First principles Exploration of Hybrid Perovskite Superlattice and Solid Solutions for Efficient and Structurally Stable Stand-Alone Hybrid Solar PV Material

Hybrid perovskite solar cells (with chemical formula ABX₃) are of great interest due to the recently measured power conversion efficiency of greater than 25% (but theoretically, 33.7%). Perovskite structures are easily customisable, with a range of options for A, B and X. This enables us to both tune the electronic band gap and the stability by varying the composition. Two promising perovskites are the CH₃NH₃PbI₃(MAPI) and CH(NH₂)₂PbBr₃ (FAPB) structures. With these, we have two choices, we can use solid state solution (mixing) approaches, or we can structure them in a desired form, such as a superlattice. Both approaches have advantages and disadvantages. In order to compromise for a more desirable bandgap for stand-alone PV and better structural stability, the hybrid perovskite constituents are show great promise in the superlattice form, with one configuration showing the low band gap of 1.29 eV.<br/> <br/>We present a theoretical investigation of the structure and electronic properties of superlattices and solid state solutions performed using first–principles density functional theory. We include the role of vibrational entropy and assess the room temperature stability of the cubic phases for these systems. For solid state solution we show that by varying the ratio of FA to MA and the ratio of I to Br, we can potentially tune the band gap and effective masses (and hence electronic transport). For MAPI(n)/FAPB(m) superlattices, our results show that the electronic direct gap is evaluated and we show that these systems in general possess a lower band gap than either of the two bulks. We also evaluate the stability of these structures, showing that the cubic phase to be favourable at room temperature. Our results show that these cubic superlattice structures present an increased stability of this phase compared to the hexagonal phase and the bulk of the constituents.