Hunter McDaniel

Highly efficient large-area colourless luminescent solar concentrators using heavy-metal-free colloidal quantum dots

Luminescent solar concentrators serving as semitransparent photovoltaic windows could become an important element in net zero energy consumption buildings of the future. Colloidal quantum dots are promising materials for luminescent solar concentrators as they can be engineered to provide the large Stokes shift necessary for suppressing reabsorption losses in large-area devices. Existing Stokes-shift-engineered quantum dots allow for only partial coverage of the solar spectrum, which limits their light-harvesting ability and leads to colouring of the luminescent solar concentrators, complicating their use in architecture. Here, we use quantum dots of ternary I-III-VI2 semiconductors to realize the first large-area quantum dot-luminescent solar concentrators free of toxic elements, with reduced reabsorption and extended coverage of the solar spectrum. By incorporating CuInSexS2-x quantum dots into photo-polymerized poly(lauryl methacrylate), we obtain freestanding, colourless slabs that introduce no distortion to perceived colours and are thus well suited for the realization of photovoltaic windows. Thanks to the suppressed reabsorption and high emission efficiencies of the quantum dots, we achieve an optical power efficiency of 3.2%. Ultrafast spectroscopy studies suggest that the Stokes-shifted emission involves a conduction-band electron and a hole residing in an intragap state associated with a native defect.

Size and Growth Rate Dependent Structural Diversification of Fe3O4/CdS Anisotropic Nanocrystal Heterostructures

To better understand the growth mechanism leading to enhanced anisotropy in nanocrystal heterostructures synthesized from nearly spherical seeds, we have examined various factors that contribute to structural diversification in Fe(3)O(4)/CdS systems. Pseudoseparation of nucleation and growth allows us to quantify how the number of heterojunctions formed varies with concentration and the size of the seed nanocrystals. A careful examination of the size dependence of the maximum number of CdS particles that can nucleate per seed nanocrystal suggests strain induced limitations. By increasing the growth rate, we observe an enhancement of spatial anisotropy in rods-on-dot heterostructures without the need for rod promoting capping molecules such as phosphonic acids. Crystallographic details allow us to identify three distinct morphologies that can arise in rods-on-dot heterostructures due to zinc blende/wurtzite polytypism in CdS. In all three cases, the junction planes contain identical or nearly identical coincidence sites.

Effects of Lattice Strain and Band Offset on Electron Transfer Rates in Type-II Nanorod Heterostructures.

Type-II nanorod heterostructures (NRHs) exhibit efficient directional charge separation and provide the potential to control this flow of charges through changes in structure and composition. We use transient-absorption spectroscopy to investigate how the magnitude of band offset and lattice strain alters dynamics of photogenerated electrons in CdSe/CdTe type-II NRHs. In the absence of alloying and strain effects, electron transfer occurs in ∼300 fs. Reducing the conduction band offset by means of alloying leads to an even shorter charge-separation time (<200 fs), whereas curved NRHs with pronounced strain exhibit a longer charge-separation time of ∼700 fs.

Integration of type II nanorod heterostructures into photovoltaics.

High-quality epitaxial interfaces and delicate control over shape anisotropy make nanorod heterostructures (NRHs) with staggered band offsets efficient in separating and directing photogenerated carriers. Combined with versatile and scalable wet chemical means of synthesis, these salient features of NRHs are useful for improving both the performance and the cost-effectiveness of photovoltaics (PVs). However, inefficient carrier transport and extraction have imposed severe limitations, outweighing the benefits of enhanced charge separation. Hence integration of type II NRHs into PVs has thus far been unfruitful. Here, we demonstrate PVs that utilize NRHs as an extremely thin absorber between electron and hole transporting layers. In the limit approaching monolayer thickness, PVs incorporating NRHs have up to three times the short circuit current and conversion efficiency over devices made from their single-component counterparts. Comparisons between linear and curved NRHs are also made, revealing the importance of internal geometry and heterointerfacial area for enhanced contribution of charge-separated state absorption to photocurrent and in contacting charge transport layers.

Anisotropic Strain-Induced Curvature in Type-II CdSe/CdTe Nanorod Heterostructures

Type-II band-offset CdSe/CdTe nanorod heterostructures with curved and linear shapes have been synthesized and examined with atomic-resolution transmission electron microscopy techniques. Strain from growth of larger-lattice CdTe partly on the sides of CdSe nanorod seeds is shown to lead to an overall curvature in the rods. Lattice expansion from the inner to the outer portion of the curved region exceeds the expected lattice mismatch between the two materials because of the buildup of an unusual compressive strain in the CdSe. In contrast, exclusive tip growth results in linear barbell-shaped heterostructures that do not exhibit strain-induced curvature. The ability to vary the anisotropic lattice strain should allow control over the underlying electronic structure, providing new approaches to directing photogenerated carriers that may facilitate incorporation of nanorod heterostructures in various energy applications.

Engineered core/shell quantum dots as phosphors for solid-state lighting

Light-emitting diodes (LEDs) for solid state light ing (SSL) typically combine a blue or near- ultraviolet drive LED with one or more dow nconverting phosphors to produce “white” light. Further advances in both efficiency and wh ite-light quality will re quire new phosphors with narrow-band, highly efficient emission, particul arly in the red. A team led by principal investigator Dr. Victor Klim ov of Los Alamos National Labo ratory proposes to develop engineered semiconductor nanocrystal quantum dots (QDs) that combine optimal luminescent properties with long-term stability under ty pical downconverting conditions to enable new performance levels in SSL. The white LED phosphor industry is estimated to have sales of roughly

Prospects for Strained Type-II Nanorod Heterostructures

400 million in 2018 and would significantly benefit from the development of bright and narrow red-emitting QD phosphors because they woul d enable warmer whites without wasting energy by emission of light beyond the response of the human eye. In order to capitalize on the market opportunity, the LANL team is partnering with a local company called UbiQD that will facilitate US manufacturing.