The impact of molecular planarity on electronic devices in thienoisoindigo-based organic semiconductors†
The influence of molecular planarity on field-effect-transistor and photovoltaic cell performance in thienoisoindigo derivatives has been studied. Thienoisoindigo derivatives end-capped with benzothiophene TII(SB)2 and benzofuran TII(OB)2 together with benzothiophene-capped isoindigo II(SB)2 are prepared, and their electronic properties are investigated. The crystal structures of TII(SB)2 and TII(OB)2 are determined by single-crystal X-ray structure analyses. The redox and optical measurements as well as the molecular orbital calculation indicate that thienoisoindigo-based molecules TII(SB)2 and TII(OB)2 have higher HOMO levels and smaller band gaps than II(SB)2. The single-crystal structure analysis reveals that TII(SB)2 and TII(OB)2 have flat form, agreeing well with the structure optimized by the density functional theory (DFT) calculation, and TII(SB)2 and TII(OB)2 form slipped one-dimensional stacks with the alkyl chains extending out of the molecular plane. As an active layer of organic field-effect transistors, TII(SB)2 and TII(OB)2 show one order of magnitude larger p-type carrier mobility than that of II(SB)2. It is noted that TII(SB)2 and TII(OB)2 fabricated on a tetratetracontane (TTC) modified substrate show balanced ambipolar properties (μh ≈ μe ≈ 10−2 cm2 V−1 s−1), where the carrier balance comes from well delocalized frontier molecular orbitals (FMOs). The photovoltaic properties of TII(SB)2, TII(OB)2, and II(SB)2 are investigated in bulk heterojunction devices using PC71BM. The devices show a photovoltaic efficiency up to 2.4% for TII(OB)2 and 1.4% for TII(SB)2. The device performance is closely associated with the flat structure of the thienoisoindigo unit, which effectively minimizes the steric interference of the benzothiophene and benzofuran units to facilitate the slipped co-facial π–π stacking.