Amorphous photoelectric converting device

An amorphous photoelectric converting device that remains efficient despite exposure to heat over long periods of time is formed by placing one on top of the other a plurality of photovoltaic elements each comprising a thin film of p-i-n structure. The p-type layer and the n-type layer of adjacent elements are made of microcrystalline silicon so that good ohmic contact is established, and the p-type layer of microcrystalline silicon contains boron in an amount sufficient to neutralize the donor atoms which diffuse from the adjacent n-type layer when the device is left to stand at high temperatures for a long period of time. The amount of boron, however, is limited to such an extent that the boron atoms do not adversely affect the initial desired characteristics of the device. A preferred range of boron levels is 3.times.10.sup.20 to 1.times.10.sup.21 atoms/cm.sup.3.

BACKGROUND OF THE INVENTION 
The present invention relates to an amorphous photoelectric converting 
device in which a plurality of semiconductor thin films or elements of 
p-i-n structure are placed one on top of the other in the direction 
perpendicular to the bonded surface, such as in a multijunction solar 
cell. 
Solar cells and photosensors are among the applications of a photovoltaic 
element composed of an amorphous semiconductor film, especially an 
amorphous silicon (a-Si:H) film formed through decomposition of a silane 
gas by glow discharge or ultraviolet light. Current research relating to 
solar cells is aimed at the improvement of their conversion efficiency and 
reliability, which is necessary for the cells to be put to practical use. 
A multijunction solar cell is known to generate a high output voltage. It 
is made up of a-Si:H elements, each element having the p-i-n structure, as 
shown in FIG. 2. In FIG. 2 there are shown a transparent substrate 1, a 
transparent electrode layer 2; a first element composed of a first p-type 
layer 31, a first i-type layer 41, and a first n-type layer 51; a second 
element composed of a second p-type layer 32, a second i-type layer 42, 
and a second n-type layer 52, and a reverse metal electrode layer 6, which 
are placed one on top of the other. The adjacent first n-type layer 51 and 
second p-type layer 32 are both made of microcrystalline silicon 
(.mu.C-Si:H) which permits increased contact between the two layers. 
Additionally, a positive terminal 21 is formed on the exposed surface of 
the transparent electrode layer 2. 
A solar cell of this type is improved in efficiency if the second i-type 
layer 42 is made of a material having a narrower optical band width than 
that for the first i-type layer 41. For example, a solar cell having a 
first i-type layer of a-Si:H and a second i-type layer of a-SiGe:H can 
utilize sunlight more efficiently. Further studies are being made on solar 
cells of multiple layer type formed by laminating together or more units 
of p-i-n structure. 
Unfortunately, the solar cell of multiple layer type suffers from a 
disadvantage that when it is allowed to stand at high temperatures for a 
long time, it becomes seriously deteriorated in characteristic properties 
such as open circuit voltage, curve factor, and conversion efficiency. The 
deterioration is caused by the diffusion of impurities into the interface 
between the n-type semiconductor layer of one element and the p-type 
semiconductor layer of the other adjacent element. The diffused impurities 
impair the interfacial ohmic contact between the two adjacent layers. 
SUMMARY OF THE INVENTION 
It is thus an object of the present invention to provide an amorphous 
photoelectric converting device of multijunction type which is free of the 
above-mentioned disadvantage. Even after being exposed to high temperature 
for a long period of time, the claimed converting device does not suffer 
deterioration of photoelectric converting characteristics by the 
degradation of ohmic contact between the adjacent p-type layer and n-type 
layer of consecutive layered photovoltaic elements of p-i-n structure. 
The obove-mentioned object is achieved by an amorphous photoelectric 
converting device comprising a plurality of photovoltaic elements 
laminated one on top of the other, each of said elements being made up of 
thin films of p-type layer, i-type layer, and n-type layer and having the 
p-i-n structure, with the n-type layer and p-type layer of adjacent 
elements being made of microcrystalline silicon and the remaining other 
layers being made of amorphous silicon, wherein the concentration of boron 
in said p-type layer of microcrystalline silicon (.mu.C-Si:H) ranges from 
3.times.10.sup.20 to 2.times.10.sup.21 atoms/cm.sup.3.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
According to the present invention, the p-type layer of .mu.C-Si:H in each 
element contains a sufficient amount of boron (B) so as to protect the 
ohmic contact between layers from becoming deteriorated by the diffusion 
of donor impurities, such as phosphorus (P) from the n-type layer of the 
adjacent element, even when the device is allowed to stand at high 
temperatures for a long period of time. The upper limit of the boron 
concentration is established so that the initial desired characteristics 
are not adversely affected by boron. 
An embodiment of the present invention will be described with reference to 
FIG. 1, in which the same reference characters are used for the 
corresponding parts in FIG. 2. An amorphous silicon solar cell shown in 
FIG. 1 was made up of a transparent glass substrate 1, a transparent 
conductive film layer 2 of indium tin oxide (ITO) or SnO.sub.2, a p-type 
amorphous silicon carbide (a-SiC:H) layer 31 having a large band gap, an 
i-type a-Si:H layer 41, an n-type .mu.C-Si:H layer 71, a p-type .mu.C-Si:H 
layer 82 in which the boron concentration is limited to between 
3.times.10.sup.20 to 2.times.10.sup.21 atoms/cm.sup.3, preferably 
5.times.10.sup.20 to 1.times.10.sup.21 atoms/cm.sup.3, an i-type a-Si:H 
layer 42, an n-type a-Si:H layer 52, and a back electrode 6, which are 
formed one on top of the other. 
The amorphous silicon solar cell constructed as mentioned above was allowed 
to stand at 50.degree. C. and 140.degree. C. for 3000 hours to see how it 
changes in characteristic properties. For comparison, the same experiment 
as above was carried out with a conventional amorphous silicon solar cell 
in which the p-type .mu.C-Si:H layer 82 contains less than 1% of boron. 
FIGS. 3 and 4 show a part of the results of the experiments. The boron 
concentration is indicated by a circle sign for 7.times.10.sup.20 
atoms/cm.sup.3, by a cross sign for 1.5.times.10.sup.20 atoms/cm.sup.3, 
and by a square sign for 1.0.times.10.sup.20 atoms/cm.sup.3.The effect of 
the boron concentration is shown in terms of the change with time of the 
normalized efficiency for the initial conversion efficiency. In the 
experiment at 50.degree. C. as shown in FIG. 3, the conversion efficiency 
remained unchanged over the period tested. In the experiment at 
140.degree. C. as shown in FIG. 4, as the boron concentration was 
decreased, the more the conversion efficiency decreased with time. 
FIG. 5 shows the relation between the conversion efficiency and the boron 
concentration in the p-type .mu.C-Si:H layer. In FIG. 5, the broken line 
11 represents the initial conversion efficiency, and the solid line 12 
represents the conversion efficiency after standing at 140.degree. C. for 
3000 hours. It is noted that if the boron concentration in the p-type 
.mu.C-Si:H layer is 3.times.10.sup.20 to 2.times.10.sup.21 atoms/cm.sup.3, 
the conversion efficiency after 3000 hours is higher than 10%, and that if 
the boron concentration in the p-type .mu.C-Si:H layer is 
5.times.10.sup.20 to 1.times.10.sup.21 atoms/cm.sup.3, the conversion 
efficiency after 3000 hours remains higher than 11%. With the boron 
concentration higher than 2.times.10 .sup.21 atoms/cm.sup.3, the initial 
conversion efficiency is low and hence the conversion ratio after 3000 
hours is also low. 
The foregoing indicates that the solar cell in this example withstands the 
high temperature test at 140.degree. C. for 3000 hours if the boron 
concentration is in the range of 3.times.10.sup.20 atoms/cm.sup.3 to 
2.times.10.sup.21 atoms/cm.sup.3. The concept of the present invention may 
be applied to a solar cell in which the p-type .mu.C-Si:H layer is made 
from hydrogenated silicon such as SiH.sub.4 and Si.sub.2 H.sub.6 or from 
fluorinated silicon such as Si.sub.2 H.sub.2 F.sub.4 and SiF.sub.4, and 
also to a solar cell in which the p-type layer forming the interface is a 
microcrystalline silicon germanium film. 
Another embodiment of the present invention is shown in FIG. 6. It differs 
from the first one shown in FIG. 1 in that the p-type layer forming the 
interface between the first layer element and the second layer element is 
of double layer structure. In other words, the p-type layer is formed from 
two sublayers, a first sublayer 82 which comprises boron doped 
.mu.C-silicon and a second sublayer 31 which comprises a material such as 
a-SiC:H that has a band gap greater than a-Si:H. This structure produces 
the effect of increasing the photoelectric conversion efficiency of the 
p-i-n junction forming the second layer element.