In Situ Sulfur-Supplementation Toward Efficient PbS Quantum Dot Photovoltaics for Tandem Applications

Efficient narrow-bandgap PbS quantum dot (QD) solar cells are promising candidates for bottom subcells in multi-junction tandem photovoltaics. However, solution-phase ligand exchange inevitably disrupts surface stoichiometry by introducing sulfur vacancies and excess lead iodide (PbI 2 ), which could act as non-radiative recombination sites and ion migration pathways. Herein, we develop an in situ sulfur-supplementation strategy utilizing N,N -Diphenylthiourea (DPTU). In the butylamine-mediated alkaline environment, DPTU releases sulfide ions that spontaneously repair sulfur vacancies and scavenge excess PbI 2 . Driven by the significant difference in solubility product constants between PbS and PbI 2 in N,N -Dimethylformamide, this process produces stoichiometric PbS ink with suppressed defect density. Combined with vacuum-assisted annealing, the 0.95 eV PbS QD devices achieve a record efficiency of 14.34% and a silicon-filtered efficiency of 1.56%, with excellent near-infrared photodetector performance and aging stability. Furthermore, optimized PbS devices contributed additional efficiencies of 6.37%, 2.56%, and 2.22% beneath 1.77, 1.26, and 1.77/1.26 eV perovskite filters, respectively. This work offers a robust in situ bulk passivation strategy to suppress vacancy defects and ion migration in PbS QD photovoltaics, paving the way for their application in high-performance tandem solar cells.