Development of Quinoxaline-based polymer acceptors for efficient all-polymer solar cells

Abstract

A conjugated polymer is an organic material that can demonstrate optical and electrical properties resulting from the alternative structure of single and double bonds with various advantages such as light-weight, flexibility, solution processibility for large-scale production. Based on these characteristics, the conjugated polymers have been actively applied as key materials of organic solar cells that can convert light into electricity as a sustainable next-generation energy source. In particular, all-polymer solar cells (all-PSCs), composed of both p-type and n-type conjugated polymers for the active layer, realize excellent mechanical stabilities through entanglement effect of polymer chains. This key merit makes all-PSCs suitable for a next-generation power source with wearability and stretchability. However, to simultaneously achieve robustness as well as high performance of all-PSCs, it is imperative to develop efficient conjugated polymers. In this dissertation, quinoxaline (Qx)-based n-type conjugated polymers were reported, for the first time, and the effect of modification of their chemical structure on the photovoltaic performance of all-PSCs is carefully investigated in depth. In order to enhance electron deficiency of Qx for constructing n-type conjugated polymers, nitrile functional groups which have a strong electron-withdrawing effect are introduced. As a result, Qx-based n-type conjugated polymers are successfully developed and exhibit unipolar n-type characteristics in organic field-effect transistor devices. Also, Qx-based conjugated polymers show great potential as n-type components for application to high-performance all-PSCs based on their high absorption coefficient and vertical electron transport ability. Qx has another advantage over other n-type building blocks, in detail, it has multiple positions for the incorporation of side chains. Therefore, the change of the side chain positions (e.g. meta and para position of pendant benzene ring) can provide more room for controlling various properties beyond conventional side chain engineering. Selectively meta-positioned side chain induces electron-withdrawing effect and desirable electron transport ability, enabling the Qx-based polymer to work well as an n-type component in all-PSCs. In contrast, when simply changing the side chain position from meta to para, electron-donating effect and poor charge conducting pathway result in inferior performance in corresponding all-PSCs. These results demonstrate the importance of introducing alkoxy side chains at a proper location in the Qx-based polymer for their application to all-PSCs. Finally, continuous research and the careful optimization of the Qx-based n-type conjugated polymers will realize high-performance all-PSCs in near future.