Solution-processed bulk heterojunction photovoltaic cells from gradient π-conjugated thienylene vinylene dendrimers

A series of gradientπ-conjugated dendrimers and their corresponding models based on 5,5,10,10,15,15hexahexyltruxene moieties as nodes and oligo(thienylene vinylene) (OTVs) units with different lengths as branching arms are synthesized in good yields through Wittig-Horner reactions. All new compounds are fully characterized by ¹H and ¹³C NMR spectroscopy, elemental analysis, and MALDI-TOF MS or ESI-MS. Investigation of their photophysical properties reveals that the gradient dendritic scaffold not only results in a higher molar absorption coefficient and broader absorption region than those of their corresponding model compounds, but also improves the PL quantum yields relative to the corresponding OTVs. The suitable HOMO and LUMO levels as well as excellent film forming properties make these molecules potential candidates for organic solar cells. Solution-processed bulk heterojunction solar cells using these dendrimers as donor and [6,6]-phenyl-C₆₁ butyric acid methyl ester as acceptor are prepared and tested. The power conversion efficiency of the devices based on G0-4-2 is 0.40% under illumination of air mass 1.5 and 100 mWcM^-2. This is the highest record value for OTV-based materials to date. Although the absorption band of dendrimer G0-4-2 is much narrower than that of poly(3-hexylthieny-lene vinylene) (P3HTV), the efficiency of its solar cell device is almost twice that of the device based on P3HTV. This result shows clearly the advantage of gradient dendritic structures as active materials for photovoltaic cells.