Enhanced Phase Stability and Reduced Bandgap for CsPbI₃ Perovskite through Bi³⁺ and Cl^– Co-Doping

All-inorganic perovskite CsPbI₃ is emerging as a thermally more stable alternative to organic-inorganic hybrid perovskites. However, CsPbI₃ perovskite suffers from poor phase stability at ambient temperature, and its bandgap is a bit too large as light-harvesting materials in both single-junction and perovskite-on-silicon tandem solar cells. In this study, we propose an electrically neutral co-doping strategy that equimolar Bi³⁺ (occupying the Pb site) and Cl^– (occupying the interstitial site) are incorporated into CsPbI₃. Unlike the individual Bi³⁺ or Cl^– doping, the neutral co-doping can avoid stimulating the formation of the detrimental native defects. Our first-principles calculations suggest that the co-doped systems are stable at ambient temperature and possess narrower bandgaps compared with the undoped CsPbI₃. Moreover, the electron and hole states are spatially separated in these multiple-ion compounds.