Dual-Phase Ligand Engineering Enables 18.21% FAPbI Quantum Dot Solar Cells.

Formamidinium lead triiodide (FAPbI) perovskite quantum dot (PQD) are promising candidate for high-performing quantum dot photovoltaic due to its narrow bandgap, high ambient stability, and long carrier lifetime. However, the carrier transport blockage and nonradiative recombination loss, originating from the high-dielectric ligands and defects/trap states on the FAPbI PQD surface, significantly limit the efficiency and stability of its photovoltaic performance. In this work, through exploring dual-site molecular ligands, namely 2-thiophenemethylammonium iodide (2-TM) and 2-thiopheneethylammonium iodide (2-TE), a dual-phase synergistic ligand exchange (DSLE) protocol consisting of both solution-phase and solid-state ligand engineering is demonstrated. The DSLE strategy effectively replaces the native long insulating ligands and simultaneously passivate surface defects in hybrid FAPbI PQDs, leading to enhanced electronic coupling for efficient charge transport. Consequently, the FAPbI PQD solar cell based on DSLE strategy achieves a notable enhanced efficiency from 15.43% to 17.79% (2-TM) and 18.21% (2-TE), respectively. Besides, both 2-TM and 2-TE engineered devices exhibit enhanced stability, maintaining over 80% of its initial efficiency after aging in ambient environment (20-30% humidity, 25 °C) for over 1400 h. It believes these findings will provide a new protocol to precisely regulate the surface chemistry of hybrid PQDs toward high-performance optoelectronic applications.