Unveiled Effects of Methylammonium Chloride Additives on Formamidinium Lead Halide: Expediting Carrier Injection from the Photoabsorber to Carrier Transport Layers through Spontaneously Modulated Heterointerfaces in Perovskite Solar Cells

Perovskite solar cells (PSCs) based on narrow-bandgap formamidinium lead halide (FAPbI3) photoabsorber have garnered substantial attention owing to their high photovoltaic (PV) performances. Extensive studies have established that the introduction of methylammonium chloride (MACl) significantly improves the bulk quality of FAPbI3 so commonly used; while heating the photoabsorber’s precursor film, the incorporated MACl facilitates the crystalline growth of FAPbI3, simultaneously volatilizing and dissipating from the perovskite layer. However, not only the photoabsorber's bulk quality but also heterointerfaces between the photoabsorber and carrier transport materials importantly contribute to the PV performances. Paradoxically, the MACl effects on FAPbI3 heterointerfaces have been sparingly explored and consequently remained elusive. Herein, the effects of MACl on these heterointerfaces are unveiled by time-resolved photoluminescence spectroscopy and time-resolved microwave conductivity. The MACl additive accelerates carrier injections from FAPbI3 to the carrier transport materials presumably owing to the occurrence of spontaneous modulation of the heterointerfaces. In particular, at the heterointerface between FAPbI3 and the titanium oxide (TiO2) electron transport layer, an emissive interlayer that is chloride-containing and wide-bandgap (FAPbI3-xClx) is formed spontaneously, which most likely serves multiple advantages leading to the observed PV performance enhancement: hole blocking and facilitation of electron injection from FAPbI3 to TiO2 without additional hole trapping. Consequently, the present results provide novel insights into the widely employed MACl additive in FAPbI3 photoabsorbers and thereby propels the further advancement of PSCs. Furthermore, this study demonstrates effective investigation of carrier dynamics with regard to the heterointerface, which is challenging, using potent time-resolved spectroscopies, so will promote development in the material sciences.