A Korean research team has created semi-transparent perovskite solar cells in a step towards the future of solar windows. The cells demonstrate high-power conversion efficiency and transmit visible light while blocking infrared light, meaning they could potentially generate electrical energy while enabling smart heat management.

Typical solar cells are made of crystalline silicon, but it is difficult to make them translucent and therefore practical for use in windows. Perovskites are hybrid organic-inorganic halide-based photovoltaic materials, which are cheap to produce and easy to manufacture. Using perovskites, a Korean research team led by Professor Seunghyup Yoo of the Electrical Engineering School at the Korea Advanced Institute of Science and Technology (KAIST) and Professor Nam-Gyu Park of the Chemical Engineering School at Sungkyunkwan University, developed a semi-transparent solar cell that is highly efficient and, additionally, functions effectively as a thermal-mirror.

South Korea’s Ministry of Knowledge and Economy has increased investment in renewable energy, including solar, wind and biofuels, to reduce reliance on foreign oil imports. South Korea is a fast-growing gigawatt-market for photovoltaics (PV), the conversion of light into electricity using semiconducting materials. This research was supported mainly by the Climate Change Research Hub Program of KAIST, and echoes the institute’s goal of contributing to the ‘green growth’ of Korea.

The team has developed a top transparent electrode (TTE) that works well with perovskite solar cells – a multi-layer stack consisting of a metal film sandwiched between a high refractive-index layer and an interfacial buffer layer. Unlike conventional transparent electrodes focusing only on transmitting visible light, the proposed TTE plays the dual role of passing through visible light while reflecting infrared rays. The semi-transparent solar cells made with the proposed TTEs exhibited average power conversion efficiency as high as 13.3 per cent with 85.5 per cent infrared rejection.

An experiment used a halogen lamp to illuminate an object for five minutes through three mediums: a window of bare glass, automotive tinting film, and the proposed semi-transparent perovskite solar cell. An infrared (IR) camera took thermal images of the object as well as that of each window’s surface. The object’s temperature rose to 36.8 Celsius degrees when exposed through the glass window, whereas both the tinting film and the cell allowed the object to remain below 27 Celsius degrees. However, the tinting film, which absorbs light to block solar energy, became hot; the solar cell stayed cool since it rejects solar heat energy by reflection, rather than by absorption. The total solar energy rejection (TSER) of the proposed cell was as high as 89.6 per cent.

Professor Yoo of KAIST said: “The major contributions of this work are to find transparent electrode technology suitable for translucent perovskite cells and to provide a design approach to fully harness the potential it can further deliver as a heat mirror in addition to its main role as an electrode. The present work can be further fine-tuned to include coloured solar cells and to incorporate flexible or rollable form factors, as they will allow for greater design freedom and thus offer more opportunities for them to be integrated into real-world objects and structures such as cars, buildings, and houses.”