A group of inversion layer MOS solar cells has been fabricated. The highest value of open-circuit voltage obtained for the cells is 0.568V. One of the cells has produced a short-circuit current of 79.6 mA and an open-circuit voltage of 0.54V. It is estimated that the actual area AMO efficiency of this cell is 6.6 percent with an assumed value of 0.75 for its fill factor. Efforts made for fabricating an IL/MOS cell with reasonable efficiencies are reported. Future work for 4 sq cm IL cells and 25 sq cm IL cells is discussed

Ho, Fat Duen

English

NASA Technical Reports Server

N00-15621
/
FURTHER STUDY OF INVERSION LAYER MOS SOLAR CELLS
by
Fat Duen Ho
Associate Professor of Electrical and Computer Engineering
The University of Alabama in Huntsville
Huntsville, Alabama
ABSTRACT
A group of inversion layer MOS solar cells has been fabricated. The
highest value of open-circuit voltage obtained for the cells is 0.568V. One
of the cells has produced a short-circuit current of 79.6 mA and an open-
circuit voltage of 0.54V. It is estimated that the actual area AMO efficiency
of this cell is 6.6% with an assumed value of 0.75 for its fill factor.
Efforts made for fabricating an IL/_OS cell with reasonable efficiencies
are reported. Future work for 4 cm_ IL cells and 25 cm_ IL cells is
discussed
XXI-i
https://ntrs.nasa.gov/search.jsp?R=19880006239 2020-03-20T07:37:47+00:00Z
ACKNOWLEDGEMENTS
The author wishes to express his gratitude to Mr. M. D. Martin and
Mr. T. M. Edge for their continuous support and encouragement for this
research. He wishes to acknowledge Mr. P. D. McManus for his work with
the fabrication of the IL/MOS solar cells. He would like to thank
Mr. O. A. Norton, Sr., for helping i_st the devices, Mr. J. B. Farrier
for bonding the IL cells into packages, Mr. C. Bahr for his assistance
in metalization using the vacuum systems and Mr. P. Belcher for doing
some machine shop work for the project. He would like to express his
appreciation to Mrs. M. S. McCartney for her help. He is grateful to
Ms. Georgann Ricks for her assistance and for her typing of this
manuscript. His thanks also go to Dr. Gerald Karr, Ms. Ernestine Cothran,
and other members of the 1987 NASA/ASEE coordinating team for their work
for this program.
XXI- ii
LIST OF TABLES
Table
Number
2
3
Title
Experimental results of the
fabricated in Run 2
Experimental results of the
fabricated in Run No. 3
Experimental results of the
fabricated in Run No. 4
IL/MOS
IL/MOS
IL/MOS
solar cel Is
solar cel Is
solar cells
XXI-8
XXl-9
XXl- 10
XXI-iii
INTRODUCTION
In the previous report (1), we presented the experimental results
for three IL/MOS solar cells fabricated in the NASA/MSFC laboratory in
1986. The best open-circuit voltage recorded then was 0.465V for one
of the cells, while its short-circuit current only had a value of 0.02 mA.
In the same report (1), we pointed out that the reasons of getting low
short-circuit currents were probably due to (1) the thin interfacial oxide
formed by sputtering was too thick and (2) the thermal oxide became too
thin at the edge of aluminum to strongly invert the surface.
Efforts have been made to control more accurately the thickness of
the thin layer of oxide between aluminum and silicon of the MIS contacts.
Using the furnace with 450°C, an oxide layer about 20A can be grown on
a <111> p-type silicon wafer in 20 minutes. This process has improved
the open-circuit voltages and the short-circuit currents of the IL cells
substantially. One of the 4 cm 2 cells in the fourth run of the experiments
has produced 79.6 mA short-circuit current and a value of 0.54V for the
open-circuit voltage. An actual area (3.56 cm 2) AMO efficiency of 6.6%
has been achieved with the assumption that the fill factor equals 0.75.
XXI-1
OBJECTIVES
The objectives of this study are (I) fabrication of 4 cm2 IL/MOS
solar cells with good efficiencies (13% AMO efficiency or better), (2)
development of a simple, inexpensive, low-temperature process for fabricating
high-efficiency IL solar cells and (3) preliminary study of large-area
(25 cm 2) IL/MOS solar cells and arrays.
XXI-2
BACKGROUND AND OVERVIEW
The operational principle and the design consideration of IL/MIS
solar cells have been discussed in some details in other papers and reports
(1)-(7). Here, only a brief review is given.
A. Advantages of the IL Cells
The advantages of IL/MIS solar cells are that the processing
is mainly of low temperature and the diffusion-induced crystal damage
inherent in diffused p-n junction cells can be avoided. In IL cells, very
shallow junctions are formed with high electric field which helps to collect
electron-hole pairs generated by short-wavelength light. There is no
"dead layer" in this kind of cells. Therefore, their responses for the
ultraviolet light are better than those of diffused cells. The IL cells
may be possible to have nearly ideal diode properties, which in turn can
produce a larger value of open-circuit voltage. The cost of producing
the cells can be cut down by using the low temperature processing
tec hnol ogy.
B. Design Consideration of IL cells
To have the highest possible efficiency, the structure
of an IL cell must be optimized. The most important parameters of the
cells for design consideration are (1) the resistivity of the substrate,
XXI-3
(2) the fixed charge in the oxide or other insulators, (3) the number
of grids per unit length (usually per cm), and (4) the thickness of the
interfacial layer of the thin oxide in an MIS diode.
For an IL cell, the substrate must be p-type with the
111> orientation. The wafers should be doped properly. If it is doped
too heavily, it may have difficulty to invert the thin inversion layer
beneath the oxide or other insulator; and if it is too lightly doped,
the sheet resistance of the inversion layer would be too large. The sheet
resistance of the inversion layer can be reduced by increasing the number
of grid lines. But too many grid lines would cover too large an area,
which would reduce the area exposed to the sunlight. A compromise between
these two parameters should be reached by balancing their opposite effects.
The thickness of the thin oxide in the MIS contacts plays a very critical
role in processing the IL cells. The insulator must be thin enough to
guarantee the tunneling effect is sufficient (8). But too thin interfacial
layer of the oxide would lead to low open-circuit voltages and would affect
the general performance of the cell. Therefore, the thickness of the
insulating thin layer should be optimized.
C. Fabrication Process
The fabrication procedures of the IL cells are s_merized
as follows:
0
(1) Thermal oxide with a thickness of approximately 1500A
is grown on p-type silicon substrate with <111> crystal orientation.
XXI-4
(2) Metal contact regions are defined using the first mask.
(3) Aluminum is deposited on the back surface and sintered.
(4) The thin oxide in an MIS contact is grown using 450°C furnace
for 20 minui_s.
(5) Aluminum is evaporated onto the front part of the sample.
(6) Aluminum grid patterns are defined using a second mask.
XXI-5
RESULTSANDDISCUSSION
There were three runs of experiments for the IL cells in the ten
weeks period of the 1987 Summer Faculty Fellowship Program. The fourth
run was ended on September 15, 1987, about one month after the program
was over. The results of the first run were poor due to the accidental
etching of the aluminum on the backs of the wafers. In addition the thin
layers of oxide in MIS contacts may be too thick because there was no
accurate control of the thin oxide at that time. The results of the second
run are listed in Table 1 while those of the third run are in table 2.
Both open-circuit voltages and short-circuit currents are much better
in the third run than those corresponding values obtained in the first
and second runs. In the third run, the aluminum on the back sides was
also accidently etched. The worsening contacts in the back sides of the
IL cells definitely contributed the bad effects on the short-circuit currents,
which are even smaller than their counterparts in the cells fabricated
in 1986.
The fourth run was much more successful. The open-circuit voltage
of one of the cells equals 0.54V while having a short-circuit current
of 79.6 mA. The total area of the cell is about 4 cm 2. The measurements
are done under AMO illumination. An actual area (3.56 cm 2) efficiency
of 6.8% was estimated with an assumed value of 0.75 for the fill factor
of the cell. The result are listed in Table 3.
XXI-6
Note that the oxidation processes with HCl gettering and those without
HCI gettering did not make significant difference for the performance
of the cells. Therefore, from Run #4 on, we have been using HCI gettering
in all oxidation processes.
XX1-7
TABLE1: EXPERIMENTALRESULTSOFTHE IL/MOSSOLARCELLS
FABRICATEDIN RUNNO. 2
Cell
No.
1A
1B
2A
2B
Type and N_nber
Res istivity of Grids ISC
of Substrate per cm Oxide (mA)
P <111> 121 1544A 0.002
1.19 _ -cm (Thermal)
6
P <111> 121 1545A 0.001
1.19 _ -cm (Thermal)
P <111> 121 1218A 0.005
1.19 _-cm (Sputtering)
O
P <111> 121 1227A 0.005
1.19 _-cm (Sputtering)
VOC
(v)
O.27
O.29
0.32
O.002
XX1-8
TABLE2: EXPERIMENTALRESULTSOFTHEIL/MOSSOLARCELLS
FABRICATEDIN RUNNO. 3
Cel 1
NO.
Type and N_ber
Resistivity of Grids ISC
of Substrate per cm Oxide (mA)
0
P <111> 121 1435A 0.752
1.19 _-cm (With HCI
get tering )
@
P <111 > 121 1845A 0.388
1.19 _-cm (With HCf
gettering)
&
P <111> 121 1460A 0.814
1.19 _-cm (With HCf
gettering)
VOC
(v)
0.539
0.283
0.568
XX1-9
TABLE3: EXPERIMENTALRESULTSOFTHEIL/MOSSOLARCELLS
FABRICATEDIN RUNNO. 4
Cell
No.
W1-1
W1-2
W2-2
W2-3
Type and Number
Resistivity of Grids ISC
of Substrate per cm Oxide (mk)
P <111> 69 1530_ 79.6
1.1 _-cm (With HCI
Gettering)
Q
P <111> 69 1535A 75.2
1.1 _-cm (With HCI
Gettering)
P <111> 41 1506X 1.6
17 _-cm (With HCI
Gettering)
e
P <111> 41 1409A 52.6
17 _-cm (With HCI
Gettering)
VOC
(v)
0.54
0.56
0.38
0.482
XXI-IO
CONCLUSION AND RECOI_NDATION
A. Conclusion
A group of IL/MOS solar cells have been fabricated in the NASA/MSFC
laboratory. One of the cells has a short-clrcuit current of 79.6 mA and
an open-circuit voltage of 0.54V. The actual area (3.56 cm 2) AMO efficiency
is estimated to be 6.6% with an assumed value of 0.75 for its fill factor.
We conclude that the thin oxide in MIS contacts of the cells has been
playing a very critical role to improve the short-circuit currents and
even the open-circuit voltages. More efforts, therefore, should be made
0
to control the thickness of the thin oxide (less than 20A) more accurately.
We also conclude that HCI purge is irrelevant to the inversion layer induced
in IL cells. Hence, HCf purge is recommended for all oxidation processes.
B. Rec ommendati ons:
The future work for the IL cells is recommended as follows:
1. Improvement of the performance of the IL ce]Is so that the 4 cm 2
cells with about 13% AMO efficiency or better cou]d be fabricated. In
addition, the cells using sputtering SiO 2 or chemical vapor deposition
(CVD) of SiO 2 should be made to get higher oxide charge densities. Also,
we have planned to use other insulators such as SiO and Si3N 4 to establish
the inversion layer of an IL cell.
XXI-11
2. Study of smaller area (1 cm 2) IL cells for comparison.
3. Study of large-area (25 cm2) IL cells and arrays. It is believed
that the large area cells can provide large area arrays at lower assembly
cost. To study the feasibility of IL/MIS cells for large-scale application,
the ability to scale up to large areas for these cells must be explored.
Up to now, to the best of our knowledge, there is no literature which
has dealt with large-area IL/MIS cells, and the IL cells fabricated are
no larger than 4 cm 2. It is believed that our efforts will be significant
in developing large silicon solar cells in general and in designing IL/MIS
solar cell arrays in particular.
4. Study the large-area IL/MOS solar cell array for space application.
The work on 4 cm2 and 25 cm2 cells will lay the groundwork of the research
for IL solar cell arrays.
XXI-12
REF ERENCE S
Ii
.
.
.
5.
.
7.
.
F. D. Ho. Research Reports for 1986 NASA/ASEE Summer Fellowship
Program, XXii (1986).
G. C. Salter and R. E. Thomas, Proceedings, 11th Photovoltaic Specialists
Conference, Arizona, 364 (1975).
P. van Halen, R. E. Thomas, R. Mertens, and R. van Overstraeten,
"Procedings, 12th IEEE Photovoltaic Specialists Conference, 907 (1976).
G. C. Salter and R. E. Thomas, Solid State Electronics 20, 95 (1977).
C. E. Norman and R. E. Thomas, IEEE Transactions on Electronic Devices,
ED-27, 731 (1980).
D. K. Yeh and T. A. DeMassa, Solid State Electronics 27, 283 (1984).
R. Hezel and R. P. Vollersten, Proceedings, 5th E. C. Photovoltaic
Solar Energy Conference, 1113 (1983).
R. Godfrey and M. Green, IEEE Transactions on Electronic Devices,
ED-27, 737 (1980).
XXI-13


Further study of inversion layer MOS solar cells

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