8 research outputs found
Controlling Microstructure of Pentacene Derivatives by Solution Processing: Impact of Structural Anisotropy on Optoelectronic Properties
The consideration of anisotropic structural properties and their impact on optoelectronic properties in small-molecule thin films is vital to understand the performance of devices incorporating crystalline organic semiconductors. Here we report on the important relationship between structural and optoelectronic anisotropy in aligned, functionalized-pentacene thin films fabricated using the solution-based zone-casting technique. The microstructure of thin films composed of 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-pentacene) and 6,13-bis(triethylsilylethynyl)pentacene (TES-pentacene) is systematically controlled by varying the casting speed. By controlling the structural alignment, we were able to experimentally decouple, for the first time in these films, an intramolecular absorption transition dipole (at ∼440 nm) oriented close to the pentacene short axis and an intermolecular absorption transition dipole (at ∼695 nm) oriented predominantly along the conjugated pentacene–pentacene core stacking axis (crystallographic <i>a</i>-axis) in both films. Using the intermolecular absorption as a signature for intermolecular delocalization, much higher optical dichroism was obtained in TES-pentacene (16 ± 6) than TIPS-pentacene (3.2 ± 0.1), which was attributed to the 1D packing structure of TES-pentacene compared to the 2D packing structure of TIPS-pentacene. This result was also supported by field-effect mobility anisotropy measurements of the films, with TES-pentacene exhibiting a higher anisotropy (∼21–47, depending on the casting speed) than TIPS-pentacene (∼3–10)
Ultrafast Intraband Spectroscopy of Hot-Carrier Cooling in Lead-Halide Perovskites
The
rapid relaxation of above-band-gap “hot” carriers
(HCs) imposes the key efficiency limit in lead-halide perovskite (LHP)
solar cells. Recent studies have indicated that HC cooling in these
systems may be sensitive to materials composition, as well as the
energy and density of excited states. However, the key parameters
underpinning the cooling mechanism are currently under debate. Here
we use a sequence of ultrafast optical pulses (visible pump–infrared
push–infrared probe) to directly compare the intraband cooling
dynamics in five common LHPs: FAPbI<sub>3</sub>, FAPbBr<sub>3</sub>, MAPbI<sub>3</sub>, MAPbBr<sub>3</sub>, and CsPbBr<sub>3</sub>.
We observe ∼100–900 fs cooling times, with slower cooling
at higher HC densities. This effect is strongest in the all-inorganic
Cs-based system, compared to the hybrid analogues with organic cations.
These observations, together with band structure calculations, allow
us to quantify the origin of the “hot-phonon bottleneck”
in LHPs and assert the thermodynamic contribution of a symmetry-breaking
organic cation toward rapid HC cooling
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Highly Luminescent Encapsulated Narrow Bandgap Polymers Based on Diketopyrrolopyrrole
We present the synthesis
and characterization of a series of encapsulated
diketopyrrolopyrrole red-emitting conjugated polymers. The novel materials
display extremely high fluorescence quantum yields in both solution
(>70%) and thin film (>20%). Both the absorption and emission
spectra
show clearer, more defined features compared to their naked counterparts
demonstrating the suppression of inter and intramolecular aggregation.
We find that the encapsulation results in decreased energetic disorder
and a dramatic increase in backbone colinearity as evidenced by scanning
tunnelling microscopy. This study paves the way for diketopyrrolopyrrole
to be used in emissive solid state applications and demonstrates a
novel method to reduce structural disorder in conjugated polymers
Recommended from our members
Highly Luminescent Encapsulated Narrow Bandgap Polymers Based on Diketopyrrolopyrrole
We present the synthesis
and characterization of a series of encapsulated
diketopyrrolopyrrole red-emitting conjugated polymers. The novel materials
display extremely high fluorescence quantum yields in both solution
(>70%) and thin film (>20%). Both the absorption and emission
spectra
show clearer, more defined features compared to their naked counterparts
demonstrating the suppression of inter and intramolecular aggregation.
We find that the encapsulation results in decreased energetic disorder
and a dramatic increase in backbone colinearity as evidenced by scanning
tunnelling microscopy. This study paves the way for diketopyrrolopyrrole
to be used in emissive solid state applications and demonstrates a
novel method to reduce structural disorder in conjugated polymers
Effect of Fluorination on the Properties of a Donor–Acceptor Copolymer for Use in Photovoltaic Cells and Transistors
Two novel indacenodithiophene (IDT) based donor–acceptor
conjugated polymers for use in organic field effect transistors and
photovoltaic devices are synthesized and characterized. The effect
of inclusion of two fluorine atoms on the acceptor portion of the
polymer is thoroughly investigated via a range of techniques. The
inductively withdrawing and mesomerically donating properties of the
fluorine atoms result in a decrease of the highest occupied molecular
orbital (HOMO), with little effect on the lowest unoccupied molecular
orbital (LUMO) as demonstrated through density functional theory (DFT)
analysis. Inclusion of fluorine atoms also leads to a potentially
more planar backbone through inter and intrachain interactions. Use
of the novel materials in organic field effect transistor (OFET) and
organic photovoltaic (OPV) devices leads to high mobilities around
0.1 cm<sup>2</sup>/(V s) and solar cell efficiencies around 4.5%
Stable Mott Polaron State Limits the Charge Density in Lead Halide Perovskites
Large polarons are known to form in lead halide perovskites
(LHPs).
Photoinduced isolated polarons at low densities have been well-researched,
but many-body interactions at elevated polaron densities, exceeding
the Mott criterion (i.e., Mott polaron density), have remained elusive.
Here, employing ultrafast terahertz spectroscopy, we identify a stable
Mott polaron state in LHPs at which the polaron wavefunctions start
to overlap. The Mott polaron density is determined to be ∼1018 cm–3, in good agreement with theoretical
calculations based on the Feynman polaron model. The electronic phase
transition across the Mott density is found to be universal in LHPs
and independent of the constituent ions. Exceeding the Mott polaron
density, excess photoinjected charge carriers annihilate quickly within
tens to hundreds of picoseconds, before reaching the stable and long-lived
Mott state. These results have considerable implications for LHP-based
devices and for understanding exotic phenomena reported in LHPs
Confinement and Exciton Binding Energy Effects on Hot Carrier Cooling in Lead Halide Perovskite Nanomaterials
The relaxation of
the above-gap (“hot”) carriers
in lead halide perovskites (LHPs) is important for applications in
photovoltaics and offers insights into carrier–carrier and
carrier–phonon interactions. However, the role of quantum confinement
in the hot carrier dynamics of nanosystems is still disputed. Here,
we devise a single approach, ultrafast pump–push–probe
spectroscopy, to study carrier cooling in six different size-controlled
LHP nanomaterials. In cuboidal nanocrystals, we observe only a weak
size effect on the cooling dynamics. In contrast, two-dimensional
systems show suppression of the hot phonon bottleneck effect common
in bulk perovskites. The proposed kinetic model describes the intrinsic
and density-dependent cooling times accurately in all studied perovskite
systems using only carrier–carrier, carrier–phonon,
and excitonic coupling constants. This highlights the impact of exciton
formation on carrier cooling and promotes dimensional confinement
as a tool for engineering carrier–phonon and carrier–carrier
interactions in LHP optoelectronic materials
Isostructural, Deeper Highest Occupied Molecular Orbital Analogues of Poly(3-hexylthiophene) for High-Open Circuit Voltage Organic Solar Cells
We present the synthesis and characterization
of two novel thiazole-containing
conjugated polymers (<b>PTTTz</b> and <b>PTTz</b>) that
are isostructural to poly(3-hexylthiophene) (P3HT). The novel materials
demonstrate optical and morphological properties almost identical
to those of P3HT but with HOMO and LUMO levels that are up to 0.45
eV deeper. An intramolecular planarizing nitrogen–sulfur nonbonding
interaction is observed, and its magnitude and origin are discussed.
Both materials demonstrate significantly greater open circuit voltages
than P3HT in bulk heterojunction solar cells. <b>PTTTz</b> is
shown to be an extremely versatile donor polymer that can be used
with a wide variety of fullerene acceptors with device efficiencies
of up to 4.5%. It is anticipated that this material could be used
as a high-open circuit voltage alternative to P3HT in organic solar
cells