Modulation of Photoinduced Iodine Expulsion in Mixed Halide Perovskites with Electrochemical Bias

Hole trapping at iodine (I) sites in MAPbBr(1.5)I(1.5) mixed halide perovskites (MHP) is responsible for iodine migration and its eventual expulsion into solution. We have now modulated the photoinduced iodine expulsion in MHP through an externally applied electrochemical bias. At positive potentials, electron extraction at TiO2/MHP interfaces becomes efficient, leading to hole buildup within MHP films. This improved charge separation, in turn, favors iodine migration as evident from the increased apparent rate constant of iodine expulsion (k(expulsion) = 0.0030 s(-1)). Conversely, at negative potentials (-0.3 V vs Ag/AgCl) electron-hole recombination is facilitated within MHP, slowing down iodine expulsion by an order of magnitude (k(expulsion) = 0.00018 s(-1)). The tuning of the E-Fermi level through external bias modulates electron extraction at the TiO2/MHP interface and indirectly controls the buildup of holes, ultimately inducing iodine migration/expulsion. Suppressing iodine migration in perovskite solar cells is important for attaining greater stability since they operate under internal electrical bias.

Automated summary

This study demonstrates that external electrochemical bias can modulate photoinduced iodine expulsion in mixed halide perovskites, with efficient electron extraction at positive potentials favoring iodine migration, and electron-hole recombination at negative potentials slowing down iodine expulsion. The tuning of the E-Fermi level through external bias controls charge separation and ultimately influences iodine migration/explusion in perovskite solar cells, which is crucial for their stability under internal electrical bias.