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

• Published in: Journal of materials chemistry. A, Materials for energy and sustainability Vol. 2; no. 38; pp. 15781 - 15787
• Main Authors: Chou, Wei-Yang, Yen, Chia-Te, Wu, Fu-Chiao, Cheng, Horng-Long, Liu, Shyh-Jiun, Tang, Fu-Ching
• Format: Journal Article
• Language: English
• Published: 01.01.2014
• ISSN: 2050-7488; 2050-7496
• DOI: 10.1039/C4TA03447C

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 oc was measured when the substrate was bent under various curvatures. The V oc was increased and decreased by using tensile and compressive stresses, respectively. The ratio of increase for V oc is 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 oc as 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 oc shifts. 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 oc cannot be neglected in the application of flexible OSCs on a flexible loading circuit.