Improvement of charge mobility and photovoltaic performance of small molecule oligothiophene donors through self-assembly

Abstract

Through molecular engineering of cyanopyridone functionality, herein, two new, small molecule oligothiophene donors, coded as CP7 and CP8, are reported which were designed, synthetically developed, and used in solution-processed bulk-heterojunction devices with the conventional acceptor, PC61BM. Both CP7 and CP8 comprise a common donor, i.e., triphenylamine, a common π-linker, i.e., bis-thiophene, and a cyanopyridone electron accepting group with different substituents at the nitrogen atom of cyanopyridone functionality. The presence of terminal acid group helped to achieve self-assembly of CP7 via hydrogen bonding, and as a result, its pristine and blend film morphologies were superior when compared with the film morphologies based on CP8 (donor oligothiophene comprising terminal ester functionality), thus, affording better device outcome (8.12% (CP7) vs 5.40% (CP8)). The results demonstrated that the molecular engineering of cyanopyridone-based oligothiophene donors to induce self-assembly and to improve film morphology was a key factor for the enhancement of photovoltaic performance. That being said, the power conversion efficiency of >8% is the highest efficiency number reported in the current literature for cyanopyridone-based small molecule oligothiophene donors.