Could solar windows harvest energy from the rain?

In recent years, renewables have gone from playing a relatively minor role in the world’s energy landscape, to a central one. Thanks to growing economies of scale, solar, wind and hydroelectric generation is now price competitive with (and often cheaper than) fossil fuel options. Efforts to make these technologies ever more efficient, or to redesign them to suit a wider range of applications continue apace. In the solar sector, silicon-based photovoltaics cells still dominate the market. But there’s a growing interest in transparent and semi-transparent solar cells, thanks to their potential for integration into existing structures, such as windows and glass roofs.

An international team, led by materials engineers at the Ocean University of China, has taken a unique approach to this. Writing in NanoEnergy [DOI: 10.1016/j.nanoen.2022.107776], they report on their development of a proof-of-concept hybrid energy harvesting device. Combining semi-transparent polymer solar cells (ST-PSCs) and liquid−solid triboelectric nanogenerators (TENGs), they say it can generate electricity from both sunlight and raindrops. Could ‘rainy day’, see-through solar panels be a step closer?

The team started by making a series of opaque PSCs in both conventional and inverted configurations. The thickness and microstructure of the active layer was varied to optimise the output. From this, they found the best performing blend for the active layer, which was then used to manufacture semi-transparent polymer solar cells (ST-PSCs) on rigid and flexible substrates. These devices demonstrated stable performance under sustained bending, and achieved photovoltaic efficiencies of 17.4% and 15.7% (for rigid and flexible substrates, respectively). A dielectric/metal/dielectric was added to further improve the transmittance and colour-rendering properties of the rigid ST-PSC.

The next step was to add transparent TENGs to the top surface of the two devices. This was done by combining a 130µm-thick layer of fluorinated ethylene propylene (FEP) and a platinum electrode. Negative charges would be generated on the FEP surface due to the triboelectric effect between water droplets (which are conductive) and FEP, while positive charges would be generated on the PSC top electrode. A continuous stream of water droplets was used to characterise the performance of the TENG. The maximum electrical power output measured under this test was 2.62 W m−2. The authors write that “… one waterdrop (volume: 35 μL, height: 15 cm) can light more than 20 light-emitting diodes” in one of their hybrid devices. The TENG also retained a voltage of ~80V under conditions equivalent to heavy rain.

The TENG’s relatively high effective transmittance (87.75%) for wavelengths of 350−1000 nm allowed light to pass through it and reach the ST-PSC. And while this reduced the light-to-electricity conversion of the solar cell, values of 10.1% and 8.4% under 1 sun were still achieved. The maximum electrical power output was 101 W m−2 on sunny days.

They conclude, “Our work paves an intriguing prospect of developing ST-PSC/TENG hybrid systems for solar and raindrop energy conversion, not merely scaling up the green electricity production under different weather conditions, but also evaluating their integrability, transparency, amenity and sustainability for versatile window-integrated applications.”


Tong Liu, Yang Zheng, Yunxiang  Xu, Xianjie Liu, Chuanfei Wang, Liangmin Yu, Mats Fahlman, Xiaoyi Li, Petri Murto, Junwu Chen, Xiaofeng Xu. “Semitransparent polymer solar cell/triboelectric nanogenerator hybrid systems: Synergistic solar and raindrop energy conversion for window-integrated applications,” Nano Energy, Vol 103 A (2022), 107776. DOI: 10.1016/j.nanoen.2022.107776