Improving the efficiency of organic solar cell with a novel ambipolar polymer to form ternary cascade structure

This study describes the utilization of a novel conjugated copolymer, namely, poly[2,3-bis(thiophen-2-yl)-acrylonitrile-9,9′-dioctyl-fluorene] (FLC8) for organic solar cell application for the first time. The highest occupied molecular orbital and the lowest unoccupied molecular orbital of FLC8 are −5.68 and −3.55 eV, respectively, which lie between the corresponding values of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methylester (PCBM). In addition, both electron and hole mobilities of FLC8 are in the range of 10−4 (cm2/V s), making it an excellent ambipolar polymer. Such unique properties make FLC8 a good candidate to form a ternary cascade bulk-heterojunction organic solar cell when blending with P3HT and PCBM. The power conversion efficiency (PCE) of the ternary cascade solar cell can be increased by up to 30% as compared with the reference cell without FLC8. We suspect that this enhancement of PCE is caused by the additional charge separation offered by the cascade structure and the fast charge transfer due to the ambipolarity of FLC8.