3 research outputs found
Evolution of Nonmirror Image Fluorescence Spectra in Conjugated Polymers and Oligomers
The
nonmirror image relationship between absorption and fluorescence
spectra of conjugated polymers contrasts with most organic chromophores
and is widely considered a signature of interchromopohore energy funneling.
We apply broad-band ultrafast fluorescence spectroscopy to resolve
the evolution of fluorescence spectra for dilute solutions of conjugated
oligothiophenes, where no energy transfer is possible. Fluorescence
spectra evolve from a mirror image of absorption, which lacks vibronic
structure, toward a spectrally narrower and vibronically structured
species on the hundreds of femtosecond to early picosecond time scale.
Our analysis of this fluorescence spectral evolution shows that a
broad distribution of torsional conformers is driven to rapidly planarize
in the excited state, including in solid films, which is supported
by Raman spectroscopy and quantum chemical modeling. Our data have
important implications for understanding different energy-transfer
regimes that are delineated by structural relaxation
The Evolution of Quantum Confinement in CsPbBr<sub>3</sub> Perovskite Nanocrystals
Colloidal
nanocrystals (NCs) of lead halide perovskites are considered
highly promising materials that combine the exceptional optoelectronic
properties of lead halide perovskites with tunability from quantum
confinement. But can we assume that these materials are in the strong
confinement regime? Here, we report an ultrafast transient absorption
study of cubic CsPbBr<sub>3</sub> NCs as a function of size, compared
with the bulk material. For NCs above ∼7 nm edge length, spectral
signatures are similar to the bulk material–characterized by
state-filling with uncorrelated charges–but discrete new kinetic
components emerge at high fluence due to bimolecular recombination
occurring in a discrete volume. Only for the smallest NCs (∼4
nm edge length) are strong quantum confinement effects manifest in
TA spectral dynamics; focusing toward discrete energy states, enhanced
bandgap renormalization energy, and departure from a Boltzmann statistical
carrier cooling. At high fluence, we find that a hot-phonon bottleneck
effect slows carrier cooling, but this appears to be intrinsic to
the material, rather than size dependent. Overall, we find that the
smallest NCs are understood in the framework of quantum confinement,
however for the widely used NCs with edge lengths >7 nm the photophysics
of bulk lead halide perovskites are a better point of reference
Impact of Acceptor Fluorination on the Performance of All-Polymer Solar Cells
Here, we systematically study the effect of fluorination on the performance of all-polymer solar cells
by employing a naphthalene diimide (NDI)-based polymer acceptor with
thiophene-flanked phenyl co-monomer. Fluorination of the phenyl co-monomer
with either two or four fluorine units is used to create a series
of acceptor polymers with either no fluorination (PNDITPhT), bifluorination
(PNDITF2T), or tetrafluorination (PNDITF4T). In blends with the donor
polymer PTB7-Th, fluorination results in an increase in power conversion
efficiency from 3.1 to 4.6% despite a decrease in open-circuit voltage
from 0.86 V (unfluorinated) to 0.78 V (tetrafluorinated). Countering
this decrease in open-circuit voltage is an increase in short-circuit
current from 7.7 to 11.7 mA/cm<sup>2</sup> as well as an increase
in fill factor from 0.45 to 0.53. The origin of the improvement in
performance with fluorination is explored using a combination of morphological,
photophysical, and charge-transport studies. Interestingly, fluorination
is found not to affect the ultrafast charge-generation kinetics, but
instead is found to improve charge-collection yield subsequent to
charge generation, linked to improved electron mobility and improved
phase separation. Fluorination also leads to improved light absorption,
with the blue-shifted absorption profile of the fluorinated polymers
complementing the absorption profile of the low-band gap PTB7-Th