Organometal halide perovskite solar cells have recently become a hot topic within the research community due to the rapid rise in their efficiency, with champion devices now reaching power conversion efficiencies of over 15%. The solution processable nature of these materials is of great appeal since it should allow the use of low cost roll-to-roll manufacturing techniques on large area, flexible substrates. To date, however, there have been very few reports on perovskite layers fabricated using roll-to-roll compatible techniques, and those which there are have typically focused on vacuum deposition rather than cheap solution processing routes such as printing or spray-coating. We have used ultra-sonic spray-coating as a deposition method for the perovskite layer of planar heterojunction solar cells, successfully demonstrating the fabrication of efficient devices by a solution processable roll-to-roll compatible technique.

The formation of a continuous layer of perovskite, free from significant pinholes, has previously been found to be important for the fabrication of efficient photovoltaic devices. To this end we have identified and optimised a number of parameters which influence the morphology of the photoactive active layer of our devices, which are cast from a solution of methylammonium iodide and lead chloride.

Casting solvent volatility plays a crucial role in film formation, and we have found that spray-coating from the solvent DMF (boiling point 153°C) produces a far more continuous film than when the solvent DMSO (boiling point 189°C) is used. We find that films which dry either too slowly or too rapidly form more incomplete layers, and thus substrate temperature during film deposition is also a critical parameter. In addition, the temperature of the subsequent thermal annealing treatment – used to crystallise the deposited materials into the mixed halide perovskite – also has a significant effect on film morphology. Optimisation of these parameters in the fabrication of photovoltaic devices with the architecture ITO/PEDOT:PSS/CH₃NH₃PbI_3-xCl_x/PCBM/Ca/Al leads to average power conversion efficiencies of 7.8% and a champion efficiency of 11.1%. Further work is currently underway to investigate and optimise further parameters including the nitrogen pressure used in the spraying process and the concentration of the perovskite precursor solution.