CA1245330A

CA1245330A - Back reflector system and devices utilizing same

Info

Publication number: CA1245330A
Application number: CA000423034A
Authority: CA
Inventors: Vincent Cannella; Ralph Mohr; David D. Allred
Original assignee: Energy Conversion Devices Inc
Current assignee: Energy Conversion Devices Inc
Priority date: 1982-03-18
Filing date: 1983-03-07
Publication date: 1988-11-22
Also published as: GB8306327D0; FR2523768B1; IN161241B; GB2116775B; NL8300925A; DE3308598A1; ZA831342B; IE54573B1; SE8301366D0; IT1160506B; IE830502L; IT8319923A0; GB2116775A; BR8301160A; IL67926A; FR2523768A1; AU540909B2; SE457300B; IL67926A0; AU1241583A

Abstract

Abstract of the Disclosure There is disclosed new and improved back reflector systems for use, for example, in photo-voltaic devices. The back reflector systems in-clude a layer of highly reflective material, such as a highly reflective and conductive metal of copper, gold, silver, or aluminum, or alloys thereof. Between the layer of highly reflective material and the semiconductor regions of the device is a layer of a transparent conductor. The transparent conductor can be, for example, a transparent conductive oxide such as indium tin oxide, cadmium stannate, or doped tin oxide.
The transparent conductor serves as a reflec-tion enhancement layer for the highly conductive metal layer, and a transparent barrier to prevent diffusion of the highly reflective material into the semiconductor regions.
The back reflecting systems of the present invention find particular applicability in photo-voltaic cells of the p-i-n configuration. The p-type layer preferably has a wide band gap and the active intrinsic region is preferably an amor-phous silicon alloy containing fluorine. Also disclosed is a tandem p-i-n photovoltaic cell embodying the invention.

Description

~;24S330 This invention relates to improved back re-flector systems and photovoltaic devices utilizing the same. The present invention has particular applicability to photovoltaic devices formed from layers of amorphous semiconductor alloys. The back reflector systems of the present invention provide increased reflection of unabsorbed light back into the devices in which they are employed.
One advantage of this approach is that increased photon absorption and charge carrier generation in the active layers is possible to provide increased short circuit currents. Another advantage is that the improved photoresponsive characteristics of fluorinated amorphous silicon alloys can be more fully realized in photovoltaic devices by prac-ticing the present 1nvention. The invention has its most important application in making- improved amorphous silicon alloy photovoltaic devices of ~ ` the p-i-n configuration, either as singIe cells or ;~ 20 multiple cells comprising a p1ura11ty of single cell units. Preferably, the doped layers of the p-i-n cells have low absorption coefficients in :
the wavelength regions of interest to best utilize the back reflector of the present invention.
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i2i~53i30 Silicon is the basis of the huge crystalline semiconductor inclustry and is the material which has produced expensive high efficiency ~18 per-cent) crystalline solar cells for space applica-tions. For terrestrial applications, the crystal-line solar cells typically have much lower effi-ciencies on the order of 12 percent or less. When crystalline semiconductor technology reached a commercial state, it became the foundation of the present huge semiconductor device manufacturing industry. This was due to the ability of the scientist to grow substantially defect-free ger-manium and particularly silicon crystals, and then turn them into extrinsic materials with p-type and n-type conductivity regions therein. This was ac-complished by diffusing into such crystalline material parts per million of donor (n) or accep-tor ~p) dopant materials introduced as substitu-tional impurities into the substantially pure 2n crystalline materials, to increase their electri-cal conductivity and to control their being either of a p or n conduction type. ~he fabrication pro-cesses for ~aking p-n junction crystals involve extremely complex, time consuming, and expensive procedures. Thus, these crystalline materials ' I ' `; ! j ~

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useful in solar cells and current control devices are produced under very carefully controlled con-ditions by growing individual single silicon or germanium crystals, and when p-n junctions are required, by doping such single crystals with extremely small and critical amounts of dopants.
These crystal growing processes produce such relatively small crystals that solar cells require the assembly of many single crystals to encompass the desired area of only a single solar cell panel. The amount of energy necessary to make a solar cell in this process, the limitat1on caused by the slze limitations of the silicon crystal, and the necessity to cut up and assemble such a crystalline material have all resulted in an im-possible economic barrier to the large scale use of crystalline semiconductor solar cells for energy conversion. Further, crystalline silicon has an indirect optical edge which results in poor light absorption in the material. Because of the poor light absorption, crystalline solar cells have to be at least 50 microns thick to absorb the incident sunlight. Even if the single crystal material is replaced bv polyerystalline silieon with eheaper production processes, the indireet ., 12~5330 optical edge is still maintained; hence the mate-rial thicl~ness is not reduced. The polycrystal-line material also involves the addition of grain boundaries and other defect problems, ~hich de-fects are ordinarily deleterious.
In su~ary, crystal silicon devices have fixed parameters which are not variable as de-sired, require large amounts of material, are only produceable in relatively small areas and are ex-pensive and time consuming to produce. Devicesbased upon amorphous silicon alloys can eliminate these crystal silicon disadvantages. An amorphous si1icon alloy has an optical absorption edge having properties similar to a direct gap semicon-ductor and only a material thickness of one micron ~; or less is necessary to absorb the same amount of sunlight as the 50 micron thick crystalline sili-con. Further, amorphous silicon alloys can be ~ made faster, easier and in larger areas than can crystalline silicon.
Accordingly, a considerable effort has been made to develop processes for readily depositing amorphous semiconductor alloys or films, each of ~; which can encompass relatively large areas, if desired, limited only by the size ofthe deposition :
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equipment, and which could be readily doped to form p-type and n-type materials where p-n junc-tion devices are to be made therefrom equivalent to those produced by their crystalline counter-parts. For many years such work was substantiallyunproductive. Amorphous silicon or germanium (Group IV) films are normally four-fold coordi-nated and were found to have microvoids and dan-gling bonds and other defects which produce a high density of localized states in the energy gap thereof. The presence of a high density of local-ized states in the energy gap of amorphous silicon semiconductor films results in a low degree of photoconductivity and short carrier lifetime, making such films unsuitahle for photoresponsive applications. Additionally, such films cannot be successfully doped or otherwise modified to shift the Fermi level close to the conduction or valence ~: :
bands, making them unsuitable for making p-n junc-tions for solar cell and current control~device applications.
In an attempt to minimize the aforementioned problems involved with amorphous silicon (origi-nally tho~ght to be elemental), W.E. Spear and P.G. Le Comber of Carnegie Laboratory of Physics, :

~2~S330 University of Dundee, in Dundee, Scotland, did some work on "Substitutional ~oping of Amorphous Silicon'l, as reported in a paper published in Solid State Communications, Vol. 17, pp. 1193-1196, 1975, toward the end of reducing the local-ized states in the energy gap in amorphous silicon to make the same approximate more closely intrin-sic crystalline silicon and of substitutionally doping the amorphous materials with suitable clas-sic dopants, as in doping crystalline materials, to make them extrinsic and of p or n conduction types.
The reduction of the localized states was accomplished by glow discharge deposition of amor-phous silicon films wherein a gas of silane (SiH4)was passed through a reaction tube where the gas was decomposed by an r.f. glow discharge and de-posited on a substrate at a substrate temperature of about 500-600K (227-327C). The material so deposited on the substrate was an intrinsic amor-phous material consisting of silicon and hydro-gen. To produce a doped amorphous material a gas of ~hosphine (PH3) for n-type conduction or a gas of diborane (B2H6) for p-type conduction were pre-mixed with the silane gas and passed through the ~ ~, ., :

:IL2~5330 glow discharge reaction tube under the same oper-ating conditions. The gaseous concentration of the dopants used was between about 5 x 10-6 and 10-2 parts per volume. The material so deposited was shown to be extrinsic and of n or p conduction type.
~ Ihile it was not known by these researchers, it is now known by the work of others that the hydrogen in the silane combines at an optimum tem-perature with many of the dangling bonds of thesilicon during the glow discharge deposition, to substantially reduce the density of the localized states in the energy gap toward the end of~making the electronic properties of the amorphous mate-rial approximate more nearly those o the corres-ponding crystalline material.
The incorporation of hydrogen in the above method however has limitations based upon the fixed ratio of hydrogen to silicon in silane, and various Si:H bonding configurations which intro-duce new antibonding states. Therefore, there are basic limitations in reducing the density of localized states in these materials.
Greatly improved amorphous silicon alloys

2~ having significantly reduced concentFations of ~ J

~Z~L5330 localized statea in the energy gaps thereof and high quality electronic propertiea have been prepared by glow di~charge as fully described in U.S. Patent No. 4,226,898, Amorphous Semiconductors Equivalent to Crystalline Semiconductor~, Stanford R. Ovshin~ky and Arun Madan which i~sued October 7, 1980, and by vapor depo~ition as fully de~cribed in U.S. Patent No. 4,217,374, Stanford R. Ovshin~ky and Ma~atsugu Izu, which i~sued on August 12, 1980, under the same title. A~ di~clo~ed in the~e patents, fluorine is introduced into the amorphous silicon ~emiconductor alloy to sub~tantially reduce the den~ity of localized atate~ therein. Activated fluorine e~pecially readily bonds to silicon in the amorphous body to aubstantially decrease the denaity of localized defect state~, becau~e the ~mall size high reactivity of ~pecification of chemical bonding of the fluorine atoms enables them to achieve a more defect-free amorphou~ ailicon alloy. The fluorine bonda to the dangling bond~ of the silicon and form~ what ia believed to be a predominantly ionic ~table bond with flexible bonding angle~, which results in a more stable and more efficient compen~ation kh/rn .~ .

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~Z~5330 or alteration than is formed by hydrogen and other compensating or altering agents. Fluorine also comhines in a preferable manner with silicon and hydrogen, utilizing the hydrogen in a more desir-able manner, since hydrogen has several bondingoptions. Without fluorine, hydrogen may not bond in a desirable manner in the material, causing extra defect status in the band gap as well as in the material itself. Therefore, fluorine is con-sidered to be a more efficient compensating or -altering element than hydrogen when employed alone or with hydrogen because of its high reactivity, specificity in chemical honding, and high electro-negativity, As an example, compensation may be achieved with fluorine alone or in combination with hydro-gen with the adclition of these element(s) in very small quantities (e.g., fractions of one atomic percent). ~owever, the amounts of fluorine and hydrogen most desirably used are much greater than such small percentages so as to form a silicon-hydrogen-fluorine alloy. Such alloying amounts of fluorine and hydrogen may, for example, be in the -` ~2~5330 range of 1 to 5 percent or greater. It is be-lieved that the alloy so formed has a lower den-sity of defect states in the energy gap than that achieved by the mere neutralization of dangling bonds and similar defect states. Such larger amount of fluorine, in particular, is believed to participate substantially in a new structural con-figuration of an amorphous silicon-containing material and facilitates the addition of other alloying materials, such as germanium. Fluorine, in addition to its other characteristics mentioned herein, is believed to be an organizer of local structure in the silicon-containing alloy through inductive and ionic effects. It is believed that fluorine also influences the bonding of hydroyen by acting in a beneficial way to decrease the den-sity of defect states which hydrogen contributes while acting as a density of states reducing ele-ment. The ionic role that fluorine plays in such an alloy is believed to be an important factor in terms of the nearest neighbor relationships.
Amorphous silicon alloys containing fluorine have thus demonstrated greatly improved character-istics for photovoltaic applications as compared : ~ :

''''``'':' ~Z~5330 to amorphous silicon alloys containing just hydro-gen alone as a density of states reducing ele-ment. However, in order to realize the full advantage of these amorphous silicon alloys con-taining fluorine when used to form the activeregions of photovoltaic devices, it is necessary to assure that the greatest possible portion of the available photons are absorbed therein for efficiently generating electron-hole pairs.
The foregoing is important in, for example, photovoltaic devices of the p-i-n configuration.
Devices of this type have p and n-type doped layers on opposite sides of an active intrinsic layer, wherein the electron-hole pairs are gener-a~ed. They establish a potential gradient across the device to facilitate the separation of the electrons and holes and also form contact layers to facilitate the collectlon of the electrons and holes as electrical current.
Not all of the available photons are absorbed by the active regions. While almost all of the shorter wavelength photons are absorbed, a large portion of the longer wavelength photons with energies near the absorption edge of the intrinsic semiconductor material, are not absorbed. The : :

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' ~ , :~245330 loss of these unabsorbed photons reduces the cur-rents which can be produced. To preclude the loss of these longer wavelength photons, back reflec-tors, formed from conductive metals have been employed to reflect the unused or unabsorbed light back into the active regions of the devices.
The p and n-type layers are conductive and preferably have a low absorption coefficient for wavelengths near the band edge, to decrease photon absorption in those layers. A back reflector is therefor extremely advantageous ~7hen used in con-junction with a p-type layer having for example a ; wide band gap forming one of the doped layers of such a device. Back reflecting layers therefore serve to reflect unused light back into the in-::
trinsic region of the device to permit further ; utilization of the sun energy for generating addi-tional electron-hole pairs. A back reflecting layer permits a greater portion of the available photons to pass into the active intrinsic layer and to be absorbed therein.
Unfortunately, the best back reflectors of the prior art have~been capable of reflecting only~
about 80 percent of the unused light of the wave-lengths of 1nterest back into the devices in which~

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they are employed. Noble metals such as copperand silver, and ~etals such as aluminum, because they are highly conductive, have been suggested as possible back reflector materials. However, these metals can diffuse into the semiconductor of the devices in which they are employed and, in doing so, adversely effect the photoresponsive charac-teristics of the devices. As a result, thin layers of other less conductive and less reflec-tive metals have been employed as diffusion bar-riers for such back reflectors. Such less conduc-tive and reflective metals include molybdenum and chromium. Although these metals prevent diffusion into the semiconductor of the devices, they reduce the reflectance of the more highly conductive metals. Hence, there is a need for better back reflecting systems which not only provide greater reflection of the unused light, but also preclude diffusion of the back reflector material into the devices.
Applicants herein have discovered new and improved back reflector systems which provide both increased reflection of unused light over prior back reflectors and protection from the back re-~5 flector materials dlffusing into the semiconductor

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of the devices. The back reflectors of the pres-ent invention can be utilized in both single cell photovoltaic devices of the p-i-n configuration, and multiple cell structures having a plurality of single cell units.
~ e have found that the ahove disadvantages may be overcome by utilizing the new and improved back reflector systems of the present invention which provide both increased reflection of un-absorbed light back into the active regions of thedevices in which they are employed and protection from diffusion of the back reflector materials into the devices.
The back reflector systems include a layer of a highly reflective material and a layer of a transparent material ~hich serves as both a con-ductor and a barrier layer to prevent diffusion of the reflective materials into the semiconductor regions of the devices.
` 20 The highly reflective material can be a highly reflective metallic material such as gOldr silver, copper or aluminum, or alloys or compounds thereof. The transparent conductor can be~a transparent conductive oxide or a transparent con ductive chalcogenide.

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The present invention provides new and im-proved back reflector systems for use in photo-voltaic devices. The back reflector systems of the present invention provide increased reflection of unabsorbed light back into the active regions of the devices in which they are employed while preventing diffusion of the back reflector mate-rials into the devices.
The back reflector systems include a layer of a highly reflective material and a layer of a transparent conductor. The transparent conductor layer is disposed between the device and the layer of highly reflective material.
The highly conductive material can be a high-ly reflective metallic material such as a highly ; reflective metal of gold, silver, copper or alumi-:
num, or alloys thereof. The hi~hly reflective metallic material can also be metallic compounds such as ~Nxr TiNX, ZrNx~ Hf~xl~ or MoNx-~ The transparent~conductor can be a transpar-ent conductive oxide such as indium tin oxide, ::: : :
cadmlum stannate, dop~ed tln oxide, vanadium oxide, germanium tin oxide, ferric oxide, zinc oxide, and cuprous oxide. The transparent conductor can also be a transparent conductive chalcogenide such as ::

, zinc selenide or cadmium sulfide. It can also be silicon carbide.
The transparent conductor serves to enhance reflection of the unabsorbed light back into the devices and also serves as a transparent barrier layer to prevent diffusion of the highly reflec-tive materials into the semiconductor regions of the devices. The back reflector systems of the present invention therefore provide increased bac~
reflection of unabsorbed light without de~rading the photoresponsive characteristics of the semi-conductor materials of the devices.
The back reflectors of the present invention are particularly applicable in photovoltaic de-vices of p-i-n configuration. Such devices in-clude an intrinsic active semiconductor region wherein photogenerated electron-hole pairs are created and~doped regions of opposite conductivity disposed on opposite respective sides of the in ; 20 trinsic re~ion. The active intrinsic region is preferably an amorphous silicon alIoy body or layer containing fluorine as a densîty of states reducing element. The doped regions also pref-erably include an a~orphous silicon wide band gap : :
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p-type alloy layer forming either the top or bot-tom semiconductor layer of the device. In either case, the amorphous semiconductor regions are preferably deposited on a substrate with the layer of highly conductive metal adjacent the substrate and the transparent conductive oxide disposed between the layer of highly reflective material and the bottom doped layer.
Substantially all of the shorter wavelength photons are absorbed in the active intrinsic regions while only a portion of the photons having longer wavelengths and energies near the absorp-tion edge of the intrinsic material are absorbed.
Therefore, the thickness of the transparent con~
ductor is adjusted to optimize the reflection of the longer wavelength photons. To that end, the thickness of the transparent conductor is prefera-bly determined by the relationship:

d= ~k/4 n Where: d is the layer thickness;
is the minimum photon wavelength to be reflected;
n is the index of refraction of the transparent conductor; anA
k ls an odd integral multiplier.

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The back reflector systems of the present invention can also be utilized in multiple cell devices, such as tandem cells.
Fig. 1 is a diagrammatic representation of a glow discharge deposition systern which may be utilized in practicing the method of the present invention for making the photovoltaic devices of the invention;
Fig. 2 is a sectional view of a portion of the system of Fig. 1 taken along the lines of 2-2 therein;
Fig. 3 is a sectional view of a p-l-n photo-voltaic device embodying the present invention;
and Fig. 4 is a sectional view of a multiple cell incorporating a plurality of p-i-n photovoltaie cell units arranged in tandem configuratlon em-bodying the present invention.
Referring now more partieularly to Fig. 1, ; 20 there is shown a glow discharge depositlon system 10 including a housing 12. The houslng 12 en-closes a vaeuum ehamber 14 and ineludes an inlet ehamber 16 and an outlet chamber 18. A eathode baeking member 20 is mounted in the vaeuum chsmber 11 through an insulator 22.

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The backing member 20 includes an insulating sleeve 24 circumferentially enclosing the backing member 20. A dark space shield 26 is spaced from and circumferentially surrounds the sleeve 24. A
substrate 28 is secured to an inner end 30 of the backing member 20 by a holder 32. The holder 32 can be screwed or otherwise conventionally secured to the backing member 20 in electrical contact there~ith.
The cathode backing member 20 includes a well 34 into which is inserted an electrical heater 36 for heating the backing member 20 and hence the substrate 28. The cathode backing member 20 also includes a temperature responsive probe 38 for measuring the temperature of the backing member 20. The temperature probe 38 is utilized to con-trol the energization of the heater 36 to maintain the backin~ member 20 and the substrate 28 at any desired temperature.
The system 10 also includes an electrode 40 which extends from the housing 12 into the vacuum chamber 14 spaced from the cathode backing member 20. The electrode 40 includes a shield 42 sur-rounding the electrode 40 and which in turn car-ries a substrate 44 mounted thereon. The elec-trode 40 includes a well 46 into which is inserted ~ILZ4533~) an electrode heater 48. The electrode 40 also includes a temperature responsive probe 50 for measuring the temperature of the electrode 40 and hence the substrate 44. The probe 50 is utilized to control the energization of the heater 48 to maintain the electrode 40 and the substrate 44 at any desired temperature, independently of the mem-ber 20.
A glow discharge plasma is developed in a space 52 between the substrates 28 and 44 by the power generated from a regulated ~.F., ~.C. or D.C. power source coupled to the cathode backing member 20 across the space 52 to the electrode 40 which is coupled to ground. The vacuum chamber 14 is evacuated to the desired pressure by a vacuum pump 54 coupled to the chamber 14 through a parti-cle trap 56. A pressure gauge 58 is coupled to the vacuum system and is util~zed to control the pump 54 to maintain the system 10 at the desired pressure.
The inlet chamber 16 of the housing 12 pre-ferably is provided with a plurality of conduits 60 for introducing materials into the system 10 to be mixed therein and to be deposited in ~he cham-ber 14 in the glow:discharge plasma space 52 upon , .
'`' ~Z~533(~

the substrates 28 and 44. If desired, the inlet chamber 16 can be located at a remote location and ~he gases can be premixed prior to being fed into the chamber 14. The gaseous materials are fed into the conduits 60 through a filter or other purifying device 62 at a rate controlled by a valve 64.
When a material initially is not in a gaseous form, but instead is in a liquid or solid form, it can be placed into a sealed container 66 as indi-cated at 68. The material 68 then is heated by a heater 70 to increase the vapor pressure thereof in the container 66. A suitable gas, such as argon, is fed through a dip tube 72 into the mate-rial 68 so as to entrap the vapors of the material68 and convey the vapors through a filter 62' and a valve 64' into the conduits 60 and hence into the system 10.
The inlet chamber 16 and the outlet chamber 18 preferably are provided with screen means 74 to confine the: plasma in the chamber 14 and princi-pally between the substrates 28 and 44.
The materials fed through the conduits 60 are mi~ed in the inlet chamber 16 and then fed into the glow discharge space 52 to maintain the plasma . .

~Z4533~

and deposit the alloy on the substrates with the incorporation of silicon, fluorine, oxygen and the other desired alterant elements, such as hydrogen, and/or dopants or other desired materials.
In operation, and for depositing layers of intrinsic amorphous silicon alloys, the system 10 is first pumped down to a desired deposition pres-sure, such as less than 20 mtorr prior to deposi-tion. Starting materials or reaction gases such as silicon tetrafluoride (SiF4) and molecular hydrogen (H2) and/or silane are fed into the inlet chamber 16 through separate conduits 60 and are then mixed in the inlet chamber. The gas mixture is fed into the vacuum chamber to maintain a par-tial pressure therein of about .6 torr. A plasmais generated in the space 52 between the sub-strates 28 and 44 using either a DC voltage of greater than 1000 volts or by radio frequency power of about 50 watts operating at a frequency of 13.56 ME~z or other desired frequency.
In addition to the intrinsic amorphous sili-con alloys deposited in the manner as described above, the devices of the present invention as illustrated in the various embodiments to be des-cribed herelnafter also utilize doped amorpùous .., ,, . ~

~Z~53313 silicon alloys including wide band gap p amorphous silicon alloys~ These doped alloy layers can be p, p+, n, or n+ type in conductivity and can be formed by introducing an appropriate dopant into the vacuum chamber along with the intrinsic start-ing material such as silane (SiH4) or the silicon tetrafluoride (SiF4) starting material and/or hydrogen and/or silane.
For n or p doped layers, the material can be doped with 5 to lO0 ppm of dopant materials as it is deposited. For n+ or p~ doped layers, the material is doped with 100 ppm to over l percent of dopant material as it is deposited. The n dopants can be phosphorus, arsenic, antimony, or bismuth. Preferably, the n doped layers are deposited by the glow discharge decomposition of at least silicon tetrafluoride (SiF4) and phos-phine (PH3). Hydrogen and/or silane gas (SiH4) may also be added to this mixture.
The p dopants can be boron, aluminum, gal lium, indium, or thallium. Preferably, the p doped layers are deposited by the glow discharge decomposition of at least silane and diborane (B2H6) or silicon tetrafluoride and diborane. To 533~

-2~-the silicon tetrafluoride and diborane, hydrogen and/or silane can also be added.
In addition to the foregoing, and in accor-dance with the present invention, the p-type layers are formed from amorphous silicon alloys containing at least one band gap increasing ele-ment. For example, carhon and/or nitrogen can be incorporated into the p-type alloys to increase the band gaps thereof. A wide band gap p amor-phous silicon alloy can be formed for example by agas mixture of silicon tetrafluoride (SiF4), silane (SiH4), diborane (B2H6), and methane (CH4). This results in a p-type amorphoas silicon alloy having a wide band gap.
The doped layers of the devices are deposited at various~temperatures depending upon the type of material deposited and the substrate used~ For aluminum substrates, the upper temperature should not be above about 600C and for stainless steel it could be above about 1000C. For the intrinsic and doped alloys initially compensated with hydro-gen, as for example those deposited from silane gas starting material, the substrate temperature should be less than about 400C and preferably between 250C and 350C.

~:4533(~

Other materials and alloying elements may also be added to the intrinsic and doped layers to achieve optimized current generation. These other materials and elements will be described herein-after in connection with the device configurationsembodying the present invention illustrated in Figs. 3 and 4.
~ eferring now to Fig. 3, it illustrates in sectional view a p-i-n device embodying the pres-ent invention. The device 110 includes a sub-strate 112 which may be glass or a flexible web formed from stainless steel or aluminum. The sub-strate 112 is of a width and length as desired and preferàbly 5 to 10 mils thick.
In accordance with the present invention, a layer 114 of highly reflective material is depos-ited upon the substrate 112. The layer 114 lS
deposited by vapor deposition, which is a rela-tively ~ast deposition process. The layer 114 preferably is a highly reflective metallic mate-rial such as silver, gold, alu~inum, or copper or alloys thereof. The highly reflective material can also be a highly reflective metallic compound such as WNX, TiNX, ZrNx, HfNX, or MoNx. Deposited over the layer 114 is a layer 115 of a transparent :~IL2~533(3 conductor. The transparent conductor can be a transparent conductive oxide (TC0) deposited in a vapor deposition environment and, for example, may be indium tin oxide (IT0), cadmium stannate (Cd2SnO4) zinc oxide, cuprous oxide, vanadium oxide, germanium tin oxide, ferric oxide, or tin oxide (SnO2). The transparent conductor layer 115 can also be formed silicon carbide, or a transpar-ent conductive chalcogenide such as cadmium sul-fide or zinc selenide. The layer 114 of highlyreflective material and the layer 115 of transpar-ent conductor form a back reflecting system in accordance with the present invention.
The substrate 112 is then placed in the glow discharge deposition environment. A first doped wide band gap p-type amorphous silicon alloy layer 116 is deposited on the layer 115 in accordance with the present invention. The layer 116 as shown is p+ in conductivity. The p+ region is as 20 thin as possible on the order of 50 to 500 ang-stroms in thickness which is sufficient for the p+
region to make good ohmic contact with the trans-parent conductive oxide layer 115. The p+ region also serves to establish a potential gradient across the device to facilitate the collection of '''' ' ` ' ' ~Z~5330 photo induced electron-hole pairs as electrical current. The p~ region 116 can be deposited from any of the gas mixtures previously referred to for the deposition of such material in accordance with the present invention.
A body of intrinsic amorphous silicon alloy 118 is next deposited over the wide band gap p-type layer 116. The intrinsic bod 118 is rela-tively thick, on the order of 4500A, and is depos-ited from silicon tetrafluoride and hydrogenand/or silane. The intrinsic body preferably con-tains the amorphous silicon alloy compensated with fluorine where the majority of the electron-hole pairs are generated. The short circuit current of the device is enhanced by the combined effects of the back reflector of the present invention and the wide band gap of the p-type amorphous silicon alloy layer 116.
Deposited on the intrinsic body 118 is a fur-ther doped layer 120 which is of opposite conduc-tivity with respect to the first doped layer 116.
It comprises an n~ conductivity amorphous silicon alloy and may also have a wide band gap. The ~+
layer 120 is deposited from any of the gas mix-tures previously referred to for the deposition of ,, 12~S33~

such material. The n+ layer 120 is deposited to a thickness between 50 and 500 angstroms and serves as a contact layer.
A transparent conductive oxide (TCO) layer 122 is then deposlted over the n+ layer 120. The TCO layer 122 can also be deposited in a vapor deposition environment and, for example, may be indium tin oxide (ITO), cadmium stannate (Cd2SnO4), or doped tin oxide (SnO2).
On the surface of the TCO layer 122 is depos-ited a gri~ electrode 124 made of a metal having good electrical conductivity. The grid may com-prise orthogonally related lines of conductive material occupying only a minor portion of the area of the metallic region, the rest of which is to be exposed to solar energy. For example, the grid 124 may occupy only about from 5 to 10~ of the entire area of the TCO layer 1220 The grid electrode 124 uniformly collects current from the TCO layer 122 to assure a good lo~ series resis-tance for the device.
To complete the device llO, an anti-reflec-tion (AR) layer 126 is applied over the grid elec-trode 124 and the areas of the TCO layer 122 between the grid electrode areas. The AR layer 126 has a solar radiation incident surface upon which impinges the solar radiation. For example 7 the AR layer 126 may have a thickness on the order of magnitude of the wavelength of the maximum energy point of the solar radiation spectrum/
divided by four times the index of refraction of the anti-reflection layer 126. A suitable AR
layer 126 would be zirconium oxide of about 500A
in thickness with an index of refraction of 2.1.
In an alternative form, the TCO layer 12~ can also serve as an anti-reflection layer and the anti-reflection layer 126 may then he eliminated and a suitahle encapsulant may be substituted in its place.
It is not necessary that the transparent con-ductor layer 115 and TCO layer 122 be formed from the same material. The TCO layer 122 must be able to transmit incident radiation of both short and long wavelength. ~lowever, since essentially all of the shorter wavelength radiation will be absorbed in the intrlnsic region 118 during the first pass therethrough, the transparent conductor layer 115 need only be transmissive of longer wavelength radiation, for example, light having wavelengths of about 6000A or longer.

~;4533~

The thickness of the layer 115 of transparent conductor, here a transparent conductive oxide, can be adjusted to optimize the reflectance enhancement of the layer 115. For example, the layer 115 preferably has a thickness determined by the relationship:
d= ~k/4/n Where: d is the thickness of layer 115;
~ is the minimum photon wave-length to be reflec~ed;
n is the index of refraction of the transparent conductor;
and k is an odd integral multi-plier.
Nearly all of the photons having shorter wavelengths will be absorbed by the active intrin-sic layer 118. As a result, and as previously explained, the major portion of the photons which are not absorbed have longer wavelengths. These photons may have wavelengths of about 6000A for example and~longer. For a transparent conductive oxide of, for example, indium tin oxide which has an index of refraction of about 2.0 at these longer wavelengths, and with k being preferably equal to 1, the thickness of layer 115 should be about 750A.

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Any one of the highly reflective materials previously mentioned may be used in conjunction with the indium tin oxide layer of 750A. However, of the reflective materials previously mentioned, copper is the least expensive and exhibits good reflectivity for the longer wavelengths of 6000A
or greater. With this combination of materials and thickness of the indium tin oxide of 750A, there can be expected at least 97 percent reflec-tion of all of the unused light back into thesemiconductor regions of the device 110. Addi-tionally, because the transparent conductive oxide also serves as a transparent barrier layer, dif-fusion of the copper, or any of the other highly reflective materials when employed, into the semiconductor regions of the device 110 is pre-vented.
As previously mentioned, the band gap of the intrinsic layer 118 can be adjusted for a particu-lar photoresponse characteristic with the incor-; poration of band gap decreasing elements. As a further alternative, the band gap of the intrinsic body 118 can be graded so as to be gradually increasing from the p+ layer 116 to n+ layer 120 . . , ~Z~5330 For example, as the intrinsic layer 118 is deposited, one or more band gap decreasing elements such as germanium, tin, or lead can be incorporated into the alloys in gradually decreasing concentration. Germane gas (GeH~¦ for example can be introduced into the glow discharge deposition cham~er from a relatively high concentration a-t first and gradually diminished thereafter as the intrinsic layer is deposited to a point where such introduction is terminated. The resulting intrinsic body will thus have a band ~ap descreasing element, such as germanium, therein in gradually decreasing concentrations from the p-~ layer 116 to~ards t~e n+ layer 120.
Referring now to Fig. 4, a multiple cell device 150 is there illustrated in sectional view which is arranged in tandem configuration. The device 150 comprises two single cell units 152 and 154 arranged in series relation ~s can be appreciated, plural single cell units o~ more than two can be utilized.

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The device 150 includes a substrate 156 formed fro~ a metal having good electrical conduc-tivity such as stainless steel or aluminum, for example. Deposited on the substrate 156 is a back reflector system embodying the present invention which includes a layer 157 of highly reflective material which may be formed from the materials and by the processes as previously described. To complete the back reflector, a layer 159 of a transparent conductor such as a transparent con-ductive oxide is deposited onto the highly reflec-tive material layer 157. The layer lS9 can be formed from any of the transparent conductors and deposited to an optimized thickness as previously described.
The first cell unit 152 includes a first doped p+ amorphous silicon alloy layer 158 depos-ited on the transparent conductive oxide layer 159. The p+ layer is preferably a wlde band gap p-type amorphous silicon alloy in accordance with the present invention. It can be deposited from .
any of the previously mentioned starting materials for depositing such material.

Deposited on the wide band gap p+ layer 158 25 lS a first intrinsic amorphous silicon alloy body ., . .

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-3~-160. The first intrinsic alloy body 160 is pref-erably an amorphous silicon-fluorine alloy.
Deposited on the intrinsic layer 160 is a further doped amorphous silicon alloy layer 162.
5 It is opposite in conductivity with respect to the conductivity of the first doped layer 15~ and thus is an n+ layer. It may also have a wide band gap, The second unit cell 154 is essentially iden-tical and includes a first doped p~ layer 164, an intrinsic body 166 and a further doped n+ layer 168. The device 150 is completed with a TCO layer 170, a grid electrode 172, and an antireflection layer 174.
The band gaps of the intrinsic layers are preferably adjusted so that the band gap of layer 166 is greater than the band gap of layer 160. To that end, the alloy forming layer 166 can include one or more band gap increasing elements such as nitrogen and carbon. The intrinsic alloy forming the intrinsic layer 160 can include one or more ; band gap decreasing elements such as germanium, tin, or lead.
It can be noted from the figure that the intrinsic layer 160 of the cell is thicker than the intrinsic layer 166. This allows the entire S33~3 usuable spectrum of the solar energy to be utilized for generating electron-hole pairs.
Although a tandem cell embodlment has been shown and described herein, the unit cells can also be isolate~
from one another with oxide layers for example to form a stacked multiple cell. Each cell could include a pair of collection electrodes to facilitate the series connection of the cells with external wiring.
As a further alternative, and as mentioned wîth respect to the single cells previously describedt one or more of the intrinsic bodies of the unit cells can include ~lloys having graded band gaps~ Any one or more OI the band gap increasing or decreasing elements previously mentioned can be incorporated into the intrinsic alloys for this purpose.
As can be appreciated from the foregoing, the present inven-tion provides new and improved back reflector systems for use, for example, in photovoltaic cells. The back reflectors not only c.r~

12~533~

increase the amount of unused light reflected back into the semiconductor regions of the cells, but also serve to prevent diffusion of the back reflector materials into the semiconductor regions. As examples of the effectiveness of the new and improved back reflectors of the present invention, with a transparent conductive oxide of indium tin oxide, reflectivities of 98.5 percent, 97 percent, and 90 percent are obtainable when highly reflective metals of silver, copper, and aluminum, respectively are used therewith as com-pared to reflectivities of 80% for silver alone, 74% for copper alone, and 70~ for aluminum alone.
For each embodiment of the invention des-cribed herein, the alloy layers other than the intrinsic alloy layers can be other than amorphous layers, such as polycrystalline layers. (By the term "amorphous" is meant an alloy or material which has long range disorder, although it may have short or intermediate order or even contain at times some crystalline inclusions.) Modifications and variations of the present invention are possible in light of the above teachings. It is therefore, to be understood that 1~4533~

within the scope of the appended claims the inven-tion may be practiced otherwise than as specif-ically described.

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Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In a photovoltaic device formed from semiconductor material including at least one active region upon which radiation can impinge to produce charge carries, an improved back reflector means for reflecting unused radiation back into said active region, said back reflector means comprising:
a first layer formed from a transparent material;
a second layer adjacent said first layer on the side thereof opposite said active region, said second layer formed from a highly reflective material; and said first layer of transparent conductor being of a thickness which renders the reflectivity or said back reflector means greater than the reflectivity of said highly reflective material alone.

2. A device as defined in claim 1 wherein said transparent material comprises a transparent conductor.

3. A device as defined in claim 2 wherein said transparent conductor comprises a transparent conductive oxide.

4. A device as defined in claim 3 wherein said transparent conductive oxide is formed from one of the group consisting of indium tin oxide, cadmium stannate, zinc oxide, vanadium oxide, ger-manium tin oxide, ferric oxide, cuprous oxide or tin oxide.

5. A device as defined in claim 2 wherein said transparent conductor comprises silicon car-bide.

6. A device as defined in claim 2 wherein said transparent conductor comprises a transparent conductor chalcogenide.

7. A device as defined in claim 6 wherein said transparent conductive chalcogenide comprises cadmium sulfide or zinc selenide.

8. A device as defined in claim 1 wherein said highly reflective material comprises a highly reflective metallic material.

9. A device as defined in claim 8 wherein said highly reflective metallic material is one of the group consisting of aluminum, silver, gold, and copper or alloys thereof.

10. A device as defined in claim 8 wherein said highly reflective metallic material comprises a metallic compound.

11. A device as defined in claim 10 wherein said metallic compound is one of the group con-sisting of WNx, TiNx, ZrNx, HfNx, and MoNx.

12. A device as defined in claim 1 wherein said semiconductor material is formed from amor-phous silicon alloys.

13. A device as defined in claim 1 wherein said active region comprises an intrinsic amor-phous silicon alloy including at least one density of states reducing element, said element being fluorine.

14. A device as defined in claim 13 wherein said intrinsic amorphous silicon alloy includes a second density of states reducing element incor-porated therein, said element being hydrogen.

15. A device as defined in claim 1 wherein said semiconductor material is formed from super-imposed layers of amorphous silicon alloys includ-ing an active intrinsic amorphous silicon alloy layer, a first doped amorphous silicon alloy layer between said intrinsic layer and said back reflec-tor means, and a second doped amorphous silicon alloy layer adjacent said intrinsic layer on the side thereof opposite said first doped layer and being of opposite conductivity with respect to said first doped layer.

16. A device as defined in claim 15 wherein said first doped layer comprises a wide band gap p-type amorphous silicon alloy.

17. A device as defined in claim 16 wherein said transparent conductor layer is between said wide band gap p-type layer and said layer of high-ly reflective material.

18. A photovoltaic device as defined in claim 1 wherein said transparent layer comprises a transparent barrier layer between said second layer and said active region to enhance reflection of unused radiation back into said active region and to preclude diffusion of said highly reflec-tive material into said active region.

19. A device as defined in claim 18 wherein said transparent barrier layer is a transparent conductive oxide.

20. A device as defined in claim 19 wherein said transparent barrier layer is formed from one of the group consisting of indium tin oxide, cad-mium stannate, zinc oxide, cuprous oxide or tin oxide.

21. A device as defined in claim 18 wherein said semiconductor material is formed from amor-phous silicon alloys.

22. A device as defined in claim 1 wherein said active region is an intrinsic amorphous sili-con alloy including at least one density of states reducing element, said element being fluorine.

23. A device as defined in claim 22, wherein said intrinsic amorphous silicon alloy includes a second density of states reducing element incor-porated therein, said element being hydrogen.

24. A device as defined in claim 18 wherein said semiconductor material is formed from super-imposed layers of amorphous silicon alloys includ-ing an active intrinsic amorphous silicon alloy layer, a first doped amorphous silicon alloy layer between said intrinsic layer and said back reflec-tor means, and a second doped amorphous silicon alloy layer adjacent said intrinsic layer on the side thereof opposite said first doped layer and being of opposite conductivity with respect to said first doped layer.

25. A device as defined in claim 24 wherein said first doped layer comprises a wide band gap p-type amorphous silicon alloy.

26. A device as defined in claim 25 wherein said transparent barrier layer is between said wide band gap p-type layer and said layer of high-ly reflective material.

27. A device as defined in claim 18 wherein said transparent barrier layer is a transparent conductive oxide.

28. A device as defined in claim 27 wherein said transparent conductive oxide is one of the group consisting of indium tin oxide, cadmium stannate, zinc oxide, cuprous oxide, and tin oxide.

29. A device as defined in claim 18 wherein said highly reflective material is one of the group consisting of silver, gold, aluminum, and copper.

30. A multiple cell photovoltaic device formed from multiple layers of amorphous semicon-ductor alloys deposited on a substrate, said device comprising:
a plurality of single cell units arranged in series relation including a bottom cell unit, each said single cell unit comprising a first doped amorphous semiconductor alloy layer, a body of intrinsic amorphous semiconductor alloy deposited on said first doped layer, a further doped amor-phous semiconductor alloy layer deposited on said intrinsic body and being of opposite conductivity with respect to said first doped amorphous semi-conductor alloy layer and a back reflector between said bottom cell unit and said substrate comprising a first layer formed from a transparent material adjacent said bottom cell unit and a second layer between said first layer and said substrate, said second layer being formed from a highly reflective material; and said first layer being of a thickness which renders the reflectivity of said back reflector means greater than the reflectivity of said highly reflective material alone.

31. A device as defined in claim 30 wherein said transparent material comprises a transparent conductor.

32. A device as defined in claim 31 wherein said transparent conductor comprises a transparent conductive oxide.

33. A device as defined in claim 32 wherein said transparent conductive oxide is one of the group consisting of indium tin oxide, cadmium stannate, zinc oxide, cuprous oxide, and tin oxide.

34. A device as defined in claim 30 wherein said highly reflective material is a highly reflective metallic material.

35. A device as defined in claim 34 wherein said highly reflective metallic material is one of the group consisting of aluminum, silver, gold, and copper or alloys thereof.

36. A device as defined in claim 30 wherein said first doped layer of said bottom cell com-prises a wide band gap p-type amorphous silicon alloy.

37. A device as defined in claim 30 wherein said plurality of cell units includes a top cell unit, and wherein said further doped layer of said top cell unit comprises a wide band gap p-type amorphous silicon alloy.

38. A multiple cell photovoltaic device as defined in claim 30 wherein said transparent mate-rial comprises a transparent barrier layer between said bottom cell unit and said first layer for en-hancing reflection of unused light back into said device and to preclude diffusion of said highly reflective material into said device.

39. A device as defined in claim 38 wherein said transparent barrier layer is a transparent conductive oxide.

40. A device as defined in claim 39 wherein said transparent conductive oxide is one of the group consisting of indium tin oxide, cadmium stannate, zinc oxide, cuprous oxide, and tin oxide.

41. A device as defined in claim 30 wherein said highly reflective material is a highly re-flective metallic material.

42. A device as defined in claim 41 wherein said highly reflective metallic material is one of the group consisting of aluminum, silver, gold, and copper or alloys thereof.

43. A device as defined in claim 30 wherein said first doped layer of said bottom cell com-prises a wide band gap p-type amorphous silicon alloy.

44. A device as defined in claim 30 wherein said plurality of cell units includes a top cell unit, and wherein said further doped layer of said top cell unit comprises a wide band gap p-type amorphous silicon alloy.

45. A device as defined in claims 3, 19, or 32 wherein said transparent conductive oxide has a thickness determined by the expression:

Where: d is the layer thickness;
.lambda. is the minimum photon wavelength to be re-flected;

n is the index of refrac-tion of the transparent conductor; and k is an odd integral multi-plier.