2 research outputs found
Dipole Moment and Polarizability of Tunable Intramolecular Charge Transfer States in Heterocyclic π‑Conjugated Molecular Dyads Determined by Computational and Stark Spectroscopic Study
The annulation of
two redox-active molecules into a compact and
planar structure paves the way toward a new class of electronically
versatile materials whose physical properties can be tuned via a substitution
of one of the constituting moieties. Specifically, we present tetrathiafulvalene–benzothiadiazole
donor–acceptor molecules. The critical role played by the dielectric
properties of these molecules is evident by the large spectral shifts
of the ground-state absorption spectra in a range of solvents. Stark
spectroscopy is performed to determine experimentally dipole and polarizability
change over transitions in the visible range with particular attention
to the transition from the highest-occupied molecular orbital (HOMO)
to the lowest-unoccupied molecular orbital (LUMO). The experimental
results are compared to the results of time-dependent density functional
theory calculations, and we reciprocally validate results from calculation
and experiment. This allows us to filter out effective models and
reveal important insights. The calculations are initially performed
in the gas phase and subsequently a polarizable continuum model is
adopted to probe the influence of the solvent on the molecular dielectric
properties. The results show a large charge displacement from the
HOMO to the LUMO and confirm the intramolecular charge transfer nature
of the lowest-energy transition. Substitution of the acceptor moiety
with electron-withdrawing groups results in changes to the experimentally
determined molecular properties consistent with the effects predicted
by computational results. The dominant contribution to the electroabsorption
signal is due to the change in dipole moment, which is measured to
be roughly 20 D for all samples and forms a small angle with the transition
dipole moment in a toluene solvent environment
Refractive indices of layers and optical simulations of Cu(In,Ga)Se<sub>2</sub> solar cells
<p>Cu(In,Ga)Se<sub>2</sub>
-based solar cells have reached efficiencies close to 23%. Further knowledge-driven improvements require accurate determination of the material properties. Here, we present refractive indices for all layers in Cu(In,Ga)Se<sub>2</sub> solar cells with high efficiency. The optical bandgap of Cu(In,Ga)Se<sub>2</sub> does not depend on the Cu content in the explored composition range, while the absorption coefficient value is primarily determined by the Cu content. An expression for the absorption spectrum is proposed, with Ga and Cu compositions as parameters. This set of parameters allows accurate device simulations to understand remaining absorption and carrier collection losses and develop strategies to improve performances.</p