On interface recombination, series resistance, and absorber diffusion length in BiI3 solar cells

Abstract

Bismuth triiodide is a lead-free direct wide-bandgap solution-processable semiconductor that could be an alternative to lead-based perovskites in tandem or multijunction solar cells. However, the power conversion efficiency of single-junction BiI3 solar cells remains low. Here, we determine the main loss mechanisms of BiI3 solar cells in both n-i-p and p-i-n architectures. Overall, p-i-n devices have higher power conversion efficiency than that of n-i-p. It is found that n-i-p devices have higher (and significant) non-radiative recombination at the interface of the BiI3/transport layer, resulting in a lower open-circuit voltage than p-i-n devices. Moreover, the high series resistance (>70 Ω cm2) and a low average electron–hole diffusion length (∼60 nm) contributes to the low short-circuit current density (<5 mA/cm2) and fill factor (<40%) in all devices. In addition, interface recombination also reduces short-circuit current density. Finally, we demonstrate that lithium doping of BiI3 can increase the diffusion length of BiI3 to improve the performance of BiI3 solar cells. Solar cells with the configuration ITO/NiOx/Li:BiI3/PC61BM/bis-C60/LiF/Ag obtain a power conversion efficiency of 1.3% under AM 1.5 G illumination. The deep understanding of the main loss mechanisms of this work paves the way for future optimization of BiI3 solar cells.