Following pioneering work, solution-processable organic-inorganic hybrid perovskites, such as CH3NH3PbX3 (X = Cl, Br, I), have gained attention as light-harvesting materials for mesoscopic solar cells. So far, the perovskite pigment has been deposited in a single step onto mesoporous metal oxide films using a mixture of PbX2 and CH3NH3X in a common solvent. However, the uncontrolled precipitation of the perovskite produces large morphological variations, resulting in a wide spread of photovoltaic performance in the resulting devices, which hampers the prospects for practical applications. Here Prof. Michael Grätzel and his research team described a sequential deposition method, published on Nature, for the formation of the perovskite pigment within the porous metal oxide film. PbI2 was first introduced from solution into a nanoporous titanium dioxide film and subsequently transformed into the perovskite by exposing it to a solution of CH3NH3I. They found that the conversion occurred within the nanoporous host as soon as the two components came into contact, permitting much better control over the perovskite morphology than was possible with the previously employed route. Using this technique for the fabrication of solid-state mesoscopic solar cells greatly increases the reproducibility of their performance and allows them to achieve a power conversion efficiency of approximately 15 per cent (measured under standard AM1.5G test conditions on solar zenith angle, solar light intensity and cell temperature, see Figure below). The power conversion efficiency of 15% achieved with the best device is amongst the highest for solution-processed photovoltaics and sets a new record for organic or hybrid inorganic–organic solar cells in general. Their findings open new routes for the fabrication of perovskitebased photovoltaic devices, because other preformed metal halide mesostructures may be converted into the desired perovskite by the simple insertion reaction. On the basis of their results, they believe that this new class of mesoscopic solar cells will find widespread application and will eventually lead to devices that rival conventional silicon-based photovoltaics.

Source: Nature, 2013, 499, 316