13 research outputs found
Engineered CuInSe<sub><i>x</i></sub>S<sub>2–<i>x</i></sub> Quantum Dots for Sensitized Solar Cells
Colloidal CuInSe<sub><i>x</i></sub>S<sub>2–<i>x</i></sub> quantum dots (QDs) are an attractive less-toxic
alternative to PbX and CdX (X = S, Se, and Te) QDs for solution-processed
semiconductor devices. This relatively new class of QD materials is
particularly suited to serving as an absorber in photovoltaics, owing
to its high absorption coefficient and near-optimal and finely tunable
band gap. Here, we engineer CuInSe<sub><i>x</i></sub>S<sub>2–<i>x</i></sub> QD sensitizers for enhanced performance
of QD-sensitized TiO<sub>2</sub> solar cells (QDSSCs). Our QD synthesis
employs 1-dodecanethiol (DDT) as a low-cost solvent, which also serves
as a ligand, and a sulfur precursor; addition of triakylphosphine
selenide leads to incorporation of controlled amounts of selenium,
reducing the band gap compared to that of pure CuInS<sub>2</sub> QDs.
This enables significantly higher photocurrent in the near-infrared
(IR) region of the solar spectrum without sacrificing photovoltage.
In order to passivate QD surface recombination centers, we perform
a surface–cation exchange with Cd prior to sensitization, which
enhances chemical stability and leads to a further increase in photocurrent.
We use the synthesized QDs to demonstrate proof-of-concept QDSSCs
with up to 3.5% power conversion efficiency
Auger Recombination of Biexcitons and Negative and Positive Trions in Individual Quantum Dots
Charged exciton states commonly occur both in spectroscopic studies of quantum dots (QDs) and during operation of QD-based devices. The extra charge added to the neutral exciton modifies its radiative decay rate and also opens an additional nonradiative pathway associated with an Auger process whereby the recombination energy of an exciton is transferred to the excess charge. Here we conduct single-dot spectroscopic studies of Auger recombination in thick-shell (“giant”) CdSe/CdS QDs with and without an interfacial alloy layer using time-tagged, time-correlated single-photon counting. In photoluminescence (PL) intensity trajectories of some of the dots, we resolve three distinct states of different emissivities (“bright”, “gray”, and “dark”) attributed, respectively, to the neutral exciton and negative and positive trions. Simultaneously acquired PL lifetime trajectories indicate that the positive trion is much shorter lived than the negative trion, which can be explained by a high density of valence band states and a small hole localization radius (defined by the QD core size), factors that favor an Auger process involving intraband excitation of a hole. A comparison of trion and biexciton lifetimes suggests that the biexciton Auger decay can be treated in terms of a superposition of two independent channels associated with positive- and negative-trion pathways. The resulting interdependence between Auger time constants might simplify the studies of multicarrier recombination by allowing one, for example, to infer Auger lifetimes of trions of one sign based on the measurements of biexciton decay and dynamics of the trions of the opposite sign or, alternatively, estimate the biexciton lifetime based on studies of trion dynamics
Auger Up-Conversion of Low-Intensity Infrared Light in Engineered Quantum Dots
One
source of efficiency losses in photovoltaic cells is their
transparency toward solar photons with energies below the band gap
of the absorbing layer. This loss can be reduced using a process of
up-conversion whereby two or more sub-band-gap photons generate a
single above-gap exciton. Traditional approaches to up-conversion,
such as nonlinear two-photon absorption (2PA) or triplet fusion, suffer
from low efficiency at solar light intensities, a narrow absorption
bandwidth, nonoptimal absorption energies, and difficulties for implementing
in practical devices. Here we show that these deficiencies can be
alleviated using the effect of Auger up-conversion in thick-shell
PbSe/CdSe quantum dots. This process relies on Auger recombination
whereby two low-energy, core-based excitons are converted into a single
higher-energy, shell-based exciton. Compared to their monocomponent
counterparts, the tailored PbSe/CdSe heterostructures feature enhanced
absorption cross-sections, a higher efficiency of the “productive”
Auger pathway involving re-excitation of a hole, and longer lifetimes
of both core- and shell-localized excitons. These features lead to
effective up-conversion cross-sections that are more than 6 orders
of magnitude higher than for standard nonlinear 2PA, which allows
for efficient up-conversion of continuous wave infrared light at intensities
as low as a few watts per square centimeter
Response of Semiconductor Nanocrystals to Extremely Energetic Excitation
Using a combination of transient photoluminescence and
transient
cathodoluminescence (trCL) we, for the first time, identify and quantify
the distribution of electronic excitations in colloidal semiconductor
nanocrystals (NCs) under high-energy excitation. Specifically, we
compare the temporally and spectrally resolved radiative recombination
produced following excitation with 3.1 eV, subpicosecond photon pulses,
or with ionizing radiation in the form of 20 keV picosecond electron
pulses. Using this approach, we derive excitation branching ratios
produced in the scenario of energetic excitation of NCs typical of
X-ray, neutron, or gamma-ray detectors. Resultant trCL spectra and
dynamics for CdSe NCs indicate that all observable emission can be
attributed to recombination between states within the quantum-confined
nanostructure with particularly significant yields of trions and multiexcitons
produced by carrier multiplication. Our observations offer direct
insight into the transduction of atomic excitation into quantum-confined
states within NCs, explain that the root cause of poor performance
in previous scintillation studies arises from efficient nonradiative
Auger recombination, and suggest routes for improved detector materials
Simple yet Versatile Synthesis of CuInSe<sub><i>x</i></sub>S<sub>2–<i>x</i></sub> Quantum Dots for Sunlight Harvesting
Common approaches to synthesizing
alloyed CuInSe<sub><i>x</i></sub>S<sub>2–<i>x</i></sub> quantum dots (QDs)
employ high-cost, air-sensitive phosphine complexes as the selenium
precursor. Such methods typically offer low chemical yields and only
moderate emission efficiencies, particularly for selenium-rich compositions.
Here we demonstrate that such hazardous and air-sensitive selenium
precursors can be completely avoided by utilizing a combination of
thiols and amines that is very effective at reducing and then complexing
with elemental selenium to form a highly reactive selenium precursor
at room temperature. The optical properties of the CuInSe<sub><i>x</i></sub>S<sub>2–<i>x</i></sub> QDs synthesized
by this new approach can be finely tuned for optimal sunlight harvesting
through control of QD size and composition. In order to demonstrate
the importance of such material tunability, we incorporate QDs into
liquid-junction Grätzel solar cells and study correlations
between varied QD size and composition and the resulting device performance.
We also investigate charge transport in films of CuInSe<sub><i>x</i></sub>S<sub>2–<i>x</i></sub> QDs by incorporating
them into bottom-gate field effect transistors. Such films exhibit
measurable <i>p</i>-type conductance even without exchange
of the long native surface ligands, and the film’s conductance
can be improved by more than 3 orders of magnitude by replacing native
ligands with shorter ethanedithiol molecules. The results of this
study indicate the significant promise of CuInSe<sub><i>x</i></sub>S<sub>2–<i>x</i></sub> QDs synthesized by
this method for applications in photovoltaics utilizing both sensitized
and <i>p</i>–<i>n</i> junction architectures
Mn<sup>2+</sup>-Doped Lead Halide Perovskite Nanocrystals with Dual-Color Emission Controlled by Halide Content
Impurity doping has
been widely used to endow semiconductor nanocrystals
with novel optical, electronic, and magnetic functionalities. Here,
we introduce a new family of doped NCs offering unique insights into
the chemical mechanism of doping, as well as into the fundamental
interactions between the dopant and the semiconductor host. Specifically,
by elucidating the role of relative bond strengths within the precursor
and the host lattice, we develop an effective approach for incorporating
manganese (Mn) ions into nanocrystals of lead-halide perovskites (CsPbX<sub>3</sub>, where X = Cl, Br, or I). In a key enabling step not possible
in, for example, II–VI nanocrystals, we use gentle chemical
means to finely and reversibly tune the nanocrystal band gap over
a wide range of energies (1.8–3.1 eV) via postsynthetic anion
exchange. We observe a dramatic effect of halide identity on relative
intensities of intrinsic band-edge and Mn emission bands, which we
ascribe to the influence of the energy difference between the corresponding
transitions on the characteristics of energy transfer between the
Mn ion and the semiconductor host
Controlled Alloying of the Core–Shell Interface in CdSe/CdS Quantum Dots for Suppression of Auger Recombination
The influence of a CdSe<sub><i>x</i></sub>S<sub>1‑<i>x</i></sub> interfacial alloyed layer on the photophysical properties of core/shell CdSe/CdS nanocrystal quantum dots (QDs) is investigated by comparing reference QDs with a sharp core/shell interface to alloyed structures with an intermediate CdSe<sub><i>x</i></sub>S<sub>1‑<i>x</i></sub> layer at the core/shell interface. To fully realize the structural contrast, we have developed two novel synthetic approaches: a method for fast CdS-shell growth, which results in an abrupt core/shell boundary (no intentional or unintentional alloying), and a method for depositing a CdSe<sub><i>x</i></sub>S<sub>1‑<i>x</i></sub> alloy layer of controlled composition onto the CdSe core prior to the growth of the CdS shell. Both types of QDs possess similar size-dependent single-exciton properties (photoluminescence energy, quantum yield, and decay lifetime). However the alloyed QDs show a significantly longer biexciton lifetime and up to a 3-fold increase in the biexciton emission efficiency compared to the reference samples. These results provide direct evidence that the structure of the QD interface has a significant effect on the rate of nonradiative Auger recombination, which dominates biexciton decay. We also observe that the energy gradient at the core–shell interface introduced by the alloyed layer accelerates hole trapping from the shell to the core states, which results in suppression of shell emission. This comparative study offers practical guidelines for controlling multicarrier Auger recombination without a significant effect on either spectral or dynamical properties of single excitons. The proposed strategy should be applicable to QDs of a variety of compositions (including, <i>e.g</i>., infrared-emitting QDs) and can benefit numerous applications from light emitting diodes and lasers to photodetectors and photovoltaics
Thickness-Controlled Quasi-Two-Dimensional Colloidal PbSe Nanoplatelets
We
demonstrate controlled synthesis of discrete two-dimensional
(2D) PbSe nanoplatelets (NPLs), with measurable photoluminescence,
via oriented attachment directed by quantum dot (QD) surface chemistry.
Halide passivation is critical to the growth of these (100) face-dominated
NPLs, as corroborated by density functional theory studies. PbCl<sub>2</sub> moieties attached to the (111) and (110) of small nanocrystals
form interparticle bridges, aligning the QDs and leading to attachment.
We find that a 2D bridging network is energetically favored over a
3D network, driving the formation of NPLs. Although PbI<sub>2</sub> does not support bridging, its presence destabilizes the large (100)
faces of NPLs, providing means for tuning NPL thickness. Spectroscopic
analysis confirms the predicted role of thickness-dependent quantum
confinement on the NPL band gap
Shape-Controlled Narrow-Gap SnTe Nanostructures: From Nanocubes to Nanorods and Nanowires
The rational design and synthesis
of narrow-gap colloidal semiconductor
nanocrystals (NCs) is an important step toward the next generation
of solution-processable photovoltaics, photodetectors,
and thermoelectric devices. SnTe NCs are particularly attractive
as a Pb-free alternative to NCs of narrow-gap lead chalcogenides.
Previous synthetic efforts on SnTe NCs have focused on spherical nanoparticles.
Here we report new strategies for synthesis of SnTe NCs with shapes
tunable from highly monodisperse nanocubes, to nanorods
(NRs) with variable aspect ratios, and finally to long, straight nanowires
(NWs). Reaction at high temperature quickly forms thermodynamically
favored nanocubes, but low temperatures lead to elongated particles.
Transmission electron microscopy studies of reaction products at various
stages of the synthesis reveal that the growth and shape-focusing
of monodisperse SnTe nanocubes likely involves interparticle
ripening, while directional growth of NRs and NWs may be initiated
by particle dimerization via oriented attachment
Design and Synthesis of Heterostructured Quantum Dots with Dual Emission in the Visible and Infrared
The unique optical properties exhibited by visible emitting core/shell quantum dots with especially thick shells are the focus of widespread study, but have yet to be realized in infrared (IR)-active nanostructures. We apply an effective-mass model to identify PbSe/CdSe core/shell quantum dots as a promising system for achieving this goal. We then synthesize colloidal PbSe/CdSe quantum dots with shell thicknesses of up to 4 nm that exhibit unusually slow hole intraband relaxation from shell to core states, as evidenced by the emergence of dual emission, <i>i</i>.<i>e</i>., IR photoluminescence from the PbSe core observed simultaneously with visible emission from the CdSe shell. In addition to the large shell thickness, the development of slowed intraband relaxation is facilitated by the existence of a sharp core–shell interface without discernible alloying. Growth of thick shells without interfacial alloying or incidental formation of homogeneous CdSe nanocrystals was accomplished using insights attained <i>via</i> a systematic study of the dynamics of the cation-exchange synthesis of both PbSe/CdSe and the related system PbS/CdS. Finally, we show that the efficiency of the visible photoluminescence can be greatly enhanced by inorganic passivation