Intrinsic Defect Limit to the Electrical Conductivity and a Two-Step p-Type Doping Strategy for Overcoming the Efficiency Bottleneck of Sb2S3-Based Solar Cells

The photovoltaic efficiency increase in Sb2S3-based solar cells has stagnated for 5 years since the highest efficiency of 7.5% was achieved in 2014. One important bottleneck is the high electrical resistivity of Sb2S3. The first-principle calculations reveal that the high-resistivity results from the compensation between the intrinsic donor V-S and acceptors V-Sb, Sb-S, and S-Sb which have comparably high concentration (low formation energy). The compensation also limits the improvement of conductivity through direct extrinsic doping. Further calculations of O dopants show that O-S has low formation energy, so the dominant intrinsic donor V-S can be passivated by O and thus the p-type doping limit imposed by V-S can be overcome. Meanwhile, other p-type limiting and recombination-center donor defects can be suppressed under the S-rich condition, which explains why the highest efficiency is achieved in O-doped Sb2S3 after sulfurization. Given the unexpected beneficial effects of O doping and sulfurization, a two-step doping strategy is proposed for overcoming the efficiency bottleneck: 1) use O to passivate the V-S and S-rich condition to suppress other detrimental defects, making p-type doping feasible and minority carrier lifetime long; 2) introduce other p-type dopants to increase hole carrier concentration.