Steady-state photocurrent measurement

The basic idea of most diffusion length measurement techniques is to generate a certain number of minority carriers inside the bulk Si, for example by illumination, and to measure the fraction of these carriers that diffuse to a collecting interface. This fraction can be determined capacitively [Bo6], as well as by measurements of the steady-state photocurrent [Dr2, Lei 1], The parameter obtained by these measurements is the minority carrier diffusion length ID of electrons in... 

More detailed studies are required in order to check whether the above correlation, established on the basis of relatively rapid potential sweep measurements, holds also for the steady-state photocurrents, i.e., in the situation when the Ti02 surface becomes covered with the peroxo-titanate species. These should also include water photocleavage experiments onto titanium dioxide powders loaded with some of the catalysts investigated by Contractor and Bockris. The difficulty, associated with the fact that most of... 

If krec = 0, the photocurrent simply follows the illumination step and contains no information about the rate of charge transfer at the interface. The comparison shows that, unlike IMPS and PEIS, light modulated microwave reflectivity measurements still provide kinetic information at high band bending where recombination is negligible and the steady state photocurrent is described by the Gartner equation. 

In the steady state the left-hand side of Eq. (13) is equal to 0. Measurables are the steady state photocurrent, photovoltage, light and dark current voltage characteristics, and the quantum efficiency spectrum (or IPCE). If the physical origin of in is known, the dependence of such DC measurements on variations in intensity, wavelength, and bias, deliver the parameters controlling (for example, the values of the diffusion length and diffusion constant in the case of diffusion limited transport). 

In a steady-state photocurrent experiment, a monochromatic light source is used to excite the semiconductor and the photocurrent and photovoltage are measured (Green [1992]). If the load resistance is zero, the short-circuit photocurrent is mea-smed and the photovoltage is zero. If the load is infinite, the photocurrent is zero and the open-circuit photovoltage is measured. 

The temperature dependence of steady state photocurrents in Ceo samples has been reported by a number of authors [9,12-15,19,20,26,28,31,35,37,39,41,43,45-47,51,53-55,57]. The magnitude and the details of this temperature dependence vary considerably among published studies. At room temperature, for example, measured values of the photo and dark conductivities cover the ranges 10 -10 (ficm) and 10 -10 (Dcm) , respectively. The large variations... 

Steady-state wavelength-specific photocurrents were measured for the Ti-Fe-0 films in a two-electrode arrangement at different applied voltages. Incident photon-to-current efficiencies (IPCE) are calculated using the following equation ... 

When an ion-pair recombines, it may form an excited state which can luminesce. The intensity of luminescence is a direct monitor of the competing ratio of recombination and luminescence. With steady-state conditions, the luminescence intensity is proportional to the rate of recombination. For instance, Morrow et al. [380] have radio lysed solutions of pyrene in cyclohexane. Solvated electrons and pyrene cations are produced. On recombination, an excited singlet state is produced which can fluoresce. If two pyrene molecules are in (or near) contact when one or other molecule is in the excited singlet state, then excimer fluorescence may be observed. The intensity of fluorescence can be decreased by application of an electric field, since fewer ion-pairs recombine to form the excited state. Jarnagin [381] and Holroyd and Russell [382] have photoionised iVjA iV. iV -tetramethyl-p-phenylenedia-mine (TMPD) with light (of photon energy 5.5—6 eV) in hydrocarbon solvents and measured the photocurrent at various electric field strengths. 

Fig. 34 Photocurrent (steady state) versus light intensity for PTS-polydiacetylene single crystals. Data for F II b taken under illumination with 457 nm line of Ar laser F L b measured using a tungsten lamp (457 nm). Electric field, F = 4 x 103 V cm-1. (After Lochner et al., 1976a)...

The above-discussed photocurrent measurements were performed under the steady-state illumination of electrodes. In the pulse illumination mode, the photo-... 

This section considers the competition between electron transfer and surface recombination under steady state conditions, and shows that no kinetic information can be obtained from the measured photocurrent. 

At t = 0, Qs = 0 and the measured photocurrent is entirely due to charging. At longer times the surface charge tends towards its steady state value, and the charging current falls to zero. The current due to charge transfer is proportional to Q so it is zero at t = 0 and increases with time towards its final steady state value. Since the recombination term also depends on Qs, the fraction of charge carriers at the surface that are transferred is determined by the ratio ktr/(ktr + krcc) for all values of t. 

Current-potential measurements, in the dark and under illumination of the semiconductor working electrode, are extremely useful for first defining the charge-transfer behavior across the interface before more sophisticated experiments are undertaken. The irradiation can be either continuous or intermittent (chopped) the latter mode has the distinct advantage that both the dark and light behavior can be examined in the same scan [55, 58]. Even some dynamic information can thus be extracted under the nominally steady-state conditions typical of a cyclic or linear potential sweep experiment. Another useful steady-state experiment is photocurrent spectroscopy (performed at a fixed DC potential) [55], although this can also be dynamically performed via IMPS (see below). Such measurements not only yield the so-called photoaction spectrum of the semiconductor electrode, but also afford information on surface recombination and surface state activity at the interface as discussed below. 

Another consequence of the presence of the series capacitance, Cp, is the reduction of the source impedance of the membrane in the high-frequency range. Because of this peculiar effect, a short-circuit measurement is harder to achieve in the high frequency range than in a steady state because a lower input impedance is required to achieve a short-circuit condition. Sometimes a commercial picoammeter is used to measure displacement photocurrents under a presumed short-circuit condition. However, such instruments often have an input impedance on the order of 100 kft in the megahertz frequency range. In view of the much reduced source impedance of a reconstituted purple membrane, these measurement conditions are actually closer to an open circuit than to a short-circuit condition. The measurement is often accompanied by a telltale observation that the measured current is the first time derivative of the measured photovoltage (26). A detailed analysis of this problem is presented elsewhere (19, 20). 

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