Device lifetime

The utility and importance of multi-layer device structures was demonstrated in the first report of oiganic molecular LEDs [7]. Since then, their use has been widespread in both organic molecular and polymer LEDs [45, 46], The details of the operating principles of many multi-layer structures continue to be investigated [47—49], The relative importance of charge carrier blocking versus improved carrier transport of the additional, non-luminescent layers is often unclear. The dramatic improvements in diode performance and, in many cases, device lifetime make a detailed understanding of multi-layer device physics essential. 

Although even lower WF can be achieved with, e.g., Yb (0 = 2.4 eV), the low reflectivity index of the latter makes it less suitable for OLED applications. The active metal Ca (0 = 2.60 eV) often has to be accompanied with other metals such as Al to increase the device lifetime. It is worth noting that the WF of the metals can be affected by their purity, their deposition method, and the surface structure, and the crystal orientation of the deposited films. 

Once again, the most recent developments have been in the area of green phosphorescent materials where phenomenal efficiencies are now beginning to be coupled with good device lifetimes. The prototypical emitter of this type is iridium-tris-2-phenylpyridine (Ir(ppy)3) used as a dopant (Scheme 3.98), usually diluted into a carbazole type host, because it is prone to serious self-quenching problems. 

If there is one clear need in the field of OLED materials it continues to be in the area of blue emitters. A blue emissive material with good color coordinates CIE (0.10, <0.10) coupled with long device lifetime (>10,000 h) and high electrical efficiency (>5 cd/A) is the holy grail of materials chemists in this field. A major effort to find such materials continues in many laboratories including our own and the current sets of available materials may be supplanted at any time. However, the current best candidate blue emitters in the SMOLED area compromise many desirable properties — the most troublesome being long lifetime. 

Barrier metals, as opposed to alloys like AuGeNi, are employed in many thin film metallization systems to promote adhesion and prevent interdiffusion. Gold is an excellent conductor, however, it has very poor adhesion to both Si and GaAs. Gold also shortens the device lifetime when it diffuses into an active region of the device. For this reason it is used in multilayered structures such as Ta/Pt/Ta/Au (50), W/Au (50) and Cr/Au (51). SIMS, AES and RBS have all been used effectively in studying metal-metal interdiffusion, to extract diffusion coefficients, and to estimate device lifetimes. 

GaN-based laser diodes have now been shown to be reliable, potentially extremely useful devices [1], These laser diodes have been developed within a very short time span of about two years. Starting with pulsed operation at room temperature [2] mid later CW operation [3], we have experienced an exponentially increasing device lifetime. 

Enhancement in the performance of OLEDs can be achieved by balanced charge injection and charge transport. The charge transport is related to the drift mobility of charge carriers. Liu et al. [166] reported blue emission from OLED based on mixed host structure. A mixed host structure consists of two different hosts NPB and 9,10-bis(2 -naphthyl)anthracene (BNA) and one dopant 4,4 -bis(2,2-diphenylvinyl)-l,l -biphenyl (ethylhexyloxy)-l,4-phenylene vinylene (DPVBi) material. They reported significant improvement in device lifetime compared to single host OLEDs. The improvement in the lifetime was attributed to the elimination of heterojunction interface and prevention to formation of fluorescence quenchers. Luminance of 80,370 cd/m2 at 10 V and luminous efficiency of 1.8 cd/A were reported. 

Whether or not a chemical process step has been successful is difficult to measure, since there are few on-line measurable electrical properties. For example, film thickness and grain structure of polycrystalline silicon can be measured after a deposition step. However, their effect on device performance might not show up until subsequent doping or patterning steps fail. Similarly, it is possible to measure etch rates on-line by laser interferometry, but the etch profiles must be checked by electron microscopy. Unexpected mask undercutting or undiscovered etch residues can result in subsequent contact and device lifetime problems. 

On the other hand, it may be useful for some applications to have short conjugated segments all of the same length. Obtaining blue electroluminescence may be an example Large photon energies require short conjugation, but small molecules tend to crystallize, and the device lifetime is short. A copolymer may solve the problem a first example has been reported recently [50]. 

The number of polymeric materials in which EL has been demonstrated is increasing rapidly. Very different molecular formulas have been reported. The main aims are to obtain all colors and to improve the yields, the quantum yield expressed in photons per electron as well as the power yield, and to improve device lifetime. We shall not describe the chemistry involved, merely hint at some underlying principles. 

It is evident from the above discussion that the threshold voltage, current density, power efficiency, luminous efficiency and, to some extent, device lifetime of OLEDs using organic low-molar-mass compounds, oligomers and polymers depends on intrinsic molecular properties, such as HOMO and LUMO energy levels, efficiency of hole and electron injection and subsequent transport, efficiency of singlet formation and fluorescence efficiency. The... 

OLEDs are nowadays the most important type of light source for artificial lighting, making them potential candidates in the development of full-color flat panel display devices. Challenging problems to be addressed are emission color, emission efficiency and device lifetime. The emission color problem results from the broad emission bands exhibited by electroluminescent devices containing organic emitting layers, since pure and sharp emission bands from these materials, a requisite for display applications, are... 

Type Ref. Example materiaP Poling field [V pm" ] Required poling field intensity [mW cm" ] Index contrast, An (DFWM or 2BC)" Grating growth speed [s] Optical quality Device lifetime [months]... 

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