ET04.01.02 : Understanding the Role of Surfaces on Halide Segregation in Mixed-Anion Perovskites

8:30 AM–8:45 AM Nov 26, 2018 (US - Eastern)

Hynes, Level 3, Room Ballroom C

Rebecca Belisle1 Kevin Bush1 Luca Bertoluzzi1 Aryeh Gold-Parker1 Michael Toney1 Michael McGehee1

1, Stanford Univ, Stanford, California, United States

Photo-induced halide-segregation currently limits the perovskite chemistries available for use as high bandgap semiconductors for tandem solar cells. This is particularly problematic for perovskite-perovskite tandems, where semiconductors with bandgaps in excess of 1.8 eV are needed for optimal pairing with the low-bandgap tin-rich perovskites. And while the problem of halide-segregation is well documented in the literature, strategies to circumvent this problem are largely lacking. In this study we present a new method for hindering this process – surface modifications. By varying the surface chemistry of mixed-anion perovskites and monitoring the evolution of their photoluminescence and X-ray diffraction patterns under illumination, we link changes in the perovskite surface to changes in the rate of halide segregation. We observe that we can both reduce non-radiative recombination and dramatically slow the onset of halide segregation by applying specific post-deposition surface treatments to CH3NH3PbI2Br films. Additionally we demonstrate that the surface sensitivity of halide segregation extends to perovskite/selective contact interfaces as well, and that halide segregation is suppressed at specific perovskite/selective contact heterojunctions. Finally, by using these observations and an in depth knowledge of the perovskite surface chemistry, we present a model by which tuning surface chemistry can prevent halide segregation in the bulk of the perovskite. In short, we propose carrier trapping at perovskite surfaces as the driver for halide migration and subsequent halide segregation, and that by reducing the trapping at surface states with targeted surface treatments this process can be abated if not completely stopped. Overall, this work presents both a deeper understanding of the halide-segregation process in perovskites as well as a pathway towards stable high bandgap perovskites for high efficiency perovskite tandems.