15 research outputs found
Real-Time Tracking of Polymer Crystallization Dynamics in Organic Bulk Heterojunctions by Raman Microscopy
State-of-the-art organic photovoltaic active layers typically undergo post-treatment such as thermal or solvent vapor annealing to increase their performance by tuning the bulk heterojunction morphology. Molecular crystallinity is one of the key factors that determine the morphology. Real-time tracking of the crystallinity during the post-treatment is strongly desired for understanding the physics of the crystallization process and for optimizing the post-treatment protocol. Here, we report on the cold crystallization (CC) dynamics of the polymer in the temperature range of 50-150 degrees C in polymer:fullerene blends based on poly(3-hexylthiophene) with various fullerene-based acceptors (C-60, PC61BM, PC71BM, bisPC(61)BM, HBIM, AIM8, and IrC60) in real-time by Raman microscopy. We also reveal how different solvents, fullerene acceptors, and temperatures affect CC during thermal annealing. We further demonstrate a correlation between the fullerene derivative weight and the polymer crystallinity for the as-cast films and also a correlation of the polymer crystallinity before and after annealing. Our findings are essential for developing efficient strategies of morphology optimization in emerging organic photovoltaic devices with real-time Raman microscopy tracking as a valuable tool
Highly Luminescent Solution-Grown Thiophene-Phenylene Co-Oligomer Single Crystals
Thiophene-phenylene co-oligomers (TPCOs) are among the most promising materials for organic light emitting devices. Here we report on record high among TPCO single crystals photoluminescence quantum yield reaching 60%. The solution-grown crystals are stronger luminescent than the vapor-grown ones, in contrast to a common believe that the vapor-processed organic electronic materials show the highest performance. We also demonstrate that the solution grown TPCO single crystals perform in organic field effect transistors as good as the vapor-grown ones. Altogether, the solution-grown TPCO crystals are demonstrated to hold great potential for organic electronics.</p
Interplay Between Mixed and Pure Exciton States Controls Singlet Fission in Rubrene Single Crystals
Singlet fission (SF) is a multielectron process in which one singlet exciton
S converts into a pair of triplet excitons T+T. SF is widely studied as it may
help overcome the Shockley-Queisser efficiency limit for semiconductor
photovoltaic cells. To elucidate and control the SF mechanism, great attention
has been given to the identification of intermediate states in SF materials,
which often appear elusive due to the complexity and fast timescales of the SF
process. Here, we apply 10fs-1ms transient absorption techniques to high-purity
rubrene single crystals to disentangle the intrinsic fission dynamics from the
effects of defects and grain boundaries and to identify reliably the fission
intermediates. We show that above-gap excitation directly generates a hybrid
vibronically assisted mixture of singlet state and triplet-pair multiexciton
[S:TT], which rapidly (<100fs) and coherently branches into pure singlet or
triplet excitations. The relaxation of [S:TT] to S is followed by a relatively
slow and temperature-activated (48 meV activation energy) incoherent fission
process. The SF competing pathways and intermediates revealed here unify the
observations and models presented in previous studies of SF in rubrene and
propose alternative strategies for the development of SF-enhanced photovoltaic
materials
Diagrammatic theory for Anderson Impurity Model. Stationary property of the thermodynamic potential
A diagrammatic theory around atomic limit is proposed for normal state of
Anderson Impurity Model. The new diagram method is based on the ordinary Wick's
theorem for conduction electrons and a generalized Wick's theorem for gtrongly
correlated impurity electrons. This last theorem coincides with the definition
of Kubo cumulants. For the mean value of the evolution operator a linked
cluster theorem is proved and a Dyson's type equations for one-particle
propagators are established. The main element of these equations is the
correlation function which contains the spin, charge and pairing fluctuations
of the system. The thermodynamic potential of the system is expressed through
one-particle renormalized Green's functions and the corelation function. The
stationary property of the thermodynamic potential is established with respect
to the changes of correlation function.Comment: 7 pages, 6 figures, Submitted to PR
Long-range exciton transport in brightly fluorescent furan/phenylene co-oligomer crystals
The design of light-emitting crystalline organic semiconductors for optoelectronic applications requires a thorough understanding of the singlet exciton transport process. In this study, we show that the singlet exciton diffusion length in a promising semiconductor crystal based on furan/phenylene co-oligomers is 24 nm. To achieve this, we employed the photoluminescence quenching technique using a specially synthesized quencher, which is a long furan/phenylene co-oligomer that was facilely implanted into the host crystal lattice. Extensive Monte-Carlo simulations, exciton-exciton annihilation experiments and numerical modelling fully supported our findings. We further demonstrated the high potential of the furan/phenylene co-oligomer crystals for light-emitting applications by fabricating solution-processed organic light emitting transistors
Acceptor-Enhanced Local Order in Conjugated Polymer Films
Disorder
in conjugated polymers is a general drawback that limits
their use in organic electronics. We show that an archetypical conjugated
polymer, MEH-PPV, enhances its local structural and electronic order
upon addition of an electronic acceptor, trinitrofluorenone (TNF).
First, acceptor addition in MEH-PPV results in a highly structured
XRD pattern characteristic for semicrystalline conjugated polymers.
Second, the surface roughness of the MEH-PPV films increases upon
small acceptor addition, implying formation of crystalline nanodomains.
Third, the low-frequency Raman features of the polymer are narrowed
upon TNF addition and indicate decreased inhomogeneous broadening.
Finally, the photoinduced absorption and surface photovoltage spectroscopy
data show that photoexcited and dark polymer intragap electronic states
assigned to deep defects disappear in the blend. We relate the enhanced
order to formation of a charge-transfer complex between MEH-PPV and
TNF in the electronic ground state. These findings may be of high
importance to control structural properties as they demonstrate an
approach to increasing the order of a conjugated polymer by using
an acceptor additive
Easily Processable Highly Ordered Langmuir-Blodgett Films of Quaterthiophene Disiloxane Dimer for Monolayer Organic Field-Effect Transistors
Self-assembly of highly soluble water-stable
tetramethyldisiloxane-based
dimer of α,α′-dialkylquaterthiophene on the water–air
interface was investigated by Langmuir, grazing incidence X-ray diffraction,
and X-ray reflectivity techniques. The conditions for formation of
very homogeneous crystalline monolayer Langmuir-Blodgett (LB) films
of the oligomer were found. Monolayer organic field-effect transistors
(OFETs) based on these LB films as a semiconducting layer showed hole
mobilities up to 3 × 10<sup>–3</sup> cm<sup>2</sup>/(V
s), on–off ratio of 10<sup>5</sup>, small hysteresis, and high
long-term stability. The electrical performance of the LB films studied
is close to that for the same material in the bulk or in the monolayer
OFETs prepared from water vapor sensitive chlorosilyl derivatives
of quaterthiophene by self-assembling from solution. These findings
show high potential of disiloxane-based LB films in monolayer OFETs
for large-area organic electronics
Molecularly Smooth Single-Crystalline Films of Thiophene–Phenylene Co-Oligomers Grown at the Gas–Liquid Interface
Single
crystals of thiophene–phenelyne co-oligomers (TPCOs)
have previously shown their potential for organic optoelectronics.
Here we report on solution growth of large-area thin single-crystalline
films of TPCOs at the gas–liquid interface by using solvent–antisolvent
crystallization, isothermal slow solvent evaporation, and isochoric
cooling. The studied co-oligomers contain identical conjugated core
(5,5′-diphyenyl-2,2′-bithiophene) and different terminal
substituents, fluorine, trimethylsilyl, or trifluoromethyl. The fabricated
films are molecularly smooth over areas larger than 10 × 10 μm<sup>2</sup>, which is of high importance for organic field-effect devices.
The low-defect structure of the TPCO crystals is suggested from the
monoexponential kinetics of the PL decay measured in a wide dynamic
range (up to four decades) and from low crystal mosaicity assessed
by microfocus X-ray diffraction. The TPCO crystal structure is solved
using a combination of X-ray and electron diffraction. The terminal
substituents affect the crystal structure of TPCOs, bringing about
the formation of a noncentrosymmetric crystal lattice with a crystal
symmetry <i>Cc</i> for the bulkiest trimethylsilyl terminal
groups, which is unusual for linear conjugated oligomers. Comparing
the different crystal growth techniques, it is concluded that the
solvent–antisolvent crystallization is the most robust for
fabrication of single-crystalline TPCOs films. The possible nucleation
and crystallization mechanisms operating at the gas–solution
interface are discussed
Luminescent Organic Semiconducting Langmuir Monolayers
In
recent years, monolayer organic field-effect devices such as transistors
and sensors have demonstrated their high potential. In contrast, monolayer
electroluminescent organic field-effect devices are still in their
infancy. One of the key challenges here is to create an organic material
that self-organizes in a monolayer and combines efficient charge transport
with luminescence. Herein, we report a novel organosilicon derivative
of oligothiophene–phenylene dimer <b>D2-Und-PTTP-TMS</b> (D2, tetramethyldisiloxane; Und, undecylenic spacer; P, 1,4-phenylene;
T, 2,5-thiophene; TMS, trimethylsilyl) that meets these requirements.
The self-assembled Langmuir monolayers of the dimer were investigated
by steady-state and time-resolved photoluminescence spectroscopy,
atomic force microscopy, X-ray reflectometry, and grazing-incidence
X-ray diffraction, and their semiconducting properties were evaluated
in organic field-effect transistors. We found that the best uniform,
fully covered, highly ordered monolayers were semiconducting. Thus,
the ordered two-dimensional (2D) packing of conjugated organic molecules
in the semiconducting Langmuir monolayer is compatible with its high-yield
luminescence, so that 2D molecular aggregation per se does not preclude
highly luminescent properties. Our findings pave the way to the rational
design of functional materials for monolayer organic light-emitting
transistors and other optoelectronic devices