Science http://doi.org/z88 (2015)

Perovskites may be poised to take the solar-energy industry by storm, but do we really understand them? One thing is certain: any candidate material needs to have hole and electron diffusion lengths in excess of a micrometre to function efficiently in a solar cell. Any shorter and the thin geometry of the device prohibits effective charge transport. So how far can they go? Qingfeng Dong and co-workers have given us striking evidence that single crystals of methylammonium lead iodide support diffusion lengths up to nearly double this lower functional limit under standard illumination conditions.

The carrier diffusion lengths in this oft-studied mixed-halide perovskite are expected to be sensitive to defects. And most defects tend to be surface proximal, which creates problems in a thin-film architecture. Dong et al. provided a method for growing single crystals without the high trap densities associated with their thin-film analogues. The exceptional diffusion lengths they report may even render these materials useful for X- and gamma-ray sensing, in addition to light harvesting.