Thiazole-Induced Quinoid Polymers for Efficient Solar Cells: Influence of Molecular Skeleton, Regioselectivity, and Regioregularity

Abstract

Fluorination strategy has been regarded as a promising approach to improve the photovoltaic performance in polymer solar cells. However, the synthesis is relatively tedious and costly for most fluorinated momomers. In this work, which is different from works where fluorine atoms are usually incorporated on the side chain, we successfully developed a thiazole-induced strategy to construct efficient photovoltaic materials via inserting one thiazole unit into the backbone of a nonfluorinated quinoid polymer, which would enhance the intermolecular interactions and decrease the ionization potential (IP) of the resulted polymers, benefiting from the desirable molecular skeleton. And then, considering the asymmetry nature of acceptor segments, the influence of molecular regioselectivity with different orientations of the thiazole unit on optoelectronic properties was systematically investigated. Encouragingly, a superior power conversion efficiency (PCE) of 9.36% for PBTzT-4-based photovoltaic device was obtained, higher than that of the isomer polymer PBTzT-6 (PCE = 8.52%) and a signficant increase of 50% compared to the widely reported analogue polymer PBDTTT-E-T (PCE = 6.21%) just without a thiazole unit, which can be ascribed to more planar molecular conformation, stronger crystallinity, and excellent phase separation. More interestingly, compared with random polymers, the regioregular copolymers exhibit enhanced red-shifted absorption and better crystallinity and compatibility with PC71BM, leading to more desirable efficicency for PBTzT-4R-based devices (PCE = 9.63%) with higher JSC of 17.56 mA/cm2, which can be comparable to the typical polymer PTB7-Th. This work not only provides a new strategy to improve the intermolecular interaction through backbone design but also reveals that the orientations of the asymmetric unit (that is, regioselectivity and regioregularity) along the polymer backbone play a crucial role and should be taken into account in future molecule design.