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Resonance Raman Spectroscopy and Imaging of Franck–Condon Vibrational Activity and Morphology in Conjugated Polymers for Solar Cells
journal contribution
posted on 2019-08-01, 17:36 authored by John K. GreyConspectusVibrational reorganization influences photophysical
outcomes in
conjugated polymers used as active materials for optoelectronic devices.
Excited state geometric rearrangements typically involve many displaced
vibrations, yet most materials design schemes rely solely on pure
electronic models with limited predictive capability. Although the
coupling of vibrational motions to electronic processes occurs over
a broad range of time scales, resolving structural displacements immediately
following photon absorption can be particularly insightful for understanding
the intrinsic stabilities of excited states. These Franck–Condon
vibrational relaxation processes occur on time scales of <1 ps
in polymers and mainly involve high-frequency skeletal motions. Establishing
correlations between Franck–Condon vibrational reorganization
and steady-state material properties could generate new avenues for
informing materials design, which is especially important in the fast-paced
field of organic photovoltaics (OPV) where seemingly elegant strategies
often fail but molecular-level insights are usually lacking.The goal of this Account is to highlight relationships between
molecular structure, packing, and vibrational reorganization in OPV
systems, such as blends of conjugated polymers with fullerenes. Resonance
Raman spectroscopy (RRS) is a sensitive probe of Franck–Condon
activity in OPV materials, and signals are bolstered by large resonance
enhancements and low backgrounds from quantitative fluorescence quenching.
Our group has undertaken extensive RRS investigations of heterogeneous
OPV materials in functioning device environments to uncover new insights
of the multidimensional excited state potential energy landscape and
fluctuations with local morphology. Time-dependent quantum mechanical
approaches facilitate this effort by providing an intuitive theoretical
framework to access dynamical perspectives of Raman transitions. Moreover,
dynamics regimes of Franck–Condon excited state structural
evolution can be selected simply by tuning excitation energies. This
excitation detuning approach also reveals structurally and electronically
distinct conformers with unique Franck–Condon signatures typically
concealed under inhomogeneously broadened absorption line shapes.
Interestingly, long and rich progressions of overtone and combination
transitionsrare for large molecules with multiple displaced
modesare frequently resolved that exhibit strong sensitivity
to the local chromophore environment. These harmonic features encode
useful dynamics information by serving as internal “clocks”
of Franck–Condon vibrational activity in addition to enabling
quantitative estimates of mode-specific displacements. RRS attributes
may be further exploited to perform noninvasive imaging of functioning
OPV devices in concert with variable frequency electrical imaging
probes. This approach generates direct spatial correlations between
morphology-dependent Franck–Condon vibrational activity and
material performance metrics (e.g., photocurrent generation) on submicrometer
size scales.
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OPV materialsmaterials design schemesvibrationalmaterial performance metricsFrancktime scalesResonance Raman spectroscopySolar Cells ConspectusVibrational reorganization influences photophysical outcomesRRSsubmicrometer size scalespolymerabsorption line shapesResonance Raman Spectroscopyexcitation detuning approachdevice
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