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Hybrid organic-inorganic lead halide perovskite photovoltaic cells have already surpassed 20% conversion efficiency in the few years that they have been seriously studied. However, many fundamental questions still remain unanswered as to why they are so good. One of these is "Is the organic cation really necessary to obtain high quality cells?" In this study, we show that an all-inorganic version of the lead bromide perovskite material works equally well as the organic one, in particular generating the high open circuit voltages that are an important feature of these cells.

Direct and inverse photoemission spectroscopies are used to determine materials electronic structure and energy level alignment in hybrid organic-inorganic perovskite layers grown on TiO 2 . The results provide a quantitative basis for the analysis of perovskite-based solar cell performance and choice of an optimal hole-extraction layer.

Direct comparison between perovskite-structured hybrid organic-inorganic methylammonium lead bromide (MAPbBr3) and all-inorganic cesium lead bromide (CsPbBr3), allows identifying possible fundamental differences in their structural, thermal and electronic characteristics. Both materials possess a similar direct optical band gap, but CsPbBr3 demonstrates a higher thermal stability than MAPbBr3. In order to compare device properties, we fabricated solar cells, with similarly synthesized MAPbBr3 or CsPbBr3, over mesoporous titania scaffolds. Both cell types demonstrated comparable photovoltaic performances under AM1.5 illumination, reaching power conversion efficiencies of ∼6% with a poly aryl amine-based derivative as hole transport material. Further analysis shows that Cs-based devices are as efficient as, and more stable than methylammonium-based ones, after aging (storing the cells for 2 weeks in a dry (relative humidity 15-20%) air atmosphere in the dark) for 2 weeks, under constant illumination (at maximum power), and under electron beam irradiation.

The liquid junction dye-sensitized solar cell (DSSC) has reached laboratory solar efficiencies of 11%. In contrast, the semiconductor-sensitized analogue (SSSC) has, up to now, exhibited a maximum efficiency of 2.8%. This begs the questions: is this difference fundamental? Will SSSCs always be inferior to DSSCs? We discuss the differences between the two types of cells, considering typical charge transfer times for the various current generating and recombination processes. Three main factors that could contribute to differences between the two types of cells are discussed: multiple layers of absorbing semiconductor on the oxide, the different electrolytes normally used for the two types of cell, and charge traps in the absorbing semiconductor. Entropic effects and the irreversible electron injecting nature of the normally used Ru dye to TiO2 are also briefly considered. We conclude that although the DSSC does possess some fundamental advantages, we can expect large improvements in efficiency of the SSSC, possibly reaching values comparable to the DSSC.

Developments in organic-inorganic lead halide-based perovskite solar cells have been meteoric over the last 2 years, with small-area efficiencies surpassing 15%. We address the fundamental issue of how these cells work by applying a scanning electron microscopy-based technique to cell cross-sections. By mapping the variation in efficiency of charge separation and collection in the cross-sections, we show the presence of two prime high efficiency locations, one at/near the absorber/hole-blocking-layer, and the second at/near the absorber/electron-blocking-layer interfaces, with the former more pronounced. This 'twin-peaks' profile is characteristic of a p-i-n solar cell, with a layer of low-doped, high electronic quality semiconductor, between a p-and an n-layer. If the electron blocker is replaced by a gold contact, only a heterojunction at the absorber/hole-blocking interface remains.

Organic-inorganic hybrid semiconductors may provide the basis for the next generation of thin-film solar cells. [Also see Reports by Stranks et al. and Xing et al. ]