Thermodynamic Band Gap Model for Photoinduced Phase Segregation in Mixed-Halide Perovskites

Abstract

Provided is a comprehensive description of a band gap thermodynamic model, which predicts and explains key features of photosegregation in lead-based, mixed-halide perovskites. The model gives a prescription for illustrating halide migration driven by photocarrier energies. Where possible, model predictions are compared with experimental results. Free energy derivations are provided for three assumptions: (1) halide mixing in the dark, (2) a fixed number of photogenerated carriers funneling to and localizing in low band gap inclusions of the alloy, and (3) the statistical occupancy of said inclusions from a bath of thermalized photocarriers in the parent material. Model predictions include excitation intensity (I exc)-dependent terminal halide stoichiometries (x terminal), temperature-independent x terminal, excitation intensity thresholds (I exc,threshold) below which photosegregation is suppressed, and reduced segregation in nanocrystals as compared to thin films. The model also offers insight into kinetically manipulating photosegregation rates via control of underlying mediators and rationalizes asymmetries in forward and reverse photosegregation rate constants/activation energies. What emerges is a cohesive framework for understanding ubiquitous photosegregation in mixed-halide perovskites and a rational basis by which to manage the phenomenon.

In Situ PL Tracking of Halide Exchange at 3D/QD Heterojunction Perovskite Solar Cells

“…This is strong evidence that, during the halide exchange at the interface, the Br – originating from the QD can passivate the undercoordinated Pb 2+ (i.e., the halide vacancies) via a vacancy-mediated exchange mechanism and, consequently, reduce the nonradiative recombination pathways, increasing the PL intensity (detailed in the Supporting Information). ,− Under thermal annealing, the redshift of QD emission is accentuated (Figure g-j), suggesting, as one might have expected, temperature activation of the halide exchange reaction (individual spectra evolution is shown in Figure S3). The PL color plots also indicate that the PL related to the QD tends to reach a common emission wavelength, i.e., to reach a compositional equilibrium.…”