Temperature dependence of electroluminescence in a tris-(8-hydroxy) quinoline aluminum (Alq/sub 3/) light emitting diode

Organic electroluminescent devices using tris-(8-hydroxy) quinoline aluminum (Alq/sub 3/) as the emissive layer and N,N'-diphenyl-N,N' bis (3-methylphenyl)-[1-1'-biphenyl]-4-4'-diamine as the conventional hole transport layer have been fabricated. The temperature- and field-dependent quantum efficiency have been investigated over the temperature range from 1 to 300 K using a model developed by Shen et al. to explore the physics at the organic heterointerface in the present device structure with the formation of an accumulation layer. It has been observed that electron luminescence intensity decreases with decreasing temperature down to 160 K, then saturates in the low-temperature region. The quantum efficiency increases with decreasing temperature and finally reaches an almost constant value. From the analysis, it is seen that the model can explain the luminescence behavior of the device satisfactorily down to 120 K but fails to explain the low-temperature behavior. The efficiency has also been studied with voltage and it is seen that there is an optimum voltage required to get the maximum efficiency.