Excitonic Properties of Low-Band-Gap Lead-Tin Halide Perovskites

The MAPb(1-x)Sn(x)I(3) (x = 0-1) (MA = methylammonium) perovskite family comprises a range of ideal absorber band gaps for single- and multijunction perovskite solar cells. Here, we use spectroscopic measurements to reveal a range of hitherto unknown fundamental properties of this materials family. Temperature dependent transmission results show that the temperature of the tetragonal to orthorhombic structural transition decreases with increasing tin content. Through low-temperature magnetospectroscopy, we show that the exciton binding energy is lower than 16 meV, revealing that the dominant photogenerated species at typical operational conditions of optoelectronic devices are free charges rather than excitons. The reduced mass increases approximately proportionally to the band gap, and the mass values (0.075-0.090m(e)) can be described with a two-band k.p perturbation model extended across the broad band gap range of 1.2-2.4 eV. Our findings can be generalized to predict values for the effective mass and binding energy for other members of this family of materials.