The Curious Electronic Properties of Hybrid Halide Perovskite Solar Cells

Mark van Schilfgaarde

King's College London

The performance of organometallic perovskite solar cells has rapidly surpassed those of both traditional dye-sensitised and organic photovoltaics, e.g. solar cells based on CH3NH3PbI3 have recently reached 15% conversion efficiency. We analyse its electronic structure and optical properties within the quasiparticle self-consistent GW approximation (QSGW). Quasiparticle self-consistency is essential for an accurate description of the band structure: bandgaps are much larger than what is predicted by the local density approximation (LDA) or GW based on the LDA. As a consequence of spin-orbit coupling valence band dispersions are modified in a very unusual manner: it splits the valence band maximum at the R point into two maxima nearby (similarly for the conduction band minimum, also at R). This can be interpreted in terms of a large Dresselhaus term, which vanishes at R but small excursions about R varies linearly in k along one direction. Effective hole masses become temperature dependent. The average hole mass is small, which partially accounts for the long diffusion lengths recently observed.
In addition, hybrid perovskites exhibit spontaneous electric polarisation, which can be tuned through judicious choice of the organic cation. The presence of ferroelectric domains will result in internal junctions that aid separation of photoexcited electron and hole pairs. The combination of high dielectric constant and low effective mass facilitates both Wannier-Mott exciton separation and effective ionisation of donor and acceptor defects. The photoferroic effect can be exploited to produce higher voltages and may contribute to the current-voltage hysteresis found in perovskite solar cells.