Low Bandgap Diketopyrrolopyrrofe Based Small Molecules Containing Acetylene Linkage: Synthesis And Photovoltaic Properties

In this paper, a series of organic small molecular photovoltaic materials based ondiketopyrrolopyrrole （DPP） central core and with triple bond as the fridge have beendesigned and synthesized. In order to investigated the influence of molecule structureto photovoltaic property, two structures of molecules were synthesized, that isA-π-A-π-A typed small molecule A1, A2, A3and A4, and D-π-A-π-D typed smallmolecules D1, D2, D3and M1. In molecule M1, DPP is connected with the end groupcarbazole through a single bond, and in the other molecules, DPP was connected withthe end acceptor group （2-thiophenecarboxaldehyde, benzaldehyde, phenylacetonitrile,benzotrifluoride） or donor group （carbazole, fluorene, triphenylamine） through a triplebound. The purpose of such design was to study the difference between two forms ofbridge bonds for organic photovoltaic materials. The density functional theory （DFT）calculations, photophysical and electrochemical properties of new materials wereexamined to help research their photophysical and electrochemical properties.The studies have shown that, all the new molecules were nice photovoltaicmaterials with suitable energy levels and excellent light absorbing. A-π-A-π-A typedsmall molecules were not only good donor but also acceptor materials because of theirlow HOMO and good LUMO energy levels. While D-π-A-π-D typed small moleculeswere good donor materials for organic solar cells. At the same time, research findingthat compound D1showed lower HOMO energy level and narrow band gap, and hasstrong molecular planarity than compound M1. In our paper, fluorescence quenchingexperiments were taken on materials M1and D1with PC₆₁BM, respectively. In orderto investigate the photovoltaic performance, the solution processable bulkheterojunction （BHJ） photovoltaic devices were fabricated with the structure ofITO/PEDOT: PSS/donor: PC₆₁BM/LiF/Al using M1and D1as the donor materials,respectively. Compared with M1, compound D1with triple bond linkage exhibited a lower HOMO energy level, resulting an increased Voc of0.73V with powerconversion efficiency （PCE） of0.38%, under an illumination of AM1.5G （100mW/cm²）.