Conventional c-Si PV cells are structurally rigid and fragile, so they can break apart when they are under extreme external forces. Furthermore, rectangular PV modules that contain c-Si PV cells can only be deployed on a flat surface such as the roof of a building. Scientists and engineers that are involved in the field of solar PV have therefore been working on flexible and foldable PV cells that could become renewable energy solutions for a wider range of applications.

The latest example of this kind of technology comes from Pusan (Busan) National University in South Korea. An international team of scientists working at the institution has recently developed a PV cell that technically can be folded in half without breaking. At the same time, this foldable PV cell has achieved a fairly decent conversion efficiency rate of 15.2%.

Although the latest PV modules that contain c-Si cells boast higher efficiency and greater power output than ever before, they still lack the structural flexibility and resiliency that would lead to greater convenience in transportation and installation. Conventional PV cells are easily damaged when they bump into other hard objects, and they cannot be laminated on curved or bendable surfaces. Hence, more flexible solutions have to be developed in order for solar PV to become a viable option for powering cars, smartphones, or even clothes.

In recent years, curved and thin-film PV cells have been created with special materials such as organic dyes and graphene. A more mainstream material technology for thin-film cells is copper indium gallium selenide (CIGS). Flexible modules comprising CIGS cells have been offered in the market.

However, most flexible cells and modules that now exist on the market cannot be folded in half without breaking. Reaching this level of bendability may seem extreme, but the scientists at Pusan National University want to push the limit of what is possible and create a fully foldable PV module.

Professor Il Jeon, who participated in the development of the new foldable PV cell and co-authored the study on it, said that “foldable” devices are not the same as “flexible” electronics. Specifically, the former are able to achieve a folding radius as small as 0.5mm, whereas the latter cannot. The professor pointed out that flexible PV cells have been made using ultra-thin glass substrates and transparent metal oxide conductors, but these devices are not fully foldable. At most, the current generation of flexible PV cells can wrap around curved but still fairly rigid surfaces.

The scientists at Pusan National University have developed a prototype of a foldable PV cell using single-walled carbon nanotubes (SWNTs) that are 0.6-1.8nm in diameter and serve as the conducting layer. SWNTs are embedded into a polyimide (PI) substrate that fills the gap between the nanotubes. The thin, transparent, and fully foldable film is then doped with molybdenum oxide to further enhance conductivity and the adhesion of nanotubes to the substrate.

The SWNT-PI cell has a thickness of just 7µm and is able to achieve a folding radius of 0.5mm. The team at Pusan National University said that the prototype is demonstrated to have undergone 10,000 folding cycles without breaking. The conversion efficiency rate of the prototype reaches 15.2%, which is high for a flexible or foldable PV cell. The team also pointed out that their technology can be used to make “solar windows” as the transparency of the SWNT-PI cell is 80%.

Il Jeon added that the SWNT-PI cell should be regarded as the best so far among all types of flexible and foldable PV cells in terms of mechanical stability and conversion efficiency.

(Source: Pusan National University)

 (The image at the top of article shows conventional solar cells and is for reference only. Source: Pixabay.)