Radiative efficiency

Quantum well interface roughness Carrier or doping density Electron temperature Rotational relaxation times Viscosity Relative quantity Molecular weight Polymer conformation Radiative efficiency Surface damage Excited state lifetime Impurity or defect concentration... 

The overall efficiency of LED emission depends on three factors which vary between the different types of LEDs. These are the efficiency of electron-hole production, the radiative efficiency of recombination, and the efficiency of extraction of the optical signal from the junction. 

When the inhomogeneity in crystal quality is sufficiently small compared to carrier diffusion length, carriers diffuse into poor-quality areas due to the slope of the carrier density. (In fact, the actual mechanism may be more complicated, because the bandgap inhomogeneity exists as well.) hi this case, radiative efficiency is determined by the poor-quality areas, and consequently the sample exhibits relatively more homogeneous but poor radiative efficiency in total. This results in both higher transparent current density and lower differential gain. 

The light output of a LED is given by the injection current and the radiative efficiency and is governed by two primary loss mechanisms. A large fraction of the recombination at room temperature is by non-radiative transitions at defects (see Section 8.3.5). The thermal quenching of the photoluminescence is lower in the alloys than in... 

EM radiative efficiency = F / F. All these decay rates can instead... 

For the plane and the sphere, Rad > lifetime is dominated hy non-radiative emission, and the EM radiative efficiency (approximate modified quantum yield) is small. We also note that is then... 

Efficient recombination occurs in direct-gap semiconductors. The recombination probability is much lower in indirect-gap semiconductors because a phonon is required to satisfy momentum conservation. The radiative efficiency of indirect-gap semiconductors can be increased by isoelectronic impurities, e. g. N in GaP. Isoelectronic impurities can form an optically active deep level that is localized in real space (small Ax) but, as a result of the uncertainty relation, delocalized in k space (large Ak), so that recombination via the impurity satisfies momentum conservation. 

Crystal defects, surfaces, and some impurities create deep levels in the bandgap. These levels are separated from the conduction and valence bands by more than approximately 5 ksT. Deep levels act as neither good acceptor nor good donor levels because they are not usually ionized at room temperature. Deep levels provide an alternative mechanism for the recombination of holes and electrons and thus affect radiative efficiency. Transitions where phonons are not involved are called direct transitions indirect transitions involve phonons and are less radiatively efficient. 

Earlier observations by Cesario et al. [60] of a decay in fluorescence for arrays of Au nanoparticles spaced above a Ag film by a Si02 layer of increasing thickness, were interpreted as due to the finite vertical extent of the evanescent fields associated with a surface plasmon. In this model the coupling results in an enhanced interaction between individual localized plasmons at individual nanostructures [61] and thus an enhancement in the radiative efficiency increasing the spacer layer thickness moves the nanowires out of the evanescent field of the surface plasmon. A possible physical mechanism for the experimentally observed decay involves nonradiative decay of the excited states. The aluminum oxide deposited in these experiments was likely to be nonstoichio-metric, and defects in the oxide could act as recombination centers. Thicker oxides would result in higher areal densities of defects, and decay in fluorescence. A definitive assignment of the mechanism for the observed fall off of fluorescence would require determination of the complex dielectric function of our oxides (as deposited onto an Ag film), and inclusion in the field-square calculations. Finally it should be noted that at very small thicknesses quenching of the fluorescence is expected [38,62] consistent with observations of an optimum nanowire-substrate spacer thickness. 

Formula Compound Code Radiative Efficiency (W/mVppb) Atmospheric Lifetime (years) Global Warming Potential... 

Gss 2011 Atmospheric Mixing Ratio " 2011 Radiative Forcing (W/m ) 2011 Radiative Efficiency (W/ (m ppb(v))) Atmospheric Residence Time (years) Change in Mixing Ratio from 2005 to 2011... 

CFCs, which have a major role in stratospheric ozone destruction (Section 4.6.4), are also of concern because they are radiatively active in portions of the infrared spectrum that are not already strongly attenuated by water vapor, CO2, CH4, or N2O. The radiative efficiency of a mole of CFCs is about 10,000 times as much as a mole of CO2 CO2 has a radiative efficiency of approximately 1.4 x 10 W/(m ppb(v)), whereas the radiative efficiencies of CFCs are on the order of 0.3 W/(m ppb(v)) (IPCC, 2013). CFCs also have long atmospheric residence times, ranging from 45 years for CFC-11 to 1700 years for CFC-115 (CCIF2CF3) (IPCC, 2007). The locations of some CFC absorbance bands are shown in Fig. 4.48. 

HFCs, another class of CFC substitutes, do not contribute to stratospheric ozone destruction because they do not contain chlorine or bromine. Some HFCs have long atmospheric residence times, ensuring their presence for many decades to come. Like HCFCs, the radiative efficiencies of HFCs are on the order of 0.2 W/(m ppb(v)) (IPCC, 2013), only slightly less than those... 

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