GeI 2 additive for high optoelectronic quality CsPbI 3 quantum dots and their application in photovoltaic devices

Trioctylphosphine (TOP)-based syntheses of CsPbI3 perovskite quantum dots (QDs) yield unprecedented high photoluminescence quantum yield (PL QY), lower Stokes shifts, and longer carrier lifetimes due to their enhanced crystallinity. This synthetic route relies on a heavily Pb-rich condition or a large Pb:Cs molar ratio in precursor solution to produce QDs with appropriate stoichiometry as well as to guarantee a good colloidal stability. The high Pb condition is achieved by a high concentration of PbI2 prepared in TOP. Here we find such Pb-rich strategies can be avoided by providing additional iodine ions using other metal halide salts. In particular GeI2, which contrary to PbI2, readily dissolves in TOP. CsPbI3 QDs prepared using PbI2/GeI2 combination show near-unity PL QY and improved chemical stability compared to the previous synthetic route. Furthermore, we find no sign of Ge incorporation in the QDs (compositionally or energetically). The ensuing QD solar cells deliver power conversion efficiency of 12.15% and retain 85% of its peak performance after storage over 90 days. The PbI2/GeI2 dual-source iodine synthetic approach presented here represents a more rational and robust route to high-quality CsPbI3 QDs.