Illumination quantum efficiency

While the cell efficiencies of these films were not specifically investigated, best parameters of 2 mAcm (ca. AMI illumination quantum efficiencies increased with decreasing illumination intensity due to diffusion limitations in the nanoporous film) 0.5 V and ca. 50% fill factor were obtained. However, great variation in these parameters were obtained one reason for this can be seen from a consideration of Figure 9.9. If a CdSe film is etched, but less than optimally (shorter time, more dilute HCl), it is clear that after a certain, unique etch treatment, zero net photocurrent will be obtained. The actual photocurrent (and other ontpnt parameters) of the film is a balance between photoanodic and photoca-thodic cnrrents. 

ZnS colloids were also used by Kuwabata et al. to photoreduce C02 to formate [130]. In this system, the ZnS colloids also reduced pyrroloquinoline quinone which served as an electron mediator to the enzyme, methanol dehydrogenase, which could then reduce formate to methanol. In C02-saturated aqueous solution at pH 7 and under far-UV (280nm) illumination, quantum efficiencies of 7% and 6% were achieved for formate and methanol, respectively. 

We recently demonstrated that photocatalyzed destruction rates of low quantum efficiency contaminant compoimds in air can be promoted substantially by addition of a high quantum efficiency contaminant, trichloroethylene (TCE), in a single pass fixed bed illuminated catalyst, using a residence time of several milliseconds [1-3]. Perchloroethylene (PCE) and trichloropropene (TCP) were also shown to promote contaminant conversion [2]. These results establish a novel potential process approach to cost-effective photocatalytic air treatment for contaminant removal. 

Significant reverse currents at semiconductor electrodes are not only observed for breakdown but also under illumination. For the latter case a quantum efficiency q, the number of exchanged holes and electrons n e per incident photons np, can be defined ... 

A p-type electrode is in depletion if a cathodic bias is applied. Illumination generates one electron per absorbed photon, which is collected by the SCR and transferred to the electrolyte. It requires two electrons to form one hydrogen molecule. If the photocurrent at this electrode is compared to that obtained by a silicon photodiode of the same size the quantum efficiencies are observed to be the same for the solid-state contact and the electrolyte contact, as shown in Fig. 4.13. If losses by reflection or recombination in the bulk are neglected the quantum efficiency of the electrode is 1. 

If an oxide-free, hydrogen-terminated, n-type electrode is anodized under illumination in an electrolyte free of HF (for example HC1), a quantum efficiency of close to 2 is observed for the initial contact of the electrode to the electrolyte. During the oxidation of the first hydrogenated monolayer the quantum efficiency decreases to 1 and remains at that value during the formation of the first few nanometers of anodic oxide, as indicated by filled triangles in Fig. 4.13. For a further increase of oxide thickness the quantum efficiency decreases to values significantly below 1 [Chl4]. 

A crystal activated with Ti + ions presents an absorption that peaks at 514 nm and the corresponding emission spectrum peaks at 600 nm. A sample of this crystal, which has an optical density of 0.6 at the absorption peak, is illuminated with an Ar laser emitting at 514 nm with a power of 2 mW. (a) Determine the laser power of the beam after it passes through the crystal, (b) If the quantum efficiency isrj = 0.6, determine the intensity (in photons per second) emitted as luminescence and the power dissipated as heat in the crystal. 

A photodiode is illuminated with a green beam (532 nm) whose power is unknown. The photodiode is operating in the photovoltaic regime at room temperature. After illumination, the voltage induced in the photodiode is 34 mV. Calculate the incident power if the quantum efficiency is 0.65 and if the electrical current generated in the photodiode in the absence of illumination is 1 mA. 

Calculate the current induced in a photodiode with an inhinsic quantum efficiency of 0.90 when it is illuminated at room temperature with a 0.35 mW light beam whose wavelength is 1140 nm. The photodiode is working in the photoconductor regime, and in the absence of illumination no electrical current is generated by this photodiode. What happens if the photodiode is cooled down to 5 °C ... 

A host material is activated with a certain concentration of Ti + ions. The Huang-Rhys parameter for the absorption band of these ions is 5 = 3 and the electronic levels couple with phonons of 150 cm . (a) If the zero-phonon line is at 522 nm, display the 0 K absorption spectrum (optical density versus wavelength) for a sample with an optical density of 0.3 at this wavelength, (b) If this sample is illuminated with the 514 nm line of a 1 mW Ar+ CW laser, estimate the laser power after the beam has crossed the sample, (c) Determine the peak wavelength of the 0 K emission spectrum, (d) If the quantum efficiency is 0.8, determine the power emitted as spontaneons emission. 

Figure 5.38 illustrates the experimental setup for water photoelectrolysis measurements with the nanotuhe arrays used as the photoanodes from which oxygen is evolved. The 1-V characteristics of 400 nm long short titania nanotuhe array electrodes, photocurrent density vs. potential, measured in IM KOH electrolyte as a function of anodization hath temperature under UV (320-400 nm, lOOmW/cm ) illumination are shown in Fig. 5.39. The samples were fabricated using a HF electrolyte. At 1.5V the photocurrent density of the 5°C anodized sample is more than three times the value for the sample anodized at 50°C. The lower anodization temperature also increases the slope of the photocurrent—potential characteristic. On seeing the photoresponse of a 10 V 5°C anodized sample to monochromatic 337 nm 2.7 mW/cm illumination, it was found that at high anodic polarization, greater than IV, the quantum efficiency is larger than 90%.

The transient current response of photo-electrodes to stepped-illumination changes suggests itself as a method of mechanistically interpreting this quantum efficiency problem. Though such transients have been studied for p-type GaP (1) and a number of n-type transition metal compounds (2, 3, 5, 6), published... 

Matsuoka used a different photosensitizer, p-terphenyl, with a cobalt(III) cyclam as the catalyst [37-39], In a C02-saturated acetonitrile/methanol solution with either TEOA or TEA as the sacrificial reductant, the quantum efficiencies for CO and formic acid production were 15% and 10%, respectively, under 313 nm illumination. Again, however, the TONs and production rates for macrocyclic complexes were low. 

More recently, the use of a pyridinium mediator in an aqueous p-GaP photo-electrochemical system illuminated with 365 nm and 465 nm light has been reported [125], In this case, a near-100% faradaic efficiency was obtained for methanol production at underpotentials of 300-500 mV from the thermodynamic C02/methanol couple. Moreover, quantum efficiencies of up to 44% were obtained. The most important point here, however, was that this was the first report of C02 reduction in a photoelectrochemical system that required no input of external electrical energy, with the reduction of C02 being effected solely by incident fight energy. 

Variables 3 and 4 are found in the familiar relationship, i.e., that fluorescence in dilute solutions is directly proportional to the absorbance of the solute AX at the wavelength selected for excitation, times the quanta of light IoX available at the wavelength selected for excitation, times the fluorescence quantum efficiency X for the solute. Oftentimes the wavelengths available for maximum illumination in an excitation source do not... 

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