Charge transfer at the semiconductor-electrolyte interface

A semiconductor is characterised by its energy bands, Le. by the conduction and valence band and its Fermi level. In the bulk of the semiconductor, the position of the Fermi level depends on the doping. It is related to the electron and hole densities by 

Taking a simple one-step redox couple as an example, the electrochemical potential of electrons is given by the Nernst equation 

As explained in Appendix lA, the absolute energy scale is related to the redox potential Uredox on the SFIE scale by 

At equilibrium, the Fermi levels are equal on both sides of the interface. 

The actual position of the energy bands at the semiconductor surface can be experimentally determined by investigating the charge and potential distribution across the interface. The latter is composed essentially of the potential drop 0h across the Helmholtz layer and 4 sd across the space-charge layer below the semiconductor surface. Thus, the total potential difference across the interface is given by 

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While many of the standard electroanalytical techniques utilized with metal electrodes can be employed to characterize the semiconductor-electrolyte interface, one must be careful not to interpret the semiconductor response in terms of the standard diagnostics employed with metal electrodes. Fundamental to our understanding of the metal-electrolyte interface is the assumption that all potential applied to the back side of a metal electrode will appear at the metal electrode surface. That is, in the case of a metal electrode, a potential drop only appears on the solution side of the interface (i.e., via the electrode double layer and the bulk electrolyte resistance). This is not the case when a semiconductor is employed. If the semiconductor responds in an ideal manner, the potential applied to the back side of the electrode will be dropped across the internal electrode-electrolyte interface. This has two implications (1) the potential applied to a semiconducting electrode does not control the electrochemistry, and (2) in most cases there exists a built-in barrier to charge transfer at the semiconductor-electrolyte interface, so that, electrochemical reversible behavior can never exist. In order to understand the radically different response of a semiconductor to an applied external potential, one must explore the solid-state band structure of the semiconductor. This topic is treated at an introductory level in References 1 and 2. A more complete discussion can be found in References 3, 4, 5, and 6, along with a detailed review of the photoelectrochemical response of a wide variety of inorganic semiconducting materials. 

There is a growing tendency to invoke surface states to explain electron transfer at semiconductor-electrolyte interfaces. Too frequently the discussion of surface states is qualitative with no attempt to make quantitative estimates of the rate of surface state reactions or to measure any of the properties of these surface states. This article summarizes earlier work in which charge transfer at the semiconductor-electrolyte interface is analyzed as inelastic capture by surface states of charge carriers in the semiconductor bands at the surface. This approach is shown to be capable of explaining the experimental results within the context of established semiconductor behavior without tunneling or impurity conduction in the bandgap. Methods for measuring the density and cross section of surface states in different circumstances are discussed. 

As discussed above only surface charging is of real importance for the interpretation of charge transfer at the semiconductor/electrolyte interface. The formation of a conducting layer leads to a solid state device, for which there is no need to be placed directly into the electrolyte. An insulating layer, on the other hand, can not improve charge transfer, except perhaps for very thin layers that allow electron tunneling. The band position at oxidized parts of the surface is not known. 

Tafalla, D. and Salvador, P. 1987. Mechanisms of charge transfer at the semiconductor-electrolyte interface oxygen electroreduction at naked and platinized n-Ti02 electrodes. Ber. Bunsenges. Phys. Chem., 91,475M79. 

FF = 0.68, and rj = 11.7%. The improvement in the photoelectrochemical solar cell properties has been ascribed to the formation of n-CdSe/n-WOs heterojunctions, which enhances the charge transfer at the semiconductor/electrolyte interface. These results indicated for the first time the interesting effects of STA and PTA on chemically deposited CdSe films. This opens up a new method for fabricating mixed electrodes with improved physical properties and photoelectrochemical solar cell performances. 

Similar photovoltaic cells can be made of semiconductor/liquid junctions. For example, the system could consist of an n-type semiconductor and an inert metal counterelectrode, in contact with an electrolyte solution containing a suitable reversible redox couple. At equilibrium, the electrochemical potential of the redox system in solution is aligned with the Fermi level of the semiconductor. Upon light excitation, the generated holes move toward the Si surface and are consumed for the oxidation of the red species. The charge transfer at the Si/electrolyte interface should account for the width of occupied states in the semiconductor and the range of the energy states in the redox system as represented in Fig. 1. 

The semiconductor electrode must be ideally polarizable over the potential range of interest. This means that there is no leakage current or Faradaic reaction to allow charge transfer across the semiconductor-electrolyte interface. This restriction is not too important if measurements are taken at sufficiently high frequency that the effects of Faradaic reactions are suppressed. 

Nishida M (1980) A Theoretical treatment of charge transfer via surface states at the semiconductor electrolyte interface. Analysis of water electrolysis process. 

The electron transfer reactions at the semiconductor/electrolyte interface occur either via the conduction band or the valence band. The total current is therefore given by the sum of four partial currents, denoted as represent electron transfer via the conduction anc valence bands, respectively, and the superscripts, a and c, indicate anodic anc cathodic processes, respectively. Let us assume nereafter that the electron transfer occurs only via the conduction band. In a simple case where the concentration of the electrolyte is sufficiently high and only the overvoltages at the Helmholtz layer (tjh) and in the space charge layer (rjsc) are important, the ica and cc can be given as follows4)... 

We have thus far talked about the chemisorption of ions at the semiconductor/electrolyte interface and charge transfer in the semiconductor surface layer. The main charge transfer process of interest is the transfer of electrons and holes across the semiconductor/electrolyte interface to the desired electrolyte species resulting in their oxidation or reduction. For any semiconductor, electrode charge transfer can occur with or without illumination and with the junction biased in the forward or reverse direction. 

Recombination in the depletion layer can become important when the concentration of minority carriers at the interface exceeds the majority carrier concentration. Under illumination minority carrier buildup at the semiconductor-electrolyte interface can occur due to slow charge transfer. Thus surface inversion may occur and recombination in the depletion region can become the dominant mechanism accounting for loss in photocurrent. 

Unpinning of band edges at the semiconductor/electrolyte interface is understood as a common phenomenon for n- and p-type materials. Thus, the band edge positions as obtained from Hatband potential measurements in the dark, cannot be taken as a fixed value for the interpretation of charge transfer processes. More investigations in this direction are necessary. 

FIGURE 1.10. An equivalent circuit for the electrical components at the semiconductor/electrolyte interface in the absence of an oxide. represents the resistance of the electrolyte Ch is the capacity of the Helmholtz double layer and Rf is the charge transfer resistance 0, and Ru are the capacitance and resistance associated with the space charge layer in the semiconductor C, and are the capacitance and resistance of the surface states. 

Surface states and crystal imperfections have been found to play an important role in charge-transfer and redox reactions at the semiconductor-electrolyte interface (see Refs. 161-173). Mathematical and conceptual relationships have been developed which describe electrochemical reactions at the semiconductor-electrolyte interface in terms of surface states and potentials (see, e.g., Refs. 17, 71, and 174-182). Electrochemical reaction via surface states has been included within an analytic model,183 but this model is still limited by the restrictions described above. 

For n-type semiconductors, the fifth requirement implies that hole transfer across the semiconductor/electrolyte interface should be sufficiently fast in order to compete with the anodic decomposition reaction. More generally, interfacial charge transfer should be fast enough to avoid the accumulation carriers at the surface, as this would lead to a decrease of the electric field and a concomitant increase in electron-hole recombination. To improve the kinetics of charge transfer, catalyti-cally active surface species can be added. Examples of effective oxygen evolution catalysts are Ru02 [87], IrO [88], and Co-based compounds [64], whereas Pt, Rh [89], Cr-Rh, RuOa, or NiO [73, 90] are usually employed as a catalysts for hydrogen evolution. 

Nanostructured electrode morphologies can be used to address some of the intrinsic material s limitations and trade-offs mentioned above. The most obvious advantage of a nanostructured morphology is the increase in specific surface area. The concomitant increase in the number of surface sites greatly enhances the overall charge transfer kinetics at the semiconductor/electrolyte interface. 

Diffusion of I3 ions to the counter electrode (see below) must be rapid. Electron transport through the semiconductor to the working electrode must be faster than charge recombination at the semiconductor-sensitizer interface. Electrons flow around the outer circuit, performing electrical work. On reaching the counter electrode, electrons are transferred into the electrolyte, regenerating 1 (right-hand side of Fig. 28.7) ... 

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