ALSNews is a biweekly electronic newsletter to keep users and other interested parties informed about developments at the Advanced Light Source, a national user facility located at Lawrence Berkeley National Laboratory, University of California. To be placed on the mailing list, send your name and complete internet address to ALSNews@lbl.gov. We welcome suggestions for topics and content.

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ALSNews           Vol. 90           November 12, 1997

Table of Contents


    1. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS
    2. A STEP TOWARD INEXPENSIVE, EFFICIENT SOLAR CELLS
    3. JANUARY SHUTDOWN CANCELED
    4. OPERATIONS UPDATE

1. CALL FOR INDEPENDENT INVESTIGATOR PROPOSALS

The ALS is now accepting proposals from scientists who wish to conduct research at the facility as independent investigators during April-September 1998. The deadline for proposals is January 15, 1998. Because the period scheduled in our last proposal cycle (December 1997 to March 1998) was short, all proposals that were not granted beam time for that period will automatically roll over to this proposal cycle.

The proposal form for independent investigators is available in Portable Document Format (PDF) on the Web. Information on the proposal process is available at the same location.

To request a proposal form by mail, contact:
Elizabeth Saucier, ALS Administrator
Tel: (510) 486-6166
Fax: (510) 486-4960
Email: alsuser@lbl.gov

For information on beamlines available to independent investigators, contact:
Fred Schlachter, ALS Program Support
Tel: (510) 486-4892
Fax: (510) 486-6499
Email: fred_schlachter@lbl.gov

2. A STEP TOWARD INEXPENSIVE, EFFICIENT SOLAR CELLS
(contact: samchugo@lbl.gov)

Solar energy offers promise for environmental reasons and as a power source for remote locations. However, contamination by metals (which inevitably occurs during manufacturing) can lower silicon solar cells' efficiency so much that they are not economically viable. Scott McHugo, investigating this problem at Beamline 10.3.1, has characterized how copper and nickel contaminants cluster in silicon and observed their partial disappearance under thermal treatment.

Silicon solar cells contain regions of so-called n-type and p-type material (produced by adding phosphorus and boron respectively) and p-n junctions where the two types join. Electrical charge is carried primarily by electrons in n-type material and by positively charged electron vacancies (holes) in p-type material. Any holes in n-type material or electrons in p-type material are called minority carriers. When a solar cell absorbs sunlight, extra electrons and holes are created on both sides of the p-n junctions. Minority carriers on each side then diffuse toward junctions, where they can lower their energy by crossing to the other side; in so doing they generate a useful electrical current.

A reduction in this electrical current, and thus in the solar cell's efficiency, occurs when minority carriers recombine with majority carriers before reaching a junction. The distance the carriers travel before recombination is called diffusion length. Metal contaminants decrease diffusion length, so it is essential to remove them. Previous efforts have produced 23% efficiency in ultrapure solar cells made of single-crystal silicon, but this material is too expensive for commercial solar power.

Polycrystalline silicon (poly-Si, many small silicon crystallites separated by grain boundaries) is potentially much less expensive. However, the 12% efficiency of poly-Si cells can be too low for economic viability and does not improve significantly in the purification scheme known as gettering. This process, highly effective with single-crystal silicon, involves heating to several hundred degrees Celsius in the presence of a metal reservoir that draws the contaminants out of the silicon.

McHugo used multiple experimental techniques to investigate the behavior of metal contaminants in poly-Si and to search for more effective purification methods. He measured surface photovoltages on a poly-Si wafer to locate regions with low minority-carrier diffusion lengths. When he examined a 100x100-micron region of the wafer with the x-ray fluorescence microprobe at Beamline 10.3.1, he found that copper and nickel were the only contaminants detectable in that region and that together they formed micron-size aggregates of nanometer-size clusters. He also used a scanning electron microscope to locate grain boundaries and dislocations (a type of structural defect). Dislocations, but not grain boundaries, matched up with the copper/nickel locations.

Heating the wafer and checking the results with the microprobe was the next step. Heating without a metal reservoir dissolved most of the copper/nickel clusters. Subsequent gettering with an aluminum reservoir reduced copper and nickel to undetectable levels. New photovoltage measurements showed an increase from 10 to 60 microns in diffusion length. This increase is good, but not as good as one might expect based on the reduction in copper and nickel contamination. The discrepancy may be due to copper, nickel, or other contaminants such as iron or chromium remaining outside the microprobe scan area. Diffusion-length measurements on a scale to match that of the microprobe are now being taken to address this possibility. Future experiments will also examine the behavior of iron contaminants.

This research was conducted by Scott McHugo (Berkeley Lab), using the Center for X-Ray Optics x-ray fluorescence microprobe at Beamline 10.3.1. Funding: Office of Basic Energy Sciences of the U.S. Department of Energy.

3. JANUARY SHUTDOWN CANCELED
(contact: rmmiller@lbl.gov)

The shutdown previously planned for January 3-11, 1998, has been canceled. The installations scheduled for this shutdown will be performed during the April-May 1998 shutdown, which will be one week longer to accommodate the extra work. Users will gain beam time on January 7-12 as a result of this change.

ALS management and the Users' Executive Committee are working out a revised operations schedule for January-May 1998. ALSNews will report on the schedule when it is available.

4. OPERATIONS UPDATE
(contact: rmmiller@lbl.gov)

Beam reliability for the last two weeks was 96.9% overall and 96.4% for user shifts. A Lab-wide power failure occurred at 2:55 a.m. Friday November 7th, causing a ten-hour interruption to user operations.

There are now two groups using the "camshaft" spike (the additional bunch inserted in the gap of a multibunch fill) on an alternating schedule. To accommodate the timing needs of those groups and the needs of another group that requires a gap with no beam, we have changed the fill pattern to 277 buckets (554 ns) followed by a gap of 70 ns, a single bunch in bucket 312, and a 32-ns gap. This change should be transparent to other users.

Operations Summary for November 12 - December 1

Nov 12, 00:00- Nov 17, 07:15  1.9-GeV/400-mA/278-bunch user operations
                              (camshaft mode)
Nov 17, 07:30- Nov 18, 24:00  Maintenance & startup
Nov 19, 00:00-24:00           Accelerator physics
Nov 20, 00:00-08:00           User scrubbing & special operations
Nov 20, 08:00- Nov 26, 23:15  1.9-GeV/400-mA/287-bunch user operations
Nov 27, 00:00- Nov 28, 24:00  Thanksgiving holiday
Nov 29, 00:00- Dec 01, 08:00  No operations
Dec 01, 08:00-24:00           Maintenance & startup
Dec 02, 00:00-24:00           Accelerator physics
Dec 03, 00:00-08:00           User scrubbing & special operations
Dec 03, 08:00- Dec 08, 07:15  1.9-GeV/400-mA/287-bunch user operations