The emergence of charge coherence in soft molecular organic semiconductors via the suppression of thermal fluctuations

Abstract

Organic semiconductors are already widely used in electronics. Nevertheless, their fundamental properties are still being debated. In particular, charge transport, which determines the performance of organic light-emitting diodes, solar cells and organic field-effect transistors, has been described by a wide range of complementary but incompatible theories. These theories involve either localized charge carriers hopping from molecule to molecule, leading to incoherent charge transport, or delocalized charge carriers moving around freely in the semiconductor, leading to coherent transport. In this communication, we reveal the first experimental evidence that charge coherence can be tuned from partially to fully coherent in one and the same material—pentacene—showing a continuous transition from one transport mechanism to the other. Microscopically, the transport mechanism depends on the overlap of adjacent molecular orbitals, which in turn is sensitive to molecular thermal fluctuations. We control these fluctuations through moderate variations of pressure and temperature, leading to a mobility increase of 75%. We quantify the influence of these thermal fluctuations by estimating the critical value below which fully coherent charge transport emerges. The ability to control thermal fluctuations and therefore to effectively tune the charge coherence is an important key to improving charge transport in soft molecular materials. A team in Japan experimentally shows that charge transport in an organic semiconductor can be tuned by varying the temperature and pressure. Charge transport in organic semiconductors has been much debated, with incompatible theories invoking either localized charge carriers hopping from molecule to molecule (incoherent transport) or delocalized charge carriers moving freely among the molecules (coherent transport) being put forward. Jun Takeya and colleagues at the University of Tokyo and the University of Tsukuba have realized a continuous change from one transport mechanism to the other in the aromatic hydrocarbon pentacene. They found that the transport mechanism depends on the degree to which the orbitals of neighbouring molecules overlap, which is sensitive to thermal fluctuations. Pushing molecules closer together through raising the pressure and lowering the temperature enhanced electronic coupling, boosting pentacene's mobility by 75 per cent. Our study reveals the first experimental evidence that charge transport can be tuned from partially to fully coherent. We used pentacene, a soft organic material, to measure transport properties depending on temperature and pressure. Under ambient conditions, charge transport was only partially coherent. At 1 GPa and below 220 K fully coherent charge transport emerged. Microscopically, we find that the thermal fluctuations are reduced so that the charge carriers start moving around freely leading to a coherent charge transport.