Tandem solar cell

A tandem solar cell device includes an upper solar cell, a lower solar cell, and an intervening buffer layer. A short wavelength region of the incident light is absorbed by the upper solar cell while the light having passed through the upper solar cell is absorbed by the lower solar cell. The buffer layer is a semiconductor layer having a larger band gap energy than the upper solar cell, a crystalline lattice match with the upper solar cell, and a tunnel junction.

FIELD OF THE INVENTION 
The present invention relates to a tandem solar cell device and, more 
particularly, to a buffer layer inserted between solar cells of a tandem 
solar cell device including different kinds of solar cells. 
BACKGROUND OF THE INVENTION 
A tandem structure solar cell device comprising a GaAs solar cell serially 
disposed on a Si solar cell utilizes solar light more effectively than 
either a Si solar cell or a GaAs solar cell alone. 
FIG. 2 is a cross-sectional view of a prior art tandem solar cell device. 
In FIG. 2, a Si solar cell (lower solar cell) 10 includes an n-type Si 
substrate 11 about 100 to 200 microns thick and a p-type Si layer 12 less 
than 1 micron thick disposed thereon. Light having wavelengths of 0.4 to 
1.1 microns is absorbed by and converted to electricity by the Si solar 
cell 10. A GaAs solar cell (upper solar cell) 20 includes an n-type GaAs 
layer 21 about 2 microns thick and a p-type GaAs layer 22 about 1 micron 
thick disposed thereon. Light having wavelengths of 0.4 to 0.9 microns is 
absorbed by and converted to electricity by the GaAs solar cell 20. A 
buffer layer 30 less than several hundred angstroms thick is inserted 
between the upper solar cell 20 and the lower solar cell 10, improving the 
lattice matching of the Si and GaAs crystals. This buffer layer 30 
includes a tunnel junction 33 and a high impurity concentration p.sup.+ 
-type Ge layer 31 and an n.sup.+ -type Ge layer 32 sandwiching the tunnel 
junction. 
An n side ohmic contact electrode 1 is disposed on the rear surface of the 
n-type Si substrate 11. A p side ohmic contact electrode 2 is disposed on 
part of the front surface of the p-type GaAs layer 22. An anti-reflection 
film 40 comprising a silicon nitride film having a thickness of 600 to 700 
angstroms is disposed on the exposed surface of GaAs layer 22. 
Thus, a tandem solar cell device 100 includes the upper solar cell 20, the 
lower solar cell 10, the buffer layer 30, the anti-reflection film 40, the 
p side electrode 2, and the n side electrode 1. 
In the tandem solar cell device described, light of relatively short 
wavelengths, i.e., of 0.4 to 0.9 microns, from the solar light spectrum of 
0.4 to 2 micron wavelengths which is incident on the device from above is 
converted into electricity by the upper GaAs solar cell 20, and the light 
that passes through the upper solar cell 20 is converted into electricity 
by the lower Si solar cell 10. The charge carriers generated at the 
respective solar cells 10 and 20 are extracted through the electrodes 1 
and 2 as a photocurrent that passes through the thin buffer layer 30 
disposed between the two solar cells. 
In the prior art tandem solar cell device, the lattice matching buffer 
layer 30 cuts off light which could be converted to electricity by the 
lower solar cell 10 and little electricity is produced in the lower solar 
cell 10. 
In more detail, when a plurality of semiconductor layers having different 
lattice constants are serially disposed, crystalline defects are produced 
and a semiconductor layer of good crystallinity cannot be obtained. The 
movement of charge carriers in the semiconductor layer is obstructed by 
the crystalline defects, thereby decreasing device efficiency. Therefore, 
in this prior art device, the Ge buffer layer 30 whose crystal lattice 
constant matches that of GaAs is inserted between the p-type Si layer 12 
and the GaAs layer 21, thereby improving the crystallinity of the GaAs. 
Furthermore, in order to improve the electrical junction between the upper 
GaAs solar cell 20 and the lower Si solar cell 10, the buffer layer 30 
includes a p.sup.+ -type Ge layer 31 and an n.sup.+ -type Ge layer 32 
containing a high concentration of dopant impurities and a tunnel junction 
30a is produced within the buffer layer 30. 
The energy band gap of the Ge buffer layer 30 is narrower than that of Si, 
i.e., the band gap energy of Si is 1.11 eV while that of Ge is 0.66 eV. As 
a result, light that can be converted into electricity by the Si layer is 
absorbed by the Ge buffer layer 30 and hardly reaches the Si solar cell 
10; that is, the light-to-electricity conversion takes place not in the Si 
layer having a high light-to-electricity conversion efficiency but in the 
Ge layer having a low light-to-electricity conversion efficiency. This 
lowers the light-to-electricity conversion efficiency of the entire 
device. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a tandem solar cell 
device having an upper solar cell with improved crystallinity without 
reducing the light transmissivity of a buffer layer disposed between the 
upper and lower solar cells. 
It is another object of the present invention to provide a tandem solar 
cell device having a high efficiency which includes an improved electrical 
connection between the upper and the lower solar cells. 
It is still another object of the present invention to provide a method of 
producing such a tandem solar cell device. 
Other objects and advantages of the present invention will become apparent 
from the detailed description given hereinafter. It should be understood, 
however, that the detailed description and specific embodiments are given 
by way of illustration only, since various changes and modifications 
within the spirit and scope of the invention will become apparent from the 
detailed description. 
According to an aspect of the present invention, a buffer layer is inserted 
between an upper solar cell and a lower solar cell of a tandem solar cell 
device for improved lattice matching of the crystals of the solar cell 
layers. This buffer layer comprises a semiconductor material having a 
larger energy band gap than that of a semiconductor material comprising 
the upper solar cell which enhances the light transmissivity of the buffer 
layer without adversely affecting the crystallinity of the upper solar 
cell. Thus, the solar light will not be absorbed by the buffer layer and 
is effectively converted to electricity by the lower solar cell. 
Since a tunnel junction is disposed within the buffer layer, an improved 
electrical connection is realized between the upper solar cell and the 
lower solar cell, thereby reducing the resistance loss which would 
otherwise occur at the junction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 is a cross-sectional view of a tandem solar cell device according to 
a first embodiment of the present invention. In FIG. 1, a tandem solar 
cell device 200 includes a lower Si solar cell 10 and an upper GaAs solar 
cell 20 disposed thereon. The lower Si solar cell 10 includes an n-type Si 
substrate 11 having a resistivity of 2 .OMEGA..cndot.cm, a thickness of 
200 microns, and a diameter of 3 inches, and a p-type Si diffusion layer 
12 having a thickness of 0.15 to 0.3 micron disposed on the Si substrate. 
A buffer layer 50 of a semiconductor material having a larger band gap 
energy than that of GaAs comprising the upper solar cell 20 is disposed on 
the p-type Si layer 12. Herein, ZnSe having a band gap energy of 2.67 eV, 
larger than the band gap energy of GaAs, i.e., 1.43 eV, is used for the 
buffer layer. This buffer layer 50 includes a p.sup.+ -type ZnSe layer 51 
and an n.sup.+ -type ZnSe layer 51 comprising a tunnel junction. The upper 
solar cell 20 comprising III-V compound semiconductors including GaAs 
includes an n-type GaAs layer 21 about 1.5 to 3 microns thick and a p-type 
GaAs layer 22 about 0.5 micron thick. The other elements are the same as 
those of FIG. 2. 
In this embodiment, when light, such as solar light, is incident on the 
device from above, light of relatively short wavelengths is absorbed by 
the pn junction of the n-type GaAs layer 21 and the p-type GaAs layer 22, 
and the light of relatively long wavelengths which has passed through the 
GaAs layer 20 and the ZnSe buffer layer 50 is converted to electricity by 
the pn junction of the n-type Si substrate 11 and the p-type Si layer 12. 
The charge carriers generated at the respective solar cells 20 and 10 are 
extracted from the n side electrode 1 and the p side electrode 2 as a 
photocurrent through the ZnSe layer 50. 
To construct the tandem solar cell device 200, the surface of the Si 
substrate 11 is treated with an acid mixture (a solution of sulfuric acid 
and aqua regia) which is well known as an etchant for Si. Thereafter, a 
p-type Si diffusion layer 12 having a thickness of 0.15 to 0.3 micron is 
produced by a thermal diffusion using BBr.sub.3 as a diffusion source at a 
diffusion temperature of 1050.degree. C. and a diffusion time of 40 to 60 
minutes. 
Next, a p.sup.+ -type ZnSe layer 51 and an n.sup.+ -type ZnSe layer 52 
having carrier concentrations of about 10.sup.-3 cm.sup.-3 are 
successively grown to thickness of 500 to 1000 angstroms on the Si 
substrate 11 by the metal organic chemical vapor deposition (MOCVD) 
process using dimethylzinc (DMZn) and hydrogen selenide (H.sub.2 Se) to 
form the buffer layer 50. The MOCVD temperature is 300.degree. C. and the 
pressure is 0.4 Torr. The ZnSe layer 50 produces a tunnel junction between 
the lower Si solar cell 10 and the upper GaAs solar cell 12 which produces 
an improved electrical connection between the solar cells. 
Next, an n-type GaAs layer 21 having a carrier concentration of 
1.times.10.sup.17 cm.sup.-3 and containing Se or S as a dopant and a 
p-type GaAs layer 22 having a carrier concentration of 5.times.10.sup.18 
cm.sup.-3 and containing Zn as a dopant are successively grown on the ZnSe 
buffer layer 50 to thicknesses of 1.5 to 3 microns and 0.5 microns, 
respectively, by MOCVD using trimethylgallium (TMGa) and arsine 
(AsH.sub.3) to form the upper solar cell 20. The conditions of this MOCVD 
process are a growth temperature of 750.degree. C. and a pressure of about 
120 Torr. 
Thus, the light-to-electricity regions, i.e., the silicon solar cell 10 and 
the GaAs solar cell 20, are produced. Thereafter, a silicon nitride 
(Si.sub.3 N.sub.4) film 40 as an anti-reflection film is deposited on the 
p-type GaAs layer 22 to a thickness of 700 to 800 angstroms by a plasma 
CVD method or by thermal decomposition of silane gas and ammonia gas at 
700.degree. C. 
Thereafter, a titanium layer, which has good adhesion to semiconductors, is 
deposited on the rear surface of Si substrate 11 as well as on the front 
surface of p-type GaAs layer 22 to a thickness of about 500 angstroms by 
electron beam deposition (MBE) or by sputtering. Subsequently, silver is 
deposited on the Ti layers to a thickness of several microns by electron 
beam deposition for connection with the external lead terminals, forming a 
p side ohmic contact electrode 2 on the p-type GaAs layer 22 and an n side 
ohmic contact electrode 1 on the n-type Si substrate 11. 
In this embodiment, since a ZnSe buffer layer 50 having a larger energy 
band gap than that of the GaAs layer is inserted between the lower Si 
solar cell 10 and the upper GaAs solar cell 20, the long wavelength light 
which can be converted to electricity by the Si layer 10 is not absorbed 
by the buffer layer 50. Thus, since the ZnSe buffer layer 50 has a band 
gap energy of 2.67 eV, larger than that of silicon, and is transparent in 
a long wavelength region, the long wavelength light is incident on the 
lower solar cell 10 and is converted to electricity with high efficiency. 
Since ZnSe matches GaAs in lattice constant, no crystal defects are created 
during crystal growth of the GaAs layer on the ZnSe buffer layer 50, and a 
relatively perfect GaAs crystalline structure can be obtained. Therefore, 
the charge carriers at the upper GaAs solar cell 20 have a higher degree 
of freedom in their movement, which increases efficiency. 
Furthermore, since a tunnel junction 55 is produced inside the ZnSe buffer 
layer 50, the electrical junction between the lower Si solar cell 10 and 
the upper GaAs solar cell 20 is improved, thereby reducing the resistance 
loss which would otherwise occur at the junction. As a result, the 
light-to-electricity conversion efficiency of the entire device is 
improved. 
In the above-illustrated embodiment, a ZnSe layer having a larger energy 
band than that of the BaAs layer of the upper solar cell 20 is used, but 
an AlGaAs layer having a larger band gap energy than that of GaAs may be 
used. 
While in the above-illustrated embodiment the upper solar cell 20, is GaAs, 
it may be an Al.sub.x Ga.sub.1-x As (0&lt;x&gt;1) layer. In this case, a high 
quality crystal can be produced not only on the AlGaAs buffer layer but 
also on the ZnSe buffer layer. 
As shown in FIG. 6, when a window layer 60 of Al.sub.x Ga.sub.1-x As (x=0.8 
to 0.9) is disposed on the p-type GaAs layer 22 of GaAs solar cell 20, 
surface recombination due to defects at the surface of the GaAs layer can 
be reduced. This window layer 60 is grown to a thickness of below 0.1 
micron in order to suppress the absorption of short wavelength light 
therein. This window layer 60 can be produced successively without 
semiconductor layers using a method similar to the production method for 
the GaAs cell, that is, MOCVD, MBE, or liquid phase epitaxy (LPE). 
While in the above-illustrated embodiment the tandem solar cell device 200 
has a single upper solar cell 20, a plurality of light-to-electricity 
conversion regions may be produced as an upper solar cell. Such a 
light-to-electricity conversion device having an upper solar cell 
including a tandem structure is described according to second and third 
embodiments of the present invention. 
FIG. 3 shows a tandem solar cell device according to a second embodiment of 
the present invention. In FIG. 3, an upper solar cell 20 is obtained by 
successively depositing first to third Al.sub.x Ga.sub.1-x As 
light-to-electricity conversion regions 20a to 20c on a ZnSe buffer layer 
50. The farther from the buffer layer 50 the Al.sub.x Ga.sub.1-x As 
light-to-electricity conversion region is, the larger the Al composition 
ratio x is. These regions 20a, 20b, and 20c respectively include p-type 
Al.sub.x Ga.sub.1-x As layers 21a, 21b, 21c and n-type Al.sub.x Ga.sub.1-x 
As layers 22a, 22b, 22c. The other elements are the same as in FIG. 1. 
In this device, in addition to the effects of the above-described first 
embodiment, the light-to-electricity conversion efficiency of the upper 
solar cell 20 is enhanced; that is, the solar light which has passed 
through region 20c or 20b without being absorbed can be absorbed by the 
lower region 20b or 20a, respectively. Therefore, light-to-electricity 
conversion can be effectively achieved for the short wavelength components 
of solar light. Furthermore, the respective light-to-electricity 
conversion regions can be easily produced by varying the composition ratio 
of AlGaAs during its growth. 
FIG. 4 shows a tandem solar cell device according to a third embodiment of 
the present invention. In this device, in addition to the first ZnSe 
buffer layer 50 disposed between the lower solar cell 10 and the upper 
solar cell 20, second and third ZnSe buffer layers 50b and 50c are 
disposed between the respective light-to-electricity conversion layers of 
upper solar cell 20. The respective buffer layers 50a to 50c include 
respective tunnel junctions 55a to 55c including first to third n.sup.+ 
-type ZnSe layers 52a to 52c and first to third p.sup.+ -type ZnSe layers 
51a to 51c, respectively. Herein, the buffer layer may comprise AlGaAs 
instead of ZnSe. 
In this embodiment, the electrical junction between the respective 
light-to-electricity conversion regions can be improved whereby the 
resistance loss at the junction can be reduced and the 
light-to-electricity conversion efficiency can be enhanced relative to the 
second embodiment. 
While in the above-described second and third embodiments, the same 
semiconductor material is used for the multilayer light-to-electricity 
conversion regions of the upper solar cell, GaAs may be used as the main 
light-to-electricity conversion region and AlGaAs may be used as the 
auxiliary light-to-electricity conversion regions. 
FIG. 5 shows a tandem solar cell according to a fourth embodiment of the 
present invention. In FIG. 5, an upper auxiliary AlGaAs solar cell 70 is 
disposed on the GaAs solar cell 20 with a second buffer layer 50b. The 
AlGaAs solar cell 70 includes an n-type AlGaAs layer 71 and a p-type 
AlGaAs layer 72 disposed thereon. The second buffer layer 50b includes a 
p.sup.+ -type AlGaAs layer 51b and an n.sup.+ -type AlGaAs layer 52b. 
Herein, a p.sup.+ -type GaAs layer may be used in place of the p.sup.+ 
-type AlGaAs layer 51b. 
In the upper auxiliary AlGaAs solar cell 70, the range of wavelengths that 
can be absorbed is increased toward the shorter wavelengths, i.e., to a 
range of 0.3 to 0.9 micron, compared to a range of 0.4 to 0.9 micron for 
the upper solar cell 20. Therefore, the natural short wavelength light can 
be effectively converted to electricity, thereby improving the conversion 
efficiency. Furthermore, the resistance loss at the electrical junction 
between the main and auxiliary solar cells can be suppressed. 
As is evident from the foregoing description, according to the present 
invention a buffer layer for lattice matching is inserted between an upper 
cell and a lower cell of a tandem solar cell device. This buffer layer 
includes a semiconductor material having a larger band gap energy than 
that of the semiconductor material in the upper solar cell and includes a 
tunnel junction. Therefore, the crystallinity of the upper solar cell and 
the solar light permeability of the buffer layer are improved. 
Furthermore, since the buffer layer includes a tunnel junction, a good 
electrical junction between the respective light-to-electricity conversion 
regions is realized, thereby reducing the resistance loss which would 
otherwise occur at the respective junctions. As a result, the total 
light-to-electricity conversion efficiency of the device is improved.