Electronic Band Engineering via MI3 (M = Sb, Bi) Doping Remarkably Enhances the Air Stability of Perovskite CsSnI3

CsSnI3 is a representative all-inorganic and less toxic perovskite material. However, extreme structural and chemical instability of perovskite CsSnI3 makes its optoelectronic applications highly challenging. Upon exposure to air and moisture, it immediately undergoes a phase transition to a thermodynamically more stable, but optoelectronically inactive, one-dimensional polymorph near ambient temperature and ultimately deforms into Cs2SnI6. To prohibit this undesirable process, perovskite CsSnI3 has to be stored and treated restrictively in an inert atmosphere and encapsulated hermitically. Here, we demonstrate an unusual strategy to markedly enhance the air stability of perovskite CsSnI3. Namely, MI3 (M = Sb, Bi) doping modifies the electronic band structure of perovskite CsSnI3. As a result, it is remarkable that its heat of formation reduces, being lower than that of its competing polymorph. Accordingly, otherwise thermodynamically unfavorable perovskite CsSnI3 becomes more stable than the latter energetically, thereby preventing the undesirable phase transition. SbI3 (3 mol %)-doped CsSnI3 retains 96% of its perovskite structure, whereas pristine CsSnI3 retains only 12% after 12 h of exposure to air with 45–55% relative humidity. MI3 doping also reduces the energy band gap of perovskite CsSnI3. We employed first-principles density functional theory calculations to explain the origin of the enhanced stability and red-shifted band gaps. Our current work demonstrates that electronic band structure engineering by chemical doping can be an effective means of controlling the phase stability of polymorphs, which are otherwise difficult to stabilize or unattainable. This strategy can be widely applied to materials with low stability but high technological importance.