Physicists from the Cavendish Laboratory have discovered groundbreaking methods to enhance the performance of organic semiconductors. By innovating ways to remove more electrons from these materials than previously possible and leveraging unique properties within a non-equilibrium state, they have achieved significant improvements for electronic devices.
Enhanced Electron Removal and Non-Equilibrium States
“Our goal was to understand the impacts of heavy doping in polymer semiconductors,” stated Dr. Dionisius Tjhe, a Postdoctoral Research Associate at the Cavendish Laboratory. Doping, the process of adding or removing electrons in a semiconductor, enhances its ability to conduct electrical current. In a recent Nature Materials paper, Tjhe and his colleagues explained how these novel insights could significantly enhance the performance of doped semiconductors.
Energy Bands and Advanced Doping Levels
Electrons in solids are organized into energy bands, with the valence band playing a crucial role in properties like electrical conductivity and chemical bonding. Typically, doping in organic semiconductors involves removing a small fraction of electrons from the valence band, creating holes that conduct electricity.
“Typically, only 10 to 20 percent of electrons are removed from the valence band of an organic semiconductor,” Tjhe explained. “However, in our study, we succeeded in completely emptying the valence band in two polymers. Even more remarkably, in one of these materials, we were able to extract electrons from the band beneath the valence band, which may be an unprecedented achievement in the field.”
Higher Conductivity in Deeper Energy Levels
Interestingly, the conductivity is significantly higher in the deeper valence band compared to the top one. Dr. Xinglong Ren, a co-first author of the study, noted, “Higher conductivity in deep energy levels could lead to more powerful thermoelectric devices, which convert heat into electricity. By finding materials with higher power output, we can make waste heat conversion into electricity more viable.”
Polymer Benefits and Broader Implications
While researchers believe the valence band emptying effect could be replicated in other materials, it is most easily observed in polymers. “The polymer’s energy band arrangement and disordered chains facilitate this effect,” Tjhe pointed out. “Achieving this in other semiconductors like silicon is more challenging. Understanding how to replicate this in other materials is our next crucial step.”
Innovative Methods to Enhance Thermoelectric Performance
Doping increases hole numbers but also raises ion counts, potentially limiting power. However, using a field-effect gate electrode allows researchers to control hole density without affecting ion numbers.
“By utilizing the field-effect gate, we discovered that modifying the hole density produced unusual results,” according to Dr. Ian Jacobs, a Royal Society University Research Fellow at the Cavendish Laboratory. “Surprisingly, adjusting the hole density with the field-effect gate, whether by adding or removing holes, always resulted in increased conductivity.”
Exploiting Non-Equilibrium State Properties
The researchers linked these surprising effects to a ‘Coulomb gap,’ a seldom-seen characteristic in disordered semiconductors. This phenomenon vanishes at room temperature but was observable at -30°C.
“Coulomb gaps are difficult to detect as they only manifest when the material cannot achieve its most stable state,” Jacobs explained. “In our material, ions become frozen at relatively high temperatures.” When electrons are added or removed in this frozen state, the material enters a non-equilibrium state, revealing the Coulomb gap.”
This non-equilibrium state allows both thermoelectric power output and conductivity to increase together, a significant improvement. The current limitation is that the field-effect gate only affects the material’s surface. Affecting the bulk could enhance power and conductivity even further.
Future Prospects
Despite remaining challenges, the researchers have outlined a clear path to improving organic semiconductor performance. Their work opens doors for further investigation, particularly in energy applications. “Tapping into transport within these non-equilibrium states continues to offer a promising approach for improving organic thermoelectric devices,” said Tjhe.
With these advancements, the potential for organic semiconductors in electronic devices and energy applications looks more promising than ever.
Source: University of Cambridge
