Investigation of the Optoelectronic Properties of Organic Light-Emitting Transistors Based on an Intrinsically Ambipolar Material

The realization of organic light-emitting transistors (OLETs) based on R,?-dihexylcarbonylquaterthiophene(DHCO4T), an intrinsically ambipolar and luminescent semiconductor, is reported. In this device structure,optimization of the hole/electron density ratio in the channel region has been identified as the major issue tooptimize light emission.. Therefore, the focus of this study is to understand how DHCO4T optoelectronicresponse vary with semiconductor film growth conditions as well as the selection of the gate dielectric andmetal contact materials. Our results demonstrate that DHCO4T hole and electron mobilities and the IDS-VDShysteresis mainly depend on the gate dielectric material composition. Atomic force microscopy analysis ofthe semiconductor film reveals a layer-by-layer growth mechanism, giving rise to the formation of a continuousand homogeneous charge transport layers. With Au as the source and drain contact material, the best carriermobilities have been measured for the poly(methyl methacrylate)-coated SiO2 gate dielectric devices. Metalswith Fermi energy ranging from -5.1 to -2.87 eV have also been investigated. Metal deposition on thesemiconductor film does not significantly affect film morphology as evidenced by the topography of theelectrode top surface. However, for a given dielectric material, the OLET performance strongly depends onthe metal/dielectric combination employed and marginally correlates with the contact Fermi energy.Electroluminescence has been observed in DHCO4T-based OLETs but principally in concert with unipolartransport. The hole and electron large gate threshold voltage values have been identified as the principallimitation to high electroluminescence performances.