Improved OLEDs

To further enhance the performance of the OLED-based sensing platform, approaches to narrow the EL emission bands and enhance the brightness, efficiency, and lifetime of the OLEDs are desired. Such approaches will include the following  

Tandem stacked phosphorescent OLEDs (SPhOLEDs) (with EL decay times l J,s) [92]. The SPhOLEDs structure is 

OLEDs with a mixed hole, electron, and emitting material layer [93]. Several studies have shown that such mixed layers enhance the stability of the devices. 

Microcavity OLEDs fabricated on distributed Bragg reflectors (quarter wave stacks) [94]. Such OLEDs are fabricated on dielectric layers with significant dielectric contrast, so they narrow the emission spectrum by constructive interference. The narrow emission spectra also result in more efficient and more stable devices than regular OLEDs. The emission spectrum can be tailored to the specific sensor requirements (i.e., the absorption peak of the sensing element). 

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Lee J, Jung B-J, Lee J-I, Chu HY, Do L-M, Shim H-K (2002) Modification of an ITO anode with a hole-transporting SAM for improved OLED device characteristics. J Mater Chem 12 3494... 

A different approach has been proposed to improve carrier transport and to increase emission efficiency by exploiting conformational properties of conjugated polymers indifferent solvents. MEH-PPV in tetrahydrofuran (THF) gives rise to coils that partially suppress interchain interactions. A different situation occurs in chlorobenzene (CB) solutions where a more extended conformation is detected resulting in an efficient aggregates formation [150,151]. Different polymer chain conformations in solutions are likely retained in films spun from them and then exploited to improve LED performances. Since their operation is a complicated balance of transport and emission properties, a multilayer structure with MEH-PPV films spun from different solvents could improve OLED performances. Schwartz and coworkers exploited different conformations of MEH-PPV in CB and THF... 

UV/Ozon or plasma treatment of ITO surfaces is commonly used to improve surface wetting and the quality of subsequently deposited layers. The cleaning steps of ITO may have additional consequences that have not been fully understood. In contrast to expectations it was foimd that an oxygen plasma treatment improves OLED performance despite an intermediate PEDOTPSS buffer layer. It is assumed that ITO treatment might have a beneficial influence on PEDOTPSS film formation and morphology in that specific device configuration. 

I ole is obtained by distilling the ammonium salt of muclc acid, preferably in the presence of glycerol which leads to an improved yield CHOHCHOHCOONH4 CH=CH ... 

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... 

The performance of AMOLEDs is improved drastically in the past years. In contrast to the data shown in Table 1.2 (which representing development stage in 2002), a set of recent data of a 14.1" WXGA AMLCD made with solution-processed OLED emitters is shown in Table 1.3 [163,175,176], The color gamut is improved to over 60% with respect to NTSC. The luminous and power efficiencies at white point (x 0.28, y 0.31) are >8 cd/A and >5 lm/W. The power efficiency surpasses the performance of AMLCDs, plasma displays, and all other known flat-panel displays in commercial market or under development. A photo of the 14.1" AMOLED display is shown in Figure 1.25b. 

Closely related to the anode modifications described above, the use of a HIL material to improve charge injection into the OLED device has spawned a number of materials, which have been shown to provide benefits, particularly in terms of lower operating voltages and extended lifetimes of devices. 

The performance of OLED devices employing CuPc as a HIL is unstable due to thermally induced HTM crystallization on the CuPc surface [27]. One approach to improve the hole injection and enhance the device stability is to overcoat the CuPc or else to directly deposit... 

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