Eun-Mi Han

Observation of degradation processes of Al electrodes in organic electroluminescence devices by electroluminescence microscopy, atomic force microscopy, scanning electron microscopy, and Auger electron spectroscopy

Degradation of top electrodes is one of the most important factors to determine the lifetimes of organic electroluminescence (EL) devices. An organic EL device [indium thin oxide (ITO)/N,N’‐diphenyl‐N,N’‐bis(3‐methylphenyl)‐(1,1’‐biphenyl)‐4, 4’‐diamine (TPD)/tris(8‐hydroxy‐ quinoline)aluminum (Alq3)/Al] was prepared and a morphological change of the Al top electrode was observed during and/or after applying voltage by atomic force microscopy and scanning electron microscopy (SEM). The change in the electrode surface, i.e., the increase in surface roughness was observed during the current flow. The degradation process started from faint dark core parts and propagated into disks with different rates depending on the magnitude of applied voltage. Degraded sites of the Al electrode, which were analyzed as aluminum oxide by Auger electron spectroscopy, protruded into the air on the organic layers. In SEM images of a life‐end electrode, discontinuities due to crevasse formation in the organic layers sandwiched...

Growth of dark spots by interdiffusion across organic layers in organic electroluminescent devices

In order to clarify the propagation mechanism of dark spots in an organic multilayered electroluminescent device, in situ electroluminescence microscopy as well as photoluminescence (PL) microscopy and Auger electron spectroscopy of the degraded device was carried out. The difference in local PL spectra between the dark spots and the normal surface area was also observed by using scanning near‐field optical/atomic force microscopy. The growing mechanism of the dark spots was proposed from these observations.

Crystallization of organic thin films for electroluminescent devices

The interfacial change of a vapor-deposited tris(8-hydroxyquinoline) aluminum complex (Alq3)N,N′-diphenyl-N,N′-bis(3-methyl-phenyl) - (1,1′-biphenyl) -4,4′-diamine (TPD) (Alq3TPD) bilayer and the surface morphology of the Alq3TPD bilayer and the mixed single layers of co-deposited TPD and Alq3 were characterized by fluorescence microscopy and spectroscopy, and atomic force and friction force microscopy in the range of temperature from room temperature to 80 °C under air. In this study was found enhanced suppression of the crystallization of TPD in a molecularly dispersed system such as the co-deposited Alq3-TPD mixed film in comparison with the single-component TPD film. It was also found that the crystallization of Alq3 in the interface of the bilayer or in the co-deposited single layer proceeded before the crystallization of TPD.

Oriented thin films of conjugated polymers : polysilanes and polyphenylenes

Abstract In this paper, preparation of uniaxially oriented films of conjugated polymers, polysilanes and poly(p-phenylenes) (PPP) is investigated. Highly oriented thin films of poly(dimethylsilane) (PDMS) were prepared by the friction transfer method. The properties of PDMS films strongly depend on the preparation temperature. At a temperature higher than 210°C, uniform ultrathin films with extremely high orientation were obtained. It is considered that the friction transfer is related to the disorder (hexagonal) phase of PDMS. Other polysilanes, such as poly(diethylsilane) (PDES), poly(di-n-hexylsilane) (PDHS), poly(di-n-butylsilane) and poly(methylphenylsilane) also afforded oriented films by the same method. Oriented films of PPP were prepared by the friction transfer method. The thin films of the polymers were characterized mainly by ultraviolet absorption spectroscopy. The friction-transferred PDMS and PPP had abilities of orienting other materials. The films of polysilanes, such as PDHS, which were cast on the oriented PDMS from solution, were oriented along the PDMS chain direction. Oriented films of some oligophenylenes were obtained by vapor deposition or cast on the friction-transferred PPP. When the friction-transferred PPP film was immersed into the reaction mixture, a newly oriented PPP film were polymerized on the friction-transferred PPP films.

Scanning force microscopy of organic thin‐film amorphous hole transport materials

Amorphous organic thin films of N,N′‐diphenyl‐N,N′‐bis(3‐methylphenyl)‐(1,1′‐biphenyl)‐ 4,4′‐diamine (TPD) and 4,4′,4″‐tris(3‐methylphenylphenylamino)triphenylamine (m‐MTDATA) which function as hole transport materials were fabricated on slide glass plates by vapor deposition. By heating the films in air, the change in photoluminescence (PL) spectra for TPD was little, while that for m‐MTDATA was significant and a new broad peak appeared in the longer‐wavelength range. The latter change was pronounced in an oxygen atmosphere, but was suppressed in a nitrogen atmosphere. To clarify the difference in the effect of the storage conditions on the PL characteristics between TPD and m‐MTDATA, these two films were also studied by the scanning force microscopy techniques of atomic force microscopy (AFM) and friction force microscopy (FFM). It was found from the observation by AFM that the m‐MTDATA films showed no change in topography, while the TPD films were crystallized easily at room temperature in the air. How...

Study of Interfacial Degradation and Morphological Change of the Vapor-Deposited Bilayer of Alq3/TPD for Organic Electroluminescent Devices by AFM and PL Technique

Abstract The morphological change of vapor-deposited films of TPD (N, N′-diphenyl-N, N′-bis-(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine) and Alq3 (Tris-(8-hydroxyquinoline) aluminum) and the interfacial degradation of Alq3/TPD bilayer were characterized by atomic force microscopy (AFM) and fluorescence spectroscopy, respectively, in the temperature range from room temperature to 80°C under an ambient atmosphere. The TPD and the Alq3 film are widely used as a hole transport and an emitter layer, respectively, in an organic electroluminescent device (an indium tin oxide (ITO) coated glass/TPD/Alq3/metal). As the substrate plate, an ITO coated glass or a slide glass was used in this work. The relative photoluminescence (PL) intensities of Alq3 to that of TPD in an Alq3/TPD and a TPD/Alq3 bilayer were increased with time. The changes were accelerated with increasing temperature and ascribed to quenching of photo-excited TPD by Alq3 diffusing into the TPD layer.

Thermal Stabilities of Organic Layer in Electroluminescent Devices

Abstract The thermal stability of organic EL devices was investigated by using an infrared thermal imaging radiometer, SEM, AFM, and fluorescence spectroscopy. The morphology of the electroluminescent (EL) devices was changed by high current flow which led to the increase in temperature of the devices. The interdiffusion between two organic layers was accelerated with the increase in temperature. The stability of the organic layers was improved by co-deposition of TPD-Alq as the hole transport layer.

Temperature Change in Structure of Bilayers of ALQ/TPD-Doped Polycarbonate Films for Organic Electroluminescent Devices

Abstract The change of surface and interface in bilayers of Alq (Tris-(8-hydroxyquinoline) aluminum) / TPD (N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-(1,1′-biphenyl)-4,4′-diamine)-doped polycarbonate (PC) films for organic electroluminescent devices was characterized by atomic force microscopy (AFM), SEM, and fluorescence microscopy, in the temperature range from R.T. to 250 °C under the air. As single layer films, the dip-coated TPD-doped PC film on indium-tin oxide (ITO)-coated glass and the vapor-deposited Alq films on various substrates showed good thermal stability compared with the vapor-deposited TPD film itself. However, in the bilayered structure of Alq / TPD-doped PC films, the Alq film as the overlaying layer was deteriorated readily, which was accelerated with increasing temperature and TPD concentration in the TPD-doped PC film.

Study of energy transfer from excited TPD to Alq in organic electroluminescent devices by time-resolved fluorescence spectroscopy using a scanning near-field optical atomic force microscope

We demonstrate the direct measurement of molecular diffusion at organic/organic interfaces of organic electroluminescence devices by use of a scanning near‐field optical atomic force microscope. Our preliminary study shows that the degradation of an electroluminescence device is partly caused by crystallization of the organic layers. Because the initial stage of degradation cannot be observed by microscopic methods, nanoscale optical properties of the interface in multilayer systems are currently receiving a great deal of attention. Defects of organic electroluminescence devices were investigated using a scanning near‐field optical atomic force microscope. This instrument is capable of measuring both a topographic and a fluorescence image at the same time. The defect area and other areas are clearly observed and time‐resolved near‐field fluorescence spectra demonstrate emission of the different species. These results suggest that defects occur at the organic solid interface, and that energy transfer occurs from excited TPD, as donor, to Alq, as acceptor.

Fabrication and Characterization of Reduced Graphene Oxide Counter Electrode for Dye-Sensitized Solar Cells

To improve performance of dye-sensitized solar cell(DSSC), thermally reduced graphene oxide(rGO) was investigated as Pt-free counter electrodes(CE) material. The DSSCs with rGO were fabrcated by simple spin coating method under different temperature and ambient atmospheres like Ar, N2 and air. When the rGO CE was reduced at 450°C under the Ar atmosphere, it was obtained the power conversion efficiency of 6.02%, which showed the high catalytic activity and performance of DSSC as much as Pt. This value means the thermally reduced rGO CEs under the Ar gas will be suitable as a substitute for Pt CEs.