Degradation, OLEDs

Despite the potential significance of shallow traps in the operational degradation of OLED devices, there are no established general techniques for their effective detection and measurement in such an environment. The application of methods, such as electron spin resonance [43], electrically detected magnetic resonance [44], and charge modulation spectroscopy [45,46], is hindered by their limited sensitivities and selectivities in operationally degraded OLED devices. 

The paper is oiganized to describe, first, the materials that have been used in OLEDs, then the device structures that have been evaluated. After a description of the methods used to characterize and evaluate materials and devices, we summarize the current stale of understanding of the physics of device operation, followed by a discussion of the mechanisms which lead to degradation and failure. Finally, we present the issues that must be addressed to develop a viable flat-panel display technology using OLEDs. Space and schedule prevent a comprehensive review of the vast literature in this rapidly moving field. We have tried to present... 

An additional limit to the size of a passive array relates to the current which flows in an OLED when it is under reverse bias [189]. When a given pixel is turned on in the array, there are many possible parallel paths for the current, each involving two diodes in reverse bias and one forward. Hence, as the number of rows and columns increases, there is a higher level of background emission from non-selected pixels that limits the contrast ratio of the array. As a result, the contrast degrades as N increases. 

ZD Popovic and H Aziz, Reliability and degradation of small molecule-based organic light-emitting devices (OLEDs), IEEE J. Sel. Top. Quant. Electron., 8 362-371, 2002. 

A similar power law for degradation has also been observed in a-Si H [26] and OLEDs [27]. 

Scheme 5.13 Mechanism proposed for the formation of2-acetylpyrido 3,4-d imidaz,ole via Strecker degradation of histidine with 2-oxopropanaP44...

As mentioned above and treated in detail below, crystallization of compounds such as TPD is one of the main degradation processes in OLEDs (see Sec. 1.9 below). Doping of these compounds enhances stability by inhibiting the crystallization process and by localizing the excitation energy on the dopant or guest molecule (see Sec. 1.6 below). 

These generally blue-emitting materials were studied extensively by Hosokawa and coworkers.73 Among them, 4,4 -bis(2,2 -diphenylvinyl)-l,l -biphenyl (DPVBi) (see Figs. 1.1 and 1.3) has proven to be a particularly promising material for blue OLEDs. The degradation of OLEDs based on this material is apparently due to its crystallization, which results from its relatively low Tg 64° C. Indeed, the related spiro-DPVBi, with Tg 100° C, yields considerably more stable devices.70... 

The stability of OLEDs is obviously a key element in determining their technological impact. The common degradation mechanisms were recently summarized by Sato et al.150 We now briefly review these mechanisms. 

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