Li Weizhi

Study on temperature characteristics of organic light-emitting diodes based on tris-(8-hydroxylquinoline)-aluminum

Both single-layer and double-layer organic light-emitting devices based on tris-(8-hydroxylquinoline)-aluminum (Alq3) as emitter are fabricated by thermal vacuum deposition. The electroluminescent characteristics of these devices at various temperatures are measured, and the temperature characteristics of device performance are studied. The effect of temperature on device current conduction regime is analyzed in detail. The results show that the current-voltage (I-V) characteristics of devices are in good agreement with the theoretical prediction of trapped charge limited current (TCLC). In addition, both the charge carrier mobility and charge carrier concentration in the organic layer increase with the rise of temperature, which results in the monotonous increase of Alq3 device current. The current conduction mechanisms of two devices at different temperatures are identical, but the exponent m in current-voltage equation changes randomly with temperature. The device luminance increases slightly and the efficiency decreases monotonously due to the aging of Alq3 luminescent properties caused by high temperature. A tiny blue shift can be observed in the electroluminescent (EL) spectra as the temperature increases, and the reduction of device monochromaticity is caused by the intrinsic characteristics of organic semiconductor energy levels.

Blue and green organic light-emitting devices with various film thicknesses for color tuning

Blue and green organic light-emitting devices with a structure of indium tin oxide (ITO) /N,N′-bis-(1-naphthyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/aluminum(III) bis(2-methyl-8-quinolinato)4-phenylphenolato (BAlq)/tris(8-hydroxyquinolate)-aluminum (Alq3)/Mg:Ag have been fabricated. Blue to green light emission has been achieved with the change of organic film thickness. Based on energy band diagram and charge carrier tunneling theory, it is concluded that the films of different thicknesses play a role as a color-tuning layer and the color-variable electroluminescence (EL) is ascribed to the modulation function within the charge carrier recombination zone. In the case of heterostructure devices with high performance, the observed EL spectra varies significantly with the thickness of organic films, which is resulted from the shift of recombination region site. It has not been hitherto indicated that the devices compose of identical components could be implemented to realize different color emission by changing the film thickness of functional layers.