Response of single junction GaAs/GaAs and GaAs/Ge solar cells to multiple doses of 1 MeV electrons

A comparison of the radiation tolerance of MOCVD-grown GaAs cells and GaAs/Ge cells was undertaken using 1 MeV electrons. The GaAs/Ge cells are somewhat more tolerant of 1 MeV electron irradiation and more responsive to annealing than are the GaAs/GaAs cells examined in this study. However, both types of cells suffer a greater degradation in efficiency than has been observed in other recent studies. The reason for this is not certain, but it may be associated with an emitter thickness which appears to be greater than desired. The deep level transient spectroscopy (DLTS) spectra following irradiation are not significantly different for the GaAs/Ge and the GaAs/GaAs cells, with each having just two peaks. The annealing behavior of these peaks is also similar in the two samples examined. It appears that no penalty in radiation tolerance, and perhaps some benefit, is associated with fabricating MOCVD GaAs cells on Ge substrates rather than GaAs substrates

Emerging material systems for solar cell applications

Primary effort to date involves modifications and additions to the vacuum system in order to produce films of InP by the plasma-induced deposition process

Development of copper sulfide/cadmium sulfide thin-film solar cells

The most important accomplishments during this period were to demonstrate and to elucidate further the complex effects that occur during the aging of Cu/sub 2/S/CdS thin-film solar cells in flowing wet oxygen. There are two distinct effects. At constant illumination, the short-circuit current of cells aged at room temperature consistently decreases with time. The second effect, related to diode opposing current, is more involved and may result from several competing mechanisms. Over the short term (approx. 4 to 5 hours), the magnitude of diode opposing current decreases. After approx. 20 hours of aging, opposing current generally returns to the level achieved after hydrogen annealing which immediately preceded the aging sequence. Optical measurements of the spectral transmission of the Cu/sub 2/S layers in a cell content have been made using a silicon detector epoxied to the back of a CdS cell after the copper foil substrate was removed. There is no significant change in Cu/sub 2/S transmission behavior for wavelengths ranging from 525 to 1000 nm during wet-oxygen aging for periods of 2 to 36 hours. This suggests that the decrease in J/sub SC/ at constant illumination, for the aging experiments in a flowing wet-oxygen ambient, arises because of changes in minority-carrier transport properties of the Cu/sub 2/S. Before developing a method for using an epoxied silicon detector to measure optical behavior of the Cu/sub 2/S layer, we explored the possibility of using a junction-containing wafer of silicon as a substrate for deposited CdS films. Some monolithic structures were successfully fabricated. Comparisons were made of CdS grain structure details in the junction detector area and in an adjacent metallized area

Cadmium sulfide/copper sulfide heterojunction cell research. Technical progress report No. 2, January 1--March 31, 1978

Poor adhesion of CdS films to zinc-plated copper foil substrates has been eliminated by depositing somewhat thinner films than earlier and by using the mat (as opposed to the smooth) side of the electroformed copper foil for deposition. Particle spatter effects have been reduced and cells of 4 cm/sup 2/ area have been produced with average efficiency of 5.4%. Use of Zn--plated Cu, Mo, Zn--plated Mo and glass substrates appears to influence the etching behavior of CdS films in dilute chromic acid. The results suggest a sensitivity to the presence of zinc. Laser scanned reflection and photocurrent response studies reveal some influence of substrate morphology. Direct correlations are weak, presumably because of strong local variations in topography, due to texturing and in carrier transport or junction collection efficiency. Similar types of studies on single crystal-based cells have established the feasibility of using localized photoresponse to characterize minority carrier transport in special test structures. Compositional analysis of Cu/sub 2/S layers on single crystal CdS have been attempted using ESCA and ion etching methods. A major problem is the formation of copper nodules, presumably due to the ion etching method, which do not permit obtaining a true profile of this element. Films of (Zn/sub x/Cd/sb 1-x/)S alloys, deposited using electron beam evaporation, have been made with composition corresponding to that of the evaporant sources

Development of copper sulfide/cadmium sulfide thin-film solar cells. Thirteenth technical progress report, July 1, 1982-September 30, 1982

During this period we modified the characteristics of a completed cell by laser treatment in order to alter the sensitivity of the heterojunction behavior to the aging effects of oxygen and moisture. Previously we associated the changes in opposing current which affect the open-circuit voltage of the cells during aging with an increased value of the electric field in the space charge region. The present work demonstrates a strong correlation between the short-circuit current behavior and the density of deep donor states which control open-circuit voltage behavior. Practically speaking, the general degradation trends in the short-circuit current and the opposing current behavior, which we observed in control cells earlier, occur after substantial delay in the laser-treated cell. The laser treatment reduced the rate of the degradation effects in question by at least one order of magnitude. Furthermore, during the early stages of aging in wet oxygen, the laser-treated cell performance was enhanced in terms of both short-circuit current and opposing current

Investigation of photovoltaic mechanisms in polycrystalline thin-film solar cells. Interim technical report, November 1, 1980-July 31, 1981

Effort is reported on measurement technique development to assess the utility of Deep-Level Transient Spectroscopy (DLTS) methods in characterizing polycrystalline silicon that was deliberately doped with Ti during growth. Difficulties encountered with lateral DLTS measurements are discussed. In this approach, modulation of the grain boundary, double-depletion region produces the entire DLTS signal. Major effort has been applied in grain boundary characterization and control. The most significant accomplishments to date have involved laser scanning of slices of Wacker SILSO polysilicon having nearly identical grain structure. By using various kinds of treatments and by comparing treated and untreated substrates having nearly identical grain structure, control of grain boundary photocurrent suppression (..delta..I/sub ph/) over the range 1% less than or equal to ..delta..I/sub ph/ less than or equal to 40% was demonstrated

Phase 2 of the Array Automated Assembly Task for the Low Cost Silicon Solar Array Project. First quarterly report, October 1, 1977--December 31, 1977

A process sequence is proposed and analyzed with regard to yield and cost of direct materials for fabrication. Cells of at least 13% conversion efficiency and panels of 10% efficiency have been assumed. Material costs, excluding silicon, were estimated as low as $0.19/watt (1975). Use of the JPL Interim Price Estimation Guidelines gives a projected contribution to selling price of$0.24/watt (1975) for the direct materials used in processing. If silicon is included, the contribution to selling price is $0.46/watt (1975). Development effort during this quarter has been applied to junction information, anti-reflection coating and laser scribing. Junction formation effort was in two areas: (1) establishing the process parameter ranges for the back surface diffusion from a boron-doped oxide and for the front junction diffusion using POCl/sub 3/ and (2) determining if the purity of the POCl/sub 3/ source material is critical to cell performance. Anti-reflection coatings of TiO/sub 2/ have been prepared on cells formed in dendritic web silicon by spin-on application of a liquid metallo-organic precursor.Achievement of 12 to 13% conversion efficiency after single layer coating was common. Laser scribing to remove dendrites from the ribbon and to delineate cell areas has been demonstrated. The presence of photoresist, AR or metallic films on the ribbons does not significantly affect scribing. Laser scribing has been found to degrade cell performance, presumably by damaging the front collecting junction. Proper control of laser penetration during back-surface scribing is expected to solve this problem

Phase 2 of the Array Automated Assembly Task for the Low Cost Silicon Solar Array Project. Second quarterly report, January 1--March 31, 1978

Development effort has been applied in these areas during the second quarter: effects of back surface field (BSF) region diffusions, application of anti-reflection coatings and photoresist layers to web silicon cells by dipping and cell separation by laser scribing. In addition, consideration has been given to selecting appropriate adhesives for encapsulation of completed modules. A review has been made of the properties of several organic materials as candidates for the adhesives in preparing modules with cell arrays encapsulated between sheets of glass. Room temperature curing materials have been chosen to eliminate the need for a costly and time consuming curing step

Cadmium sulfide/copper sulfide heterojunction cell research. Technical progress report No. 3, April 1--June 30, 1978

Extensive studies have been made of the structural details of Cu/sub 2/S films on evaporated polycrystalline CdS layers. Examinations of free standing films reveal Cu/sub 2/S penetration along grain boundaries of at least 5 ..mu..m on unetched CdS films and of 10 ..mu..m or more on etched films. Preliminary structural comparisons have been made on films yielding low (< 3%) and higher efficiency (approx. > 5%) cells. Minority carrier diffusion length values of about 0.3 ..mu..m have been obtained from photocurrent response to a laser spot scanned along a Cu/sub 2/S layer of graded thickness. Doubling the time of exposure to a 200/sup 0/C nitrogen ambient used in forming the Cu/sub 2/S layer does not significantly change the diffusion length. Anomalously high values of diffusion length (approx. > 1 ..mu..m) are indicated for a region associated with a crack in the CdS. Compositional profiling of Cu, S and Cd by ion scattering spectroscopy (ISS) and Auger electron spectroscopy (AES) have yielded generally similar results for Cu/sub 2/S films prepared on single crystal CdS substrates. Somewhat greater detail appears in certain of the profiles obtained with ISS which is attributed to less influence of depth averaging than in the AES case

Development of copper sulfide/cadmium sulfide thin-film solar cells

The purpose of this work has been to identify aspects of cell fabrication and treatment which are critical for achieving high efficiency Cu/sub 2/S/CdS solar cells. In approaching the problem several comparisons were made of the effects of specific steps in two methods of cell fabrication. These methods had previously given cells of about 6% and a maximum of 9% efficiency. Three areas requiring special attention and specific means to achieve acceptable results were identified. (1) The Cu/sub 2/S/CdS heterojunction area must be minimized. If single source evaporations of CdS are made on substrates whose temperatures (approx. 220/sup 0/C) are monitored and controlled using welded thermocouples, the CdS films will have adequately large grains (grain diameter greater than or equal to 2 ..mu..m) and will not develop significant etch pits during texturing in a mild etchant solution. (2) The termination of the wet barrier processing steps must be done carefully. An acceptable termination involves minimizing the amount of cuprous chloride retained on the cell surface during transfer to a rinsing stage while providing adequate exclusion of air from the space above the surface of the cuprous chloride solution. (3) Once formed, the Cu/sub 2/S layer should not be exposed to high temperatures (>100/sup 0/C) for long periods of time (> 5 min) if surface adsorbed moisture or oxygen are present. Heat treatments in ampoules under flowing hydrogen atmospheres should be preceded and followed by periods of at least 30 minutes at room temperature in the reducing ambient. If all these precautions are taken, wet chemical barrier processing of thermally evaporated CdS films on zinc-plated copper foil substrates yields cells of nearly 8% conversion efficiency without AR coating