J. Chan

Change of the emission spectra in organic light-emitting diodes by layer thickness modification

Electroluminescence and photoluminescence of organic light-emitting diodes consisting of an indium tin oxide anode, N,N8-di(naphthalene-1-yl)-N,N8-diphenyl-benzidine as a hole transport layer, tris (8-hydroxyquinoline) aluminum as emitting layer, and an Ag cathode were measured for different layer thickness values. It was found that, for a certain range of thickness values, multiple peak emission can be achieved. In addition, the emission spectra were dependent on the viewing angle. For the optimized thickness values, normal incidence chromaticity coordinates achieved were 0.32 and 0.43. Possible explanations for observed unexpected behavior are discussed.

Optimization of microcavity OLED by varying the thickness of multi-layered mirror

We optimized the emission efficiency from a microcavity OLEDs consisting of widely used organic materials, N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transport layer and tris (8-hydroxyquinoline) (Alq3) as emitting and electron transporting layer. LiF/Al was used as a cathode, while metallic Ag was used as an anode material. A LiF/NPB bi-layer or NPB layer on top of the cathode was considered to alter the optical properties of the top mirror. The electroluminescence emission spectra, electric field distribution inside the device, carrier density, recombination rate and exciton density were calculated as a function of the position of the emission layer. The results show that for optimal capping layers thicknesses, light output is enhanced as a result of the increase in both the reflectance and transmittance of the top mirror. Once the optimum structure has been determined, the microcavity OLED devices were fabricated and characterized. The experimental results have been compared to the simulations and the influence of the thickness of the mirror layers, emission region width and position on the performance of microcavity OLEDs was discussed.

Response to “Comment on ‘Change of the emission spectra in organic light-emitting diodes by layer thickness modification’” [Appl. Phys. Lett. 86, 186101 (2005)]

In our recent paper, 1 we presented the study of the emission spectra of triss8-hydroxyquinolined aluminum sAlqd based organic light-emitting diodes sOLEDsd as a function of organic layer thickness. Both calculations and experimental results were presented. The discrepancy between the calculated and measured emission spectra was noted, and possible reasons were discussed. Finally, it was concluded that further study is needed to conclusively establish whether any other phenomena in addition to simple interference play a role in the obtained results. In his recent comment, 2 Shore claimed that our experimental data can be entirely explained by simple interference phenomena, and presented calculations which qualitatively “reproduce” basic behavior of our devices. However, the calculations in our work fFig. 1sbdg also show similar behavior as those presented in the comment by Shore sFig. 2d. Since Shore 2 changed only the Alq layer thickness, while we considered devices with different N, N8-disnaphthalene-1-yl d-N,N8-diphenylbenzidinesNPBd and Alq thicknesses, direct comparison can only be made for the device with 65 nm NPB and 139 nm Alq. It is obvious from Fig. 1sbd in our letter and Fig. 2 in the recent comment 2 that both calculations show essentially the same features. Yet, quantitative agreement between the experimental data and the calculated results is lacking. Since the thicknesses of organic layers were verified by step profiler and spectroscopic ellipsometry after deposition, thickness errors in the fabricated devices are unlikely. Another possible reason for the discrepancy, as correctly identified by Shore, 2 is different emission region thickness. Figure 1 illustrates the influence of the assumed emission region thickness on the calculated electroluminescence sELd spectra. Further studies with confined emitting layers of known thickness are in progress to experimentally establish the influence of the emitting layer thickness. We would also like to point out that, regardless of the assumed emission layer thickness, the calculated spectra always show two peaks, while some of the experimental spectra showed clear shoulders in addition to two peaks. Comparison between the calculated and the experimental spectra for the 65/153s65 nm NPB and 153 nm Alq d device is shown in Fig. 2sad. It can be observed that the calculated spectrum, exhibiting a two-peak structure, does not describe the experimental spectrum well. The calculation for the same

Electrical and Optical Simulation of Tris (8-hydroxyquinoline) Aluminium-Based Microcavity Organic Light Emitting Diode (MOLED)

A detailed examination of the emitted radiation spectrum from tris(8-hydroxyquinoline) aluminum (Alq) based OLEDs on optical and electrical models have been presented. The OLED structure is examined as a function of choice of anode material and position of the NPB/Alq interface. The simulation results have been compared to those obtained from experiments, showing good agreement in both electrical and optical characteristics. The enhancement in light emission by aligning antinode of the stand wave pattern with effective carrier recombination region has been observed

Angular dependence of the emission from low Q-factor organic microcavity light emitting diodes

Microcavity light emitting diodes based on materials with broad emission spectrum, such as tris (8-hydroxyquinoline) aluminum (Alq3), typically exhibit a significant blue shift of the emission wavelength with increasing viewing angle. In this work, we investigate the influence of the organic layer thickness on the angular dependence of the emission spectrum in low Q-factor microcavity OLEDs. We demonstrate that for different organic layer thicknesses qualitatively different emission wavelength dependences on the viewing angle can be obtained in microcavity devices consisting of two organic layers, N,N′-di(naphthalene-1-yl)-N,N′-diphenylbenzidine (NPB) as a hole transport layer and Alq3 as emitting layer. The devices with different organic layer thickness were characterized by electroluminescence, photoluminescence, reflectance, and transmission measurements and results compared with the model.