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

ABSTRACT 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.