2020同步年報

Physics and Materials Science 013 Two Steps Are Better than One Using grazing-incidence wide-angle X-ray scattering, researchers have studied the precisely con- trolled kinetics of crystallization of a benchmark FAPbI 3 -based perovskite film with sequential A-site doping of Cs + and GA + ; the results show how a process of two steps instead of a single step yields higher-quality perovskite solar cells with enhanced efficiency of 23%. P erovskite solar cells (PSC) typically consist of a hybrid organic-inorganic lead or tin-halide-based material as a light- harvesting active layer, and have become increasingly important because of power-conversion efficiencies (PCE) over 25% in recent times. Whereas the first MAPbI 3 -based (MA = methyl ammonium; CH 3 NH 3 ) PSC were fabricated via a one-step spin-coating process in 2009, 1 subsequent reports showed the importance of a two-step method 2 as well as an anti-solvent- assisted one-step method to improve PCE. 3 On the other hand, replacing MA cations with FA cations (FA = formamidinium, (CH(NH 2 ) 2 + ) also showed that PCE improved up to 19%. 4 Recent studies have shown that doping the A-site with cations such as Cs + , Rb + and guanidinium (GA + ; (C(NH 2 ) 3 + )) has resulted in further improvements with decreased defect densities and non-radiative recombination. 5 It has been reported earlier that the kinetics of crystal growth of the one-step process is complicated and sensitive to time, as it generally involves a dynamic competition between multiple phases and intermediate phases. 6 Similar studies on the two-step process, specifically addressing the sequential A-site doping, have, however, not been reported in the literature. To understand the mechanism of formation of a perovskite film, researchers have now undertaken a systematic set of time- resolved grazing incidence wide-angle X-ray scattering (GIWAXS) in-situ experiments at the small- and wide-angle X-ray scattering (SWAXS) beamline, TLS 23A1 of NSRRC. 7 The wavelength of the X-rays was 1.240 Å (energy 10 keV); the scattered signals were collected with a C9728DK area detector, which was placed at distance ≈ 169 mm from the sample. The distance from sample to detector was calibrated with a lanthanum hexaboride (LaB 6 ) sample. The spin-coating and annealing proce- dures were conducted with a custom-built spin-coating chamber and a remotely controlled hotplate chamber. As shown schematically in Fig. 1 , in experiments undertaken in the first step, a PbI 2 layer is deposited by spin-coating a PbI 2 precursor solution, followed by thermal annealing at 70 °C. For experiments in the second step, a solution of propanol-2 (IPA) containing organic salts (formamidinium iodide (FAI), methylammonium bromide (MABr) and methylammonium chloride (MACl)) was injected onto the PbI film through a motorized syringe on top of the film formed in the first step. This treatment creates a perovskite film through the interdiffusion of PbI 2 and organic salts. The entire process was monitored with GIWAXS. The spinning speed of both steps was maintained at 1500 rpm, which is the same value as in the device fabrication, thus representing the actual conditions used to make real devices. Fig. 1 : Schematics of two-step fabrication and synchrotron-based GIWAXS measurements in situ . (a) Two-step fabrication of perovskite films doped with Cs + and GA + during the first and second steps, respectively. (b) Sketch of a map of synchrotron-based GIWAXS measurements in situ . [Reproduced from Ref. 7]