Distribution of bromine in mixed iodide-bromide organolead perovskites and its impact on photovoltaic performance

Mixed iodide-bromide (I-Br) organolead perovskites are of great interest for both single junction and tandem solar cells since the optical bandgap of the materials can be tuned by varying the bromine to iodine ratio. Yet, it remains unclear how bromine incorporation modifies the properties of the perovskite solar cells. Here we use methylammonium lead iodide-bromide [MAPb(I1-xBrx)(3) (x = 0-0.11)] as a model system to study the question. Through elemental analysis we found that bromine exhibits an increased concentration towards the interface between perovskite and TiO2, and that such interface bromine accumulation is more pronounced when a HI acid additive is used to produce the perovskite films. Opto-electrical characterization results further suggest that the carrier lifetime in the perovskite and the charge extraction at the perovskite/TiO2 interface are improved in the mixed halide perovskite samples. Accordingly, the optimal thickness of the perovskite active layer is increased from 330 nm for MAPbI(3) to 510 nm for MAPb(I0.89Br0.11)(3), resulting in an average efficiency increase of 1.6%. The study highlights the role of bromine in passivating defect states at grain boundaries and interfaces in mixed halide perovskite solar cells and demonstrates the use of solvent additives in tuning the electronic properties of the mixed halide perovskite materials