Apparatus used to measure the properties of the semiconductors. Credit: Dr Martin Statzs, Sirringhaus Lab

Scientists have enhanced organic semiconductors by achieving groundbreaking electron removal and leveraging non-equilibrium state properties, potentially boosting thermoelectric device efficiency.

Cavendish physicists have discovered two new ways to improve organic semiconductors. They found a way to remove more electrons from the material than previously possible and used unexpected properties in an environment known as the non-equilibrium state, boosting its performance for use in electronic devices.

“We really wanted to hit the nail and figure out what is happening when you heavily dope polymer semiconductors,’ said Dr Dionisius Tjhe, Postdoctoral Research Associate at the Cavendish Laboratory. Doping is the process of removing or adding electrons into a semiconductor, increasing its ability to carry electrical current.

In a recent paper published in Nature Materials, Tjhe and his colleagues detail how these novel insights could be helpful in improving the performance of doped semiconductors.

Energy bands at unprecedented levels of doping

Electrons in solids are organized into energy bands. The highest-energy band, referred to as the valence band, controls many of the 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, the absence of electrons, can then flow and conduct electricity.

Emptying of valence and deeper-valence bands through doping. Credit: Sirringhaus Lab

“Traditionally, only ten to twenty percent 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,” said Tjhe. “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 is significantly larger in the deeper valence band, compared to the top one. “The hope is that charge transport in deep energy levels could ultimately lead to higher-power, thermoelectric devices. These convert heat into electricity,” said Dr Xinglong Ren, Postdoctoral Research Associate at the Cavendish Laboratory and co-first author of the study. “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.”

Why was this observed in this material?

Although the researchers believe that the emptying of the valence band should be possible in other materials, this effect is perhaps 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,” said Tjhe. “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.”

Is there another way to increase the thermoelectric performance?

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

“Using the field-effect gate, we found that we could adjust the hole density, and this led to very different results,” explained Dr Ian Jacobs, Royal Society University Research Fellow at the Cavendish Laboratory. “Conductivity is normally proportional to the number of holes, increasing when the number of holes are 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!”