Investigations Of Novel Organic Light-emitting Diodes And Carrier Control Mechanism

Organic light-emitting diodes (OLEDs) have been attracting more and moreattention as an important flat panel display technology. In order to obtain OLEDsdevices with high performance, the improvement of carrier control is of particularimportance. The interest of this dissertation focuses on the carrier control and novelOLEDs structure design, especially on the hole injection and hole transportationcontrol. In the present work, based on hole carrier control, polytetrafluoroethylene(Teflon) buffer layer and organic alternative multilayer (AML) structure have beendesigned, and four types of novel OLEDs, double-layer OLEDs with Teflon bufferlayer, single-active layer OLEDs, OLEDs without hole-transporting layer and OLEDswith AML hole-transporting layer, have been successfully fabricated. Therelationship between device structure and the device performance has also beenstudied in detail. The main contents of this report are as follows: Teflon buffer layer has been successfully obtained by vacuum thermal depositionmethod. Compared with the devices with the conventional copper phthalocyanine(CuPc) buffer layer, the double-layer OLEDs with Teflon buffer layer proved to behigher efficiency at the same current density level. By introducing Teflon film as the buffer layer, and tris(8-hydroxyquinoline)aluminum (Alq3) as the single-active material, novel single-active layer OLEDs withhigh brightness and efficiency have been successfully fabricated. The investigationindicates that a wider recombination zone exists in the single-active layer OLEDscompared with the conventional double-layer OLEDs, and the device performancecould be greatly enhanced by doping dipolar material in the emissive layer. By using Teflon film as the buffer layer, Alq3 as the emissive layer, andAluminum (III) bis (2-methyl-8-quinolinato) 4-phenylphenolate (BAlq) as theelectron-transporting and hole-blocking layer, novel highly efficient OLEDs withouthole-transporting layer have been successfully obtained. A luminance of 88000cd/m2and an efficiency of 21 cd/A (external quantum efficiency of 5.5%) could be obtained - II -ABSTRACTat current density of 4000 A/m2. Among all the reported OLEDs with N,N-dimethylquinacridone (DMQA) as the dopant, the OLEDs in our work show thehighest efficiency and brightness. Further study indicates that the high externalefficiency is attributed to singlet Alq3 excitons generated through triplet-tripletannihilation of Alq3 triplet excitons. By introducing N, N'-bis-(1-naphthyl)-N, N'-diphenyl-1, 1' biphenyl 4, 4'-diamine (NPB)/CuPc AML structure as the hole-transporting layer, the AMLOLEDs with high efficiency have been fabricated. Such an improvement in thedevice performance could be attributed to the improved hole-electron balance due tothe energy gap at the interface of NPB/CuPc. Further investigation indicates that theOLEDs with emissive hole-transporting layer could be obtained by introducingNPB/5,6,11,12-tetraphenylnaphthacen (rubrene) AML structure in hole-transportinglayer. The results demonstrate that organic AML structure can not only efficientlycontrol the carrier transporting, and thus conducive to achieving an electron-holebalance, but also help to adjust the emitting zone, and the emission color. Based on the results obtained above, two theoretical models about the holecarrier control mechanism of Teflon buffer layer and the AML structure have alsobeen proposed.