Thermionic Devices

Diamond based thermionic solar cellDiamond has a negative electron affinity, and this means that electrons can be emitted from its surface with little or no loss of energy. If a high voltage is used to extract the electrons, this is called 'field emission', and the electrons can be accelerated and made to strike a phosphor screen. This is the basis of so-called cold cathode field emission displays, which may be a competing technology for the lucrative LCD or plasma screen market.

Alternatively, the electrons can be extracted from the surface simply by heating the diamond to temperatures above around 350°C. This 'boiling' off of electrons is known as 'thermionic emission', and occurs at a much lower temperature in diamond than in other materials. The emitted electrons can be made to strike an electrode, so completing an electric circuit. Thus, heat has been converted into electricity which is exactly what is required for solar cells. The light/heat from the sun will be focused down using lenses onto a diamond thermionic device, and the resulting heat used to generate electricity. The hope is that this process will be significantly more efficient than existing silicon-based solar cells, making solar power a viable energy source at last!

At Bristol, we are experimenting with various types of diamond for use as the thermionic emission layer, including Li-doped nanodiamond particles which are simply inkjet-printed onto flat sheets of glass. We have already achieved very promising current levels at temperatures <400°C. The energy company EON have just given us a 1M euro grant to try to build a working solar panel based on this novel technology.

Surface Termination

The hydrogenated diamond surface has a negative electron affinity (NEA) which means that the bottom of the conduction band lies above the vacuum level. Thus, any electrons which can be promoted into the conduction band can be emitted readily into vacuum. However, this hydrogenated surface begins to desorb at temperatures above about 500°C. We are studying a number of alternative termination chemistries which provide NEA as well as being termperature stable. These involve various metal oxides, the most promising of which is LiO. The diamond surface is first oxidised using an O2 plasma or ozone lamp, and then a few nm of Li metal is evaporated onto this surface, and annealled in vacuum at about 400°C for 30mins. This causes the Li to react with the oxygen to form an almost ionic LiO bond on the surface. Simply rinsing in water washes away the excess (unreacted) Li, leaving a monolayer of LiO_terminated diamond. This has a huge NEA of ~ -4eV, while being stable in vacuum to 1000°C. Other potential terminations include MgO, VO, and various other transition metal oxides. Some of this work is being done in collaboration with Bob Nemanich's group at Arizona State University.

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