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

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.