Hole blocking layer

In bilayer LEDs the field distribution within the device can be modified and the transport of the carriers can be controlled so that, in principle, higher efficiencies can be achieved. On considering the influence of the field modification, one has to bear in mind that the overall field drop over the whole device is given by the effective voltage divided by the device thickness. If therefore a hole-blocking layer (electron transporting layer) is introduced to a hole-dominated device, then the electron injection and hence the efficiency of the device can be improved due to the electric field enhancement at the interface to the electron-injection contact, but only at expense of the field drop at the interface to the hole injection contact This disadvantage can be partly overcome, if three layer- instead of two layer devices are used, so that ohmic contacts are formed at the interfaces [112]. 

Figure 11-18. Calculated current (solid line) and iccuinbiiuuion current (dashed line) density as a function of voltage bias for a single-layer structure, a two-layer structure with a hole-blocking layer and a two-layer structure in which the hole-blocking layer also serves as an electron transport layer.

Due to the electronegative atom, polypyridines are good electron acceptors from UPS and UV-vis absorption spectra, Yamamoto and coworkers [669] estimated /iA 3.5 eV, /P = 6.3eV. Chen and coworkers [670] reported L a 2.9 cV, /p = 5.7 eV based on electrochemical measurements. For the double-layer ITO/PPV/562/A1 device, in which 562 acts as electron transport and hole-blocking layer, Chen and coworkers [670] reported a EL efficiency of 0.12cd/A that is 17 times higher than for an single-layer PPV-based PLED. The improvement in d by a factor of 60 (from 0.004 to 0.25%) for this device configuration was demonstrated by Monkman and coworkers [665]. 

M.-Y. Hwang, M.-Y. Hua, and S.-A. Chen, Poly(pyridine-2,5-diyl) as electron-transport/hole blocking layer in poly(phenylene vinylene) light-emitting diode, Polymer, 40 3233-3235, 1999. 

T. Lee, O.O. Park, L. Do, T. Zyung, T. Ahn, and H. Shim, Polymer light emitting devices using ionomers as an electron injecting and hole blocking layer, J. Appl. Phys., 90 2128-2134 (2001). 

Y. Sato, S. Ichinosawa, T. Ogata, M. Fugono, and Y. Murata, Blue-emitting organic EL devices with a hole blocking layer, Synth. Met., 111-112 25-29 (2000). 

ETL materials that are used most often are emissive metal complexes, especially aluminium but also beryllium and lanthanides such as europium and terbinm, of ligands such as 8-hydroxyquinoUne, benzoquinolines and phenanthroUne, whilst other effective compounds inclnde extended conjugated compounds, e.g. distyrylarylene derivatives. Some ETL materials are chosen because they are non-emissive to act as combined ET and hole blocking layers. A selection of these ETL materials is illustrated in Figure 3.35. 

Fig. 32 Typical electrophosphorescent multilayer device architecture composed of a hole injection layer (HIL), a hole transport layer (HTL), a emission layer (EL), a hole blocking layer (HBL), an electron blocking layer (EBL), and finally an electron transporting layer (ETL). Shown are the HOMO and LUMO energy levels of the corresponding molecules [77]. The vacuum level is assumed to be constant...

To avoid these problems, other types of blocking contacts have been proposed (Oda et al., 1981, Hatanaka et al., 1982 Ishioka et al., 1983). In these cases, a wide-gap transparent thin film such as SiOz or Si N,., is used as a hole-blocking layer in place of the n layer. A chalcogenide, such as Sb2S3 or As2Se1.5TeI.5, or an oxide such as Ce02 thin film, is then used instead of the p layer. 

Figure 5 illustrates an example of the schematic structure for an a-Si H target. On a glass substrate, a tin oxide transparent electrode layer is deposited by CVD. On this layer, a very thin Si02, as a hole-blocking layer, a photoconductive a-Si H layer, and an electron-blocking Sb2S3 layer are added, in that order. The thickness of the a-Si H layer is 2-4 pm. 

Using a hole blocking layer, holes can be confined in hole transporting or in hole injecting materials and they can be used as emitters. It minimizes the possibility of... 

Figure 34 Comparison of the single organic layer geometry (a), with the double layer obtained by insertion of an electron transporting, hole blocking layer (b). The energy-level positions are highly simplified. Recombination should occur preferentially within the hole accumulation layer. (From Ref. 288.)...

Bian and coworkers reported the fabrication of a four-layer device by using the vacuum deposited film of the Eu + complex as the emitting layer, Alq3 as electrontransporting layer, bcp as hole-blocking layer and tpd as a hole-transporting layer. This device, with a luminance of ca 230 cdm , was more efficient than the more common ones containing two or three layers. 

Figure VII-4 The band diagram of a polymer LED with a hole blocking layer.

Cathode Materials and Electron-Injection/Hole-Blocking Layer. 78... 

It behaved as a hole-blocking layer for increasing hole-electron recombination probability... 

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