Heterostructural Interface Atomic-Structure Predictions for SnO2/CdTe with CdCl2 Treatment in Photovoltaics

The electronic structure properties of interfaces are of central importance to photovoltaics. Computational approaches can provide information complementary to experimental characterization. In particular, first principles methods can establish an unambiguous link between atomic structures features and their electronic consequences. This strength is also a challenge, because realistic atomic structure models must be available for such calculations. Only for highly idealized situations, such as the epitaxial interface between two reasonably lattice matched materials with the same crystal structure (coherent interface), can the atomic interface structure be trivially constructed. The situation becomes much more complex for a general interface problem, including highly lattice mismatched systems or materials with different stoichiometries or crystal structures.<br/>We adopted the kinetically limited minimization (KLM) approach, previously developed for unconstrained bulk crystal structure prediction [J Chem Phys 154, 234706 (2021)], to the substrate/thin-film interface problem in slab geometry with vacuum separation. Using density functional theory (DFT) total energy calculations, we sample atomic structures for SnO2/CdTe interfaces. In CdTe photovoltaics, CdCl2 treatment is a necessary process step for acceptable device efficiencies. Therefore, we predict the interface structure both without and in the presence of CdCl2. The simulations treat SnO2 as a substrate with a given crystal and surface structure, whereas for the film (CdTe and CdCl2) there are no initial assumptions about the crystal structure other than choosing suitable coincidence site lattices that can accommodate the CdTe zinc-blende structure with acceptable strain. Besides the unknown interface structure, the well-established CdTe bulk and back-surface structures are correctly reproduced by the algorithm. The addition of CdCl2 results in improved bonding and periodicity, strongly reduces the total interface energy. Electronically, the CdCl2 treatment results in an almost perfect conduction band alignment and dramatically reduces the defect density of states in the CdTe band gap, highlighting its beneficial effects for solar cell performance.