[Influence of parameters of ZnS film on the organic/inorganic composite luminescence devices].

In the present paper, to fabricate electroluminescent devices CdSe QDs were used as active materials, TPD (N,N'-biphenyl-N,N'-bis-(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine) was used as a hole transport layer, and ZnS was used as an electron transport layer. The electroluminescent properties of the organic/inorganic composite ITO/TPD/CdSe QDs/ZnS/Ag light emitting devices were studied. Both TPD and CdSe QDs thin films were spin-coated and ZnS thin films were deposited by magnetron sputtering. The surfaces of the devices are smooth. The luminescence (EL) peak of the CdSe QDs is at 580 nm which is assigned to the band-edge exciton emission. Compared to the previous EL device of ITO/ZnS/CdSe QDs/ZnS/Ag, it is seen that the new devices do not display surface state related emission peaks and EL intensity is about 10 folds that of the previous device. The enhancement of luminescence efficiency is attributed to both of the excitation of CdSe QDs by accelerated electron collision and carriers injection into QDs: (1) electrons are accelerated by the ZnS layer and collide with CdSe QDs, which excites electrons in QDs to excited states and allows them to emit photons; (2) the holes injected into QDs recombine with some of electrons excited in the QDs. The authors further studied the influence of thickness variation of ZnS on the luminescent properties. ZnS thin films are of 80, 120, and 160 nm thickness, respectively. It was found that as the thickness of ZnS increases the threshold voltage rises and EL intensity increases, but breakdown voltage decreases. The EL peak position blue shifts when the thickness of ZnS decreases. The explanation of underlying mechanism is given.