However, in contrast to mild bending with curvature radius of several millimeters, folding generates the crease with extreme curvature radius of sub-millimeter, resulting in the appearance of large strain and stress. As a result, it is highly challenging to realize robustly foldable and highly efficient solar cells.
It is widely accepted that folding is the extreme condition of bending which generating crease with extreme low curvature radius of sub-millimeter. Thus, foldable solar cells meet the requirements of size compactness and shape transformation for many emerging applications.
It was usually observed in reported papers that when flexible solar cells were prepared on ITO electrodes, they degraded after bending with radius of 4 mm or even lower, [51, 52] not to mention the folding process. Thus, flexible transparent electrodes as alternatives is important for realizing foldable solar cells.
The key requirements to construct highly foldable solar cells, including structure design based on tuning the neutral axis plane, and adopting flexible alternatives including substrates, transparent electrodes and absorbers, are intensively discussed.
By now, carbon nanotube, graphene, ultrathin metal, metal nanowire, metal grids, conductive polymer, and their complex, have been widely applied in the robust bendable and foldable solar cells.
As a result, cracks will be formed in the functional layers or delamination will be occurred at the interface as soon as the strain exceeds the crack onset strain, leading to the degradation or even failure of the solar cells under repeated folding. Therefore, it is highly challenging to realize robustly foldable solar cells.