Researchers have intensified organic semiconductors by achieving revolutionary electron extraction and exploiting non-equilibrium state characteristics, feasibly enhancing thermoelectric device efficiency.
The physicists at Cavendish have unearthed two innovative methods to optimize organic semiconductors. They devised a technique to eliminate more electrons from the material than previously feasible and utilized unforeseen characteristics in a setting known as the non-equilibrium state, amplifying its effectiveness for utilization in electronic devices.
“Our primary aim was to investigate deeply and uncover the mechanisms when you intensely dope polymer semiconductors,” remarked Dr Dionisius Tjhe, Postdoctoral Research Associate at the Cavendish Laboratory. Doping involves the removal or addition of electrons into a semiconductor, augmenting its capacity to conduct electrical current.
In a recently published article in Nature Materials, Tjhe and his comrades elaborate on how these innovative perceptions could be beneficial in advancing the performance of doped semiconductors.
Energy bands at unparalleled levels of doping
Electrons in solids are categorized into energy bands. The top-energy band, termed the valence band, governs several crucial physical traits such as electrical conductivity and chemical bonding. Doping in organic semiconductors is achieved by extracting a minor fraction of electrons from the valence band. Consequently, holes, which represent the absence of electrons, can flow and facilitate electricity conduction.
“Historically, only ten to twenty percent of the electrons in an organic semiconductor’s valence band were extracted, significantly higher than the parts per million levels customary in silicon semiconductors,” conveyed Tjhe. “In two of the polymers that we investigated, we successfully emptied the valence band entirely. Even more remarkably, in one of these materials, we could delve deeper and extract electrons from the underlying band. This could mark a milestone achievement!”
Remarkably, the conductivity is considerably higher in the deeper valence band compared to the upper one. “The aspiration is that charge transport in deep energy levels might eventually lead to more potent thermoelectric devices. These devices convert heat into electricity,” expounded Dr Xinglong Ren, Postdoctoral Research Associate at the Cavendish Laboratory and co-first author of the research. “By discovering materials with enhanced power output, we can convert more of our wasted heat into electricity and transform it into a more sustainable energy source.”
What caused this phenomenon in this substance?
Although the researchers surmise that clearing the valence band should be achievable in other substances, this phenomenon is perhaps most discernible in polymers. “We speculate that the arrangement of the energy bands in our polymer, as well as the disordered structure of the polymer chains, enable us to accomplish this,” ascertained Tjhe. “In contrast, other semiconductors, like silicon, are presumably less predisposed to these effects, as it is more challenging to evacuate the valence band in these materials. Understanding how to replicate this outcome in other materials is the pivotal next phase. It’s a thrilling era for us.”
Is there an alternative approach to magnify the thermoelectric efficiency?
Doping causes a rise in the number of holes, but it also escalates the number of ions, which restricts the power. Fortunately, researchers can regulate the number of holes without influencing the number of ions by employing an electrode termed a field-effect gate.
“Through the utilization of the field-effect gate, we observed that we could manipulate the hole density, leading to diverse outcomes,” explicated Dr Ian Jacobs, Royal Society University Research Fellow at the Cavendish Laboratory. “Normally, the conductivity is directly proportional to the number of holes, escalating when the hole count is increased and decreasing when they are abated. This trend emerges when the hole count is modified by injecting or removing ions. Conversely, with the field-effect gate, a distinct effect emerges. Enhancing or decreasing the hole count invariably induces a conductivity rise!”
Harnessing the potency of the non-equilibrium state
The scientists managed to attribute these unforeseen effects to a ‘Coulomb gap’, a widely recognized, albeit seldom observed feature in disordered semiconductors. Intriguingly, this effect dissipates at room temperature, and the anticipated pattern is reestablished.
“Coulomb gaps are notoriously elusive to detect in electrical assessments, as they become discernible solely when the material is unable to attain its most stable configuration,” added Jacobs. “In contrast, we managed to detect these effects at markedly higher temperatures than anticipated, only about -30°C.”
“It transpires that in our material, the ions crystallize; this can occur at relatively elevated temperatures,” disclosed Ren. “When we introduce or remove electrons while the ions are crystallized, the material enters a non-equilibrium state. The ions would desire to rearrange and stabilize the system, yet they are immobilized due to the crystallization. This enables us to observe the Coulomb gap.”
Typically, there exists a compromise between thermoelectric power output and conductivity, where one amplifies while the other dwindles. However, due to the Coulomb gap and the non-equilibrium impacts, both can burgeon simultaneously, augmenting the performance. The sole constraint is that the field-effect gate solely influences the surface of the material currently. If the bulk of the material can be influenced, the power and conductivity could reach even greater magnitudes.
While the team still has more groundwork to cover, the research paper delineates a definitive approach to ameliorate the performance of organic semiconductors. Holding promising opportunities in the energy domain, the team has opened the door for further exploration of these traits. “Transport within these non-equilibrium states has, once again, demonstrated a promising pathway for enhanced organic thermoelectric devices,” asserted Tjhe.
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