Polymeric OLEDs

J Blochwitz, M Pfeiffer, M Hofman, and K Leo, Non-polymeric OLEDs with a doped amorphous hole transporting layer and operating voltages down to 3.2 V to achieve 100 cd/m2, Synth. Met., 127 169-173, 2002. 

Since the pioneering work of Tang and VanSlyke [128], LEDs made with organic materials (OLEDs) have rapidly attracted interest in the scientific literature, industry and the media. The potential use of OLEDs in flat panel displays has encouraged numerous scientific and industrial efforts to realize this new technology. The introduction of conjugated polymers as emitting materials by Friend and co-workers [129], which led to polymeric OLEDs or PLEDs, was one of the milestones in the development of OLEDs. 

The surface treatment of ITO has an effect on the performance of polymeric OLEDs even when a PEDOTPSS buffer layer is incorporated. Especially an oxygen plasma treatment of ITO prior to PEDOTPSS deposition will increase efficiency and lifetime. This is surprising as the interface between ITO and PEDOTPSS is found to be always ohmic. Because oxygen plasma treatment changes the ITO morphology and reduces the contact angle of water, it is assumed that this drives the phase separation of PEDOT and PSS in ways more favorable for electroluminescence. 

Ho et al. improved the photometric efficiency of polymeric OLEDs by increasing the work function of PEDOTiPSS with layer thickness to form a graded layer. The authors deposited PEDOTiPSS together with cationic poly(p-xylylene-a-tetrahydrothiophenium) (PXT) to dedope PEDOT. The concentration of PXT was increased from one to the next successively deposited layer to achieve a continuous increase of the work function and the electron blocking properties over the HIL. 

The influence of the ratio of PEDOT to PSS on device performance was studied on polymeric OLEDs. A significant increase in device efficiency was found when a layer of PEDOT PSS with a ratio of PEDOT to PSS of 1 20 was coated on a high conducting polymeric anode with a ratio of 1 2.5. The high PSS content is necessary to form a PSS-enriched surface to block electrons and to prevent excitons from being quenched, as discussed earlier. 

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... 

An alternative approach utilizes polymeric analogs of PBD. The oxadiazole unit may be in the polymer main chain or attached as a side chain. A reasonable device performance has been demonstrated in poly(aromatic oxadia/ole)s [71—74. ... 

DuPont is an active player in OLED technology. Polymers used in devices as emitting materials are poly(p-phenylenevinylene), poly(arylenevinylene)s, poly(p-phenylene), poly(arylene)s, polyquinolines, and polyfluorenes. In some cases, an anionic surfactant such as lithium nonylphenoxy ether sulfate was added to the above-mentioned polymeric emitters... 

Ziegler catalysis involves rapid polymerization of ethylene and a-ole-fins with the aid of catalysts based on transition-element compounds, normally formed by reaction of a transition-element halide or alkoxide or alkyl or aryl derivative with a main-group element alkyl or alkyl halide (1,2). Catalysts of this type operate at low pressures (up to 30 atm), but often at 8-10 atm, and, in special cases, even under reduced pressure, and at temperatures up to 120°C, but often as low as 20-50°C. Approximately 2,200,000 tons of polyethylene and 2,900,000 tons of polypropylene are produced per year with the aid of such catalysts. The polyeth-... 

Doped PVK thin films display intense electroluminescence from the Ndm ion and OLED devices fabricated with this active material have a maximum irradiance of 8.5 nW mm-2 and an external quantum yield of 0.007%. Further refinement of the processing will hopefully lead to a still better optimization of the performance of these Ndm-doped polymeric emissive layers (O Riordan et al., 2006). 

If we compare the data in Table 8.5 with the barrier requirements set in polymer electronics (Fig. 8.11), it is evident they cannot be met with metallized films, not even with ultra-high-barrier films, multi-layer structures from metal evaporation, and polymeric layers. For transparent barriers, as required for OLEDs, displays, organic solar cells, etc., evaporated oxide layers are even further from meeting the values required. 

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