Temperature Dependent Electroluminescence of Alq$_{3}$ Based OLEDs

The temperature dependent I-V characteristics and electroluminescence (EL) of an ITO/PEDOT/NPD/Alq$_{3}$/LiF--Al OLED is investigated. The I-V measurements reveal a trap charge limited current behavior with characteristic trap energy of 53 meV. The EL of the device is compared with the photoluminescence (PL) of a 50 nm thick Alq$_{3}$ film on Si (001) in the range from 10 to 320 K. The EL efficiency of the device shows similar temperature dependence as the PL intensity obtained from the Alq$_{3}$ film. The OLED brightness saturates at 10,000 cd/m$^{2}$ (T = 300 K; V = 15 V). For a constant forward bias (8 V) the maximum EL efficiency of the device is 2.2 cd/A obtained at 180 K. The maximum PL efficiency of the Alq$_{3}$ film is observed at the same temperature (180 K). Furthermore, both the EL and PL spectra reveal a maximum redshift at 180 K which is tentatively attributed to the formation of self-trapped excitons within the Alq$_{3 }$layer.