Enhancing Perovskite Solar Cell Performance by Interface Engineering Using CH 3 NH 3 PbBr 0.9 I 2.1 Quantum Dots

Transcription

1 Supporting Information for Enhancing Perovskite Solar Cell Performance by Interface Engineering Using CH 3 NH 3 PbBr 0.9 I 2.1 Quantum Dots Mingyang Cha,, Peimei Da,, Jun Wang, Weiyi Wang, Zhanghai Chen, Faxian Xiu, Gengfeng Zheng,* and Zhong-Sheng Wang,* Department of Chemistry, Laboratory of Advanced Materials, Collaborative Innovation Center of Chemistry for Energy Materials, Fudan University, 2205 Songhu Road, Shanghai , China State Key Laboratory of Surface Physics, Key Laboratory of Micro and Nano Photonic Structures (Ministry of Education), Department of Physics, Collaborative Innovation Center of Advanced Microstructures, Fudan University, 220 Handan Road, Shanghai , China M.C. and P.D. contributed equally to this work * Corresponding authors: zs.wang@fudan.edu.cn (Z.-S.Wang) and gfzheng@fudan.edu.cn (G.Zheng) S1

2 Materials PbI 2, methylamine in ethanol (40wt%), hydrobromide acid and hydroiodic acid (57wt% in water) were purchased from Alfa Aesar. Spiro-OMeTAD was purchased from Luminescence Technology Corp. All purchased chemicals were used without further purification. CH 3 NH 3 X (denoted as MAX, MA = CH 3 NH 3, X = I, Br) was synthesized by reaction of the methylamine with the corresponding acid as follows: 25 ml methylamine in ethanol and 15 ml acid were mixed at 0 C and stirred for 2 h. The precipitate was obtained by evaporation of solvent at 50 C. The crude product was washed with diethyl ether three times and finally dried in vacuum for 24 h. Methods Preparation of MAPbBr 3-x I x QDs Colloidal MAPbBr 3-x I x QDs were prepared following the literature method. S1 In a typical synthesis of MAPbBr 3 QDs, a mixture of 0.16 mmol MABr and 0.2 mmol PbBr 2 was dissolved in 5 ml of DMF with 20 µl of n-octylamine to form a precursor solution. 0.1 ml of precursor solution was slowly dropped into 10 ml of anhydrous chlorobenzene with a pipette under vigorous stirring (Figure S1). A bright yellow green colloidal solution was obtained after centrifugation at 7000 rpm for 10 min. The above synthetic procedure was also used to prepare the other four MAPbBr 3-x I x QDs using different precursor solutions. The MAPbBr 1.2 I 1.8 precursor solution contains a mixture of mmol MABr, mmol MAI, 0.08 mmol PbBr 2 and 0.12 mmol PbI 2 dissolved in 5 ml of DMF with 20 µl of n-octylamine. The MAPbBr 0.9 I 2.1 precursor solution contains a mixture of mmol MABr, mmol MAI, 0.06 mmol PbBr 2 and 0.14 mmol PbI 2 dissolved in 5 ml of DMF with 20 µl of n-octylamine. The MAPbBr 0.7 I 2.3 precursor solution contains a mixture of mmol MABr, mmol MAI, mmol PbBr 2 and mmol PbI 2 dissolved in 5 ml of DMF with 20 µl of n-octylamine. The MAPbBr 0.4 I 2.6 precursor solution contains a mixture of mmol MABr, mmol MAI, mmol PbBr 2 and mmol PbI 2 dissolved in 5 ml of DMF with 20 µl of n-octylamine. S2

3 Fabrication of Devices The device fabrication has been reported elsewhere. S2 FTO substrates were patterned using Zn and HCl. The TiO 2 precursor was spin-coated on cleaned FTO substrates at 2500 rpm for 30 s, followed by annealing at 500 C for 1 h. After cooling down to RT, the TiO 2 film was immersed in 0.04 M aqueous TiCl 4 solution at 70 C for 30 min and was annealed again at 450 C for 30 min. Fabrication of MAPbI 3 Active Layer: 90 µl of MAPbI 3 solution was first dropped onto the as prepared TiO 2 -coated FTO substrate and then spin coated at 4000 rpm for 30 s, during which anhydrous chlorobenzene (1 ml) was fast dropped onto the center of the substrate. To obtain high quality perovskite films with the anti-solvent method, it is very important to control the dropping time of anhydrous chlorobenzene during the spin coating of MAPbI 3 suspension. It was found that high-quality perovskite films could be prepared when anhydrous chlorobenzene was fast dropped on the perovskite film at the sixth second, as judged from the thin film photographs (Figure S10). The MAPbI 3 perovskite films were then dried at 95 C for 10 min to be used for next processing. The MAPbI 3 perovskite can crystallize to form a film completely through the above-described processes. Fabrication of MAPbI 3 /MAPbBr 3-x I x Films: The MAPbBr 3-x I x QDs dispersed in chlorobenzene is spin coated onto the above MAPbI 3 perovskite layer at 4000 rpm for 30 s followed by annealing at 90 C for 2 min, forming the MAPbI 3 /MAPbBr 3-x I x film. The as-prepared MAPbI 3 perovskite film is already crystallized within 30 s followed by annealing, which guarantees the MAPbBr 3-x I x QDs to be formed on the surface of MAPbI 3 film but not doped in the MAPbI 3 film. Fabrication of QDs mixed MAPbI 3 Films: The MAPbBr 3-x I x QDs mixed active layer was fabricated using the same procedures for MAPbI 3 perovskite, in which the anti-solvent chlorobenzene was replaced with MAPbBr 3-x I x QDs dispersed in chlorobenzene, as shown in the schematic fabrication process (Figure S6). At the sixth second, the MAPbI 3 perovskite has not crystallized to form a film completely from the precursor so that MAPbBr 3-x I x QDs are doped in the MAPbI 3 perovskite film during the spin coating of the MAPbBr 3-x I x QDs. A doped spiro-ometad solution in chlorobenzene was spin-coated on the above films S3

4 at 4000 rpm for 30 s, forming an HTM layer. As the MAPbBr 3-x I x QDs can be dispersed but not dissolved in chlorobenzene, and the perovskite thin films are solidified under heating in advance, the anhydrous chlorobenzene cannot destroy the MAPbBr 3-x I x QDs during the spin coating of spiro-ometad solution. Finally, a Cr buffering layer and an 80-nm thick Au layer were sequentially deposited on the HTM layer by magnetron sputtering. Finally, fabrications of various PSC devices are completed. Measurements A Keithley-2420 source meter in conjunction with a Sol3A class AAA solar simulator equipped with an AM1.5G filter and a 450 W Xenon lamp was used to measure the current-voltage curves. The intensity of white light was calibrated with a reference silicon solar cell (Oriel-91150). To avoid stray light, a black mask with an aperture area of cm 2 was put on the surface of devices during measurements. Incident monochromatic photon-to-electron conversion efficiency (IPCE) as a function of wavelength was recorded on a SM-250 system (Bunkoh-keiki, Japan). The UV-vis spectra were recorded on Shimadzu UV-2550 UV-vis spectrometer. Electrochemical impedance spectroscopy (EIS) and intensity-modulated photovoltage spectroscopy (IMVS) measurements were executed on an electrochemical workstation (ZAHNER ZENNIUM CIMPS-1, Germany). The EIS was measured under illumination of AM1.5G simulated solar light (100 mw cm -2 ) at open circuit in a frequency range of 0.1 Hz 1 MHz. The IMVS was recorded under LED white light in a frequency range of 0.1 Hz 1 MHz at open circuit. The photoluminescence spectra of the QDs are recorded on Picoquant PicoHarp 300. The time-resolved photoluminescence spectra were collected using pulse laser as an optical excitation source (wavelength: 635 nm, pulse width: 50 ps, and repetition rate: 10 MHz). S4

5 Table S1. Average photovoltaic performance parameters of PSCs fabricated by spin coating MAPbBr 0.9 I 2.1 QDs for different times on the annealed MAPbI 3 perovskite active layer. Concentration of QD Number of spin coating a V oc (mv) J sc (ma cm -2 ) FF PCE mm 1 928± ± ± ± mm 1 913± ± ± ± mm 2 891± ± ± ± mm 3 885± ± ± ±0.18 a The perovskite film was heated after each spin coating of QDs at 90 C for 2 min before the next spin coating. (%) Table S2. Average photovoltaic parameters of MAPbI 3 /MAPbBr 0.7 I 2.3 -QDs/HTM and MAPbI 3 /MAPbBr 0.4 I 2.6 -QDs/HTM solar cells Device V oc (mv) J sc (ma cm -2 ) FF PCE MAPbI 3 /MAPbBr 0.7 I 2.3 -QDs/HTM 895± ± ± ±0.24 MAPbI 3 /MAPbBr 0.4 I 2.6 -QDs/HTM 885± ± ± ±0.43 (%) Table S3. Photovoltaic parameters of QD mixed MAPbI 3 PSCs. Device V oc (mv) J sc (ma cm -2 ) FF PCE (%) Average PCE (%) MAPbI 3 -MAPbBr ±0.62 MAPbI 3 -MAPbBr 1.2 I ±1.30 MAPbI 3- MAPbBr 0.9 I ±0.28 Control ±0.71 S5

6 Figure S1. Schematic illustration of the reaction system for MAPbBr 3-x I x QDs Figure S2. UV-Vis absorption spectra of MAPbBr 3-x I x QDs S6

7 Figure S3. UPS for MAPbBr3-xIx QDs. UPS measurements were performed in an ultra- high vacuum chamber with a He(I) (21.2 ev) discharge lamp. Valence band positions can be extracted from the spectra according to literatures3. Figure S4. Top-view SEM images of MAPbI3 layer (a) before and (b) after deposition of MAPbBr3-xIx QDs. S7

8 Figure S5. Photoluminescence spectra of MAPbBr 0.7 I 2.3 and MAPbBr 0.4 I 2.6 QD solutions with excitation wavelength of 500 and 550 nm, respectively. Figure S6. Evolution of power conversion efficiency with time for PSCs with (red dots) and without MAPbBr 0.9 I 2.1 QDs (i.e. the control PSC, black dots). S8

9 Figure S7. UV-Vis absorption spectra of MAPbBr 3-x I x QDs deposited perovskite films. Figure S8. The schematic fabrication process of mixing the MAPbBr 3-x I x QDs inside the MAPbI 3 layer. S9

10 Figure S9. Photoluminescence decay curves of perovskite films mixed with MAPbBr 0.9 I 2.1 QDs. Figure S10. Photographs of MAPbI 3 films obtained by fast dropping of anhydrous chlorobenzene at the 3 rd, 6 th and 9 th seconds. References [S1] Zhang, F.; Zhong, H.; Chen, C.; Wu, X.-g.; Hu, X.; Huang, H.; Han, J.; Zou, B.; Dong, Y. ACS Nano 2015, 9, [S2] Da, P.; Cha, M.; Sun, L.; Wu, Y.; Wang, Z.-S.; Zheng, G. Nano Lett. 2015, 15, [S3] Carlson, B.; Leschkies, K.; Aydil, E. S.; Zhu, X.-Y. J.Phys. Chem. C 2008, 112, S10