The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitized and organic photovoltaics. High-power conversion efficiency can be realized 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 polarization; 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 ionization 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.

Solar cells comprising methylammonium lead iodide perovskite (MAPI) are notorious for their sensitivity to moisture. We show that hydrated crystal phases are formed when MAPI is exposed to water vapour at room temperature and that these phase changes are fully reversed when the material is subsequently dried. The reversible formation of CH 3 NH 3 PbI 3 •H 2 O followed by (CH 3 NH 3 ) 4 PbI 6 •2H 2 O (upon long exposure times) was observed using time resolved XRD and ellipsometry of thin films prepared using 'solvent engineering', single crystals, and state of the art solar cells. In contrast to water vapour, the presence of liquid water results in the irreversible decomposition of MAPI to form PbI 2 . MAPI changes from dark brown to transparent on hydration; the precise optical constants of CH 3 NH 3 PbI 3 •H 2 O formed on single crystals were determined, with a bandgap at 3.1 eV. Using the single crystal optical constants and thin film ellipsometry measurements, the time dependent changes to MAPI films exposed to moisture were modelled. The results suggest that the mono-hydrate phase forms independently of the depth in the film suggesting rapid transport of water molecules along grain boundaries. Vapour phase hydration of an unencapsulated solar cell (initially J sc ≈ 19 mA cm -2 and V oc ≈ 1.05 V at 1 sun) resulted in more than a 90 % drop in short circuit photocurrent and around 200 mV loss in open circuit potential, but these losses were fully reversed after the cell was exposed to dry nitrogen for 6 hours. Hysteresis in the current-voltage characteristics was significantly increased after this dehydration, which may be related to changes in the defect density and morphology of MAPI following recrystallization from the hydrate. Based on our observations we suggest that irreversible decomposition of MAPI in the presence of water vapour only occurs significantly once a grain has been fully converted to the hydrate phase.

In past decades the scientific community has been looking for a reliable first-principles method to predict the electronic structure of solids with high accuracy. Here we present an approach which we call the quasiparticle self-consistent approximation. It is based on a kind of self-consistent perturbation theory, where the self-consistency is constructed to minimize the perturbation. We apply it to selections from different classes of materials, including alkali metals, semiconductors, wide band gap insulators, transition metals, transition metal oxides, magnetic insulators, and rare earth compounds. Apart from some mild exceptions, the properties are very well described, particularly in weakly correlated cases. Self-consistency dramatically improves agreement with experiment, and is sometimes essential. Discrepancies with experiment are systematic, and can be explained in terms of approximations made.

Solar cells based on a light absorbing layer of the organometal halide perovskite CH 3 NH 3 PbI 3 have recently surpassed 15 % conversion efficiency, though how these materials work remains largely unknown. We analyse the electronic structure and optical properties within the quasiparticle self-consistent GW approximation. While this compound bears some similarity to conventional sp semiconductors, it also displays unique features. Quasiparticle self-consistency is essential for an accurate description of the band structure: band gaps are much larger than what is predicted by the local density approximation (LDA) or GW based on the LDA. Valence band dispersions are modified in a very unusual manner. In addition, spin-orbit coupling strongly modifies the band structure and gives rise to unconventional dispersion relations and a Dresselhaus splitting at the band edges. The average hole mass is small, which partially accounts for the long diffusion lengths observed. The surface ionisation potential (workfunction) is calculated to be 5.7 eV with respect to the vacuum level, explaining efficient carrier transfer to TiO 2 and Au electrical contacts.

We have developed a new type of self-consistent scheme within the GW approximation, which we call quasiparticle self-consistent GW (QSGW ). We have shown that QSGW describes energy bands for a wide-range of materials rather well, including many where the local-density approximation fails. QSGW contains physical effects found in other theories such as LDA+U , SIC and GW in a satisfactory manner without many of their drawbacks (partitioning of itinerant and localized electrons, adjustable parameters, ambiguities in double-counting, etc.). We present some theoretical discussion concerning the formulation of QSGW , including a prescription for calculating the total energy. We also address several key methodological points needed for implementation. We then show convergence checks and some representative results in a variety of materials.

We present a new kind self-consistent GW approximation (scGW) based on the all-electron, fullpotential LMTO method. By iterating the eigenfunctions of the GW Hamiltonian, self-consistency in both the charge density and the quasiparticle spectrum is achieved. We explain why this form of self-consistency should be preferred to the conventional one. Then some results for Si are shown as a representative semiconductor, to establish agreement with a prior scGW calculation. Finally we consider many details in the electronic structure of the antiferromagnetic insulators MnO and NiO. Excellent agreement with experiment is shown for many properties, suggesting that a Landau quasiparticle (energy band) picture of MnO and NiO provides a reasonable description of electronic structure even in these correlated materials. The GW approximation (GWA) of Hedin[1] is generally believed to accurately predict excited-state properties, and in particular improve on the local density approximation (LDA), whose limitations are well known, e.g. to underestimate bandgaps semiconductors and insulators. Usually GWA is computed as 1-shot calculation starting from the LDA eigenfunctions and eigenvalues; the self-energy Σ is approximated as Σ = iG LDA W LDA , where G LDA is a bare Green function constructed from LDA eigenfunctions, and W LDA is the screened Coulomb interaction constructed from G LDA in the random phase approximation (RPA). However, establishing the validity of the 1-shot approach has been seriously hampered by the fact that nearly all calculations to date make further approximations, e.g. computing Σ from valence electrons only; the plasmon-pole approximations; and the pseudopotential (PP) approximation to deal with the core. Only recently when reliable all-electron implementations have begun to appear, has it been shown that the 1-shot GWA with PP leads to systematic errors [2,3,4]. There is general agreement among the all-electron calculations (see Table I) that the Γ-X transition in Si is underestimated when Σ = iG LDA W LDA . And we have shown previously[2] that the tendency for Σ = iG LDA W LDA to underestimate gaps is almost universal in semiconductors. This is reasonable because W LDA overestimates the screening owing to the LDA small band gaps. G constructed from quasiparticles (QP) with a wider gap (e.g. a self-consistent G) reduces the screening, and therefore generates GW with a wider gap. However, there are many possible ways to achieve selfconsistency. The theoretically simplest (and internally consistent) is the fully self-consistent scheme (scGW), which is derived through the Luttinger-Ward functional with the exchange-correlation energy approximated as the sum of RPA ring diagrams. Then W is evaluated as W = v(1 − vP ) −1 with the proper part of the polarization function P = −iG × G. However, such a construction may not give reasonable W [5], resulting in a poor G, for the following reason. If Σ is ω-dependent, G can be partitioned into a QP part and a residual satellite part. The QP part consists of terms whos...

The optical constants of methylammonium lead halide single crystals CH3NH3PbX3 (X = I, Br, Cl) are interpreted with high level ab initio calculations using the relativistic quasiparticle self-consistent GW approximation (QSGW). Good agreement between the optical constants derived from QSGW and those obtained from spectroscopic ellipsometry enables the assignment of the spectral features to their respective inter-band transitions. We show that the transition from the highest valence band (VB) to the lowest conduction band (CB) is responsible for almost all the optical response of MAPbI3 between 1.2 and 5.5 eV (with minor contributions from the second highest VB and the second lowest CB). The calculations indicate that the orientation of [CH3NH3](+) cations has a significant influence on the position of the bandgap suggesting that collective orientation of the organic moieties could result in significant local variations of the optical properties. The optical constants and energy band diagram of CH3NH3PbI3 are then used to simulate the contributions from different optical transitions to a typical transient absorption spectrum (TAS).

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