Charge carrier transport (CT) in organic semiconducting materials is at the heart of many exciting and revolutionising technologies ranging from organic transistors, light-emitting diodes, curved or flexible displays and photovoltaic cells. Yet, our fundamental understanding of CT in the active layers of these devices that could rationalise experimental observations and guide further advances in the field is still rather limited, in stark contrast to inorganic semiconductors and metals. Charge carriers in organic semiconductors (OSs) are often described by one of two extremes: by a travelling wave propagating through the material or by a particle hopping from one molecular fragment to the next. Here we show that neither of these simplified pictures applies. Solving the time-dependent electronic Schrodinger equation coupled to nuclear motion for eight different organic molecular crystals, we find that the excess charge carrier forms a polaron delocalized over 10-20 molecules in the most conductive crystals. The polaron propagates through the material by diffusive jumps over several lattice spacings (nanometers) at a time during which it expands to about twice its size. Computed localization lengths and charge mobility are in excellent agreement with experimental estimates, where available, and exhibit strong positive correlation in agreement with the recently proposed transient localization theory. We show that off-diagonal electron-phonon coupling slows down charge propagation up to 7-fold in existing OSs suggesting that conformational locking in geometries with high pi-orbital overlap holds the key for new high mobility OSs.

Mechanism of charge carrier propagation through organic semiconductors from non-adiabatic molecular dynamics

2019 Fall National Meeting

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