Structural optimization of large acceptor–donor–acceptor-type molecules for improved performance of fullerene-free polymer solar cells†
To control the molecular energy levels of highly π-extended n-type molecules, we synthesized two acceptor–donor–acceptor (A–D–A)-type molecules with indacenodithiophenes (IDTs) or IDT–benzodithiophene (BDT)–IDT as donating cores and 2-(2,3-dihydro-3-oxo-1H-inden-1-ylidene)propanedinitrile (IM) as terminal accepting units. These molecules showed different optical and electrochemical properties, indicating that the energy levels can be easily tuned by changing the structure of the donating core. Among two molecules, IM-BDTIDT2 showed a relatively blue shifted absorption spectrum and low-lying highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) levels. Although IM-IDT3 and IM-BDTIDT2 have a highly π-extended conjugated structure, no clear crystalline behaviour was observed in their thin films. When applied to polymer solar cells (PSCs), the device based on IM-BDTIDT2 displayed a higher PCE (5.33%) than the device bearing IM-IDT3 owing to the lower-lying energy levels of IM-BDTIDT2. Thus, the use of BDT as a donating core unit is favorable for limiting high-lying energy levels in highly π-extended A–D–A-type molecules.