Enhancing optical absorption by reducing the trap states in polymer semiconductor processed for solar cell

Abstract

Current-voltage characteristics of an organic solar cells can be tuned by tailoring the active layer material’s polymer chain ordering which in turn decides the PV performance. The suppression of exciton recombination, improved charge carrier separation, large carrier diffusion length can be achieved by improving the packing density of the π-π stacking, polymer chain length and chain ordering. The high packing density π-π stacking can be formed through enhancing the percentage of crystallinity of the semiconducting polymer layer. This can be achieved by optimizing the processing parameters. This work gives some insights on the spinning time of active layer (P3HT:PC61BM) on the structural, optical and solar cell performance. P3HT:PC61BM crystalline properties were studied through grazing incidence X-ray diffraction (GI-XRD). Optical absorption of the active layer was studied by the UV–visible spectrometer. The trap density of states of the solar cells processed at various active layer spinning condition is evaluated. From the obtained result it is noted that device fabricated with longer spinning time showed higher power conversion efficiency over shorter spinning duration. These changes in the device performance could be due to solvent evaporation kinetics, chain alignment in the polymer thin film. Further, the mechanism of trap states formation based on the active layer processing condition is explained.