The impact of alkyl side‐chain substituents on conjugated polymers on the photovoltaic properties of bulk heterojunction (BHJ) solar cells has been studied extensively, but their impact on small molecules has not received adequate attention. To reveal the effect of side chains, a series of star‐shaped molecules based on a triphenylamine (TPA) core, bithiophene, and dicyanovinyl units derivatized with various alkyl end‐capping groups of methyl, ethyl, hexyl and dodecyl is synthesiyed and studied to comprehensively investigate structure‐properties relationships. UV‐vis absorption and cyclic voltammetry data show that variations of alkyl chain length have little influence on the absorption and highest occupied molecular orbital (HOMO)‐lowest unoccupied molecular orbital (LUMO) levels. However, these seemingly negligible changes have a pronounced impact on the morphology of BHJ thin films as well as their charge carrier separation and transportation, which in turn influences the photovoltaic properties of these small‐molecule‐based BHJ devices. Solution‐processed organic solar cells (OSCs) based on the small molecule with the shortest methyl end groups exhibit high short circuit current (Jsc) and fill factor (FF), with an efficiency as high as 4.76% without any post‐treatments; these are among the highest reported for solution‐processed OSCs based on star‐shaped molecules.

Here we report the application of the Electron Spin Resonance (ESR) spectroscopy as a highly sensitive analytical technique for assessment of the electronic quality of organic semiconductor materials, particularly conjugated polymers. It has been shown that different batches of the same conjugated polymer might contain substantially different amounts of radical species which were attributed to structural defects and/or impurities behaving as traps for mobile charge carriers. Good correlations between the concentrations of radicals in various batches of conjugated polymers and their performances in organic solar cells have been revealed.

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Dynamic disorder manifested in fluctuations of charge transfer integrals considerably hinders charge transport in high-mobility organic semiconductors. Accordingly, strategies for suppression of the dynamic disorder are highly desirable. In this...

We present a structural comparison of monolayers on a SiO2 substrate of two asymmetrically substituted sexithiophenes (6T). Molecule 1 consists of 6T with a branched alkyl chain at one end only and shows a crystalline structure. In molecule 2, the bifunctional 6T has in addition at the other end a linear alkyl chain. It displays thermotropic liquid crystalline (LC) behavior. Both compounds form readily single molecular layers from solution. Remarkably, full monolayer coverage can be achieved before multilayer growth starts. LC properties promote preordering near the interface as well as exchange of molecules between the growing domains, thus regulating the domain sizes. As a result, the LC compound 2 forms single-molecule islands with larger domain sizes compared to compound 1. Surface X-ray investigations indicate that the 6T cores are tilted relative to the layer normal. The tilt angle is as large as 54° for compound 2 compared to 28° for compound 1. For molecule 2, interfacial constraints and packing requirements because of the asymmetric substitution cause a rather loosely organized core structure.