Quantum efficiency internal

Internal quantum efficiency (IQE) is defined as the ratio of the number of charge carriers produced by the cell to the actual number of photons absorbed by the cell. It is also a pure number varying between 0 and 1 but larger than EQE as not all incident photons are absorbed by the photocatalytic fuel cell. A way to express IQE is as follows (Kaneko et al., 2009b)  

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Efficiency. Efficiency of a device can be reported in terms of an internal quantum efficiency (photons generated/electrons injected). The external quantum efficiency often reported is lower, since this counts only those photons that escape the device. Typically only a fraction of photons escape, due to refraction and waveguiding of light at the glass interface (65). The external efficiency can be increased through the use of shaped substrates (60). 

The internal quantum efficiency of a LED is governed by the relative radiative and nonradiative recombination rates. The total recombination rate,... 

Thushigh internal quantum efficiency requires short radiative and long nonradiative lifetimes. Nonradiative lifetimes are generally a function of the semiconductor material quaUty and are typically on the order of microseconds to tens of nanoseconds for high quahty material. The radiative recombination rate, n/r, is given by equation 4 ... 

This confinement yields a higher carrier density of elections and holes in the active layer and fast ladiative lecombination. Thus LEDs used in switching apphcations tend to possess thin DH active layers. The increased carrier density also may result in more efficient recombination because many nonradiative processes tend to saturate. The increased carrier confinement and injection efficiency faciUtated by heterojunctions yields increasing internal quantum efficiencies for SH and DH active layers. Similar to a SH, the DH also faciUtates the employment of a window layer to minimise absorption. In a stmcture grown on an absorbing substrate, the lower transparent window layer may be made thick (>100 /tm), and the absorbing substrate subsequendy removed to yield a transparent substrate device. 

For a simplified case, one can obtain the rate of CL emission, =ft GI /e, where /is a function containing correction parameters of the CL detection system and that takes into account the fact that not all photons generated in the material are emitted due to optical absorption and internal reflection losses q is the radiative recombination efficiency (or internal quantum efficiency) /(, is the electron-beam current and is the electronic charge. This equation indicates that the rate of CL emission is proportional to q, and from the definition of the latter we conclude that in the observed CL intensity one cannot distii pish between radiative and nonradiative processes in a quantitative manner. One should also note that q depends on various factors, such as temperature, the presence of defects, and the... 

In electroluminescent applications, electrons and holes are injected from opposite electrodes into the conjugated polymers to form excitons. Due to the spin symmetry, only the antisymmetric excitons known as singlets could induce fluorescent emission. The spin-symmetric excitons known as triplets could not decay radiatively to the ground state in most organic molecules [65], Spin statistics predicts that the maximum internal quantum efficiency for EL cannot exceed 25% of the PL efficiency, since the ratio of triplets to singlets is 3 1. This was confirmed by the performance data obtained from OLEDs made with fluorescent organic... 

There is no reason why the same principle cannot be applied for light-emitting polymers as host materials to pave a way to high-efficiency solution-processible LEDs. In fact, polymer-based electrophosphorescent LEDs (PPLEDs) based on polymer fluorescent hosts and lanthanide organic complexes have been reported only a year after the phosphorescent OLED was reported [8]. In spite of a relatively limited research activity in PPLEDs, as compared with phosphorescent OLEDs, it is hoped that 100% internal quantum efficiency can also be achieved for polymer LEDs. In this chapter, we will give a brief description of the photophysics beyond the operation of electrophosphorescent devices, followed by the examples of the materials, devices, and processes, experimentally studied in the field till the beginning of 2005. 

As a result, power efficiency is a function of the internal quantum efficiency, TjInt the light extraction, rjout. and the voltage, V. Thus, to improve device performance, advances in these three key areas are required. Examples of strategies used to maximize power efficiency are described below. 

RJ Nelson and RG Sobers, Minority-carrier lifetime and internal quantum efficiency of surface-free GaAs, J. Appl. Phys., 49 6103-6108, 1978. 

Although one would prefer to know the internal quantum efficiency, it is only possible to measure the external quantum efficiency. Much of the light generated by an OLED is wave-guided out from the edges of the device. 

Apx is the chemical potential of the excited state relative to the ground state and (j) is the internal quantum efficiency which is the fraction of the excited states utilized for the generation of a useful product. The chemical potential is related Gibbs energy by 0=1,1 jV where pi is the chemical potential of the i state and Ni... 

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