Unusual Band Gap Bowing in CsCdPb_(1-Br₃ Alloys Due to a Chemical Mismatch

In this work, the band structure of CsCdPb_1-Br₃ alloys is investigated through first-principles calculations. An unusual upward band gap bowing is revealed, which is consistent with the experimental observations of the blue-shifted gap in Cd doped CsPbBr₃. The gap bowing is found to be mainly contributed to by the conduction band minimum. Based on symmetry analysis, it is demonstrated that, at the Pb-rich or Cd-rich regime, the hybridization between the Pb(6p)-driven and Cd(5s)-driven conduction bands is strongly suppressed due to their different symmetries. Such a chemical mismatch leads to an almost independent evolution of the Pb(6p) and Cd(5s) bands. Then, a model of band shrinking and broadening is proposed to explain upward gap bowing. The results highlight the critical role of symmetry in determining the electronic properties of alloys consisting of materials with distinct band edge characters.