External quantum efficiency dependence

The incident monochromatic photon-to-current conversion efficiency (IPCE), also called external quantum efficiency, is defined as the number of electrons generated by light in the external circuit divided by the number of incident photons as a function of excitation wavelength. It is expressed in Equation (7).29 In most cases, the photoaction spectrum overlaps with the absorption spectrum of the sensitizer adsorbed on the semiconductor surface. A high IPCE is a prerequisite for high-power photovoltaic applications, which depends on the sensitizer photon absorption, excited state electron injection, and electron transport to the terminals ... 

The luminance reaches 100 cd/m2 at 2.5 V with EL efficiency of 2.5 cd/ A. The corresponding external quantum efficiency is about 2% ph/el. At —10 V bias, the photosensitivity at 430 nm is around 90 mA/W, corresponding to a quantum yield of 20% el/ph [135], The carrier collection efficiency at zero bias was relatively low in the order of 10-3 ph/el. The photosensitivity showed a field dependence with activation energy of 10 2 eV [135], This value is consistent with the trap distribution measured in the PPV-based conjugated polymers [136,137],... 

The luminance (cd m ) depends on the applied bias voltage and can achieve values of 10 with external quantum efficiencies (percentage of photons per electron) of 2-3%. The PTCDA/Alqs discussed above (Pig. 4.32) exhibits a low luminescence efficiency because the relative position of the LUMOs is inadequate to confine electrons in the emissive Alqs layer. 

Figure VB-1 Temperature dependence of the external quantum efficiency QEj (EL) of an LED fabricated from OClClO-PPV -i- Bu-PBD (20%) open circles data from an identical device fabricated with pure OCICIO-PPV (without Bu-PBD) are shown for comparison. Data were obtained at 6.7 mA / cm. (Taken from ref. 158)...

Fig. 4.21 shows the thickness dependence of the external quantum efficiency for electroluminescence, QFext(FL) as a function of the film thickness for two closely related PPV derivatives, OCIClO-PPVand MEH-PPV. The quantum efficiencies of these devices were measured using an integrating sphere. 

This is a key issue not only for obvious energy-consumption considerations, but also for its effect on the lifetime of the devices (this last parameter depends strongly on the ability to operate at low voltage to obtain a given luminance). Each process at stake in the OLED during the long way from charge injection to photon emission contributes to decrease this ratio far below unity. A simple way to put this into equation is to decompose the external quantum efficiency as follows ... 

Therefore, the different complexes affect the external quantum efficiency of dye doped organic light-emitting diodes. This arises, because the efficiency of Forster energy transfer from the matrix to the dye is dependent on the degree of overlap between the EL spectrum of the matrix material and the absorption spectrum of the dye. ... 

Figure 6.4 Device performances depending on different processing methods. Coiors of symbols are as in Figure 6.3 and yeliowcoior is added for devices made by the roll-to-roll process. Aii data were measured at AM 1.5C/100mWcm . (a) /-V plots, (b) External quantum efficiency, (c) Power conversion efficiency and short circuit current density (j ). The error bars represent standard deviation.

S3/2 I15/2 = 550 nm (Fig. 16.19). These emission bands match well with the absorption of c-Si. Note that the exact emission wavelength can vary by 10 nm depending on the host material. Shalav et al. in 2005 reported the application of NaYp4 20 %Er phosphors as the upconverters in a bifacial c-Si solar cell [48]. These phosphors were mixed into an optically transparent acrylic adhesive medium at a concentration of 40 wt% and then placed on the rear of a bifacial c-Si solar cell. Reflective white paint was used as a reflector on the rear of the system. An external quantum efficiency of 2.5 % was obtained for the solar cell under excitation at 1523 nm with a 5.1-mW laser. More recently, Fischer et al. also investigated the... 

TABLE 6 Dependence of External Quantum Efficiency of Single-Layer EL Devices for MEH-PPV on the Work Function of the Cathode [151]... 

An important performance parameter of an optical detector at a given wavelength is the external quantum efficiency It is defined as the ratio of the number of electrons generated to the number of incident photons before any photogain occurs. Here rj takes into account the surface reflection loss and other losses in the detector. In general, rj depends on the absorptivity of the materials and the dimensions of the absorption region. The photogain M can result from carrier injection in semiconductor materials, as in the case of photoconductive devices (see Sec. 9.4.3), or from impact ionization, as in the case of avalanche photodiodes (see Sec. 9.4.2). 

The N3 and N719 dyes (Table 38.1) show the highest incident photon-to-current conversion efficiency (IPCE) as compared with other dyes. When the optical properties of the dyes are taking into consideration, there are two quantum efficiencies (QEs), that is, an external quantum efficiency (EQE) and an internal quantum efficiency (IQE) [43]. EQE includes the effect of optical losses by transmission and reflection, while IQE refers to the efficiency of the photons that are not reflected or transmitted out of the cell [43]. From the reflection and transmission of a solar cell, the EQE curve can be corrected to obtain the internal quantum efficiency curve [43]. IPCE is related to EQE and therefore IPCE depends on the absorption of light as well as the collection of charges. 

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