Photocurrent evaluation

Band gaps in semiconductors can be investigated by other optical methods, such as photoluminescence, cathodoluminescence, photoluminescence excitation spectroscopy, absorption, spectral ellipsometry, photocurrent spectroscopy, and resonant Raman spectroscopy. Photoluminescence and cathodoluminescence involve an emission process and hence can be used to evaluate only features near the fundamental band gap. The other methods are related to the absorption process or its derivative (resonant Raman scattering). Most of these methods require cryogenic temperatures. 

A very useful parameter for evaluating the performance of a photoelectrolysis cell is the incident photon to current conversion efficiency (IPCE). This is a measure of the effectiveness in converting photons incident on the cell to photocurrent flowing between the working and counter electrodes. IPCE is also called the external quantum efficiency. 

Besides evaluating photoelectrodes for use in PECs, photoelectrochemical characterization can be used for other purposes. For example, photocurrent spectra of CD CdS has been used to measure the semiconductor bandgap (as f vs. hv), and agreement between the bandgap values measured by this method and by absorption spectroscopy for as-deposited and annealed films was found [78]. 

In the photoelectric method, the measured average work function is always less than the true since patches of high work function tend to be excluded from the emission process. Thus, the nonuniform distribution of adsorbate on a patch surface may cause a slight discrepancy in the evaluation of A. Experimentally, the photoelectric method has various limitations. Photocurrents of the order of 10 A. must be measured accurately in the region of vo, and for films of work function greater than 5 v., the threshold frequency lies in the far ultraviolet—a practical disadvantage. Furthermore, the method is inapplicable at pressures in excess of 10 mm. Hg because of ionization of the gas by collision. 

Fig. 4.9. (a, top) The 8iph/iph vs. v 1 dependence for W03 electrode sensitized by Dye II in monomeric form ( ) partially aggregated by coprecipitation with PD IV (O). The excitation wavelength 560 nm. / = 20 s. The total surface concentration of Dye II 10 8 mol cm 2. Electrolyte 0.25 M Na2S04. (b, bottom) The potential-time programme and corresponding photocurrent-time curves used for x evaluation. Hatched areas indicate the exposure periods. 

The exciton migration within aggregates of cyanine dyes and the possibility of oxygen diffusion into the porous dye film result in a bulk generation of photocurrent [80]. Photoholes produced due to the oxidation of excitons by molecular oxygen diffuse to the back contact. The diffusion coefficient of charge carriers in dye layer (Dc) can be evaluated from the potential-step chronoamperometric measurements in the indifferent electrolyte. Considering dye film as a thin-layer cell, the current vs. time dependence can be described as follows [81] ... 

The oxide layer of a metal such as copper may be seen as a semiconductor with a band gap, which may be measured by absorption spectroscopy or photocurrent spectroscopy and photopotential measurements. Valuable additional data are obtained by Schottky Mott plots, i.e. the C 2 E evaluation of the potential dependence of the differential capacity C. For thin anodic oxide layers usually electronic equilibrium is assumed with the same position of the Fermi level within the metal and the oxide layer. The energetic position of the Fermi level relative to the valence band (VB) or conduction band (CB) depends on the p- or n-type doping. Anodic CU2O is a p-type semiconductor with cathodic photocurrents, whereas most passive layers have n-character. 

Measurements of the photocurrent-voltage (/-F) and photovoltage properties describe the energy conversion properties of dye-sensitized photoelectrochemical cells and are probably the best fundamental method for their evaluation. This data can be acquired in a two- or three-electrode configuration. Figure 8 shows sche-... 

This equation holds for the case of small applied voltages when the extension of the scl is smaller than the oxide thickness. Evaluation of the slope of the 1 /C2cl (Arp, ) curve allows for the determination of the product erND. Once the sd reaches completely through the oxide film, the capacitance is determined by df, again yielding the conventional potential independent capacitance C/A = r o/df- A schematic representation of the relevant potential drops as well as the band structure is shown in Figure 1.3 for the case of the Ti/Ti02 system. An example of an illuminated surface (induced photocurrent) is also shown here, which is required later. 

Figure 12.24 shows a set-up for the measurements, including a photocalorimetric cell with 1 jjm to 1 mm thick electrodes. The relative changes of the sensor signal (and hence the relative change of the internal quantum efficiency of the photocurrent) are evaluated as a function of U. Figure 12.25 is an example of such a measurement. 

Deeper traps can in some cases be emptied by irradiation with light. Figure 8.42 shows as an example the excitation spectrum of the photocurrent in an anthracene crystal which was doped with 10 tetracene molecules and whose hole traps were previously filled at a lower temperature, so that the anthracene crystal contained tetracene radical-cations before the excitation. The excitation spectrum shows the 0,0 transition and the vibronic series of the energetically lowest doublet-doublet transition Di Dq in the tetracene radical-cation. The combined evaluation of the thermally and the optically-stimulated currents yielded in this special case a value = 0.42 eV for the depth of the hole traps represented by tetracene in an... 

Fig. 6.9 The case of two photoanodes (A and B) with distinctly different j-V responses that would yield similar efficiencies if analyzed as half-cells (a) whereas cOTrect evaluation in a full device (b) would clearly distinguish B as the better component that leads to a greater jsc- Also shown are the effects of shadowing on the photocathode. In comparison to a standalone photocathode (c), the photocurrent produced by a photocathode placed in tandem with photoanode B (d) will be greater than if it were placed in tandem with photoanode A (e)...

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