Minimizing Scaling Losses in High-Performance Quantum Dot Luminescent Solar Concentrators for Large-Area Solar Windows.

While luminescent solar concentrators (LSCs) have been researched for several decades, there is still a lack of commercially available systems, mostly due to scalability, performance, aesthetics, or some combination of these challenges. These obstacles can be overcome by the systematic optimization of a laminated glass LSC design, demonstrated herein. In particular, we first show that it is possible to improve optical and electrical efficiencies of an LSC by fine-tuned optimization of the constituent fluorophore-containing interlayer resin. Further still, an increased understanding of commercially available solar cells allows us to establish a direct correlation between the device's optical and electrical efficiency. Next, optical characterization of LSCs of varying sizes allows us to elucidate the main loss mechanisms in our LSCs, as well as ways to mitigate them. Altogether these optimization steps create opportunities for high-performance multi-interlayer LSC devices with demonstrated electrical power conversion efficiency as high as 1.1% to 4.9% at visual light transmission of 74% to 5%. Furthermore, careful examination of different blue-color (red-band absorbing) dyes provides a path for color-tunability of LSC windows toward neutral regimes. Design iterations of multiple device form factors enabled a color-neutral prototype without significant performance losses by separating color-neutralizing and LSC layers into different panes of an insulated glass unit. This work demonstrates the importance of LSC design optimization in achieving high-performance solar window technology with commercially acceptable aesthetics.

Synthetic Ligand Selection Affects Stoichiometry, Carrier Dynamics, and Trapping in CuInSe2 Nanocrystals.

CuInSe2 nanocrystals exhibit tunable near-infrared bandgaps that bolster utility in photovoltaic applications as well as offer potential as substitutes for more toxic Cd- and Pb-based semiconductor compositions. However, they can present a variety of defect states and unusual photophysics. Here, we examine the effects of ligand composition (oleylamine, diphenylphosphine, and tributylphosphine) on carrier dynamics in these materials. Via spectroscopic measurements such as photoluminescence and transient absorption, we find that ligands present during the synthesis of CuInSe2 nanocrystals impart nonradiative electronic states which compete with radiative recombination and give rise to low photoluminescence quantum yields. We characterize the nature of these defect states (hole vs electron traps) and investigate whether they exist at the surface or interior of the nanocrystals. Carrier lifetimes are highly dependent on ligand identity where oleylamine-capped nanocrystals exhibit rapid trapping (<20 ps) followed by diphenylphosphine (<500 ps) and finally tributylphosphine (>2 ns). A majority of carrier population localizes at indium copper antisites (electrons), copper vacancies (holes), or surface traps (electrons and/or holes), all of which are nonemissive.

An investigation into the potential for wind turbines to cause barotrauma in bats.

The high rates of bat mortality caused by operating wind turbines is a concern for wind energy and wildlife stakeholders. One theory that explains the mortality is that bats are not only killed by impact trauma, but also by barotrauma that results from exposure to the pressure variations caused by rotating turbine blades. To date, no published research has calculated the pressure changes that bats may be exposed to when flying near wind turbines and then used these data to estimate the likelihood that turbines cause barotrauma in bats. To address this shortcoming, we performed computational fluid dynamics simulations of a wind turbine and analytical calculations of blade-tip vortices to estimate the characteristics of the sudden pressure changes bats may experience when flying near a utility-scale wind turbine. Because there are no data available that characterize the pressure changes that cause barotrauma in bats, we compared our results to changes in pressure levels that cause barotrauma and mortality in other mammals of similar size. This comparison shows that the magnitude of the low-pressures bats experience when flying near wind turbines is approximately 8 times smaller than the pressure that causes mortality in rats, the smallest mammal for which data are available. The magnitude of the high-pressures that bats may experience are approximately 80 times smaller than the exposure level that causes 50% mortality in mice, which have a body mass similar to several bat species that are killed by wind turbines. Further, our results show that for a bat to experience the largest possible magnitude of low- and high-pressures, they must take very specific and improbable flight paths that skim the surface of the blades. Even a small change in the flight path results in the bat being hit by the blade or experiencing a much smaller pressure change. Accordingly, if bats have a physiological response to rapid low- and high-pressure exposure that is similar to other mammals, we conclude that it is unlikely that barotrauma is responsible for a significant number of turbine-related bat fatalities, and that impact trauma is the likely cause of the majority of wind-turbine-related bat fatalities.

Transient Lattice Response upon Photoexcitation in CuInSe2 Nanocrystals with Organic or Inorganic Surface Passivation.

CuInSe2 nanocrystals offer promise for optoelectronics including thin-film photovoltaics and printed electronics. Additive manufacturing methods such as photonic curing controllably sinter particles into quasi-continuous films and offer improved device performance. To gain understanding of nanocrystal response under such processing conditions, we investigate impacts of photoexcitation on colloidal nanocrystal lattices via time-resolved X-ray diffraction. We probe three sizes of particles and two capping ligands (oleylamine and inorganic S2-) to evaluate resultant crystal lattice temperature, phase stability, and thermal dissipation. Elevated fluences produce heating and loss of crystallinity, the onset of which exhibits particle size dependence. We find size-dependent recrystallization and cooling lifetimes ranging from 90 to 200 ps with additional slower cooling on the nanosecond time scale. Sulfide-capped nanocrystals show faster recrystallization and cooling compared to oleylamine-capped nanocrystals. Using these lifetimes, we find interfacial thermal conductivities from 3 to 28 MW/(m2 K), demonstrating that ligand identity strongly influences thermal dissipation.

Silicon Nanocrystal Superlattice Nucleation and Growth.

Colloidal dodecene-passivated silicon (Si) nanocrystals were dispersed in hexane or chloroform and deposited onto substrates as face-centered cubic superlattices by slowly evaporating the solvent. The uniformity of the nanocrystals enables extended order; however, the solvent and the evaporation protocol significantly influence the self-assembly process, determining the morphology of the films, the extent of order, and the superlattice orientation on the substrate. Chloroform yielded superlattices with step-flow growth morphologies and (111)SL, (100)SL, and (110)SL orientations. Hexane led to mostly island morphologies when evaporated at room temperature with exclusively (111)SL orientations. Higher evaporation temperatures led to more extensive step-flow deposition. A model for the surface diffusion of nanocrystals adsorbed on the superlattice surface is developed.

Bubble Assemblies of Nanocrystals: Superlattices without a Substrate.

A method was developed to create free-standing nanocrystal films in the form of solidified bubbles. Bubbles of octadecanethiol-capped gold nanocrystals were studied by in situ grazing incidence small-angle X-ray scattering (GISAXS) to determine how the absence of an underlying substrate influences a disorder-order transition of a nanocrystal superlattice. We find that the presence of the substrate does not significantly change the nature of the disorder-order transition but does lead to reduced interparticle separation and reduced thermal expansion. Bubble assemblies of silicon and copper selenide nanocrystals are also demonstrated.

Synthesis of bis-α,α'-amino acids through diastereoselective bis-alkylations of chiral Ni(II)-complexes of glycine.

The Ni(II) complex derived from glycine Schiff base with (S)-N-(benzylprolyl)-2-aminobenzophenone can be effectively alkylated with α,ω-dibromide reagents to furnish the corresponding bis-alkylated products. This method presents a direct approach for the preparation of the corresponding bis-α,α'-amino acids with high biological importance. Heterogeneous (phase-transfer) as well as homogeneous conditions for the alkylation reactions have been investigated and the latter proved to be more efficient in terms of stereochemical outcome. In particular, alkylation of the glycine Schiff base Ni(II) complex with 1,3-dibromopropane followed by acid-catalysed hydrolysis of the resulting bis-alkylation product, allowed for the preparation of naturally occurring (2S,6S)-diaminopimelic acid in high overall yield.