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Cheng-Min Tsai1 Eric Wei-Guang Diau1

1, National Chiao Tung University, Hsinchu, , Taiwan

All-solid-state organic-inorganic lead perovskites have attracted much attention, due to its rapid and tremendous development within the photovoltaic field. However, toxic lead-containing still a big issue of this field. We developed and characterized methylammonium tri-halide tin perovskites (MASnIBr2-xClx) for carbon-based mesoscopic solar cells free of the hole-transporting layers. There is a question whether iodide and chloride co-crystalize together in a perovskite structure. We found that the iodide and chloride can form the stable single phase, but based on certain amounts of bromide within the structure. Three different halides (Cl, Br and I) co-crystalized inside the crystal of tin perovskites, MASnIBr2-xClx, according the stoichiometric ratios, from x = 0 (SnBr2/SnCl2 = 100/0) to x = 0.5 (SnBr2/SnCl2 = 75/25). When the SnCl2 ratio was higher than 25 % (x > 0.5), phase separation occurred to generate MASnI3-yBry and MASnCl3-zBrz these two phases. The MASnIBr1.8Cl0.2 (SnCl2 = 10 %) device showed the best device performance with VOC/mV = 380, JSC/mA cm-2 = 14.0, FF = 0.573 and PCE/% = 3.1 with great long-term stability and reproducibility. Transient PL decay measurements were carried out to show the intrinsic problem of tin-based perovskites with the averaged lifetimes less than 100 ps, for which the MASnIBr1.8Cl0.2 film featured the longest lifetime over the others to account for its greater device performance than the others. What is more, we also investigated the effect of a bi-functional ammonium cation, EOAI, on structure and energy levels of a tin-based perovskite, FASnI3. Following the increase of EOAI ratio within the EOAyFA(1-y)SnI3, the valence band maximum of tin perovskite can be modified linearly from deficient -4.91 eV to appropriated -5.50 eV and the crystal structure will transform from 3D to a 2D/3D hybrid structure. Finally, the power conversion efficiency of tin-based carbon electrode perovskite solar cells improved 5 times, which that compared with the FASnI3 reference device. X-ray diffraction (XRD) analysis, ultraviolet photoelectron spectra (UPS) and electrochemical impedance spectra (EIS) were performed to understand the crystal structures and the photovoltaic performances of the devices for this series of tin perovskite solar cells.

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