Apr 27, 2017

In a recent study, EPFL researchers demonstrated how light affects perovskite film formation in solar cells. The team showed that, in the two most common methods used today, light can greatly accelerate the formation of perovskite films and control the morphology of their crystals, influencing the efficiency of the resulting solar cell.

The team used confocal laser scanning fluorescence microscopy and scanning electron microscopy to examine how direct light affects the crystal formation when depositing perovskites in layers, a common stage in building a solar cell. The goal is to ensure homogeneity across the perovskite film, as this is linked to superior photovoltaic performance.

The researchers explained that most efforts seem to be focused on improving perovskite solar-cell efficiency by optimizing their architecture and the fabrication processes. However, fundamental aspects of perovskite formation, such as the mechanism or the factors controlling the reaction are poorly understood, which results in batch-to-batch inconsistencies in performance. This is why understanding the light effect can help reproducibility and scaling-up of perovskite photovoltaics to commercial production.

The EPFL scientists studied the effect of light on the two most common methods used today for perovskite film deposition. The first method, 'sequential deposition', involves depositing lead iodide onto a titanium dioxide scaffold, and then dipping them together in a methylammonium iodide solution to make the final methylammonium lead iodide perovskite. The second approach is the 'anti-solvent method', in which a perovskite precursor solution is coated uniformly onto a spinning flat base to create a thin layer. An anti-solvent is then dripped on it and, finally, the whole sample is heated to form the perovskite.

Using both the microscopy techniques during the sequential deposition method, the scientists found that when light is present, it significantly accelerates the overall formation of the perovskite. The effect is enhanced when light intensity is increased to solar levels.

The team explains that the morphological differences in the film are not the result of heating from the light, but the product of a quantum effect: the lead iodide film crystallizes as the first step in perovskite formation, and it forms surface traps. These traps capture photo-generated holes, which lowers the surface tension. As a result, more lead iodide crystals grow in the film, ultimately producing more perovskite crystals.

On the other hand, the researchers found that the anti-solvent method actually benefits more from the absence of light, and it gives best results in the dark.

The study shows that controlling the light present during fabrication can help achieve the best performance from perovskite solar cells. 'Surprisingly and strikingly we find that light strongly affects the rate of formation and the morphology of perovskites in conventional methods used where no light effects have been remarked before,' says Michael Grätzel, the renowned researcher.