Pushing organic semiconductors even further

The researchers identified a way to remove more electrons from the material than previously possible through leveraging unexpected properties in a non-equilibrium state environment, boosting its performance for electronic devices.

Post doctoral Research Associate at the Cavendish Laboratory, Dr Dionisius Tjhe, stated: “We really wanted to hit the nail and figure out what is happening when you heavily dope polymer semiconductors.” Doping is the method of removing or adding electrons to a semiconductor, which increases its electrical conductivity.

Published in the Nature Materials journal, Tjhe and his colleagues detailed how these novel insights could improve doped semiconductor’s performance.

New energy band doping heights

Electrons in solids are organised into energy bands. The highest-energy band, known as the valence band, controls many important physical properties, such as electrical conductivity and chemical bonding. Doping in organic semiconductors is achieved by removing a small fraction of electrons from the valence band. Holes – which are the absence of electrons – can then flow and conduct electricity.

Tjhe explained: “Traditionally, only 10-20% of the electrons in an organic semiconductor’s valence band are removed, which is already much higher than the parts per million levels typical in silicon semiconductors. In two of the polymers that we studied, we were able to completely empty the valence band. More surprisingly, in one of these materials, we can go even further and remove electrons from the band below. This could be the first time that’s been achieved!”

Interestingly, the conductivity was significantly larger in the deep valence band compared to the top one, Dr Xinglong Ren, Postdoctoral Research Associate at the Cavendish Laboratory & Co-Author of the Study, noted: “The hope is that charge transport in deep energy levels could ultimately lead to higher-power thermoelectric devices. These convert heat into electricity. By finding materials with a higher power output, we can convert more of our waste heat into electricity and make it a more viable energy source.”

Can it be applied across materials?

The researchers firmly believe that the emptying of the valence band should be possible in other materials, but this effect is perhaps most easily seen in polymers. As Tjhe explores: “We think that the way the energy bands are arranged in our polymer, as well as the disordered nature of the polymer chains, allows us to do this. In contrast, other semiconductors, such as silicon, are probably less likely to host these effects, as it is more difficult to empty the valence band in these materials. Understanding how to reproduce this result in other materials is the crucial next step. It’s an exciting time for us.”

Other methods of increasing thermoelectric performance

Doping increases the number of holes, but it also increases the number of ions, which limits the power. Researchers can control the number of holes without affecting the number of ions by using an electrode known as a field-effect gate.

Dr Ian Jacobs, Royal Society University Research Fellow at the Cavendish Laboratory, explains: “Using the field-effect gate, we found that we could adjust the hole density, and this led to very different results. Conductivity is normally proportional to the number of holes, increasing when the number of holes is increased and decreasing when they are removed. This is observed when we change the number of holes by adding or removing ions. However, when using the field-effect gate we see a different effect. Adding or removing holes always causes a conductivity increase!”

Harnessing non-equilibrium

The researchers traced these unexpected effects to a ‘Coloumb gap,’ a well-known, though rarely observed feature in disordered semiconductors. This effect disappears at room temperature and the expected trend is recovered.

Jacobs adds: “Coulomb gaps are notoriously hard to observe in electrical measurements because they only become visible when the material is unable to find its most stable configuration. On the other hand, we were able to see these effects at much higher temperatures than anticipated, only about -30°C.”

Ren continues: “It turns out that in our material, the ions freeze; this can happen at relatively high temperatures. If we add or remove electrons when the ions are frozen, the material is in a non-equilibrium state. The ions would prefer to rearrange and stabilise the system, but they can’t because they are frozen. This allows us to see the Coulomb gap.”

Usually, there is a trade-off between thermoelectric power output and conductivity: one increases whilst the other decreases. However, due to the Coulomb gap and the non-equilibrium effects, both can be increased together, improving performance. The only limitation is that the field-effect gate currently only affects the surface of the material. If the bulk of the material can be affected, it would increase the power and conductivity to even larger magnitudes.

Though the group still has some headway to make, the research paper outlined a clear method to improve the performance of organic semiconductors. With exciting prospects in the energy field, the group left the door open for further investigation of these properties. Tjhe concluded: “Transport in these non-equilibrium states has once again proved to be a promising route for better organic thermoelectric devices.”