Chemical engineering of methylammonium lead iodide/bromide perovskites: tuning of opto-electronic properties and photovoltaic performance

Abstract

Hybrid (organic–inorganic) lead trihalide perovskites have attracted much attention in recent years due to their exceptionally promising potential for application in solar cells. Here a controlled one-step method is presented where PbCl₂ is combined with three equivalents methylammonium halide (MAX, with X = I and/or Br) in polar solvents to form MAPb(I_1−xBr_x)₃ perovskite films upon annealing in air at 145 °C. The procedure allows for a linear increment of the semiconductor bandgap from 1.60 eV to 2.33 eV by increasing the Br content. A transition from a tetragonal to a cubic structure is found when the Br fraction is larger than 0.3. X-ray photoelectron spectroscopy investigations shows that the increase of Br content is accompanied by a shift of the valence band edge to lower energy. Simultaneously, the conduction band moves to higher energy, but this shift is less pronounced. Time-resolved single-photon counting experiments of the perovskite materials on mesoporous TiO₂ show faster decay kinetics for Br containing perovskites, indicative of improved electron injection into TiO₂. Interestingly, kinetics of MAPbI_2.7Br_0.3Cl_y on TiO₂ scaffold became faster after prolonged excitation during the measurement. In solar cell devices, MAPbI_2.7Br_0.3Cl_y yielded best performance, giving more than 14% power conversion efficiency when used in combination with an n-type contact consisting of fluorine-doped tinoxide glass coated with dense TiO₂ and a mesoporous Al₂O₃ scaffold, and a p-type contact, spiro-MeOTAD/Ag.