Organic-Inorganic Perovskites (OIPs) have demonstrated bandgap tunability over a 400-800nm range, making them attractive for optoelectronic applications. This bandgap may be tuned by substituting the halides of the OIP, which has prompted much research into the phase stability of mixed-halide OIPs. A miscibility gap was predicted for mixed I-Br OIPs, consistent with the spontaneous phase separation observed both at equal I:Br composition and under illumination, for a wider range of alloys.
This work investigates phase stability and halide migration in single-halide and mixed-halide OIPs by examining changes in the interface profiles of OIP-OIP thin film lateral heterostructures. The effects of heat, light, and defect concentration in the OIP thin films on halide migration (or lack thereof) are systematically investigated. Methylammonium (MA)PbI3-MAPbBr3, MAPb(I0.88Br0.12)3-MAPbBr3 and MAPb(I0.3Br0.7)3-MAPbBr3 heterostructures were synthesized by halide exchange and their interface profiles after exposure to heat/light were characterized using Energy-Dispersive X-ray analysis, optical microscopy, and confocal microscopy. None of the interface profiles changed after extreme and lengthy exposure to heat. The profile of the MAPb (I0.3Br0.7)3-MAPbBr3 interface did not change after lengthy exposure to light (1 Sun). The roles of defect concentration and interface morphology are explored as explanations for this unexpected behavior. A comparison of our results on the phase stability at lateral heterojunctions to the phase stability of alloy thin films will be presented.