Two methods have been found to lower cost of polycrystalline silicon solar cells. Successive layers of polycrystalline silicon are deposited over supporting substrates of relatively inexpensive metallurgical-grade polycrystalline silicon, graphite, or steel

Chu, T. L.

English

NASA Technical Reports Server

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December 1975
	 B75-10293 
• 
NASA TECH BRIEF 
Goddard Space Flight Center 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage 
commercial application. Tech Briefs are available on a subscription basis from the National Technical Information 
Service, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Office, NASA, Code KT, Washington, D.C. 20546. 
Low-Cost Thin-Layer Silicon Solar Cells 
.
The problem: 
The silicon solar cell is currently the most-well-
known photovoltaic device utilized to convert solar 
energy to electric energy. The technology has 
advanced to the point where silicon solar panels used 
in spacecraft are capable of supplying several 
kilowatts of power to the craft instruments. Silicon 
solar cells, though, are still very costly to manufac-
ture. Although production costs of single crystalline 
cells have dropped from $100 to $20 per peak watt, 
further cost reductions of one order of magnitude are 
required before they can be considered alternate 
energy sources. 
The solution: 
Two methods have been found to lower the cost of 
polycrystalline silicon solar cells. The first simplifies 
the manufacturing process directly; the second 
provides a means of bonding the silicon to differing 
substrates of less expensive material. 
How it's done: 
Successive layers of polycrystalline silicon contain-
ing appropriate dopants are deposited over support-
ing substrates of metallurgical-grade polycrystalline 
silicon, graphite, or steel. These materials are coated 
with a diffusion barrier of silica, borosilicate, or 
phosphosilicate to create a pn junction. When 
graphite is used as the substrate, a diffusion barrier of 
silicon carbide is formed between the graphite and the 
first overlying silicon layer. When steel is used, a 
diffusion barrier of silica, borosilicate, or phospho-
silicate is deposited prior to the first polycrystalline 
layer in order to avoid the diffusion of contaminating 
iron atoms. 
The pn junction must be very shallow. The n-layer 
(or upper layer) is 0.2 to 0.4 um thick; the p-layer (or 
inner layer) is 10 to 100 pm thick. The junction is
prepared by: (1) thermally decomposing silane, (2) by 
the reduction of trichlorosilane or silicon tetrachloride 
with hydrogen at temperatures ranging from 900° to 
1,200° C, or (3) by the thermal decomposition of 
dichlorosilane. Several such layers are deposited and 
appropriately doped to form the junction. 
The figure shows the resistivity profile of the 
structured device (except for the n-layer which is 
obtained by the spreading resistance technique). The 
plot shows a high resistivity area in the silicon 
adjacent to the graphite-silicon interface and is 
apparently caused by the formation of silicon carbide. 
Thickness Above Graphite Substrate (Micrometers)

Resistivity Profile of Polycrystalline Structure 
(continued overleat) 
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https://ntrs.nasa.gov/search.jsp?R=19750000293 2020-03-19T20:17:01+00:00Z
Fabricating the cell by decomposing polycrystalline 
silicon onto a steel substrate is complicated by several 
problems. One is the difference between thermal 
expansion coefficients of the materials. Silicon has an 
expansion coefficient of 4x10/°C, while most steels 
have coefficients of 14 to 16x10/°C. A diffusion 
barrier is used to minimize the difference in thermal 
expansion characteristics of the layers and is 
chemically inert to steel and silicon at high 
temperatures. Layers of films of silica, borosilicate, 
phosphosilicate, and aluminosilicate form suitable 
diffusion barriers between steel and silicon. Silica, 
borosilicate, and phosphosilicate are all inert toward 
silicon and steel at high temperatures and have the 
added advantage of being relatively soft. The thermal 
expansion coefficient of borosilicate also can be 
changed as a function of its composition. 
The methods by which the silica and borosilicate 
diffusion barriers are deposited on the steel substrates 
are not critical. The thickness of the deposited 
diffusion barrier is 1 to 5 Mm. Normally, the 
borosilicate diffusion layers contain from 10 to 18 
percent boron, while phosphosilicate layers contain 
from 10 to 20 percent phosphorous. By using the 
above diffusion barrier materials, it is possible to 
prepare integral and tightly formed structures, such 
as silicon/silica/steel, silicon/borosilicate/steel, and 
silicon/silica-borosilicate/steel.
Suitable steels used for the substrates of the present 
cells include Armco silicon steel and USS Vitrenamel 
I. Prior to deposition of the first junction layer, the 
surface of the steel can be scavenged of oxygen by 
treating it with a reducing agent such as hydrogen at a 
temperature from 600° to 1,000° C. 
Note: 
Requests for further information may be directed 
to:
Technology Utilization Officer 
Goddard Space Flight Center 
Code 704.1 
Greenbelt, Maryland 20771 
Reference: TSP75-10293 
Patent status: 
This invention is owned by NASA, and a patent 
application has been filed. Inquiries concerning 
nonexclusive or exclusive license for its commercial 
development should be addressed to: 
Patent Counsel 
Goddard Space Flight Center 
Code 204 
Greenbelt, Maryland 20771 
Source: Ting L. Chu of
Southern Methodist University 
(GSC- 12023)
S 
S 
Categories: 04 (Materials) 
08 (Fabrication Technology) 
01 (Electronics - Components 
and Circuitry) . B75-10293


Low-Cost thin-layer silicon solar cells

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