Unraveling the effect of PbI₂ concentration on charge recombination kinetics in perovskite solar cells

CH₃NH₃PbI₃ perovskite solar cells have rapidly risen to the forefront of emerging photovoltaic technologies. A solution-based, two-step method was reported to enhance the reproducibility of these solar cells. In this method, first a coating of PbI₂ is applied by spin-coating onto a TiO₂-coated substrate, followed by a dip in a methylammonium iodide solution, leading to conversion to CH₃NH₃PbI₃. The concentration of PbI₂ in the spin-coating solution is a very important factor that affects the infiltration of the perovskite and the amount deposited. The best solar cell performance of 13.9% was obtained by devices prepared using 1.0 M of PbI₂ in dimethylformamide. These devices also had the longest electron lifetime and shortest carrier transport time, yielding lowest recombination losses. Rapid quenching of the perovskite emission is found in device-like structures, suggesting reasonably good efficient carrier extraction at the TiO₂ interface and quantitative extraction at the spiro-OMeTAD interface.