6 research outputs found
Self-Assembled PbSe Nanowire:Perovskite Hybrids
Inorganic semiconductor nanowires
are of interest in nano- and
microscale photonic and electronic applications. Here we report the
formation of PbSe nanowires based on directional quantum dot alignment
and fusion regulated by hybrid organic–inorganic perovskite
surface ligands. All material synthesis is carried out at mild temperatures.
Passivation of PbSe quantum dots was achieved via a new perovskite
ligand exchange. Subsequent <i>in situ</i> ammonium/amine
substitution by butylamine enables quantum dots to be capped by butylammonium
lead iodide, and this further drives the formation of a PbSe nanowire
superlattice in a two-dimensional (2D) perovskite matrix. The average
spacing between two adjacent nanowires agrees well with the thickness
of single atomic layer of 2D perovskite, consistent with the formation
of a new self-assembled semiconductor nanowire:perovskite heterocrystal
hybrid
All-Quantum-Dot Infrared Light-Emitting Diodes
Colloidal quantum dots (CQDs) are promising candidates for infrared electroluminescent devices. To date, CQD-based light-emitting diodes (LEDs) have employed a CQD emission layer sandwiched between carrier transport layers built using organic materials and inorganic oxides. Herein, we report the infrared LEDs that use quantum-tuned materials for each of the hole-transporting, the electron-transporting, and the light-emitting layers. We successfully tailor the bandgap and band position of each CQD-based component to produce electroluminescent devices that exhibit emission that we tune from 1220 to 1622 nm. Devices emitting at 1350 nm achieve peak external quantum efficiency up to 1.6% with a low turn-on voltage of 1.2 V, surpassing previously reported all-inorganic CQD LEDs
Origins of Stokes Shift in PbS Nanocrystals
Stokes
shift, an energy difference between the excitonic absorption
and emission, is a property of colloidal quantum dots (CQDs) typically
ascribed to splitting between dark and bright excitons. In some materials,
e.g., PbS, CuInS<sub>2</sub>, and CdHgTe, a Stokes shift of up to
200 meV is observed, substantially larger than the estimates of dark–bright
state splitting or vibronic relaxations. The shift origin remains
highly debated because contradictory signatures of both surface and
bulk character were reported for the Stokes-shifted electronic state.
Here, we show that the energy transfer among CQDs in a polydispersed
ensemble in solution suffices to explain the excess Stokes shift.
This energy transfer is primarily due to CQD aggregation and can be
substantially eliminated by extreme dilution, higher-viscosity solvent,
or better-dispersed colloids. Our findings highlight that ensemble
polydispersity remains the primary source of the Stokes shift in CQDs
in solution, propagating into the Stokes shift in films and the open-circuit
voltage deficit in CQD solar cells. Improved synthetic control can
bring notable advancements in CQD photovoltaics, and the Stokes shift
continues to provide a sensitive and significant metric to monitor
ensemble size distribution
Halide Re-Shelled Quantum Dot Inks for Infrared Photovoltaics
Colloidal
quantum dots are promising materials for tandem solar cells that complement
silicon and perovskites. These devices are fabricated from solution
phase; however, existing methods for making infrared-bandgap CQD inks
suffer agglomeration and fusion during solution exchange. Here we
develop a ligand exchange that provides robust surface protection
and thereby avoids aggregation. First, we exchanged long oleic acid
ligands to a mixed system comprising medium-chain ammonium and anionic
chloride ligands; we then reshelled the surface using short halides
and pseudohalide ligands that enabled transfer to a polar solvent.
Absorbance and photoluminescence measurements reveal the retention
of exciton sharpness, whereas X-ray photoelectron spectroscopy indicates
halide capping. The best power conversion efficiency of these devices
is 0.76 power points after filtering through silicon, which is 1.9×
higher than previous single-step solution-processed IR-CQD solar cells
Gradient-Doped Colloidal Quantum Dot Solids Enable Thermophotovoltaic Harvesting of Waste Heat
Electromagnetic
radiation emitted from hot objects represents a
sizable source of energy, one that in most applications is not harvested
efficiently. Even for a blackbody at 800 °C, the radiation intensity
peaks near 2.7 μm wavelength, and this requires a semiconductor
absorber having a band gap in the short-wavelength infrared and beyond
to enable thermophotovoltaic (TPV) heat recovery. Here we report the
first solution-processed TPV device to harvest efficiently 800 °C
heat. The active layer consists of colloidal quantum dots (CQDs),
infrared-absorbing nanoparticles synthesized using a scalable solution-based
method, having 0.75 eV band gap. We construct rectifying junction
devices based on controllably p- and n-doped CQD solids that benefit
from a gradient in electron affinity that extends over the devices’
thickness. The gradient-doped architecture relies on engineered charge
carrier drift and overcomes the existing limitations of small band
gap CQD solids. The devices provide 2.7% efficiency in the conversion
of optical power from above-band gap photons from a blackbody source
at 800 ± 20 °C into electrical power. The cells were thermally
stable up to 140 °C, increasing the promise of CQD solids for
TPV applications
Enhanced Mobility-Lifetime Products in PbS Colloidal Quantum Dot Photovoltaics
Colloidal quantum dot (CQD) photovoltaics offer a promising approach to harvest the near-IR region of the solar spectrum, where half of the sun’s power reaching the earth resides. High external quantum efficiencies have been obtained in the visible region in lead chalcogenide CQD photovoltaics. However, the corresponding efficiencies for band gap radiation in the near-infrared lag behind because the thickness of CQD photovoltaic layers from which charge carriers can be extracted is limited by short carrier diffusion lengths. Here, we investigate, using a combination of electrical and optical characterization techniques, ligand passivation strategies aimed at tuning the density and energetic distribution of charge trap states at PbS nanocrystal surfaces. Electrical and optical measurements reveal a more than 7-fold enhancement of the mobility-lifetime product of PbS CQD films treated with 3-mercaptopropionic acid (MPA) in comparison to traditional organic passivation strategies that have been examined in the literature. We show by direct head-to-head comparison that the greater mobility-lifetime products of MPA-treated devices enable markedly greater short-circuit current and higher power conversion efficiency under AM1.5 illumination. Our findings highlight the importance of selecting ligand treatment strategies capable of passivating a diversity of surface states to enable shallower and lower density trap distributions for better transport and more efficient CQD solar cells