Two steps towards efficiency

Nature 499, 316–319 (2013)

Perovskites (CH₃NH₃PbX₃, with X = Cl, Br, I) have recently gained attention as sensitizers for efficient, solution-processable photovoltaic devices. Such air-stable, fast electron-transporting materials have been used as a coating on mesoporous metal-oxide electrodes, leading to enhanced power-conversion efficiencies in solar cells. However, the standard coating method based on the deposition of a solution containing both the perovskite components PbX₂ and CH₃NH₃X suffers from poor control over the precipitation process and thus limited reproducibility of the photovoltaic performance. Michael Grätzel and colleagues now report a two-step deposition method: first, PbI₂ infiltrates uniformly into the nanopores of a TiO₂ film, and second, the film is dipped into a CH₃NH₃I solution and the PbI₂ is converted into perovskite pigments with a uniform size distribution. Solar cells with an improved photocurrent — due to the higher loading of the perovskite pigment in the TiO₂ film and an increased light scattering — and a power-conversion efficiency as high as 15% were realized.