The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitised and organic photovoltaics. High power conversion efficiency can be realised in both mesoporous and thin-film device architectures.
We address the origin of this success in the context of the materials chemistry and physics of the bulk perovskite as described by electronic structure calculations. In addition to the basic optoelectronic properties essential for an efficient photovoltaic device (spectrally suitable band gap, high optical absorption, low carrier effective masses), the materials are structurally and compositionally flexible. As we show, hybrid perovskites exhibit spontaneous electric polarisation; we also suggest ways in which this can be tuned through judicious choice of the organic cation. The presence of ferroelectric domains will result in internal junctions that may aid separation of photoexcited electron and hole pairs, and reduction of recombination through segregation of charge carriers. The combination of high dielectric constant and low effective mass promotes both Wannier-Mott exciton separation and effective ionisation of donor and acceptor defects. The photoferroic effect could be exploited in nanostructured films to generate a higher open circuit voltage and may contribute to the current-voltage hysteresis observed in perovskite solar cells.
The performance of perovskite solar cells recently exceeded 15% solar-to-electricity conversion efficiency for small-area devices. The fundamental properties of the active absorber layers, hybrid organic-inorganic perovskites formed from mixing metal
and organic halides [textit{e.g.} (NH$_4$)PbI$_3$ and (CH$_3$NH$_3$)PbI$_3$], are largely unknown. The materials are semiconductors with direct band gaps at the boundary of the first Brillouin zone. The calculated dielectric response and band gaps show an orientation dependence, with a low barrier for rotation of the organic cations. Due to the electric dipole of the methylammonium cation, a photoferroic effect may be accessible, which could enhance carrier collection.
