On the mechanism of charge transport in pentacene

Abstract

Terahertz transient conductivity measurements are performed on pentacene single crystals, which directly demonstrate a strong coupling of charge carriers to low frequency molecular motions with energies centered around 1.1 THz. We present evidence that the strong coupling to low frequency motions is the factor limiting the conductivity in these organic semiconductors. Our observations explain the apparent paradox of the "bandlike" temperature dependence of the conductivity beyond the validity limit of the band model. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.2955462͔Recent years have seen much interest in the conductivity of pentacene and other molecular semiconductors of the oligoacene family, fueled by the unique properties of these materials. Oligoacenes are flexible, cheap, and exhibit relatively high charge carrier mobilities, 1 allowing for novel applications in electronic devices. [2][3][4] Despite this interest, the nature of charge transport in the materials has remained the subject of intense debate.Crystalline organic semiconductors exhibit charge mobilities that decrease with temperature. [5][6][7][8][9] In analogy to inorganic semiconductors, this has been interpreted as an indication of delocalized charge carriers and a "bandlike" transport mechanism. 10-13 Theoretical models based on polaronic band conduction 14 indeed provide a good qualitative description of the bandlike mobility of charge carriers in pentacene, but the factors determining the absolute charge mobility remain unknown. For instance, an analysis of the high temperature mobility has indicated that the mean free path of charge carriers is of the same magnitude as the intermolecular distance. 15 It is not clear as to why the signature of delocalized bandlike transport persists at these temperatures, where the mean free path is smaller than the intermolecular distances. Because of the "softness" of these van der Waals bonded organic crystals, it is possible that the coupling of the carriers with the intermolecular vibrations could play a key role in the description of the charge carrier dynamics. Troisi and Orlandi 16 have recently formulated a model ͑TO model͒ that states that the charge mobility in oligoacenes is limited by large fluctuations of the electronic coupling between adjacent organic molecules due to thermal molecular motions, in particular, the low energetic phonon modes with energies around 1 THz. 17 This model correctly describes the bandlike temperature dependence of the mobility, including its absolute value, outside the validity of delocalized transport models. 16 Direct experimental evidence for the various proposed models has been lacking. In particular, experimental insights into the role of intermolecular low frequency vibrational modes in determining the charge transport mechanism have been missing to date.Terahertz spectroscopy 18 provides the opportunity of studying the response of charge carriers in the frequency range of these modes, and has proven useful for investigating photocarrier dynamics in...

Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

“…This was directly verified in a terahertz experiment on pentacene where on charge injection a pronounced absorption peak at 1.1 THz was observed (Laarhoven et al, 2008). This peak was attributed to a charge-induced activation of a soft vibrational mode at that frequency that couples strongly to the charge carrier dynamics.…”

“…(6) under the neglect of the reflection contribution, which is not altered by photoexcitation for this case. Equation (7) applies to a broad range of THz pump-probe experiments where the condition of ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ð þ Á Þ p 2 l 0 =c ( 1 is fulfilled [see, e.g., (Hendry, Koeberg, O'Regan, and Bonn (2006) and Laarhoven et al (2008)]. …”

Trap density of states in small-molecule organic semiconductors: A quantitative comparison of thin-film transistors with single crystals

“…[13,15] Interestingly, the importance of thermal disorder is supported by recent tetrahertz transient conductivity measurements on pentacene crystals. [16] In essence, the band broadening due to the thermal motion of the molecules is expected to result in electronic trap states which would be related to the intrinsic nature of small molecule semiconductors. [17] Clearly, trap states can also be due to extrinsic defects and these traps can completely dominate the charge transport resulting in an effective mobility µ ef f .…”

The Transient Localization Scenario for Charge Transport in Crystalline Organic Materials

“…From the experimental point of view, several key experiments are in agreement with the transient localization scenario, and an increasing number of results is now discussed in terms of these ideas. [20,21,22,23,24,25] Despite a number of review articles and books published on the subject of charge transport in organic semiconductors, this important theoretical framework has not been comprehensively described in any review yet (although some aspects are mentioned in Refs. [21,26,27]).…”