Multi-layered thin film solar cell

A multi-layered thin film solar cell is provided, which includes a substrate, a plurality of transparent electrodes, and a plurality of groups of photoelectric conversion elements formed of semiconductor material and forming successive photoelectric conversion layers. The layers have optical band gaps which decrease successively in a direction away from a side of the cell adapted to receive incident light. The groups of elements forming the layers are laminated on the substrate. Each of the elements belonging to one of the groups is connected in series with one of the elements belonging to another of the groups and is connected in parallel with other elements in the group to which it belongs.

BACKGROUND OF THE INVENTION 
This invention relates to a multi-layered thin film solar cell in which 
solar light successively enters plural photoelectric conversion layers 
having different light sensitivities. 
For improving the efficiency of a thin film solar cell such as an amorphous 
silicon type solar cell, effective utilization of the solar light spectrum 
is indispensable. Since the conversion efficiency is restricted in a thin 
film solar cell using a single photoelectronic conversion layer, it is 
necessary to laminate two or more photoelectric conversion layers as shown 
in FIG. 3 in order to increase the utilization efficiency of the solar 
light by dividing the sensitivity region to the solar light spectrum. In 
FIG. 3, for light 10 passing through a light permeable substrate 1 and a 
transparent electrode 2, the shorter wavelength portion is absorbed by a 
first photoelectric conversion layer 31 with greater optical band gap 
(Eg), the longer wavelength portion is absorbed by a third photoelectric 
conversion layer 33 with smaller optical band gap (Eg), and the medium 
wavelength portion is absorbed by a second photoelectric conversion layer 
32 with an intermediate optical band gap (Eg). 
The power of the solar cell having a laminated structure of photoelectric 
conversion layers of different sensitivity regions is outputted through 
the transparent electrode 2 and the back electrode 4. It is shown by 
theoretical calculations that a conversion efficiency of about 20% can be 
obtained for an amorphous silicon type solar cell and various studies have 
been made to obtain a multi-layered thin film solar cell. 
From a practical point of view, however, the structure shown in FIG. 3, in 
which a plurality of photoelectric conversion layers are successively 
laminated on a substrate, has several problems. At first, since each of 
the photoelectric conversion layers is laminated successively, the device 
structure has to be designed such that equal electrical currents are 
generated in each of the photoelectric conversion layers. Further, if the 
spectrum of the solar light varies depending on the season or site 
location, design adaptability can no longer be attained and the advantage 
obtained by adopting the multi-layered structure is reduced due to the 
non-uniformity of the current. Secondly, since an n-p or p-n junction is 
formed at the interface between each of the photoelectric conversion 
layers, recombination loss of carriers occurs or reverse voltage is 
generated at the junction, causing a reduction in the cell power. 
As a countermeasure, a thin film solar cell module as shown in FIG. 4 has 
been proposed in Japanese Laid Open Pat. No. Sho 60-30163. That is, a 
group of solar cell units each comprising a laminated transparent 
electrode 2, a photoelectric conversion layer 31 and a transparent 
electrode 51 are connected in series on one transparent insulating 
substrate 1, whereas solar cell units each comprising a metal electrode 4, 
a photoelectric conversion layer 32 and a transparent electrode 52 are 
connected in series on the substrate 11. These cell groups are opposed to 
each other with both of the substrates being on the outside, coupled by 
way of frames 61 and sealed with transparent resins 62. 
Similarly to the case shown in FIG. 3, the optical band gap Eg of the 
photoelectric conversion layer 31 is greater than that of the 
photoelectric conversion layer 32. Both of the serially connected solar 
cells are further connected in parallel with each other by connecting 
terminals 63 with 64 and terminals 65 with 66, respectively. However, such 
a module has a drawback that solar cells have to be formed on two 
substrates separately and the structure is complicated and expensive as 
well. 
SUMMARY OF THE INVENTION 
The object of this invention is to provide a multi-layered thin film solar 
cell which is capable of overcoming the foregoing problems, free from the 
restriction of equalizing the electric current generated in each of the 
photoelectric conversion layers and, further, capable of being formed on a 
single substrate. 
The foregoing object can be attained in accordance with this invention by a 
multi-layered thin film solar cell, comprising groups of photoelectric 
conversion elements forming photoelectric conversion layers consisting of 
semiconductors laminated on the substrate, wherein the optical band gaps 
are successively decreased from the side of light incidence in the 
laminating direction for more effective utilization of the solar light 
spectrum, each photoelectric conversion element belonging to one of the 
groups forming a photoelectric conversion layer is connected in series 
with a conversion element belonging to another of the groups forming 
another photoelectric conversion layer and each photoelectric conversion 
element belonging to one of said groups is connected in parallel with 
other photoelectric conversion elements in the same group. According to 
this structure, the solar cell is free from the restriction of equalizing 
the generated current because elements consisting of the same 
semiconductor and generating the same current are connected in series. 
In the thin film solar cell according to the present invention, respective 
said groups of the photoelectric conversion elements are successively 
laminated such that a group is displaced laterally relative to the group 
lying thereunder by one element at one end of the cell, a transparent 
electrode is disposed between vertically adjacent photoelectric conversion 
layers, the electrode at the substrate side of each element is connected 
to the electrode at the counter-substrate side of the adjacent element in 
the same group positioned at the other end side of the aforesaid element, 
and the photoelectric conversion layer of the element at the cell's other 
end in one of the groups is formed on an extension of the transparent 
electrode on the substrate side of an element positioned at the cell's 
other end in the underlying group of elements. In this embodiment, 
parallel-serial matrix arrangement and connection on one identical 
substrate can be attained with ease.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
This invention will now be described by way of its preferred embodiment. 
FIGS. 1(a) to 1(g) illustrate the manufacturing steps of one embodiment 
according to this invention, in which the portions identical with those 
shown in FIGS. 3 and 4 are designated by the same reference numerals. 
In FIG. 1(a), a transparent conductive film made of SnO.sub.2 or 
ITO/SnO.sub.2 with a thickness from 2,000 to 4,000 .ANG. is formed by 
electron beam vapor deposition on the entire surface of a glass substrate 
1 of 10 cm square and is divided into seven regions, each of 7 to 8 mm 
width and separated by a gap of 100 .mu.m to 2 mm, by means of a 
photolithographic process to form transparent electrodes 2. Only the 
transparent electrode 21 at one end has a width greater than the other 
electrodes by a factor of three or more. 
In FIG. 1(b), first photoelectronic conversion layer 31 is formed by the 
combined use of a glow discharge method, a photo CVD method, etc. The gap 
between each of the transparent electrodes 2 is filled by layer 31. By 
means of patterning by a photolithographic process, layer 31 is divided 
into six photoelectric conversion regions each separated by a gap of 100 
.mu.m to 2 mm at the opposite side of electrode 2. The first photoelectric 
conversion layer 31 is a p i n amorphous silicon film in which a-SiC:H 
having an Eg of 1.9 eV is used as a p-film. 
In FIG. 1(c), an ITO film or ZnO film of 4,000 to 6,000 .ANG. thickness is 
formed over the entire surface, and patterned by a photolithographic 
process into six intermediate transparent electrodes 71, which are in 
contact at the ends thereof with the transparent electrodes 2. The 
intermediate transparent electrode with such an increased thickness can 
reduce the electric power loss and decrease the deleterious effects on the 
characteristics. In this way, six photoelectronic conversion elements 
forming the first photoelectric conversion layer 31 are connected in 
series with each other. 
Then, as shown in FIG. 1(d), a second photoelectric conversion layer 32 
using a p i n amorphous silicon film with Eg=1.7 eV is deposited over the 
entire surface and patterned to form six photoelectronic conversion 
regions except for the upper portion of the first photoelectric conversion 
layer 31 at the leftmost end in the drawing. The second photoelectric 
conversion layer 32 at its rightmost end is formed on the transparent 
electrode 21 while being adjacent to the first photoelectric conversion 
layer 31 on its rightmost end. 
Further, as shown in FIG. 1(e), intermediate transparent electrodes 72 
similar to the intermediate transparent electrodes 71 are formed on the 
second layer and brought into contact with the intermediate transparent 
electrodes 71. 
In FIG. 1(f), a third photoelectric conversion layer 33 using an amorphous 
silicon-germanium alloy film with Eg=1.5 eV is formed in the same manner 
as the second photoelectric conversion layer 32 while being displaced by 
one element rightwardly. 
Finally, as shown in FIG. 1(g), back electrodes 4 are formed by metal 
deposition and patterning. As a result, as shown by the equivalent circuit 
in FIG. 2, there is provided a thin film solar cell comprising six 
photoelectric conversion elements A using the first photoelectric 
conversion layer 31, six photoelectric conversion elements B using the 
second photoelectric conversion layer 32, and six photoelectric conversion 
elements C using the third photoelectric conversion layer 33 connected in 
parallel and series with each other. 
Explanation will now be made as to the result of the comparison between the 
efficiency of the multi-layered type thin film solar cell according to 
this invention and the efficiency of the solar cell having a structure as 
illustrated in FIG. 3. 
In the case of a conventional type device as shown in FIG. 3 formed with 
two layers, that is, a first photoelectric conversion layer with Eg=1.9 eV 
and a second photoelectric conversion layer with Eg=1.7 eV, if the 
thickness of the first layer is 2,300 .ANG. and the thickness of the 
second layer is 7,000 .ANG., there can be obtained a short circuit 
electric current density Jsc=8 mA/cm.sup.2, an open voltage Voc=1.65 V and 
an efficiency .eta.=8.58%. On the other hand, according to an embodiment 
of this invention in which the third photoelectric conversion layer 33 is 
omitted, Jsc=10 mA/cm.sup.2, Voc=0.85 V and .eta.=5.95% in the first 
layer, Jsc=6 mA/cm.sup.2, Voc=0.8 V and .eta.=3.26% in the second layer 
while selecting the film thickness relatively optionally, and an overall 
efficiency of 8.84% is obtained. In this way, the efficiency for the 
entire assembly cannot be expressed as a simple sum unless the voltages 
between the two layers are not unified. Further, if there is a voltage 
difference between photoconversion layers of greater than 0.2 V, the 
effect obtainable by adopting the multi-layered structure is also reduced. 
However, for a two-layer device of amorphous silicon, the effect of the 
multi-layered structure according to this invention is significant and the 
freedom of device design is enhanced. Also, since the p-n junction formed 
at the interface between the two different photoelectric conversion layers 
is out of the effective photoelectric conversion region, it does not 
deleteriously affect the power. 
Comparison with the conventional type a device was made for the 
three-layered structure of the invention as described above. The 
respective characteristics from the first layer to the third layer were: 
Voc=0.85 V, Jsc=8 mA/cm.sup.2, .eta.=4.76% for the first layer, Voc=0.8 V, 
Jsc=5 mA/cm.sup.2, .eta.=2.72% for the second layer, and Voc=0.76 V, Jsc=5 
mA/cm.sup.2 and .eta.=2.62% for the third layer, and the overall 
efficiency of the entire device was .eta.=9.09%. These characteristics are 
superior to those of the conventional three-layered structure in which 
Voc=2.31 V, Jsc=6 mA/cm.sup.2, .eta.=8.73%. When the total power output 
per one sunny day were compared by using the two devices as described 
above, the three-layered device according to this invention showed a 
better result by 12%. 
Although a glass substrate is used in this embodiment, it is of course 
possible to form the device easily by using a substrate made of stainless 
steel or flexible polymeric film by reversing the laminating order thereby 
obtaining similar effects. Further, although the example shows the case of 
using amorphous silicon type material for constituting the multi-layers, 
it is of course possible to use polycrystalline and single crystal 
silicon, as well as Group III-V compounds such as GaAs, InP and AlP, and 
Group II-VI compounds such as CdS, CdTe and ZnSe, and CuInSe.sub.2, etc., 
may be combined to obtain similar effects. 
According to this invention, since groups of photoelectric conversion 
elements comprising photoelectric conversion layers, the optical band gap 
of which is decreased successively from the side of light incidence, are 
laminated on the substrate and connected in a matrix arrangement, it is 
free from the design restriction of equalizing the optically generated 
current in the laminating direction and, therefore, respective 
photoelectric conversion layers having photoelectric regions of different 
band gaps can be prepared under optically optimum conditions to thereby 
provide a multi-layered thin film solar cell with high efficiency. In such 
a matrix arrangement and connection, even if defects, for example, 
short-circuits due to pinholes in the photoelectric conversion layer occur 
in a part of the elements, the output power of the entire device is not 
subject to substantial effects, contrary to the conventional type solar 
cell. 
Great advantages can be obtained by the multi-layered thin film device 
according to this invention, since each of the elements can easily be 
connected in a matrix arrangement by laminating photoelectric conversion 
elements connected in series such that each of the layers is displaced 
relative to the underlying layer by one element, and since an additional 
advantage can also be obtained in that electric power can be reliably 
outputted even in a case where the vertically stacked elements in each of 
the layers are simultaneously in a non-generating condition due to shadows 
or the like.