Article pubs.acs.org/JPCC
Computer Simulation of Shallow Traps Created by Impurity Molecules in Anthracene Crystal Alexey V. Odinokov*,† and Alexander A. Bagaturyants†,‡ †
Photochemistry Center, Russian Academy of Sciences, Moscow, Russian Federation National Research Nuclear University MEPhI (Moscow Engineering Physics Institute), Moscow, Russian Federation
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ABSTRACT: Conductive properties of crystalline organic semiconductors depend to a great extent on the presence of traps. One of the possible sources of traps are impurity molecules incorporated into the crystal lattice. The equilibrium crystal structure around an impurity in anthracene and the corresponding depth of shallow traps are calculated using simulations with a polarizable force field, quantum chemistry, and perturbation theory. Two possible causes of the trap formation are considered: the first one is the electrostatic potential created by the permanent dipole of the impurity molecule, and the second one is the alteration of the polaron binding energy in the vicinity of a point defect. Changes in intermolecular couplings produced by the lattice distortion make an unexpectedly small contribution to the polaron energy. The dominant influence is exerted by the absence of the resonance level in the lattice site occupied by the impurity. This factor inhibits the formation of traps and partially compensates for the action of the electrostatic potential. These results provide insight into the mechanisms that govern charge localization on point defects in organic semiconductors.
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idea of the current switch based on polar dopants.10,11 However, the exact mechanism of trap formation and interplay between the lattice distortion and electrostatic field effects remain unclear. In this work, we use computer simulations to investigate the disturbing effect of an impurity molecule residing in the crystal of an organic semiconductor and to estimate the depth of the resulting traps. Recently, computer simulations using density functional theory or polarizable force fields have been successfully applied to the problem of charge transfer in crystalline organic semiconductors.12,13 These simulations have employed first-principle calculations combined with the theory of charge transfer and have been able to describe many experimental trends. Here we do not intend to compute the microscopic parameters responsible for the “intrinsic” conductivity of the pure crystal. Instead, we try to find their changes near the impurity molecule and to explore the effect of these changes on the energy of charge carriers. We chose anthracene (AC) as a prototypical organic semiconductor and considered two types of impurities: 9,10-dihydroanthracene (dhAC) and acridine (AD). These molecules display different charge distributions. While AD possesses a substantial dipole moment, the first nonzero electric moment for dhAC is quadrupole. Using AD and dhAC as test objects, we cover both dipolar and quadrupolar types of impurities, emphasizing the role of an electrostatic potential. In addition, these substances
INTRODUCTION Organic materials for electronic and optoelectronic applications have attracted growing attention in recent years.1 Great efforts are made to design new materials and efficient device architectures. Obtaining high charge conductivity of crystalline organic materials produced on an industrial scale remains a challenging issue. Charge trapping is one of the factors that governs the dynamics of charge carriers and often prevents high transport rates.2,3 This effect arises from the binding of the charge carriers (holes or electrons) with specific sites in the bulk of the material. According to the binding energy, traps are usually divided into two groups, deep traps (>0.1 eV) and shallow traps (
