A multijunction silicon solar cell of high efficiency is provided by providing a tunnel junction between the solar cell junctions to connect them in series. The tunnel junction is comprised of p+ and n+ layers of high band gap 3-5 or 2-6 semiconductor materials that match the lattice structure of silicon, such as GaP (band gap 2.24 eV) or ZnS (band gap 3.6 eV). Each of which has a perfect lattice match with silicon to avoid defects normally associated with lattice mismatch

Daud, Taher

Kachare, Akaram H.

English

NASA Technical Reports Server

United States Patent [I91 p i ]  Patent Number: 4,631,352 
Daud et al. [45] Date of Patent: Dec. 23, 1986 
[54] HIGH BAND GAP 11-VI AND 111-V 
TUNNELING JUNCTIONS FOR SILICON 
MULTIJUNCI‘ION SOLAR CELLS 
[75] Inventors: Taher Daud, La Crescenta, Akaram 
H. Kachare, La Canada, both of 
Calif. 
1731 Assignee: The United States of America as 
represented by the Administrator of 
the National Aeronautics and Space 
Administration, Washington, D.C. 
[21] Appl. No.: 809,975 
[22] Filed: Dec. 17,1985 
[51] Int. CI.4 ...................... HOlL 31/06 HOlL 27/14 
[52] US. Cl. .................................................... 136/249 
[58] Field of Search ................................... 136/249 TJ 
[561 References Cited 
U.S. PATENT DOCUMENTS 
3,046,459 7/1962 Anderson et al. .................... 357/12 
4,128,843 12/1978 Chiang .................................. 357/30 
4,179,702 12/1979 Lamorte ................................ 357/30 
4,278,474 7/1981 Blakeslee et al. ................... 136/249 
4,295,002 10/1981 Chappell et al. .................... 136/244 
4,332,974 6/1982 Fraas ................................... 136/249 
4,387,265 6/1983 Dalal ................................... 136/249 
I ”  
4,404,421 12/1983 Fraas ................................... 136/249 
4,461,922 7/1984 Gay et al. ........................... 146/249 
4,451,691 5/1984 Fraas ................................... 136/249 
4,479,027 10/1984 Todorof ........................ 136/249 TJ 
OTHER PUBLICATIONS 
P. D. Sparks et al., Conference Record, 17th IEEE 
PhotovoIatic Specialists Conference (1984), pp. 726-728. 
M. S. Bae, R. V. DAiello, “P+/N High-Efficiency 
Silicon Solar Cells”, Applied Physics Letters, vol. 3 1, 
Primary Examiner-Aaron Weisstuch 
Attorney, Agent, or Firm-Paul F. McCaul; John R. 
Manning; Thomas H. Jones 
e571 ABSTRACT 
A multijunction silicon solar cell of high efficiency is 
provided by providing a tunnel junction between the 
solar cell junctions to connect them in series, where the 
tunnel junction is comprised of p+ and n+ layers of high 
band-gap 111-V or 11-VI semiconductor materials that 
match the lattice structure of silicon, such as Gap 
(band-gap 2.24 ev) or ZnS (band-gap 3.6 eV), each of 
which has a perfect lattice match with silicon to avoid 
defects normally associated with lattice mismatch. 
NO. 4, 15 Aug. 1977, pp. 285-287. 
2 Claims, 2 Drawing Figures 
I f i  15 14 13 12 IO 
https://ntrs.nasa.gov/search.jsp?R=19870007966 2020-03-23T15:12:45+00:00Z
U.S. Patent 
Si 
16 15 
/ 
Si 
Dec. 23,1986 
p t  n-- 
4,63 1,352 
P P+ n 
v - w -  
Cell I Tunnel Junct. Cell 2 
G P . I I ~ - v  or 11-VI FIG. I (Gap or ZnS) 
FIG. 2 
a 
4,631,352 
1 L 
ZnS (bandgap 3.6 eV), each of which has a perfect 
lattice match with silicon to avoid defects normally 
associated with lattice mismatch. 
The novel features that are considered characteristic 
5 of this invention are set forth with particularity in the 
appended claims. The invention will best be understood 
from the following description when read in connection 
with the accompanying drawings. 
HIGH BAND GAP 11-\G AND 111-V TUNNELING 
JUNCTIONS FOR SILICON MULTIJUNCTION 
SOLAR CELLS 
ORIGIN OF INVENTION 
The invention described herein was made in the per- 
formance of work under a NASA contract, and is sub- 
ject to the provisions of Public Law 96-517 (35 USC 
202) in which the Contractor has elected not to retain lo 
title. 
DESCRIPT1oN OF THE 
FIG. 1 illustrates schematically the geometry of a 
multijunction solar cell having a tunnel junction con- 
necting the solar cell junctions in series. 
FIG. 2 illustrates in a graph the effect of imperfect 
reflections at the tunnel junction of the multijunction 
o f ~ I ~ .  1 when the tunnel junction is made of 
BACKGROUND OF THE INVENTION 
This invention relates to silicon Solar cells, and more 
particularly monolithic Silicon mUltiJUnCtiOn S o h  Cells 
with 111-V or 11-VI tunnel junctions to connect solar 
cells of a stack. silicon. 
Silicon is the least expensive semiconductor material 
available for solar cells. Silicon solar cell conversion 
efficiency has been continuously improved from about 20 
5% in 1956 to more than 16% at the present time. Im- 
processing has been largely responsible for this. How- 
ever, a compromise between shallower junctions and 
solar 
DESCRIPTION OF THE PREFERRED 
EMBODIMENTS 
The geometry of the multijunction solar cell applica- 
vention is illustrated in FIG. 1 as a p substrate, 10, that 
provements both in and in ble both to the prior art solar cells and the present in- 
lower series resistance of the front diffused region soon 25 is 200 pm thick, 
reached its limits. (Losses in the diffused region as well 
the metal grid are proportional to the square of the 
in resistivity, polished* and 
l1 through 
Beam Epitaxy wBE)  
Other layers Of doped 
16* are grown using 
current). similarly, a limit has been reached in a requiring a substrate temperature of only 600"-650" C. 
promise between That temperature is low enough to avoid any redistribu- 
more collection at the front junction. Such a compro- 30 tion of dopants in the layers €Town SUCC~SiVelY. A 
mise needs longer minority carrier diffusion lengths. vaccum of 10-10 torr is used during all MBE process- 
The quest for higher efficiency has led to stacking ing. 
solar cells. Because of improvements in techniques of A back contact layer 11 can be formed by thermal 
epitaxial silicon growth, with finer control of doping diffusion of boron with a surface concentration of 
possible, a multilayered silicon structure of two or more 35 - 1020/cc and a junction depth (high-low) of about 0.5 
p/n junctions can be produced with heavily doped pm, or it can be formed by MBE growth with Ga dop- 
n+/p+ tunnel junction between them acting as low ing to 5 x  1018/cc and a thickness of 0.5 to 1 pm. A 50 
resistance connections. The stack is characterized by to 100 nm layer 12 of silicon, with an antimony (Sb) 
high open circuit voltage, which is the sum of the open concentration of - 1016atoms/cc, is grown as an n layer 
circuit voltages of the individual cells thus stacked. 40 which forms a junction J2 for cell 2. For an all-silicon 
One would think that heavfiY doped silicon layers solar cell of the prior art configuration, the concentra- 
could be used as tunneling junctions in multijunction tion is then increased to 1019/cc to form an n+ layer 13. 
silicon solar cells. However, analysis has shown that the mis layer is made very shallow (-300 to 400 A). 
interface recombination velocity at the silicon tunneling other very shallow layer 14 is then deposited with as 
junction is greater than 104 cm/sec, which degrades 45 high a G~ doping as possible (2 to sX  1oi8/cc) to fom 
performance below that of a conventional monojunc- a p+ junction 
tion silicon cell. To minimize this recombination loss, Jr joining the Pear cell The doping 
conductor) with gll-v Of 'Om- thick p base layer 15 of cell 1. The junction J2 of cell 1 pounds which are uniquely suited for the tunneling 50 is then fomd 
lO19/cc for a 500 A layer 16. This structure is then junction in the multijunction silicon solar cells. Because of the technological advancements of silicon removed from the MBE system and standard tech- compared with 111-V semiconductor compounds, a niques are employed to metalize (Al) the back surface number of opto-electronic devices are made from sili- 
con wafers. The present invention provides a viable 55 and to provide I' and '* vi- 
high voltage source which has excellent compatibility Pd-Ag). 
with other devices fabricated on silicon chipse m i s  The tunnel junction acts as a low resistance connec- 
compatibility may not be with solar cells tion between cells 1 and 2 but when the layers 13 and 14 
based on 111-V compounds. are made of silicon, a large (105 cm/sec) interface re- 
60 combination velocity, S ,  at the tunnel junction de- 
SUMMARY OF THE INVENTION grades efficiency below that of a conventional single 
In accordance with the present invention, a multi- silicon junction cell indicated by data in the graph of 
junction silicon solar cell of high efficiency is provided FIG. 2. The graph is of calculated AMO (Air Mass 
by tunnel junctions between the solar cell junctions to Zero) efficiency as a function of Senfor thc series con- 
connect them in series. The tunnel junctions are com- 65 nected cells, where Seffis defined as the effective sur- 
prised of p+ and n+ layers of a high bandgap 111-V or face recombination velocity at the p/p+ interface. For 
11-VI semiconductor material that matches the lattice reference, the eficiency of a single cell is shown at 
structure of silicon, such as GaP (bandgap 2.24 eV) or 17.25%. 
absorption with thicker cells and 
ne n+ and p+ layers form a 
to a front cell the present invention combines silicon (group IV semi- concentration is reduced to 1016/ce to form the 4.5 pm 
Sb doping concentration 
and back 
4.63 1,352 
3 
Multijunction solar cells of the same general geome- 
try have been proposed for GaAs solar cells, using 
GaAs tunnel junctions for interconnections between 
cells, or other group 111-V compound material, such as 
GaInP for a high band-gap cell of GaInP in front of a 
low band-gap cell of GaInAs, as shown in FIG. 3 of 
US. Pat. No. 4,179,702. Such a high band-gap tunnel 
junction improves the efficiency of the connection, and 
lattice mismatch between the tunnel junction and the 
two cells is minimized, but the materials used for the 
cells are more costly then silicon, and are not compati- 
ble with other devices fabricated in silicon. 
The solution to the problem of maintaining high effi- 
ciency in stacked solar cell junctions and compatibility 
with silicon devices fabricated in a chip is to use a high 
band-gap 111-V or 11-IV semiconductor compound for 
the tunnel junction connecting the silicon solar cell 
junctions, as indicated in FIG. 1. GaP and ZnS have a 
lattice match with silicon which avoids defects nor- 
mally associated with lattice mismatch. A multijunction 
solar cell made using MBE growth, or other suitable 
epitaxy growth technique, may thus be readily fabri- 
cated with a high band-gap tunnel junction between 
cells. A high band-gap material will form B high barrier 
4 
-4 E Eg3" 
3qha =zero 
Tt = exp 
5 
where 
m* (effective mass)-0.2~9.1 x 10-3' kg 
Eg (energy gap band)= 1.2X 1.6X 10-19Joules 
q (electric charge)= 1.6X 10- 19 coulomb 
h (Planck's constant)= 1.05x 10-34j-sec 
€(electric field) = 6 X 106 V/meter 
10 
This will reduce the recombination at that interface to 
less than lo3 cm/sec. In addition to good lattice match 
with silicon, such a high band-gap has a very low opti- 
Although particular embodiments of the invention 
have been described and illustrated herein, it is recog- 
nized that modifications and variations may readily 
occur to those skilled in the art. Consequently, it is 
20 intended that the claims be interpreted to cover such 
modifications and variations. 
15 cal loss due to its wider band-gap. 
What is claimed is: 
1. A multijunction silicon solar cell comprising a 
plurality of p/n junction solar cells epitakially grown in 
25 layers to receive solar energy in series through the 
n-type layer of each solar cell and having the p-type 
layer of one connected to the n-type layer of the next in 
semiconductor material of higher band-gap than silicon, 
as defined in 
claim 1 wherein said high band-gap material is G ~ P  of 
the 111-V group or ZnS of the 11-VI group. 
to minofity carrier diffusion to the p+/n+ interface of 
the 
series by a P+/n+ tunnel junction of 1II-V or II-'cpp 
junction* A WKB approximation (Wentzel- 30 and having a near perfect lattice match with silicon. 
Kramers-Brillouin method) shows near zero probability 
of tunneling for the minority carriers at the Si/GaP or 
Si/ZnS barrier (p/p+). The WKB tunneling probability 
Tt is calculated from the following equation 
2. A multijunction silicon solar 
* * * * *  
35 
45 
50 
55 
45 


High band gap 2-6 and 3-5 tunneling junctions for silicon multijunction solar cells

Abstract

Similar works

Full text

Available Versions

NASA Technical Reports Server