Open-circuit voltage shifted by the bending effect for flexible organic solar cells

The shift of the open-circuit voltage (V_oc) of flexible organic solar cells (OSCs) under bending conditions was investigated by fabricating bi-layer heterojunction and polymer-based OSCs on flexible polyethylene terephthalate (PET) substrates. To realize the performance variations of flexible solar cells characterized by important parameters, V_ocwas measured when the substrate was bent under various curvatures. The V_ocwas increased and decreased by using tensile and compressive stresses, respectively. The ratio of increase for V_ocis larger than the ratio of reduction, thus indicating that the intermolecular distance of an organic semiconductor is difficult to change because of the strong electrostatic repulsive force. A quantitative analysis of energy levels by the photoluminescence spectrum, UV-visible absorption spectrum, and quantum chemical calculation at various bending states was used to explain the V_ocas a function of bending curvature. The peak shifts of UV-visible absorption and photoluminescence spectra provide direct evidence of the variation in energy levels when devices are bent, which causes V_ocshifts. For bent organic semiconductor films, the bending curvature-dependent intermolecular distance was studied by Raman spectroscopy by analyzing the intermolecular coupling energy. This study shows that the change of V_occannot be neglected in the application of flexible OSCs on a flexible loading circuit.