Publication date: 2nd November 2020
Trapping of holes at iodide sites in mixed halide perovskite, MHP (MAPbBr1.5I1.5 ) films cause iodide ions to migrate toward grain boundaries, thus inducing the formation of iodide rich phases and bromide rich phases. When mixed halide perovskite films are in contact with solution, the migration of iodine extends beyond phase segregation as it gets expelled into solution. The selective removal of iodine from MHP upon continued irradiation transforms the perovskite film into a bromide-rich perovskite film. Substituting A-site cation of MHP with cesium (Cs) slows down iodide expulsion due to increased thermodynamic stabilization of MHP lattices. Similar lattice stabilization has also been observed in Cl-alloyed perovskites. Furthermore, photoinduced iodide expulsion process in MHPs can be modulated through externally applied electrochemical bias. At anodic potentials, electron extraction at TiO2/MHP interfaces becomes more efficient, leading to hole build-up within MHP films. This improved charge separation, in turn, favors iodine migration as evident from the increased apparent rate constant of iodine expulsion (kexpulsion = 0.0030 s-1). Conversely, at cathodic bias (-0.3 V vs. Ag/AgCl potential) electron-hole recombination is facilitated within MHP films, slowing down iodine expulsion by an order of magnitude (kexpulsion = 0.00018 s-1). The tuning of the EFermi level through external bias modulates electron extraction at the TiO2/MHP interface indirectly controls the build-up of holes, which ultimately induces iodine migration/expulsion. Suppressing iodine migration in perovskite solar cells is important for attaining greater stability since they operate under internal electrical bias.
P.V.K. acknowledges the support of the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences of the U.S. Department of Energy, through award DE-FC02-04ER15533.
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