Control Over Ligand Exchange Reactivity in Hole Transport Layer Enables High-Efficiency Colloidal Quantum Dot Solar Cells

Colloidal quantum dot (CQD) solar cells are solution-processed photovoltaic devices that exhibit promise in harvesting the infrared solar spectrum. Solid-state ligand exchange is the method employed to fabricate the CQD hole transport layer (HTL) in these cells: insulating oleic acid ligands are substituted with short thiol ligands (1,2-ethanedithiol) to create conductive p-type CQD solids. Thiols' high reactivity with the CQD surface results in rapid exchange, giving rise to aggregates of dots and unpassivated sites on dots, each contributing to sub-bandgap trap states. Here we report a strategy to minimize trap states in the CQD HTL by controlling the solvent type in the exchange. By employing a less volatile solvent, we achieve a slower reaction, leading to increased order and a 2 times reduced trap density in CQD solids. These improvements enable a power conversion efficiency of 13.1 +/- 0.1% in CQD solar cells compared to control devices showing 12.4 +/- 0.1%.