Solution-processed semiconductors have opened promising avenues for next-generation semiconductor and optoelectronic industries. Colloidal quantum dots (QDs) as one of the most typical materials are widely utilized for the design and development of light-emitting diodes, photodetectors, and solar cells. Recently, an emerging process of PbS QD ink has been employed to attain world record power conversion efficiency by surface passivation using a PbI2 ligand to form PbI2-PbS and the process optimization in the field of photovoltaics. However, the bonding and debonding of the ligands on the surface of PbS QDs are dynamic reversible processes in an ink environment. The uncoordinated Pb2+ defects induced by unbonded PbS QDs serve as the recombination sites. Thus, the present ink process leaves much room for the enhancement by surface passivation of PbS QDs. Herein, we devise an efficient strategy with a supplementary phenethylammonium iodide (PEAI) ligand for the formation of the PEAI-PbS interface in PbS QD ink-processed solar cells. This newly developed method can not only improve the passivation on the QD surface by iodine ions but also simultaneously enhance the carrier collection efficiency by a graded energy alignment between PbI2-PbS and PEAI-PbS layers. The corresponding power conversion efficiency of the optimized device has significantly increased by approximately 20% more than the control device. As a result, such a robust and efficient method regarded as a strategic candidate can overcome the bottleneck of imperfect passivation caused by a large specific surface area and loose bonding ligands, eventually promoting the industrial application of QDs.
Keywords: PbS quantum dots; band alignment; ink process; solar cells; supplementary ligand strategy.