Optoelectronic properties of hyperbranched polythiophenes

Branched conjugated architectures should possess the advantage of isotropic charge transport compared to conventional linear conjugated polymers, as for example poly(3-hexylthiophene) (P3HT) which is commonly used in organic solar cells. This contribution investigates the optoelectronic properties of branched poly(thiophene)s p3T and p4T synthesized in a straightforward one-pot procedure by oxidative coupling of branched trithiophene and tetrathiophene monomers with FeCl 3. These polymers can be regarded as model systems for ideal amorphous conjugated materials. Optical characterization in solution and in thin films together with cyclic voltammetry data suggests the applicability of these materials for the use in organic solar cell devices. In particular, the HOMO and LUMO levels of the branched polythiophenes are shifted to lower energy values as compared to linear P3HT. Field effect mobilities are in the order of 10 -4 cm 2/(V s). A first optimization of solar cell devices based on the branched polythiophene materials in combination with PCBM as acceptor resulted in efficiencies of 0.6% with open-circuit voltages being about 30% higher (up to 714 mV) than normally found with P3HT.