The structure of transparent solar cell based on TiO2/NiO heterostructure and an image of the device showing its high transparency. Credit: Joondong Kim from Incheon National University.
The structure of transparent solar cell based on TiO2/NiO heterostructure and an image of the device showing its high transparency. Credit: Joondong Kim from Incheon National University.

Transparent photovoltaics could turn windows, mobile electronics, and displays into energy generators, according to researchers from Incheon National University in Korea. Together with colleagues at Sungkyunkwan University, Duy Tan University in Vietnam, and Thapur Institute of Engineering and Technology in India, the team has constructed novel transparent photovoltaic cells from metal oxides TiO2 and NiO [Nguyen et al., Journal of Power Sources 481 (2021) 228865, https://doi.org/10.1016/j.powsour.2020.228865].

“Developing photodetectors is highly desirable in terms of energy-efficient devices, device architecture simplicity, and power generators for operating electrical appliances by converting UV light to electricity,” explains Joondong Kim, who led the effort with Thanh Tai Nguyen and Malkeshkumar Patel.

Photovoltaic devices harness solar energy to produce electricity cleanly and efficiently in a range of applications from self-powered electronics to transport and hydrogen energy production. Metal oxides, and in particular TiO2, which is widely used already in photovoltaics, possess excellent optoelectronic properties, typically have a wide bandgap for efficient photon absorption, are environmentally benign and non-toxic. In addition to TiO2, which acts as a UV light absorber, the researchers investigated the addition of NiO to devices. NiO is highly transparent and easy to manufacture, while Ni is one of the most abundant metals. The thin layers of p-type NiO and n-type TiO2 form an efficient heterojunction on a glass substrate with fluorine-doped tin oxide and silver nanowires serving as the bottom and top electrodes.

The new transparent photovoltaic cell selectively absorbs UV photons for power generation while remaining largely transparent, allowing more than 57% of visible light through. This value easily exceeds the required 40% transparency for practical window-type applications, point out the researchers. While the power conversion efficiency is only just over 2% in the proof-of-concept devices, the novel photovoltaics can operate a small motor when illuminated. As well as generating power, the device can also function as a self-powered UV-sensitive photodetector. Unlike other metal oxide devices, these photovoltaics also boast a fast response time of around 4 ms.

“While this innovative solar cell is still very much in its infancy, our results strongly suggest that further improvement is possible for transparent photovoltaics by optimizing the cell’s optical and electrical properties,” says Nguyen. “Our all-oxide photovoltaic cell could resolve the problem of electrolyte leakage and unstable performance, which is faced in wet solar cells.

The researchers are now working on boosting device efficiency. In the long run, for commercial applications in windows or electronic devices such as UV sensors, these metal oxides have the additional advantages of being widely available and highly stable under atmospheric conditions.

“We are very excited about transparent solar cells,” says Patel. “We could be freed from wired electricity, at least for electronic gadgets such as sensors and displays.”