Photovoltaic cell of semi-conducting diamond

A diamond or diamond-like material containing a p-type semi-conducting layer, an n-type semi-conducting layer and a p-n junction for use as a photovoltaic cell, particularly a solar cell. The material may be included in a suitable unit for generating electric power when exposed to appropriate radiation.

BACKGROUND OF THE INVENTION 
This invention relates to semi-conducting diamond. 
Solar cells are photovoltaic cells which convert solar energy into 
electrical energy. These cells contain a p-n junction in a suitable 
semi-conductor material. Exposure of the p-n junction to radiant energy 
from the sun causes a potential difference to develop which will generate 
a current in a circuit in which the cell is included. 
The semi-conductor materials which are frequently used for solar cells are 
silicon and germanium. However, such cells suffer from the disadvantage 
that a very small percentage of the radiant energy absorbed is converted 
into electric energy. 
SUMMARY OF THE INVENTION 
The present invention provides a diamond or diamond-like material 
containing a p-type semi-conducting layer and an n-type semiconducting 
layer and a p-n junction for use as a photovoltaic cell. 
According to another aspect of the invention there is provided a unit for 
generating electric power including a diamond or diamond-like material 
containing a p-type semi-conducting layer, an n-type semi-conducting layer 
and a p-n junction, one of the layers being exposed to radiation, ohmic 
contacts on each layer and an electrical conductor connecting the two 
contacts to an electrical circuit.

DETAILED DESCRIPTION OF THE INVENTION 
Diamond materials containing p-type and n-type semi-conducting properties 
are known. Diamonds having p-type semi-conducting properties are known 
naturally, i.e. diamonds of type IIb, and can also be produced 
synthetically by doping non-conducting diamond with dopant atoms such as 
atoms of Group IIIa. Diamonds having n-type semi-conducting properties may 
be produced by doping non-conducting diamond, for example ion implanting 
lithium or by damaging the non-conducting diamond by ion implanting with 
carbon or other ions. 
The material is preferably thin, having a thickness of less than 0.5 mm. 
The photovoltaic cell will typically be a solar cell, i.e. a cell which 
converts solar radiation into electric energy. 
In the unit, it is preferred that the p-type conducting layer is exposed to 
the radiation. 
It has been found that photovoltaic cells containing diamond p-n junctions 
develop a very much higher voltage than similar cells utilising silicon or 
germanium as the semi-conductor material. 
The diamond p-n junction can be produced by taking diamond having p- or 
n-type semi-conducting properties and creating a layer of diamond thereon 
having the other type of semi-conducting property. 
One preferred method of producing a diamond p-n junction is to take diamond 
of type IIb and implant carbon ions into a surface thereof at a suitable 
temperature. The implanted layer has n-type semiconducting properties due 
to the re-structuring of the metastable diamond. Implanting such carbon 
ions in a surface of the diamond is a known technique involving bombarding 
the diamond with energetic ions, which are preferably positive ions. The 
ions penetrate the diamond substrate and also form a thin film on and in 
the surface of the substrate. The apparatus for carrying out the method 
includes a source for producing a beam of ions, an accelerator for 
accelerating the ions and a filter for screening out unwanted ions from 
the beam. A wide range of ion energies from a few eV to MeV can be 
employed. The ion implantation takes place at a temperature below the 
temperature at which diamond growth occurs for carbon ions), excluding the 
temperature generation by the beam itself. 
The restructured n-type layers may also be obtained by implanting other 
suitable ions which at similar dose and temperature will create a similar 
intermediate n-type conducting structure. Even rare gas ions may be used. 
Ions of the Group Va adds to the n-type conductivity when used. 
The n-type layer produced in this way may have a color which varies from 
light brown to pitch black and therefore one achieves absorption of light 
over the full spectrum. By a suitable choice of dose and temperature of 
diamond, post-implantation annealing and/or ion energies, the n-type layer 
may be created to have exactly the correct absorption coefficient in order 
to absorb all the radiation left after absorption of the ultraviolet 
radiation in the p-type layer preceding it. Furthermore, the cell can be 
engineered to ensure that all this absorption occurs in the depletion 
layer on the n-type side of the p-n junction, thus making it more 
efficient. 
A thin film of semi-conducting diamond can be produced by depositing a thin 
film of a diamond-like material on a substrate, using for example, the 
method described in U.S. Pat. No. 3,961,103, at the same time doping the 
diamond-like material with atoms suitable to give p- or n-type 
semi-conducting properties, and subjecting the diamond-like material to 
temperatures and pressures in the diamond stable region of a carbon phase 
diagram. The substrate is thus provided with a thin layer of 
semi-conducting diamond which can then have a layer of the other type of 
semi-conducting material produced thereon. Doping of the diamond-like 
layers with boron, phosphorus, or arsenic may be achieved using diborane, 
phosphine, or arsine in a glow discharge of a carbon-containing gas such 
as methane. 
As an example, a type IIb diamond of thickness 0.5 mm was implanted with 
carbon ions at energies of 170, 106, 60, and 30 kev at doses of 
3.44.times.10.sup.16 ions/cm.sup.2, 2.99.times.10.sup.16 ions/cm.sup.2, 
2.30.times.10.sup.16 ions/cm.sup.2 and 1.5.times.10.sup.16 ions/cm.sup.2, 
respectively. The diamond was held at a temperature of 300.degree. C. 
Electrical contacts were made using gold on the n-type side and silver on 
the p-side. When held in the sunlight an open circuit voltage larger than 
1.4 volts was registered. By thinning the diamond and providing better 
electrical contact the efficiency can be improved. 
The diamond of type IIb may be of natural origin or it may be produced 
synthetically. 
The diamond or diamond-like material described above may be included in an 
electrical circuit which is capable of producing useful electric energy. 
An example of such a circuit is illustrated by the attached drawing. 
Referring to this drawing, there is shown a diamond or diamond-like 
material 10 having a p-type semi-conducting layer 12 and an n-type 
semi-conducting layer 14 in contact with each other along line 16. The 
material 10 is thus a photovoltaic cell, with a p-n junction along line 
16. Each layer has an ohmic contact 18 and the two contacts 18 are 
electrically connected to an electrical circuit through conductor 20. In 
use, it is the p-type semi-conducting layer 14 which is exposed directly 
to the radiant energy.