Optical and electronic properties of mixed halide (X = I, Cl, Br) methylammonium lead perovskite solar cells

We report on the fabrication and opto-electronic characterization of solution-processed planar heterojunction perovskite solar cells based on methylammonium (MA) lead halide derivatives, MAPbI3−xYx (Y = Cl, Br, I). Dissolving equimolar amounts of lead iodide (PbI2) and methylammonium iodide (H3CNH3I) together with various amounts of additional methylammonium halides, perovskite precursor solutions were obtained, which were used in the fabrication of three perovskite systems, MAPbI3, MAPbI3−xClx and MAPbI3−xBrx. The effect of the halide ratio in the perovskite formulations processed via a one-step deposition technique on optoelectronic properties and on photovoltaic performance of the formed perovskites was investigated. The perovskite film morphology, temperature-dependent photoluminescence properties, hysteresis behaviour in current-voltage characteristics and the photovoltaic performance as a function of chemical composition were studied using microscopic, spectroscopic and photovoltaic characterization techniques. The power conversion efficiency was found to be dependent on MAPbI3−xYx (Y = Cl, Br, I) perovskite film morphology. By controlling perovskite precursor composition and stoichiometry, highest power conversion efficiencies of 9.2, 12.5 and 5.4% were obtained for MAPbI3, MAPbI3−xClx and MAPbI3−xBrx devices, respectively. In addition, the physical parameters of the mixed halide perovskites such as the exciton binding energy, exciton-phonon interaction and bandgap were determined via temperature-dependent photoluminescence spectroscopy. Exciton binding and optical phonon energies of MAPbI3−xYx (Y = Cl, Br, I) were found to be in the ranges of 49-68 meV and 29-32 meV respectively. The solution-processed MA lead halide derivatives form highly crystalline materials with chemical versatility allowing the tuning of their optical and electronic properties depending on the nature and the ratio of the halides employed.