Infrared (IR) solar cells are promising devices for significantly improving the power conversion efficiency of common solar cells by harvesting the low-energy IR photons. PbSe quantum dots (QDs) are superior IR photon absorbing materials due to their strong quantum confinement and thus strong interdot electronic coupling. However, the high chemical activity of PbSe QDs leads to etching and poor passivation in ligand exchange, resulting in a high trap-state density and a high open circuit voltage (VOC) deficit. Here we develop a hybrid ligand co-passivation strategy to simultaneously passivate the Pb and Se sites; that is, halide anions passivate the Pb sites and Cd cations passivate the Se sites. The cation and anion hybrid passivation substantially improves the quality of PbSe QD solids, giving rise to an excellent trap-state control and prolonged carrier lifetime. A high VOC and a high short circuit current density (JSC) are achieved simultaneously in the IR QD solar cells based on this hybrid ligand treatment. Finally, a IR-PCE of 1.31% under the 1100-nm-filtered solar illumination is achieved in the PbSe QD solar cells, which is the highest IR-PCE for PbSe QD IR solar cells at present. Additionally, the PbSe QD devices show a high external quantum efficiency of 80% at ∼1295 nm.
Keywords: PbSe quantum dots; hybrid ligands; infrared solar cells; passivation; trap states.