The liquid solution of perovskite precursor, solvent and additive flows from a slit-shaped nozzle onto the glass substrate below. Image: HZB/Jinzhao Li.
The liquid solution of perovskite precursor, solvent and additive flows from a slit-shaped nozzle onto the glass substrate below. Image: HZB/Jinzhao Li.

A long-cherished dream of materials researchers is a solar cell that can convert sunlight into electrical energy as efficiently as silicon but that can be easily and inexpensively fabricated from abundant materials. Scientists at the Helmholtz-Zentrum Berlin (HZB) in Germany have now come a step closer to achieving this, by improving a process for vertically depositing a solution made from an inexpensive perovskite solute onto a moving substrate below.

Not only have the scientists discovered the crucial role played by one of the solvents used in this process, but they have also taken a closer look at the aging and storage properties of the solution. They report their findings in a paper in Advanced Energy Materials.

Solar cells made of crystalline silicon still account for the lion's share of roof installations and solar farms. But other technologies have long since become established as well – such as those that convert sunlight into electrical energy through the use of extremely thin layers of a photovoltaic material deposited upon a substrate.

The perovskite solar cells that Eva Unger and her team at HZB are researching belong to this group. "These are the best solar cells to date that can be made using a 2D ink," Unger explains. "And now their efficiencies are approaching those for cells made of crystalline silicon."

Many methods have been developed for fabricating small test cells in the laboratory, where they can be studied and improved. But industrial-scale fabrication is still a long way off. "Unfortunately, processes that are optimized for fabricating small surface areas cannot always be scaled up," says Unger.

In other words, not everything that works perfectly in the lab also necessarily works economically on the factory floor. "That's why we are taking the next step and developing scalable methods," Unger explains. "This means our team is focusing on processes for coating larger surfaces." At the Hybrid Silicon Perovskite Research, Integration & Novel Technologies (HySPRINT) Innovation Lab, a center for collaborations between HZB and industry, Unger and her team are concentrating on fabrication processes that have already proven their importance to industry.

"We have experimented here with slot-die coating," she says. In this process, the 'ink' – as the thin liquid solution of perovskite precursor, solvent and additive is known in the trade – flows from a slit-shaped nozzle and falls like a curtain onto a glass substrate below that will later become a solar cell.

After application, crystallization begins, causing an ultra-thin layer of a semiconducting perovskite structure to grow on the glass substrate. Unger and her team have now discovered that the exact amount of an organic solvent called dimethyl sulfoxide (DMSO) in the ink is critical for this process. Unger uses DMSO as an additive because it has an amazing effect on the ink.

"DMSO induces crystallization nuclei for the perovskite," she says. Crystallization nuclei are tiny grains that help jump-start a crystal and promote its growth. "During X-ray diffraction experiments at BESSY II, we saw quite a big difference between inks with and without DMSO added."

However, as her team has found out in many experiments, the amount added plays a decisive role. More DMSO favours crystal growth – up to a certain point. If this point is exceeded, other processes come into play and the resulting microstructure reduces the performance of the solar cells.

"It's like seasoning a soup," says Unger. "If you add too little, it remains bland. If you add too much, it won't taste good either. So you need to add just the right amount to make it best." In addition to the optimal composition, the HZB team has also thoroughly investigated the ageing processes and thus the storage life of the inks. "This is an aspect that has received little attention so far," Unger explains. "The age of a perovskite precursor ink can influence device performance. This is an important factor that must be considered when developing inks and processes."

This story is adapted from material from Helmholtz-Zentrum Berlin, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.