Recombination surface photovoltage

The light-induced creation of recombination centers causes the diffusion length to decrease with exposure time as shown in Fig. 3 (Carlson et al., 1984b). The diffusion length was measured by the surface photovoltage method (Dresner et al., 1980), and similar results were obtained from an analysis of device characteristics (Faughnan et al., 1983). 

There are numerous techniques to measure the recombination lifetime. Some of the better known are photoconductive decay (13). diode reverse recovery (14). diode open circuit voltage decay (15). surface photovoltage (JL ) and forward-biased pn junction I-V characteristic (17. I will describe one particular photoconductive decay method, because it is a relatively new, non-contact method that requires no junctions. This makes it very suitable for a large number of measurements as for a process sequence characterization tool. 

The lower limit for short lifetimes in this technique is determined by the optical excitation source turn-off time to about 0.1 gs. For shorter lifetimes steady-state diffusion length measurements are more suitable. The diffusion leyth is related to the recombination lifetime by the equation L l/(Dt ). Suitable techniques are surface photovoltage and scanning electron microscope electron beam induced current. They lend themselves to lifetimes down to the nano-second range. 

To detomine the surface photovoltage is measured as a function of a. Vspv is kept constant during the measurement by adjusting O. Holding V py constant keeps the surface recombination velocity approximately constant during the measurement since the surface potential is approximately constant. If we further assume L and R to be independent of wavelength then we can rewrite Eq.(18) as... 

If c = 0, then VJ,h gives a measure of the flat-band potential provided r/re(i()x is known. In fact, this formula is very rarely obeyed in practice and deviations are both common and complex. Detailed theories of the potential distribution at the semiconductor-electrolyte interface have been presented, based on photovoltage measurements, but immense care needs to be taken in the interpretation of the photovoltage since kinetic effects apparently play a major role. This is especially true if surface recombination plays an important role [172]. 

In most cases, the iodide-triiodide redox couple has been employed (presumably because of its success in shuttling the photo-oxidized dye in the sensitization experiments) although other redox electrolytes e.g., SCN /(SCN)2 , [342] have also been employed. For the chalcogenide films, sodium selenosulfite was employed [319]. It must be noted that, aside from losses due to the surface recombination and back-reactions, an additional loss component from the increase in film resistance must also be recognized especially as the film thickness is increased. The resistance loss manifests as a deterioration in the photovoltage and fill factor. 

The photovoltages of 0.50 to 0.75 V obtained in the present work are fairly larger than those obtained in aqueous solutions ( 0.5 V) [2-4]. This may be due to a decrease in surface carrier recombination centers (such as surface penetrated H atoms) in p-Si by a decrease in the H concentration in solution, or due to a beneficial effect of TEA cations on the CO2 reduction reported in the literature [7-9]. Further studies are now in progress. 

In Section 2.4.1, we saw how the photovoltage of a photoelectrochemical cell can be maximised. There is, however, a thermodynamic limit, often called the detailed balance limit, on the photovoltage and consequently of the conversion efficiency. Corresponding theories have been pubhshed (Ross and Hsiao, 1977 Ross and Collins, 1980 Bolton et al, 1980). These theories are apphcable for photovoltaic cells as well as for photoelectrolysis ceUs, and yield a lower limit of a recombination rate which cannot be surpassed. The basic concept of the theory is as foUows. At equilibrium in the dark, the recombination fluxp.dark of radiative transitions across any plane in an ideal ceU is equal to the photon flux emitted by unit surface area of a blackbody, i.e. 

A second technique that has been used is the measurement of photovoltage. The basis of this technique is that, on illumination under open-circuit conditions, the potential distribution will be modified so as to eliminate the potential drop within the depletion layer. In fact, as has been demonstrated by Kautek and Gerischer [7], the theory of the photovoltage effect is far from straightforward, especially in the presence of surface states. The effect is a steady-state rather than equilbrium phenomenon the potential distribution will change until the flux of holes to the surface is equal to the flux of electrons and the potential at which this occurs will depend on the recombination kinetics at the surface. Only when these kinetics are slow, i.e. when the surface states are slow and the main surface state equilbrium is with the redox couple in solution, is the technique likely to give results that can be interpreted within a consistent framework. 

The experiment of White, Abruna and Bard (46a) sheds some light on how polymers may enhance the PEC response of semiconductor electrodes. n-WSe2 and n-MoSe2 showed quite variable oxidation waves when Immersed in aqueous solutions of Fe(CN) / and 13 / couples. In general, those electrodes with numerous surface imperfections (edges, steps) showed the poorest photovoltages and photocurrents. Surface Imperfections historically have been considered to be centers for the recombination of (e /h ) pairs, and they may act as catalysts for dark "back... 

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