Organic Light-Emitting Transistor as an Effective Photonic Device Platform: System Engineering for Tuning the Optoelectronic Performance

Abstract

The increasing interest in organic electronics is connected with the easy processability and the possibility of molecular tailoring of the organic semiconductor materials. Ranging from dry deposition techniques in high vacuum to wet processes and nanopatterting techniques, small molecule and polymeric π-conjugated materials have been implemented in a plethora of optoelectronic device applications. Among the other, Organic Light-Emitting Transistors (OLETs) are emerging as an innovative class of multifunctional devices able to integrate the electronic properties of a transistor and the light generation capability. In this thesis, we aim at investigating the photonic and optoelectronic performance of suitable-engineered devices based on different field-effect transistors architecture. Both the active layer and the gate dielectric layer of the device were investigated in order to increase the device performance in terms of brightness, color coordinate and external quantum efficiency. Starting from the study of the active layer in an ambipolar single-layer OLET, we succeeded in controlling the solid-state phases of the oligothiophene derivative namely NT4N. By means of three different deposition techniques (thermal sublimation, supersonic molecular beam deposition, lithographically controlled wetting) we investigated the influence of the different molecular packing motifs on the field-effect charge mobility. Given the limited number of efficient electroluminescent organic small molecules with high field-effect charge mobility, we adopted another approach for enhancing the figures of merit of OLET devices by implementing a multilayer heterostructure comprised by a charge-transport layer and a light-emitting layer. By introducing a newly-synthetized anthracene-based twisted oligomer as emissive layer a deep blue emitting unipolar OLET was realized. Finally, the integration of a high-capacitance hybrid photonic crystal as gate dielectric into the single-layer ambipolar OLET based on NT4N permitted to achieve low gate threshold voltages, and consequently intense brightness, together with modulation of the spectral and spatial characteristics of the emitted electroluminescence.