Synthesis and photovoltaic properties of functional dendritic oligothiophenes

Novel p-type and low bandgap functional dendritic oligothiophenes bearing hole-transporting carbazole as peripheral substituents and an electron-withdrawing dicyanovinyl core group, namely, DCT(n)-DCN, where n = 1 or 2 for solution-processable photovoltaic (PV) applications have been synthesized. With electron-donating carbazole surface-functionalized moieties conjugated with dicyanovinyl core group, the optical bandgap of these functional dendritic oligothiophene thin-films greatly reduces to 1.74 eV with a strong spectral broadening and a high ionization potential at ∼5.5 eV as determined by UV photoelectron spectroscopy. The bulk heterojunction PV cells fabricated from these dendrimers blended with PC₇₁BM as an acceptor showed a power conversion efficiency up to 1.64% with an open circuit voltage of (V _oc) = 0.93 V in the annealed device. We have demonstrated that the desirable molecular and PV properties of dendritic oligothiophenes can be obtained/tuned by the incorporation of functional group(s) onto peripheral of the dendron and into the core. In addition, these functional dendritic oligothiophenes show superior functional properties even at low dendritic generation as compared to the unsubstituted higher generation dendritic oligothiophenes as a p-type, low-bandgap semiconductor for solution-processable bulk heterojunction PV cells.