Enhanced performance of organic solar cells based on thiophene/phenylene co-oligomers

•TPCOs have been investigated as a major p-type layer of organic solar cells.•The highest PCE reached ∼3.0% with VOC of 1.0V by a simple planar heterojunction.•The variation in the HOMO levels of TPCOs provides various VOC. We investigated solar cells in which various thiophene/phenylene co-oligomers (TPCOs) were used as the major photoelectric-conversion component and obtained enhanced performance. These simple planar heterojunction solar cells consisted of TPCO as the p-type semiconductor and fullerenes as the n-type semiconductor. The variation in the highest occupied molecular orbital levels of the TPCOs caused systematic modulation of the open-circuit voltage (VOC) without incurring a large change in the short-circuit current. We obtained a maximum power-conversion efficiency (PCE) of 3.0%, with VOC reaching 1.0V. This PCE exceeds that obtained for a device containing zinc phthalocyanine, a conventional p-type photoelectric-conversion material.