Experiments and theory on pentacene in the thin film phase: structural, electronic, transport properties, and gas response to oxygen, nitrogen, and ambient air

Pentacene Excitons in Strong Electric Fields

Electroluminescence spectroscopy of organic semiconductors in the junction of a scanning tunneling microscope (STM) provides access to the polarizability of neutral excited states in a well-characterized molecular geometry. We study the Stark shift of the self-trapped lowest singlet exciton at 1.6 eV in a pentacene nanocrystal. Combination of density functional theory (DFT) and time-dependent DFT (TDDFT) with experiment allows for assignment of the observation to a charge-transfer (CT) exciton. Its charge separation is perpendicular to the applied field, as the measured polarizability is moderate and the electric field in the STM junction is strong enough to dissociate a CT exciton polarized parallel to the applied field. The calculated electric-field-induced anisotropy of the exciton potential energy surface will also be of relevance to photovoltaic applications.

Low-lying excited states in armchair polyacene within Pariser-Parr-Pople model: a density matrix renormalization group study

We studied the nature of the ground state and low-lying excited states of armchair polyacene oligomers (Polyphenanthrene) within long-range Pariser-Parr-Pople model Hamiltonian with up to 14 monomers using symmetrized density matrix renormalization group technique. The ground state of all armchair polyacenes studied is found to be singlet. The results show that lowest singlet dipole allowed excited state has higher energy for armchair polyacenes as compared to linear fused polyacenes. Moreover, unlike linear fused polyacenes, the lowest singlet excited state of these oligomers is always found to lie below the lowest dipole forbidden two-photon state indicating that these armchair polyacene oligomers strongly fluoresce. The calculations of low-lying excitations on singly and triply electron doped armchair polyacene oligomers show a low energy band with strong transition dipole moment that coupled to charge conductivity. This implies armchair polyacene posses novel field-effect transistor properties.

Multiscale study of charge mobility of organic semiconductor with dynamic disorders

The impact of dynamic disorder arising from the thermal fluctuations on the charge transport in organic semiconductors is studied by a multi-scale approach combining molecular dynamics, electronic structure calculations and kinetic Monte Carlo simulations for pentacene crystal of thin-film phase. It is found that for 1-D arrays, such fluctuations severely reduce charge mobility as temperature increases. However, when going from an 1-D array to an 2-D herringbone layer, for a wide range of temperatures, the charge transport property is found to be unaffected by such disorders from our multiscale computational study. And in some extreme cases, when the fluctuations of the hopping integral are even larger than their average values, the dynamic disorders can increase the charge mobility. In addition, we point out that the "band-like" behavior concluded by the experiment can be reproduced by quantum charge transfer involving nuclear vibration tunneling effects within a hopping model.