Electronic structures and charge transport mobilities in hybrid organic-inorganic mixed Sn-Pb alloyed perovskites

An all-perovskite tandem cell based on narrow-bandgap mixed tin-lead (Sn-Pb) alloyed perovskites is a potential photovoltaic device whose power conversion efficiency can exceed the Shockley-Queisser limit of a single-junction solar cell, 33%. However, comprehensive descriptions of the charge-carrier mobilities and transport mechanisms in the mixed Sn-Pb perovskite system remain elusive. Herein, we integrate density functional theory (DFT) calculations with charge transport models to provide more insight into the electronic structures and transport behaviors of these materials. We specifically employ a model using large polaron transport based on LO phonon scattering and ionized impurity scattering due to the oxidation of Sn²⁺ to Sn⁴⁺. DFT simulations revealed that there is a crossover in the electronic properties of pure-Sn and pure-Pb perovskites at Pb contents of more than 0.5, which causes increasing reduced effective mass as the Pb content increases. Theoretical calculations for charge-carrier mobility were in agreement with the experimental data between 23 and 89 cm² V^-1 s^-1 at room temperature. In mixed Sn-Pb perovskites, LO phonon scattering predominates. However, in pure-Sn perovskites, transport scatterings based on LO phonon and ionized impurity scatterings are important. This discovery advances our knowledge of the transport mechanisms underlying the system of mixed Sn-Pb compounds.