Introduction
The National Photovoltaics Program managed by the United States Department of Energy
has the goal of establishing solar photovoltaic energy as a significant energy source for
the United States. Some of the means for reaching this goal include advanced and applied
research in photovoltaic cells to advance the state of technology, and support of
education programs for training of future professionals in solar photovoltaic energy.
Sandia National Laboratories has responsibility for development of crystalline silicon
cell technology within the National Photovoltaics Program. As part of this program, Sandia
National Laboratories is supporting the University Center of Excellence for Photovoltaics
Research and Education in Crystalline Silicon Solar Cells at the Georgia Institute of
Technology. The purpose of the Center is to advance the state of technology in
crystalline-silicon solar cells through research, to investigate industrially relevant
processes that may result in lower-cost processing techniques, to collaborate with U.S.
manufacturers of crystalline silicon cells to improve their performance and reduce their
cost, and to provide educational assistance to university students through the operation
of a silicon solar cell fabrication laboratory. Hence, the Center will help train
undergraduate and graduate students in the field of crystalline-silicon solar cells, both
at Georgia Tech and at other Universities through joint research efforts.
GOALS 1995-2000
High-Efficiency Cells on Commercial Silicon Substrates
The goal is to determine which processing techniques work best for each of the many
types of solar-grade silicon which is commercially in use. In this way, different
variations of processing techniques or entirely different processes can be developed to
optimize fabrication of high-efficiency cells on PV-industry Cz silicon and
multicrystalline silicon, and to optimize techniques for light trapping in
multicrystalline silicon.
High Throughput Cell Technologies
The goal is to develop processing techniques that combine the functions of several
distinct conventional furnace processes into fewer rapid thermal processes which can be
performed in a much shorter interval of time. The cells so produced should perform as well
as or better than cells conventionally produced. The rapid processing techniques should be
limited to those which can be scaled up to mass production.
- Reduce cell processing time from 5 days to 8-10 hours by RTP/PECVD/ photolithography
(PL) - Fabricate high-efficiency RTP/PECVD/PL cells on float zone Cz silicon.
- Fabricate high-efficiency RTP/PECVD/PL cells on multicrystalline silicon.
- Develop and integrate screen-printed contacts (SPC) for RTP/PECVD cells and fabricate
RTP/PECVD/SPC cells single-crystal float zone silicon. - Conduct fundamental studies on PECVD coatings to enhance bulk and surface
passivation. - Develop screen-printed contact (SPC) through PECVD coating and reduce cell processing
time to less than 2 hours/batch.
Thin-layer Crystalline Silicon Growth on Foreign Substrates
The purpose is to take advantage of some recent innovative solar cell design concepts
that would allow the fabrication of moderate-efficiency large-area cells on relatively
low-quality thin crystalline-silicon layers. This could reduce the cost of silicon solar
cells considerably since presently the largest component of cell cost is due to the growth
of ingots and their cutting into silicon wafers.
Because of the high degree of difficulty involved in accomplishing this, we are joined
in partnership with members of the U.S. PV industry or other Universities who are pursuing
similar approaches.
1. Develop titanium boride (TiB2) coated graphite substrates for thin silicon cells and
grow thin CVD Silicon layers on these substrates.
2. Fabricate thin silicon cells on foreign substrates by conventional and RTP/PECVD/SPC
techniques.
Novel furnace-based technologies for high efficiency solar cells
The goal is to implement novel approaches to solar cell manufacturing that will not
require the purchase of new capital equipment, and will significantly enhance the
performance of existing cell designs. Such techniques should be capable of batch
processing, resulting in high throughput for all stages of cell fabrication. In addition,
the number of thermal cycles required for junction formation, oxidation and gettering
should be sharply reduced to further increase throughput.
- Use existing diffusion furnaces and chemicals to simultaneously diffuse P and B or P and
Al, and grow passivating oxide (in-situ). - Develop process models and characterization techniques to quantify junction formation
and quality of in-situ oxide passivation. - Fabricate high efficiency n+pp+ solar cells on float zone, Cz and
multicrystalline silicon, in one thermal cycle. - Incorporate complementary high-throughput processing techniques, such as screen printing
and cassette loading to novel simultaneous diffusion technique.
Industry/University Collaborative Research and Development
The purpose is to collaborate with U.S. manufacturers of crystalline-silicon cells to
help them understand how to improve their device designs or material quality to improve
cell performance and reduce cost. This relationship will characterize and improve their
material quality, develop and transfer appropriate gettering and passivation techniques
which would improve cell performance, design and fabricate high-efficiency cells on
industry materials, provide guidelines to industry for reducing cost and improving cell
performance, and publish joint technical papers.
Educational Support Program (ESP)
This is a university-level educational program in solar cells,operated through a
silicon solar cell fabrication laboratory. The laboratory fabricates silicon solar cells
according to runsheets and materials submitted by students in solar cell cources. Any
university that meets the ESP prerequisites is eligible to participate in this program
(University Participant). The prerequisites are that the university offer a course on
solar cells or on semiconductor device fabrication at a senior/graduate level, and that
this course have a prerequisite of a course on semiconductor device physics. The right is
reserved to limit the number of University Participants based on the capacity and the
capabilities of the laboratory. The management of the ESP ensures that information on the
program is disseminated to prospective universities. The ESP offers at least one classroom
course on solar cells. It provides hands-on training to undergraduate and graduate
students in fabricating high efficiency cells and supports other uiversity PV programs in
the U.S. through collaborations and technical assistance. There is a process line that
routinely produces high efficient celss on single crystal float zone silicon in order to
provide prompt inter-university research support.