Improved efficiency of organic solar cells based on crystals oriented by the friction transfer method

For organic photovoltaic devices (OPVDs), controlling the molecular orientation is crucial for obtaining high efficiency. In the present research, highly oriented crystalline films of thiophene-phenylene co-oligomer (TPCO) material were grown on a thin orientation template of friction-transferred polythiophene (PT). To further increase the orientation, thermal treatments were performed to induce recrystallisation of the TPCO layer. XRD, SEM, and polarised absorption analyses confirmed a high level of orientation, demonstrating that the annealing could further enhance the orientation. Moreover, the annealed samples exhibited a promising block-like crystal structure, which has the potential for creating an interpenetrated bulk heterojunction. Based on this structure, OPVDs were fabricated having TPCO molecules and C70 as the organic semiconductors, and, subsequently, their electrical properties were evaluated. However, the results indicated a decrease in the power conversion efficiency (PCE) of the devices when friction transfer and annealing were performed. Investigations of the causes and possible solutions were proposed in order to find a way to increase the PCE.