Emissive layer

The simplest polymer-based EL device consists of a single layer of semiconducting fluorescent polymer, c.g., PPV, sandwiched between two electrodes, one of which has to be transparent (Fig. 1-1). When a voltage or bias is applied to the material, charged carriers (electrons and holes) are injected into the emissive layer and these earners arc mobile under the influence of the high (> 105 V enr1) elec-... 

Top Metal Contact Polymer Emissive Layer Transparent Contact... 

In contrast with conjugated polymers, such as PPV, devices employing CN-PPV 47 as the emissive layer can achieve respectable internal efficiencies (ca. 0.2%) with both calcium and aluminum electrodes. EL efficiency may be further improved by employing a hole-transporting layer such as PPV in conjunction with... 

Figure 9-4. Color conversion (a) shows the components needed for external color conversion, (b) for internal color conversion - lop riglu enlarged view of the emission layer which consists of a matrix and color-converting dopaiuls (black dots).

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... 

G. He, M. Pfeiffer, K. Leo, M. Hofmann, J. Birnstock, R. Pudzich, and J. Salbeck, High-efficiency and low-voltage p-i-n electrophosphorescent organic light-emitting diodes with double-emission layers, Appl. Phys. Lett., 85 3911-3913 (2004). 

B.W. D Andrade, R.J. Holmes, and S.R. Forrest, Efficient organic electrophosphorescent white-light-emitting device with a triple doped emissive layer, Adv. Mater., 16 624—628 (2004). 

In the vapor-deposited OLED community, a number of approaches have been employed to produce white light emission. White OLEDs have been demonstrated based on multilayer structures, e.g., stacked backlights [153,168], multidoping of single-layer structures [145], phosphorescent monomer-excimer emission layers [169] and on doping of phosphorescent materials into separate bands within the emission zone, called a tri-junction [170]. The trijunction device has produced the highest white OLED efficiency of 16% external quantum efficiency demonstrated thus far [171]. 

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. 

A blue light-emitting electroluminescent device was claimed with an emission layer comprising a polymer matrix and a chromophoric component, which was either blended with or covalently attached to the polymer matrix. The chromophoric components were... 

Figure 3.26. Structure of an OLED. S = substrate (glass), ANO = anode (e.g., ITO — indium tin oxide), HIL = hole injection layer (e.g., Cu phthalocyanine), HTL = hole transport layer, EML = emission layer, ETL = electron transport layer, EIL = electron injection layer (e.g., LiF), KAT = cathode (e.g., Ag Mg, Al). The light that is generated by the recombination of holes and electrons is coupled out via the transparent anode.

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