Dithienosilole-Based Small-Molecule Organic Solar Cells with an Efficiency over 8%: Investigation of the Relationship between the Molecular Structure and Photovoltaic Performance

Two new acceptor–donor–acceptor (A-D-A) small molecules with 2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b′]-dithiophene (DTC) and (4,4′-bis(2-ethylhexyl) dithieno[3,2-b:2′,3′-d]silole)–2,6-diyl (DTS) as the central building block unit and 3-ethyl-rhodanine as the end-capping groups have been designed and synthesized. The influence of the bridging atoms on the optical, electrochemical properties, packing properties, morphology, and device performance of these two molecules was systematically investigated. Although with only the difference of one atom on the central core units, the two molecules showed great different properties such as film absorption, molecular packing, and charge transport properties. The optimized device based on molecule DR3TDTS exhibited a power conversion efficiency (PCE) of >8%.