462 doi:10.1017/S143192761801454X Microsc. Microanal. 24 (Suppl 2), 2018 © Microscopy Society of America 2018 Mapping Highly Efficient Mixed-cation Pseudohalide-perovskite Solar Cells with a Scanning Transmission X-ray Microscope Ming-Wei Lin 1 , Hung-Wei Shiu 1 , Kuo-Chin Wang 2 , Ming-Hsien Li 2 , Yu-Ling Lai 1 , Takuji Ohigashi 3 , Nobuhiro Kosugi 3 , Tzung-Fang Guo 2 , Peter Chen 2 , and Yao-Jane Hsu 1, 2 * 1 National Synchrotron Radiation Research Center, Hsinchu, Taiwan, R.O.C. 2 Department of Photonics, National Cheng Kung University, Tainan, Taiwan, R.O.C. 3 UVSOR Synchrotron, Institute for Molecular Science, Okazaki, Japan. * Yao-Jane Hsu, yjhsu@nsrrc.org.tw The organic−inorganic hybrid perovskite structure has rapidly attracted much attention because of the highly efficient photovoltaic cells in which it serves as a light sensitizer [1]. The perovskites are typically metal-organic frameworks of form ABX3, in which A is CH3NH3 + (MA) or CH3(NH2)2 + (FA) or Cs + , B is Pb 2+ or Sn 2+ , and X is I - or Cl - or Br - . For example, methylammonium lead iodide (MAPbI3) and its analogues exhibit novel properties such as ambipolar charge transport, a large absorption coefficient and an effective electron- or hole-diffusion length, making them suitable for photovoltaic applications. Among various perovskites, the mixed-cation lead pseudohalide perovskites were particularly successful with a large power-conversion efficiency (PCE) and satisfactory stability against moisture [2-3]. The large crystalline size with few trap states, increased entropy or decreased formation of solid solution are experimentally or theoretically proposed to interpret the origin of this high efficiency and stability. The variation of film morphologies is typically examined with a conventional TEM or SEM that provides only the bulk structure in a cross-sectional view or the surface topography in a top view. The necessity arises to improve the mapping capability to inspect the multilayer structures with a scaffold of an electron- transport layer (ETL)/perovskite/hole-transport layer (HTM) in perovskite-based solar cells. We have investigated the origin of the large PCE and the stability of mixed-cation pseudohalide perovskite solar cells using a scanning transmission x-ray microscope (STXM). In our experiments to fabricate perovskite solar cells, the mixed cation with formamidinium (FA: HC(NH)2) and Cs cations replaced methylammonium. To improve the PCE efficiency and structural stability, we used lead thiocyanate (Pb(SCN)2) as dopant so that the thiocyanate (SCN − ) anions replaced the halide ions in the tetragonal sites of the unit lattice of perovskite FA0.9Cs0.1PbI3 solar cells. The cell devices with mesoscopic titania as scaffold/electron-transport layer and spiro-OMeTAD as hole-transport layer were examined for the photovoltaic performance under standard illumination conditions. PCE greater than 16 % was achieved with optimized Pb(SCN)2 doping relative to 13.9 % for cells without Pb(SCN)2. To understand the origin of this improved efficiency and structural stability, we used a STXM to examine the chemical structure and morphology. The samples of FA0.9Cs0.1PbI3 cuboids and with varied Pb(SCN)2 concentration in molar ratios 5%, and 10% were prepared in our laboratory. The NEXAFS absorption spectra and STXM images at varied absorption edges, such as the C, O, N K-edge and Ti L-edge, were recorded at BL 09A2 spectroscopy of Taiwan Light Source at NSRRC and at BL4U in UVSOR Synchrotron in Japan, respectively. To study the effect of doping of Pb(SCN)2 on the efficiency and stability, we prepared pristine perovskite FA0.9Cs0.1PbI3 and Pb(SCN)2-doped FA0.9Cs0.1PbI3 and spin cast them on mesoporous TiO2 layers (mp- TiO2) that were pre-cast on SiN membranes. Figure 1 displays STXM images of the C K-edge acquired at 287.6 eV, which is attributed to a C-N * resonance of FA (HC * (NH)2) in FA0.9Cs0.1PbI3 with Pb(SCN)2 doped at 5 % (Fig. a) and 10 % (Fig. b). Compared to pristine FA0.9Cs0.1PbI3 (that shows no contrast), the https://doi.org/10.1017/S143192761801454X Downloaded from https://www.cambridge.org/core. IP address: 181.214.251.121, on 25 Apr 2020 at 11:47:30, subject to the Cambridge Core terms of use, available at https://www.cambridge.org/core/terms.