Current solar cells are restricted to rigid, flat panels that are difficult to store in large numbers and integrate into everyday appliances including cellphones, windows, vehicles, or indoor devices. One problem hampers this technology: to be integrated into these items, solar cells need to be foldable and bend at will repeatedly without breaking. Traditional conducting materials used in solar cells lack flexibility, creating an obstacle to developing fully foldable cells.
A key requirement for an efficient foldable conductor is the ability to withstand the pressure of bending within a very small radius while maintaining its integrity and other desirable properties. In short, a thin, flexible, transparent, and resilient conductor material is needed. Unlike merely flexible electronics, foldable devices are subject to much harsher deformations, with folding radii as small as 0.5 mm. This is not possible with conventional ultra-thin glass substrates and metal oxide transparent conductors, which can be made flexible but never fully foldable.
A team of researchers has identified a promising candidate to answer all of these requirements: single-walled carbon nanotube (SWNT) films, which possess high transparency and mechanical resilience. The SWNTs, however, struggle to adhere to the substrate surface when force is applied (such as bending) and require chemical doping. To address this problem, the team embedded the conducting layer into a polyimide (PI) substrate, filling the void spaces in the nanotubes.
To ensure maximum performance, they also doped the resulting material to increase its conductivity. By introducing small impurities (in this case, withdrawn electrons to molybdenum oxide) into the SWNT-PI nanocomposite layer, the energy needed for electrons to move across the structure is much smaller, and hence, more charge can be generated for a given amount of current.
The resulting prototype far exceeded the team’s expectations. Only 7 micrometers thick, the composite film exhibited exceptional resistance to bending, almost 80 percent transparency, and a power conversion efficiency of 15.2 percent, the most ever achieved in solar cells using carbon nanotube conductors.