(Invited) Breaking Centro-Symmetry in Formamidinium Lead Triiodide (FAPbI3) Films with Preferred Oriented Grains

Formamidinium lead triiodide (FAPbI3) is gaining much interest with an optimal band gap of ~ 1.5 eV. Although α-FAPbI3 is the most thermodynamically stable photoactive phase, it can kinetically stabilize δ-FAPbI3 in ambient. Many attempts were studied to stabilize the black α-phase, for example, pseudo-halide anion engineering, ionic liquid engineering, and incorporation of additives. Incorporating with methylammonium chloride (MACl) as an additive not only induces the stabilized α-FAPbI3, but also affects the crystallization kinetics of α-FAPbI3. Along with the role of methylammonium chloride (MACl) as a ‘stabilizer’ in the formation ofα-FAPbI3, we pointed out the additional role as a ‘controller’ in the crystallization kinetics. Herein, we examined the crystallization process of MACl, as higher concentration of MACl induces slower crystallization kinetics. Microscopic observations, such as electron back-scattered diffraction (EBSD), confocal photoluminescence (PL), time-resolved photoluminescence (TRPL) mapping, and conductive atomic force microscopy (C-AFM), were conducted to examine the optoelectronic properties of [001] preferentially oriented grains. Reduction in the non-radiative recombination, a longer lifetime of charge carriers and less photocurrent deviation between each grain induce higher short-circuit current density (Jsc ) and fill factor (FF) in α-FAPbI3 based perovskite solar cells (PSCs) with MACl40%.It highlights the importance of crystallization kinetics resulting in desirable microstructures for device engineering.