2 film by a nitrogen gas atomizing spray nozzle. After spin-coating PbI 2 solution, the PbI 2 film was directly preheated at the same temperature of the following Solid/Mist reaction. No extra drying procedure was applied. To control the quality of mist, the S-2

3 spraying speed of MAI solution was constant (5 µl/s) and the flow rate of N 2 gas was also constant (200 L/h). Therefore, the corresponding spraying time was 10 s, 20 s, 40 s, 80 s, 120 s, and 200 s for 50 µl, 100 µl, 200 µl, 400 µl, 600 µl and 1000 µl MAI solution, respectively. All of these MAI solutions were sprayed continuously in one time. The working distance between spray nozzle and PbI 2 film was 8 cm. The area of one PbI 2 film sample and its substrate was 1.2 cm * 1.5 cm and 1.5 cm *1.5 cm, respectively. The spraying area on the hot plate was about 24 cm 2. Different volumes of MAI solution and different reaction temperatures were applied. After another 10 min heating, the film was washed with isopropanol and annealed at 70 o C for 30 min. The hole conductor layer was then deposited by spin-coating at 4,000 rpm for 20 sec. The spincoating solution was prepared by dissolving 72.3 mg (2,2,7,7 -tetrakis(n,n-di-pmethoxyphenylamine)-9,9-spirobifluorene) (spiro-meotad), 28.8 ul 4-tert-butylpyridine and 17.5 ul lithium bis(trifluoromethylsulphonyl)imide solution (520 mg/ml in acetonitrile) in 1 ml chlorobenzene. After spiro-meotad being aged in the dried air condition of a brown desiccator for 12 hours, silver electrode was deposited to complete the device. All these operations were performed in air condition. Characterization: The SEM images were investigated by FEI MAGELLAN 400 Scanning Electron Microscope. The X-ray diffraction (XRD) spectra were obtained using a Rigaku D/max-2550 X-ray diffractometer. Photocurrent density-photovoltage characteristics were recorded from 1.15 V to 0 V by a CHI660 electrochemical workstation. The active area of solar cells was 0.15 cm 2, which defined by a mask. AM1.5 illumination was provided by a 3A class S-3

4 solar simulator (UHE-16, ScienceTech Inc.), which was calibrated to one sun by a KG5 filtered Si reference solar cell (certificated by VLSI Standards Inc., traceable to National Renewable Energy Laboratory). IPCE spectra are measured in the DC mode by a controlled monochrometer (BOCIC Inc.) with a 300W Xe light source (NBet Inc.). The certificated Si cell (VLSI Standards Inc.) is used as the reference. S-4

5 2. Table Table S1. The photovoltaic parameters of the devices prepared by MAI solutions with different volumes Volume Device NO. J sc (ma/cm 2 ) V oc (V) Fill factor PCE (%) 50 ul 100 ul 200 ul 400 ul 600 ul 1000 ul S-5

6 Table S2. The photovoltaic parameters of the devices prepared at different reaction temperatures Temperature Device NO. J sc (ma/cm 2 ) V oc (V) Fill factor PCE (%) 40 o C 60 o C 80 o C 100 o C 120 o C 140 o C S-6

7 3. Figure Figure S1. X-ray diffraction (XRD) patterns of MAI/TiO 2 /FTO glass, PbI 2 /TiO 2 /FTO glass and TiO 2 /FTO glass. Figure S2. Top-view SEM images of perovskite film obtained by solid/liquid interface reaction. 1 The prepared PbI 2 film was dipped into 10 mg/ml MAI in isopropanol solution for 60 s, and then annealed at 70 o C for 30 min. S-7

8 Figure S3. The photocurrent density-photovoltage characteristic of the device via different scanning directions. Reverse scan is the first scan direction (from 1.1 V to 0 V), and the scan rate is 0.1 V/s. Figure S4. The photocurrent density-photovoltage characteristic of the best-performing planar junction device. S-8

9 Figure S5. Top-view SEM images (a-e) and device efficiency (PCE) (f) of the MAPbI 3 films prepared with various concentrations of MAI solution. For SEM images, MAI solution concentrations are 1 mg/ml (a), 5 mg/ml (b), 10 mg/ml (c), 15 mg/ml (d) and 20 mg/ml (e), and the scale bar is 1 µm. The SEM images of the MAPbI 3 films prepared by different concentrations of MAI solution are shown in Figure S5, while the volume of MAI solution is 400 ul and the reaction temperature is 80 o C. As shown in Figure S5a and b, low concentration MAI solutions (1 mg/ml and 5 mg/ml) cannot produce compact MAPbI 3 films. The MAPbI 3 film prepared by 10 mg/ml MAI solution exhibits compact structure and well crystalized grains (Figure S5c). After further increasing the concentration (15 mg/ml and 20 mg/ml), the prepared films still have compact structure, but the size of the crystal grains is smaller than that prepared by 10 mg/ml MAI solution (Figure S5d and e). The PCEs of the devices based on the MAPbI 3 films prepared by different concentration S-9

10 MAI solutions are summarized in Figure S5f. For 1 mg/ml MAI solution, low average efficiency of 7.9% is obtained. After increasing the concentration to 5 mg/ml, the devices achieve an average efficiency of 14.1%. The devices prepared by 10 mg/ml MAI solution achieve the best average efficiency of 15.5%, which comes from the compact structure and the high crystallization of MAPbI 3 harvester layers. For MAI solutions with high concentration (15 mg/ml and 20 mg/ml), the corresponding devices achieve relatively low average efficiency (14.3% and 12.6%). Figure S6. (a) The photocurrent density-photovoltage characteristics of the devices with Ag and Au top electrodes, (b) The relationship between device efficiency (PCE) and scan times, (c) The relationship between device efficiency (PCE) and storage time. The photocurrent density-photovoltage characteristics of the devices with Ag and Au top electrodes are shown in Figure S6a. The device with Ag electrode exhibits a slightly higher efficiency than the device with Au electrode. It is likely because Ag electrode can reflect light more effectively than Au electrode. The measurement-stability of the devices with Ag and Au top electrodes are shown in Figure S6b. The devices are scanned ten times with 1 min interval. As shown in Figure S6b, the devices show good stability in the repeated measurements. Therefore, the data of devices with Ag top electrodes in our work is credible. S-10

11 The long term stability of the devices with Ag and Au top electrodes are shown in Figure S6c. The devices are stored in a brown desiccator with silica-gel desiccant. As shown in Figure S6c, the device based on Au electrode shows better stability than that based on Ag electrode during the long term stability test. Therefore, Au top electrode is necessary for the measurement with long period. Figure S7. IPCE spectrum (black solid circle) and integrated photocurrent (red line) of the best-performing device. The photocurrent density from I-V measurement under solar simulator is 20.7 ma/cm 2, and the photocurrent density from integrating IPCE spectrum is 19.8 ma/cm 2. The mismatch between these two measurements is less than 5%. S-11

12 References (1) Burschka, J.; Pellet, N.; Moon, S.; Humphry-Baker, R.; Gao, P.; Nazeeruddin, M.; Grӓtzel, M. Sequential Deposition as a Route to High-Performance Perovskite-Sensitized Solar Cells. Nature 2013, 499, S-12