Trapping OLEDs

The picture presented above for confinement of the excitons within the device is for the EM layer sandwiched between the HTL and ETL. The EM need not be a discrete layer in the OLED, however, for exciton confinement to occur. Alternatively, the EM can consist of a luminescent molecule doped (- 1%) into a polymeric or molecular host material (40,41,54,55). So long as the energy gap (or band gap) of the host is higher than that of the EM dopant, excitons will be effectively trapped or confined on the dopant molecules leading to improved EL efficiency. An example of such a dopant-based device... 

On the experimental front, Burrows and Forrest 155] have measured the electric field and thickness dependence of the current and radiance from bilayer devices with various HTLs and Alqs as the ETL. The data were analyzed in temis of trap-limited transport in the Alq t layer, with the assumption that the voltage drop across the HTL is negligible. However, this assumption was challenged by Vestweber and Riess [ I56 and Giebcler et al. 1157], who demonstrated that HTL plays an important role in determining the efficiency of bilayer OLEDs. 

Yersin H (2004) Triplet Emitters for OLED Applications. Mechanisms of Exciton Trapping and Control of Emission Properties. 241 1-6 Yeung LK,see Crooks RM (2001) 212 81-135... 

The most commonly used HTL materials are triarylamine compounds. These compounds were developed as HTMs for photoconductive applications such as xerography [69]. They naturally have been selected as HTMs for OLED applications largely because of their ready availability and their good electrochemical and thermal stabilities. The hole mobilities of these materials are also adequate for OLED applications. In addition, high purity, so as to ensure low hole-trap contamination, is believed necessary for long-lived OLED performance and such materials may often be train sublimed to very high purity. 

Important electrical informations about OLEDs, such as charge transport, charge injection, carrier mobility, etc., can be obtained from bias-dependent impedance spectroscopy, which in turn provides insight into the operating mechanisms of the OLED [14,15,73,75 78]. Campbell et al. reported electrical measurements of a PLED with a 50-nm-thick emissive layer [75], Marai et al. studied electrical measurement of capacitance-voltage and impedance frequency of ITO/l,4-Mv-(9-anthrylvinyl)-benzene/Al OLED under different bias voltage conditions [76], They found that the current is space-charge limited with traps and the conductivity exhibits power-law frequency dependence. 

The appreciable levels of asymmetric induction observed in the catalytic ARCM reactions discussed above suggest a high degree of enantio-differentiation in the association of ole-finic substrates to chiral Mo complexes. Such stereochemical induction may be exploited in asymmetric ring-opening metathesis (AROM). Catalytic ROM transformations [20] -although less explored than the related RCM processes - offer unique and powerful methods for the preparation of complex molecules in a single step [2d, 2g], The chiral Mo-alkylidenes that are products of AROM can be trapped either intramolecularly (RCM) or intermolecularly (cross metathesis, CM) to afford an assortment of optically enriched adducts. 

A concern with PVA layers, however, is that the moisture trapped in the PVA could be detrimental to OLED lifetime. Some dry passivation methods are attracting more interest for this application. 

Yersin, H. (2004)Triplet emitters for OLED apphcations mechanisms of exciton trapping and control of emission properties. Topics in Current Chemistry, 24, 1—26. 

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