DE3308598A1 - REAR REFLECTOR SYSTEM FOR BARRIER PHOTO ELEMENTS - Google Patents

Description

Rear reflector system for barrier layer photo elements

The invention relates to improved retroreflector systems as well as barrier photo elements using them, especially those made from layers of amorphous semiconductor alloys are formed. The back reflector systems according to the invention provide for an increased reflection of unabsorbed Light back into the photo elements in which they are inserted. One advantage of this approach is that that an increased photon absorption and charge carrier generation is possible in the active areas, so that increased Short-circuit currents are generated. Another advantage is that it has improved photosensitivity properties fluorinated amorphous silicon alloys can be better realized by applying the invention in barrier layer photo elements are. The primary application of the invention is in the manufacture of improved photo barrier elements of the pin type made of amorphous silicon alloys, either as single cells or as multiple cells with a Plurality of single cell units. The doped layers of the pin cells preferably have low absorption coefficients in the wavelength ranges of interest, so that the back reflector according to the invention is optimal can be used.

Silicon is the foundation of the vast crystalline semiconductor industry and the material with which expensive high-efficiency (18%) crystalline solar cells for space travel were manufactured. For use on the The crystalline solar cells typically have significantly lower efficiencies in the order of magnitude of the earth

12% or less. As the technology of crystalline When semiconductor reached the stage of industrial exploitation, it became the basis of today's huge semiconductor device manufacturing industry. This relied on the ability of the scientist to be essentially flawless To pull germanium and especially silicon crystals and these to non-intrinsically conductive materials with contained therein To make p- and ■ n-conductive zones. This became through it achieved that in the crystalline material ppm amounts of than Donor (s) or as acceptor (p) acting dopants as substitutional impurities in the essentially pure crystalline materials were diffused to to increase their electrical conductivity and to control their conductivity type, i.e. p or η conductivity. the Manufacturing process for the manufacture of crystals with PN junctions involve extremely complex, time-consuming and expensive processes. So these crystalline materials, which can be used in solar cells and as current regulating devices, under very carefully controlled conditions produced by pulling single silicon or germanium single crystals and, if pn junctions are required, these single crystals are doped with extremely small and critical amounts of dopants.

With this crystal pulling process, so become relatively small Crystals made solar cells require the assembly of many single crystals to create the desired area for only a single solar cell panel is covered. The process required to manufacture a solar cell using this process Amount of energy, the limitations imposed by the size restrictions of the silicon crystal and the necessary

ability to cut such a crystalline material and put together have led to the industrial Use of crystalline semiconductor solar cells for energy conversion at the insurmountable cost barrier fails. Furthermore, crystalline silicon has an indirect absorption edge, which results in poor light absorption of the material results. Due to the poor light absorption, crystalline solar cells must have a minimum thickness of 50 jum to absorb the incident sunlight. Even if the single crystal material is through If polycrystalline silicon is replaced with correspondingly cheaper manufacturing processes, the indirect one remains Absorption edge still preserved; thus the material thickness is not reduced. In the case of the polycrystalline material there are also problems with grain boundaries and other defects which are usually disadvantageous.

In summary, it can be said that crystalline silicon components have fixed parameters which cannot be changed in the desired manner, that they require large amounts of material, can only be produced over a relatively small area and are expensive and time-consuming to produce. The use of components based on amorphous silicon alloys can eliminate these disadvantages of crystalline silicon. An amorphous silicon alloy has an absorption edge with properties similar to those of a direct bandgap semiconductor and only 1 µm or less of material is required to absorb the same amount of sunlight that is absorbed by the 50 µm thick crystalline silicon. Furthermore, amorphous silicon alloys can be produced faster, more easily and with a larger area than crystalline silicon.

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Thus, significant efforts have been devoted to methods of easily depositing amorphous semiconductor alloys or to develop films, each of which should cover relatively large areas, if desired only are limited by the size of the deposition apparatus, and a simple doping is used to form p- and η-conductive materials can take place when using pn-junction components are to be produced therefrom, which are equivalent to their crystalline counterparts. Over many years these efforts were essentially unproductive. Amorphous silicon or germanium films (i.e. those of group IV) are normally four-coordinate and have been found to have micro-vacancies and dangling bonds as well as other defects which produce a high density of local states in their band gap. The presence a high density of local states in the band gap of amorphous silicon semiconductor films results in a low one Light conductivity and a short carrier life, making such films suitable for light receiving applications are unsuitable. Furthermore, such films cannot be successfully doped or otherwise modified will cause the Fermi level to be shifted close to the conduction or valence bands, making the films unsuitable for the production of pn junctions for solar cells and current regulating devices.

In trying to minimize the aforementioned problems, found in amorphous silicon (which was originally thought to be elemental) have been reported by W.E. Spear and P.G. Le Comber of the Carnegie Laboratory of Physics, University of Dundee, Scotland, is working on a substit-

functional doping of amorphous silicon carried out ("Substitutional Doping of Amorphous Silicon", published in Solid State Communications, Vol. 17, 1193-1196, 1975) for the purpose of reducing local conditions in the Band gap in amorphous silicon, so that it can be more closely approximated to intrinsically conductive crystalline silicon would, and for substitutional doping of the amorphous materials with suitable classical dopants, such as when doping crystalline materials in order to make them non-intrinsically conductive and p- or η-conductive.

The reduction in local conditions was due to glow discharge deposition achieved by amorphous silicon films, whereby a silane gas (SiH.) is sent through a reaction tube in which the gas is decomposed by an HF glow discharge and on the substrate at a substrate temperature from approx. 500-600 K (227-327 C) was deposited. The material thus deposited on the substrate was intrinsically conductive amorphous material consisting of silicon and hydrogen. To create a doped amorphous material became a phosphine gas (PH_) for the n-conductivity or a diborane gas (B-Hj.) for the p-conductivity with the Silane gas premixed and passed through the glow discharge reaction tube sent under the same operating conditions.

The gas concentration of the dopants used was

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between approx. 5 χ 10 and 10 parts by volume. The material so deposited was non-intrinsically conductive and of the n- or p-line type.

Today, through the work of others, we know what these scientists did not know, namely that hydrogen

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in the silane itself at an optimal temperature with many of the free bonds of silicon during glow discharge deposition connects, whereby the density of the local states in the band gap is significantly reduced, so that the electronic properties of the amorphous material better can be approximated to those of the corresponding crystalline material.

The incorporation of hydrogen in the aforementioned process however, it has limits on the invariable hydrogen / silicon ratio in the silane as well as on various Si: H bond configurations that lead to new loosening states to lead. There are therefore fundamental restrictions on reducing the density of local areas States in these materials.

Substantially improved amorphous silicon alloys with significantly reduced concentrations of local conditions in their band gaps and with electronic properties high quality were obtained by glow discharge (see US Pat. No. 4,226,898) and by vapor deposition (see US Pat. No. 4,217,374) manufactured. As explained in these patents, fluorine is incorporated into the amorphous silicon semiconductor alloy built in to significantly reduce the density of local conditions in this. Activated fluorine binds particularly easy on silicon in the amorphous body, so that the density of local defect states in this is considerable is reduced because of the small size, the high reactivity and specificity of the chemical bond of the Fluorine atoms enable them to achieve a more defect-free amorphous silicon alloy. The fluorine

binds to the free bonds of silicon and forms - as is assumed - a mainly ionic stable Binding with flexible binding angles, which results in a more stable and effective compensation or change, than it is formed by hydrogen and other compensating or modifying agents. Fluorine combines furthermore preferably with silicon and hydrogen, the hydrogen being used in a more advantageous manner because hydrogen has multiple bonding options. Without fluorine, hydrogen cannot develop in the desired way bind in the material, but causes an additional defect state in the band gap as well as in the material itself. Hence Fluorine is considered to be a more effective compensating or altering element than hydrogen when it is either on its own or with hydrogen because of its high reactivity, specificity of the chemical bond and high electronegativity is used.

For example, compensation can only be made with fluorine or in conjunction with hydrogen by adding this element or these elements can be achieved in very small quantities (e.g. fractions of 1 atom%). The amounts of fluorine and hydrogen, which are used with particular preference, however, are considerably greater than these low percentages, see above that a silicon-hydrogen-fluorine alloy is formed. Such alloy amounts of fluorine and hydrogen are z. B. in the range of 1-5% or more. The alloy thus produced is believed to have a lower density of defect states in the energy band gap, than they can be achieved by the mere neutralization of dangling bonds and similar defect states. Further

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it is believed that such a larger amount of fluorine is significant on a new structure configuration of an amorphous silicon-containing material and facilitates the addition of other alloys, such as germanium. In addition to Its other properties already mentioned, fluorine is believed to be a local organizer Structure in the silicon-containing alloy by inductive and ionic effects is. It is believed that fluorine also affects the bonding of hydrogen by being in acts favorably in the direction of a decrease in the density of defect states caused by hydrogen while it acts as the density of states reducing element. The ionic role that fluorine in one such alloy plays is considered to be an important factor regarding of the relationships between nearest neighbors.

Amorphous silicon alloys containing fluorine thus have significantly improved characteristics for the Application as barrier photo elements versus amorphous silicon alloys shown as the density of states reducing element contain only hydrogen. However, to take full advantage of these amorphous silicon alloys, containing fluorine when used to form the active areas of barrier photovoltaic elements it must be ensured that the largest possible proportion of the available photons is absorbed in it, so that electron-hole pairs are effectively generated.

These processes are e.g. B. in junction photo elements with pin configuration of particular importance. Such photo

elements have ρ- and η-doped layers on opposite sides Pages of an active intrinsic layer in which the electron-hole pairs are generated. they form a voltage gradient across the device to facilitate the separation of electrons and holes, and also form contact layers, which simplifies the collection of electrons and holes as an electric current will.

The active areas do not absorb all of the available photons. It is true that practically all photons become shorter Wavelength absorbed, but a large proportion of the photons with a longer wavelength, whose energies are close to the absorption edge of the intrinsic semiconductor material is not absorbed. Loss of this unabsorbed Photons reduce the short-circuit currents that can be generated. To the loss of these photons with greater wavelength to exclude, back reflectors formed from electrically conductive metals are used to remove the unused or reflect unabsorbed light back into the active areas of the components.

The p- and η-conductive layers are conductive and have preferably a large band gap to reduce the photon absorption in these layers. Thus, is a Back reflector is extremely advantageous when it is used in conjunction with a p-conducting layer with a large band gap, which forms one of the doped layers of such a component is used. Retroreflective Layers thus serve to reflect the unused light back into the intrinsic area of the photo element,

where another use of solar energy for generation additional electron-hole pairs is possible. A retroreflective one Layer allows a larger proportion of the available photons to enter the active intrinsic layer and get absorbed there.

Unfortunately, the best of the known rear reflectors can only Approx. 80% of the unused light of the wavelengths of interest are reflected back into the components in which they are are used. Precious metals like copper and silver and metals like aluminum were considered as being highly conductive suggested possible materials for rear reflectors. However, these metals can be found in the semiconductor of the photo elements, in which they are used, diffuse and thereby the light response properties of the photo elements adversely affect. As a result, other, less highly conductive and reflective reflectors were used as rear reflectors Metals used. These are e.g. B. molybdenum and chromium. It is true that these metals form a diffusion into the semiconductor of the photo elements, however, can reduce the reflectance of the more highly reflective metals. It There is thus a need for better back reflection systems that not only have a stronger reflection of the unused Light takes place, but also exclude diffusion of the back reflector material into the photo elements.

There have now been new and improved retroreflector systems found that both result in an increased reflection of unused light compared to known back reflectors than also protection against diffusion of the rear reflector materials in the semiconductor of the photo elements offer. The back reflective

reri according to the invention can be used in single-cell blocking RchichL photo elements of the pin type as well as in multiple cell structures with a plurality of single cell units can be used.

The present invention provides new and improved retroreflective systems for use in barrier photo elements specified. The back reflector systems result in increased reflection of unabsorbed light back into the active areas of the photo elements in which they are provided and at the same time prevent diffusion of the rear reflector materials in the photo elements.

The retroreflective systems comprise a layer of a highly reflective material and a layer of one translucent conductor material. The translucent conductive layer is between the photo element and the highly reflective one Shift provided.

The highly reflective material can be a highly reflective one metallic material such as a highly reflective metal, e.g. B. gold, silver, copper or aluminum or alloys these materials. The highly reflective metallic material can be made from metallic compounds such as WN,

Ti, ZrN, HfN . or MoN. χ χ x1 χ

The translucent conductor can be a translucent conductive oxide such as indium tin oxide, cadmium stannate, doped Tin oxide, vanadium oxide, germanium tin oxide, iron (III) oxide, zinc oxide or copper (I) oxide. The translucent one Head can also be translucent conductive

Be a chalcogenide such as zinc selenide or cadmium sulfide. It can are also silicon carbide.

The translucent conductor has the function of reflecting to amplify unabsorbed light back into the photo elements, and also acts as a light permeable barrier layer, which is a diffusion of the highly reflective materials in the semiconductor areas of the photo elements prevented. The retroreflector systems according to the invention thus offer increased retroreflection of non-absorbed light without a deterioration in photosensitivity properties the semiconductor materials of the photo elements.

The rear reflectors according to the invention are specifically shown in FIG Pin-type barrier photo elements, usable. Such photo elements have an intrinsically conductive active semiconductor area in which light-induced electron-hole pairs are generated, and doped zones of opposite conductivity are provided on opposite sides of the intrinsic area. The active intrinsic The region is preferably an amorphous silicon alloy body or a fluorine as a density of states reducing element containing layer. The doped regions furthermore preferably comprise a p-conducting amorphous silicon alloy layer with a large band gap, which forms either the top or the bottom semiconductor layer of the photo element. In In both cases, the amorphous semiconductor regions are preferably deposited on a substrate, the layer being composed of highly reflective metal adjoins the substrate and the translucent conductive oxide between the highly reflective Material layer and the lowermost doped layer lies.

Essentially all photons with shorter wavelengths are absorbed in the active intrinsic areas, whereas only a part of the photons with longer wavelengths and energies close to the absorption edge of the intrinsic Material is absorbed. Therefore, the thickness of the transparent conductor is adjusted so that the reflection of the Photons with a longer wavelength is optimized. For this purpose, the thickness of the transparent conductor is preferred determined by the following relationship:

d = '

with d = the layer thickness,

Tl = the smallest to be reflective

Photon wavelength,
η = the refractive index of the translucent

Head, and
k = an odd integer multiplier.

The back reflector systems according to the invention are also shown in FIG Multi-cell photo elements, e.g. B. in tandem cells, can be used.

The back reflector system of the invention for a barrier photo element made of semiconductor material with at least one active area on which radiation is incident Generation of charge carriers, with the back reflector system reflecting unused radiation back into the active area, is characterized by one of a translucent

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casual material formed first layer and by a second layer attached to the first layer on top of the active Area on the opposite side of the same, the second layer being made of a highly reflective material consists.

A multi-cell junction photo element according to the invention made from a plurality of layers of amorphous semiconductor alloys, deposited on a substrate is characterized by a plurality of single cell units including a lower cell unit are provided in a row arrangement, each individual cell unit a first doped amorphous semiconductor alloy layer, an intrinsic amorphous semiconductor alloy body formed on the first doped layer is deposited, another doped amorphous layer deposited on the intrinsic body Semiconductor alloy layer which is opposite to the first doped amorphous semiconductor alloy layer from the opposite Conductivity type, and a back reflector located between the lowermost cell unit and the substrate and a first layer made of a light transmissive material adjacent the lower cell unit and a second layer between the first layer and the substrate, wherein the second layer of a highly reflective material consists, includes.

The invention is explained in more detail, for example, with the aid of the drawing. Show it:

1 shows a diagrammatic representation of a glow discharge separation device, the to

implementation of the method according to the invention for the production of the barrier layer photo elements can be used;

Figure 2 is a sectional view 2-2 of part of the device according to Fig. 1;

Fig. 3 is a sectional view of a pin-type barrier photo element embodying the invention; and

Fig. 4 is a sectional view of a multiple cell which comprises a plurality of pin junction photocell assemblies which are in a twin or tandem configuration are arranged.

1 shows a glow discharge deposition device 10 with a housing 12. This encloses a vacuum chamber 14 and comprises an inlet chamber 16 and an outlet chamber 18. A cathode carrier 20 is fastened in the vacuum chamber 14 via an insulator 22.

The cathode carrier 20 comprises an insulating sleeve 24 which surrounds it circumferentially. A dark room shield 26 is at a distance from the sleeve 24 this circumferentially surrounding. A substrate 28 is at an inner end 30 of the Cathode carrier 20 secured by means of a holder 32. The bracket 32 can be screwed or otherwise conventionally secured to the cathode carrier 20 in electrical contact therewith.

The cathode carrier 20 has a recess 34 into which an electrical heating element 36 for heating the cathode carrier 20 and thus the substrate 28 is installed. The cathode carrier 20 also includes a temperature sensor 38,

which measures the temperature of the cathode carrier 20. The temperature sensor 38 is used to activate the heating element 36 to maintain the cathode carrier 20 and the Substrate 28 at any desired temperature.

The device 10 further includes an electrode 30, which extends from the housing 12 into the vacuum chamber 14 at a distance from the cathode carrier 20. The electrode 40 includes them surrounding shielding 42, which in turn carries a substrate 44 attached to it. The electrode 40 has a recess 46, in which an electrode heating element 48 is built. The electrode 40 also has a temperature sensor 50, which measures their temperature and thus that of the substrate 44. The temperature sensor 50 is used to activate of heating element 48 to maintain electrode 40 and substrate 44 at any desired temperature regardless of the cathode carrier 20 to control.

A glow discharge plasma is in a space 52 between the Substrates 28 and 44 formed by power from a regulated RF, AC, or DC power supply, which is coupled to the cathode carrier 20 and through the space 52 to the electrode 40, which in turn is connected to earth. The vacuum chamber 14 is evacuated to the desired pressure by a vacuum pump 54 connected to the vacuum chamber 14 is coupled via a particle trap 56. A manometer 58 is connected to the vacuum system and is used Control of the pump 54 in such a way that the device 10 is kept at the desired pressure.

The inlet chamber 16 of the housing 12 preferably has a plurality of lines 60 for introducing materials into the

Device 10 in which they are mixed and in the vacuum chamber then deposited onto substrates 28 and 44 in glow discharge plasma space 52. If desired For example, the entry chamber 16 may be located at a remote location and the gases may prior to their introduction into the Vacuum chamber 14 are premixed. The gaseous materials are in the lines 60 through a filter or another cleaning device 62 is initiated with a throughput determined by a shut-off element 64.

If a material is not originally in gaseous form, but instead in solid or liquid form, it can be placed in a hermetically sealed container 66, as indicated at 68. The material 68 is then of a heating element 70 to increase its vapor pressure in the container 66. A suitable gas, ζ. Β. Argon, is introduced into the material 68 through a dip tube 72 and entrains and conveys the vapors of the material 68 through a filter 62 ″ and a shut-off element 64 ′ into the lines 60 and from there into the device 10.

The entry chamber 16 and the exit chamber 18 preferably have shields 74 so that the plasma in the Vacuum chamber 14, and there mainly between the substrates 28 and 44, is held.

The materials supplied through the lines 60 are mixed in the entry chamber 16 and then into the glow discharge space 52 to maintain the plasma and deposit the alloy on the substrates using silicon, Fluorine, oxygen and the other desired modification

elements such as hydrogen and / or dopants or other desired materials are incorporated.

For the deposition of layers of intrinsically conductive amorphous silicon alloys, the device 10 is first to a desired deposition pressure, e.g. B. evacuated less than 20 mTorr. Starting materials or reaction gases such as silicon tetrafluoride (SiF.) And molecular hydrogen (H ₂ ) and / or silane are introduced into the inlet chamber 16 through separate lines 60 and then mixed in the inlet chamber. The gas mixture is fed into the vacuum chamber in order to maintain a partial pressure of approx. 0.6 Torr in it. A plasma is generated in the space 52 between the substrates 28 and 44 using a direct voltage of more than 1000 V or by using RF energy of approximately 50 W, operating at a frequency of 13.56 MHz or another desired frequency .

In addition to the intrinsic amorphous silicon alloys deposited in the manner explained above, the photo elements described here also use doped amorphous silicon alloys, including those with a large band gap p. These doped alloy layers can be p-, p + -, n- or n + -conducting and by introducing a suitable dopant into the vacuum chamber together with the intrinsic starting material such as silane (SiH ₄ ) or silicon tetrafluoride (SiF.) And / or hydrogen and / or silane can be introduced.

For n- or p-doped layers, the material can be used with 5-100 ppm dopant doped during the deposition

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( ¹ H. For tu- or p + -doped layers, the material is doped with 100 ppm to more than 1% dopant during its deposition. The n-dopants are, for example, phosphorus, arsenic, antimony or bismuth η-doped layers deposited by the glow discharge decomposition of at least silicon tetrafluoride (SiF.) and phosphine (PH_). Hydrogen and / or silane gas (SiH ₄ ) can also be added to this mixture.

The p-type dopants are e.g. B. boron, aluminum, gallium, indium or thallium. The doped layers are preferably deposited by the glow discharge decomposition of at least silane and diborane (B ₀ H, -) or silicon tetrafluoride and diborane. Hydrogen and / or silane can also be added to the silicon tetrafluoride and diborane.

In addition, according to the invention, the p-type layers made of amorphous silicon alloys containing at least one bandgap-increasing element. For example, carbon and / or nitrogen can be used in the p-type alloys to increase their band gaps to be built in. An amorphous silicon alloy with a large band gap ρ can e.g. B. by a gas mixture of silicon tetrafluoride (SiF.), Silane (SiH.), Diborane (B_H ,.)

4 fi ί Ο

and methane (CH.) are formed. This results in a p-type amorphous silicon alloy with a large band gap.

The doped layers of the photo elements are deposited at different temperatures depending on the one Material type and the substrate used deposited.

In the case of aluminum substrates, the maximum temperature should not be above approx. 600 ^° C., while it can be ^{above approx. 1000 °} C. in the case of stainless steel. In the case of intrinsically conductive and doped alloys that were originally compensated with hydrogen, e.g. B. from silane gas as the raw material deposited alloys, should the substrate temperature below about 400 ⁰ C, preferably between 250 and 350 ⁰ C, are. -,
■■■■. ■ [

Other materials and alloy elements can be intrinsically conductive and doped layers can also be added in order to achieve optimized power generation. These further materials and elements are described below in connection with the exemplary embodiments of the photo elements according to FIGS. 3 and 4 explained.

Fig. 3 shows in section a pin photo element according to the invention. The photo element 110 includes a substrate 112 made of glass or a flexible stainless steel or aluminum sheet. The substrate 112 has a desired length and length Width and a thickness of preferably 0.12-0.25 mm.

According to the invention, on the substrate 112 is a highly reflective Material layer 114 deposited. Layer 114 is applied by vapor deposition, which is a relatively fast application process is. Layer 114 is preferably a highly reflective metallic material such as Silver, gold, aluminum or copper or alloys of these materials. The highly reflective material can also be a highly reflective metallic compounds such as WN, TiN,

X Λ

ZrN, HfN “or MoN. On the layer 114 is

a translucent leather layer 115 is applied. This can be a translucent conductive oxide (TCO) deposited in a vapor deposition environment; she is z. B. indium tin oxide (ITO), cadmium stannate (Cd-SnO ^), zinc oxide, copper (I) oxide, vanadium oxide, germanium tin oxide, iron (III) oxide or tin oxide (SnO ₂ ). The transparent conductive layer 115 can also consist of silicon carbide or a transparent chalcogenide such as cadmium sulfide or zinc selenide. The highly reflective layer 114 and the light-transmissive conductor layer 115 form a back reflection system according to the invention.

The substrate 112 is then placed in the glow discharge deposition facility brought in. A first doped p-type amorphous silicon alloy layer 116 with a wide band gap is deposited on layer 115 in accordance with the invention. The illustrated layer 116 is p + -type. The pH zone is made as thin as possible, their thickness is between 50 and 500 Å, which is sufficient for a good ohmic Establish contact between the p + region and the transparent conductive oxide layer 115. The pH zone is used also to form a potential gradient through the photo element, whereby the trapping of light-induced Electron-hole pairs as an electric current is facilitated. The p + ~ zone 116 can be any of the previously indicated Gas mixtures are deposited.

Then an intrinsic amorphous silicon alloy body is made 118 on the p-type layer 116 with a large Band gap deposited. The intrinsic body 118 is relatively thick, on the order of 4500 8, and becomes

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made of silicon tetrafluoride and hydrogen and / or silane deposited. The intrinsic body preferably contains the amorphous silicon alloy compensated with fluorine, since this is where most of the electron-hole pairs are generated. The short-circuit current of the photo element is combined by the Effects of the back reflector according to the invention and the large band gap of the p-type amorphous silicon alloy layer 116 increased.

Another doped one is doped on the intrinsic body 118 Layer-120. deposited, which is opposite to the first doped layer 116 of the opposite conductivity type. she consists of an n + -conducting amorphous silicon alloy and can also have a large band gap. the n + layer 120 is deposited from any of the gas mixtures previously mentioned in this context. It is deposited in a thickness between 50 and 500 Å and serves as a contact layer.

A transparent conductive oxide layer (TCO layer) 122 is then deposited on the n + layer 120. This can also be applied in a vapor deposition environment and is e.g. B. indium tin oxide (ITO), cadmium stannate (Cd ₂ SnO.) Or doped tin oxide (SnO ₂ ).

On the surface of the TCO layer 122 is a grid electrode 124 made of a metal with good electrical conductivity. The grid can be perpendicular to each other Comprise lines of conductive material that take up only a small proportion of the surface of the metallic area, exposing the rest of the surface to solar energy

is. For example, the grid 124 can only be about 5-10% of the total area of the TCO layer 122 cover. The grid electrode ensures that electricity is evenly collected from the TCO layer 122, so that a good low series resistance for the photo element is ensured.

To complete the photo element 110, refer to the Grid electrode 124 and the surface areas of the TCO layer 122 a reflection-reducing element between the grating areas or AR layer 126 applied. The AR layer has a surface that is exposed to solar radiation hits. For example, the AR layer 126 has a thickness on the order of the wavelength of the maximum energy point the solar radiation spectrum divided by four times the refractive index of the anti-reflective layer 126. A suitable one AR layer 126 is e.g. B. zirconium oxide with a thickness of about 500 A and a refractive index of 2.1. Alternatively can the TCO layer 122 can also act as an anti-reflective layer, in which case the anti-reflective Layer 126 is omitted and a suitable casting compound is provided for it.

The transparent conductor layer 115 and the TCO layer 122 do not need to be made of the same material. The TCO layer 122 must be capable of incident To transmit radiation of both short and long wavelengths. However, there is essentially all of the radiation shorter wavelength in intrinsic region 118 is absorbed through it during the first pass, the light-transmissive conductor layer 115 only needs for To be transparent to radiation of longer wavelengths, e.g. B. for light with wavelengths of about 6000 A or longer.

The thickness of the transparent conductive layer 155, which in the present case is a transparent conductive oxide, can be adjusted to optimize the increase in the reflectance of layer 115. E.g. that is The thickness of the layer 115 is preferably determined by the following relationship

d = A k / 4 / n

with d = thickness of layer 115,

V = smallest photon wavelength to be reflected, η = refractive index of the translucent conductor, and k = an odd integer multiplier.

Almost all photons of shorter wavelengths are from of the intrinsic active layer 118 is absorbed. As a result, as already explained, the vast majority of unabsorbed photons have longer wavelengths. These photons can have wavelengths of around 6000 A and longer to have. In the case of a translucent conductive oxide, e.g. B. indium tin oxide with a refractive index of about 2.0 in these longer wavelengths, where k is preferably equal to 1, the thickness of the layer 115 should be approx. 750 8.

Any of the aforementioned highly reflective materials can be used in conjunction with the indium tin oxide layer of 750 Ä can be used. From the aforementioned reflective Materials, however, copper is the most cost-effective and has long wavelengths of 6000 A or more a good reflectance. With this combination

from Workft: <- fPen and the thickness of the indium tin oxide from 750 K can reflect at least 97% of all unused light back into the semiconductor areas of the photo element 110 can be expected. Furthermore, since the translucent Conductive oxide also acts as a translucent barrier layer, diffusing the copper or one of the other highly reflective materials in the Prevents semiconductor regions of the photo element 110 ..

As already mentioned, the band gap of the intrinsic layer 118 can be based on a specific light sensitivity characteristic can be adjusted by installing the band gap reducing elements. Alternatively, the band gap of the intrinsic body 118 be graded so that it is steadily larger from the p + -layer 116 to the n + -layer 120 (See, e.g., U.S. Patent Application Serial No. 427,756 filed Sept. 29, 1982). While the intrinsic layer 118 is deposited, z. B. one or more bandgap reducing elements such as germanium, tin or Lead can be built into the alloys in a steadily decreasing concentration. For example, Monogermangas (GeH.) Can be added to the Glow discharge deposition chamber with one first relative high concentration and then, while the intrinsic layer is being deposited, down to a point at which the initiation is terminated. The resulting intrinsic body thus contains a Bandgap-reducing element such as germanium in a concentration that decreases steadily from the pH layer 116 to the n + layer 120.

4 shows a multi-cell photo element 150 in tandem or Twin arrangement. Photo element 150 includes two

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Single cell units 152 and 154 arranged in series are provided. Of course, more than two can Single cell units are used.

The photo element 150 includes a substrate 156 of a electrically conductive metal such as stainless steel or aluminum. On the substrate 156 is a back reflector system deposited according to the invention, which a layer 157 from highly reflective material made from one of the materials described above and with one of the explained Process can be produced includes. To complete the back reflector, a layer 159 is made of one translucent conductor, e.g. B. a translucent conductive oxide, on the highly reflective layer 157 deposited. The layer 159 can consist of one of the described light-permeable conductor materials and with be deposited to an optimal thickness.

The first cell unit 152 comprises a first doped one p + -type amorphous silicon alloy layer 158 deposited on the transparent conductive oxide layer 159 is. According to the invention, the p + layer is preferably a p-conducting amorphous silicon alloy with a large band gap. It can be deposited from any of the starting materials already mentioned for this purpose.

There is a first on the wide band-spaced p + layer 158 intrinsic amorphous silicon alloy body 160 deposited. This is preferably an amorphous silicon-fluorine alloy .

The second cell unit 154 is essentially identical formed and comprises a first doped .pH layer 164, an intrinsic body 166 and a further doped n + layer 168. The photo element 150 is made up of a TCO layer 170, a grid electrode 172 and an anti-reflective layer 174 are completed.

The band gaps between the intrinsic layers are preferred adjusted so that the band gap of layer 166 is greater than that of layer 160. For this purpose, the alloy forming layer 166 has one or more bandgap-enlarging elements such as nitrogen and carbon contain. The intrinsic alloy making up intrinsic layer 160 may include one or more of the bandgap contain reducing elements such as germanium, tin or lead.

It can be seen from the figure that the intrinsic layer 160 of the cell is thicker than the intrinsic layer 166. As a result, the entire usable spectrum of solar energy can be used to generate electron-hole pairs.

A tandem photocell was shown and explained above; but the cell units can also through Oxide layers are isolated from one another, so that a multi-cell stack is formed. Each cell unit could have two collecting electrodes to simplify the series connection of the cell units with external circuits.

As another alternative, which is already in relation to the Single cells mentioned can be one or more of the

intrinsic body of the cell units with alloys include graduated band gaps. Any of the aforementioned increasing or decreasing the band gap For this purpose, elements can be individually or combined in the intrinsic alloys are installed (cf. e.g .. the U.S. patent application serial no. 427 757).

As can be seen from the above explanation, new and improved back reflector systems are created by the invention, the z. B. in barrier photo elements can be used. The rear reflectors not only increase that Amount of reflected back into the semiconductor areas of the photocells unused light, but also prevent diffusion of the back reflector materials into the semiconductor areas. The efficiency of the new and improved rear reflectors results from a translucent conductive one Oxide like indium tin oxide from the reflectivity of 98.5%, 97% or 90% when using highly reflective Metals such as silver, copper or aluminum compared to degrees of reflection of 80% for only silver, 74% for only Copper or 70% for aluminum only.

In each of the exemplary embodiments described, with the exception of the intrinsic alloy layers, the Alloy layers not to be amorphous, but can e.g. B. be polycrystalline. ("Amorphous" means an alloy or a material with far-reaching disorder, although it is also one with a near or an intermediate degree of order or even crystalline inclusions can be present from time to time.)

Claims (45)

P a t e η t a η s ρ r ü_c _h__e / Ί /. Back reflector system for a barrier photo element Semiconductor material with at least one active area on the radiation strikes to generate charge carriers, the back reflector system in the unused radiation active area reflected back, indicated by a first layer formed from a translucent material (115; 159); and a second layer (114; 157) adjacent to the first layer on the opposite of the active area (118; 160, 166) Side of the same adjoins and consists of a highly reflective material. 2. rear reflector system according to claim 1, characterized in that that the translucent material comprises a translucent electrical conductor. 3. back reflector system according to claim 2, characterized, that the translucent electrical conductor is a translucent conductive oxide. 4. rear reflector system according to claim 3, characterized in that that the translucent conductive oxide indium tin oxide, Cadmium stannate, zinc oxide, vanadium oxide, germanium tin oxide, Is iron (III) oxide, copper (I) oxide or tin oxide. 5. Rü;: kti, * flcktousystem according to claim 2, characterized in that the transparent electrical conductor comprises silicon carbide. 6. rear reflector system according to claim 2, characterized in that that the translucent electrical conductor is a translucent conductive chalcogenide. 7. back reflector system according to claim 6, characterized in that the transparent conductive chalcogenide is cadmium sulfide or zinc selenide. 8. back reflector system according to one of claims 1-7, characterized in that the highly reflective material is a highly reflective one metallic material is. 9. back reflector system according to claim 8, characterized in that the highly reflective metallic material aluminum, Is silver, gold or copper or their alloys. 10. back reflector system according to claim 8, characterized in that the highly reflective metallic material is a metallic compound includes. ' 11. back reflector system according to claim 10, characterized, that the metallic compound WN-. TiN _v , ZrN _v , HfN _v or MoN. 12. Back reflector system according to one of claims 1-11, characterized in that the semiconductor material consists of amorphous silicon alloys consists. 13. Rear reflector system according to claim 12, characterized in that that the active area (118; 160, 166) is an intrinsic amorphous silicon alloy with at least one density of states reducing element, this element being fluorine. ■ 14. rear reflector system according to claim 13, characterized in that a second element reducing the density of states is built into the intrinsically conductive amorphous silicon alloy where this element is hydrogen. 15. Rear reflector system according to one of claims 1-14, characterized in that the semiconductor material consists of superposed Amorphous silicon alloy layers and comprises: an active intrinsic amorphous silicon alloy layer (118; 160, 166), a first doped amorphous silicon alloy layer (116; 158, 164) between the intrinsic layer and the back reflector system and a second doped amorphous silicon alloy layer (120; 162, 168) attached to the intrinsic layer on its to -A- ■ the first doped layer adjoins the opposite side and is of opposite conductivity relative to the first doped layer. 16. Back reflector system according to claim 15, characterized in that that the first doped layer (116; 158, 164) is a p-type is amorphous silicon alloy with a wide band gap. 17. Rear reflector system according to claim 16, characterized in that that the transparent conductor layer (115; 159, 170) between the p-conductive layer (116; 158, 164) with large Band gap and the highly reflective layer (114; 157) lies. 18. Barrier photo element with back reflector system according to claim 1, characterized, that the transparent layer (115; 159, 170) between the second layer (120; 162, 168) and the active area (118; 160, 166) comprises a light transmissive barrier layer that prevents the reflection of unused radiation back into the active area is amplified and the diffusion of the highly reflective Excludes material in the active area. 19. rear reflector system according to claim 19, characterized, that the translucent barrier layer (118; 160, 166) is a translucent conductive oxide. . · ^: ** ·· * ■ · "'- ■ ■ - ■ ~ 3303598 20. Rear reflector system according to claim 19, characterized in that that the transparent barrier layer (118; 160, 166) indium tin oxide, cadmium stannate, zinc oxide, copper (I) ~ oxide or is tin oxide. 21. Rear reflector system according to one of claims 18-20, characterized in that the semiconductor material consists of amorphous silicon alloys consists. 22. Back reflector system according to one of claims 18-21, characterized in that the active area is an intrinsic amorphous silicon alloy with at least one element which reduces the density of states ", this element being fluorine. 23. Back reflector system according to claim 22, characterized in that a second element reducing the density of states is built into the intrinsically conductive amorphous silicon alloy where this element is hydrogen. 24. Back reflector system according to one of claims 18-23, characterized in that the semiconductor material consists of superposed amorphous silicon alloy layers comprising an active intrinsic amorphous silicon alloy layer (118; 160, 166), a first doped amorphous silicon alloy layer (116; 158, 164) between the intrinsic layer and the back reflector and a second doped amorphous Silicon alloy layer (120; 162 _r 168) which adjoins the intrinsic layer on its opposite side to the first doped layer and has opposite conductivity with respect to the first doped layer. 25. Rear reflector system according to claim 24, characterized in that the first doped layer (116; 158, 164) is a p-type is amorphous alloy with a large band gap. 26. Rear reflector system according to claim 25, characterized in that that the transparent barrier layer (115; 159, 170) between the p-type layer (116; 158, T6.4) with large Band gap and the highly reflective layer (114; 157) lies. 27. Back reflector system according to one of claims 24-26, characterized in that the light transmissive barrier layer (115; 159, 170) is a is translucent conductive oxide. 28. Back reflector system according to claim 27, characterized in that the transparent conductive oxide indium tin oxide, Cadmium stannate, zinc oxide, cupric oxide or tin oxide is. 29. Rear reflector system according to one of claims 24-28, characterized in that ' that the highly reflective material silver, gold, aluminum or copper. 30. A multi-cell junction photo element made from a plurality of layers of amorphous semiconductor alloys deposited on a substrate;
characterized by a plurality of single cell units provided in series including a lower cell unit, each single cell unit comprising: a first doped amorphous semiconductor alloy layer, an intrinsic amorphous semiconductor alloy body deposited on the first doped layer, - A further doped amorphous semiconductor alloy layer deposited on the intrinsically conductive body, which is shown in is of the opposite conductivity type with respect to the first doped amorphous semiconductor alloy layer, and - a back reflector between the bottom cell unit and the substrate and a first layer consisting of a light-permeable material adjacent lower cell unit and a second layer between the first layer and the substrate having, wherein the second layer consists of a highly reflective material. 31. The multi-cell junction photovoltaic element of claim 30, characterized, that the translucent material comprises a translucent electrical conductor. 32. Multi-cell junction photo element according to claim 31, characterized in that the translucent conductor is a translucent conductive oxide. 33. Multi-cell junction photo element according to claim 32, characterized in that the translucent conductive oxide indium tin oxide, Cadmium stannate, zinc oxide, cupric oxide or tin oxide 34. Multi-cell barrier layer photo element according to one of claims 30-33, characterized in that the highly reflective material is a highly reflective one metallic material is. 35. Multi-cell junction photo element according to claim 34, characterized in that the highly reflective metallic material aluminum, Silver, gold or copper or alloys of these metals 36. Multi-cell barrier layer photo element according to one of claims 30-35, characterized in that the first doped layer (116; 158, 164) of the lowermost Cell unit comprises a p-type amorphous silicon alloy with a large band gap. 37. Multi-cell junction photo element according to any one of Claims 30-36, characterized, that the plurality of cell units is a topmost cell unit has, and that the further doped layer (116; 158, 164) of the top Cell unit a p-type amorphous silicon alloy with a large band gap. 38. Multi-cell junction photo element according to claim 30, characterized, that the translucent material forms a translucent barrier layer (115; 159, 170) between the lowermost cell unit and the first layer (116; 158, 164) which is the reflection of wasted light back into the photo element (110; 150) and prevents diffusion of the highly reflective material into the photo element. 39. Multi-cell junction photo element according to claim 38, characterized, that the transparent barrier layer (115; 159, 170) a is translucent conductive oxide. 40. Multi-cell junction photo element according to claim 39, characterized, that the transparent conductive oxide is indium tin oxide, cadmium stannate, zinc oxide, cupric oxide or tin oxide 41. Multi-cell junction photo element according to any one of Claims 30-40, characterized in that the highly reflective material is a highly reflective metallic material is. 42. Multi-cell junction photo element according to claim 41, characterized in that the highly reflective metallic material aluminum, Silver, gold or copper or alloys of these metals 43. Multi-cell junction photo element according to one of claims 30-42, characterized in that the first doped layer (116; 158, 164) of the lowermost Cell unit with a p-type amorphous silicon alloy large band gap is. 44. Multi-cell junction photo element according to claim 30, characterized in that the plurality of cell units are a topmost cell unit comprises, and that the further doped layer (116; 158, 164) of the uppermost Cell unit is a p-type amorphous silicon alloy with a large band gap. 45. Multi-cell barrier layer photo element according to one of claims 3, 19, 27, 32 or 39, characterized in that the thickness of the transparent conductive oxide is through the following relationship is determined: with d = layer thickness,. > - = smallest photon wavelength to be reflected, η = refractive index of the translucent conductor, and k = an odd integer multiplier.