Heterovalent Substitution in Mixed Halide Perovskite Quantum Dots for Improved and Stable Photovoltaic Performance

One of the strategies to improve the device performance of metal-halide perovskite solar cells is to alter the electronic and optical properties of the perovskite lattice by metal ion doping. In this work, aliovalent doping of silver at the lead site of CsPbBr 1.5 I 1.5 quantum dots (QDs) shows striking prospects in improving the photovoltaic (PV) device performance. Lattice doping could be achieved only up to ∼3.5 atom % Ag + with respect to Pb 2+ which has significant impact on the electronic and optical properties of the QDs. The band gap could be reduced from 2.15 eV for pristine QDs to 2.12 eV with 3.5 atom % Ag-doping, beyond which midband gap states are created along with the formation of a secondary AgI phase with higher Ag + substitution. Density functional theory (DFT) shows enhanced optical absorption coefficient and p-type character of the Ag-doped QDs. With the QDs having 3.5 atom % Ag, a significant ∼20% enhancement in photoconversion efficiency (PCE) is observed up to 9.67 ± 0.12% due to the reduction of surface and intrinsic defects, decrease in nonradiative recombination leading to an increase in carrier lifetime, and increase in charge transfer. Although in general all-inorganic perovskite (AIPSK) QDs are attractive because of their higher crystallinity, better optical property, and easy solution processability, the solution stability of AIPSK QDs is not the same as that in PV devices. After 3.5 atom % Ag-doping, our PV devices show extremely promising ambient stability for 575 h (24 days) with less than 5% drop in PCE in comparison to 12% drop for pristine QD devices.