A simple one-step solvent-bathing process based on solvent–solvent extraction, is demonstrated for the controlled room-temperature crystallization of uniform, ultra-smooth hybrid-perovskite thin films for high-efficiency solar cells.

ABSTRACT:The crystal morphology of organolead trihalide perovskite (OTP) light absorbers can have profound influence on the perovskite solar cells (PSCs) performance. Here we have used a combination of conventional transmission electron microscopy (TEM) and high-resolution TEM (HRTEM), in cross-section and plan-view, to characterize the morphologies of a solution-processed OTP (CH 3 NH 3 PbI 3 or MAPbI 3 ) within mesoporous TiO 2 scaffolds and within capping and planar layers. Studies of TEM specimens prepared with and without the use of focused ion beam (FIB) show that FIBing is a viable method for preparing TEM specimens. HRTEM studies, in conjunction with quantitative X-ray diffraction, show that MAPbI 3 perovskite within mesoporous TiO 2 scaffold has equiaxed grains of size 10−20 nm and relatively low crystallinity. In contrast, the grain size of MAPbI 3 perovskite in the capping and the planar layers can be larger than 100 nm in our PSCs, and the grains can be elongated and textured, with relatively high crystallinity. The observed differences in the performance of planar and mesoscopic-planar hybrid PSCs can be attributed in part to the striking differences in their perovskite-grain morphologies. S olar cells based on solution-processed organometal trihalide perovskite (OTP) have emerged as a new "player" in the photovoltaics (PVs) field over the past few years. 1−4Since Miyasaka and coworkers 5 first reported the use of methylammonium lead triiodine (CH 3 NH 3 PbI 3 or MAPbI 3 ) perovskite as light harvester, the power conversion efficiency (PCE) of perovskite solar cells (PSCs) has topped 20% within a short period of time.6 On the basis of the architecture of the photoactive OTP layer, the PSC embodiments are broadly classified as 1−4,7 (i) mesoscopic, (ii) planar, and (iii) mesoscopic-planar hybrid. In the mesoscopic PSC the photoactive layer consists of a thick (typically 300 to 1000 nm) mesoporous oxide (TiO 2 , Al 2 O 3 ) scaffold fully infiltrated by the OTP. The scaffold helps anchor the OTP, and in the case of TiO 2 it provides electron-conduction pathways. 8 The planar PSC is based on a thin film of OTP without the mesoporous oxide scaffold, and it has gained popularity due to the enhanced light absorption and simpler architecture; however, planar PSCs are generally prone to photocurrent (J)−voltage (V) hysteresis issues. 9 The third emerging type of PSCs with mesoscopicplanar hybrid structure aspires to combine the two structures and their desirable attributes into one. These PSCs typically use a mesoporous oxide scaffold that is fully infiltrated by the OTP and then topped with a planar OTP "capping" layer, which enhances light absorption, 10,11 resulting in PSCs with hysteresis-free PCE of >20%. 12There have been numerous studies on controlling the overall morphology/coverage (see, e.g., ref 13) and crystallinity (see e.g. ref 14) of OTP thin films in the different PSC embodiments and their effects on OTP properties and performance of the PSCs; however, there is paucity of detailed characteriz...