Light can boost perovskite solar cell formation

26 Apr 2017

Efficiency of perovskite-based photovoltaic cells depends on controlling its deposition into films.

Scientists at the Ecole Polytechnique Federale de Lausanne (EPFL), Switzerland, have demonstrated how exposure to light can affect perovskite film formation in solar cells. The formation process is a critical factor in using perovskites in new forms of cost-effective and energy-efficient photovoltaics. The work has been published in Nature.

EPFL explained the significance of the work of the laboratory, which is led by Michael Grätzel, a pioneer in new forms of solar cells, commenting: “Perovskites are materials of immense interest in photovoltaic technology, as they promise to bring down the cost of solar cells to very low levels.”

The efficiency of perovskite-based cells depends on controlling its deposition into films, but the factors that influence the synthesis of perovskites are not yet well understood, added EPFL. Now its scientists have now shown that certain exposure to light can significantly accelerate the formation of perovskite films and control the morphology of their crystals.

Grätzel’s laboratory

Micahel Grätzel is world-renowned for inventing the first dye-sensitized solar cells (“Grätzel cells”) that revolutionized solar-energy science. In the Nature paper, PhD student Amita Ummadisingu and colleagues used confocal laser scanning fluorescence microscopy and scanning electron microscopy to examine how direct light affects the crystal formation when depositing perovskites in layers, as is usually done to build a solar cell.

The usual objective is to ensure homogeneity across the perovskite film, as this is linked to superior photovoltaic performance. Understanding effects of light can help reproducibility and scaling-up of perovskite photovoltaics to commercial production. Because of this, the EPFL scientists studied the effect of light on the two most common methods used today for perovskite film deposition.

The first method is called “sequential deposition”, which involves depositing lead iodide onto a titanium dioxide scaffold, and then dipping the combination into a methylammonium iodide solution to create methylammonium lead iodide perovskite. The second approach is called the “anti-solvent method”. Here, 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 EPFL 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.

Quantum effect

The morphological differences in the film are not the result of heating from the light; rather they are 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.

Conversely, 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,” said Michael Grätzel.

“Light is a major factor in the main deposition methods and for many perovskite compositions,” added Amita Ummadisingu. “So awareness and control of the light effect is crucial when fabricating state-of-the-art perovskite devices, and should always be considered when preparing perovskite films for opto-electronic applications such as solar cells and LEDs.” The work was funded by the Swiss National Science Foundation (SNSF).