External quantum yield

However, the method will not enhance the external quantum yield of the LED, moreover, proper optical coupling between the layers has to be achieved in order not to decrease the QY loo much [61. One of the obvious problems is the principal requirement of having the ITO-covered glass substrate between the color converter and the cmitLer layer. This can also lead to color bleeding, an effect where the emitter layer excites not only the conversion layer, it is supposed to address but also neighboring ones. 

Fig. 21 Wavelength dependence of the external quantum yield for various sensitizers in dye sensitized solar cells (F L7 = T-dicarboxy-Z 7-bipyridine) [118]...

Doped PVK thin films display intense electroluminescence from the Ndm ion and OLED devices fabricated with this active material have a maximum irradiance of 8.5 nW mm-2 and an external quantum yield of 0.007%. Further refinement of the processing will hopefully lead to a still better optimization of the performance of these Ndm-doped polymeric emissive layers (O Riordan et al., 2006). 

When a reverse bias of 2-5 V is applied to the devices with polymer-C o blends, the external photosensitivity is 0.2-0.3 AAV and the external quantum yield is 50-80% el/ph. Because polymer photodiodes exhibit a relatively flat response over a broad spectral range, they are especially suitable as detectors for spectroscopic applications. They are particularly advantageous compared to silicon for detecting light in the blue and ultraviolet regions of the spectrum. At 370 nm the photosensitivity is approximately 0.3 AAV for a polymer photodiode and 0.05 AAV for a UV-enhanced silicon photodiode [69]. 

For nc-Si/SiO2 structures of type 1 the PL band maximum shifts from 1.3 to 1.7 eV when d decreases from 4.5 to 1.5 nm the intrinsic PL of nc-Si is commonly explained by the radiative recombination of excitons confined in nc-Si, while the size dependent spectral shift is attributed to the quantum confinement effect [21]. A considerable width of the PL band can be explained by nc-Si size distribution [21] as well as by phonon-assisted electron-hole recombination [22]. The external quantum yield of the exciton PL was found to reach -1 % for the samples with d = 3 - 4 nm at room temperature [18]. The lower quantum yield of the nc-Si/SiO2 structure in comparison with that observed for single Si quantum dots [22] and for III-V and II-VI compounds [22] can be explained by lower probability of the optical transitions, which are still indirect in nc-Si [21], as well as by the exciton energy migration in the assembly of closely packed nc-Si [18]. 

The external quantiun yield or I PC E (incident photon to current efficiency) is defined as the quotient of the number of incident photons and the number of charge carriers output to the external circuit. It is smaller than the internal quantum yield for conversion of the absorbed photons into charge carriers within the cell, because it takes into account losses due to reflection, recombination, and scattering. In contrast to the internal quantum yield, which can attain values of nearly 100% (see above), the value of the external quantum yield can be measured directly from the short-circuit current density jsc. with jsc = Isc/A where A is the active area of the cell, and the incident light intensity is lo- At a given wavelength k, we have... 

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