Semi-transparent solar cells achieve record-breaking energy conversion

High-efficiency solar cells are needed as the world looks to phase out its dependence on fossil fuels for its energy demands. Conventionally deployed solar cells have an energy conversion efficiency of only 20 percent, meaning we cannot tap into 80 percent of light incidents on existing solar infrastructure.

To improve this energy conversion ratio, researchers have been working with materials such as perovskites that can deliver high energy efficiencies. Some teams want to tap into more solar energy by repurposing windows and glass facades as solar panels.

The team at the Photovoltaics Research Department, along with the Energy AI and Computational Science Lab at KIER, have now combined these two approaches to deliver a highly durable perovskite cell that is also highly efficient.

Making semi-transparent solar cells

To make semi-transparent solar cells, the team decided to replace the metal electrodes that make conventional solar cells opaque with transparent ones that allow the light to pass through them. While this might sound like a straightforward thing to do, making these changes have a detrimental effect.

The high-energy particles received by the solar cell with transparent electrodes result in the degradation of the performance of the hole transport layer. This layer is crucial in a perovskite cell since it allows the transfer of photogenerated holes to the external circuit.

To prevent this performance degradation, a metal oxide layer was used as a buffer between the hole transport and the transparent electrode layer, which caused a reduction in charge that could be transported and the stability of the device.

Lithium to the rescue

With inputs from the Energy AI and Computational Science laboratory, the team was able to recognize the cause of performance degradation while also finding out how lithium could help resolve the issue.

When Lithium ions are introduced in the hole transport layer to increase its conductivity, they also diffuse into the metal oxide layer, thereby changing the latter's electronic structure.

To resolve this, the team found that converting lithium ions into lithium oxide prevented their diffusion into the metal oxide layer, making it much more stable.

Record-breaking perovskite-based solar cells

The researchers found that the resultant solar cell not only had a 21.68 percent energy, the highest recorded in all transparent solar cells so far, but the stability of the solar cell was also recorded at 400 hours in dark storage and more than 240 hours in continuous operational conditions, another feat for perovskite-based solar cells.

The team used its newly developed solar cell as the top layer in tandem solar cells, enabling a bifacial solar cell to work using light from front and rear surfaces.

The energy conversion efficiency of the solar cell was recorded at 31.5 percent for a four-terminal cell, whereas it was 26.4 for a two-terminal configuration.

"This study represents a significant advance in the field by examining the degradation process occurring at the interface of organic compound and metal oxide buffer layer which is unique to semi-transparent perovskite solar cells," said Ahn Sejin, the project lead at KIER's Photovoltaics Research Department in the press release.

"Our solution is readily implementable, demonstrating great potential for the future use of the technologies we have developed."

The research findings were published in the journal Advanced Energy Materials.