Mechanism of Charge Transport along Columnar Stacks of a Triphenylene Dimer

Abstract

We have measured the mobility of charge carriers along the one-dimensional conducting pathways provided by columnar stacks of triphenylene units in the liquid crystalline dimer 1,10-di-[3′,6′,7′,10′,11′-pentabutyloxytriphenylenyl-2-oxy]decane using the pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) and the time-of-flight (TOF) techniques. The high frequency (30 GHz) and TOF intracolumnar mobilities approach the same value (ca. 0.01 cm 2 V -1 s -1 ) at the highest temperatures studied (about 400 K). When the temperature is lowered, the high-frequency mobility remains almost constant within the range (1.5 ( 0.5) × 10 -2 cm 2 V -1 s -1 down to 170 K. The value of µ TOF in contrast decreases dramatically at lower temperatures, reaching a value as low as 2 × 10 -6 cm 2 V -1 s -1 at 130 K. The different temperature dependences found are attributed to structural disorder within the columnar stacks. The experimental data are compared with predictions of the influence of static disorder on charge transport using different transport models. Our analytical and computer simulation studies show that the only transport mechanism consistent with both sets of experimental data is one involving thermally activated jumps over barriers with an exponential distribution of barrier heights. The experimental mobility values could be reproduced with a mean barrier height of 0.024 eV and an attempt frequency for jumping equal to 1 ps -1 . Our experimental and theoretical findings illustrate the added insights into the underlying mechanism of charge transport in complex molecular materials that can be gained from the combined results of TOF and high-frequency mobility measurements.

Charge transport in columnar stacked triphenylenes: Effects of conformational fluctuations on charge transfer integrals and site energies

“…16 In the microwave conductivity measurements charge transport within relatively ordered microdomains is probed, since the charge carriers can follow the oscillating electric field by moving between barriers for charge transport. 18 In contrast, in TOF measurements charge carriers need to traverse the inter- electrode distance of the order of several m and the probability that a charge carrier encounters a large structural fluctuation becomes more likely. For instance a large lateral slide of a molecule leads to a relatively small charge transfer integral and consequently a low mobility.…”

“…26 -31 Structural disorder has a detrimental effect on the charge carrier mobility and gives rise to disperse photocurrent transients and a frequency dependence of the charge carrier mobility. 18,[21][22][23][24] The presence of disorder causes stacks of triphenylene molecules to have a non-periodic structure and consequently electronic band structure calculations, which can be used to describe charge transport in molecular crystals, 32 are inapplicable. Quantitative insight into the effects of structural disorder on the mobility of charge carriers requires an alternative theoretical treatment of charge transport.…”

“…Quantitative insight into the effects of structural disorder on the mobility of charge carriers requires an alternative theoretical treatment of charge transport. Theoretical studies based on incoherent hopping models 18,33,34 or ͑partially͒ coherent motion 20 including effects of dynamic or static structural fluctuations have been used to estimate hole mobilities in triphenylene derivatives. The theoretical mobility values presented until now are at least one order of magnitude higher than the experimental values for liquid crystalline triphenylene derivatives.…”

Conductive response of a photo-excited sample in a radio-frequent driven resonance cavity

“…22 The study of conjugate polymers was boosted with the introduction of laser pulses as an ionizing source. 23,24 TRMC is now being used routinely as a method to determine transient conductivities of a wide variety of materials, such as silicon-based composites for solar cells, [25][26][27][28] compound semiconductors, 29,30 columnar stacked molecular aggregates, [31][32][33][34][35][36][37][38][39] bulk solids or thin films of conjugated polymers, [40][41][42][43][44][45][46][47] and molecular wires in dilute solution.…”

Charge-carrier mobilities in binary mixtures of discotic triphenylene derivatives as a function of temperature

“…Of particular relevance to their possible use in electronic device applications are charge-carrier mobility measurements, of which quite a number have been published. [5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21] The two main methods that have been used for this purpose are time-of-flight ͑TOF͒ and pulse-radiolysis time-resolved microwave conductivity ͑PR-TRMC͒, 22,23 which are both nonperturbative, time-resolved techniques.…”