Hole-injection layer

Figure 11-14 shows the calculated hole density (upper panel) and the electric field (lower panel) as a function of position for the three structures. For the devices with a hole barrier there is a large accumulation of holes at the interface. The spike in the hole density at the interface causes a rapid change in the electric field at the interface. The field in the hole barrier layer is significantly larger than in the hole injection layer. For the 0.5 eV hole barrier structure, almost all of the... 

B Werner, J Posdorfer, B Webling, H Becker, S Heun, H Vestweber, and T Hassenkam, Polyaniline as hole injection layer for OLEDs and PLEDs, SID Digest Tech. Pap., 33 603-605, 2002. 

Generally, silyl substituents seem to retard the hole-transporting ability of PPV. As a result, devices fabricated from silyl-substituted PPVs suffer from a high turn-on voltage. To improve the EL efficiency of PLEDs fabricated from Si-PPVs, the introduction of additional hole injection layer or copolymerization with electron-rich comonomers is required. 

T.M. Brown, J.S. Kim, R.H. Friend, F. Cacialli, R. Daik, and W.J. Feast, Built-in electroabsorption spectroscopy of polymer light-emitting diodes incorporating a doped poly(3,4-ethylene diox-ythiophene) hole injection layer, Appl. Phys. Lett., 75 1679-1681, 1999. 

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... 

When an electric field is applied between the anode and the cathode, electrons and holes are injected from the cathode and the anode, respectively, into the organic layers. With a matched energy barrier between the electron and the hole injection layers (EILs and HILs) and the cathode and the anode, electrons and holes are efficiently injected into the ETL and HTL. 

X. Zhou, M. Pfeiffer, J. Blochwitz, A. Werner, A. Nollau, T. Fritz, and K. Leo, Very-low-operating-voltage organic light-emitting diodes using a p-doped amorphous hole injection layer, Appl. Phys. Lett., 78 410-412 (2001). 

X. Gong, D. Moses, A.J. Heeger, S. Liu, and A.K.-Y. Jen, High-performance polymer light-emitting diodes fabricated with a polymer hole injection layer, Appl. Phys. Lett., 83 183-185 (2003). 

S Tokito, K Nada, and Y Taga, Metal oxides as a hole-injecting layer for an organic electroluminescent device, J. Phys. D Appl. Phys., 29 2750-2753, 1996. 

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.

Baytron , a conducting polymer [154] derived from 3,4-ethylenedioxythiophene, is a commercially available product that can be used as an antistatic or electrostatic coating of plastics and glass. Moreover, it has successfully been applied as counterelectrode in capacitors and as a hole-injection layer in organic light-emitting diodes [155]. 

Electro-optical modulators are other examples whose efficiency is enhanced in the presence of ion-radicals. These devices are based on the sandwich-type electrode structures containing organic layers as the electron/hole-injecting layers at the interface between the electrode and the emitter layer. The presence of ion-radicals lowers the barrier height for the electron or hole injection. Anion-radicals (e.g., anion-radicals from 4,7-diphenyl-l,10-phenanthroline—Kido and Matsumoto 1998 from tetra (arylethynyl) cyclooctatetraenes—Lu et al. 2000 from bis (1-octylamino) perylene-3,4 9,10-bis (dicarboximide)s— Ahrens et al. 2006) or cation-radicals (e.g., cation-radicals from a-sexithienyl—Kurata et al. 1998 l,l-diphenyl-2-[phenyl-4-A/,A- /i(4 -methylphenyl)] ethylene— Umeda et al. 1990, 2000), all of them are electron or hole carriers. 

Many other polymeric systems are of interest in polymer LEDs. Polythiophenes have been known for some time but it was not until improved synthetic methods were developed that their potential was realised. The process involves the reaction of the substituted monomer with FeClj in chloroform solution. After polymerisation has occurred the product precipitates and is isolated and washed. Further special purification methods are required to obtain satisfactorily pure materials. One product, of commercial interest, developed by Bayer is poly(ethylenedioxy)thiophene, known as PEDOT (3.110). This product when doped with polystyrene sulfonate, sold as Baytron P, has been found to be effective as a conducting, hole-injecting layer on the ITO electrode. ... 

A transparent electrode substrate was prepared by coating indium tin oxide (ITO) on a glass substrate and washing the substrate. ITO was then patterned using a photoresist resin and an etchant to specified patterns and the substrate washed. A hole injection layer was formed by coating a selected experimental agent dissolved in toluene to a thickness of about 50 nm and baking at 110°C for 1 hour. 

A typical multilayer device architecture containing triplet emitters is described in Fig. 31, and the resulting energy level scheme is shown in Fig. 32. ITO (indium tin oxide)-coated glass substrate is used as anode and on top of it a lOnm thick CuPc (copper phthalocyanine) hole injection layer is... 

It is possible to reduce the operating voltage of the doped LEDs with PtOEP concentrations of 2 wt% and 4 wt% below 8 V by optimizing the thickness of individual layers and incorporating a hole injection layer... 

The phosphorescent organic light emitting diodes (PHOLEDs) based on Ir(dmp>py)3 complexes were fabricated by the vacuum deposition technique with the following configuration ITO/copper phthalocyanine (CuPc, 10 nm) as hole injection layer/4,4 -bis[(l-naphthyl)(phenyl)-amino]-l,l -biphenyl (NPD, 40 nm) as hole transport layer/CBP Ir(dmppy)3 (8%) (20 nm) as emissive layer/2,9-dimethyl-4,7-diphenyl-l,10-phenanthroline (BCP, 10 nm) as a hole blocking layer/ tris-(8-hydroxyquinoline)aluminum (Alqs, 40 nm) as an electron transport layer/LiF (1 nm) as electron injection layer/ A1 (100... 

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