Solving the Doping Problem: Improving Performance in Organic Semiconductors

“We really wanted to get to the heart of the matter and find out what happens when you heavily dope polymer semiconductors,” said Dr. Dionisius Tjhe, a research assistant professor at the Cavendish Laboratory. Doping is the process of removing or adding electrons to a semiconductor, increasing its ability to conduct electricity.

In a recently published article Natural materialsTjhe and his colleagues describe in detail how these novel findings could help improve the performance of doped semiconductors.

Energy bands with an unprecedented level of doping

Electrons in solids are organized into energy bands. The highest energy band, called 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. The holes, the missing electrons, can then flow and conduct electricity.

“Traditionally, you remove only ten to twenty percent of the electrons from the valence band of an organic semiconductor, which is already much higher than the typical parts-per-million levels in silicon semiconductors,” Tjhe said. “In two of the polymers we studied, we were able to completely empty the valence band. Even more surprisingly, in one of these materials we can go even further and remove electrons from the band below it. This may be the first time this has been achieved!”

Interestingly, conductivity is much greater in the deeper valence band compared to the upper one. “We hope that charge transport at deep energy levels could eventually lead to more powerful thermoelectric devices. They convert heat into electricity,” said Dr. Xinglong Ren, a research assistant professor at the Cavendish Laboratory and co-author of the first study. “By finding materials with higher output power, we can convert more waste heat into electricity and make it a more cost-effective energy source.”

Why was this observed in this material?

Although the researchers believe that emptying the valence band should be possible in other materials, the effect is probably most easily seen in polymers. “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,” Tjhe said. “In contrast, other semiconductors, such as silicon, are likely to be less likely to exhibit these effects because it is more difficult to empty the valence band in these materials. A key next step is to understand how to reproduce this result in other materials. This is an exciting time for us.”

Is there another way to increase thermoelectric efficiency?

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

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

Harnessing the Power of Disequilibrium

The researchers were able to trace these unexpected effects to the “Coulomb gap,” a well-known but rarely observed feature in disordered semiconductors. Interestingly, the effect disappears at room temperature, and the expected trend is restored.

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

“It turns out that in our material, the ions are freezing; this can happen at relatively high temperatures,” Ren said. “If you add or remove electrons while the ions are frozen, the material is in a non-equilibrium state. The ions would rather rearrange themselves and stabilize the system, but they can’t because they’re frozen. This allows us to see the Coulomb gap.”

There is usually a trade-off between thermoelectric output power and conductivity, one increases and the other decreases. However, due to the Coulomb gap and non-equilibrium effects, both can be increased together, meaning that efficiency can be improved. The only limitation is that the field gate currently affects only the surface of the material. If most of the material could be affected, this would increase the power and conductivity to even greater amounts.

Although the group still has some progress to make, the research presents a clear path to improving the performance of organic semiconductors. With exciting prospects for the energy field, the group has left the door open for further research into these properties. “Transport in these nonequilibrium states has once again proven to be a promising path to better organic thermoelectric devices,” Tjhe said.