Illumination-Induced Halide Segregation in Gradient Bandgap Mixed-Halide Perovskite Nanoplatelets

Efficient energy funneling has exhibited great contribution to the high performance of perovskite-based optoelectronic devices. Here, formamidinium (FA⁺, HC(NH₂)₂ ⁺) lead mixed-halide nanoplatelets (FAPb(Br_xI_{1− x})₃) with gradient bandgap are fabricated using chemical vapor deposition followed with bromide–iodide substitution by exposure to FABr vapor. The as-fabricated perovskite nanoplatelets exhibit pure bromide phase in the thin nanoplatelet (tens of nanometers) and a gradual bromide–iodide composite, thus with gradient bandgap (2.29–1.56 eV), in the thick nanoplatelet (more than hundreds of nanometers). Accordingly, photoluminescence (PL) spectra are observed at 540, 560/610, and 735/790 nm, respectively. In such gradient bandgap structures, photogenerated carriers can effectively transfer and emit in the low-bandgap region by energy funneling. With illumination, the PL spectrum of Br-rich phase exhibits blueshift and therefore 610 nm band disappears. In contrast, redshift is observed in I-rich phase due to the decrease of 735 nm band while an increase of 790 nm band. It is demonstrated that irreversible and stable phases are formed with illumination in both Br-rich and I-rich nanoplatelets. This investigation develops a method to fabricate gradient bandgap perovskites with designed energy funneling, and also provides significant insight into the halide segregation in such special perovskites, which greatly benefits their future optoelectronic applications.