Recombination luminescence efficiency

In electroluminescence devices (LEDs) ionized traps form space charges, which govern the charge carrier injection from metal electrodes into the active material [21]. The same states that trap charge carriers may also act as a recombination center for the non-radiative decay of excitons. Therefore, the luminescence efficiency as well as charge earner transport in LEDs are influenced by traps. Both factors determine the quantum efficiency of LEDs. 

Time-resolved measurements of photogenerated (very intense illumination, up to 0.56 GW/cm ) electron/hole recombination on CD (selenosulphate/NTA bath) CdSe of different crystal sizes has shown that the trapping of electrons, probably in surface states, occurs in ca. 0.5 ps, and a combination of (intensity-dependent) Auger recombination and shallow-trapped recombination occurs in a time frame of ca. 50 ps. A much slower (not measured) decay due to deeply trapped charges also occurred [102]. A different time-resolved photoluminescence study on similar films attributed emission to recombination from localized states [103]. In particular, the large difference in luminescence efficiency and lifetime between samples annealed in air and in vacuum evidenced the surface nature of these states. 

Chapter 1 focuses on the characteristics of deep states in wide band-gap III-V compound semiconductors, particularly the recombination properties which control minority-carrier lifetime and luminescence efficiency. These properties are significant for many optoelectronic devices, including lasers, LEDs, and solar cells. While this review emphasizes areas of extensive recent development, it also provides references to previous comprehensive reviews. The compilation of levels reported in GaAs and GaP since 1974 is an important contribution, as is the discussion of the methods used to characterize these levels. 

The changes are explained as follows The density of surface states within the band gap on freshly cleaved InP is high. As a result, the surface recombination velocity is high and the luminescence efficiency is low. Chemisorption of oxygen splits the surface states, as large band gap, colorless InP04 is formed.19... 

The lunar transient events could be excited by protons in the solar wind but experiments with silicate minerals in proton beams show that the process is inefficient, quantum efficiencies from lxl0 4 to 1x10 , and given the concentration of protons in the solar wind the mechanism cannot account for the intensity of the observed luminescence (33). Another possibility is that neutral particles in the background solar wind or associated with disturbances on the sunfs surface provide the excitation source (34). This would be a process very similar if not identical to the candoluminescence and radical recombination luminescence observed in the laboratory. 

Ej is a demarcation energy, similar to that defined in the analysis of dispersive transport (see Section 3.2.1). It is assumed that all carriers which are thermally excited recombine non-radiatively, but the same result is obtained if some fraction are subsequently retrapped and recombine radiatively. The luminescence efficiency is given by the fraction of carriers deeper than E, . An exponential band tail density of states proportional to exp (E/kf,) results in a quantum efficiency of... 

The recombination is modified in a multilayer structiu e whose layer spacing is similar to the carrier tunneling distance and is observed in photoluminescence measurements (Tiedje 1985). Fig. 9.22(a) shows that the luminescence intensity of a-Si H/nitride multilayers decreases as the layer thickness drops below about 500 A. The interface states and bulk nitride defect states cause non-radiative recombination because the electron-hole pairs are never far from an interface. The model of non-radiative tunneling developed in Section 8.4.1 can be adapted for recombination in thin layers. When the layer thickness is less than the critical transfer radius, the luminescence efficiency is (see Eq. (8.52)). 

Various approximate reaction times are indicated for the reactions steps. Also worth noting is that luminescence produced by charge recombination in [P680 (I) ] has been found to require an activation energy of only 0.06-0.08 eV This low activation energy favors a high quantum efficiency for the recombination luminescence. 

Phosphorescent EL materials are shown in Fig. 14-19 [116]. Singlet and triplet excitons are formed in the ratio of 1 3 by the recombination of electrons and holes. For this reason, fluorescent materials can give a maximum luminescence yield of only 25% internal efficiency. Since the yield of transmission of the luminescence through the layer is around 20%, the total external efficiency is around 5%. Phosphorescent compounds can utilize the triplet excited state to emit luminescence, so that the use of both fluorescent and phosphorescent compounds could give theoretically 100% internal quantum efficiency. Actually a device with 15% external luminescence efficiency has been reported. One of the topics in EL-device research is how to realize luminescence of 100% internal quantum efficiency. The efficiency of fluorescence is often near 100%, but that of phosphorescence is usually not very high. Eu or Tb ions give efficient phosphorescence, and so some complexes of Eu or Tb have been studied for EL devices, but there has been no report of efficient luminescence. 

Luminescence efficiency of QDs has been sufficiently improved by a surface passivation technique[13]. Surface passivation is used to reduce non-radiative surface recombination of charge carriers, which behaving as non-radiative relaxation centers for the electron-hole recombination.By applying proper surface passivation ligands to eliminate surface traps that aroused by dangling bonds, it will lead to luminescence enhancement [14-21]. 

From the comparative emission spectra of the as-obtained Ca4.96(P04)3Cl Ce %.o2>Tb Vo2 sheaves and the microrods (Fig. 9.10), it can be seen that the two samples show similar spectral patterns without any emission band shift, but one can clearly observe that the nanorods have a higher PL intensity than sheaves. It is well known that surface area of materials increases along with decrease in size. The larger the surface area is, the more defects will be introduced into the phosphor crystal. Defects have a serious drawback in PL intensity for phosphors as they provide nonradiative recombination routes for electrons and holes. In addition, the complication of the morphology of the sheaf-like hierarchical architectures reduces the luminescence efficiency by reflecting part of the luminescence in the inner part of the architectures, which leads to the absorption of the defects in the sheaf-Uke hierarchical architectures. So the luminescence intensity of Ca4 96(P04)3Cl Ce Vo2 Tb o.o2 microrods is higher than that of the Ca4 96(P04)3Q Ce Vo2>... 

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