### 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.