The most widely employed strategy for band gap engineering is in alternating donor–acceptor (D–A) copolymers, where the energy of the highest occupied molecular orbital (HOMO) is typically dictated by the donor unit whilst the energy of the lowest unoccupied molecular orbital (LUMO) is dictated by the acceptor. Their effectiveness as donor materials in the bulk heterojunction of organic photovoltaics (OPVs) and in transporting charge carriers in organic field-effect transistors (OFETs) is reliant on the energetic tailoring of frontier molecular orbital (MO) energy levels and thus the band gap between them. Introduction Organic semiconducting polymers have received large attention in the past two decades due to their low cost, mechanical flexibility, and ease of processing.
Quantum chemical calculations provide insight into this behaviour, suggesting that, whilst conjugation along the HOMO is indeed inhibited via molecular design, these materials possess highly delocalized LUMOs, facilitating high n-type charge transport. The cross-conjugated polymers exhibited relatively high n-type charge transport performance in organic field-effect transistors, a rare characteristic for this type of polymer. By employing cross-conjugation within the polymer backbone as a synthetic strategy, we are able to engineer optical gaps such that the novel materials absorb over the entire visible spectrum. Herein, we present two novel organic semiconducting polymers synthesised from an ancient dye.
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