Stable photovoltaic devices and method of producing same

An improved photovoltaic device characterized by long term stability in its photoconversion ability. The device is adapted to absorb incident light throughout a substantial portion of the bulk of the photoactive region thereof in a substantially uniform manner. Said uniform absorption of light is provided by grading the band gap of at least a portion of the semiconductor material of the photoactive region thereof such that the graded portions most proximate the light incident surface of the photovoltaic device have a wider band gap than do those portions more distal from the light incident surface. The band gap gradation may be smooth or stepped, and may be accomplished by compositional variation of the semiconductor materials forming the photoactive region. A method for fabricating the stable photovoltaic device of the instant invention is also provided.

FIELD OF THE INVENTION 
This invention relates generally to photovoltaic devices, and more 
particularly to thin film photovoltaic devices which (1) include a body of 
semiconductor material adapted to provide a photoactive region for 
generating charge carrier pairs in response to photons of incident light, 
and (2) are characterized by long term photogenerative stability under a 
variety of operating conditions. The principles of the instant invention 
are particularly well adapted for the fabrication of p-i-n type 
photovoltaic devices which include, as their photoactive element, at least 
one layer of amorphous semiconductor alloy material containing silicon 
and/or germanium. 
BACKGROUND OF THE INVENTION 
According to the principles of the instant invention, there are disclosed 
photovoltaic devices and method for the fabrication of photovoltaic 
devices which exhibit a substantially uniform absorption of photons from 
the solar spectrum (typically Global AM 1.5 illumination), and 
consequently a uniform distribution of charge carriers throughout at least 
a substantial portion of the bulk of the photoactive region thereof. As a 
result, the peak rate of charge carrier (defined as electron-hole pair) 
recombination is reduced and the photovoltaic devices are rendered less 
sensitive to the effects of light induced defects formed therein. Long 
term photoconversion stability is thereby improved. 
Recently, considerable efforts have been made to develop systems for 
depositing amorphous semiconductor materials, each of which can encompass 
relatively large areas, and which can be doped to form p-type and n-type 
materials for the production of p-i-n type photovoltaic devices which are, 
in operation, substantially equivalent to their crystalline counterparts. 
It is to be noted that the term "amorphous", as used herein, includes all 
materials or alloys which have no long range order, although they may have 
short or intermediate range order or even contain, at times, crystalline 
inclusions. 
It is now possible to prepare amorphous silicon alloys by glow discharge or 
vacuum deposition techniques, said alloys possessing (1) acceptable 
concentrations of localized defect states in the energy gaps thereof, and 
(2) high quality electrical and optical properties. Such deposition 
techniques are fully described in U.S. Pat. No. 4,226,898, entitled 
Amorphous Semiconductors Equivalent To Crystalline Semiconductors, issued 
to Stanford R. Ovshinsky and Arun Madan on Oct. 7, 1980; U.S. Pat. No. 
4,217,374, of Stanford R. Ovshinsky and Masatsugu Izu, which issued on 
Aug. 12, 1980, also entitled Amorphous Semiconductors Equivalent To 
Crystalline Semiconductors; and U.S. Pat. No. 4,517,223 of Stanford R. 
Ovshinsky, David D. Allred, Lee Walter, and Stephen J. Hudgens entitled 
Method Of Making Amorphous Semiconductor Alloys And Devices Using 
Microwave Energy, which issued on May 14, 1985. As disclosed in these 
patents, which are assigned to the assignee of the instant invention and 
the disclosures of which are incorporated by reference, fluorine 
introduced into the amorphous silicon semiconductor layers operates to 
substantially reduce the density of the localized defect states therein 
and facilitates the addition of other alloying materials, such as 
germanium. 
The concept of utilizing multiple cells, to enhance photovoltaic device 
efficiency, was described at least as early as 1955 by E. D. Jackson in 
U.S. Pat. No. 2,949,498 issued Aug. 16, 1960. The multiple cell structures 
therein disclosed utilized p-n junction crystalline semiconductor devices. 
Essentially, the concept employed different band gap devices to more 
efficiently collect various portions of the solar spectrum and to increase 
open circuit voltage (Voc). The tandem cell device (by definition) has two 
or more cells with the light directed serially through each cell. In the 
first cell a large band gap material absorbs only the short wavelength 
light, while in subsequent cells smaller band gap materials absorb the 
longer wavelengths of light which pass through the first cell. By 
substantially matching the generated currents from each cell, the overall 
open circuit voltage is the sum of the open circuit voltage of each cell, 
while the short circuit current thereof remains substantially constant. 
However, it is virtually impossible to match crystalline lattice constants 
as is required in the multiple cell structures of the prior art. 
Therefore, tandem cell structures cannot be practically fabricated from 
crystalline materials in a manner which would have commercial production 
ramifications. As the assignee of the instant invention has shown, 
however, such tandem cell structures are not only possible, but can be 
economically fabricated in large areas by employing amorphous materials. 
The multiple cells preferably include a back reflector for increasing the 
percentage of incident light reflected from the substrate back through the 
semiconductor layers of the cells. It should be obvious that the use of a 
back reflector, by increasing the use of light entering the cell, 
increases the operational efficiency of the multiple cells. Accordingly, 
it is important that any photoresponsive layer of semiconductor material 
deposited atop the light incident surface of the substrate be transparent 
so as to pass a high percentage of incident light from the reflective 
surface of the back reflector through the layers of semiconductor 
material. 
Unlike crystalline silicon which is limited to batch processing for the 
manufacture of solar cells, amorphous silicon alloys can be deposited in 
multiple layers over large area substrates to form solar cells in a high 
volume, continuous processing system. Such continuous processing systems 
are disclosed in the following U.S. Pat. No. 4,400,409, for A Method Of 
Making P-Doped Silicon Films And Devices Made Therefrom; U.S. Pat. No. 
4,410,588, for Continuous Amorphous Solar Cell Production System; U.S. 
Pat. No. 4,438,723, for Multiple Chamber Deposition And Isolation System 
And Method; U.S. Pat. No. 4,492,181 for Method and Apparatus For 
Continuously Producing Tandem Amorphous Photovoltaic Cells; and U.S. Pat. 
No. 4,485,125 for Method and Apparatus For Continuously Producing Tandem 
Amorphous Photovoltaic Cells; and pending U.S. patent application Ser. No. 
244,386 filed Mar. 16, 1981 for continuous Systems For Depositing 
Amorphous Semiconductor Material. As disclosed in these patents and 
application, a substrate may be continuously advanced through a succession 
of deposition chambers, wherein each chamber is dedicated to the 
deposition of a specific semiconductor material. In making a photovoltaic 
device of p-i-n type configurations, the first chamber is dedicated for 
depositing a p-type semiconductor alloy, the second chamber is dedicated 
for depositing an intrinsic amorphous semiconductor alloy, and the third 
chamber is dedicated for depositing an n-type semiconductor alloy. Since 
each deposited semiconductor alloy, and especially the intrinsic 
semiconductor alloy, must be of high purity; every possible precaution is 
taken to insure that the sanctity of the vacuum envelope formed by the 
various chambers of the deposition apparatus remains uncontaminated by 
impurities, regardless of origin. 
The layers of semiconductor alloy material thus deposited in the vacuum 
envelope of the deposition apparatus may be utilized to form 
photoresponsive devices, such as, but not limited to photovoltaic cells 
which include one or more p-i-n cells or one or more n-i-p cells, Schottky 
barriers, photodiodes, phototransistors, or the like. Additionally, by 
making multiple passes through the succession of deposition chambers, or 
by providing an additional array of deposition chambers, multiple stacked 
cells of various configurations may be obtained. 
As should be apparent by the foregoing, thin film amorphous semiconductor 
materials offer several distinct advantage over crystalline materials, 
insofar as they can be easily and economically fabricated into large area 
photoresponsive devices by newly developed mass production processes. 
However, heretofore produced amorphous silicon based semiconductor 
materials were prone to degrade as a result of prolonged exposure to 
light. This process, termed "photodegradation", or "Staebler-Wronski 
degradation," although not fully understood, is believed to be due to the 
fact that long-term exposure to a photon flux tends to break the bonds 
between the constituent atoms of the semiconductor material, thereby 
resulting in the formation of defect states in the band gap, such as 
dangling bonds, which are detrimental to the photovoltaic efficiency of a 
photoresponsive device which incorporates the degraded semiconductor 
material. It has further been observed that photogenerated defects may be 
annealed out of a sample of degraded semiconductor material by exposing 
said sample to elevated temperatures; for example, temperatures of 
approximately 150.degree. degrees for several hours. Samples of 
semiconductor material, thus degraded by operational exposure to light and 
subsequently annealed, are restored to approximately the same level of 
photovoltaic performance which they exhibited prior to said operational 
degradation. 
It is somewhat parodoxical that the higher the initial (pre-operational) 
quality of the photovoltaic semiconductor material, (1) the greater the 
effect of photodegradation thereupon, and (2) the greater the 
operation-dependent loss of efficiency exhibited by a photovoltaic device 
incorporating such higher quality semiconductor materials. The reason for 
this phenomenon is that lower quality photovoltaic semiconductor material 
initially includes a relatively high number of defect states therein and 
consequently, the formation of additional defect states in the energy gap 
thereof via photodegradation is not as significant as for a higher quality 
semiconductor material which is initially characterized by a relatively 
low number of defect states. Because of the fact that the assignee of the 
instant invention is now able to commercially manufacture high quality 
photovoltaic semiconductor materials exhibiting a low initial density of 
defect states, in a high volume, continuous production process, the 
problem of photodegradation of photovoltaic devices fabricated therefrom 
has become increasingly significant. The practical ramifications of the 
foregoing is that the consumer is not interested in expending large sums 
of money on photovoltaic energy generating systems which will lose upwards 
of 20% efficiency over their operating life. 
Heretofore, the effects of photodegradation were dealt with by either (1) 
annealing the semiconductor material to remove the defect states and 
restore its electrical generating capacity, or (2) ignoring the defect 
states and allowing the semiconductor material to operate at less than 
full efficiency. Neither of the aforementioned options is commercially 
acceptable. While annealing does restore photodegraded cells to their 
initial operating efficiency, it necessitates the inclusion of additional 
hardware in a photovoltaic power system at additional cost to the 
consumer, and might also entail the periodic expenditure of labor. Several 
methods of annealing have been proposed. In one such method the annealing 
procedure may be instituted on a cyclic basis wherein the semiconductor 
material is periodically, typically at an interval of months to years, 
heated to an elevated temperature for a period of time sufficient to 
remove the defect states therein and restore the initial efficiency 
thereof. The heating may be carried out in situ by including a heat source 
in the photovoltaic installation, or the semiconductor material may be 
removed from the point of use and heated in an oven. In an alternative 
process, the semiconductor material may be continuously annealed by 
incorporating said material into a solar collector panel, which panel is 
adapted to collect and retain the solar thermal energy incident thereupon. 
In such an arrangement the semiconductor material is maintained, during 
normal operation, at the elevated annealing temperature and the formation 
of defect states as well as the annealing of those defect states, occurs 
simultaneously. Depending upon the operating temperature of the 
semiconductor material, overall degradation can be prevented or 
substantially slowed down. Such methods and techniques of continuous 
annealing are disclosed in U.S. patent application Ser. No. 636,172 of 
Vincent D. Cannella entitled, "Photovoltaic Panel Having Enhanced 
Conversion Efficiency Stability", filed July 31, 1984 and assigned to the 
assignee of the instant invention, the disclosure of which application is 
incorporated herein by reference. 
As mentioned supra, in the second alternative, the amorphous photovoltaic 
devices which incorporate the semiconductor material are simply allowed to 
photodegrade. The rate of photodegradation for a particular device 
configuration may be readily ascertained, and the power requirement for a 
given installation may be therefore readily specified to account for the 
degree of photodegradation expected during the operational life of the 
photovoltaic devices. For example, it may be predicted that a particular 
photovoltaic device will degrade to 80% of its initial electrical 
performance within a period of 10 years of operation; therefore a built-in 
excess capacity of 20% may be incorporated in the initial installation to 
account for this subsequent loss. While such an approach is relatively 
simple and may be acceptable for a variety of photovoltaic installations, 
it is obviously a less than adequate solution to the problem, and 
represents an intolerable solution for many other uses. In installations 
in which space for solar collection is at a premium, it is clearly 
desirable to have the photovoltaic devices operate at their maximum 
capacity at all times. In other installations reliability and consistency 
of electrical power generated by and delivered from the devices is 
required. In such installations, the photovoltaic devices must be 
fabricated from semiconductor material which is relatively consistent in 
its conversion efficiency throughout the expected operational lifetime 
thereof. 
As should be appreciated from the foregoing discussion, it would be highly 
advantageous and commerically necessary to provide a thin film amorphous 
photovoltaic device fabricated from semiconductor material which does not 
require the inclusion of extraneous hardware therewith, but which is 
nonetheless capable of maintaining a consistently high conversion 
efficiency under long term, high photon flux operating conditions, i.e., 
does not markedly degrade when exposed to light. 
As previously stated, the mechanism of photodegradation of amorphous 
photovoltaic semiconductor materials is not fully understood; however, it 
is believed that said photodegradation involves the production of a wide 
distribution of defect states in the band gap of the semiconductor 
material. The term "defects", or "defect states" as generally used by 
routineers in the field of amorphous semiconductor materials, is a broad 
term generally including all deviant atomic configurations such as: broken 
bonds, dangling bonds, bent bonds, strained bonds, vacancies, microvoids, 
etc. In a photovoltaic device, a charge carrier pair (i.e. an electron and 
a hole) is generated in response to the absorption of photons from 
incident radiation in the photoactive region of the semiconductor material 
thereof. Under the influence of an internal electrical field established 
by the doped layers of semiconductor material of the photovoltaic device, 
such as a solar cell, the charge carriers are drawn toward opposite 
electrodes of the cell causing the positively charged holes to collect at 
the positive electrode and the negatively charged electrons to collect at 
the negative electrode thereof. Under ideal operating conditions, every 
photogenerated charge carrier will be conducted to its respective 
collection electrode. However, operating conditions are not ideal and the 
loss of charge carriers occurs to some degree in all photovoltaic cells. 
The primary charge carrier collection loss is due to charge carrier 
recombination, wherein an electron and a hole reunite. Obviously, charge 
carriers that reunite or recombine are not available for electrode 
collection and the resultant production of electrical current. Defects or 
defect states that occur in the photoactive region of the semiconductor 
material of the photovoltaic device provide recombination centers which 
facilitate the reunion and recombination of electrons and holes. 
Therefore, the more defects or defect states that are present in the 
semiconductor material of a device, the higher the rate of charge carrier 
recombination therein. Accordingly, charge carrier collection efficiency 
decreases as the rate of charge carrier recombination increases within the 
photoactive region of a given semiconductor material; an increase in the 
number of defect states is therefore, at least partially responsible for 
an increase in the rate of charge carrier recombination and a 
concomanitant decrease in photovoltaic cell conversion efficiency. 
In photovoltaic cells of the type which comprise a layer of intrinsic 
amorphous semiconductor material having a layer of p-type semiconductor 
material disposed on one side thereof and a layer of n-type semiconductor 
material disposed on the other side thereof, referred to hereinafter as 
p-i-n type photovoltaic cells, applicants have observed a dramatic 
decrease in blue response (conversion of photons from the blue portion of 
the solar spectrum into electrical current) in the photoactive regions of 
the semiconductor material thereof relative to the red response 
(conversion of photons from the red portion of the solar spectrum into 
electrical current) in the photoactive regions of the semiconductor 
material thereof, upon photodegradation. That is to say, when such 
photovoltaic cells are exposed to a high intensity photon flux, the 
photoconversion efficiency measured under blue illumination decreases much 
more than the photoconversion efficiency measured under red illumination. 
The terms "blue illumination" or "blue light" are defined herein as having 
a wavelength within the approximate range of 350 to 550 nanometers; and 
the terms "red illumination" or "red light" are defined herein as photons 
having a wavelength within the approximate range of 550 to 750 nanometers. 
It is known that the absorption coefficient (the rate at which photons 
absorbed as they generate electron-hole pairs) of blue light in amorphous 
silicon alloy materials is greater than the absorption coefficient of red 
light in amorphous silicon alloy materials; therefore, under illumination 
equivalent to standard terrestrial conditions blue light is almost totally 
absorbed in the first thousand angstroms of the photoactive region of the 
semiconductor material of the photovoltaic cell, whereas the absorption of 
red light occurs more uniformly throughout the bulk of said photoactive 
region. Therefore, under blue illumination a very high density of charge 
carriers is generated in the first thousand angstroms of the photoactive 
region of the cell. In an undegraded photovoltaic cell, few defect states 
are present to provide recombination sites, and consequently the charge 
carriers are efficiently collected by the respective electrodes of the 
cell despite the high density thereof. However, if the photovoltaic cell 
is photodegraded, the high density of defect states therepresent provides 
recombination centers which facilitate the recombination of electrons and 
holes. Furthermore, the high density of electrons and holes created under 
blue illumination is conducive to, and greatly facilitates said 
recombination, since statistically, an electron and a hole are more likely 
to be reunited at a recombination center under high density conditions. 
Therefore, the collection efficiency of charge carriers, and the resultant 
overall cell performance under blue illumination is correspondingly 
decreased. 
Photons of red illumination passing through the semiconductor material of a 
photovoltaic cell are not as readily absorbed as are photons of blue 
illumination. Consequently, said red photons penetrate a further distance 
through the bulk of the photoactive region of the semiconductor material 
of the photovoltaic cell. Charge carriers generated by the absorption of 
photons from illumination by the red portion of the solar spectrum, being 
more uniformly dispersed throughout the bulk of the photoactive region of 
the semiconductor material, are concentrated at a lesser density than are 
the charge carriers generated by the absorption of photons from 
illumination of blue portions of the solar spectrum. While the total 
number charge carriers generated in the semiconductor material of the 
photovoltaic cell under steady state conditions may be substantially equal 
if equal fluxes of red or blue photons enter and are absorbed by the 
semiconductor material of the cell, the peak rate of charge carrier 
recombination under red illumination is lower, since charge carriers 
generated by red illumination are more uniformly dispersed throughout the 
bulk of the semiconductor material, and are, therefore, less likely to 
encounter a charge carrier of opposite polarity at a recombination center 
than are charge carriers generated by blue illumination. In summary: the 
effective lifetime of charge carriers is dependent upon the wavelength of 
the illumination creating them. The lifetime is minimum where 
recombination is maximum, and this point of maximum recombination will 
depend upon the absorption profile of light in the photoactive region of 
the semiconductor material of a photovoltaic device. 
Based upon the observations enumerated hereinabove, applicants conclude 
that it is necessary to promote the more uniform absorption of all photons 
of light from the solar spectrum throughout the bulk of the photoactive 
region of the semiconductor material, especially the absorption of photons 
of blue light. In this manner, more uniform generation of charge carriers 
throughout the bulk of the photoactive region of the semiconductor 
material is promoted. High charge carrier density in a narrow portion of 
the photoactive region is thus prevented, and the rate of charge carriers 
recombination at defect sites is decreased. The result is the fabrication 
of a photovoltaic cell exhibiting increased operational tolerance to 
defect states and hence improved stability (which translates into 
increased photogenerative efficiency). 
It is well known that when a beam of light passes through a homogeneous 
absorbing medium, the intensity of that beam of light decreases 
exponentially with the absorption thereof throughout the medium, the 
greatest absorption occuring most proximate the light incident surface of 
the medium. According to the principles of the instant invention, there is 
provided a photovoltaic device exhibiting uniform absorption of light 
throughout the photoactive region of the semiconductor material thereof. 
As used herein, the term "uniform absorption of light" will refer to an 
absorption of light that deviates from the aforementioned exponential 
pattern. For example, the decrease in light intensity as said light 
travels through the absorbing medium may be linear, that is to say, at a 
hypothetical point half way through the bulk of the absorbing medium, a 
beam of light will have half the intensity of said beam of light at the 
light incident surface of the absorbing medium. Of course, the term 
"uniform absorption of light" is not meant to be solely limited to light 
absorption which follows such a linear relationship, but, rather, is meant 
to include all light absorption patterns that deviate from the normal 
exponential attenuation of light through a homogeneously absorbing medium. 
It is the essence of the instant invention to promote the uniform 
absorption by forming those regions of the absorbing medium most proximate 
the light incident surface thereof more transparent to incident radiation 
than those portions of the absorbing medium more distal from said light 
incident surface. Techniques for, and photovoltaic structures produced by, 
promoting the uniform absorption of light throughout the photoactive 
region of the semiconductor material of a photovoltaic cell will be 
described in greater detail hereinbelow. 
More particularly, the instant invention provides for the uniform 
absorption of light throughout at least a substantial portion of the bulk 
of the photoactive region of the semiconductor material of a photovoltaic 
device by grading the band gap of the intrinsic layer of that device. A 
graded band gap intrinsic layer is one in which the band gap of the 
semiconductor material from which that layer is fabricated varies 
spatially; i.e., the band gap of the intrinsic semiconductor material 
taken in planes parallel to the plane of the intrinsic layer will vary 
relative to the thickness of that intrinsic layer. According to the 
principles set forth herein, the band gap of the intrinsic layer is graded 
so as to provide a relatively wide band gap region (for instance 1.9 eV) 
proximate the light incident surface of the photoactive region of the 
semiconductor material of the photovoltaic device and a narrower band gap 
region (for instance 1.7 eV) in the path of travel of the beam of incident 
light from the light incident surface. It should be noted that the term 
"graded band gap", as used herein, is intended to include any change in 
the band gap of the semiconductor material relative to the thickness 
thereof and specifically includes (1) a smooth variation in the band gap, 
be it a linear or nonlinear change, (2) a stepped variation in the band 
gap wherein said band gap varies in a series of two or more discrete 
steps, as well as (3) any combination of smooth and stepped band gap 
gradations. By grading the band gap of the intrinsic layer of 
semiconductor material of a p-i-n-type photovoltaic device, absorption of 
incident light, especially the more easily absorbed blue light, is spaced 
throughout at least a substantial portion of the bulk of the photoactive 
region thereof and the effective lifetime of charge carriers resulting 
from the absorption of photons from that illumination is increased. The 
result is the fabrication of an improved photovoltaic device exhibiting 
increased tolerance to defect states in the band gap thereof. It should be 
noted that it is not necessary to grade the band gap of the entire layer 
of intrinsic semiconductor material in order to promote said uniform 
photon absorption throughout substantially all of the bulk of the 
photoactive region. This is because the strongest photon absorption in 
amorphous silicon or amorphous silicon:germanium alloy layers occurs in a 
relatively narrow (i.e. 1000-2000 angstrom) portion of those layers; 
accordingly, by rendering this portion of the photoactive semiconductor 
layer relatively more transparent to incident radiation the uniform 
absorption of photons, as hereinabove defined, is promoted. 
As mentioned supra, in the preferred embodiment band gap grading is 
accomplished by compositionally varying the intrinsic semiconductor 
material. For example, a band gap broadening element may be added to at 
least the portion of the photoactive intrinsic semiconductor material most 
proximate the light incident surface of the photovoltaic device to render 
those portions least absorbtive of incident radiation. Alternatively, the 
layer of intrinsic semiconductor material may be initially formed of a 
relatively wide band gap semiconductor material and a band gap narrowing 
element added to those portions of the photoactive region of that 
intrinsic layer most distal from the light incident surface of said 
photovoltaic device. By controlling the concentration of the band gap 
modifying element added to any given portion of the intrinsic layer, the 
width of the band gap thereof may be controlled. By smoothly varying the 
concentration of the band gap modifying element in the intrinsic 
semiconductor material, a smoothly graded band gap may be achieved. 
Similarly, by varying the concentration of the band gap modifying element 
in the previously described stepped manner, a stepped gradation of the 
band gap of the intrinsic semiconductor material may be achieved. Such 
techniques of band gap grading (also referred to as "profiling") will be 
described in greater detail hereinbelow. 
A photovoltaic device having a varying band gap in a relatively narrow 
portion of the photoactive region of the layer of intrinsic semiconductor 
material is disclosed in a paper entitled, Achievement Of Higher 
Efficiency Amorphous Silicon-Germanium Solar Cells Using Affinity 
Gradients, presented by S. Wiedeman and E. A. Fagen at the 17th Annual 
I.E.E.E. Photovoltaic Specialists Conference held May 1-4, 1984 in 
Kissimmee, Fla. Disclosed therein is a n-i-p-type photovoltaic device 
formed of an amorphous silicon-germanium alloy in which the composition of 
the intrinsic semiconductor layer was profiled over the first few hundred 
angstroms from the light incident surface thereof. This band gap variation 
was accomplished by gradually altering the ratio of silicon to germanium 
in those few hundred angstroms. The object of such band gap variation is 
to establish an electrical field of varying strength adjacent the light 
incident surface of the intrinsic semiconductor material, which field is 
adapted to eliminate charge carrier losses at the interface of the n doped 
and intrinsic layer interface due to back diffusion of those charge 
carriers across the n and intrinsic interface. The authors of the paper 
believed that, because of the electrical field, a 29% improvement in the 
initial conversion efficiency of the photovoltaic devices was achieved. It 
should be noted that no claim was implicitedly or explicitely presented by 
the authors of the aforementioned paper for the improved long term 
stability of photovoltaic devices thus fabricated. This may be due to the 
fact that it is not possible to achieve improved long term stability from 
the reported structure since, as described hereinabove, losses in 
conversion efficiency are due to the bulk recombination of charge carriers 
at defect sites rather than a surface phenemonon such as back diffusion at 
semiconductor material layer interfaces. The method described by Wiedeman, 
et al has, as its object, the elimination of the back diffusion of charge 
carriers across the n doped and intrinsic layer interface, and, 
accordingly, compositional variation of the intrinsic semiconductor 
material is restricted to the immediate vicinity of that interface. By 
limiting the profile of the intrinsic semiconductor layers to, at best, 
the first few hundred angstroms, the majority of blue light would still be 
absorbed adjacent the light incident surface of that layer. Therefore, the 
method disclosed by Wiedeman, et al is not intended to and does not have 
any effect upon the recombination of charge carriers throughout the bulk 
of at least a portion of the intrinsic semiconductor material due to the 
presence of defect sites, such as those caused by photogradation. 
As should be apparent from the foregoing discussion, there exists a 
definite need for an amorphous thin film photovoltaic device which (1) 
exhibits long term stability in the conversion efficiency thereof, (2) 
does not require annealing to achieve that stability, and (3) does not 
necessitate the inclusion of extraneous hardware to maintain that 
stability. The instant invention provides such an amorphous thin film 
photovoltaic device in which the absorption of light in at least a 
substantial portion of the photoactive region of the semiconductor 
material thereof is substantially uniform. The photovoltaic device of the 
instant invention is tolerant of photoinduced or photogenerated defects 
formed during operation and exhibits long term stability in its 
photoconversion efficiency. 
These and many other advantages of the instant invention will be apparent 
from the drawings, the detailed description of the invention and the 
claims which follow. 
BRIEF DESCRIPTION OF THE INVENTION 
There is disclosed herein a photovoltaic device of the type comprising a 
body of semiconductor material having a first electrode in electrical 
communication with a first surface thereof, said electrode forming the 
light incident surface of the device, and a second electrode in electrical 
communication with a second surface of the semiconductor body. In one 
embodiment, the semiconductor body includes at least one triad of layers 
of semiconductor material, each triad comprising a layer of intrinsic 
semiconductor material having a layer of p-type semiconductor material 
disposed in contact with a first surface thereof and a layer of n-type 
semiconductor material disposed in contact with a second surface thereof. 
The layers of intrinsic semiconductor material of each at least one triad 
are adapted to provide a photoactive region for absorbing photons from 
incident light and generating charge carrier pairs in response thereto. 
The improvement in the photovoltaic device of the instant invention 
relates to the fabrication of the intrinsic semiconductor material in a 
manner which promotes the substantially uniform absorption of photons of 
incident light throughout at least a substantial portion of the bulk of 
the photoactive region of at least one of the triads of layers, so as to 
generate electron-hole pairs throughout said substantial portion of the 
bulk and reduce charge carrier recombination in the intrinsic 
semiconductor material, thereby providing a photovoltaic device which 
exhibits long term stability. The uniform absorption of light may be 
provided by grading the band gap throughout at least a portion of the bulk 
of the layer of intrinsic semiconductor material such that the portion of 
said intrinsic layer having the widest band gap is most proximate the 
light incident surface of the photovoltaic device. The band gap may be 
graded by varying the composition of the intrinsic layer of semiconductor 
material with respect to the distance from the light incident surface of 
that intrinsic layer. This is accomplished through the addition of band 
gap broadening elements such as nitrogen, oxygen, carbon, and mixtures 
thereof. In another preferred embodiment of the invention, wherein tandem 
photovoltaic cells are fabricated, the band gap of the intrinsic 
semiconductor material is graded by the addition of band gap narrowing 
elements such as germanium, tin, lead, and mixtures thereof. The 
principles of the instant invention are not limited to p-i-n-type cells, 
but, rather, are readily applicable to other types of photoresponsive 
devices, such as p-n devices. 
According to the method of the instant invention, tandem photovoltaic 
devices comprising several triads of layers of improved semiconductor 
material may be fabricated. The intrinsic layer of semiconductor material 
of one or more of triads is fabricated with a graded band gap so as to 
provide for uniform absorption of incident light throughout at least a 
significant portion of the bulk of the photoactive region of the 
semiconductor material so as to fabricate a high efficiency photovoltaic 
device exhibiting long term stability.

DETAILED DESCRIPTION OF THE DRAWINGS 
I. The Photovoltaic Device 
Referring now to the drawings and particularly to FIG. 1, a photovoltaic 
cell formed of a plurality of successive p-i-n layers, each of which 
includes a semiconductor alloy, is shown generally by the reference 
numeral 10. The cell 10 includes the graded band gap intrinsic layer of 
the instant invention, and thus is representative of the type of 
photoresponsive device in which the instant invention may be 
advantageously employed. 
More particularly, FIG. 1 shows a p-i-n type photovoltaic device such as a 
solar cell 10 made up of individual p-i-n type cells 12a, 12b, and 12c. 
Below the lowermost cell 12a is a substrate 11 which also functions as the 
bottom electrode of the ce11 10. The substrate 11 may be formed of a 
metallic material such as thin stainless steel or aluminum, or it may be 
formed from a thin electroformed member such as nickel. Alternatively, the 
substrate 11 may be formed from an insulating material such as glass or 
synthetic polymers, with an electrically conductive electrode layer formed 
thereupon. Although certain applications may require a thin oxide layer 
and/or a series of base contacts and/or a reflecting layer prior to the 
deposition of the semiconductor material, for purposes of this 
application, the term, "substrate" shall include any elements added 
thereto by preliminary processing. 
Each of the cells, 12a, 12b, and 12c is fabricated with an amorphous 
semiconductor body containing at least a silicon or germanium alloy. Each 
of the semiconductor bodies includes an n-type conductivity semiconductor 
layer 20a, 20b, and 20c; a graded band gap intrinsic semiconductor layer 
18a, 18b, and 18c; and a p-type conductivity semiconductor layer 16a, 16b, 
and 16c. As illustrated, cell 12b is an intermediate cell and, as 
indicated in FIG. 1, additional intermediate cells may be stacked atop the 
illustrated cells without departing from the spirit or scope of the 
present invention. 
It is to be understood that following the deposition of the layers of 
semiconductor material, a further deposition process may be either 
performed in a separate environment or as a part of a continuous process. 
In this step, a TCO (transparent conductive oxide) layer 22, formed in the 
preferred embodiment of indium tin oxide, also referred to herein as the 
top or upper electrode, is deposited atop the uppermostlayer of 
semiconductor material. An electrode grid 24 may be applied to the device 
where the cell is of a sufficiently large area, or if the conductivity of 
the TCO layer 22 is insufficient. The grid 24 shortens the charge carrier 
path through the TCO and thus increases the collection efficiency. 
While the photovoltaic cell illustrated in FIG. 1 is a stacked assembly of 
p-i-n cells, it should be apparent that the instant invention may also be 
employed with other cell arangements such as single p-i-n cells, or 
stacked or single n-i-p or p-n cells. 
II. The Device As Modified By The Instant Invention 
It has been previously stated that the recombination of charge carriers is 
a major factor in determining the losses in the conversion efficiency of 
photovoltaic devices. In p-i-n-type photovoltaic cells it has been found 
that the recombination of charge carriers near the n-i layer interface 
(i.e. the light incident surface of the intrinsic semiconductor layer) is 
controlled by hole lifetime, since holes definitely represent minority 
charge carriers in this region. Based upon laboratory observation and 
computer simulations of p-i-n photovoltaic devices, it has been determined 
that effective hole lifetime depends upon the wavelength of incident 
radiation which is responsible for the generation of that hole. Further, 
illumination from the red portion of incident radiation is responsible for 
the generation of holes characterized by longer effective lifetimes than 
the effective lifetime of holes generated by the blue portion of incident 
radiation. It should be noted that strictly speaking, carrier lifetime is 
an inherent bulk property of the particular semiconductor material from 
which the photovoltaic device is fabricated. However, since the 
semiconductor material is extremely thin, bulk properties are not 
manifested; that is to say, the device structure itself modifies the 
inherent properties of the bulk material. Hence, it is more correct to 
speak of "effective carrier lifetimes" when discussing charge carriers in 
a photovoltaic cell. Herein, the terms "carrier lifetime" and "effective 
carrier lifetime" are used interchangably with respect to electrons and 
holes. 
FIG. 2 is a graphic representation of hole (or electron) lifetime in the 
intrinsic semiconductor layer of a p-i-n-type photovoltaic device at 
various positions in that intrinsic layer (the position being taken 
relative to the light incident surface) said holes being generated by both 
red illumination (Curve A) and blue illumination (Curve B). FIG. 2 was 
derived from a computer simulation of a p-i-n photovoltaic device 
illuminated through the n layer with a flux of 10.sup.16 photons per 
cm.sup.2. Plotted along the abscissa is the position, in microns within 
the intrinsic semiconductor layer of the photovoltaic cell; the ordinate 
shows the Log.sub.1O of hole lifetime as measured in seconds. The 
intrinsic layer of amorphous silicon alloy material was determined to 
possess an 
absorption coefficient of 2.times.10.sup.3 cm.sup.-1 with respect to red 
light (see Curve A). Curve B similarly depicts the lifetime of holes 
generated by the absorption of blue radiation in the intrinsic layer of 
amorphous silicon alloy material. Said material was determined to possess 
an absorption coefficient of 2.times.10.sup.5 cm.sup.-1 for blue light. 
It is thus seen that the coefficient for the absorption of blue radiation 
in the amorphous silicon alloy material is two orders of magnitude greater 
than that of the red radiation in the same material. The conclusion to be 
drawn is that blue radiation is more strongly absorbed by the amorphous 
silicon alloy material of a photovoltaic cell than is a similar intensity 
of red radiation. For this reason, most of the blue absorption will occur 
proximate (i.e. within 1000 angstoms) of the n-i interface, (the light 
incident interface), while the red absorption will be more uniformly 
distributed throughout the bulk of the layer of intrinsic semiconductor 
material. As a result of this differential light absorption in the 
intrinsic material, there is a differential distribution of holes 
generated by the absorption of the photons of light. It is this 
differential distribution of holes generated by incident radiation which 
is responsible for the variation in the lifetime of holes generated from 
the absorption of red, vis-a-vis, blue illumination. 
The graph of FIG. 2 clearly illustrates that the lifetime of holes 
generated by the absorption of photons of red light (Curve A) is generally 
greater, and more uniform (throughout the bulk of the intrinsic material 
of the device), than the absorption of photons of blue light (Curve B). As 
was explained in detail in the Background section of this application, 
hole lifetime is dependent upon the density of charge carriers, and the 
density of charge carriers is in turn dependent upon the absorption 
characteristics of the particular semiconductor material employed. 
Obviously, if the absorption chacteristics of blue light of the layer of 
intrinsic semiconductor material could more closely approximate those 
absorption characteristics of red light, the lifetimes of holes generated 
by said blue light would then approximate the lifetimes of holes generated 
by said red light. Further, by matching the lifetimes of blue and red 
generated holes, under operating conditions the blue response of the 
photovoltaic cell would approximate the red response of the cell, which 
would result in a lower loss in efficiency upon photodegradation. That is 
to say, the absorption characteristics of the photovoltaic device would be 
changed to thereby change the effective lifetime for blue and red 
generated charge carriers. Finally, note that at point C, curve B crosses 
Curve A, thus indicating that the lifetime of holes generated by blue 
light is greater in that region of the semiconductor material of the p-i-n 
device than the lifetime of holes generated by red light. This apparently 
anomalous result is of no major significance, since at this point in the 
layer of intrinsic semiconductor material holes are no longer the minority 
carriers, and therefore their effective lifetimes do not control device 
efficiency. 
FIG. 3 is a graph representing the recombination rate for holes (or 
electrons), expressed in units of recombinations per cubic centimeter of 
semiconductor material per second at various positions taken in the 
sandwich direction of a p-i-n type photovoltaic device, for both a 
degraded and an undegraded semiconductor material, under both red and blue 
illumination. In this computer analysis, photodegradation of the 
semiconductor material was simulated by increasing both the minimum 
density of states and the charge carrier capture cross section of the 
recombination centers by an order of magnitude. More particularly, curve 
D, indicated by a dashed line, represents the hole recombination rate of 
the undegraded semiconductor material of a photovoltaic device under red 
illumination. Curve E, also indicated by a dashed line, represents the 
hole recombination rate of the same material under blue illumination. 
Curve F represents the hole recombination rate of the semiconductor 
material of a degraded device under red illumination, and Curve G 
represents the hole recombination rate of the same degraded material under 
blue illumination. As can be seen from even a cursory perusal of the 
figure, the lowest hole recombination rate is exhibited by the 
semiconductor material of an undegraded device under red illumination (see 
Curve D), which recombination rate is relatively uniform throughout the 
thickness of the semiconductor material of the intrinsic layer, neglecting 
variations due to interface defects which occur in the immediate proximity 
of the interfaces between the intrinsic and doped layers. Even after 
photodegradation, the rate of hole recombination under red illumination 
remains relatively uniform and relatively low (see Curve F). Translating 
this data into practical results, the indication is that the semiconductor 
material of the undegraded photovoltaic device under red illumination of 
approximately 10.sup.16 photons per square centimeter per second produced 
a short circuit current of approximately 0.9mA. Under the same conditions 
of illumination, the semiconductor material of the degraded photovoltaic 
device produced a short circuit current of 0.68mA. Thus, the unmistakable 
conclusion is that photodegradation produces approximately a 25% loss in 
current generated by the p-i-n photovoltaic device as measured under red 
illumination. 
Curves E and G are indicative of the photogenerative blue response of the 
intrinsic semiconductor material of the same p-i-n photovoltaic device. 
From those curves it is noted that while the rate of hole recombination is 
relatively high in the undegraded device (Curve E), the rate of hole 
recombination is much higher after degradation (Curve G) of that 
semiconductor material. Therefore, the curves teach that the rate of 
recombination for blue generated charge carriers varies greatly throughout 
the bulk (in the sandwich direction) of the semiconductor material in the 
intrinsic layer, with the highest rate of hole recombination occuring in 
the first third of the intrinsic layer as measured from the n-intrinsic 
(i.e. light incident) interface thereof. This high hole recombination rate 
is reflected in the loss of efficiency exhibited upon photodegradation of 
the photovoltaic device. In specific numerical terms, it has been found 
that the undegraded photovoltaic device represented by Curve E generates a 
short circuit current of 1.14mA upon illumination with a flux of 10.sup.16 
photons per centimeter squared, vis-a-vis, the photodegraded photovoltaic 
device which generates a short circuit current of only 0.11mA under the 
same conditions of illumination. This difference in photogenerated current 
represents a 90% loss in photoconversion efficiency which is directly 
attributable to photodegradation, when that efficiency is measured under 
blue illumination. 
As should be apparent from the foregoing discussion of FIGS. 2 and 3, the 
loss in efficiency of the photovoltaic device upon photodegradation of the 
photoactive semiconductor material thereof is primarily due to the loss of 
blue response in that material; which loss is most severe in approximately 
the first thousand angstroms of the light incident side of the layer of 
intrinsic semiconductor material and is correlatable to the higher 
absorption of photons of blue light by the semiconductor material as 
compared to red illumination. 
In contrast thereto, the instant invention provides a p-i-n-type 
photovoltaic device in which the absorption characteristics of the layer 
of intrinsic semiconductor material is tailored to provide a substantially 
uniform absorption of light therethrough, and especially a uniform 
absorption of blue light throughout a substantial portion of the bulk 
thickness thereof. A photovoltaic device, such as a solar cell, thus 
configured, will, even after degradation, exhibit an overall photoresponse 
(the photogeneration of electrical current) which approximates the 
photoresponse to incident red illumination, vis-a-vis, the photoresponse 
thereof to blue illumination. In this manner, the loss of efficiency of a 
photovoltaic device under blue light is minimized, thus allowing said 
photovoltaic device to operate at optimum efficiencies for prolonged 
periods of time without the necessity of annealing. 
According to the principles of the instant invention, the uniform 
absorption of both blue and red photons of incident illumination in the 
photoactive region of the semiconductor material of a photovoltaic device 
is promoted by grading the band gap of the semiconductor material which 
forms the intrinsic layer of the photovoltaic device. In the graded band 
gap structure, portions of the photoactive region most proximate the light 
incident surface of the photovoltaic device are specifically tailored to 
be more transparent to incident illumination than are those more distal 
portions of the photoactive region thereof. In this manner, incident light 
is absorbed more deeply into the bulk thickness of the photoactive region 
of the device, thereby avoiding the formation of a high density of charge 
carriers proximate the light incident surface thereof. In other words, 
stability of the photovoltaic device is increased by distributing the 
charge carriers more uniformly throughout the bulk thickness of the 
photoactive region thereof. 
FIG. 4 is a graphic representation of a p-i-n-type photovoltaic device, 
illustrating several variations in which the band gap of the intrinsic 
semiconductor material is graded. In prior art photovoltaic devices the 
band gap of the layer of intrinsic semiconductor material was constant, 
that is to say, the band gap of the layer did not vary with the thickness 
thereof. The energy of the band gap of such a uniformly graded layer of 
semiconductor material is represented by dashed line H, shown as extending 
horizontally from the n-doped layer, through the layer of intrinsic 
semiconductor material and into the p-doped layer. According to the 
principles of the instant invention, the band gap of the intrinsic layer 
of semiconductor material is graded either in a uniform or stepped 
fashion. Curve J is illustrative of a smoothly varying, graded band gap of 
intrinsic semiconductor material. Note that the band gap energy of the 
layer of intrinsic semiconductor material represented by Curve J, varies 
linearly from a highest initial value at the n-intrinsic layer interface 
to a lowest value represented by the line H, at a point approximately 1/3 
of the way into the bulk thickness of that intrinsic layer of 
semiconductor material, as measured from the n-intrinsic (light incident) 
interface. In an alternative embodiment, not illustrated herein, the band 
gap energy may vary smoothly, in the manner illustrated by Curve J, but in 
a nonlinear manner; i.e., the variation in band gap energy with increasing 
thickness of the intrinsic semiconductor material may be exponential. 
It is also possible to grade the band gap of the photoactive layer in a 
stepped manner, as illustrated by Curve K. In such a stepped gradation, 
the energy of the band gap varies through a plurality of levels or tiers, 
from an initially highest value at the light incident interface to 
progressively lower values as the distance from that interface increases. 
Although Curve K shows two levels of band gap energy (i.e. one step), the 
band gap energy may be varied through the use of as great a number of 
steps as deemed convenient. Curves J and K depict the band gap as graded 
through only a portion of the bulk thickness of the intrinsic 
semiconductor layer so as to emphasize the fact that the entire thickness 
of the intrinsic layer need not be band gap graded in order to promote the 
uniform absorption of photons of both red and blue light. It has been 
found that if the initial one to two thousand angstroms of the intrinsic 
layer of semiconductor material is band gap graded, uniform absorption 
will be promoted throughout a substantial portion of the bulk thickness of 
that intrinsic layer so as to secure the improved stability promised by 
the instant invention. Obviously, it would be within the scope of the 
instant invention to grade the band gap of the intrinsic layer of 
semiconductor material throughout a greater, or lesser portion of the bulk 
thickness thereof; the only proviso being that a substantially uniform 
absorption of incident radiation (as defined hereinabove) be promoted. 
The photovoltaic devices fabricated according to the principles of the 
instant invention include a photovoltaic layer of semiconductor material 
manufactured by any process of thin film deposition known to those 
ordinarily skilled in the art, provided that the composition of the 
photoresponsive layer is profiled during the deposition thereof, so as to 
grade the band gap of that layer. Among the methods which may be employed, 
without limitation, are sputtering, evaporation, chemical vapor 
deposition, or glow discharge deposition. 
It has been found that certain elements, including nitrogen, oxygen, 
fluorine, and carbon, whether taken alone or in combination, are capable 
of widening the band gap of amorphous silicon and germanium alloy 
materials. Accordingly, the improved, more stable photovoltaic device of 
the instant invention may be fabricated by incorporating increasing 
amounts of one or more band gap widening elements into the amorphous 
semicondonductor alloy material of the intrinsic layer of a p-i-n 
photovoltaic device as that intrinsic layer is being deposited. 
Alternatively, band gap narrowing elements, such as germanium, tin, lead, 
and mixtures thereof, may be employed to narrow the band gap of amorphous 
semiconductor materials such as silicon and germanium alloys. By utilizing 
this approach to band gap grading, decreasing amounts of band gap 
narrowing elements are added to the intrinsic layer of semiconductor 
material as it is deposited. Note that the patentable merit of the instant 
invention does not depend upon the manner in which the band gap is graded 
or upon the manner in which the semiconductor alloy material is deposited. 
It is only important that a photoresponsive device include therein a 
graded band gap layer of photoactive material which is specifically 
adapted to provide substantially uniform absorption of both blue and red 
light throughout a substantial portion of the bulk thickness of the 
photoactive material thereof. 
In one embodiment of the instant invention, a p-i-n-type photovoltaic 
device was prepared by glow discharge deposition techniques. A p-doped 
semiconductor layer of under approximately 200 angstroms thickness was 
deposited upon a stainless steel substrate by the glow discharge 
decomposition of a precursor process gas mixture comprising silane, 
hydrogen, and diborane gas, through the application of radio frequency 
energy of approximately 13.56 megahertz at a pressure of approximately 0.5 
torr. Upon completion of the deposition of the p-doped layer, the 
deposition chamber was purged and a process gas mixture for depositing a 
layer of intrinsic amorphous silicon alloy material was admitted 
thereinto. The precursor intrinsic process gas mixture comprised silane 
and hydrogen, and the pressure within the deposition chamber was 
maintained at approximately 0.5 torr. The decomposition of the process gas 
mixture and the deposition of the intrinsic semiconductor alloy layer was 
initiated by the application of radio frequency energy and allowed to 
proceed until approximately 4000 angstroms of intrinsic amorphous silicon 
alloy material was deposited. At that time, ammonia, equal to 30% of the 
amount of silane present, was admitted into the deposition chamber and the 
deposition process proceeded until approximately an additional 1000 
angstroms of intrinsic amorphous silicon:nitrogen alloy material was 
deposited. In this manner, a graded band gap structure comprising 
approximately 4000 angstroms of an intrinsic amorphous silicon alloy 
material and 1000 angstroms of amorphous silicon:nitrogen alloy material 
was deposited. Following the deposition of the graded intrinsic 
semiconductor layer, a layer of less than approximately 200 angstrom 
thickness of n-doped amorphous silicon alloy material was deposited atop 
the intrinsic semiconductor layer by the glow discharge decomposition of a 
process gas mixture comprising silane, hydrogen, and phosphine precursor 
gases. In a subsequent processing step, an upper transparent electrode 
formed of indium tin oxide was deposited atop the layer of n-doped 
semiconductor alloy material in a reactive evaporation process. The 
foregoing process resulted in the fabrication of a photovoltaic device 
which included a layer of intrinsic semiconductor alloy material 
characterized by a stepped band gap; obviously, a smoothly graded gap 
could likewise be provided by adding progressively greater amounts of a 
band gap widening material, such as methane, oxygen, fluorine, or nitrogen 
to the deposition atmosphere as the intrinsic layer qf semiconductor 
material is being deposited. 
The instant invention is obviously not limited to utilization of layers of 
the above described thickness. The photovoltaic devices exhibiting the 
advantages of the instant invention may, for example, be manufactured by 
utilizing thinner intrinsic regions such as, for example, 3000 angstrom 
thick intrinsic layers having a graded portion thereatop of about 700 
angstroms thickness. Among some of the band gap values having utility in 
the instant invention are band gaps of from about l.8 to 2.1 eV for the 
wide band gap graded portion and band gaps of from l.5 to l.8 eV for the 
narrower band gap portion of the intrinsic layer. It has also been found 
that tandem cells comprising several stacked p-i-n layers may be 
fabricated utilizing the principles disclosed herein in one or more of the 
intrinsic layers thereof. One such device would comprise a dual tandem 
device having a topmost (i.e., light incident) intrinsic layer which 
includes a wide band gap portion having a band gap of l.7 to 2.1 eV and a 
narrower band gap portion having a band gap of l.5 to l.7 eV; disposed 
immediately therebeneath is a second p-i-n type cell having an intrinsic 
layer with a wide band gap portion having a band gap of approximately l.5 
to l.7 eV and a narrow band gap portion having a band gap of approximately 
l.4 to l.5 eV. 
In order to demonstrate the improvement which results by utilizing the 
principles of the instant invention, a pair of p-i-n-type photovoltaic 
devices specifically including band gap graded intrinsic layers were 
fabricated by glow discharge deposition. The devices which were formed on 
stainless steel substrates comprised an aggregate of layers. Each 
aggregate of layers included an approximately 100 angstrom thick layer of 
p-type semiconductor material fabricated by the glow discharge 
decomposition of a mixture of silane and diborane; and an approximately 
100 angstrom thick layer of n-type semiconductor material fabricated by 
the glow discharge decomposition of silane and phosphine. Disposed between 
the p and n layers is the graded band gap layer of intrinsic semiconductor 
material of the instant invention. The graded band gap layer comprises a 
first approximately 4000 angstrom thick portion formed of an amorphous 
silicon:hydrogen alloy having a band gap of approximately 1.7 electron 
volts, which portion is disposed immediately atop the p-layer; and a 
second approximately 1000 angstrom thick portion formed of an amorphous 
silicon:hydrogen:nitrogen alloy having a band gap of approximately 1.9 
electron volts disposed atop the 1.7 electron volt portion. Deposition of 
both of the band gap graded intrinsic layer portions was accomplished by a 
glow discharge process. The 1.7 electron volt portion of the intrinsic 
layer was deposited by the decomposition of silane, and the 1.9 electron 
volt portion was deposited by the decomposition of a mixture of 30% 
NH.sub.3 and 70% silane. 
For purposes of comparison and control, a similar p-i-n-type photovoltaic 
device having a 5000 angstrom thick, ungraded intrinsic layer of 
semiconductor material with a constant 1.7 electron volt band gap was 
prepared in an identical glow discharge deposition process. 
It should be noted that the particular device configuration chosen, i.e., a 
relatively thick single p-i-n photovoltaic cell, is noted for 
demonstrating a high degree of photodegradation, which notoriety was borne 
out by the subsequent performance of the control device. Initial 
operational parameters, particularly short circuit current under blue 
illumination (Blue Jsc), were measured, and the devices were then 
photodegraded by prolonged exposure to AM-1 illumination. Blue Jsc was 
again measured after 6 and 72 hours of AM-1 illumination. 
It was found that significantly less degradation occurred in the Blue Jsc 
of the test devices incorporating the band gap graded intrinsic layer of 
the instant invention as compared to the ungraded control device. After 6 
hours of photodegradation, the Blue Jsc's of the test devices were 96.0% 
and 97.1% of the initial values, while the Blue Jsc of the control device 
was 73% of its original value. After 72 hours of photodegradation, the 
test devices still exhibited 91.3% and 90.3% of their initial Blue Jsc, 
while the Blue Jsc of the ungraded control device fell to 54% of its 
initial value. 
While the discussion has heretofore centered solely upon the glow discharge 
deposition of the graded intrinsic layer, other processes, such as a 
sputtering process could be similarly employed to fabricate a graded band 
gap structure for the layer of intrinsic semiconductor material. It is 
only necessary to (1) add a band gap increasing element to the sputtering 
atmosphere of the intrinsic semiconductor material as the process 
progresses to thereby reactively form a sputtered wide band gap intrinsic 
semiconductor material atop previously deposited narrower band gap 
intrinsic semiconductor intrinsic semiconductor material, or (2) change 
sputtering targets during the course of the deposition procedure so as to 
deposit wider band gap intrinsic semiconductor material. Obviously, 
similar modifications could be made in an evaporation process or a 
chemical vapor deposition process to produce the improved graded band gap 
structure for the layer of intrinsic semiconductor material of the instant 
invention. 
While the foregoing description has dealt primarily with p-i-n type 
photovoltaic devices, the instant invention is not so limited but may be 
readily adapted for use in the fabrication of stable photoactive 
semiconductor material for use in any photoresponsive device in which the 
recombination of charge carriers at photogenerated defects sites reduces 
the current generating efficiency of said device. For example, the 
foregoing analysis and description is equally applicable to n-i-p devices 
adapted for illumination from the p-doped semiconductor layer side; 
however, in such devices, the electrons represent the minority charge 
carriers and are therefore determinative of cell efficiency; accordingly, 
the previous discussion relative to holes will be applicable to electrons 
in a n-i-p device. Thus, it must be recognized that the crux of the 
instant invention is the fabrication of a graded band gap for one or more 
of the photoactive layers of the semiconductor material of a 
photoresponsive device so as to promote substantially uniform absorption 
of red and blue radiation throughout a substantial portion of the bulk 
thickness of the photoactive region thereof. For example, in a p-n type 
photovoltaic device the photoactive region, also termed the depletion 
region, thereof is formed within both the p and the n-doped layers of 
semiconductor material (proximate the interface therebetween). By 
appropriately grading the band gap of the p and/or n-doped layers thereof, 
the light absorption characteristics of the device are modified so as to 
promote substantially uniform absorption of red and blue light throughout 
that so-called depletion region. The principles of the instant invention 
may be similarly applied to yet other photoresponsive devices, such as p-n 
devices, or Schottky barrier devices, in which a photoactive region is 
formed in the semiconductor material thereof at the interface thereof with 
a metallic layer. 
The foregoing discussion, explanation, graphs and drawings are meant to 
illustrate the principles and practice of the instant invention and are 
not intended as a limitation upon the practice thereof. The scope of the 
instant invention is defined solely by the following claims, read in light 
of the specification, and including all equivalents thereof.