Charge Recombination via Intercolumnar Electron Tunneling through the Lipid-like Mantle of Discotic Hexa-alkyl-hexa-peri-hexabenzocoronenes

Abstract

The recombination of the mobile charge carriers formed in pulse-ionized hexa-alkyl-substituted hexa-peri-hexabenzocoronenes occurs mainly via intercolumnar electron tunneling through the intervening hydrocarbon mantle. This is evidenced as a dramatic increase in the time scale of the decay of the radiation-induced conductivity from a few hundred nanoseconds to close to a millisecond as the peripheral alkyl substituents increase in size from 8 to 24 carbon atoms with corresponding disk diameters, D, from 23.4 to 36.6 A. The decay kinetics are a function only of the total number of peripheral carbon atoms with no evidence for specific effects of chain branching. The 1/e decay time, tau(e), increases exponentially with D according to tau(e) = tau(e)(0) exp(betaD) with tau(e)(0) = 48 fs and beta = 0.63 A(-1). Taking into account the tilted columnar configuration of the molecules in the solid phase leads to a beta value of ca. 0.8 A(-1) for the distance dependence of intercolumnar electron tunneling. In contrast to the orders of magnitude changes in the time scale for intercolumnar charge recombination, the intracolumnar charge hopping times vary by only a factor of 4, between 40 and 160 fs, with no systematic dependence on the nature of the alkyl substituents. On the basis of the results, the time scale estimated for electron tunneling across a 40 A thick lipid membrane is estimated to be close to 1 ms.

Relaxation dynamics in the columnar liquid crystal phase of hard platelets

“…21,22 Due to the insulating effect of the lateral hydrocarbon chains, the interactions established between neighboring mesogens within the same column are significantly stronger than those between mesogens of contiguous columns, 23,24 and a (quasi) 1D charge transport is observed. [25][26][27] As a consequence of this peculiar behavior, the conducting properties of CLCDs offer the advantage to improve the performance of electronic, optoelectronic, and photovoltaic devices, such as solar cells and plastic organic field-effect transistors. [28][29][30][31][32][33][34][35] In all these applications, the liquid crystalline properties of CLCDs, and in particular their fluid-like dynamics along the columns, are of fundamental importance in the self-healing of eventual structural defects, such as grain boundaries, which might hamper the charge diffusion by trapping the charge carriers.…”

Organic Bulk‐Heterojunction Photovoltaics Based on Alkyl Substituted Discotics

“…Very long charge life-times were observed for 1b and 1c, which was explained by a suppressed charge carrier recombination resulting from the shielding of the bulky alkyl mantle. [14] For the photovoltaic device, this would hamper an efficient exciton polarization at the interface between the donor and the acceptor moiety and thus reduce the efficiency of the solar cell.…”

The Negative Effect of High‐Temperature Annealing on Charge‐Carrier Lifetimes in Microcrystalline PCBM

“…This behavior is characteristic of a stretched-exponential time dependence, which has been found to describe the disperse-charge-carrier decay kinetics in many molecular materials. [24][25][26][27] On annealing, however, the decays increasingly show an upward deviation from a powerlaw dependence at long times. This behavior can be explained if electrons and holes both contribute significantly to the conductivity but undergo localization via trapping on widely differing timescales in the annealed material.…”