Efficiency Of Photogeneration Research Articles

The efficiency of photogeneration of hydrogen at p-type III/V semiconductors

Studies have been carried out on p-GaP and other III/V p-type semiconductors in an attempt to understand the reasons for the very low efficiency of photogeneration of hydrogen at potentials just positive of flatband. The efficiency for this process only rises to significant levels at potentials >0.6 V from flatband, which prevents the effective use of these photoelectrodes in solar photoelectrolysis cells. The poor response has been found to be caused by surface hydrogen atoms formed in the first step of the photo-assisted hydrogen evolution reaction. In addition to their subsequent conversion to H 2, these atoms may be reoxidised by holes tunnelling to the surface from the valence band; this process is only suppressed at very negative potentials. Evidence for this model is provided by ac and dc experiments and a detailed impedance analysis. It has been found that the response of p-GaP may be improved dramatically by the adsorption of a layer of Ru(III) chloride. This also appears to effect a reduction in the tunnelling reaction.

On the computation of Onsager quantum efficiency

A new method of computing the Onsager quantum efficiency of photogeneration is presented. This method is shown to be much faster than and as accurate as the series method used previously. Results applicable to a-Si (dielectric constant=11.5) are given as an example.

Photoinduced voltage-decay characteristics of dye-sensitized poly-N-vinylcarbazole

Measurements of the photoinduced voltage-decay of dye-sensitized polyvinylcarbazole (PVK) films are reported. A reaction product of a benzopyrylium salt and a benzopyrane is, as the sensitizing dye, uniformly dispersed in the PVK films. The voltage-decay characteristics are analyzed on the basis of a model assuming that holes are generated by an electron transfer from PVK to photoexcited dyes. The model is confirmed by this analysis and by comparing the voltage-decay rate of the uniformly sensitized films with that of a double-layer film consisting of an undoped PVK layer and a very thin dye-sensitized PVK layer. The quantum efficiency of photogeneration of holes is found to increase linearly with the electric field and is estimated to be 0.52 at 106 V/cm. The mean range of holes at 106 V/cm is longer than 11 μ for the films containing less than 1.6 weight parts of the dye relative to 100 weight parts of PVK. The range decreases with the increase in the dye concentration.

Electronic transport in liquid and solid sulphur

The transport of generated charge carriers in sulphur has been investigated by drift mobility techniques during the melting of single crystals (at similar 114 °c) and also in the liquid prepared from molten granules of ultra-pure (6N) or laboratory reagent sulphur. The discussion of the results leads to the conclusion that in the ultra-pure liquid the transport is predominantly electronic and that both electrons and holes propagate by an intermolecular hopping mechanism. For electrons the transport mechanism is the same as in the solid, but during melting the intermolecular overlap energy drops by about a factor of 5, leading to an electron mobility μe = 10-4 cm2 s-1 v-1 at 120 °c. The hole conduction, which takes place in a small-polaron band in the solid, changes to a hopping transport upon melting. The magnitude and temperature dependence of μh is then very similar to that of μe. These results are discussed in the light of Holstein's small-polaron theory. Experiments on less pure liquid specimens show additional features which appear to arise from the ionization of impurity molecules during the transit of the generated carriers. Above 160 °c a change in the temperature dependence of μe and μh occurs, connected with the break-up and polymerization of the S8 ring molecules. The efficiency of photogeneration in the liquid is lower than in the solid, but its spectral dependence is substantially the same. The steady dark current in the ultra-pure liquid is ohmic, although on application of the field the initial current is injected under space-charge-limited conditions. The different steady dark current behaviour of the impure liquid has been attributed to the formation of heterocharge layers near the electrodes.

Field-Controlled Photogeneration and Free-Carrier Transport in Amorphous Selenium Films

We have studied the photogeneration and transport of free carriers in amorphous selenium films under varying conditions of thickness, wavelength of the exciting radiation, and applied electric field. We have found that the efficiency of photogeneration is strongly field controlled, varying by two or more powers of 10 with the field changes reported. Using a technique based on time resolving the transport of free carriers across the film, we have measured deep-hole trapping times of 10-45 \ensuremath{\mu}sec and deep-electron trapping times of 40-50 \ensuremath{\mu}sec at room temperature. In addition, we have found hole-drift mobilities of 0.13-0.16 ${\mathrm{cm}}^{2}$/V sec and electron-drift mobilities of 6.0-8.3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}3}$ ${\mathrm{cm}}^{2}$/V sec at room temperature, in good agreement with previous workers. These measurements allow an independent determination of the hole and electron range of (1.3-6.3)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}6}$ ${\mathrm{cm}}^{2}$/V and (2.4-3.1)\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}7}$ ${\mathrm{cm}}^{2}$/V, respectively. The hole range measured in this new way is 20-100 times larger than previous determinations based on an inappropriate Hecht-type analysis, which is only a measure of the product of the generation and transport efficiency and not of the transport alone. The voltage dependence of the total transported charge cannot be described by a simple range limitation. The thickness scaling laws and the independent measurement of the range require that the photogeneration efficiency must depend on the applied electric field. It is suggested that the photogeneration step involves the field-aided thermal dissociation of a tightly bound hole-electron pair. This is indicated by the electric-field and wavelength dependence of the apparent quantum efficiency.

Photogeneration of charge carriers and related optical properties in orthorhombic sulphur

The efficiency of photogeneration of electrons and holes has been studied in crystals of orthorhombic sulphur in the spectral range from 2.4 to 6 eV. The use of fast excitation pulses reduces space charge effects and makes it possible to investigate separately the contribution of either carrier to the transient photocurrent. At photon energies above 3.6 eV electron and hole generation rises sharply and approaches unit quantum efficiency near 4.8 eV. In the weekly absorbed range below 3.6 eV carrier generation is confined to the surface region. It may be associated with discrete centres and exciton diffusion. Detailed measurements of the absorption coefficient (to 3.8 eV) and the reflectivity (to 6 eV) have also been made. The results are interpreted by a model in which two absorption processes contribute to the optical properties. The non-photoconductive process leads to a band centred at 4.03 eV possessing a Gaussian shape over five orders of magnitude. This may be associated with localized excitons formed by electronic transitions coupled linearly to the vibrational modes. The photoconductive part of the absorption, as well as photogeneration and reflectivity data, place the onset of direct transitions near 4.2 eV, and suggest that the previously assigned value of 2.5 eV is incorrect.