Physicists at Cavendish have found two innovative methods to enhance the performance of organic semiconductors. They discovered a technique to extract more electrons from the material than was previously achievable, and they utilized surprising properties in a condition known as the non-equilibrium state to boost its effectiveness in electronic devices. Doping, which involves adding or removing electrons from a semiconductor, enhances its electrical current carrying capacity.
“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.
In a recent article published in Nature Materials, Tjhe and his team explain how these new findings could be beneficial in enhancing the performance of doped semiconductors.
Electrons in solid materials are arranged into energy bands
The band with the highest energy, known as the valence band, governs many crucial physical properties like electrical conductivity and chemical bonding. Doping in organic semiconductors is accomplished by removing a small percentage of electrons from the valence band. This creates “holes”, or spaces where electrons used to be, which can then move and conduct electricity.
“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.”
What is the reason for these observations in this particular 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.”
Could there be an alternative method to enhance the thermoelectric efficiency?
Doping results in a rise in the quantity of “holes”, but it also boosts the count of ions, which can restrict the power. Fortunately, scientists have found a way to manage the number of holes without influencing the number of ions. They do this by using a special type of electrode called a field-effect gate. This allows them to enhance the performance of the material without any negative impact on its power.
“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!”
Utilizing the potential of the non-equilibrium condition.
The scientists managed to link these surprising outcomes to a phenomenon known as the ‘Coulomb gap’. This is a well-recognized but seldom seen characteristic in disordered semiconductors. What’s fascinating is that this effect vanishes at room temperature, and the anticipated pattern reappears. This means that the behavior of the semiconductor returns to what is typically expected when the temperature is similar to everyday conditions.
“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,” Jacobs added. “On the other hand, we were able to see these effects at much higher temperatures than anticipated, only about -30°C.”
“It turns out that in our material, the ions freeze; this can happen at relatively high temperatures,” said Ren. “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.”
Typically, there’s a balancing act between the thermoelectric power output and conductivity in a material – when one goes up, the other tends to go down. But due to the presence of the Coulomb gap and the effects of the non-equilibrium state, both these properties can be enhanced simultaneously, leading to an overall improvement in performance. The only catch is that the field-effect gate, which is used to control these properties, currently only influences the surface of the material. If we could extend its effect to the entire volume of the material, it would result in even greater power and conductivity.
While the research team still has some progress to make, their paper presents a clear strategy for enhancing the performance of organic semiconductors. This opens up exciting possibilities in the energy sector and paves the way for further exploration of these properties.
“Transport in these non-equilibrium states has once again proved to be a promising route for better organic thermoelectric devices,” said Tjhe.
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