Photoelectrochemical interface states

Electrode Band Structure and Interface States in Photoelectrochemical Cells... 

K. Rajeshwar, Charge transfer in photoelectrochemical devices via interface states United model and comparison with experimental data, J. Electrochem. Soc. 129 (1982) 1003-1008. 

The CdSe was used as a photoelectrode in photoelectrochemical cells. The CdSe film doped with Zn, has favorable states in band gap and enhances charge-transfer kinetics at the interface. 

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. 

The promise of photoelectrochemical devices of both the photovoltaic and chemical producing variety has been discussed and reviewed extensively.Cl,, 3,4) The criteria that these cells must meet with respect to stability, band gap and flatband potential have been modeled effectively and in a systematic fashion. However, it is becomirg clear that though such models accurately describe the general features of the device, as in the case of solid state Schottky barrier solar cells, the detailed nature of the interfacial properties can play an overriding role in determining the device properties. Some of these interface properties and processes and their potential deleterious or beneficial effects on electrode performance will be discussed. 

Photoeffects at the semiconductor/RbAg Is interface were investigated with the objective of identifying the utility of this solid electrolyte material in solid-state photoelectrochemical devices with storage. 

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... 

In spite of a great number of investigations aimed at the preparation of photocatalysts and photoelectrodes based on the semiconductors surface-modified with metal nanoparticles, many factors influencing the photoelectrochemical processes under consideration are not yet clearly understood. Among them are the role of electronic surface (interfacial) states and Schottky barriers at semiconductor / metal nanoparticle interface, the relationship between the efficiency of photoinduced processes and the size of metal particles, the mechanism of the modifying action of such nanoparticles, the influence of the concentration of electronic and other defects in a semiconductor matrix on the peculiarities of metal nanophase formation under different conditions of deposition process (in particular, under different shifts of the electrochemical surface potential from its equilibrium value), etc. 

Jaegerman (1997) calls photoelectrochemical reactions that occur at high surface state configurations Bardeen model reactions because Bardeen was the first to discuss surface states. Here they are called Helmholtz reactions because the potential difference at interfaces at which they predominate is largely in the Helmholtz layer. 

There are two possible excited state interfacial electron transfer processes that can occur from a molecular excited state, S, created at a metal surface (a) the metal accepts an electron from S to form S+ or (b) the metal donates an electron to S to form S . Neither of these processes has been directly observed. The two processes would be competitive and unless there is some preference, no net charge will cross the interface. In order to obtain a steady-state photoelectrochemical response, back interfacial electron transfer reactions of S+ (or S ) to yield ground-state products must also be eliminated. Energy transfer from an excited sensitizer to the metal is thermodynamically favorable and allowed by both Forster and Dexter mechanisms [20, 21]. There exists a theoretical [20] and experimental [21] literature describing energy transfer quenching of molecular excited states by metals. How-... 

Photovoltaic devices are based on the concept of charge separation at an interface of two materials having different conduction mechanisms, normally between solid-state materials, either n- and p-type regions with electron and hole majority carriers in a single semiconductor material, heterojunctions between different semiconductors, or semiconductor-metal (Schottky) junctions. In photoelectrochemical cells, the junctions are semiconductor-electrolyte interfaces. In recent years, despite prolonged effort, disillusion has grown about the prospects of electrochemical photo-... 

Highlights of research results from the chemical derivatization of n-type semiconductors with (1,1 -ferrocenediyl)dimethylsilane, , and its dichloro analogue, II, and from the derivatization of p-type semiconductors with N,N -bis[3-trimethoxysilyl)-propyl]-4,4 -bipyridinium dibromide, III are presented. Research shows that molecular derivatization with II can be used to suppress photo-anodic corrosion of n-type Si derivatization of p-type Si with III can be used to improve photoreduction kinetics for horseheart ferricyto-chrome c derivatization of p-type Si with III followed by incorporation of Pt(0) improves photoelectrochemical H2 production efficiency. Strongly interacting reagents can alter semicon-ductor/electrolyte interface energetics and surface state distributions as illustrated by n-type WS2/I-interactions and by differing etch procedures for n-type CdTe. 

Figure 9.4 Recombination pathways of photogenerated charge carriers in an n-type semiconductor-based photoelectrochemical cell. The electron-hole pairs can recombine through a current density in the bulk of the semiconductor, the depletion region, or through defects (trap states) at the semiconductor/liquid interface, iss- Charges can also tunnel through the electric potential barrier near the surface, 4 or can transfer across the interface, The bold arrows indicate the favourable current processes in the operation of a photoelectrochemical cell. The hollow arrows indicate the processes that oppose the excess of charge carriers generated by light absorption.

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