Design and synthesis of small-molecules and polymers based on extended isoindigo for organic solar cells

Abstract

New small molecules and conjugated copolymers based on extended isoindigo (eI) and oligothiophene (nT) were synthesized (eI-(T)2, eI-(2T)2, PeI-1T and PeI-3T) to investigate the photovoltaic properties of extended isoindigo unit. Extended isoindigo can be synthesized from two oxindoles and 2,2'-bithiophene-5,5'-dicarbaldehyde by a simple Knoevenagel condensation. Extended isoindigo shows weaker electron-accepting ability than isoindigo. Therefore the small molecules and copolymers based on extended isoindigo show up-shifted LUMO levels as compared to conventional isoindigo-based materials. Also extended isoindigo-based polymers exhibit a good p-p stacking due to perfectly planar structure of eI. The photovoltaic devices based on PeI-1T blended with PC61BM exhibits power conversion efficiency (PCE) of 4.07% with a Jsc of 11.86 mA/cm^2. Extended isoindigo could be a promising building block for efficient organic solar cells.