A Twisted Thieno[3,4-b] thiophene-Based Electron Acceptor Featuring a 14-π-Electron Indenoindene Core for High-Performance Organic Photovoltaics

With an indenoindene core, a new thieno[3,4-b] thiophene-based small-molecule electron acceptor, 2,2'-((2Z, 2'Z)-((6,6'-(5,5,10,10-tetrakis(2-ethylhexyl)- 5,10-dihydroindeno[2,1-a] indene-2,7-diyl) bis(2-octylthieno[3,4-b] thiophene-6,4-diyl)) bis(methanylylidene)) bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene)) dimalononitrile (NITI), is successfully designed and synthesized. Compared with 12-pi-electron fluorene, a carbon-bridged biphenylene with an axial symmetry, indenoindene, a carbon-bridged E-stilbene with a centrosymmetry, shows elongated p-conjugation with 14 p-electrons and one more sp(3) carbon bridge, which may increase the tunability of electronic structure and film morphology. Despite its twisted molecular framework, NITI shows a low optical bandgap of 1.49 eV in thin film and a high molar extinction coefficient of 1.90 x 10(5) m(-1) cm(-1) in solution. By matching NITI with a large-bandgap polymer donor, an extraordinary power conversion efficiency of 12.74% is achieved, which is among the best performance so far reported for fullerene-free organic photovoltaics and is inspiring for the design of new electron acceptors.