Silicon semiconductor/electrolyte junction

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>).

This section and the next are dedicated to the basics of the silicon-electrolyte contact with focus on the electrolyte side of the junction and the electrochemical reactions accompanying charge transfer. The current across a semiconductor-electrolyte junction may be limited by the mass transport in the electrolyte, by the kinetics of the chemical reaction at the interface, or by the charge supply from the electrode. The mass transport in the bulk of the electrolyte again depends on convection as well as diffusion. In a thin electrolyte layer of about a micrometer close to the electrode surface, diffusion becomes dominant The stoichiometry of the basic reactions at the silicon electrode will be presented first, followed by a detailed discussion of the reaction pathways as shown in Figs. 4.1-4.4. 

Semiconductor-electrolyte interface, photo generation and loss mechanism, 458 Semiconductor-oxide junctions, 472 Semiconductor-solution interface, and the space charge region, 484 Sensitivity, of electrodes, under photo irradiation, 491 Silicon, n-type... 

Assuming that an efficient D-A type of molecule can be synthesized, it should be possible to deposit these molecules as a monolayer onto a glass slide coated with a metal such as aluminum or a wide bandgap semiconductor such as Sn(>2. With the acceptor end of the molecule near the conductor and with contact to the other side via an electrolyte solution it should be possible to stimulate electron transfer from D to A and then into the conductor, through an external circuit and finally back to D through the electrolyte. This would form the basis of a new type of solar cell in which the layer of D-A molecules would perform the same function as the p-n junction in a silicon solar cell (50). Only the future will tell whether or not this concept will be feasible but if nature can do it, why can t we ... 

This chapter is dedicated to the basics of the silicon-electrolyte contact, with emphasis on the semiconductor side of the junction. The phenomenology of the I-V curve is discussed, together with basic charge states of semiconductor electrode like accumulation, depletion and inversion. Electrostatic and electrodynamic properties will be described, with emphasis on the direct current (DC) properties of the semiconductor electrode, while alternating current (AC) properties are discussed in Section 10.2. Details of charge exchange and mass transport as well as details of the reactions at the microscopic level are considered in Chapter 4. 

A silicon electrode with a small amount of deposited noble metals such as Pt does not behave as a buried junction and the photoelectrode properties are still determined by contact of the electrolyte, not the metal. The photovoltaic response of the Pt-deposited silicon electrode is due to the incomplete coverage of the surface by Pt so that the interface energetics remains dominated by the contact of the semiconductor surface with the electrolyte. The mechanism of the hydrogen reduction processes involves first photoexcitation of electron-hole pairs followed by charging of the small Pt islands with excited electrons. The charged Pt islands react with H2O at a high rate. 

---