16 research outputs found
Macroscopic Domains within an Oriented TQ1 Film Visualized Using 2D Polarization Imaging
Large-area self-assembly
of functional conjugated polymers holds
a great potential for practical applications of organic electronic
devices. We obtained well-aligned films of polyÂ[2,3-bisÂ(3-octyloxyphenyl)Âquinoxaline-5,8-diyl-<i>alt</i>-thiophene-2,5-diyl] (TQ1) using the floating film transfer
method. Thereby, a droplet of the TQ1 solution was injected on top
of the surface of an immiscible liquid substrate, at the meniscus
formed at the edge of a Petri dish, from where the polymer solution
and the film spread in one direction. Characterization of the TQ1
film using the recently developed two-dimensional polarization imaging
(2D POLIM) revealed large, millimeter-sized domains of oriented polymer
chains. The irregular shape of the contact line at the droplet source
induced the appearance of disordered stripes perpendicular to the
spreading direction. A correlation of polarization parameters measured
using 2D POLIM revealed the microstructure of such stripes, providing
valuable information for further improvement and possible upscaling
of this promising method
Macroscopic Domains within an Oriented TQ1 Film Visualized Using 2D Polarization Imaging
Large-area self-assembly
of functional conjugated polymers holds
a great potential for practical applications of organic electronic
devices. We obtained well-aligned films of polyÂ[2,3-bisÂ(3-octyloxyphenyl)Âquinoxaline-5,8-diyl-<i>alt</i>-thiophene-2,5-diyl] (TQ1) using the floating film transfer
method. Thereby, a droplet of the TQ1 solution was injected on top
of the surface of an immiscible liquid substrate, at the meniscus
formed at the edge of a Petri dish, from where the polymer solution
and the film spread in one direction. Characterization of the TQ1
film using the recently developed two-dimensional polarization imaging
(2D POLIM) revealed large, millimeter-sized domains of oriented polymer
chains. The irregular shape of the contact line at the droplet source
induced the appearance of disordered stripes perpendicular to the
spreading direction. A correlation of polarization parameters measured
using 2D POLIM revealed the microstructure of such stripes, providing
valuable information for further improvement and possible upscaling
of this promising method
Effect of Conjugated Backbone Protection on Intrinsic and Light-Induced Fluorescence Quenching in Polythiophenes
Polythiophenes
(PTs), particularly regioregular polyÂ(3-hexylthiophene-2,5-diyl)
(rr-P3HT), are important materials in photovoltaics. The photophysical
properties of PTs are still poorly understood, because of their aggregation
tendency and formation of interchain species which can be avoided
by insulating the conjugated backbone via self-threading. We investigated
two polymers, rr-P3HT and its insulated analog, imbedded in PMMA at
low concentrations. The exciton decay dynamics and fluorescence quantum
yield were analyzed as a function of excitation power densities over
the range from 1 Ă 10<sup>â4</sup> to 100 W/cm<sup>2</sup>. For both polymers, substantial (up to 5 times) photoinduced fluorescence
quenching was observed owing to singletâtriplet annihilation
and quenching by other long-living charged photoproducts. We found
that chain insulation eliminates static (or ultrafast) fluorescence
quenching, but has no effect on slow dynamic quenching at time scales
longer than 10 ps. We propose that static quenching is solely due
to chain aggregation, whereas the dynamic quenching is a consequence
of intrachain processes
Super-Resolution Luminescence Microspectroscopy Reveals the Mechanism of Photoinduced Degradation in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite Nanocrystals
Photoinduced
degradation of individual methylammonium lead triiodide
(MAPbI<sub>3</sub>) perovskite nanocrystals was studied using super-resolution
luminescence microspectroscopy under intense light excitation. The
photoluminescence (PL) intensity decrease and blue-shift of the PL
spectrum up to 60 nm together with spatial shifts in the emission
localization position up to a few hundred nanometers were visualized
in real time. PL blinking was found to temporarily suspend the degradation
process, indicating that the degradation needs a high concentration
of mobile photogenerated charges to occur. We propose that the mechanistic
process of degradation occurs as the three-dimensional MAPbI<sub>3</sub> crystal structure smoothly collapses to the two-dimensional layered
PbI<sub>2</sub> structure. The degradation starts locally and then
spreads over the whole crystal. The structural collapse is primarily
due to migration of methylammonium ions (MA<sup>+</sup>), which distorts
the lattice structure causing alterations to the PbâIâPb
bond angle and in turn changes the effective band gap
Super-Resolution Luminescence Microspectroscopy Reveals the Mechanism of Photoinduced Degradation in CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3</sub> Perovskite Nanocrystals
Photoinduced
degradation of individual methylammonium lead triiodide
(MAPbI<sub>3</sub>) perovskite nanocrystals was studied using super-resolution
luminescence microspectroscopy under intense light excitation. The
photoluminescence (PL) intensity decrease and blue-shift of the PL
spectrum up to 60 nm together with spatial shifts in the emission
localization position up to a few hundred nanometers were visualized
in real time. PL blinking was found to temporarily suspend the degradation
process, indicating that the degradation needs a high concentration
of mobile photogenerated charges to occur. We propose that the mechanistic
process of degradation occurs as the three-dimensional MAPbI<sub>3</sub> crystal structure smoothly collapses to the two-dimensional layered
PbI<sub>2</sub> structure. The degradation starts locally and then
spreads over the whole crystal. The structural collapse is primarily
due to migration of methylammonium ions (MA<sup>+</sup>), which distorts
the lattice structure causing alterations to the PbâIâPb
bond angle and in turn changes the effective band gap
Organization of Bacteriochlorophylls in Individual Chlorosomes from Chlorobaculum tepidum Studied by 2-Dimensional Polarization Fluorescence Microscopy
Chlorosomes are the largest and most efficient natural light-harvesting systems and contain supramolecular assemblies of bacteriochlorophylls that are organized without proteins. Despite a recent structure determination for chlorosomes from Chlorobaculum tepidum (Ganapathy Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 8525), the issue of a possible large structural disorder is still discussed controversially. We have studied individual chlorosomes prepared under very carefully controlled growth condition by a novel 2-dimensional polarization single molecule imaging technique giving polarization information for both fluorescence excitation and emission simultaneously. Contrary to the existing literature data, the polarization degree or modulation depth (<i>M</i>) for both excitation (absorption) and emission (fluorescence) showed extremely narrow distributions. The fluorescence was always highly polarized with <i>M</i> â 0.77, independent of the excitation wavelength. Moreover, the fluorescence spectra of individual chlorosomes were identical within the error limits. These results lead us to conclude that all chlorosomes possess the same type of internal organization in terms of the arrangement of the bacteriochlorophyll c transition dipole moments and their total excitonic transition dipole possess a cylindrical symmetry in agreement with the previously suggested concentric multitubular chlorophyll aggregate organization (Ganapathy Proc. Natl. Acad. Sci. U.S.A. 2009, 106, 8525<i>)</i>
Single LeĚvy StatesâDisorder Induced Energy Funnels in Molecular Aggregates
Using
fluorescence super-resolution microscopy we studied simultaneous spectral,
spatial localization, and blinking behavior of individual 1D J-aggregates.
Excitons migrating 100 nm are funneled to a trap appearing as an additional
red-shifted blinking fluorescence band. We propose that the trap is
a Frenkel exciton state formed much below the main exciton band edge
due to an environmentally induced heavy-tailed LeĚvy disorder.
This points to disorder engineering as a new avenue in controlling
light-harvesting in molecular ensemble
Polarization Imaging of Emissive Charge Transfer States in Polymer/Fullerene Blends
Photoexcitation of conjugated polymerâfullerene
blends results
in population of a local charge transfer (CT) state at the interface
between the two materials. The competition between recombination and
dissociation of this interfacial state limits the generation of fully
separated free charges. Therefore, a detailed understanding of the
CT states is critical for building a comprehensive picture of the
organic solar cells operation. We applied a new fluorescence microscopy
method called two-dimensional polarization imaging to gain insight
into the orientation of the transition dipole moments of the CT states,
and the associated excitation energy transfer processes in TQ1:PCBM
blend films. The polymer phase was oriented mechanically to relate
the polymer dipole moment orientation to that of the CT states. CT
state formation was observed to be much faster than energy transfer
in the polymer phase. However, after being formed an emissive CT state
does not exchange excitation energy with other CT states, suggesting
that they are spatially and/or energetically isolated. We found that
the quantum yield of the CT emission is smaller for CT states spatially
located in the highly oriented polymer domains, which is interpreted
as the result of enhanced CT state dissociation in highly ordered
structures
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