Silicon-based solar energy conversion cells

Silicon-based solar cells devised hitherto have used crystalline silicon because of its acceptable electrical characteristics. Amorphous silicon is much cheaper and has better optical absorption characteristics, but its electrical characteristics are so poor that it cannot be used, without some additional treatment such as hydrogenation. The invention provides a method of production of solar cells using amorphous silicon in which the silicon is evaporated in a suitable vacuum onto a substrate in the presence of a positive field which opposes the migration of the positive silicon ions toward the substrate. The resulting devices are found to have much improved electrical characteristics while retaining a good absorption characteristic. A tentative hypothesis is given for the unexpected improvement.

FIELD OF THE INVENTION 
The present invention is concerned with improvements in or relating to 
methods of manufacture of silicon-based solar energy conversion cells and 
to cells produced by the methods. 
REVIEW OF THE PRIOR ART 
The field of solar energy conversion cells continues to be of considerable 
interest, since there are numerous applications in which they are already 
a practicable source of electric power, and number may be expected to 
increase as the efficiency of cells is increased, the cost of production 
is decreased, and the cost of alternative energy sources increases. It is 
well known to use silicon as the basic converter material in such cells, 
and in a known construction a wafer of appropriately-doped crystalline 
silicon has electrical contacts applied, the cell being completed by the 
application of an anti-reflection coating and any transparent protective 
layer that may be required. 
The silicon can be produced either in the amorphous or the crystalline 
form, the former having superior optical absorption properties over the 
latter, especially in the range of wavelengths found in solar radiation, 
while the crystalline form has superior electrical properties. Thus, as 
compared to the crystalline form amorphous silicon is found to have a 
large density of states in the energy gap, to the extent that it is 
difficult to dope and it has an extremely low excess carrier life time; 
consequently untreated amorphous silicon has been impractical for solar 
cell use, and proposals have therefore been made to alleviate this 
disadvantage, for example by introducing hydrogen into the silicon, which 
significantly reduces the states density, but also results in reduced 
optical absorption. Despite this reduction the optical absorption of the 
hydrogenated form is sufficiently superior to the untreated amorphous form 
to justify its commercial use, as a result of which thin and cheap silicon 
films can be employed; the crystalline form must be quite thick and such 
thick crystalline wafers are relatively expensive. Even though the maximum 
electrical reported efficiency of an amorphous device is only 10%, as 
compared to 18% obtainable with a crystalline device, the end cost of the 
amorphous device is about one tenth that of the crystalline device. 
DEFINITION OF THE INVENTION 
It is the principal object of the invention to provide a new method of 
making silicon-based solar energy conversion devices and the devices 
produced by the method. 
In accordance with the present invention there is provided a method of 
making an amorphous silicon-based solar energy conversion device 
comprising: 
placing a substrate to be coated and a source of silicon vapour in the 
enclosure and establishing a vacuum of at least 1.times.10.sup.-4 Torr in 
the enclosure, 
heating the silicon source in the said vacuum to a temperature such as to 
cause the production of heated silicon vapour within the enclosure, 
whereby the substrate is exposed in the enclosure under said vacuum to the 
heated silicon vapour for deposit thereon of a layer of amorphous silicon, 
applying an electric field between the source and the substrate with the 
substrate positive with respect to the source, so that the electric field 
opposes the deposition of positive silicon ions on the substrate to result 
in a layer of amorphous silicon having improved electrical characteristics 
as compared to a layer deposited in the absence of an electric field. 
Preferably the electric field has a value from about 10 to about 33 volts 
per cm, and preferably the potential difference has a value of from about 
200 to about 500 volts, applied over a distance of about 15 cm to about 20 
cm. The substrate may be heated to a temperature not higher than about 
350.degree. C., and preferably is heated to a temperature of about 
200.degree. C.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
A water cooled stainless steel bell jar cover 10 is mounted on a base 12 
and a vacuum is produced in its interior via an outlet 14. A carbon 
crucible 16 mounted on the base contains a charge of silicon 18 that is 
heated by heating the crucible via a surrounding water-cooled induction 
heating coil heater 20 to a temperature at which it will evaporate freely, 
usually about 1400.degree. C. The crucible is electrically grounded via a 
lead 22. A substrate member 24, for example a thin layer of stainless 
steel supported on a suitable backing member electrically insulated from 
ground, is mounted by arm 26 above the crucible and can be heated by a 
heater 28 to an elevated temperature not greater than about 350.degree. 
C., and usually about 200.degree. C. The heaters are supplied with power 
from respective terminals 30 and 31. The substrate is electrically 
connected by a lead 32 to the positive terminal of a battery 34 by which 
the substrate is positively charged relative to the crucible, so that a 
corresponding positive field is established between them and opposes the 
transfer of the evaporated positive silicon ions to the substrate. 
Nevertheless, silicon is deposited on the substrate in the form of a thin 
layer 36 thereof, and surprisingly is found to have better electrical 
characteristics than the amorphous silicon that would be deposited in the 
absence of the electric field, while retaining the excellent absorption 
characteristics of amorphous silicon. 
The silicon 18 employed is a high purity polycrystalline grade (impurities 
of a few ppb) supplied by Monsanto for the production of silicon wafers. 
Silicon of such high purity was employed to avoid the possibility of test 
results being invalidated by some particular impurity. The substrate is of 
pre-prepared glass, such as Corning 7059* Glass intended for as a 
substrate in evaporation techniques, or thin 304 stainless steel that has 
been polished and cleaned. After placement of the crucible and the 
substrate the interior of the bell jar is pumped with the substrate heated 
to about 200.degree. C. until a vacuum of at least 1.times.10.sup.-8 Torr 
has been achieved, conveniently by pumping overnight. Air alone can be 
removed by pumping for about 3-4 hours, but difficulty is obtained in 
removing sufficient of the water vapour that inherently is present and a 
longer pumping time is preferred for this reason. Removal of the water 
vapour is facilitated by cooling the metal bell jar, when the residual 
polar water vapour molecules preferably condense on its interior surface. 
It is believed that at the temperature of the process the water vapour may 
react with the carbon of the crucible to produce carbon dioxide, carbon 
monoxide and hydrogen; the carbon oxides and hydrogen are easily removed, 
but minor amounts of carbon, oxygen and hydrogen consistent with this 
hypothesis are found in the resultant films. 
FNT *Trademark 
This relatively high degree of vacuum is not essential and a value of 
1.times.10.sup.-6 Torr would be satisfactory for commercial and more usual 
application. Values as low as 1.times.10.sup.-4 might also be tolerated 
provided that other conditions are not such that a glow discharge is 
obtained. It is found in practice that there must be a sufficient distance 
between the boiling silicon and the substrate such that silicon other than 
in pure vapour form (e.g. splashes and globules) cannot be deposited on 
the substrate; a distance of 15 cm is usually sufficient for this purpose 
but can of course be reduced if other precautions are taken against this 
effect. 
In a specific embodiment therefore the distance between the substrate and 
the silicon is about 15 to 20 cm, and over this distance a potential of 
200 to 500 volts is found to be effective, which corresponds to a field of 
from about 10 to 33 volts per cm. Voltages of up to 1000 over a distance 
of 20 cm (50 volts per cm) have also been employed, but do not appear to 
be any more effective than the lower values. The silicon layer 36 can be 
quite thin and thicknesses thereof from about 0.2 to 1.5 micrometers are 
suitable. A thickness value of about 0.5 micrometer is particularly 
preferred and requires about 3 hours of operation for its deposition. 
Films of up to 3 micrometers have been produced for study of their 
absorption characteristics, but are not found to justify the additional 
costs, etc. involved in their production. 
Devices in accordance with the invention have been examined by Dr. B. G. 
Yacobi of the Solar Energy Research Institute of Golden, Colo., U.S.A. by 
electron diffraction using an electron microscope. He has reported 
informally that at the level of 20-30 Angstroms no microcrystallinity was 
observed, and it must be concluded therefore that the silicon is in 
amorphous state, but with structural order at the atomic level that has 
been enhanced by the application of the invention, as will be apparent 
from the following review of observed data. 
Thus, FIG. 2 is a graph of the conductivity of the resultant films 
(.sigma.) against the reciprocal of the respective temperature. Line 1 is 
for an evaporated amorphous first device produced without the application 
of an electric field, while line 2 is for a second device produced with a 
field of 500 volts at a distance of 20 cm (25 volts per cm). The marked 
increase in slope from line 1 to line 2 may be noted. The lower slope for 
the zero field first amorphous device shows that there is a large density 
of states in the energy gap, and it is the presence of these states that 
reduces the capacity for decrease in conductivity as the temperature 
decreases. It is therefore significant to compare this parameter of the 
second device with that of a third device in which the silicon layer has 
been hydrogenated; the characteristics of two such hydrogenated devices 
are shown as lines 3 and 3' and it will be seen that they correspond 
closely to that of a device of the invention, so that the employment of 
the invention produces an equivalent effect to hydrogenation. 
FIG. 3 is a graph of the optical absorption coefficient (.alpha.) of 
different devices plotted against the photon energy of the incident 
radiation in electron-volts. Line 4 shows the characteristic for a typical 
crystalline silicon and it will be noted that this intersects the ordinate 
at about 2.5 electron volts, which is about green on the colour scale. A 
lower value of absorption means of course that a thicker layer of the 
relatively expensive crystalline silicon is required for energy capture 
and conversion. Lines 5 and 6 are the characteristics for hydrogenated 
amorphous silicon devices and the considerable shift toward the shorter 
red end of the spectrum will be noted, signifying that there is 
correspondingly greater capture of higher energy photons. Line 7 is for an 
amorphous device produced with zero field and the much greater absorption 
than the crystalline or hydrogenated amorphous devices is to be noted. 
Line 8 is for a device of the invention with which the vaporised silicon 
again was subjected to a field of 500 volts at 20 cm; it will be noted 
that the absorption mostly is somewhat less than that of the zero field 
device. Despite the lower absorption obtained for photons of less than 2.5 
electron volts the values are about 3 to 5 times greater than for the 
hydrogenated silicon, and the electrical characteristic of the untreated 
amorphous material make it unsuited for solar cells. 
At this time I have no provable reason for this improvement in electrical 
characteristics of the vaporised silicon, and I do not intend to be bound 
by the following hypothesis, which is given solely as a possible 
explanation of this unexpected effect. Under zero field evaporation 
conditions positively charged silicon ions form part of the stream of 
silicon reaching the substrate surface. They are not particularly mobile 
when they reach the substrate surface and tend to "stick" where they land, 
so that a completely random amorphous structure results, even though the 
majority of silicon reaching the surface is in the form of mobile neutral 
atoms and negative ions. The positive field tends to reject the positive 
silicon ions while causing an accumulation of electrons at the surface. 
Neutral silicon atoms and negative ions that have a large surface mobility 
still reach the surface, any positive silicon ions "eager" for bonding 
which do reach the surface immediately bond with the available electrons 
and are electronically "neutral" silicon atoms on landing, so that they 
become much more mobile and are also able to migrate on the surface to 
bond with other neutral silicon atoms to form a more ordered structure 
than is normally obtained with evaporated amorphous material, having much 
of the superior electrical property of macro-crystalline silicon, while 
yet exhibiting superior absorption characteristics close to those of the 
amorphous form. It is believed significant for this hypothesis that 
heating of the substrate is found to improve the effect, since again 
increased mobility results from the increased temperature. The substrate 
temperature cannot be raised too far or the crystallisation will also 
proceed too far and the absorption coefficient adversely affected. The 
preferred maximum value for the substrate is therefore only 350.degree. 
C., more preferably 200.degree. C., even though the usual temperature for 
the production of the crystalline form is over 500.degree. C.