Surface states semiconductor-electrolyte interface

Fig. 3.13 Semiconductor-electrolyte interface (a) at equilibrium, (b) under reverse bias (c) under forward bias. Arrows denote direction of current flow [reduction reaction ox + e red], (d) Electron transfer mediated through surface states.

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

The study of surface states, therefore, is vital to an understanding of the semiconductor/ electrolyte interface. 

The Current-Potential Relation at a Semiconductor/Electrolyte Interface (Negligible Surface States)21... 

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. 

While the ability to treat capture cross sections theoretically is very primitive and the experimental data on capture cross sections are very limited this phenomenological parameter seems to be an appropriate meeting place for experiment and theory. More work in both of these areas is needed to characterize and understand the important role of surface states in electron transfer at semiconductor-electrolyte interfaces. 

Fig. 5.7 shows schematically the potential across a semiconductor-electrolyte interface. To understand it we have to take two additional effects into account. First, the liquid molecules usually show a preferred orientation at the surface. Their dipole moment causes a jump of the potential. Second, on a solid surface the electrons can occupy surface states. These extra electrons contribute to the potential. 

It is generally accepted that three major processes limit the photoelectrochemical current in semiconductors after a bandgap excitation [76]. These processes are schematically illustrated in the band diagram shown in Fig. 3.2. The bold arrows show the desired processes for efficient water splitting PEC cell after a bandgap excitation the transport of electrons to the back contact, the transfer of the hole to the semiconductor surface and the oxidation of water at the semiconductor/electrolyte interface. The three major limiting processes are a) bulk recombination via bandgap states, or b) directly electron loss to holes in the... 

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. 

In this section, we first consider a general model of the faradaic processes occurring at the semiconductor-electrolyte interface due to Gerischer [11]. From Gerischer s model, using the potential distribution at the interface, we may derive a Tafel-type description of the variation of electron transfer with potential and we will then consider the transport limitations discussed above. We then turn to the case of intermediate interactions, in which the electron transfer process is mediated by surface states on the semiconductor and, finally, we consider situations in which the simple Gerischer model breaks down. 

In the presence of a faradaic current, the a.c. response of a semiconductor becomes significantly more complex. Nevertheless, using the theory of Sect. 3, it is possible to derive expressions for the a.c. response of the semiconductor-electrolyte interface both in the simple case of electron transfer from CB to electrolyte and in the case where surface states play an intermediate role. 

The exponential dependence of the current on applied potential for p-type silicon and highly doped n-type silicon in the pore formation regime can be analyzed using the Gerischer model of the semiconductor/electrolyte interface [77]. In the absence of surface states, the hole current for a p-type semiconductor is given by ... 

The Mott-Schottky regime spans about 1 V in applied bias potential for most semiconductor-electrolyte interfaces (i.e., in the region of depletion layer formation of the semiconductor space-charge layer, see above) [15]. The simple case considered here involves no mediator trap states or surface states at the interface such that the equivalent circuit of the interface essentially collapses to its most rudimentary form of Csc in series with the bulk resistance of the semiconductor. Further, in all the discussions above, it is reiterated that the redox electrolyte is sufficiently concentrated that the potential drop across the Gouy layer can be neglected. Specific adsorption and other processes at the semiconductor-electrolyte interface will influence Ffb these are discussed elsewhere [29, 30], as are anomalies related to the measurement process itself [31]. Figure 7 contains representative Mott-Schottky... 

Figure 9. Three situations for an n-type semiconductor-electrolyte interface at equilibrium showing overlap of the redox energy levels with the semiconductor conduction band (a) with surface states (b) and with the semiconductor valence band (c). A discrete energy level is assumed for the surface states as a first approximation.

To make matters worse, the nonideal behavior of semiconductor-electrolyte interfaces as noted above is exacerbated when the latter are irradiated. Changes in the occupancy of these states cause further changes in Fh so that the semiconductor surface band-edge positions are different in the dark and under illumination. These complications are considered later. The surface states as considered above are shallow (with respect to the band-edge positions) and can essentially be considered as completely ionized at room temperature. However, for many oxide semiconductors, the trap states may be deep and therefore are only partially ionized. Specifically, they may be disposed with respect to the semiconductor Fermi level such that they are ionized only to a depth that is small relative to W [49]. The manifestation of such deep traps in the AC impedance behavior of semiconductor electrolyte interfaces has been discussed [14, 49]. 

Figure 21. Surface-state mediation of both minority carrier (i.e., hole) transfer and recombination for an n-type semiconductor electrolyte interface.

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. 

In an idealized case when the effect of the Helmholtz layer can be neglected, i.e., when Ch Csc, there is a negligible amount of surface states, that is. Css Qc. The total capacitance of the semiconductor/electrolyte interface described by Eq. (1.45) becomes C Csc. The interface capacitance as a function of the electrode potential then follows the Mott-Schottky equation ... 

J. N. Chazalviel and A. V. Rao, Optical absorption by surface states and atomic reorganization effects at the semiconductor/electrolyte interface, J. Electrochem. Soc. 134, 1138, 1987. 

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