Surface Oxidation as a Cause of High Open‐Circuit Voltage in CdSe ETA Solar Cells

TiO₂/CdSe/CuSCN extremely thin absorber (ETA) solar cells are found to give relatively high values of open‐circuit voltage (>0.8 V) but low currents upon annealing the cadmium selenide (CdSe) in air (500 ºC). Annealing in N₂ produces much lower photovoltages and slightly lower photocurrents. Band structure measurements show differences between the two annealing regimes that, however, appear to favor the N₂‐annealed CdSe. On the other hand, chemically resolved electrical measurements (CREM) of the cells reveal marked differences in photo‐induced charge trapping, in particular at absorber grain boundaries of the air versus N₂‐annealed systems, correlated with the formation of Cd–O species at the CdSe surface. Using transient absorption and photovoltage decay, pronounced lifetime differences are also observed, in agreement with the strong suppression of charge recombination. The results point to a multiple role of grain surface‐oxidation, which both impedes electron injection from the CdSe to the TiO_2, but, much more significantly, enhances hole injection to the CuSCN via passivation of hole traps that act as efficient recombination centers.     

GPU-accelerated Hausdorff distance computation between dynamic deformable NURBS surfaces

We present a parallel GPU-accelerated algorithm for computing the directed Hausdorff distance from one NURBS surface to another, within a bound. We make use of axis-aligned bounding-box hierarchies that bound the NURBS surfaces to accelerate the computations. We dynamically construct as well as traverse the bounding-box hierarchies for the NURBS surfaces using operations that are optimized for the GPU. To compute the Hausdorff distance, we traverse this hierarchy after culling bounding-box pairs that do not contribute to the Hausdorff distance. Our contribution includes two-sided culling tests that can be performed in parallel using the GPU. The culling, based on the minimum and maximum distance ranges between the bounding boxes, eliminates bounding-box pairs from both surfaces that do not contribute to the Hausdorff distance simultaneously. We calculate accuracy bounds for our computed Hausdorff distance based on the curvature of the surfaces. Our algorithm runs in real-time with very small guaranteed error bounds for complex NURBS surfaces. Since we dynamically construct our bounding-box hierarchy, our algorithm can be used to interactively compute the Hausdorff distance for models made of dynamic deformable surfaces.     

Exciton-plasmon interactions in quantum dot-gold nanoparticle structures

We present a self-assembly method to construct CdSe/ZnS quantum dot-gold nanoparticle complexes. This method allows us to form complexes with relatively good control of the composition and structure that can be used for detailed study of the exciton-plasmon interactions. We determine the contribution of the polarization-dependent near-field enhancement, which may enhance the absorption by nearly two orders of magnitude and that of the exciton coupling to plasmon modes, which modifies the exciton decay rate.     

Eppur si Muove: Proton Diffusion in Halide Perovskite Single Crystals

Ion diffusion affects the optoelectronic properties of halide-perovskites (HaPs). Until now, the fastest diffusion has been attributed to the movement of the halides, largely neglecting the contribution of protons, on the basis of computed density estimates. Here, the process of proton diffusion inside HaPs, following deuterium–hydrogen exchange and migration in MAPbI₃, MAPbBr₃, and FAPbBr₃ single crystals, is proven through D/H NMR quantification, Raman spectroscopy, and elastic recoil detection analysis, challenging the original assumption of halide-dominated diffusion. The results are confirmed by impedance spectroscopy, where MAPbBr₃- and CsPbBr₃-based solar cells respond at very different frequencies. Water plays a key role in allowing the migration of protons as deuteration is not detected in its absence. The water contribution is modeled to explain and forecast its effect as a function of its concentration in the perovskite structure. These findings are of great importance as they evidence how unexpected, water-dependent proton diffusion can be at the basis of the ≈7 orders of magnitude spread of diffusion (attributed to I⁻ and Br⁻) coefficient values, reported in the literature. The reported enhancement of the optoelectronic properties of HaP when exposed to small amounts of water may be related to the finding.     

Size‐Dependent Control of Exciton–Polariton Interactions in WS2 Nanotubes

Multiwall WS₂ nanotubes (and fullerene‐like nanoparticles thereof) are currently synthesized in large amounts, reproducibly. Other than showing interesting mechanical and tribological properties, which offer them a myriad of applications, they are recently shown to exhibit remarkable optical and electrical properties, including quasi‐1D superconductivity, electroluminescence, and a strong bulk photovoltaic effect. Here, it is shown that, using a simple dispersion‐fractionation technique, one can control the diameter of the nanotubes and move from pure excitonic to polaritonic features. While nanotubes of an average diameter >80 nm can support cavity modes and scatter light effectively via a strong coupling mechanism, the extinction of nanotubes with smaller diameter consists of pure absorption. The experimental work is complemented by finite‐difference time‐domain simulations, which shed new light on the cavity mode–exciton interaction in 2D materials. Furthermore, transient absorption experiments of the size‐fractionated nanotubes fully confirm the steady‐state observations. Moreover, it is shown that the tools developed here are useful for size control of the nanotubes, e.g., in manufacturing environment. The tunability of the light–matter interaction of such nanotubes offers them intriguing applications such as polaritonic devices, in photocatalysis, and for multispectral sensors.     

Self‐Assembled Hybrid Materials Based on Organic Nanocrystals and Carbon Nanotubes

Organic crystalline materials are used as dyes/pigments, pharmaceuticals, and active components of photonic and electronic devices. There is great interest in integrating organic crystals with inorganic and carbon nanomaterials to create nanocomposites with enhanced properties. Such efforts are hampered by the difficulties in interfacing organic crystals with dissimilar materials. Here, an approach that employs organic nanocrystallization is presented to fabricate solution‐processed organic nanocrystal/carbon nanotube (ONC/CNT) hybrid materials based on readily available organic dyes (perylene diimides (PDIs)) and carbon nanotubes. The hybrids are prepared by self‐assembly in aqueous media to afford free‐standing films with tunable CNT content. These exhibit excellent conductivities (as high as 5.78 ± 0.56 S m^?1), and high thermal stability that are superior to common polymer/CNT hybrids. The color of the hybrids can be tuned by adding various PDI derivatives. ONC/CNT hybrids represent a novel class of nanocomposites, applicable as optoelectronic and conductive colorant materials.     

How front side plasmonic nanostructures enhance solar cell efficiency

Dielectric roughness on the front surface enhances significantly solar cell efficiency by light trapping in the absorbing layer. However in plasmonic assisted thin silicon solar cells we show, by a detailed analysis of the various mechanisms, that front-surface plasmonic structures enhance the efficiency by a different mechanism—namely the effective broadband forward scattering into the silicon, while trapping (path length enhancement) is relatively small for these structures. The plasmonic local field enhancement contribution is even smaller in this configuration. Based on our study we optimized this “anti-reflection mechanism” by tuning the plasmonic structure such that the spectral location of the more efficiently scattering dipole and quadrupole resonances fit correctly the visible and NIR sun spectrum.     

Light-Stimulated Formation and Dissociation of Supramolecular Donor–Acceptor Complexes between Eosin and Bipyridinium Azobenzenes: Design of Molecular Electronic Devices for the Piezoelectrical Transduction of Recorded Optical Signals

The photoisomerizable electron acceptors, trans-4,4′-bis(N-methylpyridinium)azobenzene, ( 1t ), and trans-3,3′-bis(N-methylpyridinium)azobenzene, ( 2t ), exhibit photoswitchable binding affinities to eosin. While the trans isomers, ( 1t ) or ( 2t ) exhibit high association affinities (K_a = 8.3 × 10³ M⁻¹ and 3.8 × 10⁴ M⁻¹, respectively), the cis isomers ( 1c ) or ( 2c ) reveal lower binding affinities (K_a = 3.4 × 10³ M⁻¹ and 1.4 × 10⁴ M⁻¹, respectively). The formation of the supramolecular donor–acceptor complexes between the eosin and the electron acceptors is associated with an absorbance change of the chromophore. This enables the cyclic spectroscopic transduction of the optically-induced formation and dissociation of the complexes upon photoisomerization between the respective trans and cis states. An eosin monolayer was assembled onto the Au-electrodes associated with a quartz piezoelectric crystal. The eosin-functionalized crystal was employed for the microgravimetric, quartz-crystal-microbalance analyses of the association of ( 1t ), ( 2t ), ( 1c ), or ( 2c ) to the crystal interface. The microgravimetric analyses allowed the characterization of the kinetics of association of the electron acceptors to the monolayer and the association constants between the isomers ( 1t ), ( 2t ), ( 1c ), and ( 2c ) with the eosin monolayer. By cyclic photoisomerization of the electron-acceptor between the trans and cis states, reversible piezoelectric transduction of the formation of the complexes with ( 1t ) or ( 2t ) at the monolayer interface, and their dissociation upon photoisomerization to ( 1c ) or ( 2c ) was accomplished.     

Simultaneous electrical transport and scanning tunneling spectroscopy of carbon nanotubes

We performed scanning tunneling spectroscopy measurements on suspended single-walled carbon nanotubes with independently addressable source and drain electrodes in the Coulomb blockade regime. This three-terminal configuration allows the resistance to the source and drain electrodes to be individually measured, which we exploit to demonstrate that electrons were added to spin-degenerate states of the carbon nanotube. Unexpectedly, the Coulomb peaks also showed a strong spatial dependence. By performing simultaneous scanning tunneling spectroscopy and electrical transport measurements we show that the probed states are extended between the source and drain electrodes. This indicates that the observed spatial dependence reflects a modulation of the contact resistance.     

Topologically guaranteed univariate solutions of underconstrained polynomial systems via no-loop and single-component tests

We present an algorithm which robustly computes the intersection curve(s) of an underconstrained piecewise polynomial system consisting of n equations with n+1 unknowns. The solution of such a system is typically a curve in Rⁿ⁺¹. This work extends the single solution test of Hanniel and Elber (2007) [6] for a set of algebraic constraints from zero-dimensional solutions to univariate solutions, in Rⁿ⁺¹. Our method exploits two tests: a no-loop test (NLT) and a single-component test (SCT) that together isolate and separate domains D where the solution curve consists of just one single component. For such domains, a numerical curve tracing is applied. If one of those tests fails, D is subdivided. Finally, the single components are merged together and, consequently, the topological configuration of the resulting curve is guaranteed. Several possible applications of the solver, namely solving the surface–surface intersection problem, computing 3D trisector curves, flecnodal curves or kinematic simulations in 3D are also discussed.