Band Gap Narrowing in Lead-Halide Perovskites by Dynamic Defect Self-Doping for Enhanced Light Absorption and Energy Upconversion.

Metal halide perovskites (MHP) have attracted great attention in the photovoltaic industry due to their high and rapidly rising power conversion efficiencies, currently over 25%. However, hybrid organic-inorganic MHPs are inherently chemically unstable, limiting their application. All-inorganic MHPs perovskites, such as CsPbI, have many merits, but their stable conversion efficiency is lower, around 18%, due to a larger band gap causing a mismatch with the solar spectrum. Choosing α-CsPbI as a prototypical system, we demonstrate a new general concept of dynamic defects that fluctuate between deep and shallow states, and increase the range of absorbed solar photons, without accelerating the nonradiative electron-hole recombination. In their deeper energy state, the defects narrow the band gap and allow the harvesting of light with longer wavelengths. Fluctuating to shallower energies, the defects allow the escape of photogenerated charges into bands, enabling charge transport and resulting in the defect-mediated upconversion of thermal energy into electricity. Defect covalency and participation of low-frequency anharmonic vibrations decouple trapped charges from free charge carriers, minimizing nonradiative charge carrier losses. Our findings demonstrate that defect covalency and defect dynamics are unique and important properties of MHPs, and can be used to optimize MHPs for efficient solar energy harvesting and optoelectronic applications.