Inorganic-organic hybrid perovskites are a new family of solar cell materials, which have recently been used to make solar cells with efficiency approaching 20%. Here, we report the unique defect chemistry of the prototype material, CH3NH3PbI3, based on first-principles calculation. We found that both the Pb cations and I anions in this material exhibit strong covalency as characterized by the formation of Pb dimers and I trimers with strong covalent bonds at some of the intrinsic defects. The Pb dimers and I trimers are only stabilized in a particular charge state with significantly lowered energy, which leads to deep charge-state transition levels within the band gap, in contradiction to a recent proposal that this system has only shallow intrinsic defects. Our results show that, in order to prevent the deep-level defects from being effective recombination centers, the equilibrium carrier concentrations should be controlled so that the Fermi energy is about 0.3 eV away from the band edges. Beyond this range, according to a Shockley-Read-Hall analysis, the non-equilibrium carrier lifetime will be strongly affected by the concentration of I vacancies and the anti-site defects with I occupying a CH3NH3 site.

Chalcogenide perovskites are proposed for photovoltaic applications. The predicted band gaps of CaTiS3, BaZrS3, CaZrSe3, and CaHfSe3 with the distorted perovskite structure are within the optimal range for making single-junction solar cells. The predicted optical absorption properties of these materials are superior compared with other high-efficiency solar-cell materials. Possible replacement of the alkaline-earth cations by molecular cations, e.g., (NH3NH3)(2+), as in the organic-inorganic halide perovskites (e.g., CH3NH3PbI3), are also proposed and found to be stable. The chalcogenide perovskites provide promising candidates for addressing the challenging issues regarding halide perovskites such as instability in the presence of moisture and containing the toxic element Pb.

M olecular hydrogen (H 2 ) sorbents are appealing materials for storing hydrogen fuel onboard vehicles. The uptake and release of H 2 fuel in the sorbent materials can be fast and require less heat transfer. To use the H 2 sorbents at near ambient conditions, the binding energy of H 2 in these materials must be within certain range (e.g., 20Ϫ40 kJ/mol). 1,2 Theoretical studies predicted 3,4 that Kubaslike interactions between transition metal (TM) centers and coordinated H 2 could fall within this desirable energy range. Such predictions are consistent with recent experimental studies by using metalϪorganic frameworks (MOFs) with under-coordinated TM. 5Ϫ8 Attempts to anchor TM directly on carbon nanostructures, however, have not yet been successful. Recently, Hamaed et al. used organometallic precursor to successfully graft Ti onto the inner surface of mesoporous silica. 9 Though this work demonstrated the feasibility of individually dispersing Ti and the capability of binding multi-H 2 by dispersed Ti, mesoporous silica has a relatively small surface-to-volume ratio and may be too heavy for practical hydrogen storage. So far, no practical H 2 sorbent is available. Finding the right material for onboard storage is still a grand challenge. Concerning TM-based organometallic sorbents, several conditions are required at the same time: First, the substrate materials possess high surface-to-volume ratio and are lightweight. Second, the TM atoms are undercoordinated and well-exposed to accommodate multi-H 2 . Third, these unsaturated TM atoms, despite their high chemical reactivity, 10 do not form clusters. These require that the anchoring bonds between the TM atoms and the substrate are strong and the TM coverage is also optimized. Along the line of strengthening the anchoring bonds, several strategies have been suggested, such as functionalizing organic molecules, 11 employing defect sites in carbon materials, 12,13 and directly integrating metal atoms into the skeleton. 14,15 Alternatively, graphene oxide (GO) can be a potential substrate to covalently anchor TM atoms by simultaneously satisfying all these three conditions. GO has large surface-to-volume ratio and is intrinsically lightweight (condition 1). GO possesses ample O sites on the surfaces. Oxygen is the key in anchoring under-coordinated Ti (condition 2) and enhancing the TMϪ substrate binding (condition 3), as having been experimentally demonstrated on mesoporous silica. 9 Although GO has been routinely synthesized and extensively studied, 16Ϫ24 currently its precise atomic structures are still under intense investigation. In fact, the O content of GO can vary greatly, depending

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Organic-inorganic hybrid perovskite solar materials, being low-cost and high-performance, are promising for large-scale deployment of the photovoltaic technology. A key challenge that remains to be addressed is the toxicity of these materials since the high-efficiency solar cells are made of lead-containing materials, in particular, CH3NH3PbI3. Here, based on first-principles calculation, we search for lead-free perovskite materials based on the split-anion approach, where we replace Pb with non-toxic elements while introducing dual anions (i.e., splitting the anion sites) that preserve the charge neutrality. We show that CH3NH3BiSeI2 and CH3NH3BiSI2 exhibit improved band gaps and optical absorption over CH3NH3PbI3. The split-anion approach could also be applied to pure inorganic perovskites, significantly enlarging the pool of candidate materials in the design of low-cost, high-performance and environmentally-friendly perovskite solar materials.