STUDY OF CHARGE-CARRIER INJECTION AND LUMINESCENCE PROCESSES IN OLIGOTHIOPHENE-BASED LIGHT-EMITTING-DIODES

Light-emitting diodes (LEDs) with end-capped oligothiophenes (ECnT, n=5-7) as the active material were fabricated by vacuum sublimation. The metal/ECnT/ITO LEDs were studied for ECnT layers of different thicknesses (30-200 nm) and for different metals (Mg, Al, In) as top electrodes. Electroluminescence (EL) and photoluminescence (PL) spectra, current-voltage (IV) curves and quantum efficiencies were measured for a wide range of temperatures (20-300 K). The IV curves cannot be described by a standard tunnelling or thermionic injection mechanism for charge carriers at Schottky barriers alone. Instead, we have evidence that field- and temperature-dependent mobilities in the organic bulk material are important here. From EL spectra of LEDs with a double-layer film we derive that light emission arises from a region close to the cathode (less than 30 nm).