In this work, MCC researchers develop a quantum dynamical simulation approach revealing in atomistic detail how the charge carrier wavefunction moves along a temperature gradient in an organic molecular crystal resulting in thermoelectric charge transport.
Thermoelectric materials convert a temperature gradient into a voltage and have thus sparked enormous interest for use in renewable electricity generation through waste heat recovery. Good thermoelectric materials should be made from non-toxic, earth-abundant elements, have low thermal conductivity, high electrical conductivity and high Seebeck coefficients to maximize the energy-efficiency determining the figure of merit. Organic semiconductors (OS) have emerged as promising thermoelectric materials that fulfil most of these criteria, in particular low thermal conductivities. Moreover, their mechanical flexibility opens up entirely new applications inaccessible to inorganics, e.g., wearable thermoelectrics for recovery of human body heat.
A major drawback of OS compared to their inorganic counterparts are their limited Seebeck coefficients. Improving this important transport coefficient is challenging because we currently do not have a good understanding of the thermoelectric transport physics in these materials, in contrast to inorganic thermoelectrics. This is because charge carriers in OS form partially delocalized quantum objects “half way” between waves and particles that cannot be described by the standard theory used for inorganics (the Boltzmann transport equation).
Running extensive quantum dynamical simulations on ARCHER2, this work uncovers for the first time the detailed molecular-scale picture of how charge carriers propagate quantum mechanically from the hot to the cold end of an OS subject to a temperature gradient. Importantly, this study permits a deep understanding of the fundamental driving forces that causes the directed charge carrier motion and hence the build-up of a thermally-induced voltage in these materials.
In addition to providing a breakthrough in our fundamental understanding of thermoelectricity in organics, this work has important implications for the design of better organic thermoelectrics. It is proposed that the Seebeck coefficient should increase with increasing sensitivity of carrier delocalization to changes in temperature as this would lead to a stronger directional preference of charge carrier motion along the temperature gradient. The anticipated relationship between Seebeck coefficient and temperature-dependent carrier delocalization is expected to provide a powerful design rule for the discovery and development of new thermoelectric materials for use in wearable and low power electronics.
Outcomes:
New simulation methodology:
Non-adiabatic dynamics of charge carrier propagation in molecular organic semiconductors under a temperature gradient simulating the thermoelectric effect.
New Knowledge: Understanding the microscopic mechanism and thermoelectric transport in organic materials and proposals for design rules for better organic thermoelectrics.
Collaborators:
- Dr Jan Elsner, Filip Ivanovic, Aaron Dines (UCL)
- Dr Samuele Giannini (Institute for the Chemistry of OrganoMetallic compounds, CNR Pisa, Italy)
- Prof Henning Sirringhaus + group (University of Cambridge, Cavendish Laboratory)
Funding:
- EPSRC DTP studentship (EP/W524335/1)
- EPSRC (EP/W017091/1)
- MCC EPSRC (EP/l000202 and EP/R029431)
- ERC Consolidator Grant No 682539/SOFTCHARGE
- European Research council (101020872)
- Cambridge commonwealth, European & International Trust
- Chinese Scholarship council
- Royal Society (RP/R1/201082)
Publication:
- Elsner, Y. Xu, E. D. Goldberg, F. Ivanovic, A. Dines, S. Giannini, H. Sirringhaus, and J. Blumberger, “Thermoelectric transport in molecular crystals driven by gradients of thermal electronic disorder,” Sci. Adv., vol. 10, p. eadr1758, 2024. https://www.science.org/doi/10.1126/sciadv.adr1758
Contact:
Prof Jochen Blumberger, j.blumberger@ucl.ac.uk