Byrnes Steven J

Bio: Byrnes Steven J is an academic researcher from Charles Stark Draper Laboratory. The author has contributed to research in topics: Surface acoustic wave & Lens (optics). The author has an hindex of 12, co-authored 41 publications receiving 2421 citations. Previous affiliations of Byrnes Steven J include University of California, Berkeley & Harvard University.

Above-bandgap voltages from ferroelectric photovoltaic devices

Abstract: In conventional solid-state photovoltaics, electron-hole pairs are created by light absorption in a semiconductor and separated by the electric field spaning a micrometre-thick depletion region. The maximum voltage these devices can produce is equal to the semiconductor electronic bandgap. Here, we report the discovery of a fundamentally different mechanism for photovoltaic charge separation, which operates over a distance of 1-2 nm and produces voltages that are significantly higher than the bandgap. The separation happens at previously unobserved nanoscale steps of the electrostatic potential that naturally occur at ferroelectric domain walls in the complex oxide BiFeO(3). Electric-field control over domain structure allows the photovoltaic effect to be reversed in polarity or turned off. This new degree of control, and the high voltages produced, may find application in optoelectronic devices.

Photovoltaic effects in BiFeO3

Abstract: We report a photovoltaic effect in ferroelectric BiFeO3 thin films The all-oxide heterostructures with SrRuO3 bottom and tin doped indium oxide top electrodes are characterized by open-circuit voltages ∼08–09 V and external quantum efficiencies up to ∼10% when illuminated with the appropriate light Efficiencies are at least an order of magnitude larger than the maximum efficiency under sunlight (AM 15) thus far reported for ferroelectric-based devices The dependence of the measured open-circuit voltage on film thickness suggests contributions to the large open-circuit voltage from both the ferroelectric polarization and band offsets at the BiFeO3/tin doped indium oxide interface

Harvesting renewable energy from Earth’s mid-infrared emissions

Abstract: It is possible to harvest energy from Earth's thermal infrared emission into outer space. We calculate the thermodynamic limit for the amount of power available, and as a case study, we plot how this limit varies daily and seasonally in a location in Oklahoma. We discuss two possible ways to make such an emissive energy harvester (EEH): A thermal EEH (analogous to solar thermal power generation) and an optoelectronic EEH (analogous to photovoltaic power generation). For the latter, we propose using an infrared-frequency rectifying antenna, and we discuss its operating principles, efficiency limits, system design considerations, and possible technological implementations.

Multilayer optical calculations

Abstract: When light hits a multilayer planar stack, it is reflected, refracted, and absorbed in a way that can be derived from the Fresnel equations. The analysis is treated in many textbooks, and implemented in many software programs, but certain aspects of it are difficult to find explicitly and consistently worked out in the literature. Here, we derive the formulas underlying the transfer-matrix method of calculating the optical properties of these stacks, including oblique-angle incidence, absorption-vs-position profiles, and ellipsometry parameters. We discuss and explain some strange consequences of the formulas in the situation where the incident and/or final (semi-infinite) medium are absorptive, such as calculating $T>1$ in the absence of gain. We also discuss some implementation details like complex-plane branch cuts. Finally, we derive modified formulas for including one or more "incoherent" layers, i.e. very thick layers in which interference can be neglected. This document was written in conjunction with the "tmm" Python software package, which implements these calculations.

Surface Propensities of Atmospherically Relevant Ions in Salt Solutions Revealed by Phase-Sensitive Sum Frequency Vibrational Spectroscopy

Abstract: We use phase-sensitive sum-frequency vibrational spectroscopy (PS-SFVS) to study the air/water interfaces of a set of salt solutions. The spectra reveals the presence of an electric double layer fo...

Fast parallel algorithms for short-range molecular dynamics

Abstract: Three parallel algorithms for classical molecular dynamics are presented. The first assigns each processor a fixed subset of atoms; the second assigns each a fixed subset of inter-atomic forces to compute; the third assigns each a fixed spatial region. The algorithms are suitable for molecular dynamics models which can be difficult to parallelize efficiently—those with short-range forces where the neighbors of each atom change rapidly. They can be implemented on any distributed-memory parallel machine which allows for message-passing of data between independently executing processors. The algorithms are tested on a standard Lennard-Jones benchmark problem for system sizes ranging from 500 to 100,000,000 atoms on several parallel supercomputers--the nCUBE 2, Intel iPSC/860 and Paragon, and Cray T3D. Comparing the results to the fastest reported vectorized Cray Y-MP and C90 algorithm shows that the current generation of parallel machines is competitive with conventional vector supercomputers even for small problems. For large problems, the spatial algorithm achieves parallel efficiencies of 90% and a 1840-node Intel Paragon performs up to 165 faster than a single Cray C9O processor. Trade-offs between the three algorithms and guidelines for adapting them to more complex molecular dynamics simulations are also discussed.

Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells

Abstract: The performance of organometallic perovskite solar cells has rapidly surpassed that of both conventional dye-sensitized and organic photovoltaics. High-power conversion efficiency can be realized in both mesoporous and thin-film device architectures. We address the origin of this success in the context of the materials chemistry and physics of the bulk perovskite as described by electronic structure calculations. In addition to the basic optoelectronic properties essential for an efficient photovoltaic device (spectrally suitable band gap, high optical absorption, low carrier effective masses), the materials are structurally and compositionally flexible. As we show, hybrid perovskites exhibit spontaneous electric polarization; we also suggest ways in which this can be tuned through judicious choice of the organic cation. The presence of ferroelectric domains will result in internal junctions that may aid separation of photoexcited electron and hole pairs, and reduction of recombination through segregation...

A broadband achromatic metalens for focusing and imaging in the visible.

Abstract: A key goal of metalens research is to achieve wavefront shaping of light using optical elements with thicknesses on the order of the wavelength. Such miniaturization is expected to lead to compact, nanoscale optical devices with applications in cameras, lighting, displays and wearable optics. However, retaining functionality while reducing device size has proven particularly challenging. For example, so far there has been no demonstration of broadband achromatic metalenses covering the entire visible spectrum. Here, we show that by judicious design of nanofins on a surface, it is possible to simultaneously control the phase, group delay and group delay dispersion of light, thereby achieving a transmissive achromatic metalens with large bandwidth. We demonstrate diffraction-limited achromatic focusing and achromatic imaging from 470 to 670 nm. Our metalens comprises only a single layer of nanostructures whose thickness is on the order of the wavelength, and does not involve spatial multiplexing or cascading. While this initial design (numerical aperture of 0.2) has an efficiency of about 20% at 500 nm, we discuss ways in which our approach may be further optimized to meet the demand of future applications. Controlling the geometry of each dielectric element of a nanostructured surface enables frequency-dependent group delay and group delay dispersion engineering, and the fabrication of an achromatic metalens for imaging in the visible in transmission.

Perovskite oxides for visible-light-absorbing ferroelectric and photovoltaic materials

Abstract: Most known ferroelectric photovoltaic materials have very wide electronic bandgaps (that is, they absorb only high-energy photons) but here a family of perovskite oxides is described that have tunable bandgaps, allowing their use across the whole visible-light spectrum. The spontaneous electrical polarization that characterizes a ferroelectric material is attractive for solar-cell applications as the positive and negative charges generated by light absorption have a natural tendency to separate, making them easier to harvest efficiently. Unfortunately most known ferroelectrics have wide electronic bandgaps — that is they absorb only higher energy photons that make up a small fraction of the solar spectrum. Ilya Grinberg and colleagues now show that a classic ferroelectric can be chemically engineered to tune the bandgap over a broad range, achieving strong absorption and photocurrent generation across the solar spectrum. Ferroelectrics have recently attracted attention as a candidate class of materials for use in photovoltaic devices, and for the coupling of light absorption with other functional properties1,2,3,4,5,6,7. In these materials, the strong inversion symmetry breaking that is due to spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the bandgap, which may enable efficiencies beyond the maximum possible in a conventional p–n junction solar cell2,6,8,9,10. Ferroelectric oxides are also stable in a wide range of mechanical, chemical and thermal conditions and can be fabricated using low-cost methods such as sol–gel thin-film deposition and sputtering3,5. Recent work3,5,11 has shown how a decrease in ferroelectric layer thickness and judicious engineering of domain structures and ferroelectric–electrode interfaces can greatly increase the current harvested from ferroelectric absorber materials, increasing the power conversion efficiency from about 10−4 to about 0.5 per cent. Further improvements in photovoltaic efficiency have been inhibited by the wide bandgaps (2.7–4 electronvolts) of ferroelectric oxides, which allow the use of only 8–20 per cent of the solar spectrum. Here we describe a family of single-phase solid oxide solutions made from low-cost and non-toxic elements using conventional solid-state methods: [KNbO3]1 − x[BaNi1/2Nb1/2O3 − δ]x (KBNNO). These oxides exhibit both ferroelectricity and a wide variation of direct bandgaps in the range 1.1–3.8 electronvolts. In particular, the x = 0.1 composition is polar at room temperature, has a direct bandgap of 1.39 electronvolts and has a photocurrent density approximately 50 times larger than that of the classic ferroelectric (Pb,La)(Zr,Ti)O3 material. The ability of KBNNO to absorb three to six times more solar energy than the current ferroelectric materials suggests a route to viable ferroelectric semiconductor-based cells for solar energy conversion and other applications.

Organohalide lead perovskites for photovoltaic applications

Abstract: There are only few semiconducting materials that have been shaping the progress of third generation photovoltaic cells as much as perovskites. Although they are deceivingly simple in structure, the archetypal AMX3-type perovskites have built-in potential for complex and surprising discoveries. Since 2009, a small and somewhat exotic class of perovskites, which are quite different from the common rock-solid oxide perovskite, have turned over a new leaf in solar cell research. Highlighted as one of the major scientific breakthroughs of the year 2013, the power conversion efficiency of the title compound hybrid organic–inorganic perovskite has now exceeded 18%, making it competitive with thin-film PV technology. In this minireview, a brief history of perovskite materials for photovoltaic applications is reported, the current state-of-the-art is distilled and the basic working mechanisms have been discussed. By analyzing the attainable photocurrent and photovoltage, realizing perovskite solar cells with 20% efficiency for a single junction, and 30% for a tandem configuration on a c-Si solar cell would be realistic.