Direct interband transitions in tris-(8-hydroxyquinoline) aluminum thin films

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Abstract
The electroluminescent properties of three different device structures (A:ITO/SiO2/Alq3/SiO2/Al, B:ITO/Alq3/SiO2/Al, and C:ITO/SiO2/Alq3/Al) based on the tris-(8-hydroxyquinoline) aluminum (Alq3) were investigated. A blue electroluminescence at 457 nm was obtained from device (A) and (B), and the green emission at 518 nm was obtained from device (C). It is generally agreed that the green emission originates from the recombination of the singlet excitons. The blue emission, here, is attributed to the direct transitions between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital of Alq3. This is due to the electric field-induced excitons dissociation and the space charge accumulation at the interface. The high internal electric field enhances the dissociation of neutral singlet excitations into LUMO states and inhibits the formation of the singlet excitons, therefore enhances the probability for direct interband transitions of the relaxed carriers. The intensity of the blue emission is dependent on the operating frequency. This indicates that space charge accumulation time and effective internal electric field are responsible for the blue emission intensity.