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 V-oc of 0.72 V, a J(sc) of 6.88 mA/cm(2), and an FF of 49.6%, leading to a power conversion efficiency (PCE) of 2.45%.