15 research outputs found
Ultralow Threshold One-Photon- and Two-Photon-Pumped Optical Gain Media of Blue-Emitting Colloidal Quantum Dot Films
Colloidal quantum dots (QDs) offer advantageous properties as an optical gain media for lasers. Optical gain in the QDs has been shown in the whole visible spectrum, yet it has been intrinsically challenging to realize efficient amplified spontaneous emission (ASE) and lasing in the blue region of the visible spectrum. Here, we synthesize large-sized core/gradient shell CdZnS/ZnS QDs as an efficient optical gain media in the blue spectral range. In this Letter, we demonstrate for the first time that two-photon-absorption-pumped ASE from the blue-emitting QD is achievable with a threshold as low as 6 mJ/cm(2). Utilizing these QDs, we also report one-photon-absorption-pumped ASE at an ultralow threshold of similar to 60 mu J/cm(2), which is comparable to the state-of-the-art red-emitting QD-based gain media. This one-photon-pumped ASE threshold is an order of magnitude better than that of the previously reported best blue-emitting QD-based gain media
Colloidal Quantum Dot Tandem Solar Cells Using CVD Graphene as An Atomically Thin Intermediate Recombination Layer
Two-terminal tandem cell architectures are believed to be an effective way to further improve the power conversion efficiency in solution processed photovoltaics. To design an efficient tandem solar cell, two key issues need to be considered. First, subcells with well-matched currents and complementary absorption characteristics are a prerequisite for high efficiency. Second, identifying the appropriate intermediate layer (IML) to connect the subcells is necessary to minimize the optical and electronic losses. PbS colloidal quantum dots (CQDs) are a notable choice for the subcells due to their low cost, solution processability, and remarkable wide range band gap tunability. Single-layer graphene (Gr) has been proposed to be a promising IML due to its high transparency and conductivity. Here, as a proof of concept, we demonstrate a solution-processed, two-terminal PbS CQDs tandem solar cell employing chemical vapor deposited Gr as the IML. In doing so, we report a PbS CQD cell comprising subcells with bandgaps of 1.4 and 0.95 eV that delivers power conversion efficiency in excess of 7%, substantially higher than that of previously reported CQD tandem cells.Peer ReviewedPostprint (author's final draft
Continuously Tunable Emission in Inverted Type-I CdS/CdSe Core/Crown Semiconductor Nanoplatelets
The synthesis and unique tunable optical properties of core/crown nanoplatelets having an inverted Type-I heterostructure are presented. Here, colloidal 2D CdS/CdSe heteronanoplatelets are grown with thickness of four monolayers using seed-mediated method. In this work, it is shown that the emission peak of the resulting CdS/CdSe heteronanoplatelets can be continuously spectrally tuned between the peak emission wavelengths of the core only CdS nanoplatelets (421 nm) and CdSe nanoplatelets (515 nm) having the same vertical thickness. In these inverted Type-I nanoplatelets, the unique continuous tunable emission is enabled by adjusting the lateral width of the CdSe crown, having a narrower bandgap, around the core CdS nanoplatelet, having a wider bandgap, as a result of the controlled lateral quantum confinement in the crown region additional to the pure vertical confinement. As a proof-of-concept demonstration, a white light generation is shown by using color conversion with these CdS/CdSe heteronanoplatelets having finely tuned thin crowns, resulting in a color rendering index of 80. The robust control of the electronic structure in such inverted Type-I heteronanoplatelets achieved by tailoring the lateral extent of the crown coating around the core template presents a new enabling pathway for bandgap engineering in solution-processed quantum wells
Type-II Colloidal Quantum Wells: CdSe/CdTe Core/Crown Heteronanoplatelets
Solution-processed quantum wells, also known as colloidal nanoplatelets (NPLs), are emerging as promising materials for colloidal optoelectronics. In this work, we report the synthesis and characterization of CdSe/CdTe core/crown NPLs exhibiting a Type-II electronic structure and Type-II specific optical properties. Here, based on a core-seeded approach, the CdSe/CdTe core/crown NPLs were synthesized with well-controlled CdTe crown coatings. Uniform and epitaxial growth of CdTe crown region was verified by using structural characterization techniques including transmission electron microscopy (TEM) with quantitative EDX analysis and X-ray diffraction (XRD). Also the optical properties were systematically studied in these Type-II NPLs that reveal strongly red-shifted photoluminescence (up to similar to 150 nm) along with 2 orders of magnitude longer fluorescence lifetimes (up to 190 ns) compared to the Type-I NPLs owing to spatially indirect excitons at the Type-II interface between the CdSe core and the CdTe crown regions. Photoluminescence excitation spectroscopy confirms that this strongly red-shifted emission actually arises from the CdSe/CdTe NPLs. In addition, temperature-dependent time-resolved fluorescence spectroscopy was performed to reveal the temperature-dependent fluorescence decay kinetics of the Type-II NPLs exhibiting interesting behavior. Also, water-soluble Type-II NPLs were achieved via ligand exchange of the CdSe/CdTe core/crown NPLs by using 3-mercaptopropionic acid (MPA), which allows for enhanced charge extraction efficiency owing to their shorter chain length and enables high quality film formation by layer-by-layer (LBL) assembly. With all of these appealing properties, the CdSe/CdTe core/crown heterostructures having Type-II electronic structure presented here are highly promising for light-harvesting applications
Infrared SolutionâProcessed Quantum Dot Solar Cells Reaching External Quantum Efficiency of 80% at 1.35 ”m and Jsc in Excess of 34 mA cmâ2
Developing lowâcost photovoltaic absorbers that can harvest the shortâwave infrared (SWIR) part of the solar spectrum, which remains unharnessed by current Siâbased and perovskite photovoltaic technologies, is a prerequisite for making highâefficiency, lowâcost tandem solar cells. Here, infrared PbS colloidal quantum dot (CQD) solar cells employing a hybrid inorganicâorganic ligand exchange process that results in an external quantum efficiency of 80% at 1.35 ”m are reported, leading to a shortâcircuit current density of 34 mA cmâ2 and a power conversion efficiency (PCE) up to 7.9%, which is a current record for SWIR CQD solar cells. When this cell is placed at the back of an MAPbI3 perovskite film, it delivers an extra 3.3% PCE by harnessing light beyond 750 nm.Peer Reviewe
Colloidal Quantum Dot Tandem Solar Cells Using CVD Graphene as An Atomically Thin Intermediate Recombination Layer
Two-terminal tandem cell architectures are believed to be an effective way to further improve the power conversion efficiency in solution processed photovoltaics. To design an efficient tandem solar cell, two key issues need to be considered. First, subcells with well-matched currents and complementary absorption characteristics are a prerequisite for high efficiency. Second, identifying the appropriate intermediate layer (IML) to connect the subcells is necessary to minimize the optical and electronic losses. PbS colloidal quantum dots (CQDs) are a notable choice for the subcells due to their low cost, solution processability, and remarkable wide range band gap tunability. Single-layer graphene (Gr) has been proposed to be a promising IML due to its high transparency and conductivity. Here, as a proof of concept, we demonstrate a solution-processed, two-terminal PbS CQDs tandem solar cell employing chemical vapor deposited Gr as the IML. In doing so, we report a PbS CQD cell comprising subcells with bandgaps of 1.4 and 0.95 eV that delivers power conversion efficiency in excess of 7%, substantially higher than that of previously reported CQD tandem cells.Peer Reviewe
Plasmon-Enhanced Energy Transfer in Photosensitive Nanocrystal Device
FoÌrster
resonance energy transfer (FRET) interacted with
localized surface plasmon (LSP) gives us the ability to overcome inadequate
transfer of energy between donor and acceptor nanocrystals (NCs).
In this paper, we show LSP-enhanced FRET in colloidal photosensors
of NCs in operation, resulting in substantially enhanced photosensitivity.
The proposed photosensitive device is a layered self-assembled colloidal
platform consisting of separated monolayers of the donor and the acceptor
colloidal NCs with an intermediate metal nanoparticle (MNP) layer
made of gold interspaced by polyelectrolyte layers. Using LBL assembly,
we fabricated and comparatively studied seven types of such NC-monolayer
devices (containing only donor, only acceptor, Au MNPâdonor,
Au MNPâacceptor, donorâacceptor bilayer, donorâAu
MNPâacceptor trilayer, and acceptorâAu MNPâdonor
reverse trilayer). In these structures, we revealed the effect of
LSP-enhanced FRET and exciton interactions from the donor NCs layer
to the acceptor NCs layer. Compared to a single acceptor NC device,
we observed a significant extension in operating wavelength range
and a substantial photosensitivity enhancement (2.91-fold) around
the LSP resonance peak of Au MNPs in the LSP-enhanced FRET trilayer
structure. Moreover, we present a theoretical model for the intercoupled
donorâAu MNPâacceptor structure subject to the plasmon-mediated
nonradiative energy transfer. The obtained numerical results are in
excellent agreement with the systematic experimental studies done
in our work. The potential to modify the energy transfer through mastering
the excitonâplasmon interactions and its implication in devices
make them attractive for applications in nanophotonic devices and
sensors
Cation Disorder and Local Structural Distortions in AgxBi1âxS2 Nanoparticles
By combining X-ray absorption fine structure and X-ray diffraction measurements with density functional and molecular dynamics simulations, we study the structure of a set of AgxBi1âxS2 nanoparticles, a materials system of considerable current interest for photovoltaics. An apparent contradiction between the evidence provided by X-ray absorption and diffraction measurements is solved by means of the simulations. We find that disorder in the cation sublattice induces strong local distortions, leading to the appearance of short AgâS bonds, the overall lattice symmetry remaining close to hexagonalPeer ReviewedPostprint (published version
Macrocrystals of Colloidal Quantum Dots in Anthracene: Exciton Transfer and Polarized Emission
In this work, centimeter-scale macrocrystals
of nonpolar colloidal
quantum dots (QDs) incorporated into anthracene were grown for the
first time. The exciton transfer from the anthracene host to acceptor
QDs was systematically investigated, and anisotropic emission from
the isotropic QDs in the anthracene macrocrystals was discovered.
Results showed a decreasing photoluminescence lifetime of the donor
anthracene, indicating a strengthening energy transfer with increasing
QD concentration in the macrocrystals. With the anisotropy study,
QDs inside the anthracene host acquired a polarization ratio of âŒ1.5
at 0° collection angle, and this increases to âŒ2.5 at
the collection angle of 60°. A proof-of-concept application of
these excitonic macrocrystals as tunable color converters on light-emitting
diodes was also demonstrated