Junction semiconductor- redox electrolyte

Figure 2.1 Energy band schematic for an n-type semiconductor-redox electrolyte junction. Dotted lines indicate conduction and valence band for the situation of equilibration of semiconductor Fermi level Ep and solution redox energy ro solid lines indicate the semiconductor bands when a cathodic potential V Is applied, resulting in a...

Figure IV.9 Model of a photovoltaic cell with a semiconductor -redox electrolyte junction.

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... 

The simplest photoelectrochemical cells consist of a semiconductor working electrode and a metal counter electrode, both of which are in contact with a redox electrolyte. In the dark, the potential difference between the two electrodes is zero. The open circuit potential difference between the two electrodes that arises from illumination of the semiconductor electrode is referred to as the photovoltage. When the semiconductor and counter electrode are short circuited, a light induced photocurrent can be measured in the external circuit. These phenomena originate from the effective separation of photogenerated electron-hole pairs in the semiconductor. In conventional photoelectrochemical studies, the interface between the flat surface of a bulk single crystalline semiconductor and the electrolyte is two dimensional, and the electrode is illuminated from the electrolyte side. However, in the last decade, research into the properties of nanoporous semiconductor electrodes interpenetrated by an electrolyte solution has expanded substantially. If a nanocrystalline electrode is prepared as a film on a transparent conducting substrate, it can be illuminated from either side. The obvious differences between a flat (two dimensional) semiconductor/ electrolyte junction and the (three dimensional) interface in a nanoporous electrode justify a separate treatment of the two cases. 

The principle of operation of a photoelectrochemical solar cell (PECS) is shown in Figure 2.12 where the absorber is a semiconducting material. The rectifying junction is formed between redox electrolyte and semiconductor, as also shown in... 

When a semiconductor comes in contact with another material of Fermi level, different from the semiconductor, a junction is formed. This can be formed between -type and p-type semiconductors, or a metal and a semiconductor, or a semiconductor and a redox electrolyte. 

What is the role of the constituents of the redox electrolyte present at the interface of the semiconductor-electrolyte junction Choice and optimization of the electrolyte has a substantial effect on PEC solar cells [18-19]. Before a semiconductor is immersed in an electrolyte, anions and cations freely and randomly move in the solution. As a result of this movement, no specific spatial accumulation of ions occurs in the solution. This situation alters... 

In a PEC, light is incident on the n-type semiconductor/electrolyte junction (photoanode), where light absorption occurs and an electron-hole pair is formed. The pair is separated by the strong electric field found just beneath the semiconductor surface, and the hole is driven towards the interface between semiconductor and electrolyte. Charge transfer to the redox species A contained in the electrolyte results in the oxidation to Conversely, the electron is driven to the metal/electrolyte interface (counter electrode), where the redox species is reduced. No net chenaical work is done and we can extract the energy as a current from the cell. 

Following the same procedure, the kinetic constants have been determined for very different electrochemical conditions. When n-WSe2 electrodes are compared in contact with different redox systems it is, for example, found9 that no PMC peak is measured in the presence of 0.1 M KI, but a clear peak occurs in presence of 0.1 M K4[Fe(CN)6], which is known to be a less efficient electron donor for this electrode in liquid junction solar cells. When K4[Fe(CN)6] is replaced by K3[Fe(CN)6], its oxidized form, a large shoulder is found, indicating that minority carriers cannot react efficiently at the semiconductor/electrolyte junction (Fig. 31). 

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