Efficient and stable tandem solar cell developed

Solar cells convert light into energy, but they can be inefficient and vulnerable to the environment. Now, an international research team has developed a new type of solar cell that can both withstand environmental hazards and has power conversion efficiency of 26.7%, as described in the journal Science.

Led by Professor Byungha Shin from the Korea Advanced Institute of Science and Technology (KAIST), the researchers focused on developing a new class of light-absorbing material, called a wide-bandgap perovskite. The material has a highly effective crystal structure that can process the power needs, but it can become problematic when exposed to environmental hazards such as moisture. Researchers have made some progress increasing the efficiency of solar cells based on perovskite, but the material has greater potential than what was previously achieved.

To achieve better performance, Prof Shin and his team built a double-layer (or tandem) solar cell, in which two or more light absorbers are stacked together to better utilise solar energy. To use perovskite in these tandem devices, the scientists modified the material’s optical property, which allows it to absorb a wider range of solar energy. Without the adjustment, the material is not as useful in creating high-performing tandem solar cells.

The modification of the optical property of perovskite, however, comes with a downside: the material becomes hugely vulnerable to the environment — particularly, and ironically, to light. So to counteract the perovskite’s delicate nature, the researchers engineered combinations of molecules composing a two-dimensional layer in the perovskite, stabilising the solar cells.

“We developed a high-quality wide-bandgap perovskite material and, in combination with silicon solar cells, achieved world-class perovskite-silicon tandem cells,” Prof Shin said.

The development was only possible due to the engineering method, in which the mixing ratio of the molecules building the two-dimensional layer is carefully controlled. In this case, the perovskite material not only improved efficiency of the resulting solar cell but also gained durability, retaining 80% of its initial power conversion capability even after 1000 hours of continuous illumination. This is the first time such a high efficiency has been achieved with a wide-bandgap perovskite single layer alone, according to Prof Shin.

“Such high-efficiency wide bandgap perovskite is an essential technology for achieving ultrahigh efficiency of perovskite-silicon tandem (double layer) solar cells,” Prof Shin said. “The results also show the importance of bandgap matching of upper and lower cells in these tandem solar cells.”

Having stabilised the wide-bandgap perovskite material, the researchers are now focused on developing even more efficient tandem solar cells that are expected to have more than 30% of power conversion efficiency — something that no-one has yet achieved.

“Our ultimate goal is to develop ultrahigh-efficiency tandem solar cells that contribute to the increase of shared solar energy among all energy sources,” Prof Shin said.

Image caption: Structure of the perovskite-Si tandem device. Image credit: Professor Byungha Shin, KAIST.

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