Abstract
Three two-dimensional donor–acceptor conjugated copolymers consisting of a benzo[1,2-b:4,5-b′]dithiophene derivative and thieno[3,2-b]thiophene with a conjugated side chain were designed and synthesized for use in bulk heterojunction (BHJ) or nonfullerene polymer solar cells (PSCs). Through attaching various acceptor end groups to the conjugated side chain on the thieno[3,2-b]thiophene moiety, the electronic, photophysical, and morphological properties of these copolymers were significantly affected. It was found that the intermolecular charge transfer interactions were enhanced with the increase in the acceptor strength on the thieno[3,2-b]thiophene moiety. Moreover, a better microphase separation was obtained in the copolymer: PC71BM or ITIC blend films when a strong acceptor end group on the thieno[3,2-b]thiophene moiety was used. As a result, BHJ PSCs based on copolymer:PC71BM blend films as active layers exhibited power conversion efficiencies from 2.82% to 4.41%, while those of nonfullerene copolymer:ITIC-based inverted PSCs ranged from 6.09% to 7.25%. These results indicate the side-chain engineering on the end groups of thieno[3,2-b]thiophene unit through a vinyl bridge linkage is an effective way to adjust the photophysical properties of polymers and morphology of blend films, and also have a significant influence on devices performance.
Highlights
Surface morphologies of the active layers were performed by atomic force microscopy (AFM) system in the tapping mode using an SII SPA400 operating in ambient atmosphere and transmission electron microscopy (TEM) at an accelerating voltage of 120 kV using a JEOL JEM-1200EX II instrument
The energy levels of these copolymers were analyzed by cyclic voltammetry (CV) experiments, and the HOMO and LUMO energy levels were estimated from the onset of the oxidation and and the HOMO and LUMO energy levels were estimated from the onset of the oxidation and reduction reduction potential using ferrocene as the reference and assuming an absolute energy level of −4.8
Derivative, with a conjugated side chain attaching various acceptors as end groups were synthesized for use in TT, with a conjugated side chain attaching various acceptors as end groups were synthesized for use
Summary
Solar cells have attracted great attention as the most promising alternative resource of energy. Li et al reported a new polymer (designated as PBDT-S-TT), with further fine modification, where they replaced the alkylthienyl substituents on the BDT unit with alkylthiothienyl groups, and the PCE reached 8.42%. The sulfonyl group provided a strong electron-withdrawing effect that greatly influenced the LUMO level as well as the HOMO level, and this result suggests that it is possible to affect the overall electronic properties by introducing a strong electron-withdrawing group on a TT unit to improve the performance of PSCs. Peng et al recently reported two new 2D polymers based on modified BDT and TT units, denoted as PBDTTT-CN and PBDTTT-S-CN. This work demonstrates that various end groups in the copolymer structure significantly influence the absorption range, electronic energy levels, and morphologies of thin films and potentially serve as active-layer materials for PSC applications
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