Molecular origin of differences in hole and electron mobility in amorphous Alq3—a multiscale simulation study

Abstract

In order to determine the molecular origin of the difference in electron and hole mobilities of amorphous thin films of Alq(3) (meridional Alq(3) (tris(8-hydroxyquinoline) aluminium)) we performed multiscale simulations covering quantum mechanics, molecular mechanics and lattice models. The study includes realistic disordered morphologies, polarized site energies to describe diagonal disorder, quantum chemically calculated transfer integrals for the off-diagonal disorder, inner sphere reorganization energies and an approximative scheme for outer sphere reorganization energies. Intermolecular transfer rates were calculated via Marcus-theory and mobilities were simulated via kinetic Monte Carlo simulations and by a Master Equation approach. The difference in electron and hole mobility originates from the different localization of charge density in the radical anion (more delocalized) compared to the radical cation (more confined). This results in higher diagonal disorder for holes and less favourable overlap properties for the hole transfer integrals leading to an overall higher electron mobility.

Effects of energy correlations and superexchange on charge transport and exciton formation in amorphous molecular semiconductors: An ab initio study

“…This organic semiconductor shows strong spatial energy correlations due to the large electric dipole moment of the Alq 3 molecule. The materials considered by us have a vanishing molecular dipole moment and the degree of correlation is therefore significantly lower than in Alq 3 . Because of this, we use a simplified version of the method described in Ref.…”

“…IV, using stochastically generated direct transfer integrals. We used simulation boxes with sizes up to 110 × 110 × 110 nm 3 and performed averages over five disorder realizations, yielding results for the mobilities with sufficiently small error bars.…”

“…We recently established from ab initio calculations that including superexchange coupling of guest orbitals via virtual host orbitals leads to a strong enhancement and change in the guest concentration dependence of the hole mobility in two prototypical host-guest systems often used as emitting layers in OLEDs [18]: α-NPD:Ir(MDQ)2(acac) (α-NPD doped with the red-emitting dye (acelylacetonate)bis(2-methyldibenzo[f,h]quinoxinalate)iridium) and TCTA:Ir(ppy) 3 (TCTA doped with the green-emitting dye fac-tris(2-phenylpyridyl)iridium) [22]. Although in pure α-NPD and TCTA a similarly strong enhancement of the mobility is not expected, the longer hopping range associated with superexchange is expected to enhance its F dependence.…”

“…An acceleration in this understanding has occurred by the employment of ab initio calculations of the morphology and the intermolecular charge hopping rates of these semiconductors [1][2][3][4][5][6]. Recently, we developed an ab initio model for hole transport in two amorphous molecular semiconductors often used as hole transporters in OLEDs: α-NPD [N,N'-Di(1-naphthyl)-N,N'-diphenyl-(1, 1'-biphenyl)-4,4'-diamine] and TCTA [tris(4-carbazoyl-9-ylphenyl)amine] [7].…”

Ab initiocharge-carrier mobility model for amorphous molecular semiconductors

“…In a fully predictive approach the mobility function for a particular molecular semiconductor should follow from ab initio microscopic simulations of the morphology and hopping rates. In the last few years, some research groups have started to follow this road [15][16][17][18][19][20]. The improvements expected from such an approach include quantified insight in the role of the spatial decay and off-diagonal disorder of the intersite coupling.…”