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Electron energy-loss spectroscopy (EELS) can measure similar information to x-ray, UV–Vis, and IR spectroscopies but with atomic resolution and increased scattering cross-sections. Recent advances in electron monochromators have expanded EELS capabilities from chemical identification to the realms of synchrotron-level core-loss measurements and to low-loss, 10–100 meV excitations, such as phonons, excitons, and valence structures. EELS measurements are easily correlated with electron diffraction and atomic-scale real-space imaging in a transmission electron microscope (TEM) to provide detailed local pictures of quasiparticle and bonding states. This perspective provides an overview of existing high-resolution EELS (HR-EELS) capabilities while also motivating the powerful next step in the field—ultrafast EELS in a TEM. Ultrafast EELS aims to combine atomic-level, element-specific, and correlated temporal measurements to better understand spatially specific excited-state phenomena. Ultrafast EELS measurements also add to the abilities of steady-state HR-EELS by being able to image the electromagnetic field and use electrons to excite photon-forbidden and momentum-specific transitions. We discuss the technical challenges ultrafast HR-EELS currently faces, as well as how integration with in situ and cryo measurements could expand the technique to new systems of interest, especially molecular and biological samples. [ABSTRACT FROM AUTHOR]

We demonstrate that in solar cells with highly reflective back mirrors, the measured internal quantum efficiency exhibits a shift in bandgap relative to the measured external quantum efficiency. The shift arises from the fact that the measured reflectance at the front surface includes a superposition of waves reflecting from the front and back surfaces. We quantify the magnitude of the apparent shift and discuss the errors that can result in determination of quantities such as the photocurrent. Because of this apparent shift, it is important the bandgap be determined from the external quantum efficiency. [ABSTRACT FROM AUTHOR]

World-record power conversion efficiencies for Cu(In,Ga)Se2 (CIGS) solar cells have been achieved via a post-deposition treatment with alkaline metals, which increases the open-circuit voltage and fill factor. We explore the role of the potassium fluoride (KF) post-deposition treatment in CIGS by employing energy- and time-resolved photoluminescence spectroscopy and electrical characterization combined with numerical modeling. The bulk carrier lifetime is found to increase with post-deposition treatment from 255 ns to 388 ns, which is the longest charge carrier lifetime reported for CIGS, and within ∼40% of the radiative limit. We find evidence that the post-deposition treatment causes a decrease in the electronic potential fluctuations. These potential fluctuations have previously been shown to reduce the open-circuit voltage and the device efficiency in CIGS. Additionally, numerical simulations based on the measured carrier lifetimes and mobilities show a diffusion length of ∼10 lm, which is ∼4 times larger than the film thickness. Thus, carrier collection in the bulk is not a limiting factor for device efficiency. By considering differences in doping, bandgap, and potential fluctuations, we present a possible explanation for the voltage difference between KF-treated and untreated samples. [ABSTRACT FROM AUTHOR]

We present a proof-of-concept framework to independently determine the bulk Shockley-Read- Hall (SRH) lifetime and surface recombination velocity in silicon wafers self-consistently. We measure the transient decay of free-carrier absorption (FCA) using two different excitation wavelengths (1050 and 750 nm) for p-type crystalline Si (c-Si) wafers over a wide injection range and fit the FCA transients for the two excitation wavelengths in a coupled manner. In this way, we can estimate the surface recombination lifetime accurately. However, we find that the capability to uniquely measure extrinsic bulk-SRH recombination is challenging in the presence of other recombination processes and can be broadly categorized into five different regimes depending on the relative strengths of each recombination pathway. [ABSTRACT FROM AUTHOR]

We report above-bandgap ordinary optical properties of [variant_greek_epsilon]-phase GaSe single crystal. Reference-quality pseudodielectric function <[variant_greek_epsilon](E)>＝<[variant_greek_epsilon]1(E)>+i<[variant_greek_epsilon]2(E)> and pseudorefractive index ＝ +i spectra were measured by spectroscopic ellipsometry from 0.73 to 6.45 eV at room temperature for the light polarization perpendicular to the optic axis (E[accent:_right_hook_over]⊥c⁁). The <[variant_greek_epsilon]> spectrum exhibited several interband-transition critical-point structures. Analysis of second-energy derivatives calculated numerically from the measured data yielded the critical-point energy values. [ABSTRACT FROM AUTHOR]

We show that time-resolved photoluminescence measurements of completed polycrystalline CdTe solar cells provide a measure of recombination near the CdTe/CdS metallurgical interface that is strongly correlated to the open-circuit voltage in spite of complex carrier dynamics in the junction region. Oxygen in the growth ambient during close-spaced sublimation generally reduces this recombination rate; grain size does not have a strong effect. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Presents a study that explored the minority electron carrier recombination in n-type gallium arsenide heterostructures using photoluminescence decay measurements. Methodology; Examination of the low density decay constants of the samples; Analysis of the photon recycling in the samples.

In this paper, we study the integrability of a class of nonlinear non-autonomous quad graph equations compatible around the cube introduced by Boll in the framework of the generalized Adler, Bobenko, and Suris (ABS) classification. We show that all these equations possess three-point generalized symmetries which are subcases of either the Yamilov discretization of the Krichever-Novikov equation or of its non-autonomous extension. We also prove that all those symmetries are integrable as they pass the algebraic entropy test. [ABSTRACT FROM AUTHOR]

The authors report ellipsometrically determined optical properties of epitaxial cadmium oxide thin film grown by metal-organic vapor phase epitaxy on r-plane sapphire substrate. The ellipsometric data were collected from 0.74 to 6.45 eV with the sample at room temperature. Artifacts from the surface overlayers were reduced as far as possible by the premeasurement surface treatment procedures. Complex dielectric function [variant_greek_epsilon]=[variant_greek_epsilon]1+i[variant_greek_epsilon]2 and refractive index N=n+ik spectra were extracted from multilayer modeling of the data with the B-spline functions. [ABSTRACT FROM AUTHOR]

Umbral calculus can be viewed as an abstract theory of the Heisenberg commutation relation [P⁁,M⁁]=1. In ordinary quantum mechanics, P⁁ is the derivative and M⁁ the coordinate operator. Here, we shall realize P⁁ as a second order differential operator and M⁁ as a first order integral one. We show that this makes it possible to solve large classes of differential and integrodifferential equations and to introduce new classes of orthogonal polynomials, related to Laguerre polynomials. These polynomials are particularly well suited for describing the so-called flatenned beams in laser theory [ABSTRACT FROM AUTHOR]

In situ monitoring of material properties during thin-film deposition provides researchers with a valuable tool for maximizing solar cell performance, while also enabling efficient exploration of deposition parameter space. This article describes how our research team has utilized in situ real-time spectroscopic ellipsometry (RTSE) to maximize our productivity in two related projects. We are using hot wire chemical vapor deposition (HWCVD) for low-temperature (90–235 °C) deposition of very thin films of amorphous hydrogenated silicon for amorphous Si/crystal-Si heterojunction (SHJ) solar cells. We are also using HWCVD for low-temperature (200–440 °C) deposition of epitaxial films of silicon on crystal-Si substrates. We utilize RTSE as both an in situ diagnostic and a postgrowth analysis tool for SHJ solar cells and epi-Si films grown by HWCVD. Using input from RTSE analysis we have achieved a photovoltaic energy conversion efficiency of 17.1% on an Al-backed p-type float-zone c-Si wafer. Epi-Si films have been grown as thick as 500 nm utilizing parameter optimization based on RTSE analysis. [ABSTRACT FROM AUTHOR]

Analyzes the conditions necessary for the existence of generalized symmetries for a class of differential-difference equations. Application of the conditions to prove the existence of integrable equations belonging to the class; Construction of the classifying conditions; Integrability of differential-difference equations.

Studies the symmetries of discrete dynamical systems. Algorithm for calculation; Integrable Toda lattice; Continuous Lie point symmetry groups.

Two different methods of finding Lie point symmetries of differential-difference equations are presented and applied to the two-dimensional Toda lattice. Continuous symmetries are combined with discrete ones to obtain various reductions to lower dimensional equations, in particular, to differential equations of the delay type. The concept of conditional symmetries is extended from purely differential to differential-difference equations and shown to incorporate Bäcklund transformations. [ABSTRACT FROM AUTHOR]

The Kadomtsev-Petviashvili (KP) equation (u[sub t] + 3uu[sub x]/2 + 1/4u[sub xxx])[sub x] + 3σu[sub yy]/4 = 0 allows an infinite-dimensional Lie group of symmetries, i.e., a group transforming solutions amongst each other. The Lie algebra of this symmetry group depends on three arbitrary functions of time "t" and is shown to be related to a subalgebra of the loop algebra A[sup (1), sub 4]. Low-dimensional subalgebras of the symmetry algebra are identified, specifically all those of dimension n ≤ 3, and also a physically important six-dimensional Lie algebra containing translations, dilations, Galilei transformations, and "quasirotations." New solutions of the KP equation are obtained by symmetry reduction, using the one-dimensional subalgebras of the symmetry algebra. These solutions contain up to three arbitrary functions of t. [ABSTRACT FROM AUTHOR]

We report the component E[accent:_right_hook_over]⊥c⁁ of the pseudodielectric-function tensor <[variant_greek_epsilon](E)>=<[variant_greek_epsilon]1(E)>+i<[variant_greek_epsilon]2(E)> of γ-phase single-crystal InSe, obtained from 1.5 to 9.2 eV by vacuum-ultraviolet spectroscopic ellipsometry with the sample at room temperature. Overlayer artifacts were reduced as far as possible by measuring fresh surfaces prepared by cleavage. Accurate critical-point energies of observed structures were obtained by a combined method of spectral analysis. [ABSTRACT FROM AUTHOR]

Efficient crystalline silicon heterojunction solar cells are fabricated on p-type wafers using amorphous silicon emitter and back contact layers. The independently confirmed AM1.5 conversion efficiencies are 19.3% on a float-zone wafer and 18.8% on a Czochralski wafer; conversion efficiencies show no significant light-induced degradation. The best open-circuit voltage is above 700 mV. Surface cleaning and passivation play important roles in heterojunction solar cell performance. [ABSTRACT FROM AUTHOR]

Spectroscopic ellipsometry measurements of CuInSe2 (CIS) and CuIn0.8Ga0.2Se2 (CIGS) reveal that there are important differences in electronic properties between stoichiometric CIS (CIGS) and Cu-poor CIS (CIGS). We find a reduction in the absorption strength in the spectral region of 1–3 eV. This reduction can be explained in terms of the Cu 3d density of states. Cu-poor CIS (CIGS) materials show an increase in band gap due to the reduction in repulsion between Cu 3d and Se 4p states. The experimental results have important implications for the function of polycrystalline optoelectronic devices. [ABSTRACT FROM AUTHOR]

In this work, we found that the interdiffusion of the CdS and Zn[sub 2]SnO[sub 4] (ZTO) layers can occur either at high temperature (550-650 °C) in Ar or at lower temperature (400-420 °C) in a CdCl[sub 2] atmosphere. By integrating a Zn[sub 2]SnO[sub 4] film into a CdS/CdTe solar cell as a buffer layer, this interdiffusion feature can solve several critical issues and improve device performance and reproducibility of both SnO[sub 2]-based and Cd[sub 2]SnO[sub 4]-based CdTe cells. Interdiffusion consumes the CdS film from both the ZTO and CdTe sides during the device fabrication process and improves quantum efficiency at short wavelengths. The ZTO film acts as a Zn source to alloy with the CdS film, which results in increases in the band gap of the window layer and in short-circuit current density J[sub sc]. Interdiffusion can also significantly improve device adhesion after CdCl[sub 2] treatment, thus providing much greater process latitude when optimizing the CdCl[sub 2] process step. The optimum CdCl[sub 2]-treated CdTe device has high quantum efficiency at long wavelength, because of its good junction properties and well-passivated CdTe film. We have fabricated a Cd[sub 2]SnO[sub 4]/Zn[sub 2]SnO[sub 4]/CdS/CdTe cell demonstrating an NREL-confirmed total-area efficiency of 15.8% (V[sub oc]=844.3 mV, J[sub sc]=25.00 mA/cm[sup 2], and fill factor=74.82%). This high-performance cell is one of the best thin-film CdTe solar cells in the world. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Gallium arsenide layers grown by molecular beam epitaxy on (100) Si substrates were subjected to annealing under As overpressure at 650, 750, and 850 °C for 1/2 h. A substantial reduction in the dislocation density near the interface and in the bulk of the epitaxial layers was observed for the 850 °C anneal. In situ annealing at 700 °C for 1/2 h after 1/2 h of growth followed by a deposition of InGaAs/GaAs strained-layer superlattices and bulk layers also resulted in reduced dislocation densities. [ABSTRACT FROM AUTHOR]