Semiconductor interfaces, potential distribution

Fig. 4.1 Structure of the electric double layer and electric potential distribution at (A) a metal-electrolyte solution interface, (B) a semiconductor-electrolyte solution interface and (C) an interface of two immiscible electrolyte solutions (ITIES) in the absence of specific adsorption. The region between the electrode and the outer Helmholtz plane (OHP, at the distance jc2 from the electrode) contains a layer of oriented solvent molecules while in the Verwey and Niessen model of ITIES (C) this layer is absent...

Fig. 4.12 Dependence of concentrations of negative charge carriers (ne) and positive charge carriers (np) on distance from the interface between the semiconductor (sc) and the electrolyte solution (1) in an w-type semiconductor. These concentration distributions markedly differ if the semiconductor/electrolyte potential difference A cp is (A) smaller than the flat-band potential AF

A representative potential distribution across the interface is shown in Fig. 3.9(c), taking the potential of the bulk solution as zero. The potential difference across the space charge region (psd occurs over a larger distance than that of the Helmholtz layer (pn). For an n-type semiconductor, (psc results from the excess positive charge of ionized donors in the bulk of the space charge region within the... 

Figure 4.12 is an illustration of the potential distribution for n-type semiconductor particles at the semiconductor-electrolyte interface. There are two limiting cases of equation (4.8.11) for photo-induced electron transfer in semiconductors. For large particles the potential drop within the semiconductor is defined by ... 

The band bending at the semiconductor/liquid (electrolyte solution) interface can be understood by considering the potential distribution at this interface. In a case where the electrolyte solution contains a redox couple (R/Ox), which causes an electrochemical redox reaction,... 

The potential distribution in the space chaige layer can be obtained by solving the Poisson equation for a given charge distribution. For a semiconductor/elecirolyte interface such as that shown in Fig.4.2, the potential,0(x), at a distance, x, from the semiconductor surface is given as follows ... 

Fig. 4.2 Schematic illustrations of (a) the charge distribution, (b) the charge-density distribution, (c) the potential distribution, and (d) the band bending at the semiconductor/redox electrolyte interface, assuming that no surface charge nor surface dipole is present.

Study of the Potential Distribution at the Semiconductor-Electrolyte Interface in Regenerative Photoelectrochemical Solar Cells... 

Electrolyte Electroreflectance (EER) is a sensitive optical technique in which an applied electric field at the surface of a semiconductor modulates the reflectivity, and the detected signals are analyzed using a lock-in amplifier. EER is a powerful method for studying the optical properties of semiconductors, and considerable experimental detail is available in the literature. ( H, J 2, H, 14 JL5) The EER spectrum is automatically normalized with respect to field-independent optical properties of surface films (for example, sulfides), electrolytes, and other experimental particulars. Significantly, the EER spectrum may contain features which are sensitive to both the AC and the DC applied electric fields, and can be used to monitor in situ the potential distribution at the liquid junction interface. (14, 15, 16, 17, 18)... 

The Effect of Surface States on the Distribution of Potential in the Semiconductor Interface... 

In the dark, the junction between an extrinsic (doped) semiconductor and a redox electrolyte behaves as a diode because only one type of charge carrier (electrons for n-type and holes for p-type) is available to take part in electron transfer reactions. The potential distribution across the semiconductor/electrolyte interface differs substantially from that across... 

Fig. 16.1 Potential distribution at the semiconductor-electrolyte interface [Copyright Wiley-VCH Verlag GmbH Co. KGaA. Reproduced with permission from Memming (2001)]...

Photoelectrochemistry — In principle, any process in which photon absorption is followed by some electrochemical process is termed photo electro chemical, but the term has come to have a rather restricted usage, partly to avoid confusion with photoemission (q.v.). The critical requirements for normal photo electro chemical activity is that the electrode itself should be a semiconductor that the electrolyte should have a concentration substantially exceeding the density of -> charge carriers in the semiconductor and that the semiconductor should be reverse biased with respect to the solution. To follow this in detail, the differences in potential distribution at the metal-electrolyte and semiconductor-electrolyte interfaces need to be understood, and these are shown in Fig. 1, which illustrates the situation for an n-type semiconductor under positive bias. 

Fig. 4. Detailed potential distribution for a semiconductor-electrolyte interface.

The semiconductor-electrolyte interface normally encountered is far more complex than that reviewed here the most significant additional complexity that must be dealt with is the presence of uncompensated charge at the surface of the semiconductor, which may have a drastic effect on the potential distribution. In principle, this surface charge may arise in the following ways. 

Fig. 8. Potential distribution at the semiconductor-electrolyte interface for (a) a negative charge density at the surface and (b) a positive charge density at the surface.

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. 

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]. 

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