Alkali-metal-ion-doping strategy to improve the photovoltaic properties of Ag2BiI5 solar cells

Abstract

Alkali-metal-ion doping is an efficient strategy to improve the device performance of thin film solar cells. Though doping with Li⁺ or Cs⁺ doping has been reported in Ag–Bi–I solar cells, the influence of doping with other alkali metal ions on Ag–Bi–I solar cells has not been systematically studied. In this work, we investigate the effects of five alkali metal ions (Li⁺, Na⁺, K⁺, Rb⁺, and Cs⁺) on the structure, morphology, and optical and electrical properties of Ag₂BiI₅ absorber layers, and also their influence on the device performance. All alkali-metal-doped Ag₂BiI₅ thin films show high crystallinity and have similar suitable band gaps. Hall effect measurements demonstrate that the alkali-metal-doped Ag₂BiI₅ films show an improved electrical performance compared with the pristine Ag₂BiI₅ thin film, which is beneficial to achieving high-efficiency solar cells. Unexpectedly, Cs⁺ doping can significantly increase the grain size of Ag₂BiI₅ thin films, leading to a porous absorber that reduces the device efficiency. The device performance of Li⁺-, Na⁺-, K⁺- and Rb⁺-doped Ag₂BiI₅ is significantly improved compared with that of pristine Ag₂BiI₅. It is found that the order of the power conversion efficiency promotion ability is K⁺ > Rb⁺ > Na⁺ > Li⁺ > Cs⁺ under the same experimental conditions. The present approach can be extended to other Bi-based solar cells to improve their device performance, which is critical for applications in absorbers with poor electrical performance or low crystallinity.