Comprehensive Approach to Intrinsic Charge Carrier Mobility in Conjugated Organic Molecules, Macromolecules, and Supramolecular Architectures

Abstract

Si-based inorganic electronics have long dominated the semiconductor industry. However, in recent years conjugated polymers have attracted increasing attention because such systems are flexible and offer the potential for low-cost, large-area production via roll-to-roll processing. The state-of-the-art organic conjugated molecular crystals can exhibit charge carrier mobilities (μ) that nearly match or even exceed that of amorphous silicon (1-10 cm(2) V(-1) s(-1)). The mean free path of the charge carriers estimated from these mobilities corresponds to the typical intersite (intermolecular) hopping distances in conjugated organic materials, which strongly suggests that the conduction model for the electronic band structure only applies to μ > 1 cm(2) V(-1) s(-1) for the translational motion of the charge carriers. However, to analyze the transport mechanism in organic electronics, researchers conventionally use a disorder formalism, where μ is usually less than 1 cm(2) V(-1) s(-1) and dominated by impurities, disorders, or defects that disturb the long-range translational motion. In this Account, we discuss the relationship between the alternating-current and direct-current mobilities of charge carriers, using time-resolved microwave conductivity (TRMC) and other techniques including field-effect transistor, time-of-flight, and space-charge limited current. TRMC measures the nanometer-scale mobility of charge carriers under an oscillating microwave electric field with no contact between the semiconductors and the metals. This separation allows us to evaluate the intrinsic charge carrier mobility with minimal trapping effects. We review a wide variety of organic electronics in terms of their charge carrier mobilities, and we describe recent studies of macromolecules, molecular crystals, and supramolecular architecture. For example, a rigid poly(phenylene-co-ethynylene) included in permethylated cyclodextrin shows a high intramolecular hole mobility of 0.5 cm(2) V(-1) s(-1), based on a combination of flash-photolysis TRMC and transient absorption spectroscopy (TAS) measurements. Single-crystal rubrene showed an ambipolarity with anisotropic charge carrier transport along each crystal axis on the nanometer scale. Finally, we describe the charge carrier mobility of a self-assembled nanotube consisting of a large π-plane of hexabenzocoronene (HBC) partially appended with an electron acceptor. The local (intratubular) charge carrier mobility reached 3 cm(2) V(-1) s(-1) for the nanotubes that possessed well-ordered π-stacking, but it dropped to 0.7 cm(2) V(-1) s(-1) in regions that contained greater amounts of the electron acceptor because those molecules reduced the structural integrity of π-stacked HBC arrays. Interestingly, the long-range (intertubular) charge carrier mobility was on the order of 10(-4) cm(2) V(-1) s(-1) and monotonically decreased when the acceptor content was increased. These results suggest the importance of investigating charge carrier mobilities by frequency-dependent charge carrier motion for...

Monodisperse N‐Doped Graphene Nanoribbons Reaching 7.7 Nanometers in Length

“…The measurements show nearly invariable ϕ Σ μ maxima for NR‐10 (0.85×10 −4  cm 2  V −1  s −1 ), NR‐20 (0.82×10 −4  cm 2  V −1  s −1 ), and NR‐30 (0.75×10 −4  cm 2  V  −1  s −1 ), and also nearly invariable half lifetimes ( τ 1/2 ) for NR‐10 (1.4 μs), NR‐20 (1.1 μs), and NR‐30 (0.9 μs). These ϕ Σ μ values are in line with those observed on conjugated polymers such as P3HT and polyfluorene 20. The insignificant differences found in ϕ Σ μ maxima and τ 1/2 with respect to NR length are consistent with the localized HOMO/LUMO densities.…”

Monodisperse N‐Doped Graphene Nanoribbons Reaching 7.7 Nanometers in Length

“…These fSm values are in line with those observed on conjugated polymers such as P3HT and polyfluorene. [20] Thei nsignificant differences found in fSm maxima and t 1/2 with respect to NR length are consistent with the localized HOMO/LUMO densities.…”

“…To shed light on the conducting properties of the NRs, time-resolved microwave conductivity (TRMC) [20] measurements have been carried out (Figure 2d). TRMC measures the pseudo-photoconductivity (fSm,where f is the product of the quantum yield, and Sm is the sum of the charge carrier mobilities) under an oscillating microwave electric field directly from powder samples with no metal contacts.T he measurements show nearly invariable fSm maxima for NR-10 (0.85 10…”

Semimetallic and charge-ordered α−(BEDT-TTF)2I3 : On the role of disorder in dc transport and dielectric properties

“…Such a low density of carriers is in accord with distinctly small electron and hole pockets at the Fermi level instead of a metal-like, large Fermi surface. The carrier mobilities turn out to be almost the same and around 200 cm 2 /Vs which is at least two orders of magnitude larger than typical values in organic conductors 17,[41][42][43] and much closer to values found in other semimetals. 44 Note that within the limitations of our model the R H > 0 is due to µ h − µ e > 0 where the difference in mobilities is significantly below 1%.…”