abstract
Wannier excitons are often observed in semiconductors with electron hole pairs. The Elliott–Tanguy model (ET) can describe the optical absorption for Wannier excitons with single bandgap energy well. However, in polycrystalline materials, such as metalhalide perovskite (MHP) layers as applied in solar cells, the bandgap is broadened with Gaussian distribution and thus an improved description is required. In this work, we present optical permittivity of polycrystalline MHP layers described with a model with Wannier excitons and with Gaussian distributed bandgap energies, which is henceforth named the Gaussian–Elliott–Tanguy model (GET). The GET model shows Hermitian symmetry and Kramers–Kronig consistency, which correspond to the reality of response in the time domain and causality, respectively. The optical sum rule from zero to infinity is not satisfied.