Synthesis, Molecular and Photovoltaic Properties of an Indolo[3,2-b]indole-Based Acceptor-Donor-Acceptor Small Molecule

Indolo[3,2‐b]indole, containing two fused indole units, is an unexplored but promising electron‐rich molecule for constructing donor–acceptor materials due to its planar, symmetric, and extended conjugated structure. We have successfully developed a new synthetic pathway to prepare 2,7‐diboronic ester‐indolo[3,2‐b]indole, which was then reacted with dithienodiketopyrrolo‐pyrrole acceptor to afford a new acceptor–donor–acceptor (A–D–A) conjugated molecule, 2,7‐bis(dithienodiketopyrrolo‐pyrrole)indolo[3,2‐b]indole (2,7‐DPPIIDPP). II is used to stand for indolo[3,2‐b]indole in order to emphasize that this compound is constructed from two indole units. The A–D–A linkage through the 2,7‐positions of II not only preserves the phenylene units in the para‐conjugation but also renders stronger electron‐donating strength. This material exhibited good thermal stability, high crystallinity, and broad UV/Vis absorption. The solution‐processed bulk heterojunction device using the configuration of ITO/PEDOT:PSS/2,7‐DPPIIDPP:PC71BM/Ca/Al exhibited a Voc of 0.72 V, a Jsc of 6.88 mA/cm2, and an FF of 49.6 %, leading to a power conversion efficiency (PCE) of 2.45 %. A new acceptor–donor–acceptor conjugated molecule, 2,7‐bis(dithienodiketopyrrolo‐pyrrole)indolo[3,2‐b]indole (2,7‐DPPIIDPP) was designed and synthesized. This material exhibited good thermal stability, high crystallinity, and broad UV/Vis absorption. The solution‐processed bulk heterojunction device using the configuration of ITO/PEDOT:PSS/ 2,7‐DPPIIDPP:PC71BM/Ca/Al was characterized.