Engineering topics
Publications and source records attributed to Jenkins, P..
AM0 solar cell calibration laboratories throughout the world are working together to create standard methods for AM0 calibration. In an effort to compare the results of different calibration methods, five different laboratories calibrated GaAs and Si solar cells. The results of this inter-comparison are presented herein.
Single junction InGaP/GaAs solar cells displaying high efficiency and record high open circuit voltage values have been grown by metalorganic chemical vapor deposition on Ge/graded SiGe/Si substrates. Open circuit voltages as high as 980 mV under AM0 conditions have been verified to result from a single GaAs junction, with no evidence of Ge-related sub-cell photoresponse. Current AM0 efficiencies of close to 16% have been measured for a large number of small area cells, whose performance is limited by non-fundamental current losses due to significant surface reflection resulting from greater than 10% front surface metal coverage and wafer handling during the growth sequence for these prototype cells. It is shown that at the material quality currently achieved for GaAs grown on Ge/SiGe/Si substrates, namely a 10 nanosecond minority carrier lifetime that results from complete elimination of anti-phase domains and maintaining a threading dislocation density of approximately 8 x 10(exp 5) per square centimeter, 19-20% AM0 single junction GaAs cells are imminent. Experiments show that the high performance is not degraded for larger area cells, with identical open circuit voltages and higher short circuit current (due to reduced front metal coverage) values being demonstrated, indicating that large area scaling is possible in the near term. Comparison to a simple model indicates that the voltage output of these GaAs on Si cells follows ideal behavior expected for lattice mismatched devices, demonstrating that unaccounted for defects and issues that have plagued other methods to epitaxially integrate III-V cells with Si are resolved using SiGe buffers and proper GaAs nucleation methods. These early results already show the enormous and realistic potential of the virtual SiGe substrate approach for generating high efficiency, lightweight and strong III-V solar cells.
Future missions to Mars will rely heavily on solar power from the sun, various solar cell types and structures must be evaluated to find the optimum. Sunlight on the surface of Mars is altered by air-borne dust that fluctuates in density from day to day. The dust affects both the intensity and spectral content of the sunlight. The MATE flight experiment was designed for this purpose and will fly on the Mars 2001 Surveyor Lander as part of the Mars In-Situ Propellant Production Precursor (MIP) package. MATE will measure the performance of several solar cell technologies and characterize the Martian environment in terms of solar power. This will be done by measuring full IV curves on solar cells, direct and global insolation, temperature, and spectral content. The Lander is is scheduled to launch in April 2001 and arrive on Mars in January of 2002. The site location has not been identified but will be near the equator and last from 100 to 300 days. The intent of this of this paper is to describe and update the progress on MATE. MATE has four main objectives for its mission to Mars. First is to measure the performance of solar cells daily on the surface of Mars, this will determine the day to day fluctuations in sunlight and temperature and provide a nominal power output. Second, in addition to measuring solar cell performance, it will allow for an intercomparison of different solar cell technologies. Third, It will study the long term effects of dust on the solar cells. Fourth and last, it will characterize the mars environment as viewed by the solar cell, measuring spectrum, insolation, and temperature. Additional information is contained in the original extended abstract.
The first two of a planned series of international workshops concerning space solar cell calibration and measurement techniques have been held within the past year. The need for these workshops arose from the increasing complexity of space solar cells coupled with the growing international nature of the market for space cells and arrays. The workshops, jointly sponsored by NASDA, ESA and NASA, have the objective of obtaining international agreement on standardized values for the AM0 spectrum and constant, recommendations for laboratory measurement practices and the establishment of a set of protocols for making interlaboratory comparison measurements. The results of the first two workshops, held in Waikiki, Hawaii, USA in 1994 and Madrid, Spain in 1995, are presented.
Future space missions may use laser power beaming systems with a free electron laser (FEL) to transmit light to a photovoltaic array receiver. To investigate the efficiency of solar cells with pulsed laser light, several types of GaAs, Si, CuInSe2, and GaSb cells were tested with the simulated pulse format of the induction and radio frequency (RF) FEL. The induction pulse format was simulated with an 800-watt average power copper vapor laser and the RF format with a frequency-doubled mode-locked Nd:YAG laser. Averaged current vs bias voltage measurements for each cell were taken at various optical power levels and the efficiency measured at the maximum power point. Experimental results show that the conversion efficiency for the cells tested is highly dependent on cell minority carrier lifetime, the width and frequency of the pulses, load impedance, and the average incident power. Three main effects were found to decrease the efficiency of solar cells exposed to simulated FEL illumination: cell series resistance, LC 'ringing', and output inductance. Improvements in efficiency were achieved by modifying the frequency response of the cell to match the spectral energy content of the laser pulse with external passive components.
The apparently unrelated phenomena of temperature dependency, carrier removal and photoluminescence are shown to be affected by the high dislocation densities present in heteroepitaxial InP solar cells. Using homoepitaxial InP cells as a baseline, it is found that the relatively high dislocation densities present in heteroepitaxial InP/GaAs cells lead to increased volumes of dVoc/dt and carrier removal rate and substantial decreases in photoluminescence spectral intensities. With respect to dVoc/dt, the observed effect is attributed to the tendency of dislocations to reduce Voc. Although the basic cause for the observed increase in carrier removal rate is unclear, it is speculated that the decreased photoluminescence intensity is attributable to defect levels introduced by dislocations in the heteroepitaxial cells.