Electronic polarization at surfaces and thin films of organic molecular crystals: PTCDA

Abstract

The electronic polarization energies, P = P+ + P−, of a PTCDA (perylenetetracarboxylic acid dianhydride) cation and anion in a crystalline thin film on a metallic substrate are computed and compared with measurements of the PTCDA transport gap on gold and silver. Both experiments and theory show that P is 500 meV larger in a PTCDA monolayer than in 50Å films. Electronic polarization in systems with surfaces and interfaces are obtained self-consistently in terms of charge redistribution within molecules. I. TRANSPORT GAPThe electronic structure of organic molecular crystals is strikingly different from the conventional inorganic semiconductors, such as Si, in that the electronic polarization of the dielectric medium by charge carriers constitutes a major effect, with energy scale greater than transfer integrals or temperature [1,2]. The transport gap E t for creating a separated electron-hole pair has a substantial (1-2 eV) polarization energy contribution [3] and exceeds the optical gap by ∼ 1 eV. Limited overlap rationalizes the modest mobilities of organic molecular solids. Devices such as light-emitting diodes, thin film transistors, or photovoltaic cells require charge transport and are consequently based on thin films, quite often deposited on metallic substrates [4,5]. Organic electronics relies heavily on controlling films with monolayer precision, on forming structures with several thin films, and on characterizing the interfaces. The positions of transport states and mechanisms for charge injection are among the outstanding issues for exploiting organic devices. We focus here on the electronic polarization of crystalline thin films near surfaces and interfaces. We find that electronic polarization, and hence E t , in a prototypical organic crystal is significantly different at a free surface, near a metal-organic interface, in thin organic layers, and in the bulk.Weak intermolecular forces characterize organic molecular crystals, whose electronic and vibrational spectra are readily related to gas-phase transitions [1,2]. Due to small transfer integrals, charge carriers are molecular ions embedded in the lattice of neutral molecules. The transport gap E t in the crystal is derived from the charge gap for electron transfer in the gas phase, I(g) − A(g), which is the difference between the ionization potential and the electron affinity. But crystals have electrostatic interactions even in the limit of no overlap, and charge carriers are surrounded by self-consistent polarization clouds. In contrast to polaronic effects, electronic polarization is instantaneous and directly affects the positions of energy levels. Formation of polarization clouds is associated with stabilization energy P + for cations (the "holes") and P − for anions (the "electrons"). The combination P = P + + P − occurs in E t = I(g) − A(g) − P . Since Coulomb interactions are long-ranged, polarization clouds extend over many lattice constants and P depends on the proximity to surfaces and interfaces.

Site-Specific Polarization Screening in Organic Thin Films

“…4 may also explain the negative outcome of the attempts to measure surface core-level shifts for thin films of organic semiconductors [12,13]: If the crystal structure of the films is such that the polarization screening of the photohole is mainly effected by molecules located in a plane parallel to the surface, surface core-level shifts are expected to be very small. In this sense, the present data reconcile the conflicting evidence with respect to polarization screening in Refs [11][12][13].…”

“…For example, it has been shown that the chemical bonding to a metal surface may change both the electronic levels and geometric structure of organic semiconductor molecules profoundly [5][6][7][8][9]. During charge transport, the electronic states of a molecular material may also be influenced by polarization screening, i.e., the stabilization of a locally injected charge through the polarization of the surrounding molecular environment [10,11]. While the principles of polaron-formation are well understood, the new aspect which we report here, namely, the site specificity of the polarization energy in complex unit cells and at surfaces of organic materials, was not experimentally proven so far.…”

Barrier Formation at Organic Interfaces in a Cu(100)-benzenethiolate-pentacene Heterostructure

“…[19]) and a CNL located 3.5 eV above the center of the HOMO level, E HOMO . We find that, upon adsorption, image potential effects [29] strongly reduce the energy gap to around 1.9 eV, and E CNL -E HOMO to 1.0 eV. These effects are calculated using a classical image potential with a metal image plane located 1.25 Å [30] from the outermost Cu layer.…”

Influence of film thickness and air exposure on the transport gap of manganese phthalocyanine

“…Due to image potential contributions at the metal-organic contact in the nanometer range a reduction of the barrier height is expected. [31][32][33][34]1 We observe the resulting opening of the transport band gap within the first 4 nm film thickness by a value of 0.15 eV to 0.20 eV for both HOMO and LUMO regions. At the MnPc/Co interface a constant interface dipole = (−0.7 ± 0.1) eV is observed indicating charge transfer between the MnPc molecules and the metallic substrate, as already observed in the case of MnPc/Au.…”