In Situ Dual‐Ionic Charge Compensation for Cs _x FA _1‐x PbI ₃ Perovskite Quantum Dot Solar Cells with Over 18% Efficiency

Cesium-formamidinium lead triiodide perovskite quantum dots (Cs_xFA_1-xPbI₃ PQDs) receive increasing attention for new-generation solar cells due to their outstanding optoelectronic properties and solution processibility. However, during the synthesis of Cs_xFA_1-xPbI₃ PQDs, PQDs seriously suffer from the ligand detachment from the PQD surface under the polar antisolvent, leaving numerous surface vacancies that significantly compromise the surface lattice integrity and optoelectronic properties of PQDs. A facile dual-ionic charge compensation strategy is introduced through the bimolecular nucleophilic substitution (S_N2) to reinforce the surface lattice of Cs_xFA_1-xPbI₃ PQDs. The dual-ionic ligands produced during the S_N2 reaction could in situ fill the surface vacancies of PQDs in the nonpolar solvent, which significantly improves the surface lattice integrity and thus the optoelectronic properties of PQDs, substantially diminishing trap-assisted nonradiative recombination. Consequently, the PQDs solar cells show a power conversion efficiency of up to 18.17%, representing the highest efficiency in Cs_xFA_1-xPbI₃ PQD solar cells. The remarkable photovoltaic performance is attributed to the reinforced surface lattice of PQDs, suppressing the energy losses induced by the nonradiative recombination. This study provides crucial design principles for optimizing the crystalline structure integrity of PQDs, which also paves a new avenue for developing high-performance solar cells or other optoelectronic devices.