Bifacial thin-film solar cells harness sunlight from both sides for higher output

A research team successfully implemented CuInSe₂ thin-film solar cells composed of copper (Cu), indium (In), and selenium (Se) on transparent electrode substrates. Furthermore, the team developed a “bifacial solar cell technology” that receives sunlight from both the front and back sides to generate power. This technology can be fabricated at low temperatures, enabling a simpler production process, and is broadly applicable to building-integrated solar power, agricultural solar power, and high-efficiency tandem solar cells in the future.

The findings are published in the journal Advanced Energy Materials. The team was led by Daehwan Kim and Shijoon Sung at the Division of Energy & Environmental Technology (concurrently affiliated with Interdisciplinary Engineering), Daegu Gyeongbuk Institute of Science & Technology.

Recently, bifacial solar cells have garnered significant attention, as the cells can absorb light from both the front and back simultaneously, not just the front, to generate more electricity in the same area. To implement these cells, it is necessary to use transparent electrode substrates that allow light to pass through. However, when thin-film solar cells were fabricated on transparent electrodes with low heat resistance, it was difficult to achieve good properties. This is because high temperatures are required to fabricate traditional thin-film solar cells.

Principal Researchers Kim and Sung at DGIST have successfully developed the compound thin-film solar cell technology that operates at temperatures below 420°C and has a narrow band gap. Silver (Ag) was introduced during the process to form the CuInSe₂ compound, enabling high-quality compound deposition at lower temperatures.

Moreover, it increased device efficiency by inducing a gradient of gallium (Ga) at the bottom of the CuInSe₂ layer and by improving charge transport (carrier delivery) and recombination properties. This allowed the research team to enhance the performance of bifacial CuInSe₂ thin-film solar cells based on transparent substrates.

As a result, the research team’s narrow-gap thin-film solar cells on transparent substrates achieved one of the world’s best performances, with 15.3% front efficiency and 8.44% back efficiency. They also demonstrated high efficiency in bifacial performance measurements, with a bifacial power generation density (BPGD) of 23.1 mW/cm².

“This study offers a new possibility to increase efficiency for thin-film solar cells based on transparent substrates,” said Kim and Sung. “We will further expand the scope of applications for high-efficiency bifacial solar cell technology based on transparent substrates.”

Meanwhile, Kim and Sung at the Division of Energy & Environmental Technology (also affiliated with Interdisciplinary Engineering) participated in this study as the corresponding authors, while Ali Amanat, a Ph.D. student in Interdisciplinary Engineering, and Donghwan Jeon, a full-time researcher in the Division of Energy & Environmental Technology, served as the first authors.