Chemical stability, OLEDs

TCO thin films can exhibit tremendous variability in transparency, microstructure (and surface roughness), surface composition, conductivity, chemical stability at high current densities (in OLEDs, OPVs, and chemical sensors), and in their chemical compatibihty with contacting organic layers. Variability is often noticeable from production batch to production batch and within batches of the same metal oxide material [34]. Surface pretreatment conditions have also been shown to dramatically impact the electrochemical, physical, and photophysical properties of metal oxide films [8, 9, 24, 26, 27, 35]. Modification... 

The overall performance of OLEDs can be dramatically improved by the use of dye molecules. These small molecules efficiently trap electrons and holes (ie carbanions and carbocations) and have high radiative recombination rates relative to non-radiative decay processes. One can quickly see why this would be effective in a device as the dye molecule can be optimized for recombination, while the HTL and ETL layers can be optimized for carrier transport, carrier injection and chemical stability without having to worry about radiative recombination. From the discussion in Section 9.2, it is known that many molecules exhibit low radiative recombination rates due to the symmetries of their HOMO and LUMO orbitals, resulting in disallowed optical recombination pathways. The solution to the problem is to dissolve dye molecules optimized for emission in one or more of the transport layers. 

An important part of this study was to recognize and address difficulties resulting from processing on polymeric substrates. Problems encountered included substrate handling, stability of the substrate, surface smoothness of the substrates, and effects of chemical exposure. Effective encapsulation of an OLED on a flexible polymeric substrate is a challenge but not one we will address in detail here. 

In this chapter, a review of our work regarding the chemical, electrochemical, and morphological stability of vacuum-deposited Alq3 is presented. The effect of these processes on charge transport and light emission in OLEDs is also considered, and the readers may refer to some published work for more quantitative analysis.35-37 The efficacy of simple passivation schemes as well as the fabrication of crystallization-resistant blends will also be briefly discussed. 

Conventional phosphorescent OLEDs with CBP host in the EML and stable materials in blocking layers usually show better stability compared to triplet OLEDs with other host materials. Nevertheless, it was established that chemical mechanism of degradation in the CBP + Ir(ppy)3 devices is linked to homolytic cleavage of C-N bonds in CBP and includes several key steps such as the formation of CBP excited states, homolytic dissociation and formation... 

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