11 research outputs found
Temperature Dependence of Magnetophotoconductance in One-Dimensional Molecular Assembly of Hexabenzocoronene
Temperature dependencies of transient photocharge and magnetophotoconductance effect of columnar self-assemblies of the hexabenzocoronene derivative (HBC-C14), which is a prospective one-dimensional photoconductor, presented different thermal activation processes for carrier generation and transportation, respectively. Thermal equilibrium between the low-lying short distance and high-lying long-distance geminate electron–hole (e–h) pairs is the origin for activation in carrier generation. The energy difference between these e–h pairs is estimated to be 7 meV, which was mainly due to the Coulomb interaction. On the other hand, the carrier transport with thermal activation was understood by the multiple trapping model. Carrier detrapping from localized states located in the band gap causes the thermal activation in the carrier transport. The shallow energy depth at the density peak of the localized state from the mobility edge (10 ± 3 meV) is a unique nature of HBC-C14 self-assemblies. A very narrow Gaussian distribution for density of the localized states was also clarified
Recombination of Free Carriers and Space Charges in Poly(3-hexylthiophene), as Revealed by Electrically and Capacitively Detected Magnetic Resonances
Mechanism of Intersystem Crossing of Thermally Activated Delayed Fluorescence Molecules
The spin sublevel dynamics of the
excited triplet state in thermally activated delayed fluorescence
(TADF) molecules have not been investigated for high-intensity organic
light-emitting diode materials. Understanding the mechanism for intersystem
crossing (ISC) is thus important for designing novel TADF materials.
We report the first study on the ISC dynamics of the lowest excited
triplet state from the lowest excited singlet state with charge-transfer
(CT) character of TADF molecules with different external quantum efficiencies
(EQEs) using time-resolved electron paramagnetic resonance methods.
Analysis of the observed spin polarization indicates a strong correlation
of the EQE with the population rate due to ISC induced by hyperfine
coupling with the magnetic nuclei. It is concluded that molecules
with high EQE have an extremely small energy gap between the <sup>1</sup>CT and <sup>3</sup>CT states, which allows an additional ISC
channel due to the hyperfine interactions
Temperature Dependence of Magnetophotoconductance in One-Dimensional Molecular Assembly of Hexabenzocoronene
Temperature dependencies of transient
photocharge and magnetophotoconductance
effect of columnar self-assemblies of the hexabenzocoronene derivative
(HBC-C14), which is a prospective one-dimensional photoconductor,
presented different thermal activation processes for carrier generation
and transportation, respectively. Thermal equilibrium between the
low-lying short distance and high-lying long-distance geminate electron–hole
(e–h) pairs is the origin for activation in carrier generation.
The energy difference between these e–h pairs is estimated
to be 7 meV, which was mainly due to the Coulomb interaction. On the
other hand, the carrier transport with thermal activation was understood
by the multiple trapping model. Carrier detrapping from localized
states located in the band gap causes the thermal activation in the
carrier transport. The shallow energy depth at the density peak of
the localized state from the mobility edge (10 ± 3 meV) is a
unique nature of HBC-C14 self-assemblies. A very narrow Gaussian distribution
for density of the localized states was also clarified
Effect of Acceptor Lamination on Photocarrier Dynamics in Hole Transporting Hexabenzocoronene Nanotubular Self-Assembly
Measurements of transient photoconductance
under an external magnetic
field were used to investigate photocarrier dynamics in low-dimensional
hexabenzocoronene (HBC) self-assemblies, which are a promising material
group for highly efficient solar cells achieved by bottom-up technology,
and to clarify the effect of lamination with electron acceptor layer
on the surfaces of HBC nanotubes. In an HBC column without an acceptor,
the carrier generation yield from a geminate electron–hole
(e-h) pair is dependent on the external electric and magnetic fields.
The time dependence of the magnetic field effect on geminate e-h pair
dynamics in the HBC column structure was analyzed to estimate the
recombination rate constants of the singlet and triplet e-h pairs
(<i>k</i><sub>r</sub><sup>S</sup> and <i>k</i><sub>r</sub><sup>T</sup>), which were 1.5 × 10<sup>8</sup> and
1.2 × 10<sup>8</sup> s<sup>–1</sup> respectively. The
same kinetic analysis with consideration of the electric field effect
on the photocarrier generation yield provided an electric field dependent
dissociation rate constant in the range of 10<sup>7</sup>–10<sup>8</sup> s<sup>–1</sup> in the HBC column structure. However,
neither electric nor magnetic field effects on the carrier generation
process were observed in acceptor-appended HBC nanotubes. The disappearance
of the external field effects in acceptor-appended HBC indicates that
the geminate recombination is reduced substantially by a well-organized
donor/acceptor heterojunction with an interval of a few nanometers
due to some σ-bonds. However, efficient nongeminate recombination
with a ratio of <i>k</i><sub>r</sub><sup>S</sup>:<i>k</i><sub>r</sub><sup>T</sup> = 1.0:0.8 in the acceptor-appended
HBC nanotubes was also elucidated by the incident photon density and
magnetic field effects, which is an inherent nature in materials with
high carrier density
What Can Be Learned from Magnetic Field Effects on Singlet Fission: Role of Exchange Interaction in Excited Triplet Pairs
Magnetic field effects (MFEs) on
singlet fission were studied by observing fluorescence from organic
crystal of 1,6-diphenyl-1,3,5-hexatriene under magnetic fields of
up to 5 T. We found anomalous MFE dips at magnetic fields higher than
2 T, in addition to the known MFEs which saturated around 1 T. The
observed results were analyzed by using the stochastic Liouville equation
(SLE) in which a distance-dependent exchange interaction (<i>J</i>) in triplet pair, hopping of triplet, and geminate fusion
in contacted triplet pair were incorporated. The SLE analysis revealed
that the observed dips were caused by a MFE due to the level crossing
mechanism and strongly suggested that the contacted triplet pair has
a large <i>J</i>, which has been ignored in the previous
model of MFEs on the singlet fission. Present results lead to the
conclusion that the initial dissociation of the singlet exciton to
the contacted triplet pair does not show the MFE and the triplet pair
at a <i>separated</i> distance produced by hopping of the
triplet plays an important role on the generation of the MFE on the
singlet fission
Solvent Viscosity Effect on Triplet–Triplet Pair in Triplet Fusion
The effect of the solvent viscosity
dependence of time-resolved
magnetoluminescence (ML) on the delayed fluorescence of 9,10-diphenylanthracene
(DPA) sensitized by platinum octaethylporphyrin has clarified the
structure and dynamics of the triplet–triplet pair (TT), i.e.,
the transition state of triplet fusion. Phase inversion of the ML
effect with time provides evidence for the recycle dynamics of the
excited triplet state for DPA in triplet fusion. The electron spin-relaxation
by random molecular rotation causes intersystem crossing among the
different spin states of the triplet–triplet pair and allows
the <sup>3,5</sup>TT to engage in triplet fusion. Therefore, slow-down
of the molecular diffusion by an increase in the solvent viscosity
can enhance the triplet fusion yield. However, the reduction of the
ML effect observed in quite high viscosity solvents suggests that
the substantially slow rotational motion decreases the triplet fusion
yield due to steric factors in electron exchange from the triplet–triplet
pair
Carrier Dynamics in Pentacene|C<sub>60</sub> Bilayer Solar Cell Investigated through the Magnetoconductance
We demonstrate the
magnetoconductance (MC) effect originated from
depressing the spin mixing in encounter pairs under the external magnetic
field provides quantitative information about the singlet fission,
the charge recombination, and the trap-related dynamics with triplet
exciton in a bilayer device of pentacene (Pen) and fullerene (C<sub>60</sub>). Three MC effects in low-, moderate-, and high-fields were
detected in the bilayer device at room temperature. Kinetic analysis
of the low-field MC effect showed that the charge recombination yield
at the Pen|C<sub>60</sub> interface is ∼1%. Quantum mechanical
simulations for dynamics of spin-carrying pairs following the conservation
rule of spin angular momentum in recombination showed that the moderate-
and high-field MC effects are caused by, respectively, the trap-related
dynamics with triplet exciton and the singlet fission with a maximum
yield of 52% in the layers. The quantitative information obtained
by investigating the MC effect will contribute to the development
of high-efficiency organic solar cells devices