Emitting layers

Polymer LEDs are similar to thin film organic molecular LEDs first reported in 1987 17). Organic molecular LEDs utilize thin films of small organic molecules rather than polymer films as the light-emitting layer. The films of small organic molecules are undoped and have electronic properties comparable to the polymer films used in polymer LEDs. In general, the device physics of polymer LEDs is... 

Figure 9. The net OLR is emitted layer-by-layer through the atmosphere. Ts = ground surface temperature Te = effective temperature of the atmosphere...

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. 

Enhanced environmental stability was recently demonstrated for PLED with PFO 196/gold nanoparticle (5-10 nm) nanocomposite-emitting layer [337]. In addition, the gold nanoparticle-doped PLEDs (1.5 x 10-5 volume fraction of Au) demonstrated improved luminescent lifetime and 2-3 times higher QE, compared with pure PFO-based PLED. 

A better PLED performance was observed by Jenekhe and coworkers [173] for ITO/PEDOT/polymer/Al devices with quinoxaline-phenylene vinylene copolymers 586 and 587 as emitting layers. The el and maximum brightness were measured as 0.012 and 0.01%, and 120 and 35 cd/m2, respectively. The turn-on voltages of these devices were reasonably low, 6.0 and 4.0 V, respectively. The performance of PLEDs with polymer 586 was further improved by blending with 5wt% of a hole transport material, 1, l-Mstdi-d-tolylami-ii ophenyI )cycIohexane (TAPC) that enhanced the d lto 0.06% and the maximum brightness to 450 cd/m2. 

The simplest manifestation of an OLED is a sandwich structure consisting of an emission layer (EML) between an anode and a cathode. More typical is an increased complexity OLED structure consisting of an anode, an anode buffer or hole injection layer (HIL), a hole transport layer (HTL), a light-emitting layer, an electron transport layer (ETL), a cathode... 

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S Kan, X Liu, F Shen, J Zhang, Y Ma, Y Wang, and J Shen, Improved efficiency of single-layer polymer light-emitting devices with poly(vinylcarbazole) doubly doped with phosphorescent and fluorescent dyes as the emitting layer, Adv. Funct. Mater., 13 603-608, 2003. 

FIGURE 7.1 A two-layer vapor-deposited OLED first demonstrated by Tang et al. [4], The diamine acts as the hole transporting layer, Alq3 acts as the electron transporting or emitting layer. The external quantum efficiency was 1%. 

Table 3 Device architecture and performance of light-emitting diodes with PAE emitting layer... 

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