A novel method for mapping the charge density spatial distribution in organic field-effect transistors based on the electromodulation of the photoluminescence is demonstrated. In field-effect transistors exciton quenching is dominated by exciton-charge carrier interaction so that it can be used to map the charge distribution in different operating conditions. From a quantitative analysis of the photoluminescence quenching, the thickness of the charge carrier accumulation layer is derived. The injection of minority charge carriers in unipolar conditions is unexpectedly evidenced, which is not displayed by the electrical characteristics.

Organic molecular semiconductors are key components for a new generation of low cost, flexible, and large area electronic devices. In particular, ambipolar semiconductors endowed with electroluminescent properties have the potential to enable a photonic field-effect technology platform, whose key building blocks are the emerging organic light-emitting transistor (OLET) devices. To this aim, the design of innovative molecular configurations combining effective electrical and optical properties in the solid state is highly desirable. Here, we investigate the effect of the insertion of a thieno(bis)imide (TBI) moiety as end group in highly performing unipolar oligothiophene semiconductors on the packing, electrical, and optoelectronic properties of the resulting materials. We show that, regardless of the HOMO–LUMO energy, orbital distribution, and molecular packing pattern, a TBI end moiety switches unipolar and nonemissive oligothiophene semiconductors to ambipolar and electroluminescent materials. Remarkably, the newly developed materials enabled the fabrication of single layer molecular ambipolar OLETs with optical power comparable to that of the equivalent polymeric single layer devices.

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The synthesis of two new thieno(bis)imide (TBI, N) end functionalized oligothiophene semiconductors is reported. In particular, trimer (NT3N) and pentamer (NT5N) have been synthesized and characterized. Two different synthetic approaches for their preparation were tested and compared namely conventional Stille cross coupling and direct arylation reaction via C-H activation. Theoretical calculations, optical and electrochemical characterization allowed us to assess the role of the p-conjugation extent, i.e., of the oligomer size on the optoelectronic properties of these materials. In both TBI ended compounds, due to the strong localization of the LUMO orbital on the TBI unit, the LUMO energy is almost insensitive to the oligomer size, this being crucial for the fine-tailoring of the energy and the distribution of the frontier orbitals. Surprisingly, despite its short size and contrarily to comparable TBI-free analogues, NT3N shows electron charge transport with mobility up to l N = 10 À4 cm 2 V À1 s À1 , while increasing the oligomer size to NT5N promotes ambipolar behavior and electroluminescence properties with mobility up to l N = 0.14 cm 2 V À1 s À1 and to l P = 10 À5 cm 2 V À1 s À1 .

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