Abstract
Two series (PMA and PMAT) of two-dimensional donor-acceptor copolymers consisting of a 3,4-bis(4-bromophenyl)maleimide derivative and triphenylamine with a conjugated side chain were designed and synthesized to probe their structure-function relationships for use in bulk heterojunction (BHJ) polymer solar cells (PSCs). The difference between PMA- and PMAT-series is the conjugated side chain length on the triphenylamine unit. By extending the side chain length, and by attaching various acceptor end groups to the side chain, the electronic and photophysical properties of these copolymers, as well as subsequent device performance, were significantly affected. Two series of copolymers showed broad absorption in the visible region with two obvious peaks. With increasing electron-withdrawing strength of the acceptor end groups, the intramolecular charge transfer peak becomes progressively red-shifted. Highest occupied molecular orbital (HOMO) levels in each copolymer series are similar, but lowest unoccupied molecular orbital (LUMO) levels are dictated by the acceptors. BHJ PSCs composed of the copolymers as a donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as an acceptor in 1:2 weight ratio were fabricated and characterized. PSCs based on PMA- and PMAT-series copolymers had power conversion efficiencies (PCEs) ranging from 2.05–2.16% and 3.14–4.01%, respectively. These results indicate that subtle tuning of the chemical structure can significantly influence PSC device performance.
Highlights
Significant research attention has been focused recently on polymer solar cells (PSCs) based on a blended system of an electron donor polymer and an electron acceptor of fullerene derivative (for example, [6,6]-phenyl C61 butyric acid methyl ester (PC61 BM) or [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM)) with nanoscale phase-separated bulk heterojunction (BHJ) morphology, due to their low cost, light weight, and flexibility for large-area devices [1,2,3]
On the basis of the above considerations, two series (PMA and PMAT) of two-dimensional donor-acceptor conjugated copolymers, consisting of a 3,4-bis(4-bromophenyl)maleimide derivative and triphenylamine with a conjugated side chain with various acceptors attached as end groups, are designed and synthesized to probe their structure-function relationships for use in BHJ PSCs
We synthesized two series (PMA and PMAT) of two-dimensional donor-acceptor copolymers consisting of a 3,4-bis(4-bromophenyl)maleimide derivative, and triphenylamine with a conjugated side chain with various acceptors attached as end groups, for PSC applications
Summary
Significant research attention has been focused recently on polymer solar cells (PSCs) based on a blended system of an electron donor polymer and an electron acceptor of fullerene derivative (for example, [6,6]-phenyl C61 butyric acid methyl ester (PC61 BM) or [6,6]-phenyl C71 butyric acid methyl ester (PC71 BM)) with nanoscale phase-separated bulk heterojunction (BHJ) morphology, due to their low cost, light weight, and flexibility for large-area devices [1,2,3]. Two-dimensional (2D) conjugated polymers, which possess conjugated side groups or side chains that may provide broadened absorption spectra, high hole mobility, and low-lying HOMO energy levels, have significant potential as conjugated donor materials in high-performance PSCs. For example, Li and coworkers first developed the concept of 2D conjugated polythiophenes; they found the. Cao and Huang reported that the bandgap and energy levels of silafluorene-based 2D conjugated polymers were effectively tuned through changing the electron-withdrawing strength of the acceptor groups on the side chains [18]. This research demonstrates that various side chain lengths and end groups in the copolymer structure significantly influence the absorption range, electronic energy levels, and morphologies of thin films and potentially act as active layer materials for PSCs applications
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