Metal halide perovskites, ABX3, A=Cs, CH3NH3, CH2(NH3)2, M=Pb, Sn, X=I, Br, Cl, are the new kid on the block regarding materials for high-efficiency solar cells. These perovskites are direct band gap semiconductors with a high optical absorption, small exciton binding energies, and decent carrier mobilities, all of which are beneficial for application in photovoltaics. One can make complex alloys (all with the same perovskite structure) of the mentioned compounds to tune electronic properties, such as the band gap. The valence and conduction bands around the band gap are determined by the atoms in the cubic MX3 lattice. The A cation’s role is to stabilize the lattice; it does not directly influence these bands.

Very recently the optical absorption spectra of all compounds mentioned above have become available. Many of these spectra show small, but distinct, exciton peaks. The first goal of this project is to understand the shape of these spectra on the basis of an Elliot model (which describes the optical absorption of bound and of free electron-hole pairs). Fitting an Elliot-inspired lineshape to the experimental spectra should enable us to extract exciton binding energies in the different compounds.

The exciton binding energy depends on the composition of the compound. How exactly, and which physical parameters are most important, are questions we address in the second part of the project.  The second goal of the project is then to understand the trend in the exciton binding energies in the different compounds through quantum mechanical modeling. We start from the Wannier model, with parameters such as effective hole and electron masses and dielectric functions extracted from first-principles calculations. Such parameters are partially available in the literature, or can be generated. For some cases this model has to be extended to include polaron effects, i.e., effects due to the coupling of the electrons and holes to the vibrations. We will capture such effects in a modified effective radial potential.