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2D organic semiconductor single crystals comprising one or a few molecular layers of macroscopic lateral sizes are ideal materials for ultrathin, flexible, and transparent field‐effect devices—a platform for transistors and sensors. In recent years, these 2D materials have demonstrated high performance not inferior to their 3D counterparts. However, light emissive properties of 2D organic semiconductor single crystals have not yet been reported, and a combination of high charge‐carrier mobility and bright luminescence in one material is still a challenge for 2D organic optoelectronics. Emissive high‐mobility 2D organic semiconductor based on a [1]benzothieno[3,2‐b]benzothiophene (BTBT)‐derivative, 2,7‐bis(4‐decylphenyl)[1]benzothieno[3,2‐b][1]benzothiophene (DPBTBT), is presented here. DPBTBT molecules self‐organize in large‐area ultrathin single‐crystalline films consisting of one or a few molecular layers. These 2D single crystals perfectly suit as an active layer of organic field‐effect transistors in full accordance with Shockley's model and uniquely combine the high charge‐carrier mobility reaching 7.5 cm2 V–1 s–1 with prominent light emissive properties, which allow a demonstration of the first 2D organic light‐emitting transistor. The high charge‐carrier mobility and thermal stability of the crystalline phases, pronounced luminescence, and good shelf‐life stability suggest that emissive BTBT‐type molecules are a promising avenue for 2D organic optoelectronics. [ABSTRACT FROM AUTHOR]

We describe the impact of tensile strains on the structural properties of thin films composed of PffBT4T-2OD π-conjugated polymer and PC71BM fullerenes coated on a stretchable substrate, based on a novel approach using in situ studies of flexible organic thin films. In situ grazing incidence X-ray diffraction (GIXD) measurements were carried out to probe the ordering of polymers and to measure the strain of the polymer chains under uniaxial tensile tests. A maximum 10% tensile stretching was applied (i.e., beyond the relaxation threshold). Interestingly we found different behaviors upon stretching the polymer: fullerene blends with the modified polymer; fullerene blends with the 1,8-Diiodooctane (DIO) additive. Overall, the strain in the system was almost twice as low in the presence of additive. The inclusion of additive was found to help in stabilizing the system and, in particular, the π–π packing of the donor polymer chains. [ABSTRACT FROM AUTHOR]

Structure determination in high performance polymer semiconductors such as polythieno[3,4‐b]‐thiophene‐co‐benzodithiophene (PTB7) is essential to establish proper structure–property correlations. A combination of high‐temperature rubbing and isothermal crystallization leads to oriented and crystalline films of face‐on oriented PTB7 crystals. Electron diffraction indicates that crystallinity is marginal in the rubbed films but develops upon postrubbing annealing at T ≥ 250 °C. The best oriented and crystalline PTB7 films have a dichroic ratio of 12 for UV–vis absorption. The hole mobilities are improved by a factor of six along the rubbing direction over nonaligned films (µ// = 5.8 × 10−3 cm2 V−1 s−1 vs µ⊥ = 3.1 × 10−4 cm2 V−1 s−1). Structural analysis yields three possible models that share similar structural features, however; namely (i) PTB7 chains form a layered packing such as poly(2,5‐bis(3‐dodecyl‐2‐yl)thieno[3,2‐b]thiophene) with π‐stacked backbones alternating with layers of strongly interdigitated alkyl side chains, (ii) the PTB7 chains adopt a nonplanar zigzag conformation, and (iii) PTB7 chains show a mixed stacking of thieno[3,4‐b]thiophene and benzodithiophene blocks. Overall, these results highlight the key role played in polymer semiconductor crystals by the steric constraints due to the branched alkyl side chains interdigitation on the π‐stacking of conjugated backbones. A combination of high‐temperature rubbing and isothermal crystallization affords oriented films of the polymer semiconductor PTB7 with high dichroic ratio and anisotropic hole mobilities. Structural analysis yields three possible models that share similar structural features: a layered packing with strongly interdigitated side chains, a nonplanar zigzag backbone conformation and a mixed stacking of thieno[3,4‐b]thiophene and benzodithiophene blocks. [ABSTRACT FROM AUTHOR]

X-ray and electron diffraction analyses were carried out under various geometries to investigate the role of alkyl side chain length on the structural properties of regioregular poly(3-alkylthiophene) (RR-P3AT) thin films by sequentially increasing the alkyl side chain length from n-pentyl till n-octyl. P3AT thin films with different thicknesses were drop cast at room temperature (RT) and low temperature (−30 °C) for the structural analysis. A thorough X-ray diffraction, transmission electron microscopic, field effect transistor and photoluminescence (PL) investigations revealed the following: (i) texturing of RT cast P3AT thin films along the out-of-plane direction on decreasing the alkyl side chain length, (ii) poor π–π stacking of P3AT crystallites on increasing the alkyl side chain length, which results from the increased steric hindrance between the grafted alkyl side chains, (iii) such poor π–π stacking of P3ATs on increasing the alkyl side chain length eventually found to hinder the mobility of charge carriers, and (iv) evolution of well defined vibronic side bands in the PL spectra of −30 °C cast films confirm the formation of highly crystalline films as compared to the RT cast ones. Highly edge-on oriented and uniform thin films can be fabricated by optimizing the cast temperature and concentration of the solution. [ABSTRACT FROM AUTHOR]