7-Azaisoindigo as a new electron deficient component of small molecule chromophores for organic solar cells

Abstract

The use of 7-azaisoindigo as an electron accepting moiety in donor–acceptor chromophores is reported for the first time. Four chromophores were synthesized that contained either 2,2′-bithiophene or 3,3′-bis(dodecyloxy)-2,2′-bithiophene as an electron-donating sub-unit and either isoindigo or 7-azaisoindigo as an electron acceptor. The inclusion of a heteroatom in the isoindigo framework was designed to probe its effect on the LUMO, while 3,3′-bis(dodecyloxy)-2,2′-bithiophene was used as an electron rich alternative to 2,2′-bithiophene in order to similarly evaluate its effect on the HOMO level. The electronic properties of the resulting chromophores were fully characterized by UV/visible spectroscopy, cyclic voltammetry, and density functional theory. All four dyes were evaluated for their performance as electron donors in bulk heterojunction solar cells using PC₆₁BM as the electron acceptor. Devices that used the isoindigo and 3,3′-bis(dodecyloxy)-2,2′-bithiophene-based dye displayed the best performance, reaching a maximum power conversion efficiency of 0.93%.