Perovskite Quantum Dot Photovoltaic Materials beyond the Reach of Thin Films: Full-Range Tuning of A-Site Cation Composition

We present a cation-exchange approach for tunable A-site alloys of cesium (Cs+) and formamidinium (FA(+)) lead triiodide perovskite nanocrystals that enables the formation of compositions spanning the complete range of Cs(1-x)FA(x)PbI(3), unlike thin-film alloys or the direct synthesis of alloyed perovskite nanocrystals. These materials show bright and finely tunable emission in the red and near-infrared range between 650 and 800 nm. The activation energy for the miscibility between Cs+ and FA(+) is measured (similar to 0.65 eV) and is shown to be higher than reported for X-site exchange in lead halide perovskites. We use these alloyed colloidal perovskite quantum dots to fabricate photovoltaic devices. In addition to the expanded compositional range for Cs(1-x)FA(x)PbI(3) materials, the quantum dot solar cells exhibit high open circuit voltage (V-OC) with a lower loss than the thin-film perovskite devices of similar compositions.