Nonradiative charge trapping

Popovic et al. proposed the formation of charge traps that act as nonradiative recombination centers in the emissive material as one possible explanation for the relative differences in the electroluminescence and photoluminescence losses in operating OLEDs [27]. Despite the absence of direct evidence of the formation of such traps or their ability to act as nonradiative recombination centers, the proposal was quite plausible, particularly in view of a complementary notion that, in some cases, recombination on highly emissive charge traps is responsible for the predominant emission by the dopants [35,36]. The rise in voltage required to drive a constant current through a device has been cited as possible evidence of the formation of traps, but the voltage rise could be, in fact, attributed to mechanistically imrelated phenomena, such as electrode deterioration. 

The PL quantum yield r)pl. While r]pl of many dyes is close to 100% in solution, in almost all cases that yields drops precipitously as the concentration of the dye increases. This well-known concentration quenching effect is due to the creation of nonradiative decay paths in concentrated solutions and in solid-state. These include nonradiative torsional quenching of the SE,148 fission of SEs to TEs in the case of rubrene (see Sec. 1.2 above), or dissociation of SEs to charge transfer excitons (CTEs), i.e., intermolecular polaron pairs, in most of the luminescent polymers and many small molecular films,20 24 29 32 or other nonradiative quenching of SEs by polarons or trapped charges.25,29 31 32 In view of these numerous nonradiative decay paths, the synthesis of films in which r]PL exceeds 20%, such as in some PPVs,149 exceeds 30%, as in some films of m-LPPP,85 and may be as high as 60%, as in diphenyl substituted polyacetylenes,95 96 is impressive. 

Kondakov D Y, Sandifer J R, Tang C W, and Young R H, Nonradiative recombination centers and electrical aging of organic light-emitting diodes direct connection between accumulation of trapped charge and luminance loss, /. Appl. Fhys., 93... 

Fig. 13 Photo-induced processes occurring during photovoltaic energy conversion at the surface of the nanocrystalline titania films, 1 sensitizer (S) excitation by light, 2 radiative and nonradiative deactivation of the sensitizer, 3 electron injection in the conduction band followed by electron trapping and diffusion to the particle surface, 4 recapture of the conduction band electron by the oxidized sensitizer (S+), 5 recombination of the conduction band electrons with the oxidized form of the redox couple regenerating the sensitizer and transporting the positive charge to the counterelectrode. Grey spheres, titania nanoparticles, red dots, sensitizer, green and blue dots oxidized and reduced form of the redox couple. See Color Plates...

In conjugated polymers a similar nonradiative quenching process as observed for molecular crystals [2] happens if a SE encounters a trapped or free polaron—acting as a charged defect—during the migration process ... 

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