Multilayer photovoltaic solar cell with semiconductor layer at shorting junction interface

A new high efficiency, multijunction photovoltaic solar cell for use with a concentration lens. This cell comprises an elemental single crystal substrate without an internal light sensitive junction, upon which are two or more successive homogenous layers of semiconductor materials, each layer containing within it a light sensitive p/n junction of a similar polarity, each layer having essentially the same lattice constant as the single crystal substrate, each layer having a shorting junction contact with the layer immediately above and below it, each successive layer adsorbing light energy at a shorter wavelength, and each layer being of sufficient thickness and appropriate composition to develop essentially the same current as the other layers. At the junction, between the successive layers of the multilayer cell, a thin pseudo transparent low bandgap semiconductor layer is provided at the shorting junction interface. The outer surfaces of the top layer and the substrate are provided with electrical contacts for distribution of the electric current. The top contact comprises a layer of a transparent conductive material with electrical connections and the whole structure is completed with an antireflection coating over the top.

FIELD OF THE INVENTION 
This invention relates to the conversion of solar energy to electrical 
energy and particularly to a multilayer photovoltaic cell of high 
efficiency for such conversion. 
BACKGROUND OF THE INVENTION 
Several forms of photovoltaic cells have been developed for converting 
solar energy to electrical energy; however, the efficiency of known 
systems is low and, when efficiencies are improved through the use of more 
efficient converters, the cost of the converters is high. It has been 
proposed to decrease the conversion cost and to increase the conversion 
efficiency by concentrating the solar energy, through the use of optical 
systems, onto the converters. With such systems the efficiency of 
conversion can be further increased. Cost consideration on each of the 
several components of a conversion system indicates that, with light 
concentrators, more expensive photovoltaic converters may be employed; 
however, there are economic cost limits on the concentrator system. 
Further, as the concentration of light onto the converter becomes more 
intense, there is a need for dissipating the heat derived from the 
concentrated light because the efficiency of some converters drops as the 
heat of the converter increases. 
From a study of the above recited considerations, it can be shown that, 
with the use of concentrator systems that can decrease the cost of energy 
conversion by increasing conversion efficiency, the emphasis can shift 
from converter cell cost to cell efficiency. Thus, if the cell efficiency 
can be made great enough, a concentrator system can produce electricity 
more cheaply than the same area of a lower cost array. 
These observations lead to the consideration of high efficiency stacked 
multijunction solar cells with each cell responsive to a different energy 
band of solar energy and with a concentrator for concentrating the energy 
and tracking the cell toward the source of energy. However, a key 
requirement for the successful operation of a stacked multijunction solar 
cell is the requirement that the stacked junction be series-interconnected 
through low resistance interfaces thereby allowing the flow of light 
generated current from one junction to the next. 
PRIOR ART 
Multilayer photovoltaic cells have been suggested as a means for converting 
solar energy to electrical energy. One form for such a cell is described 
in my copending application Ser. No. 52,707 filed June 28, 1979 for 
"Multijunction Photovoltaic Solar Cell". The present application is 
directed to a multilayer photovoltaic cell composed of stacks of 
homojunction cells with a semiconductor layer at the interface of the 
shorting junction between layers of the cell. 
The prior art multilayer photovoltaic cells have also suggested producing 
the successive layers of semiconductor materials responsive to different 
energy bandgaps. See L. W. James, U.S. Pat. No. 4,017,332, issued Apr. 12, 
1977. In the cell disclosed in the present application several layers are 
selected to be responsive to different bandgaps, all at substantially the 
same current level, with a specific avoidance of the bandgap for light 
energy effected by fluctuations in humidity and air mass, and between the 
layers there is provided a thin low bandgap lattice-matched intermediate 
tunnel junction layer. 
The photovoltaic cell disclosed in the present application is distinguished 
from the prior art disclosure by the specific absence of a junction in the 
substrate, by the further production of layers lattice matched to the 
substrate and containing homojunctions and further by the addition of a 
semiconductor layer at the interface of the shorting junction between 
layers of the cell to produce the desired electrical conversion with the 
desired efficiency. 
BRIEF STATEMENT OF INVENTION 
The idea of obtaining very high energy conversion efficiencies by optically 
stacking solar cells with different bandgaps is known. There is, however, 
increasing motivation to consider fabricating such a stack of solar cells 
monolithically on a single wafer. This follows for space applications 
because a single wafer is lighter than a multiple wafer stack, and for 
terrestrial applications using concentrator systems, because a single 
wafer is likely to be cheaper, simpler, and more easily cooled than is a 
multiple wafer stack. There are, however, major constraints on the design 
and fabrication of such a high efficiency monlithic stacked multijunction 
solar cell. Two design constraints are, first, that the different 
semiconductor materials making up the stack be nearly lattice matched so 
that crystal integrity can be maintained, and second, if the light 
sensitive junctions are to be series connected, that the material bandgaps 
be such that the light generated current be distributed approximately 
equally between the multiple junctions. A corollary problem is that of 
providing the desired series connection of the active junctions without 
suffering unacceptable voltage losses at inactive junctions, in the 
stacked structures to be considered. 
In accordance with the foregoing, I propose a multijunction photovoltaic 
cell comprising layers of indium gallium phosphide and indium gallium 
arsenide on a germanium substrate. The successive layers include junctions 
in different absorption bands while the substrate and successive layers 
are lattice matched to less than .+-.1% variation. At the interface of the 
shorting junction between the layers I propose a thin transparent low 
bandgap semiconductor layer. With a concentrator, antireflective outer 
coatings, and top and bottom contacts the cell provides an efficient means 
for converting solar energy to electrical energy. 
The photovoltaic cell disclosed herein is an improvement on the cell 
disclosed in my copending application Ser. No. 52,707, filed June 28, 1979 
and the specification of that application is incorporated herein by 
reference.

The interconnecting shorting junction shown in FIG. 2 and characterized 
electrically in FIG. 3 can be described as a tunneling heterojunction. 
Specifically, the p+ GaInP to n+ Ge interface is a tunneling 
heterojunction. 
In earlier described inventions, the tunneling junctions were either of a 
homojunction type with high bandgap materials (James, U.S. Pat. No. 
4,017,332) or of a heterojunction type with a medium bandgap material 
(copending application Ser. No. 52,707). 
The advantage of the tunneling heterojunction follows because, in 
tunneling, the current is controlled both by the energy barrier height and 
the barrier width. An increase in either the height or width leads to a 
reduction in tunneling current density. Thus, since the barrier is quite 
large in the prior art configuration, the width must be quite narrow. A 
small barrier width or depletion width requires an extremely high doping 
concentration, so high in fact that dopant precipitation can occur, 
thereby degrading crystallinity in the layer. A small barrier width also 
requires low interdiffusion and, therefore, low temperature processing. 
For the tunneling heterojunction structure, the barrier height is reduced, 
thereby allowing a larger barrier width. 
In the invention disclosed herein I have proposed a further reduction in 
the barrier height by placing a thin semiconductor layer at the interface 
between layers of the photovoltaic cell. This reduction in barrier height 
permits a reduction in the shorting junction series resistance and thereby 
allows a multijunction cell to be operated with higher light intensities 
and therefore higher concentration ratios and potentially lower system 
cost. Furthermore, this reduction in barrier height permits a wider 
processing temperature range and a broader choice of dopants for the 
junction. 
SPECIFIC TUNNEL JUNCTION CONFIGURATIONS 
In my copending patent application, Ser. No. 52,707, I have noted that two- 
and three-color solar cells can be fabricated using InGaAsP alloy layers 
lattice-matched to a Ge substrate. A two-color solar cell of this type is 
shown in FIG. 1. For the FIG. 1 structure, the tunnel junction lies at the 
Ga.sub.1-x In.sub.x As/In.sub.1-y Ga.sub.y P interface. The barrier height 
of the interface is characterized by the bandgap of Ga.sub.1-x In.sub.x As 
of 1.2 ev. In the interface I now propose a semiconductor layer as shown 
in the enlargement of FIG. 2. 
In proposing a tunneling heterojunction, I have noted that although the low 
band gap material will absorb light, that layer can be made thin enough so 
that little photogenerated current is lost. Specifically, for an 
absorption length of 5000 A, a 500 A thick layer will only absorb 10% of 
the light. If half the carriers generated drift in the wrong direction, 5% 
of the light generated current will be lost. This same pseudotransparency 
argument will apply to a lattice-matched semiconductor with an even 
smaller bandgap. For a multicolor cell on a germanium substrate, germanium 
is the obvious choice for a thin low bandgap lattice-matched intermediate 
tunnel junction layer. For this configuration, the barrier height for 
tunneling at the interface is characterized by the bandgap of Ge of 0.6 
ev. I have found that n+ Ge forms an ohmic contact to n-type GaAs and that 
n+ Ge/p+GaAs tunnel junctions can be fabricated with current densities 
high enough for concentrator solar cells. 
FIG. 3 shows a current (I) versus voltage (V) curve for a tunnel junction 
of the type I propose for the interface between the layers of the 
photovoltaic cell of my invention. A tunnel junction of the type to which 
FIG. 3 applies is described by J. C. Mariance in IBM Journal, 280 (1960). 
DETAILED DESCRIPTION 
FIGS. 1 and 2 represent a schematic cross-sectional representation of a 
multilayer photovoltaic solar cell of the present invention. The layers of 
the cell are not to scale either vertically or horizontally except that, 
in the vertical dimension, the layers are shown in somewhat relative 
thicknesses. As illustrated, a germanium substrate 11 is provided with a 
contact surface 12 at one side and is joined to a first semiconductor cell 
13 at the other side. The cell 13 is preferably constructed of gallium, 
indium, and arsenic, having the composition of Ga.sub.0.88 In.sub.0.12 As 
and an energy bandgap of 1.25 ev. A junction 14 is illustrated overlying 
the first layer 13. A second cell 15 is shown in contact with the first 
cell 13. The cell 15 is preferably constructed of gallium, indium, and 
phosphorous, having the composition of Ga.sub.0.43 In.sub.0.57 P and an 
energy bandgap of 1.75 ev. 
Deposited on the other surface of the second cell 15 is a conductive and 
transparent layer 16 of indium tin oxide or antimony tin oxide. The 
composition of indium tin oxide and antimony tin oxide are mixtures of two 
oxides; indium oxide (In.sub.2 O.sub.3) and tin oxide (SnO.sub.2) in the 
first composition and antimony oxide (SbO.sub.2) and tin oxide in the 
second. These mixtures may be in any ratio of the two oxides, but in 
general there is from 80 to 90 mole percent indium oxide in the first 
composition and from 10 to 30 mole percent antimony oxide in the second 
composition. These compositions are conventionally indicated by the 
chemical formulae In.sub.2 O.sub.3 /SnO.sub.2 or SnO/SbO.sub.2. 
One or more contacts 17 is attached to the outer surface of layer 16. 
Electrically conductive wires 18 and 19 are attached to the contacts 12 
and 17, respectively. A transparent antireflective outer surface coating 
29 is applied over the surface layer 16 and contacts 17. 
As illustrated in FIG. 1, a concentrator 21, here shown as a concentrating 
lens, is positioned above the cell in a position to concentrate light from 
a source 22, here illustrated as the sun. 
FIG. 2 is a section through the junction of the multilayer cell of FIG. 1, 
along lines 2--2, illustrating the particular feature of the present 
invention. 
The section is enlarged to illustrate the junction as a thin semiconductor 
layer 14 of transparent low bandgap material, such as germanium. The 
semiconductor layer 14 separates the gallium indium arsenic layer 13 from 
the gallium indium phosphorous layer 15 and, in relative dimensions, is 
approximately 50 to 300 A as compared to 500 A for layer 13a and 1000 A 
for layer 15a. 
The particular quality of the germanium layer that permits it to function 
in the desired relationship is that, for the present invention, the layer 
is doped n+ while the layer 13a is doped n+ and layer 15a is doped p+. 
A preferred method for constructing the multijunction solar cell of my 
invention is to start with a single crystal substrate, for example, a 
germanium wafer. The germanium wafer substrate does not include a light 
sensitive junction by preference; firstly, because a substrate with a 
junction becomes more costly to construct because the purity of the 
germanium wafer with a functional light sensitive junction is of the order 
of .ltoreq.1 ppm whereas the wafer without a junction requires purity 
control of only .ltoreq.1000 ppm; and secondly, because a junction in the 
germanium wafer would be responsive to the light wavelength range which is 
most severely effected by fluctuations in humidity and air mass. A further 
advantage of the germanium substrate is that it is an elemental 
semiconductor like silicon and it can be grown as a ribbon, thus 
contributing to its lower cost. Further, germanium is lattice matched to 
layers 13 and 15 above to &lt;1% and should therefore permit the efficiency 
of the cell proposed herein to more nearly approach the theoretical limit. 
Further, the choice of a germanium substrate fixes the lattice constant of 
all the layers in the stack, including the low bandgap tunneling layer. 
Because of simplicity of deposition (GeH.sub.4 pyrolysis), Ge is an ideal 
low bandgap material independent of substrate choice but a germanium 
substrate makes lattice matching automatic. 
In the preferred form of the cell described herein the substrate germanium 
layer is between 200 and 300 micrometers thick and preferably 250 
micrometers. The lower limit on thickness is determined both by operating 
conditions which establish the conduction characteristics of the substrate 
and the physical strength of the substrate in its function as the base of 
the multilayer cell. The upper limit for the dimension of the substrate is 
mainly economic in that thicker substrates are more costly to make and 
include more volume of an expensive material. 
I propose a growth method which will allow in sequence III-V alloy layer 
depositions over large substrate areas. This type of deposition is known 
having been described in U.S. Pat. No. 4,128,733, issued Dec. 5, 1978 to 
L. M. Fraas et al. In copending application Ser. No. 52,707 I have shown a 
growth chamber for such a method, called low pressure metal organic 
chemical vapor deposition (MO-CVD). In this method, one introduces 
trialkyl gallium or trialkyl indium or a mixture thereof and phosphine or 
arsine or a mixture thereof into a pyrolysis chamber. These compounds 
react on the germanium substrate to form the required InGaAs or InGaP 
alloys. One example of the reaction is: 
EQU (1-x)Ga(C.sub.2 H.sub.5).sub.3 +x In(C.sub.2 H.sub.5).sub.3 
+AsH.sub.3.sup.600.degree. C. Ga.sub.(1-x) In.sub.x As+By products, 
wherein x has a value in the range of &gt;0 to &lt;1. The product is a 
semiconductor film deposited on the germainum substrate. 
The semiconductor is doped p-type by adding dialkyl zinc, dialkyl cadmium, 
or dialkyl beryllium trimethyl amine vapors and n-type by adding hydrogen 
sulfide, tetralkyl tin or dialkyl telluride vapors. All III-V alloy layers 
with the prescribed composition are grown in sequence by using a 
programmable gas flow controller. 
In fabricating the multijunction solar cell of FIG. 1, I propose stacking 
lattice-matched homojunction cells together by placing shorting tunnel 
junctions at the heterofaces. Starting with a germanium substrate 11 with 
a dopant type p+, the next layer of the cell 13 is formed by the epitaxial 
deposition of a p+ type layer of gallium indium arsenide preferably with 
an alloy composition Ga.sub.0.88 In.sub.0.12 As. During the course of the 
deposition of this semiconductor layer, the concentration of the dopant is 
reduced to produce a p- type layer and eventually the dopant is changed to 
produce a p/n junction and transition to n- type layer. Continued 
deposition increases the thickness of the first layer and a finishing 
portion is deposited with a dopant concentration such as to produce an n+ 
layer at the boundary of the first cell. 
As shown in the enlarged FIG. 2, a layer 14 of germanium with an n+ dopant 
is then deposited on the surface of the cell 13 to produce a tunnel 
junction between the layers of the multijunction cell. The germanium layer 
is epitaxially deposited on the surface of the cell 13 with the same metal 
organic chemical vapor deposition chamber via GeH.sub.4 pyrolysis to a 
preferred thickness of between 50 and 300 A. 
A second semiconductor cell 15 is then epitaxially deposited on the outside 
surface of the Ge layer on the first cell initially with a dopant material 
and concentration to produce a p+ layer at the interface. The second 
semiconductor layer 15 is an indium gallium phosphide material with a 
preferred alloy composition of In.sub.0.57 Ga.sub.0.43 P. During the 
course of the deposition of this semiconductor layer, the concentration of 
the dopant is reduced to produce a p- type layer and eventually the dopant 
is changed to produce a p/n junction and transition to n- type layer. 
Continued deposition increases the thickness of the second layer with a 
transition of dopant composition such as to produce an n+ layer at the 
boundary of the second cell. 
An outer conductive layer 16 is then deposited on the outer surface of the 
second cell 15 to complete the two-junction photovoltaic cell. The 
conductive layer may also be an antireflective coating or a separate 
coating 20 may be deposited on layer 16 and above conductor 17 in contact 
with layer 16. Preferably the conductive layer has an alloy composition of 
indium tin oxide (In.sub.2 O.sub.3 /SnO.sub.2) conventionally abbreviated 
as ITO. 
To complete the photovoltaic cell a pair of conductors 18 and 19 are 
attached one to each outer surface 12 of the substrate and the conductors 
17 under layer 20. 
It should be noted that the photovoltaically active junctions within the 
photovoltaic cell are homojunctions and that the stacked layers are 
lattice-matched. Further, there are germanium tunnel junctions at the 
heterofaces between the cells. With this method of construction more 
effective tunnel junctions are provided. 
In the multilayer photovoltaic cell just described the first layer has a 
bandgap of 1.25 ev and the second layer has a bandgap of 1.75 ev. With an 
intermediate Ge layer aiding tunneling, the tunneling barrier height will 
be 0.6 ev. 
In the preferred embodiment here described, the thickness of each deposited 
compound semiconductor layer is between 2 and 6 micrometers thick and 
preferably about 4 micrometers thick. The highly doped tunnel junction 
layer on the low bandgap side for the homojunction cell must be thin 
enough not to absorb an appreciable amount of light, i.e., &lt;1000 A. This 
criterion is not difficult to meet, since the absorption length is longer 
just above but near the band gap of a semiconductor, i.e., the region of 
interest for a multijunction cell. This layer must also be thick enough 
not to be completely depleted, i.e., &gt;50 A. 
Each layer of the multilayer cell is lattice-matched to its neighbor layer 
with a maximum lattice constant variation of plus or minus 1.0%. This 
matching is important because with poor lattice matching or non-matching, 
crystallinity of the cell system degrades and a structure having a high 
density of crystal dislocations is formed, and in the worse cases, even 
grain boundaries are formed. Such dislocations then become sites for 
recombination of the light generated charge carriers, thus reducing the 
amount of current produced. These dislocations also provide shunting 
current paths which further reduce open circuit voltages. 
Lattice-matching is accomplished by the proper choice of the composition 
and relative amounts of the materials in the different layers. The method 
of growth, with special control of the temperature is also important to 
the formation of high quality single crystal layers. 
The layers of the preferred multilayer cell deposited on the germanium 
substrate are all lattice-matched to be germanium lattice constant of 5.66 
A to within .+-.1.0%. 
The elements (except Ge) employed in the various layers of FIG. 1 and 2 are 
all found in columns IIIA and VA of the periodic table and are preferred 
for use according to the invention. However, other semiconductive 
materials can be used in accordance with the invention as defined by the 
appended claims and their legal equivalents. For example, compounds formed 
of elements in columns IIB and VIA such as CdS and CdTe could be used; 
also IB-IIIA-VIA compounds such as CuInS or variations thereof where, 
i.e., Se is substituted for S or Ga for In; also IIB-IVA-VA compounds such 
as ZnSnP. Also, other IIIA-VA compounds can be used in lieu of the 
most-preferred IIIA-VA compounds previously discussed. 
While certain preferred embodiments of the invention have been specifically 
disclosed, it should be understood that the invention is not limited 
thereto as many variations will be readily apparent to those skilled in 
the art and the invention is to be given its broadest possible 
interpretation within the terms of the following claims.