Comparison between Semiconductor- Metal junction and Semiconductor- Liquid Electrolyte junction- Dye sensitized solar cell

Comparison between Semiconductor-Metal junction and Semiconductor-Liquid Electrolyte junction.

 

Semiconductor-Liquid Electrolyte junction.
A contact between a semiconductor and a liquid electrolyte represents an electrochemical junction. The transport of charge to and from the semiconductor surface is performed by a redox couple, which is included in the electrolyte. The redox couple is two ionic species of different state of charge: oxidized species and reduced species. Like any other material, a redox couple has a Fermi level. The energy levels of the oxidized and reduced species can be associated with the conduction and valence band energies respectively. The current density in the electrolyte provides a current directed from the semiconductor to the electrolyte.
Junction formation and light effect on junction.
When an n-type semiconductor is immersed in a solution that contains a supporting electrolyte (for example, a salt) and a redox couple, charge (electrons) can move to and from the solid semiconductor to the liquid. For specification, we will consider the properties of a liquid junction with an n-type semiconductor. Due to the excess of electrons in the n-type material, electrons will generally flow from the semiconductor to the liquid. Because electrons are removed from the semiconductor, a region near its surface is formed that has net positive charge. This fixed positive charge creates an electric field within the semiconductor and is responsible for the photo activity of the semiconductor photo electrochemical system.
When light of energy greater than the semiconductor band gap energy is incident on the semiconductor, an excess electron and an excess hole will be created within the solid. Because of the electric field near the interface with the solution, the electron is driven away from the interface and the hole is driven toward the interface. When the hole reaches the interface, the hole can oxidize a species in solution. The electron is collected at the electrode’s metal contact, which is connected to external electronics or a load (for example, a light bulb) and ultimately to a counter electrode, where a reduction reaction occurs. The semiconductor working electrode is also connected to the metal counter electrode through the solution completing the circuit. The opposite argument holds for a p-type semiconductor, in which case electrons are driven to the semiconductor surface causing reduction reactions.

a)   Energy level in semiconductor and redox electrolyte shown on a common vacuum level.
b)  Semiconductor (n-type) electrolyte interface before and after equilibrium.
c)   Semiconductor (p-type) electrolyte interface before and after equilibrium.

 Semiconductor-Metal junction:-

Metal-semiconductor junction is the simplest type of charge separating junction. If we have an n-type semiconductor of work function Φ_n and metal of work function Φ_m, such that Φ_m > Φ_n, it is called a Schottky barrier.
 
 

 (a): Band profiles of n-type semiconductor and metal in isolation 

 

 (b): Band profiles of semiconductor-metal junction in equilibrium

When metal and semiconductor are separate from each other, the Fermi levels will look like in fig. (a). When they are in electronic contact, the Fermi levels will line up. This is achieved by the exchange of charge carriers across the junction, with the consequence that the layers approach the thermal equilibrium. The energy at the conduction band edge at the interface between semiconductor and metal is higher than in the bulk of the semiconductor. The electrostatic potential energy is shown in fig (b) by the change in E_vac.

The space charge region or depletion region is the region where there is a net charge.

As E_vac changes by a certain amount, so must the conduction and valence band energies, and by the same amount. This happens because the electron affinity and band gap are invariant in the semiconductor, and is called band bending.