First principles modelling of perovskite solar cells based on TiO2and Al2O3: stability and interfacial electronic structure

Interfaces have been shown to have significant impact not only on cell efficiency but also on stability and hysteresis of perovskite solar cells. Using first-principles calculations we study the interface of perovskite with TiO2 and Al2O3. We will show that the perovskite binds much stronger onto alumina than titania. We will examine the nature of the binding and prove that this is due to formation of covalent bonds to alumina versus weak non-covalent bonding to titania. We will however show that the binding onto alumina severely distorts the interface and dissociates CH3NH3 molecules. Finally we will demonstrate that the energy level alignment between perovskite and TiO2 is significantly influenced by the interfacial plane of perovskite, resulting in a much better band bending for the mesoporous cells than for the planar cells. Our findings suggest reinforcement of the perovskite/TiO2 interface as a way to increase the stability. More importantly, they indicate that Al2O3 is not suitable for perovskite solar cells and would lead to unstable cells.