Novel green solvent could help scale up fabrication of perovskite-based tandem solar cells

In recent years, engineers have been trying to develop alternative photovoltaic (PV) technologies that could be more affordable and scalable than silicon-based solar cells, while also exhibiting good power-conversion efficiencies and retaining these efficiencies over time.

These emerging PVs include perovskite-based tandem solar cells, which are made up of two layers (i.e., sub-cells) with complementary light-absorption properties stacked on top of each other.

While these solar cells achieve promising power-conversion efficiencies, to be deployed on a large-scale, without harming the environment, these cells will need to contain environmentally friendly solvents. Notably, most green solvents created so far are ineffective when applied to wide-bandgap perovskites, thus the fabrication of tandem solar cells using these materials so far relied on toxic solvents based on N,N-dimethylformamide (C₃H₇NO).

Researchers at Nanjing University and other institutes have created a new green solvent that can effectively dissolve cesium and bromide salts in perovskites and could thus be well-suited for the fabrication of both wide-gap and narrow-gap perovskite-based tandem solar cells. This solvent, presented in a paper published in Nature Energy, contains dimethyl sulfoxide, acetonitrile and ethyl alcohol.

“In previous research, our team had reached a high performance of 24.5% in all-perovskite tandem modules, which shows a huge potential for commercialization,” Ke Xiao, co-author of the paper, told Tech Xplore.

“Considering that the environmental and health safety during the fabrication process remains the most important issue for future scalable mass production, the toxic volatile nature of conventional perovskite solvents determines that they are difficult to manage in industrial production.”

The main objective of this study by Xiao and his colleagues was to identify a relatively safe and non-toxic solvent system that could reduce toxic waste resulting from the production of perovskite-based solar cells.

The solvent they created consists of dimethyl sulfoxide and acetonitrile, two chemicals that can dissolve cesium and bromide salts, along with ethyl alcohol, which prevents the degradation of the chemical compounds initiating chemical reactions in perovskites.

“In our recent work, we used the nitrogen gas blowing assisted blade-coating method to prepare large-area wide-bandgap perovskite films,” explained Xiao.

“The most notable achievement of this new work is the proposal of a green solvent system that is suitable for scalable fabrication for wide-bandgap perovskites. This solvent system is compatible not only with different wide-bandgap perovskites, but also with narrow-band gap perovskites, which means that this solvent system can be widely applied in various types of tandem cells in the future.”

The researchers used the green solvent they developed to fabricate blade-coated wide-bandgap perovskite solar cells. In initial tests, these solar cells performed well, achieving power conversion efficiencies of 19.6% (1.78 eV) and 21.5% (1.68 eV).

They then used these cells to create 20.25 cm² all-perovskite tandem solar modules that achieved a higher power-conversion efficiency of 23.8%. In the future, the new solvent could be further tested and used to fabricate other solar modules, potentially contributing to the environmentally-friendly and large-scale fabrication of tandem solar cells based on perovskites.

“In our next studies, we plan to continue trying to reduce costs and improve the efficiency of all-perovskite tandem modules, as this is very important for industrialization,” added Xiao. “Now that we [have] identified the green solvent system, the next step will be to achieve larger-area fabrication in ambient air to avoid using redundant glove box.”

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