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Register a new userQuantum simulations of charge-separation at a model donor-acceptor interface: role of delocalization and local packing
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We investigate the electronic dynamics of a model organic photovoltaic (OPV) system consisting of polyphenylene vinylene (PPV) oligomers and a [6,6]-phenyl C61-butyric acid methylester (PCBM) blend using a mixed molecular mechanics/quantum mechanics (MM/QM) approach. Using a heuristic model that connects energy gap fluctuations to the average electronic couplings and decoherence times, we provide and estimate of the state-to-state internal conversion rates within the manifold of the lowest few electronic excitations. We show that the electronic dynamics of the OPV are dramatically altered by varying the positions of the molecules simulated at the interface. The lowest few excited states of the model interface rapidly mix allowing low frequency C-C out of plain torsions to modulate the potential energy surface such that the system can sample both intermolecular charge-transfer and charge-separated electronic configurations on sub 100 fs time scales. Our simulations support an emerging picture of carrier generation in OPV systems in which interfacial electronic states can rapidly decay into charge-separated and current producing states via coupling to vibronic degrees of freedom.
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We present a novel {em ab initio} approach for computing intramolecular charge and energy transfer rates based upon a projection operator scheme that parses out specific internal nuclear motions that accompany the electronic transition. Our approach concentrates the coupling between the electronic and nuclear degrees of freedom into a small number of reduced harmonic modes that can be written as linear combinations of the vibrational normal modes of the molecular system about a given electronic minima. Using a time-convolutionless master-equation approach, parameterized by accurate quantum-chemical methods, we benchmark the approach against experimental results and predictions from Marcus theory for triplet energy transfer for a series of donor-bridge-acceptor systems. We find that using only a single reduced mode--termed the primary mode, one obtains an accurate evaluation of the golden-rule rate constant and insight into the nuclear motions responsible for coupling the initial and final electronic states. We demonstrate the utility of the approach by computing the inelastic electronic transition rates in a model donor-bridge-acceptor complex that has been experimentally shown that its exciton transfer pathway can be radically modified by mode-specific infrared excitation of its vibrational mode.
In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This effect results in distinctive signatures in the vibrational modes of the polymer. We probe polaron photo- generation dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 200 fs, which is nearly two orders of magnitude faster than exciton localisation in the neat polymer film. Surprisingly, further vibrational evolution on <50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not signif- icantly different from that in equilibrium. This suggests that charges are free from their mutual Coulomb potential, under which vibrational dynamics would report charge-pair relaxation. Our work addresses current debates on the photocarrier generation mechanism at organic semiconductor heterojunctions, and is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.
Electronic and optical properties of doped organic semiconductors are dominated by local interactions between donor and acceptor molecules. However, when such systems are in crystalline form, long-range order competes against short-range couplings. In a first-principles study on three experimentally resolved bulk structures of quaterthiophene doped by (fluorinated) tetracyanoquinodimethane, we demonstrate the crucial role of long-range interactions in donor/acceptor co-crystals. The band structures of the investigated materials exhibit direct band-gaps decreasing in size with increasing amount of F atoms in the acceptors. The valence-band maximum and conduction-band minimum are found at the Brillouin zone boundary and the corresponding wave-functions are segregated on donor and acceptor molecules, respectively. With the aid of a tight-binding model, we rationalize that the mechanisms responsible for these behaviors, which are ubiquitous in donor/acceptor co-crystals, are driven by long-range interactions. The optical response of the analyzed co-crystals is highly anisotropic. The absorption onset is dominated by an intense resonance corresponding to a charge-transfer excitation. Long-range interactions are again responsible for this behavior, which enhances the efficiency of the co-crystals for photo-induced charge separation and transport. In addition to this result, which has important implications in the rational design of organic materials for opto-electronics, our study clarifies that cluster models, accounting only for local interactions, cannot capture the relevant impact of long-range order in donor/acceptor co-crystals.
The electronic structure of ZnPc, from sub-monolayers to thick films, on bare and iodated Pt(111) is studied by means of X-ray photoelectron spectroscopy (XPS), X-ray absorption spectroscopy (XAS) and scanning tunneling microscopy (STM). Our results suggest that at low coverage ZnPc lies almost parallel to the Pt(111) substrate, in a non-planar configuration induced by Zn-Pt attraction, leading to an inhomogeneous charge distribution within the molecule and charge transfer to the molecule. ZnPc does not form a complete monolayer on the Pt surface, due to a surface-mediated intermolecular repulsion. At higher coverage ZnPc adopts a tilted geometry, due to a reduced molecule-substrate interaction. Our photoemission results illustrate that ZnPc is practically decoupled from Pt, already from the second layer. Pre-deposition of iodine on Pt hinders the Zn-Pt attraction, leading to a non-distorted first layer ZnPc in contact with Pt(111)-I $left(sqrt{3}timessqrt{3}right)$ or Pt(111)-I $left(sqrt{7}timessqrt{7}right)$, and a more homogeneous charge distribution and charge transfer at the interface. On increased ZnPc thickness iodine is dissolved in the organic film where it acts as an electron acceptor dopant.
Functionalized polcyclic aromatic hydrocarbons (PAHs) are an interesting class of molecules in which the electronic state of the graphene-like hydrocarbon part is tuned by the functional group. Searching for new types of donor and acceptor molecules, a set of new PAHs has recently been investigated experimentally using ultraviolet photoelectron spectroscopy (UPS). In this work, the electronic structure of the PAHs is studied numerically with the help of B3LYP hybrid density functionals. Using the DELTA-SCF method, electron binding energies have been determined which affirm, specify and complement the UPS data. Symmetry properties of molecular orbitals are analyzed for a categorization and an estimate of the related signal strength. While SIGMA-like orbitals are difficult to detect in UPS spectra of condensed film, calculation provides a detailed insight into the hidden parts of the electronic structure of donor and acceptor molecules. In addition, a diffuse basis set (6-311++G**) was used to calculate electron affinity and LUMO eigenvalues. The calculated electron affinity (EA) provides a classification of the donor/acceptor properties of the studied molecules. Coronene-hexaone shows a high EA, comparable to TCNQ, which is a well-known classical acceptor. Calculated HOMO-LUMO gaps using the related eigenvalues have a good agreement with the experimental lowest excitation energies. TD-DFT also accurately predicts the measured optical gap.
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