High-voltage and green-emitting perovskite quantum dot solar cells via solvent miscibility-induced solid-state ligand exchange

Advances in surface chemistry and manipulation of CsPbI₃ perovskite quantum dots (PQDs) have enabled the replacement of native long-chain ligands with short-chain ligands, leading to their photovoltaic applications; however, there are no reports on those of wide-bandgap and green-emitting CsPbBr₃ PQDs that are promising in high-voltage and colorful building-integrated photovoltaics. Binding energies required for ligand adsorption/desorption alter according to halide compositions of PQDs because of different soft/hard acid/base interactions, and therefore, the surface ligand-exchange process for CsPbBr₃ PQDs should be developed. Herein, we demonstrate the utilization of CsPbBr₃ PQDs in green light-emitting solar cells with a high open-circuit voltage (V_OC) of 1.6 V, realized via solvent miscibility-induced ligand exchange. Carboxylate esters with different alkyl chain lengths are used; longer carboxylate esters show high miscibility with hydrophobic substances, leading to more efficient ligand exchange with preserving CsPbBr₃ PQD size but at the same time undesired less film thickness because of the stripping-out of as-cast CsPbBr₃ PQDs. Based on these results, we devise a suitably optimized solvent mixture of carboxylate esters to enable efficient ligand exchange with suppressed stripping-out phenomena. Therefore, the resultant CsPbBr₃ PQD solids show a power conversion efficiency of 4.23% and a V_OC of ∼1.6 V with green electroluminescence under applied voltage.