Unveiled effects of the methylammonium chloride additive on formamidinium lead halide: expediting carrier injection from the photoabsorber to carrier transport layers through spontaneously modulated heterointerfaces in perovskite solar cells

Abstract

Perovskite solar cells (PSCs) based on a 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 quality of the FAPbI₃ bulk, and hence, this method has been commonly employed. Upon 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 remain 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 injection 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, a chloride-containing wide-bandgap FA_1−xMA_xPbI_3−yCl_y interlayer is formed spontaneously, which most likely has multiple advantages: hole blocking and facilitation of electron injection from FAPbI₃ to TiO₂ without additional hole trapping, leading to the observed PV performance enhancement. Consequently, the present results provide novel insights into the effects of the widely employed MACl additive on the FAPbI₃ photoabsorber, thereby propelling the further advancement of PSCs. Furthermore, this study demonstrates effective investigation of carrier dynamics with regard to the heterointerfaces, which is challenging, and will promote the development of the materials science field.