The Semiconductor-Electrolyte Junction

Basic aspects of the semiconductor-electrolyte junction are discussed in the references given in the previous section. Here we summarize the main points, placing particular emphasis on their relevance to the characterization of thin-fUm PV materials. For further details on semiconductor characterization in general, the reader is referred to the excellent book by Schroder [139]. 

When a semiconductor is illuminated under depletion conditions with light of sufficient energy hv Eg, where Eg is the bandgap), electron-hole pairs are created 

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Figure 27. Minority charge carrier profiles near the semiconductor/electrolyte junction. calculated for a silicon interface at two different electrode potentials. Uf- -0.25 V and Uf= 5.0 V10 ((//= forward bias = t/ - Ufl>).

Following the same procedure, the kinetic constants have been determined for very different electrochemical conditions. When n-WSe2 electrodes are compared in contact with different redox systems it is, for example, found9 that no PMC peak is measured in the presence of 0.1 M KI, but a clear peak occurs in presence of 0.1 M K4[Fe(CN)6], which is known to be a less efficient electron donor for this electrode in liquid junction solar cells. When K4[Fe(CN)6] is replaced by K3[Fe(CN)6], its oxidized form, a large shoulder is found, indicating that minority carriers cannot react efficiently at the semiconductor/electrolyte junction (Fig. 31). 

Lemasson P, Etcheberry A, Gautron J (1982) Analysis of photocurrents at the semiconductor-electrolyte junction. Electrochim Acta 27 607-614... 

Lemasson P, Boutry AE, Tiiboulet R (1984) The semiconductor-electrolyte junction Physical parameters determination by photocurrent measurement throughout the Cdi xZnxTe alloy series. J Appl Phys 55 592-594... 

By contrast, electrolyte states are much more limited in their distribution than metal conduction band states so that in many cases electron transfer through surface states may be the dominant process in semiconductor-electrolyte junctions. On the other hand, in contrast to vacuum and insulators, liquid electrolytes allow substantial interaction at the interface. Ionic currents flow, adsorption and desorption take place, solvent molecules fluctuate around ions and reactants and products diffuse to and from the surface. The reactions and kinetics of these processes must be considered in analyzing the behavior of surface states at the semiconductor-electrolyte junction. Thus, at the semiconductor-electrolyte junction, surface states can interact strongly with the electrolyte but from the point of view of the semiconductor the reaction of surface states with the semiconductor carriers should still be describable by equations 1 and 2. 

Fig. 8.2. (a) Potential distribution across the semiconductor/electrolyte junction under depletion conditions, (b) Band bending corresponding to (a). 

Fig. 8.4. Profile of light intensity at the semiconductor electrolyte junction. W is the width of the depletion layer and L, is the hole diffusion length. The penetration depth of the light...

Allongne P., Blonkowski S. and Souteyrand E. (1992), Experimental investigation of charge-transfer at the semiconductor electrolyte junction , Electrochim. Acta 37, 781-797. 

Under depletion conditions, the withdrawal of majority carriers from the interfacial region of the semiconductor/electrolyte junction creates a space-charge layer consisting of ionised donors (for w-type semiconductors) or acceptors (for p-type semiconductors). The dependence of the space-charge density gs i on the potential... 

Figure 12.1 Simplest small-signal equivalent circuit representing the semiconductor/electrolyte junction under depletion conditions. = total series resistance, t p = parallel resistance due to charge transfer, Csd = space-charge layer capacitance.

FIGURE 11.5 Energy diagrams for the semiconductor/electrolyte junction under dark conditions (a, b) before contact and (c, d) after contact and electrostatic equilibration for (a, c) n-type and (b. d) p-type semiconductors. 

This method has widely been used in electrochemical measurements. It should be stressed, however, that direct application of Eq. (29) to experimental determination of (p( is based on several assumptions (often accepted without proof) concerning the properties of the semiconductor/electrolyte junction. These assumptions have been analyzed, for example, in Reference 38). Here we formulate the most important of these assumptions ... 

Figure 9. Energy diagram of the semiconductor/electrolyte junction (a) in darkness and (b) under illumination.

In the absence of surface recombination, all minority carriers that are collected by diffusion and migration in the semiconductor/electrolyte junction will eventually either transfer to redox species in the solution or react with the semiconductor itself leading to anodic or cathodic photodecomposition. Slow interfacial kinetics will result in the build up of photogenerated carriers at the interface, but unless photocurrent multiplication occurs, the saturation photocurrent will simply be determined by the light intensity, and the quantum efficiency will be unity. This means that the photocurrent contains no information about interfacial kinetics. In reality, most semiconductor/electrolyte interfaces are non-ideal, and a substantial fraction of the photogenerated electrons or holes do not take part in interfacial redox reactions because they recombine via surface states (see section 2.3.3). It is this competition between interfacial electron transfer and surface recombination that opens the way to obtain information about the rates of interfacial processes. 

The semiconductor-electrolyte junction is essentially an analog of the junction that is formed in a solid-state device such as a solar cell. For this reason, measurements of the EQE as a function of photon energy can be used to obtain information... 

EQE spectra can be analyzed to obtain the bandgap of absorber materials. The photocurrent response (Jphoto) of the semiconductor electrolyte junction is described by the Gartner equation [148] as... 

Figures 2.1 and 2.2. The light-generated minority carriers diiFuse and drift towards the electrolyte interface where charge transfer to the respective species (oxidized electrons reduced holes) occurs. The majority carrier current results in injection of the opposite carrier (here electrons) at the counter electrode-electrolyte interface where the opposite redox reaction takes place. The semiconductor-electrolyte junction shown here is characterized by a photovoltage and a photocurrent, that is, the solar cell is operating at or near its maximum power which, in efficient devices, is rather close to the open circuit condition. This is indicated in the inset of Figure 2.12. Therefore, a residual band bending has been shown and the photovoltage under these conditions is given by the quasi-Fermi levels at the surface. Here, only the quasi-Fermi level for holes is shown because Hf( ) only marginally differs from Ef, the Fermi level without illumination.

The Efb is a property of the semiconductor interface that depends on the electrolyte in which the measurement is made. The onset of photocurrent does not necessarily define the potential because other interfacial effects may delay the onset to a point beyond the transition from accumulation to depletion. The error from such interfacial effects could be on the order of a few millivolts to over a volt. One such interfacial effect might be the kinetic overpotential required to drive the reaction. This overpotential shifts the photocurrent onset in the cathodic direction for p-type samples and in the anodic direction for n-type samples. Therefore, catalysts are often deposited onto the electrode surface to minimize the overpotentials (see section Catalyst Surface Treatments ). However, the modification of electrode surfaces with catalysts may influence the semiconductor/electrolyte junction and surface states and additionally shift the Efb in unexpected ways. Ideally, the catalyst treatment will improve the accuracy of the measured by this technique, although effects such as Fermi level pinning may introduce a change in the band structure at the surface which may negate the improvement from a reduced kinetic overpotential. 

The semiconductor-electrolyte junction should not be confused with a metal-electrolyte junction. In the metal-electrolyte junction, the potential drop occurs entirely on the solution side and practically nil on the metal side, whereas for semiconductor-electrolyte interface, the potential drop occurs on the semiconductor side as well as the solution side. 

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