On the Effect of Confinement on the Structure and Properties of Small-Molecular Organic Semiconductors

Many typical organic optoelectronic devices, such as light-emitting diodes, field-effect transistors, and photovoltaic cells, use an ultrathin active layer where the organic semiconductor is confined within nanoscale dimensions. However, the question of how this spatial constraint impacts the active material is rarely addressed, although it may have a drastic influence on the phase behavior and microstructure of the active layer and hence the final performance. Here, the small-molecule semiconductor p-DTS(FBTTh₂)₂ is used as a model system to illustrate how sensitive this class of material can be to spatial confinement on device-relevant length scales. It is also shown that this effect can be exploited; it is demonstrated, for instance, that spatial confinement is an efficient tool to direct the crystal orientation and overall texture of p-DTS(FBTTh₂)₂ structures in a controlled manner, allowing for the manipulation of properties including photoluminescence and charge transport characteristics. This insight should be widely applicable as the temperature/confinement phase diagrams established via differential scanning calorimetry and grazing-incidence X-ray diffraction are used to identify specific processing routes that can be directly extrapolated to other functional organic materials, such as polymeric semiconductors, ferroelectrics or high-refractive-index polymers, to induce desired crystal textures or specific (potentially new) polymorphs.