Gabriele D'Avino

Simulation of Vapor‐Phase Deposition and Growth of a Pentacene Thin Film on C60 (001)

Current research in organic electronics is clearly evidencing that the strive to produce efficient organic electronic devices requires high performance materials that can only be realized through a rational design [Special11]. Although polymer-based systems are at the moment the most appealing for market applications, mainly because of their solution processability, small molecule-based devices possess potential for commercialization, presenting comparable performances and a better batch-to-batch reproducibility of their properties [Walker11]. The interest in small molecules of well defined crystalline structure arises also from the fine control over final morphologies that can be achieved through vapour-phase growth techniques [Ruiz04, Rolin10], control that allows, with respect to polymer devices, a deeper understanding of the structure-electronic properties relationships. Indeed building an efficient electronic device (e. g. a solar cell) coincides to a large extent with the fine tuning of the electronic properties at the different interfaces, typically metal-organic, inorganic-organic, and organic-organic. In this communication we focus on the latter interface between two of the most studied p-and n-type molecular organic semiconductors, pentacene (A5) and C 60 fullerene. These materials have been recently employed in producing rather efficient thin film bilayer solar cells [Yoo04, Mayer04, Yoo07, Cheyns07, Dissanayake07], ambipolar field effect transistors [Kuwahara04, Yan09, Cosseddu10] and low-voltage-operating organic complementary inverters [Na09]. The relative simplicity and the good performances of C 60 /A5 heterojunctions has stimulated theoretical research on the electronic processes occurring at the interface: density functional theory [Yi09], valence-bond Hartree-Fock [Linares10] and microelectrostatic calculations [Verlaak09] have been employed in studying model interfaces of increasing complexity. These studies coherently underline the importance of relative molecular orientations and positions in determining the interface dipole and electronic couplings, i.e. the key factors governing exciton transport and fission, charge generation and separation [Rao10]. This in turn means that improving computational predictions of the molecular organization at the interface is fundamental to understanding experimental systems of great interest. This task can in principle be tackled by using classical atomistic force fields, but it is definitely not a straightforward one. Indeed, it has been recently recognized that molecular organizations at the interface depend on the preparation process and not just on thermodynamic state of the system, so that imitating the experimental preparation techniques is often necessary to produce realistic morphologies [Liu08, Cheung08, MacKenzie10, Clancy11, Beljonne11]. For this specific system, Clancy and co-workers applied classical simulations at both coarse-grained [Choudhary06] and atomistic detail [Goose07] to study some aspects of the pentacene …

Charge Dissociation at Interfaces between Discotic Liquid Crystals: The Surprising Role of Column Mismatch

The semiconducting and self-assembling properties of columnar discotic liquid crystals have stimulated intense research toward their application in organic solar cells, although with a rather disappointing outcome to date in terms of efficiencies. These failures call for a rational strategy to choose those molecular design features (e.g., lattice parameter, length and nature of peripheral chains) that could optimize solar cell performance. With this purpose, in this work we address for the first time the construction of a realistic planar heterojunction between a columnar donor and acceptor as well as a quantitative measurement of charge separation and recombination rates using state of the art computational techniques. In particular, choosing as a case study the interface between a perylene donor and a benzoperylene diimide acceptor, we attempt to answer the largely overlooked question of whether having well-matching donor and acceptor columns at the interface is really beneficial for optimal charge separation. Surprisingly, it turns out that achieving a system with contiguous columns is detrimental to the solar cell efficiency and that engineering the mismatch is the key to optimal performance.

Electrostatic phenomena in organic semiconductors: fundamentals and implications for photovoltaics

This review summarizes the current understanding of electrostatic phenomena in ordered and disordered organic semiconductors, outlines numerical schemes developed for quantitative evaluation of electrostatic and induction contributions to ionization potentials and electron affinities of organic molecules in a solid state, and illustrates two applications of these techniques: interpretation of photoelectron spectroscopy of thin films and energetics of heterointerfaces in organic solar cells.

Do charges delocalize over multiple molecules in fullerene derivatives

We address the question of charge delocalization in amorphous and crystalline fullerene solids by performing state of the art calculations encompassing force-field molecular dynamics, microelectrostatic and quantum-chemical methods. The solution of a tight-binding model built from spatially (down to atomistic scale) and time (down to fs) resolved calculations yields the density of electronic states for the charge carriers and their energy-dependent intermolecular delocalization. Both pristine C60 and the soluble PC61BM/PC71BM acceptors may sustain high-energy states that spread over a few tens of molecules irrespective of morphology, yet electrostatic disorder (mostly dipolar and static in nature) makes the thermally available electron states collapse to hardly more than one molecule in PC61BM/PC71BM, while it has a much more limited impact in the case of the bare C60. Implications of these results for charge transport and exciton dissociation at donor–fullerene interfaces are discussed.

Cooperativity from electrostatic interactions: understanding bistability in molecular crystals

A general model for explaining bistability in crystals of donor–acceptor (DA) molecules, associated with an intramolecular charge-transfer process, is thoroughly discussed. Specifically, an extension from the two- to the three-essential states model for D-bridge-A molecular units taking into account the active role of the bridge states in the charge-transfer process is proposed. We demonstrate that the phenomenon of bistability is so robust that it survives when such an extended model is adopted. Finally, we discuss the molecular and supramolecular requirements for the observation of bistability phenomena in molecular crystals.

Anomalous dispersion of optical phonons at the neutral-ionic transition: evidence from diffuse x-ray scattering.

Diffuse x-ray data for mixed-stack organic charge-transfer crystals approaching the neutral-ionic phase transition can be quantitatively explained as due to the softening of the optical phonon branch. The interpretation is fully consistent with vibrational spectra, and underlines the importance of electron-phonon coupling in low-dimensional systems with delocalized electrons.

Dielectric properties of crystalline organic molecular films in the limit of zero overlap

We present the calculation of the static dielectric susceptibility tensor and dipole field sums in thin molecular films in the well-defined limit of zero intermolecular overlap. Microelectrostatic and charge redistribution approaches are applied to study the evolution of dielectric properties from one to a few molecular layers in films of different conjugated molecules with organic electronics applications. Because of the conditional convergence of dipolar interactions, dipole fields depend on the shape of the sample and different values are found in the middle layer of a thick film and in the bulk. The shape dependence is eliminated when depolarization is taken into account, and the dielectric tensor of molecular films converges to the bulk limit within a few molecular layers. We quantify the magnitude of surface effects and interpret general trends among different systems in terms of molecular properties, such as shape, polarizability anisotropy, and supramolecular organization. A connection between atomistic models for molecular dielectrics and simpler theories for polarizable atomic lattices is also provided.

Correlated electron-hole mechanism for molecular doping in organic semiconductors

The electronic and optical properties of the paradigmatic F4TCNQ-doped pentacene in the low-doping limit are investigated by a combination of state-of-the-art many-body \emph{ab initio} methods accounting for environmental screening effects, and a carefully parametrized model Hamiltonian. We demonstrate that while the acceptor level lies very deep in the gap, the inclusion of electron-hole interactions strongly stabilizes dopant-semiconductor charge transfer states and, together with spin statistics and structural relaxation effects, rationalize the possibility for room-temperature dopant ionization. Our findings reconcile available experimental data, shedding light on the partial vs. full charge transfer scenario discussed in the literature, and question the relevance of the standard classification in shallow or deep impurity levels prevailing for inorganic semiconductors.

Accurate description of charged excitations in molecular solids from embedded many-body perturbation theory

We present a novel hybrid quantum/classical approach to the calculation of charged excitations in molecular solids based on the many-body Greens function GW formalism. Molecules described at the GW level are embedded into the crystalline environment modeled with an accurate classical polarizable scheme. This allows the calculation of electron addition and removal energies in the bulk and at crystal surfaces where charged excitations are probed in photoelectron experiments. By considering the paradigmatic case of pentacene and perfluoropentacene crystals, we discuss the different contributions from intermolecular interactions to electronic energy levels, distinguishing between polarization, which is accounted for combining quantum and classical polarizabilities, and crystal field effects, that can impact energy levels by up to ±0.6 eV. After introducing band dispersion, we achieve quantitative agreement (within 0.2 eV) on the ionization potential and electron affinity measured at pentacene and perfluoropentacene crystal surfaces characterized by standing molecules.

On the nature of the singlet and triplet excitations mediating thermally activated delayed fluorescence (Conference Presentation)

Thermally Activated Delayed Fluorescence (TADF) process is the new paradigm for Organic Light-Emitting Diodes (OLEDs). Despite all the efforts, a complete mechanistic understanding of TADF materials has not been fully uncovered yet. Part of the complexity arises from the apparent dichotomy between the need for small energy difference between the lowest singlet and triplet excited states (EST) which has to carry a significant charge transfer (CT) character; and for a significant spin-orbit coupling which according to El-Sayed rules requires the involved singlet and triplet excited states to have very different natures. In this contribution, we will show: (i) How the nature of these excited can be characterized and how this nature can be tuned by varying the nature of the electron donating (D) or accepting (A) units in D-A(-D) compounds. (ii) How this dichotomy can be resolved once accounting in a fully atomistic model of reference carbazole derivatives for thermal fluctuations of the molecular conformations and discrete electronic polarization effects in amorphous films. For both topics, we will demonstrate that, electronic excitations involved in the TADF process have a mixed CT-locally excited character being dynamically tuned by torsional vibrational modes and that overall, the conversion of triplet-to-singlet is a dynamic process gated by conformational fluctuations.