Hole blocking layers, LEDs

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

The P-LED device consists of a transparent electrode, a light-emitting polymer film, an electron-transporting or hole-blocking layer, and a negative electrode as... 

Figure 8.53 displays the output light power-current characteristics of LEDs with superstructures. Of these. Figure 8.53(a) compares the action characteristics of the two types of the superstructures which have DMQtT as the hole-blocking layer(s). Stracture B exhibits a larger differential coefficient than structure A. According to... 

The other main loss mechanism in these LEDs is from carriers which do not recombine in the i layer, but instead are transported completely through the film. Ideally, the p-type contact should comprise a blocking layer preventing electrons from being collected, but easily injecting holes, and vice versa for the n-type contact. Perhaps when the band discontinuities between the different alloys are better understood, some new and more efficient structure can be designed. 

It should be mentioned here that a6T is also used in other multilayer LEDs, e.g. in an ITO/a6T/tetra(tert-buty ) - sexiphenylene/tris(8- hydroxyquinoline)alumin -ium/Mg Ag LED, in which the quinoline acts as emitting layer whereas a6T enhances hole injection and the phenylene acts as an electron blocking layer, confining the electrons in the emitting layer [319]. 

All the polymers containing aromatic oxadiazoles were found to be easily n-doped whereas they were difficult to p-dope as revealed by cyclic voltamme, indicating that they possess electron injection and hole blocking properties which make them suitable for use as charge transporting layers in multilayer polymer LEDs. It was also shown that oxadiazole polymers can used as emissive materials. 

The HTL enhances exciton formation and recombination in the emissive layer by blocking the electrons away from the ITO anode and efficiently injecting holes into the electroluminescent layer. Polysilanes which are insulators for electrons have a good hole mobility of around lO cm A s due to a-conjugation of the electrons along the polymer chain. Moreover they are usually transparent in the whole visible region. These properties make them interesting candidates for application as hole transport layers in LEDs. 

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