The Centre for Solar Energy and Hydrogen Research Baden-Württemberg (ZSW) has made progress towards the goal of coating large-area perovskite solar cells on an industrial scale using a process that uses more benign solvents than available hazardous solvents like dimethylformamide. The team of researchers developed a coating process for perovskites that uses a single ecofriendly solvent, dimethyl sulfoxide. The ZSW team applied this method to produce a solar cell nearly as efficient as cells made with the toxic solvent.
Perovskite precursors have to first be dissolved so they can be applied in uniform layers to the substrate. This requires solvents that usually contain dimethylformamide (DMF), which is hazardous to health and the environment. This toxicity hampers efforts to scale this process up to industrial production. Manufacturers would have to produce and dispose of larger quantities of the solvent and take even more stringent occupational safety measures, also causing costs to rise. For these reasons, many researchers and manufacturers are in search of environmentally compatible solvents that are suitable for industrial applications. This use case requires chemical properties that very few substances exhibit. Dr. Jan-Philipp Becker, the head of the ZSW’s Photovoltaics: Materials Research department, worked with his team to investigate pure dimethyl sulfoxide (DMSO) to see if it could serve this purpose.
DMSO actually looks to be unsuitable for this coating process. It is a solvent with high surface tension and viscosity, which leaves an uneven layer deposited on the solar cell. DMSO also makes it difficult to control the crystallization process, which often results in small perovskite crystals and a cell that generates less solar energy.
The ZSW researchers found two workarounds for these issues. They modified the film formation process and improved the drying method to increase the efficiency of perovskite solar cells produced with DMSO. “We used a surfactant made of silicon oxide nanoparticles to coat the perovskite solar cell and adapted the drying process,” says Becker. With these two improvements, the process now produces uniform layers with large crystallites.
The 0.24-square-centimeter perovskite solar cells produced at the ZSW achieve 16.7 percent efficiency, just 0.2 percent less than the perovskite solar cells of the same size that the institute makes with DMF. The researchers used the blade coating method to this end. It is scalable to larger production numbers and therefore suitable for industrial applications.
This scalability is what distinguishes blade-coated cells from spin-coated cells that have achieved record efficiencies of 25.7 percent. Although spin coating produces more efficient cells, this method does not lend itself to industrial-scale production of large modules.
“These new research findings are an important milestone on the path to industrial production,” says Becker, clearly delighted. “Now we will further optimize the manufacturing process and produce larger modules.” ZSW researchers now aim to demonstrate perovskite solar modules as large as 30 by 30 square centimeters made with industrial-grade coating methods. With this size, all basic challenges of a further upscaling to commercial module formats would already be overcome.
