Dithienopyrrole-based dianchoring dyes: Effect of molecular design and donors on the optical and photovoltaic properties

Abstract

Two sets of dianchoring dyes featuring λ- and H-shaped molecular configurations, respectively are synthesized either by tri-functionalization of dithienopyrrole or bridging of two dithienopyrroles by fluorene. In case of λ-shaped dyes, dianchoring dithienopyrrole core is laterally functionalized via N-fluorene with different donors while in H-shaped dyes two donor-acceptor axes were linked by fluorene connecting dithienopyrrole linker across the conjugation pathway. H-shaped dyes showed broad and intense absorption, characteristic of increased chromophore density and charge transfer while the λ-shaped dyes displayed relatively weak and narrow absorption owing to the confinement of absorption to the dithienopyrrole and cyanoacrylic acid segments. Density functional theoretical calculations revealed the origin of absorption as π-π* transition in λ–shaped dyes and charge transfer transition in H-shaped dyes. Consequently, an H-shaped dye containing diphenylaminofluorene donor displayed highest power conversion efficiency in the series due to high photocurrent density and open circuit voltage. The EIS studies revealed the large recombination resistance and small charge transfer resistance for the H-shaped dyes, which is responsible for their relatively superior performance.