321 research outputs found
Low-temperature quenching of one-dimensional localised Frenkel excitons
We present a theoretical analysis of low-temperature quenching of
one-dimensional Frenkel excitons that are localised by moderate on-site
(diagonal) uncorrelated disorder. Exciton diffusion is considered as an
incoherent hopping over localization segments and is probed by the exciton
fluorescence quenching at point traps. The rate equation is used to calculate
the temperature dependence of the exciton quenching. The activation temperature
of the diffusion is found to be of the order of the width of the exciton
absorption band. We demonstrate that the intra-segment scattering is extremely
important for the exciton diffusion. We discuss also experimental data on the
fast exciton-exciton annihilation in linear molecular aggregates at low
temperatures.Comment: 7 pages, 3 figures, accepted to Chem. Phys. Let
Temperature dependent fluorescence in disordered Frenkel chains: interplay of equilibration and local band-edge level structure
We model the optical dynamics in linear Frenkel exciton systems governed by
scattering on static disorder and lattice vibrations, and calculate the
temperature dependent fluorescence spectrum and lifetime. The fluorescence
Stokes shift shows a nonmonotonic behavior with temperature, which derives from
the interplay of the local band-edge level structure and thermal equilibration.
The model yields excellent fits to experiments performed on linear dye
aggregates.Comment: 4 pages, 3 figure
Exciton Dephasing and Thermal Line Broadening in Molecular Aggregates
Using a model of Frenkel excitons coupled to a bath of acoustic phonons in
the host medium, we study the temperature dependence of the dephasing rates and
homogeneous line width in linear molecular aggregates. The model includes
localization by disorder and predicts a power-law thermal scaling of the
effective homogeneous line width. The theory gives excellent agreement with
temperature dependent absorption and hole-burning experiments on aggregates of
the dye pseudoisocyanine.Comment: 11 pages, 3 PostScript figure
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Photoluminescence Mapping over Laser Pulse Fluence and Repetition Rate as a Fingerprint of Charge and Defect Dynamics in Perovskites
Defects in metal halide perovskites (MHP) are photosensitive, making the observer effect unavoidable when laser spectroscopy methods are applied. Photoluminescence (PL) bleaching and enhancement under light soaking and recovery in dark are examples of the transient phenomena that are consequent to the creation and healing of defects. Depending on the initial sample composition, environment, and other factors, the defect nature and evolution can strongly vary, making spectroscopic data analysis prone to misinterpretations. Herein, the use of an automatically acquired dependence of PL quantum yield (PLQY) on the laser pulse repetition rate and pulse fluence as a unique fingerprint of both charge carrier dynamics and defect evolution is demonstrated. A simple visual comparison of such fingerprints allows for assessment of similarities and differences between MHP samples. The study illustrates this by examining methylammonium lead triiodide (MAPbI3) films with altered stoichiometry that just after preparation showed very pronounced defect dynamics at time scale from milliseconds to seconds, clearly distorting the PLQY fingerprint. Upon weeks of storage, the sample fingerprints evolve toward the standard stoichiometric MAPbI3 in terms of both charge carrier dynamics and defect stability. Automatic PLQY mapping can be used as a universal method for assessment of perovskite sample quality
Are Shockley-Read-Hall and ABC models valid for lead halide perovskites?
Metal halide perovskites are an important class of emerging semiconductors.
Their charge dynamics is poorly understood due to limited knowledge of defect
physics and charge recombination mechanisms. Nevertheless, classical ABC and
Shockley-Read-Hall (SRH) models are ubiquitously applied to perovskites without
considering their validity. Herein, an advanced technique mapping
photoluminescence quantum yield (PLQY) as a function of both the excitation
pulse energy and repetition frequency is developed and employed to examine the
validity of these models. While ABC and SRH fail to explain the charge dynamics
in a broad range of conditions, the addition of Auger recombination and
trapping to the SRH model enables a quantitative fitting of PLQY maps and
low-power PL decay kinetics, and extracting trap concentrations and efficacies.
Higher-power PL kinetics requires the inclusion of additional non-linear
processes. The PLQY mapping developed herein is suitable for a comprehensive
testing of theories and is applicable to any semiconductor.Comment: Supplementary Information available at
https://cloudstore.zih.tu-dresden.de/index.php/s/t5gBPJgwZiwfRR
Impact of Excess Lead Iodide on the Recombination Kinetics in Metal Halide Perovskites
Fundmental
comprehension of light-induced processes in perovskites
are still scarce. One active debate surrounds the influence of excess
lead iodide (PbI2) on device performance, as well as optoelectronic
properties, where both beneficial and detrimental traits have been
reported. Here, we study its impact on charge carrier recombination
kinetics by simultaneously acquiring the photoluminescence quantum
yield and time-resolved photoluminescence as a function of excitation
wavelength (450–780 nm). The presence of PbI2 in
the perovskite film is identified via a unique spectroscopic signature
in the PLQY spectrum. Probing the recombination in the presence and
absence of this signature, we detect a radiative bimolecular recombination
mechanism induced by PbI2. Spatially resolving the photoluminescence,
we determine that this radiative process occurs in a small volume
at the PbI2/perovskite interface, which is only active
when charge carriers are generated in PbI2, and therefore
provide deeper insight into how excess PbI2 may improve
the properties of perovskite-based devices
Repurposing Poly(3-hexylthiophene) as a Conductivity-Reducing Additive for Polyethylene-Based High-Voltage Insulation
Poly(3-hexylthiophene) (P3HT) is found to be a highly effective conductivity-reducing additive for low-density polyethylene (LDPE), which introduces a new application area to the field of conjugated polymers. Additives that reduce the direct-current (DC) electrical conductivity of an insulation material at high electric fields have gained a lot of research interest because they may facilitate the design of more efficient high-voltage direct-current power cables. An ultralow concentration of regio-regular P3HT of 0.0005 wt% is found to reduce the DC conductivity of LDPE threefold, which translates into the highest efficiency reported for any conductivity-reducing additive to date. The here-established approach, i.e., the use of a conjugated polymer as a mere additive, may boost demand in absolute terms beyond the quantities needed for thin-film electronics, which would turn organic semiconductors from a niche product into commodity chemicals
Repurposing Poly(3-hexylthiophene) as a Conductivity-Reducing Additive for Polyethylene-Based High-Voltage Insulation
Poly(3-hexylthiophene) (P3HT) is found to be a highly effective conductivity-reducing additive for low-density polyethylene (LDPE), which introduces a new application area to the field of conjugated polymers. Additives that reduce the direct-current (DC) electrical conductivity of an insulation material at high electric fields have gained a lot of research interest because they may facilitate the design of more efficient high-voltage direct-current power cables. An ultralow concentration of regio-regular P3HT of 0.0005 wt% is found to reduce the DC conductivity of LDPE threefold, which translates into the highest efficiency reported for any conductivity-reducing additive to date. The here-established approach, i.e., the use of a conjugated polymer as a mere additive, may boost demand in absolute terms beyond the quantities needed for thin-film electronics, which would turn organic semiconductors from a niche product into commodity chemicals
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