Electroluminescence process

Figure 7.5 shows a schematic example of the electroluminescent process in a typical two-layer OLED device architecture. When a voltage is applied to the device, five key processes must take place for light emission to occur from the device. 

The conversion of light to electric current in photovoltaic devices is the direct inversion of the electroluminescent process in OLEDs, thus it is not surprising that the same molecular glasses as described above have also been used for the realization of solar cells. There are at least two different types of approaches, however, that shall be described now. 

The absence of energy shift between absorption and emission thresholds indicates minimal conformational relaxation in the excited state. There is an overall similarity to the PDAs, including the presence of a very strongly bound triplet exciton. The absence of polaron-related transitions in the PIA could be explained if the singlet were as strongly bound as in PDAs. All these properties are important in the electroluminescence processes discussed in Section V.C. 

Fig. 13. Energy transfer mechanism for Zn(hpb)2 doped with phosphorescent dopant Ir(btp)2acac in electroluminescence process.

The first step in the electroluminescent process is carrier injection. Electrode materials are typically chosen to match the orbital energies of the organic materials as closely as possible. The anode materials are typically metal oxides, such as indium-tin-oxide (ITO), with work functions that are close to the HOMO energies of the materials they... 

H. Kajii, K. Takahashi, J.-S. Kim, and Y. Ohmori, Study of the transient electroluminescence process using organic light-emitting diode with a partial doping layer, Jpn. J. Appl. Phys., 45(4B), 3721-3724 (2006). 

The efficient formation of singlet excitons from the positive and negative charge carriers, which are injected via the metallic contacts and transported as positive and negative polarons (P+ and P ) in the layer, and the efficient radiative recombination of these singlet excitons formed are crucial processes for the function of efficient electroluminescence devices. 

U. Lemmer. R. F. Malm, Y. Wada, A. Greiner, H. Basslcv. E.O. Gobel, Time resolved luminescence study of recombination processes in electroluminescent polymers, Appl. Phys. Lett. 1993, 62, 2827. 

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