Realization of Foldable Polymer Solar Cells Using Ultrathin Cellophane Substrates and ZnO/Ag/ZnO Transparent Electrodes

Foldable solar cells have attracted increasing attention for portable and wearable applications. The challenge to achieve foldability is free from the formation of cracks under large strain. Our mechanical simulations suggests that a reduction in substrate thickness from 150 to 25 μm is a means to mitigate the strain in polymer solar cells under folding with extremely small curvature radius of sub-millimeter. The polymer solar cells are experimentally prepared on ultrathin 25 μm cellophane substrates with ZnO/ultrathin Ag/ZnO (OMO) transparent electrodes, instead of conventional ITO, composing a typical device structure, cellophane/OMO/ZnO/PTB7-Th:PC₇₁BM/MoO₃/Al. The solar cells exhibit a power conversion efficiency of 5.94% and a power weight ratio of 2.11 W g⁻¹. After folding the solar cells for 35 cycles, the conversion efficiency still maintained 92% of the initial value, which is mainly ascribed to the synergistic effects of ultrathin substrates and the OMO electrodes, as inferred from the stable resistance of the cellophane/ZnO/ultrathin Ag/ZnO even after 50 folding cycles. This work paves the way toward realizing superior foldability not only in solar cells, but also in other optoelectronic devices.