Anisotropic Hole Transport in ap-Quaterphenyl Molecular Crystal: Theory and Simulation

Abstract

A computational procedure is proposed for predicting the charge hopping rate in organic semiconductor crystals. The procedure is verified using a p-quaterphenyl molecular crystal as the test system, in which the thermally activated hole mobility is relatively low, its hole states are localized, and, hence, charge transport is of hopping character. The hole mobility in p-quaterphenyl is simulated by the Monte Carlo method with the hopping probability governed by a Marcus-like rate constant. The microscopic parameters of the Marcus model have been calculated by ab initio multireference quantum chemical method (XMCQDPT/CASSCF). Molecular conformation and crystal environment effects on the Marcus hopping parameters are studied. It is found that different arrangements of monomers typical for the crystal structure provide different hopping parameters and, hence, different hole mobilities in different directions. Monte Carlo simulations of the hole mobility predict that the hole mobility attains its maximum in the [100] direction, where hopping occurs through parallel monomers at the closest distance, which is lower than 0.01 cm2/(V·s).

Theoretical Study of Charge-Transfer Exciplexes in Organic Photovoltaics

“…The nonradiative recombination rate can be estimated using nonadiabatic coupling matrix elements or via a Markus-like expression using energy profiles in a way similar to our charge mobility calculations. , Since it is a sort of an inverted Marcus case, the nonradiative recombination is facilitated by the low energy gap. In PC61BM + PEDOT8 system, the gap is ∼1.4 eV, while that in NMe2-PPT + PC61BM is ∼0.7 to 0.8 eV.…”

“…Charge separation proceeds via carrier hopping between molecules of the same type (donor to donor or acceptor to acceptor). Typical charge hopping times in amorphous organic semiconductors are within tens or hundreds of picoseconds; they can also be estimated using the Marcus model. , For efficient charge separation, charge hopping between donors or acceptors in the blend should be faster than nonradiative recombination. Simulation of the competition between the nonradiative charge recombination and charge separation is a matter of further research.…”