Abstract
A central challenge of organic semiconductor research is to make cheap, disordered materials that exhibit high electrical conductivity. Unfortunately, this endeavor is hampered by the poor fundamental understanding of the relationship between molecular packing structure and charge carrier mobility. Here a novel computational methodology is presented that fills this gap. Using a melt-quench procedure it is shown that amorphous pentacene spontaneously self-assembles to nanocrystalline structures that, at long quench times, form the characteristic herringbone layer of the single crystal. Quantum dynamical simulations of electron hole transport show a clear correlation between the crystallinity of the sample, the quantum delocalization, and the mobility of the charge carrier. Surprisingly, the long-held belief that charge carriers form relatively localized polarons in disordered OS is only valid for fully amorphous structures-for nanocrystalline and crystalline samples, significant charge carrier delocalization over several nanometers occurs that underpins their improved conductivities. The good agreement with experimentally available data makes the presented methodology a robust computational tool for the predictive engineering of disordered organic materials.
Highlights
Organic semiconductors (OS) are an exciting class of materials that have enabled disruptive technologies including large area electronics and displays, organic light emitting diodes [1, 2] and flexible solar cells [3, 4]
The delocalization of the polaron, and mobility, are limited by the thermal fluctuations of electronic coupling and site energy. This picture, emerging from direct propagation of the time-dependent electronic Schrodinger equation coupled to nuclear motion, resembles closely, and gives support to, the transport scenario predicted by alternative approaches including transient localization theory (TLT)[17, 18] and delocalized charge carrier hopping based on generalized Marcus theory[19] or polaron-transformed Redfield theory[20] mapped onto kinetic Monte Carlo.[21]
Samples of bulk pentacene with various degrees of crystallinity were created through the melting of a block of 3000 pentacene molecules followed by subsequent quenching to room temperature for quench times of 0 ns, 1, 10 and 100 ns, see Section 4 for simulation details
Summary
Organic semiconductors (OS) are an exciting class of materials that have enabled disruptive technologies including large area electronics and displays, organic light emitting diodes [1, 2] and flexible solar cells [3, 4]. Several experimental as well as computational studies have indicated that charge transport in crystalline molecular OS falls into a difficult regime where the charge is neither fully delocalized over the bulk material nor completely localized on a single molecule[5, 6, 7], as had often been assumed.[8, 9, 10, 11] We have recently shown using advanced quantum dynamical simulations, that charge carriers in single-crystalline OS form “flickering polarons”, objects that are half-way between waves and particles[12, 13, 14] We found they are delocalized over up to 10-20 molecules in the most conductive crystals and constantly change their shape and extension under the influence of the thermal motion of the atoms (crystal vibrations)[12] Taking the example of bulk crystalline pentacene, we found that the excess hole is typically delocalized over 17 molecules[12, 13], in excellent agreement with experimental estimates from electron spin resonance data[15]. Recent methodological developments have made it possible to apply this novel methodology, for the first time, to large samples of disordered OS with different nanoscale morphologies
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