Abstract:
Thin films of organic semiconductors demonstrate important optical and electrical properties and are widely used in optoelectronic devices such as organic field-effect transistors (OFETs), organic light emitting diodes (OLEDs), or organic solar cells. The functionality and device properties depend on the structure of the organic semiconductive layer. The thin films of organic small molecules can be prepared by organic molecular beam deposition (OMBD) under vacuum conditions. The complex structure in such optoelectronic devices involves at least two different materials, the donor and the acceptor compounds, which can be deposited for the active layer for example simultaneously resulting in bulk heterojunctions (BHJ). The central and important topic here is the mixing behavior whether the compounds phase-separate or mix with each other. One of the fundamental processes in such optoelectronic devices relies on the molecular charge transfer (CT) which takes place at the interface of donor and acceptor molecules, for example, in molecular mixed crystals. In this context, the morphology and intermixing on the molecular level in bulk heterojunctions have a great influence on the CT effects and therefore also on the device structure and performance. The microscopic details of the CT, in particular for the different strengths of interactions, are still not fully understood.
The present work investigates structural morphology and charge transfer effects of organic binary bulk heterojunctions. As donors the organic small molecules Diindenoperylene (DIP) and Dinaphthothienothiophene (DNTT) and as acceptors, different perylene diimide (PDI) derivatives were chosen. At first, the influences of the different n-alkyl side chains and cyano groups of the various PDI acceptors with the donor DIP on structural and optical properties in these thin film systems were examined. Two of the investigated systems show well-defined co-crystal formations and strong excited-state CT effects. Moreover, the molecular CT of two different PDI:DNTT thin film systems was characterized by obtaining weak ground-state and strong excited-state CT effects. The experimental results and optical properties were compared and complemented by first-principle calculations. Furthermore, the influences of various donor molecules with a synthesized PDI acceptor molecule were investigated. Different mixing behavior as well as CT effects were obtained for two different molecule combinations in thin films. The results lead to a deeper understanding of processes being important for organic optoelectronic applications.
organic semiconductors
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