Rodney T. Hodgson

The effect of fluorescent wavelength shifting on solar cell spectral response

Abstract Fluorescent wavelength shifting has been used to enhance the spectral response and AM0 conversion efficiency of several types of solar cells. Plastic fluorescent materials are useful for devices with a sharp cut-off in response, while ruby is suitable for devices with more gradual cut-offs. Efficiency improvements of 0.5 to 2 percentage points were measured on some cells, and greater improvements can be expected for optimized optical components. The optical efficiencies (light transmitted into the solar cell compared to light incident on the fluorescent sheet) exceeded 50% for the plastic sheets and 75% for ruby.

Frequency doubling in KB5O8⋅4H2O and NH4B5O8⋅4H2O to 217.3 nm

We have produced tunable phase‐matched second‐harmonic radiation in a KB5O8⋅4H2O crystal between 217.3 and 234.5 nm. Phase matching was achieved by crystal rotation. We also studied frequency doubling in NH4B5O8⋅4H2O.

1.3- mu m P-i-N photodetector using GaAs with As precipitates (GaAs:As)

The fabrication of a GaAs detector which operates in the 1.3- to 1.5- mu m optical range is reported. The detector is a P-i-N photodiode with an intrinsic layer composed of undoped GaAs which was grown at 225 degrees C and subsequently annealed at 600 degrees C. This growth process has been demonstrated to produce a high density of As precipitates in the low-temperature grown region, which the authors show to exhibit absorption through internal photoemission. The internal Schottky barrier height of the As precipitates is found to be 0.7 eV, leading to reasonable room-temperature responsivity out to around 1.7 mu m.<<ETX>>

Magnetically filtered low loss scanning electron microscopy

An electron microscope which includes a detector which is located in the magnetic field used to focus the primary electron beam onto the sample. The focusing magnetic field is used to energy-filter and/or energy analyze the scattered electrons without the need for additional equipment, such as a retarding-field energy filter. The magnetic field of the condenser-objective lens (or of any other type of magnetic lens) of the microscope provides the filtering and/or analyzing action, and the detector can be located so as to collect only low-loss electrons.

RESONANTLY ENHANCED, NONLINEAR GENERATION OF TUNABLE, COHERENT, VACUUM ULTRAVIOLET (VUV) LIGHT IN ATOMIC VAPORS*

As is well known, atomic vapors, by virtue of their centrosymmetry, do not display quadratic optical nonlinearities. Thus, for example, second harmonic generation is ruled out in these media, provided there are no externally applied fields that destroy the symmetry. Third harmonic generation (THG) in vapors is allowed on the basis of symmetry. We briefly note that the earliest experiments were carried out with noble gas atomic vapors, and that the experiments themselves consisted in measuring the so-called hyperpolarizabilities.s2 These nonlinearities were weak compared t o those in crystalline solids by virtue of the much lower densities of the vapors and the nonresonant nature of the nonlinearities. The lowest lying excited states of noble gas atoms were far above the photon energies involved in these third harmonic generation experiments. The r e ~ e a r c h e r s ~ ~ a t Stanford University who pioneered the use of THG in gases as a practical source of coherent ultraviolet light studied atomic systems other than the noble gases-specifically, the alkali metals-using fixed-frequency input beams. They obtained for the first time practically useful third harmonic conversion efficiencies and showed generally that the large nonlinearities were due t o resonance enhancements. The advantage gained in using metal vapors stems from the fact of their generally low first ionization potentials. Consequently, various bound excited states lie in an energy range where resonance enhancement is possible with the visible input, or VUV third harmonic output beams. The work of Harris et al .3-5 stimulated the interest of various other groups,6-8 each of which then independently focused on the further advantages t o be gained in attaining exact two-photon resonance. These advantages are based upon the lack of linear absorption or dispersion associated with a twophoton resonance. Our own approach, which is characterized experimentally by the use of nitrogen-laser-pumped dye lasers t o provide the input beams, is perhaps best suited t o take advantage of two-photon and other resonance enhancements. It also enables the VUV output t o be tuned. Technical aspects of the dye lasers we employ are adequately described in Reference 9. In the present paper the emphasis will be on physical factors determining the relative VUV output in the various tuning ranges. Before actually discussing VUV generation, however, we shall describe at some length a multiphoton ionization experiment that graphically illustrates the importance of two-photon resonance in nonlinear phenomena in gases and also serves t o acquaint the reader with the arrange-

Magnetically filtered low‐loss scanning electron microscopy

The resolution of the scanning electron microscope can be improved by mounting the sample in the high‐field region of a condenser‐objective lens. Low‐loss electrons (LLEs) are scattered from the sample with an energy loss of less than a few percent of the incident energy. In the past, LLEs have been collected with a retarding‐field energy filter. A way has been found to collect LLEs using a detector located within the magnetic field of the condenser‐objective lens which provides the required energy‐filtering action. This greatly simplifies the apparatus and makes it possible to obtain LLE images with less tilt of the specimen and with a higher beam energy than before.

GaAs see the light (photodetectors)

Summary form only given. Reported are p-i-n photodetectors using GaAs with As precipitates (GaAs:As) that detect 1.3- mu m light with reasonable efficiency. Since high-performance GaAs electronic circuits can be epitaxially grown on GaAs:As, this work may open the way for an all-GaAs 1.3- mu m optical receiver chip. In the GaAs p-i-n devices, the i layer was a 1- mu m-thick layer of GaAs:As. MSM structures with GaAs:As as the optically active material were also made. >

WA-B2 improved high-energy response of Ga 1-x Al x As-GaAs solar cells using fluorescent capping layers

applications. We now have obtained AM1 efficiency values approaching 25 percent at 200 suns. The basic material’s considerations going into high efficiency 1-sun cell design have been presented previously.2 In this paper, we will describe the design criteria and results on high concentration cells. In concentrator cells, the emphasis is on achieving minimal effective series resistance. The ohmic contact has two major effects on the performance of the solar cell; it reduces the photocurrent because of the shadowing effect of the contacts on the incident light, and it introduces a series resistance because the cell current has to flow transversely through the sheet resistance of the surface layer, the contact resistance of the ohmic contact, and the sheet resistance of the metal grid before it is delivered to the load. These two effects represent opposing requirements on the grid design. A detailed analysis investigating the distributed nature of the series resistance is currently underway and the results of this study will be presented. Measurements of cell performance were made in natural sunlight at the Jet Propulsion Laboratory’s facility at Table Mountain, CA, using a Cassegrain reflecting concentrator3 on an equational mount in the tracking mode. The average insolation during the time of the measurements was %95 mW/cm2. The primary data consisted of IV curves taken at various concentrations. Simultaneously, the cell temperature, controlled by active cooling, was monitored. For the best of these cells, AM1 values of 24.6 percent at 180 suns, and 20.5 percent at 440 suns were obtained.