2, University of California, San Diego, San Diego, California, United States
3, Argonne National Laboratory, Argonne, Illinois, United States
Hybrid halide perovskites have achieved efficient operation in optoelectronic devices, but their commercialization is hindered by chemical instability under operating conditions. Understanding both the origin of this instability and its connection to performance is key to the development of perovskite optoelectronic devices beyond the laboratory. In particular, ion migration is proposed as an important limiting mechanism in hybrid perovskite devices causing chemical and performance-related instabilities such as hysteresis in scanning current-voltage measurements. Despite the concern regarding ion migration, open questions remain as to what is moving and how any stoichiometric changes relate to device performance.
Here we identify a direct relationship between halide migration and local optoelectronic quality. By using nanoprobe X-ray fluorescence with 200 nm resolution we mapped elemental changes in thin single crystals of methylammonium lead bromide (CH3NH3PbBr3) while systematically applying an electric field across the crystal in a lateral back-contact device. This experiment revealed the migration of Br- across the perovskite crystal in the opposite direction of the electric field. Photoluminescence (PL) mapping reveals PL intensity increases in halide-rich regions and decreases in halide-poor ones, with quasi-reversible variation observed over multiple voltage biasing cycles. Nudged elastic band calculations indicate that the alignment of the methylammonium cation under bias forms channels that facilitate halide migration along the field direction. The direct link between halide migration and intrinsic optoelectronic response clarifies that halide migration is a challenge that is intrinsic to the absorber and one that plays a determining role in the performance limits of perovskite devices.