22 research outputs found
Perovskite Oxide SrTiO<sub>3</sub> as an Efficient Electron Transporter for Hybrid Perovskite Solar Cells
In this work, we explored perovskite
oxide SrTiO<sub>3</sub> (STO)
for the first time as the electron-transporting layer in organolead
trihalide perovskite solar cells. The steady-state photoluminescence
(PL) quenching and transient absorption experiments revealed efficient
photoelectron transfer from CH<sub>3</sub>NH<sub>3</sub>PbI<sub>3–<i>x</i></sub>Cl<sub><i>x</i></sub> to STO. Perovskite
solar cells with <i>meso</i>-STO exhibit an open circuit
voltage of 1.01 V, which is 25% higher than the value of 0.81 V achieved
in the control device with the conventional <i>meso</i>-TiO<sub>2</sub>. In addition, an increase of 17% in the fill factor was achieved
by tailoring the thickness of the <i>meso</i>-STO layer.
We found that the application of STO leads to uniform perovskite layers
with large grains and complete surface coverage, leading to a high
shunt resistance and improved performance. These findings suggest
STO as a competitive candidate as electron transport material in organometal
perovskite solar cells
Solvent-Dependent Excited-State Hydrogen Transfer and Intersystem Crossing in 2‑(2′-Hydroxyphenyl)-Benzothiazole
The
excited-state intramolecular hydrogen transfer (ESIHT) of 2-(2′-hydroxyphenyl)
benzothiazole (HBT) has been investigated in a series of nonpolar,
polar aprotic, and polar protic solvents. A variety of state-of-the-art
experimental methods were employed, including femto- and nanosecond
transient absorption and fluorescence upconversion spectroscopy with
broadband capabilities. We show that the dynamics and mechanism of
ESIHT of the singlet excited HBT are strongly solvent-dependent. In
nonpolar solvents, the data demonstrate that HBT molecules adopt a
closed form stabilized by O–H···N chelated hydrogen
bonds with no twisting angle, and the photoinduced H transfer occurs
within 120 fs, leading to the formation of a keto tautomer. In polar
solvents, owing to dipole–dipole cross talk and hydrogen bonding
interactions, the H transfer process is followed by ultrafast nonradiative
deactivation channels, including ultrafast internal conversion (IC)
and intersystem crossing (ISC). This is likely to be driven by the
twisting motion around the C–C bond between the hydroxyphenyl
and thiazole moieties, facilitating the IC back to the enol ground
state or to the keto triplet state. In addition, our femtosecond time-resolved
fluorescence experiments indicate, for the first time, that the lifetime
of the enol form in ACN is approximately 280 fs. This observation
indicates that the solvent plays a crucial role in breaking the H
bond and deactivating the excited state of the HBT. Interestingly,
the broadband transient absorption and fluorescence up-conversion
data clearly demonstrate that the intermolecular proton transfer from
the excited HBT to the DMSO solvent is about 190 fs, forming the HBT
anion excited state
Photophysics of Organometallic Platinum(II) Derivatives of the Diketopyrrolopyrrole Chromophore
A pair
of diketopyrrolopyrrole (DPP) chromophores that are end-functionalized
with platinum containing “auxochromes” were subjected
to electrochemical and photophysical study. The chromophores contain
either platinum acetylide or ortho-metalated 2-thienylpyridinylÂ(platinum)
end-groups (<b>DPP–PtÂ(CC)</b> and <b>DPP–PtÂ(acac)</b>, respectively). The ground state redox potentials of the chromophores
were determined by solution electrochemistry, and the HOMO and LUMO
levels were estimated. The chromophores’ photophysical properties
were characterized by absorption, photoluminescence, and time-resolved
absorption spectroscopy on time scales from sub-picoseconds to microseconds.
Density functional theory (DFT) computations were performed to understand
the molecular orbitals involved in both the singlet and triplet excited
state photophysics. The results reveal that in both platinum DPP derivatives
the organometallic auxochromes have a significant effect on the chromophores’
photophysics. The most profound effect is a reduction in the fluorescence
yields accompanied by enhanced triplet yields due to spin–orbit
coupling induced by the metal centers. The effects are most pronounced
in <b>DPP–PtÂ(acac)</b>, indicating that the orthometalated
platinum auxochrome is able to induce spin–orbital coupling
to a greater extent compared to the platinum acetylide units
Real-Time Observation of Ultrafast Intraband Relaxation and Exciton Multiplication in PbS Quantum Dots
We
examine ultrafast intraconduction band relaxation and multiple-exciton
generation (MEG) in PbS quantum dots (QDs) using transient absorption
spectroscopy with 120 fs temporal resolution. The intraconduction
band relaxation can be directly and excellently resolved spectrally
and temporally by applying broadband pump–probe spectroscopy
to excite and detect the wavelengths around the exciton absorption
peak, which is located in the near-infrared region. The time-resolved
data unambiguously demonstrate that the intraband relaxation time
progressively increases as the pump-photon energy increases. Moreover,
the relaxation time becomes much shorter as the size of the QDs decreases,
indicating the crucial role of spatial confinement in the intraband
relaxation process. Additionally, our results reveal the systematic
scaling of the intraband relaxation time with both excess energy above
the effective energy band gap and QD size. We also assess MEG in different
sizes of the QDs. Under the condition of high-energy photon excitation,
which is well above the MEG energy threshold, ultrafast bleach recovery
due to the nonradiative Auger recombination of the multiple electron–hole
pairs provides conclusive experimental evidence for the presence of
MEG. For instance, we achieved quantum efficiencies of 159, 129 and
106% per single-absorbed photon at pump photoexcition of three times
the band gap for QDs with band gaps of 880 nm (1.41 eV), 1000 nm (1.24
eV) and 1210 nm (1.0 eV), respectively. These findings demonstrate
clearly that the efficiency of transferring excess photon energy to
carrier multiplication is significantly increased in smaller QDs compared
with larger ones. Finally, we discuss the Auger recombination dynamics
of the multiple electron–hole pairs as a function of QD size
Ligand-Free Nanocrystals of Highly Emissive Cs<sub>4</sub>PbBr<sub>6</sub> Perovskite
Although
ligands of long carbon chains are very crucial to form
stable colloidal perovskite nanocrystals (NCs), they create a severe
barrier for efficient charge injection or extraction in quantum-dot-based
optoelectronics, such as light emitting diode or solar cell. Here,
we report a new approach to preparing ligand-free perovskite NCs of
Cs<sub>4</sub>PbBr<sub>6</sub>, which retained high photoluminescence quantum yield (44%). Such an
approach involves a polar solvent (acetonitrile) and two small molecules
(ammonium acetate and cesium chloride), which replace the organic
ligand and still protect the nanocrystals from dissolution. The successful
removal of hydrophobic long ligands was evidenced by Fourier transform
infrared spectroscopy, ζ potential analysis, and thermogravimetric
analysis. Unlike conventional perovskite NCs that are extremely susceptible
to polar solvents, the ligand-free Cs<sub>4</sub>PbBr<sub>6</sub> NCs
show robust resistance to polar solvents. Our ligand-free procedure
opens many possibilities not only from a material hybridization perspective
but also in optimizing charge injection and extraction in semiconductor
quantum-dot-based optoelectronics applications
Generation of Multiple Excitons in Ag<sub>2</sub>S Quantum Dots: Single High-Energy versus Multiple-Photon Excitation
We explored biexciton generation
via carrier multiplication (or
multiple-exciton generation) by high-energy photons and by multiple-photon
absorption in Ag<sub>2</sub>S quantum dots (QDs) using femtosecond
broad-band transient absorption spectroscopy. Irrespective of the
size of the QDs and how the multiple excitons are generated in the
Ag<sub>2</sub>S QDs, two distinct characteristic time constants of
9.6–10.2 and 135–175 ps are obtained for the nonradiative
Auger recombination of the multiple excitons, indicating the existence
of two binding excitons, namely, tightly bound and weakly bound excitons.
More importantly, the lifetimes of multiple excitons in Ag<sub>2</sub>S QDs were about 1 and 2 orders of magnitude longer than those of
comparable size PbS QDs and single-walled carbon nanotubes, respectively.
This result is significant because it suggests that by utilizing an
appropriate electron acceptor, there is a higher possibility to extract
multiple electron–hole pairs in Ag<sub>2</sub>S QDs, which
should improve the performance of QD-based solar cell devices
Direct Femtosecond Observation of Charge Carrier Recombination in Ternary Semiconductor Nanocrystals: The Effect of Composition and Shelling
Heavy-metal free ternary semiconductor
nanocrystals are emerging
as key materials in photoactive applications. However, the relative
abundance of intra-bandgap defect states and lack of understanding
of their origins within this class of nanocrystals are major factors
limiting their applicability. To remove these undesirable defect states
which considerably shorten the lifetimes of photogenerated excited
carriers, a detailed understanding about their origin and nature is
required. In this report, we monitor the ultrafast charge carrier
dynamics of CuInS<sub>2</sub> (CIS), CuInSSe (CISSe), and CuInSe<sub>2</sub> (CISe) nanocrystals, before and after ZnS shelling, using
state-of-the-art time-resolved laser spectroscopy with broadband capabilities.
The experimental results demonstrate the presence of both electron
and hole trapping intra-bandgap states in the nanocrystals which can
be removed significantly by ZnS shelling, and the carrier dynamics
is slowed down. Another important observation remains the reduction
of carrier lifetime in the presence of Se, and the shelling strategy
is observed to be less effective at suppressing trap states. This
study provides quantitative physical insights into the role of anion
composition and shelling on the charge carrier dynamics in ternary
CIS, CISSe, and CISe nanocrystals which are essential to improve their
applicability for photovoltaics and optoelectronics
Quantum Confinement-Tunable Ultrafast Charge Transfer at the PbS Quantum Dot and Phenyl‑C<sub>61</sub>-butyric Acid Methyl Ester Interface
Quantum
dot (QD) solar cells have emerged as promising low-cost
alternatives to existing photovoltaic technologies. Here, we investigate
charge transfer and separation at PbS QDs and phenyl-C<sub>61</sub>-butyric acid methyl ester (PCBM) interfaces using a combination
of femtosecond broadband transient absorption (TA) spectroscopy and
steady-state photoluminescence quenching measurements. We analyzed
ultrafast electron injection and charge separation at PbS QD/PCBM
interfaces for four different QD sizes and as a function of PCBM concentration.
The results reveal that the energy band alignment, tuned by the quantum
size effect, is the key element for efficient electron injection and
charge separation processes. More specifically, the steady-state and
time-resolved data demonstrate that only small-sized PbS QDs with
a bandgap larger than 1 eV can transfer electrons to PCBM upon light
absorption. We show that these trends result from the formation of
a type-II interface band alignment, as a consequence of the size distribution
of the QDs. Transient absorption data indicate that electron injection
from photoexcited PbS QDs to PCBM occurs within our temporal resolution
of 120 fs for QDs with bandgaps that achieve type-II alignment, while
virtually all signals observed in smaller bandgap QD samples result
from large bandgap outliers in the size distribution. Taken together,
our results clearly demonstrate that charge transfer rates at QD interfaces
can be tuned by several orders of magnitude by engineering the QD
size distribution. The work presented here will advance both the design
and the understanding of QD interfaces for solar energy conversion
Water-Induced Dimensionality Reduction in Metal-Halide Perovskites
Metal-halide
perovskite materials are highly attractive materials
for optoelectronic applications. However, the instability of perovskite
materials caused by moisture and heat-induced degradation impairs
future prospects of using these materials. Here we employ water to
directly transform films of the three-dimensional (3D) perovskite
CsPbBr<sub>3</sub> to stable two-dimensional (2D) perovskite-related
CsPb<sub>2</sub>Br<sub>5</sub>. A sequential dissolution–recrystallization
process governs this water-induced transformation under PbBr<sub>2</sub> rich conditions. We find that these postsynthesized 2D perovskite-related
material films exhibit excellent stability against humidity and high
photoluminescence quantum yield. We believe that our results provide
a new synthetic method to generate stable 2D perovskite-related materials
that could be applicable for light emitting device applications
Water-Induced Dimensionality Reduction in Metal-Halide Perovskites
Metal-halide
perovskite materials are highly attractive materials
for optoelectronic applications. However, the instability of perovskite
materials caused by moisture and heat-induced degradation impairs
future prospects of using these materials. Here we employ water to
directly transform films of the three-dimensional (3D) perovskite
CsPbBr<sub>3</sub> to stable two-dimensional (2D) perovskite-related
CsPb<sub>2</sub>Br<sub>5</sub>. A sequential dissolution–recrystallization
process governs this water-induced transformation under PbBr<sub>2</sub> rich conditions. We find that these postsynthesized 2D perovskite-related
material films exhibit excellent stability against humidity and high
photoluminescence quantum yield. We believe that our results provide
a new synthetic method to generate stable 2D perovskite-related materials
that could be applicable for light emitting device applications