3 research outputs found
Pump-Selective Spectral Shaping of the Ultrafast Response in Plasmonic Nanostars
Plasmonic nanostructures are, to date, well-known to
offer unique
possibilities for the tailoring of light–matter interactions
at the nanoscale. Most recently, a new route to ultrafast all-optical
modulation has been disclosed by combining the resonant features of
plasmonic nanostructures with the giant third-order optical nonlinearity
of noble metals regulated by highly energetic (hot) carriers. In this
framework, a variety of nanostructures have been designed, with special
attention to shapes featuring tips, where extreme and highly sensitive
field enhancements (hot spots) can be attained. Here, we report on
a broadband pump–probe spectroscopy analysis of an ensemble
of spiky star-shaped nanoparticles, exploring both the perturbative
and nonperturbative regimes of photoexcitation. The experiments are
corroborated by semiclassical numerical simulations of the ultrafast
optical response of the sample. We found that the peculiar hot spots
supported by the star tips allow one to easily control the spectral
shape of the transient optical signal, upon tuning of the pump wavelength.
Our results elucidate the ultrafast response of hot electrons in star-shaped
nanostructures and contribute to the understanding of the tip-mediated
enhanced nonlinearities. This work paves the way to the development
of ultrafast all-optical plasmonic modulators for pump-selective spectral
shaping
Activated Singlet Exciton Fission in a Semiconducting Polymer
Singlet exciton fission is a spin-allowed
process to generate two
triplet excitons from a single absorbed photon. This phenomenon offers
great potential in organic photovoltaics, but the mechanism remains
poorly understood. Most reports to date have addressed intermolecular
fission within small-molecular crystals. However, through appropriate
chemical design chromophores capable of intramolecular fission can
also be produced. Here we directly observe sub-100 fs activated singlet
fission in a semiconducting polyÂ(thienylenevinylene). We demonstrate
that fission proceeds directly from the initial 1B<sub>u</sub> exciton,
contrary to current models that involve the lower-lying 2A<sub>g</sub> exciton. In solution, the generated triplet pairs rapidly recombine
and decay through the 2A<sub>g</sub> state. In films, exciton diffusion
breaks this symmetry and we observe long-lived triplets which form
charge-transfer states in photovoltaic blends
Panchromatic “Dye-Doped” Polymer Solar Cells: From Femtosecond Energy Relays to Enhanced Photo-Response
There has been phenomenal effort synthesizing new low-band
gap
polymer hole-conductors which absorb into the near-infrared (NIR),
leading to >10% efficient all-organic solar cells. However, organic
light absorbers have relatively narrow bandwidths, making it challenging
to obtain panchromatic absorption in a single organic semiconductor.
Here, we demonstrate that (polyÂ[2,6-(4,4-bis-(2-ethylhexyl)-4H-cyclopentaÂ[2,1-b;3,4-b0]Âdithiophene)-alt-4,7-(2,1,3-benzothiadia-zole)]
(PCPDTBT) can be “photo-sensitized” across the whole
visible spectrum by “doping” with a visible absorbing
dye, the (2,2,7,7-tetrakisÂ(3-hexyl-5-(7-(4-hexylthiophen-2-yl)ÂbenzoÂ[c]Â[1,2,5]Âthiadiazol-4-yl)Âthiophen-2-yl)-9,9-spirobifluorene)
(spiro-TBT). Through a comprehensive sub-12 femtosecond–nanosecond
spectroscopic study, we demonstrate that extremely efficient and fast
energy transfer occurs from the photoexcited spiro-TBT to the PCPDTBT,
and ultrafast charge injection happens when the system is interfaced
with ZnO as a prototypal electron-acceptor compound. The visible photosensitization
can be effectively exploited and gives panchromatic photoresponse
in prototype polymer/oxide bilayer photovoltaic diodes. This concept
can be successfully adopted for tuning and optimizing the light absorption
and photoresponse in a broad range of polymeric and hybrid solar cells