Harnessing vibration-induced enhancement of transport in functional materials with soft structural dynamics

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In inorganic semiconductors, such as silicon, the interaction of electronic excitations with lattice vibrations is an undesirable perturbation; it limits charge carrier mobilities and mediates non-radiative recombination. In low-dimensional functional materials with non-covalent bonding the structural dynamics is not a mere perturbation, it moves centre-stage: Some vibrational modes are very soft and strongly anharmonic so that electronic processes occur in a strongly fluctuating structural landscape. The traditional view is that the resulting strong electron-vibrational coupling is also detrimental: In organic semiconductors (OSCs), for example, electronic charges and neutral electron-hole pairs (excitons) are localized by a 'cloud' of lattice deformations, which causes charge mobilities and exciton diffusion lengths to be undesirably small, thus limiting performance of optoelectronic devices. We have recently discovered systems in which this traditional paradigm does not hold, but in which the structural dynamics is highly beneficial and mediates surprisingly fast, long-range excitation transport. This runs completely against models developed for traditional semiconductors such as silicon, for which phonons limit electronic transport. The mechanism involves vibrational modes coupling localized states near the band edges to highly delocalised states within the bands that can then transport charges and energy over unprecedentedly long length scales. This unique transient delocalization regime, in which excitations are effectively able to "surf on the waves" of structural lattice distortions, is not found in silicon and was first discovered in OSCs. Our goal is to explore similar physics in other functional materials with soft structural dynamics, such as hybrid organic-inorganic perovskite (HOIP) semiconductors, 2D conjugated covalent/metal organic frameworks (COFs/MOFs) and inorganic ceramics and ion conductors.VISION AND AMBITION: In the proposed programme we aim to pursue this vibration-enhanced transport (VET) regime as a general paradigm for achieving fast and long-range electronic charge, ion and energy transport in a broad class of organic and inorganic, functional materials with soft structural dynamics. We will (i) develop new experimental/theoretical methodologies to achieve a deep fundamental understanding of the underpinning mechanisms for the vibration-enhanced transport, including identification and molecular engineering of the most effective vibrational modes mediating it, (ii) design new self-assembled functional materials in which transport length scales exceeding micrometers are achievable and (iii) exploit such long length scales to enable new device architectures and transformational device performance improvements in a broad range of (bio)electronic, optoelectronic, energy storage and photocatalytic applications.