Photocarriers

In photoexcited polar semiconductors, the coherent LO phonons couple with photocarriers to form coherent LO phonon-plasmon coupled (LOPC) modes, which exhibit fundamentally different properties from those of bare phonons. Huber and coworkers revealed the ultrafast transition of an optical... 

When all these factors contribute, the situation becomes almost hopelessly complicated. The simplest realistic case is that in which the photocarriers are generated in the space-charge region and migrate to the surface, where they are immediately consumed by an electrochemical reaction. We consider this case in greater detail. Suppose that light of frequency i/, with hu > Eg, is incident on a semiconducting electrode with unit surface area under depletion conditions (see Fig. 8.8). Let Iq be the incident photon flux, and a the absorption coefficient of the semiconductor at frequency v. At a distance x from the surface, the photon flux has decreased to Iq exp(—ax), of which a fraction a is... 

Control of photocarrier conduction in LB films by redox chromophores... 

Figure 4. Principle of the measurement of thermally stimulated current (a) when the sample is irradiated at —150°C photocarriers are generated and the traps are filled with the carriers (b) measurement of the thermally stimulated current...

As already stated and shown by Fig. 8, when the film is irradiated at room temperature in air by UV-light prior to the measurement of thermally stimulated current, the current peak at 5°C increased considerably, i.e. the population of the 0.56 eV traps increased by UV-irradiation in air. The photocarriers... 

Recently, Itaya, Okamoto and Kusabayashi (14) made an estimation that the concentration of the electron-accepting photo-oxidation product of about 10 mol/mol monomer unit is necessary to interpret the high yield of the photocarrier generation of ca. 

From photocarrier grating measurements at low laser intensities (100 mW cm-2), the value of the ambipolar diffusion coefficient of meso PS is deduced to be about... 

Braun CL (1984) Electric-field assisted dissociation of charge-transfer states as amechanism of photocarrier production. J Chem Phys 80 4157... 

Solar conversion efficiencies for amorphons cells are increasing steadily, bnt again the performance of devices critically depends on the mobility and lifetime of excess photocarriers in the films. 

Figure 4.15 Traces of transient electron currents in an a-Asg ioSco 90/Se double layer as a function of the layer thickness of Aso.ioSso.go (1) d = 0.9pm, (2) 0.7pm, and (3) 0.5 pm. The applied electric field is 7.2 X lO V/cm. The inset schematically shows the spatial distribution of photocarriers at different times.

Figure 4.17 A typical transient electron current waveform in an a-Se/Aso.ioSeo 90/Se triplelayer sample. d = 0.81 pm, (I2 = 0.43pm, and = 8.40pm. E = S X 10 V/cm. The inset shows the spatial distribution of photocarriers at different times.

In this section, we propose that the transit time of transport photocarriers can be obtained by an analysis of transient current in a double layer that consists of a thin chalcogenide nnder test and another material with higher mobility snch as a-Se. Figure 4.19(a) shows the type of carrier transit pulse that is frequently enconntered in the study of amorphous semiconductors. 

The sign of the dominant charge photocarriers in most of the polymers investigated has been found to be positive. The photosensitivity increase under ultraviolet irradiation might be connected with the photoionization of polymer molecules and the creation of local, positively charged centers acting as photoelectron traps. This mechanism seems to be confirmed by the electron spin resonance method. 

From photoinduced absorption, luminescence and electron spin resonance observations, the dominant photocarriers generated in the polymer were shown to be polarons and bipolarons [189-191]. It was found that the magnitude of photoinduced absorption is rather independent of the condition of sample preparation whereas the photoluminescence intensity is strongly influenced. The results suggest that the luminescent exciton does not play a primary role in the photogeneration of polaronic species. 

Hoestery, D. C., and G. M. Letson Trapping of photocarriers in anthracene by anthraquinone, anthrone and naphthacene. J. Phys. Chem. Solids 24,1609 (1963). 

In order to optimize the photorefractive performance of BaTi03, it is necessary to control the centers involved in the ionization and trapping of photocarriers. This may be accomplished in part by intentionally doping the crystal with various transition metal or rare earth ions, which may exist in more than one valence state. 

These electrons are thermally ionized from the vacancy and may combine with an available acceptor, thus altering the charge state of the acceptor species. Experiments have shown that such a process can lead to a change in sign of the dominant photocarrier as well as modified gain and response time of the photorefractive effect. 

This model has a number of limitations, including the fact that only point defects are considered. As discussed below, there is some evidence that more complex defect pairs or clusters may occur in BaTi03. In addition, the model requires that photocarriers are associated with only a single partially filled level in thermal equilibrium and cannot account for light-induced charge redistribution among multiple levels. Several studies (13-15) have suggested the possible importance of the latter effect, based on observations of intensity-dependent absorption and the sublinear intensity-dependence of the response time in many crystals. 

Mandelis, A., Batista, J. Shaughnessy, D. Infrared photocarrier radiometry of semiconductors Physical principles, quantitative depth profilometry, and scanning imaging of deep subsurface electronic defects. Phys. Rev. B Condens. Matter 67, 205208/1-205208/18 (2003). 

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