7 research outputs found
Striking the Right Balance of Intermolecular Coupling for High-Efficiency Singlet Fission
Singlet fission is a process that splits collective excitations, or excitons, into two with unity efficiency. This exciton splitting process, unique to molecular photophysics, has the potential to considerably improve the efficiency of optoelectronic devices through more efficient light harvesting. While the first step of singlet fission has been characterized in great detail, subsequent steps critical to achieving overall highly-efficient singlet-to-triplet conversion are only just beginning to become well understood. One of the most elementary suggestions, which has yet to be tested, is that an appropriately balanced coupling is necessary to ensure overall highly efficient singlet fission; that is, the coupling needs to be strong enough so that the first step is fast and efficient, yet weak enough to ensure the independent behavior of the resultant triplets. In this work, we show how high overall singlet-to-triplet conversion efficiencies can be achieved in singlet fission by ensuring that the triplets comprising the triplet pair behave as independently as possible. We show that side chain sterics govern local packing in amorphous pentacene derivative nanoparticles, and that this in turn controls both the rate at which triplet pairs form and the rate at which they decay. We show how compact side chains and stronger couplings promote a triplet pair that effectively couples to the ground state, whereas bulkier side chains promote a triplet pair that appears more like two independent and long-lived triplet excitations. Our results show that the triplet pair is not emissive, that its decay is best viewed as internal conversion rather than triplet–triplet annihilation, and perhaps most critically that, in contrast to a number of recent suggestions, the triplets comprising the initially formed triplet pair cannot be considered independently. This work represents a significant step toward better understanding intermediates in singlet fission, and how molecular packing and couplings govern overall triplet yields
Scaling Behavior of Human Locomotor Activity Amplitude: Association with Bipolar Disorder
Scale invariance is a feature of complex biological systems, and abnormality of multi-scale behaviour may serve as an indicator of pathology. The hypothalamic suprachiasmatic nucleus (SCN) is a major node in central neural networks responsible for regulating multi-scale behaviour in measures of human locomotor activity. SCN also is implicated in the pathophysiology of bipolar disorder (BD) or manic-depressive illness, a severe, episodic disorder of mood, cognition and behaviour. Here, we investigated scaling behaviour in actigraphically recorded human motility data for potential indicators of BD, particularly its manic phase. A proposed index of scaling behaviour (Vulnerability Index [VI]) derived from such data distinguished between: [i] healthy subjects at high versus low risk of mood disorders; [ii] currently clinically stable BD patients versus matched controls; and [iii] among clinical states in BD patients
Molecular Origins of Defects in Organohalide Perovskites and Their Influence on Charge Carrier Dynamics
The
chemical origins of charge recombination centers in lead-based
organohalide perovskites were investigated using a combination of
quantitative solution chemistry, X-ray diffraction, and time-resolved
photoluminescence spectroscopy. We explored the complex, concentration-dependent
solution equilibria among iodoplumbate coordination complexes that
have been implicated as potential midgap states in organohalide perovskites.
High concentrations of PbI<sub>2</sub>, PbI<sub>3</sub><sup>–</sup>, and PbI<sub>4</sub><sup>2–</sup> were found in precursor
solutions that match those used to deposit perovskite films for solar
cell applications. We found that the concentration of tetraiodoplumbate
PbI<sub>4</sub><sup>2–</sup> is uniquely correlated with the
density of charge recombination centers found in the final perovskite
films regardless of the lead precursor used to cast the films. However,
mixed-halide perovskites commonly referred to as CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> suppressed the formation of PbI<sub>4</sub><sup>2–</sup> in comparison to perovskites that included only iodide, which is
consistent with the longer charge carrier lifetimes reported in mixed-halide
perovskites. These findings bring a molecular-level view to the chemical
origins of charge recombination centers that provides a fundamental
basis from which to understand the reported improvement in uniformity
of perovskite films and devices deposited using sequential methods.
These findings also suggest new approaches to control the formation
of defect precursors during the deposition of organohalide perovskite
absorbers
Electron–Phonon Coupling and Resonant Relaxation from 1D and 1P States in PbS Quantum Dots
Observations
of the hot-phonon bottleneck, which is predicted to
slow the rate of hot carrier cooling in quantum confined nanocrystals,
have been limited to date for reasons that are not fully understood.
We used time-resolved infrared spectroscopy to directly measure higher
energy intraband transitions in PbS colloidal quantum dots. Direct
measurements of these intraband transitions permitted detailed analysis
of the electronic overlap of the quantum confined states that may
influence their relaxation processes. In smaller PbS nanocrystals,
where the hot-phonon bottleneck is expected to be most pronounced,
we found that relaxation of parity selection rules combined with stronger
electron–phonon coupling led to greater spectral overlap of
transitions among the quantum confined states. This created pathways
for fast energy transfer and relaxation that may bypass the predicted
hot-phonon bottleneck. In contrast, larger, but still quantum confined
nanocrystals did not exhibit such relaxation of the parity selection
rules and possessed narrower intraband states. These observations
were consistent with slower relaxation dynamics that have been measured
in larger quantum confined systems. These findings indicated that,
at small radii, electron–phonon interactions overcome the advantageous
increase in energetic separation of the electronic states for PbS
quantum dots. Selection of appropriately sized quantum dots, which
minimize spectral broadening due to electron–phonon interactions
while maximizing electronic state separation, is necessary to observe
the hot-phonon bottleneck. Such optimization may provide a framework
for achieving efficient hot carrier collection and multiple exciton
generation
Dynamic Exchange During Triplet Transport in Nanocrystalline TIPS-Pentacene Films
The multiplication of excitons in
organic semiconductors via singlet
fission offers the potential for photovoltaic cells that exceed the
Shockley–Quiesser limit for single-junction devices. To fully
utilize the potential of singlet fission sensitizers in devices, it
is necessary to understand and control the diffusion of the resultant
triplet excitons. In this work, a new processing method is reported
to systematically tune the intermolecular order and crystalline structure
in films of a model singlet fission chromophore, 6,13-bisÂ(triisopropylsilylethynyl)
pentacene (TIPS-Pn), without the need for chemical modifications.
A combination of transient absorption spectroscopy and quantitative
materials characterization enabled a detailed examination of the distance-
and time-dependence of triplet exciton diffusion following singlet
fission in these nanocrystalline TIPS-Pn films. Triplet–triplet
annihilation rate constants were found to be representative of the
weighted average of crystalline and amorphous phases in TIPS-Pn films
comprising a mixture of phases. Adopting a diffusion model used to
describe triplet–triplet annihilation, the triplet diffusion
lengths for nanocrystalline and amorphous films of TIPS-Pn were estimated
to be ∼75 and ∼14 nm, respectively. Importantly, the
presence of even a small fraction (<10%) of the amorphous phase
in the TIPS-Pn films greatly decreased the ultimate triplet diffusion
length, suggesting that pure crystalline materials may be essential
to efficiently harvest multiplied triplets even when singlet fission
occurs on ultrafast time scales
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Striking the right balance of intermolecular coupling for high-efficiency singlet fission
Chemical Science.
Volume 9, Issue 29, 2018, Pages 6240-6259.© 2018 The Royal Society of Chemistry. Singlet fission is a process that splits collective excitations, or excitons, into two with unity efficiency. This exciton splitting process, unique to molecular photophysics, has the potential to considerably improve the efficiency of optoelectronic devices through more efficient light harvesting. While the first step of singlet fission has been characterized in great detail, subsequent steps critical to achieving overall highly-efficient singlet-to-triplet conversion are only just beginning to become well understood. One of the most elementary suggestions, which has yet to be tested, is that an appropriately balanced coupling is necessary to ensure overall highly efficient singlet fission; that is, the coupling needs to be strong enough so that the first step is fast and efficient, yet weak enough to ensure the independent behavior of the resultant triplets. In this work, we show how high overall singlet-to-triplet conversion efficiencies can be achieved in singlet fission by ensuring that the triplets comprising the triplet pair behave as independently as possible. We show that side chain sterics govern local packing in amorphous pentacene derivative nanoparticles, and that this in turn controls both the rate at which triplet pairs form and the rate at which they decay. We show how compact side chains and stronger couplings promote a triplet pair that effectively couples to the ground state, whereas bulkier side chains promote a triplet pair that appears more like two independent and long-lived triplet excitations. Our results show that the triplet pair is not emissive, that its decay is best viewed as internal conversion rather than triplet-triplet annihilation, and perhaps most critically that, in contrast to a number of recent suggestions, the triplets comprising the initially formed triplet pair cannot be considered independently. This work represents a significant step toward better understanding intermediates in singlet fission, and how molecular packing and couplings govern overall triplet yields