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This paper discusses the mechanism of generation of free charges in organic photovoltaic cells (OPV) from electrostatically bound electron–hole pairs. The efficiency of this process is explained when interfacial charge-transfer (CT) states are generated by direct optical excitation. We used semiclassical quantum dynamics at a short time scale (∼100 fs) and Redfield theory at a relatively long time scale (∼10–100 ps) to cover both the process of dissociation and the relaxation to the lowest energy state. Our calculations suggest that a CT state with an intermediate electron–hole separation can evolve into a charge-separated (CS) state on ultrafast time scales (∼100 fs) as a result of quantum diffusion. On long time scales, however, the CS states ultimately relax to the low-energy CT states due to the interaction with the thermal bath, indicating that the yield of free charge carrier generation is determined by the interplay between ultrafast charge separation, due to quantum diffusion, and the much slower quantum relaxation process.

The feasibility of calculating photoinduced intramolecular electron transfer rate constants in realistic molecular donor–acceptor systems via Fermi’s golden rule, using inputs obtained from state-of-the-art electronic structure techniques, is demonstrated and tested. To this end, calculations of photoinduced electron transfer rate constants were performed on two benchmark systems: (1) phenylacetylene-bridged carbazole-naphthalimide (meta and para) and (2) C60-(N,N-dimethylaniline). Intramolecular input parameters such as normal-mode frequencies, Huang–Rhys factors, and electronic coupling coefficients were obtained via ground state, time-dependent, and constrained density functional theory. Good agreement between the intramolecular Fermi’s golden rule rate constants and the experimental rate constants is found for both systems without accounting for the solvent reorganization. The relative roles of intramolecular vs intermolecular modes at promoting electron transfer and the validity of several limits of ...

Vibrational properties of the gallium monohydrides SrGaGeH, BaGaSiH, BaGaGeH, and BaGaSnH (AeGaTtH) have been investigated by means of inelastic neutron scattering (INS) and first principles calculations. The compounds contain separated Ga–H units being part of a two dimensional polyanionic layer, [TtGaH]2− (Tt=Si, Ge, Sn). The INS spectra show internal Ga–H bending and stretching modes at frequencies around 900 and 1200 cm−1, respectively. While the stretching mode is virtually invariant with respect to the variable chemical environment of the Ga–H unit, the bending mode frequency varies and is highest for BaGaSiH and lowest for BaGaSnH. The stretching mode is a direct measure of the Ga–H bond strength, whereas the bending mode reflects indirectly the strength of alkaline earth metal–hydrogen interaction. Accordingly, the terminal Ga–H bond in solid state AeGaTtH is distinct, but—compared to molecular gallium hydrides—very weak.

We report theoretical simulations of the effects of different binding sites and metal contact geometries on the conductance through single alkanedithiol molecules on gold. Examined are the current-voltage (I- V) characteristics and the projected density of states when a one-atom-thick filament is formed at the molecule-metal interface. Filaments similar to these are believed to be formed in gold STM break-junction experiments. I- V characteristics with these contacts are compared to those of planar gold contacts at hollow or on-top sites. The results reveal that a gold filament contact produces a large change in the Fermi level lineup which significantly affects its I-V curve. The conductance decreases by a factor of 4 ∼ 6 when gold atoms are placed between the molecule and the gold surface to form a filament, and non-linearities develop. The hollow and on-top planar contacts show similar conductivities to each other and produce linear I-V curve.

Charge transport characteristics of short double-strand (ds) DNA including mismatches are stud- ied within a methodology combining molecular dynamics (MD) simulations and electronic structure calculations based on a fragment orbital approach. Electronic parameters and transmission prob- abilities are computed along the MD trajectory. We find that in the course of the MD simulation the energetic position of frontier orbitals may be interchanged. As a result, the highest occupied molecular orbital (HOMO) can temporarily have a large weight on the backbones as a function of time. This shows that care must be taken when projecting the electronic structure onto effective low-dimensional model Hamiltonians to calculate transport properties. Further, the transport cal- culations indicate a suppression of the charge migration efficiency when introducing a single GT or AC mismatch in the DNA sequence.

Superconducting AlB2-type silicides CaAlSi, SrAlSi, and BaAlSi (MAlSi) absorb hydrogen and form semiconducting monohydrides where hydrogen is exclusively attached to Al. This induces a metal-nonmet ...

The rates of interfacial charge transfer and recombination between the donor and acceptor layers play a key role in determining the performance of organic photovoltaic cells. The time scale and mechanism of these processes are expected to be impacted by the structure of the interface. In this paper we model the kinetics of those processes within the framework of a subphthalocyanine/fullerene donor/acceptor dimer model. Two likely configurations (on-top and hollow) in which the interfacial charge transfer and recombination may occur are studied. The corresponding rate constants are calculated within the fully quantum-mechanical framework of Fermi’s golden rule. All the input parameters (excitation energies, electronic coupling coefficients, normal-mode frequencies and coordinates, and Huang–Rhys factors) are obtained from density functional theory calculations with density functionals designed to yield accurate results in the case of noncovalently bound systems and charge transfer states. Multiple π–π* and...