Abstract
As a unique class of molecular electronic materials, organic donor–acceptor complexes now exhibit tantalizing prospect for heat–electricity interconversion. Over the past decades, in design of these materials for thermoelectric applications, consistent efforts have been made to synthesize a wide variety of structures and to characterize their properties. However, hitherto, one of the paramount conundrums, namely lack of systematic molecular design principles, has not been addressed yet. Here, based on ab initio calculations, and by comprehensively examining the underlying correlation among thermoelectric power factors, non-intuitive transport processes, and fundamental chemical structures for 13 prototypical organic donor–acceptor complexes, we establish a unified roadmap for rational development of these materials with increased thermoelectric response. We corroborate that the energy levels of frontier molecular orbitals in the isolated donor and acceptor molecules control the charge transfer, electronic property, charge transport, and thermoelectric performance in the solid-state complexes. Our results demonstrate that tailoring a suitable energy-level difference between donor’s highest occupied molecular orbital and acceptor’s lowest unoccupied molecular orbital holds the key to achieving an outstanding power factor. Moreover, we reveal that the charge-transfer-caused Coulomb scattering governs the charge and thermoelectric transport in organic donor–acceptor complexes.
Highlights
The ever-growing need for flexible, low-cost, and eco-friendly wearable power supply and cooling devices has sparked the booming market of organic-based thermoelectric (TE) materials1,2.The performance of a TE material is dictated by the dimensionless figure of merit, zT 1⁄4 PF κ T, where PFS2σ is the power factor, S is the Seebeck coefficient, σ is the conductivity, κ is the total thermal conductivity, and T is the temperature3
We find that the conduction bands (CBs) for 10, 11, and 12 display very narrow dispersions from R2–U2 and X–V2 (Supplementary Fig. 3h–j), which can result in large density of states near band edge; their valence bands (VBs) all show very dispersive characteristic, naturally leading to small density of states near band edge
We find that for the studied organic donor–acceptor complexes, the Coulomb scattering rather than the lattice vibration scattering plays a leading role in the hole and electron mobilities, because the evaluated mobilities caused by the Coulomb scattering are at least two orders of magnitude lower than those caused by the lattice vibration scattering, and the formers are much closer to the total mobility (Fig. 4a, b and Supplementary Table 11)
Summary
The ever-growing need for flexible, low-cost, and eco-friendly wearable power supply and cooling devices has sparked the booming market of organic-based thermoelectric (TE) materials. One of the current unsolved major puzzles is the lack of systematic materials design guidelines for this rising class of systems, and knowing very little about the basic structure–property relationships and the fundamental physical processes is the root cause of this problem. To deal with these issues, we carry out comprehensive first-principles computational investigations on the TE properties of 13 prototypical organic donor–acceptor complexes by using density functional theory, Boltzmann transport equation, Brooks–Herring approach, and deformation potential (DP) theory. We create an intuitive and general molecular roadmap for rational design of organic donor–acceptor complexes with high-performance TE response and, concurrently, we provide a unified understanding of correlation among their power factors, nontrivial physical processes, and elementary chemical structures
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