Solution processable donor-acceptor oligothiophenes for bulk-heterojunction solar cells

Novel p-type and low bandgap penta- and hexa-thiophenes asymmetrically endcapped with solubilizing triarylamine or triarylamino-substituted carbazole dendron and dicyanovinyl groups, namely, PhNOF-OT(n)-DCN and G(2)-OT(n)-DCN, respectively where n = 5-6 for solution-processable photovoltaic applications have been synthesized and investigated. With the incorporation of a solubilizing electron-donating group, the highly extended oligothiophenes are highly soluble in common organic solvents and solution-processable. Upon extending the oligothiophene backbone, there is a strong increase in the absorption around 420 nm in addition to the intramolecular charge-transfer absorption (520 nm) which results in a strong spectral broadening. The optical band-gap of these donor-acceptor oligothiophene thin-films greatly reduces to 1.85 eV. The solution-processed bulk heterojunction PV cells fabricated from these materials blended with PCBM as an acceptor showed a PCE up to 1.72% with V-oc = 0.79 V in an as-fabricated device. Our findings also suggest that highly extended donor-acceptor oligomers can be useful for a p-type, low-bandgap semiconductor for solution-processable bulk heterojunction PV cells.