Thin OLEDs

Indicator lights, fixed pattern, and segmented displays are applications which have been suggested for OLED deployment. Manufacturers of automobile components have shown interest in OLED indicators for the dashboard, where the primary considerations are those of cost, form factor, brightness, and stability over a wide range of ambient conditions. Power consumption is not particularly critical. The possibility of molding a thin light into a curved dashboard is attractive. 

To date, most small molecule-based OLEDs are prepared by vapor deposition of the metal-organic light-emitting molecules. Such molecules must, therefore, be thermally stable, highly fluorescent (in the solid state), form thin films on vacuum deposition, and be capable of transporting electrons. These properties limit the number of metal coordination compounds that can be used in OLED fabrication. 

The discovery of the use of A1Q3 as an electron-transport-emitting layer is undoubtedly the most significant achievement in the research that led to the development of stable OLEDs.180,181 It is very stable and can be sublimed without decomposition at 350 °C,188 and its thin-film PL quantum efficiency at room temperature is about 32%, independent of film thickness between 10 nm and 1,350 nm.189... 

A typical multilayer thin film OLED is made up of several active layers sandwiched between a cathode (often Mg/Ag) and an indium-doped tin oxide (ITO) glass anode. The cathode is covered by the electron transport layer which may be A1Q3. An emitting layer, doped with a fluorescent dye (which can be A1Q3 itself or some other coordination compound), is added, followed by the hole transport layer which is typically a-napthylphenylbiphenyl amine. An additional layer, copper phthalocyanine is often inserted between the hole transport layer and the ITO electrode to facilitate hole injection. 

As a class of n-type organic semiconductors, PBI derivatives have received considerable attention for a variety of applications [312, 313], for example, for organic or polymer light-emitting diodes (OLEDs and PLEDs) [314, 315], thin-film organic field-effect transistors (OFETs) [316, 317], solar cells [318, 319], and liquid crystals [320]. They are also interesting candidates for single-molecule device applications, such as sensors [321], molecular wires [322], or transistors [141]. 

Other materials such as gold (< = 4.9 eV), aluminum (< = 4.2 eV), indium-doped zinc oxide, magnesium indium oxide, nickel tungsten oxide, or other transparent conductive oxide materials, have been studied as anodes in OLEDs. Furthermore, the WF of ITO can be varied by surface treatments such as application of a very thin layer of Au, Pt, Pd, or C, acid or base treatments, self-assembly of active surface molecules, or plasma treatment. 

Bis(dimesitylboryl)-2,2 -bithiophene (BMB-2T, 242) forms a stable amorphous glass and emits pure blue color with a high fluorescence QE of 86% in THF solution [270]. However, an OLED with ITO/m-MTDATA/TPD/BMB-2T/Mg Ag emits with a broad emission due to an exciplex with TPD. The exciplex can be prevented by insertion of a thin layer of 1,3,5-tris(biphenyl-4-yl)benzene (TBB) between TPD and BMB-2T, leading to a pure blue emission. It seems that the boron complex or boron-containing compounds easily form an exciplex with common HTMs. Other similar blue emitter materials also demonstrate such behavior. 

Cheon and Shinar demonstrated that by deposition of a thin layer of the blue emitter DPVBI on the DCM-2-doped NPD device (Figure 3.12), an efficient white OLED with a brightness of over 50,000 cd/m2 and a power efficiency of 4.1 lm/W (external efficiency of 3.0%) could be achieved [274]. 

The concept of using HBMs in OLEDs started with the pioneering work of Kijima et al. when they were trying to get pure blue emission from an EL device with Alq3 as ETM and NPD as an EML [346], An undesired green emission color from Alq3 was suppressed when a thin layer of BCP was added between the NPD and Alq3 layers. 

It is known that most metals possess lower gas permeability than plastics by 6-8 orders of magnitude. An unbreakable and lightweight thin stainless steel foil substrate has been used for flexible OLEDs [75]. Therefore, a several micrometers thick metal layer can serve as a highly effective barrier to minimize the permeation of oxygen and moisture. Hence, the... 

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