Synthesis And Properties Of Triphenylamine-based Four-armed Star-shaped Small Molecular Photovoltaic Materials

In recent years, generating energy directly from green pollution-free sunlightradiation has become people one of the important ways to solve the global energy andenvironmental crisis. Solar cells based on photoelectric conversion have been paidmore and more attention. Among them, researchers focus more and more attention onorganic solar cells with cost-efficient, flexible, light-weight, transparency andlarge-area manufacturing compatibility. Organic solar cell materials are divided intoorganic polymer and organic small molecules. Compared with polymer, smallmolecules possess easier purification, amenability to mass-scale production,monodisperse in nature with well-defined chemical structures and betterbatch-to-batch reproducibility. Although the photoelectric conversion efficiency ofsmall molecular organic solar cells has been more than10%, the overall efficiency ofsmall molecule photovoltaic materials is lower than that of the same polymermaterials.Developing novel D and A moiety style (D-A style) small molecule materials arean efficient method to maximize the photovoltaic parameters. Because this structurecan not only broaden the absorption bandwith, but also control the HOMO-LUMOenergy levels through employing different D or A moieties into the molecules. Atpresent, the linear D-A small molecular materials are more explored, and get betterphotovoltaic properties. However, the triphenylamine four-armed star-shapedmolecules are relatively less explored.In this paper, a series of novel triphenylamine-based four-armed star-shapedmolecules had been designed and synthesized for BHJOSCs, named BT series andBT-T series, consisting of BT as the core and A moiety, and incorporating TPA as thearms and D moiety. Benzene modified with different units as end moietis. Besides, alkyl thiophene groups were introduced into the molecular backbones of BT-T seriesas the conjugated π-bridge. The synthesis of target molecules are calculated by thedensity functional theory calculations, ultraviolet-visible absorption test, cyclicvoltammograms and tesed their photovoltaic properties though assembling bulkheterojunction organic solar cell.The results show that different groups in the molecules have an importantinfluence on the photovoltaic properties. Electron-withdrawing group as the trailingend group of molecule decreases the HOMO of the molecule and increases the VOCbeneficially; Electron-donating group as the trailing end group of molecule helpsenhance the intramolecular ICT and increases the light absorption and JSC; Alkylthiophene groups are introduced into the molecular backbones of BT-T series as theconjugated π-bridge, which increases the molecular conjugate chain and materialsabsorb light, so as to increase the JSC.BT-TOM has an optimum photovoltaic property among the eight molecules. Inthe organic solar cell assembled with BT-TOM as donor material and PC61BM as anacceptor material, test shows JSC=5.50mA cm-2, VOC=0.89V, FF=0.38, PCE=1.88%.