Method for bandgap interpolation of perovskite's spectral complex refractive index.

Lead halide perovskites are a promising class of materials for solar cell applications. The perovskite bandgap depends on the material composition and is highly tunable. Opto-electrical device modelling is commonly used to find the optimum perovskite bandgap that maximizes device efficiency or energy yield, either in single junction or multi-junction configuration. The first step in this calculation is the optical modelling of the spectral absorptance. This requires as input the perovskite's complex refractive index N as a function of wavelength λ. The complex refractive index consists of real part n(λ) and imaginary part k(λ). For the most commonly used perovskites, n and k curves are available from spectroscopic ellipsometry measurements, but usually only for a few discrete bandgap energies. For solar cell optimization, these curves are required for a continuous range of bandgap energies. We introduce new methods for generating the n and k curves for an arbitrary bandgap, based on interpolating measured complex refractive index data. First, different dispersion models (Cody-Lorentz, Ullrich-Lorentz and Forouhi-Bloomer) are used to fit the measured data. Then, a linear regression is applied to the fit parameters with respect to the bandgap energy. From the interpolated parameters, the refractive index curve of perovskite with any desired bandgap energy is finally reconstructed. To validate our method, we compare our results with methods from literature and then use it to simulate the absorptance of a single junction perovskite and a perovskite/silicon tandem cell. This shows that our method based on the Forouhi-Bloomer model is more accurate than existing methods in predicting the complex refractive index of perovskite for arbitrary bandgaps.