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]. 

Fig. 9. Spectral response for a-Si H photodiode using Si3N4 and p-a-Si H blocking layers (solid curve). The emission intensity spectrum of the yellow-green LED is also shown (dotted curve). The applied voltage for the photodiode is 5 V. [From S. Kaneko el at., Amorphous Si H contact linear image sensor with Si3N4 blocking laye r. Technical Digest—International Electron Device Meeting. Copyright 1982 IEEE.]...

Fig. 10. Photoresponse characteristics of a-Si H photodiode using Si3N4 and p-a-Si H blocking layers. Upper wave form is the photoresponse. Lower wave form is the LED driving pulse. A yellow-green LED (35 lux) modulated by 500 Hz is used. Voltage applied to the photodiode is 5 V. Horizontal scale is 0.5 msec/division and vertical scale is 5 mV/division.

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. 

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

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]. 

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... 

Figure 8.52. Schematic diagrams of two types of the superstructure LEDs, (a) Structure A has one blocking layer (b) structure B two blocking layers. Both DMQtT and DMQqT can be effectively used as the blocking layer, when the DMSxT is chosen as the emitting layer. Reprinted with permission from Reference 227. Copyright 1994 Materials Research Society.

Figure 8.53. Output light power-current characteristics of the superstructure LEDs, (a) Comparison of strucmres A and B. DMQtT is used for the blocldng layer, (b) Comparison of different kinds of oligomers (i.e. DMQtT and DMQqT) that are chosen as the blocking layers in the structure B. The plots for DMQtT are the same data that appear in (a). Reprinted with permission from reference 227. Copyright 1994 Materials Research Society.

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