WO2008010205A2

WO2008010205A2 - Thin-film photovoltaic conversion device and method of manufacturing the same

Info

Publication number: WO2008010205A2
Application number: PCT/IL2007/000847
Authority: WO
Inventors: Boris Sigalov
Original assignee: Solaroll Ltd
Priority date: 2006-07-16
Filing date: 2007-07-08
Publication date: 2008-01-24
Also published as: WO2008010205A3; IL176885A0

Abstract

A thin-film photovoltaic conversion device, formed on a substrate, preferably made of a flexible plastic, having first and second conductive layers as electrodes, n-type and p-type layers, graded (varizone) band gap layer including pure silicon and silicon in chemical compositions selected from a group, consisting of SixGe1-x, SixCy, SixNy and SixOyNz, all these chemical compositions being simultaneously comprised in graded band gap layer and smoothly changing from one to the other. The photovoltaic device additionally comprises reflective layer, an anti-reflective layer and a protective laminating layer. The device is manufactured on the basis of at least one vacuum chamber according to two embodiments of proposed method.

Description

THIN-FILM PHOTOVOLTAIC CONVERSION DEVICE AND METHOD OF MANUFACTURING THE SAME

FIELD OF THE INVENTION

[0001] The present invention relates to thin-film photovoltaic (photoelectric) conversion devices and to a method of their manufacturing, more particularly, to a method of manufacturing which can provide the performance of a conversion device, reduce its cost, enhance the flexibility of manufacturing steps and improve manufacturing efficiency.

BACKGROUND OF THE INVENTION

[0002] In recent years, semiconductor thin film photovoltaic conversion devices as represented by a solar cell have been diversified, and crystalline silicon thin film solar cells have been developed in addition to conventional amorphous thin film solar cells. Furthermore, a tandem (hybrid)-type thin film solar cell having a stack thereof has come into practical use.

[0003] In general, a silicon thin film photovoltaic conversion device includes a first electrode, one or more semiconductor thin film photovoltaic conversion units and a second electrode stacked in sequence on a substrate at least a surface portion of which is insulated. Further, one photoelectric conversion unit includes an i-type layer sandwiched between a p-type and an n-type layer.

[0004] A major portion of the thickness of the thin film photovoltaic conversion unit is occupied by the i-type layer of a substantially intrinsic semiconductor layer, and photovoltaic conversion occurs mainly in the i-type layer. Accordingly, it is preferable that i-type layer as a photovoltaic conversion layer has a greater thickness for the purpose of light absorption, though increase of the thickness increases costs and time for deposition of the i-type layer.

[0005] The p-type and n-type conductive layers serve to produce a diffusion potential within the photovoltaic conversion unit, and magnitude of the diffusion potential affects the value of open-circuit voltage which is one of the important properties of a thin photovoltaic conversion device. However, these conductive layers are inactive layers, which do not directly contribute to photovoltaic conversion. That is, light absorbed by these inactive layers is a loss, which does not contribute to electric power generation. Consequently, it is preferable to minimize the thickness of the p-type and n-type conductive layers as far as they provide a sufficient diffusion potential.

[0006] For this reason, regardless of whether p-type and n-type conductivity type layers included in a photovoltaic conversion unit or photovoltaic conversion device are amorphous or crystalline, one whose i-type photovoltaic conversion layer which occupies a major portion of the conductivity type layer is amorphous is called an amorphous unit or an amorphous photovoltaic conversion device, and one whose i-type layer is crystalline is called a crystalline unit or a crystalline photovoltaic conversion device. .

[0007] There are known and widely used in technology solar cells for conversion of solar energy into electric power using amorphous silicon, these cells being based on the structures of p-n photovoltaic junctions, heteroj unctions, Schottky diodes, p- i-n junctions. The cheapest and most promising thereof, having the highest factor of electric energy conversion are solar cells based on the structures of p-i-n junctions formed on a substrate of glass, metal or flexible plastic. The i-type layer in these solar cells is a generally contains amorphous hydrogenated silicon (a- Si:H). The coefficient of solar energy conversion does not usually exceed 5%. [0008] There are further known tandem (cascade) solar cells consisting at least of two ρ-i-n units. Examples of such solar cells and methods of their manufacturing are described in European patent application EP 1198013 A2 and U.S. patent application US 2005/0181534 Al. [0009] European patent application EP 1198013 A2 by "Kaneka Corporation" (Japan) describes a solar cell including a plurality of photoelectric conversion units stacked on a substrate, each having a p-type layer, an i-type photoelectric conversion layer and an n-type layer deposited in this order from a light-incident side of the solar cell, and at least a rear unit among the photoelectric conversion units that is furthest from the light-incident side being a crystalline unit including a crystalline i-type photoelectric conversion layer. Manufacturing method includes the steps of forming at least one of the units on a substrate by plasma CVD and immediately thereafter forming an i-type boundary layer to a thickness of at most 5 nm by plasma CVD, and thereafter removing the substrate into the atmosphere and then forming a crystalline unit on the i-type boundary layer by plasma CVD. [0010] Masashi Yoshimi et al. (Japan) in U.S. patent application 2005/0181534 Al describe a method of manufacturing a tandem-type thin film photoelectric conversion device including the steps of forming at least one photoelectric conversion unit on a substrate in a deposition apparatus, taking out the substrate having the photoelectric conversion unit from the deposition apparatus to the air, introducing the substrate into a deposition apparatus and carrying out plasma exposure processing on the substrate in an atmosphere of a gas mixture containing an impurity for determining the conductivity type of the same, as the uppermost conductivity type intermediate layer is formed by additionally supplying semiconductor raw gas to the deposition apparatus, and then forming a subsequent photoelectric conversion unit.

[0011] The two solar cells described above differ in composition and structure of their i-layer. It is known that the i-layer is a basic layer in solar cells absorbing sunlight, in this case it is an i-layer of amorphous silicon a-Si:H generating electrons and holes. The forbidden energy gap in described solar cells measures about 1.7 eV. This means that sunlight having a smaller energy (Eg) than the forbidden energy gap will not be absorbed by such solar cells. Doping the silicon which forms the i-layer of a solar cell by germanium allows to lower the energy threshold Eg to 1.5 - 1.6 eV. Another way of lowering the threshold of absorbed energy Eg is the use of such tandem (cascade) solar cells wherein each p-i-n unit absorbs its range of the solar spectrum.

[0012] Qi Wang and Eugene Iwaniczko in US 2004/0168717 Al and in WO 03/017384 Al disclose a thin-film solar cell, which comprises an a~SiGe:H (1.6 eV) p-i-n solar cell having a deposition rate of at least ten (10) A°/second for the a- SiGe:H intrinsic layer by hot wire chemical vapor deposition. A method for fabricating a thin film solar cell comprises depositing a p-i-n layer at a deposition rate of at least ten (10) A°/second for the a-SiGe:H intrinsic layer. The same thin- film a-Si:H semiconductor material for photovoltaic and other devices and method for its production is described by Archie H. Mahan et al. in U.S. Pat. 6,468,885. [0013] There are further known solar cells comprising multi-layer p-i-n structures, when each of p-i-n structures includes silicon in a different state, such as microcrystalline silicon (mc-Si), amorphous silicon (a-Si:H), polycrystalline silicon (poly silicon). Examples of such photovoltaic devices and methods of their manufacturing are given in U.S. patents and patent applications filed by "Canon Kabushiki Kaisha" (Japan): U.S. 2005/0028860 Al, U.S. 2003/0213515 Al, U.S. 2003/0079771 Al, U.S. Pat. 6,835,888, U.S. Pat. 6,339,873, U.S. Pat. 6,268,233. [0014] The typical aforementioned photovoltaic device is described by Masafumi Sano and Tetsuro Nakamura from "Canon Kabushiki Kaisha" in Pat. application U.S. 2003/0079771 Al: a stacked photovoltaic device comprises at least three p-i-n junction constituent devices superposed in layers, each having a p-type layer, an i- type layer and an n-type layer which are formed of silicon type non-single-crystal semiconductors. An amorphous silicon layer is used as the i-type layer of a first p- i-n junction, a microcrystalline silicon layer is used as the i-type layer of a second p-i-n junction and a microcrystalline silicon layer is used as the i-type layer of a third p-i-n junction, the first to third layers being in order from the light-incident side. The drawbacks of these solar cells are a small forbidden energy gap which ranges from 1.55 to 1.75 eV, and voltage loss in each layer of the p-i-n structure. [0015] Inventions described by Stanford R. Ovshinsky et al. in US Pat. 6,723,421 and U. S. Pat. Applications U.S. 2004/0231590 Al and U.S.2006/0024442 Al, are close to the present invention.

[0016] U.S. Pat. 6,723,421 describes a non-single semiconductor material including coordinatively irregular structures characterized by distorted chemical bonding, reduced dimensionality and novel electronic properties. A process for forming the material permits variation of size, concentration and spatial distribution of coordinatively irregular structures. The electronic properties of the material can be changed by controlling the characteristics of the coordinatively irregular structures.

[0017] Aforementioned patent applications U.S. 2004/0231590 Al and U.S.2006/0024442 Al by Stanford R. Ovshinsky describe a deposition apparatus and method for continuously depositing a polycrystalline material such as polysilicon or polycrystalline SiGe-layer on a mobile discrete or continuous web substrate. The apparatus includes a pay-out unit door dispensing a discrete or continuous web substrate and a deposition unit that receives the discrete or continuous web substrate and deposits a series of one or more thin film layers thereon in a series of one or more deposition or processing chambers. In a preferred embodiment, polysilicon is formed by first depositing a layer of amorphous or miciOcrystalline silicon using PECVD and transforming this layer to polysilicon through heating or annealing with one or more lasers, lamps, furnaces or other heat sources. Laser annealing utilizing a pulsed exciter is a preferred embodiment. By controlling the processing temperature, temperature distribution within a layer of amorphous or microcrystalline silicon etc., the instant deposition apparatus affords control over the grain size of polysilicon. Passivation of polysilicon occurs through treatment with a hydrogen plasma. Layers of polycrystalline SiGe may be formed likewise. The instant deposition apparatus provides for continuous deposition of electronic devices and structures that include a layer of a polycrystalline material such as polysilicon and/or polycrystalline SiGe. Representative devices include photovoltaic devices and thin film transistors. The instant deposition apparatus also provides for continuous deposition of chalcogenide switching or memory materials alone or in combination with another metal, insulating, and/or semiconducting layers.

SUMMARY OF THE INVENTION

[0018] In accordance with the present invention, a thin-film photovoltaic conversion device is formed of a substrate; a first conductive layer; a first doped layer; a graded (varizone) band gap layer having two side faces and including pure silicon and silicon in chemical compositions with germanium, carbon, nitrogen, or nitrogen and oxygen, of the formula selected from a group, consisting of Si_x Ge_1-X , Si_xCy, Si_xNy, and Si_xOyN₂ , all these chemical compositions being simultaneously comprised in graded band gap layer, and disposed successively in direction from one said side face to the other and smoothly changing from one to the other; a second doped layer; and a second conductive layer. The substrate of this conversion device is made of metal, glass or plastic and a graded band gap layer mainly comprising silicone.

[0019] Besides, the thin-film photovoltaic conversion device additionally comprises a reflective layer near the substrate on a light-incident substrate side, an anti-reflective layer disposed above said layers on the light-incident side of the device, and a protective laminating layer disposed on the photovoltaic conversion device on the side opposite to the substrate.

[0020] The substrate of the thin-film photovoltaic conversion device is made of flexible plastic. The first conductive layer and the second conductive layer are electrodes. Besides, the first conductive layer or the second conductive layer, disposed on the light-incident side relative to the substrate, is transparent . [0021] In this thin-film photovoltaic conversion device the first doped layer and second doped layer are disposed on both sides of the graded band gap layer, one of them being of a p-type, and the other of an n-type. Finally, the band gap layer has two side faces and includes pure silicon and silicon in chemical compositions with germanium, carbon, nitrogen, or nitrogen and oxygen, of the formula selected from a group, consisting of Si_x Ge_1-X , Si_xCy, Si_xNy, and Si_xO_yN_z. Chemical composition of the graded band gap layer is smoothly transitioning in sequence: Si_x Ge_1-X , Si, Si_xCy, Si_xNy, Si_xOyN₂, in a light direction. Energetic limits of the chemical compositions comprised in this graded band gap layer change from 0,9 eV (infrared region) to 3,5 eV (ultra-violet region).

[0022] According to a method of manufacturing a thin-film photovoltaic conversion device of the present invention, this device is made by sequentially forming on a substrate a first conductive layer; a first doped layer; a graded band gap layer including pure silicon and silicon in chemical compositions with germanium, carbon, nitrogen, or nitrogen and oxygen, of the formula selected from the group consisting of Si_x Ge_1-X , Si_xCy, Si_xNy, and Si_xO_yN_z; a second doped layer and a second conductive layer. Besides, a thin-film photovoltaic conversion device is made by additionally forming a reflective layer disposed near said substrate on a light-incident substrate-side, an anti-reflective layer disposed above said layers on light-incident side of the device, as well as a protective laminating layer disposed on the photovoltaic conversion device on the side opposite to the substrate. [0023] The deposition of films forming a first doped layer, a graded band gap layer and a second doped layer is performed by one of the methods of reactive chemical deposition from a gas phase selected from a group including CVD - Chemical Vapor Deposition, LPCVD - Low Pressure Chemical Vapor Deposition, PECVD - Plasma Enhanced Chemical Vapor Deposition, HF PECVD - High Frequency Plasma Enhanced Chemical Vapor Deposition, VHF PECVD - Very High Frequency Plasma Enhanced Chemical Vapor Deposition, HWCVD - Hot Wire Chemical Vapor Deposition. The latter is used in the proposed method most frequently.

[0024] According to the proposed method, to form a graded band gap layer, hydrogen-, chlorine- and fluorine-containing gases are used, and change in composition of a graded band gap layer being achieved by smoothly changing the composition of gases and their volume consumption. To produce silicon carbide Si_xCy , methane CH₄ or another carbon-containing gas is additionally introduced. To produce nitride Si_xNy , nitrogen N₂ or ammonia NH3 are added. To produce Si_xOyNz nitrogen and oxygen - N₂ and O2 are added. Finally, to produce the first doped layer, graded band gap layer and second doped layer, hydrogen is used, which allows to reduce the number of split atomic bonds in the films of layers of the photovoltaic conversion device to a level characterized by density of the defect states within the range of 10¹⁶ - 10¹⁷ cm ^~3 and remove the defects of split bonds. [0025] In sequentially forming on the substrate a first conductive layer, a first doped layer as a n-type layer, the graded band gap layer, a second doped layer as a p-type layer, and a second conductive layer, these layers are formed by one of the methods of physical or chemical deposition from gas phase, as PVD - Physical vapor Deposition, including Magnetron Sputtering, or CVD - Chemical Vapor Deposition, substantially HWCVD - Hot Wire Chemical Vapor Deposition. [0026] The n-type layer is formed on the substrate substantially by HWCVD via attacking by gas mixture comprising SiH₄ fed at flow rate of 20 - 150 seem, as well as additional gas mixture - 5% PH₃ and 95% H₂, fed at a flow rate of 1 - 5 seem. Therewith the substrate is heated to a temperature T_SUb of 150 - 300 C⁰, and the forming process is performed at a pressure of 30 - 600 mT (milliTorr), filament current of 30 - 100 A during the deposition time - 300 - 600 sec. [0027] To form a graded band gap layer over the already formed n-type layer, by the same method of reactive chemical deposition, substantially HWCVD, there is attacked a gas mixture comprising silane - SiH₄ fed at a flow rate up to 150 seem, as well as a mixture of gases germane - GeH₄ , methane - CH₄ or another carbon- containing gas, nitrogen N₂ or ammonia - NH₃ and oxygen - O₂ , fed at a flow rate of 0 - 100 seem. Therewith the substrate is heated to a temperature T_sub of 150 - 300 C⁰ , and the forming process is performed at a pressure of 30 - 600 inT, filament current of 30 - 100 A during the deposition time - 600 - 1200 sec. [0028] To form a p-type layer over the already formed graded band gap layer, by the same HWCVD, there is attacked a gas mixture comprising silane - SiH₄ fed at a flow rate of 5 - 40 seem, as well as a mixture of gases 1,5% diborane - B₂H₆ and 98,5% hydrogen - H2 fed at a flow rate of 1 - 5 seem. Therewith the substrate is heated to a temperature T_sub of 150 - 300 C⁰, and the forming process is performed at a pressure of 30 - 600 mT, filament current of 30 - 100 A during the deposition time - 300 - 600 sec.

[0029] The proposed thin-film photovoltaic conversion device is manufactured on the basis of at least one vacuum chamber wherein hydrogen-, chlorine- and fluorine-containing gases and when necessary, methane or another carbon- containing gas, nitrogen or ammonia and oxygen are fed, and smooth changing of composition of these gases and their volume consumption causes smooth composition changing of the graded band gap layer.

[0030] The proposed thin-film photovoltaic conversion device may also be manufactured on the basis of several vacuum chambers. In this case the n-type layer is formed in one of the vacuum chambers wherein the substrate is placed, the graded band gap layer is formed in the second of vacuum chambers wherein the substrate with the n-type layer already formed thereon is inserted, and then over this n-type layer a graded band gap layer is formed, and finally, the p-type layer is formed in the third of aforesaid vacuum chambers, wherein the substrate is inserted with n-type layer and graded band gap layer already deposited thereon, and then the p-type layer is deposited on this substrate over the graded band gap layer. [0031] In the proposed thin-film photovoltaic conversion device first conductive layer, reflective layer, second conductive layer, anti-reflective layer, as well as protective laminating layer, disposed on the photovoltaic conversion device on the side opposite to the substrate, are formed by the method of physical deposition PVD - Physical Vapor Deposition, including Magnetron Sputtering, in a fourth, additional vacuum chamber.

[0032] The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033] FIG. 1 is a cross section of a thin-film photovoltaic conversion device according to one embodiment of the present invention;

[0034] FIG. 2 is a diagram of the inner part of a thin-film photovoltaic conversion device and its energetic (zone) diagram;

[0035] FIG. 3 is a diagram of the relationship between gases consumption

(cm³ /min) and composition of the graded band gap layer;

[0036] FIG. 4 is the diagram of a vacuum chamber for manufacturing a thin-film photovoltaic conversion device;

[0037] FIG. 5 is the diagram of a four-chamber basic plant for manufacturing the thin-film photovoltaic conversion device;

[0038] FIG. 6 is a cross section of a thin-film photovoltaic conversion device according to a second embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0039] In accordance with the present invention, as depicted in FIG. 1 , a thin-film photovoltaic conversion device 1 is provided having a substrate 2, a first conductive layer 3 together with a reflective layer 4 deposited on the surface of the latter, a first doped n-layer 5, a graded band gap layer 6, a second doped p-layer 7 and a second conductive transparent layer 8. Substrate 2 of this conversion device 1 may be made of metal, glass or plastic.

[0040] In the described embodiment of the thin-film photovoltaic conversion device 1 (FIG. 1) substrate 2 is made from a flexible plastic, substantially polyimide. First conductive layer 3 may have a reflective layer 4, made substantially from aluminium or silver and deposited on the surface of this layer near substrate 2, on the side opposite to light incidence direction. First conductive layer 3 and first reflective layer 4 may be also formed as a single, substantially aluminium layer. First conductive layer and second conductive layer, 3 and 8 respectively, of thin-film photovoltaic conversion device 1 are electrodes. One of these electrodes - second conductive layer 8 disposed relative to the layer 2 on the light incident side, as shown in FIG.l, is transparent. Besides, thin-film photovoltaic conversion device 1 additionally comprises an anti-reflective layer 9 disposed relative to substrate 2 on the light incident side, a laminating protective layer 10, disposed on photovoltaic conversion device 1 on the side opposite to substrate 2, as well as a current pick-off grid 11 for tapping electric current from proposed device 1.

[0041] In this thin-film photovoltaic conversion device 1 (FIG. 1), the first n- doped layer 5 and second doped p-layer 7 are disposed on both sides of side surfaces 12, 13 of graded band gap layer 6, one of them, the first n-doped layer 5, being disposed near side face 12 of graded band gap layer 6 on the side of substrate 2, and the second, p-type layer 7 - near side face 13, on the light incident side of device 1.

[0042] The most important member of proposed thin-film photovoltaic conversion device 1 (FIG. 1) is the graded band gap layer 6 having two side faces, 12, 13 respectively, and including pure silicon and silicon in chemical compositions with germanium, carbon, nitrogen, or nitrogen and oxygen, of the formula selected from the group consisting of: Si_x Ge_1-X , Si_xC_y, Si_xNy, and Si_xOyN₂, wherein all these chemical compositions are simultaneously comprised in graded band gap layer 6, being disposed in layers and smoothly changing from one to the other. Therewith the composition of graded band gap layer 6 is smoothly changing in light direction in aforesaid succession from side face 13 of a corresponding Si_x Ge_1-X -layer to side face 12 of a corresponding Si_xO_yN_z -layer.

[0043] Energy limits (Eg) of chemical composition layers, comprised in graded band gap layer 6 are changing, as shown in the diagram of FIG. 2, from 0,9 eV, which corresponds to the infrared region of the spectrum, to 3,5 eV, which corresponds to the ultraviolet region of solar spectrum. Abbreviations in FIG.2 signify: Eg - energetic limit, Ec- conductivity zones, Ef - Fermi zone, Ev - valency zone.

[0044] According to the proposed invention, the described method of manufacturing a thin-film photovoltaic conversion device 1 is performed on the basis of at least one vacuum chamber 101, the schematic diagram whereof is shown in FIG. 4. Vacuum chamber 101 comprises a body 103, nozzle 105 with a gas inlet 107 for gas feeding and channel 109 connected with a pump 111. The upper part of the chamber encloses a heater 113, whereon substrate 115 is secured, and over this substrate are successively deposited thin films 117. In the embodiment of vacuum chamber 101 shown in FIG. 4 films 117 forming the proposed p-layer 7, graded band gap layer 6 and n-layer 5 are deposited by one of the methods of physical or chemical deposition from gas stage, selected from a group including PVD - Physical Vapor Deposition, including Magnetron Sputtering, CVD - Chemical Vapor Deposition, LPCVD - Low Pressure Chemical Vapor Deposition, PECVD - Plasma Enhanced Chemical Vapor Deposition, HF PECVD - High Frequency Plasma Enhanced Chemical Vapor Deposition, VHF PECVD - Very High Frequency Plasma Enhanced Chemical Vapor Deposition (30 - 300 MHz), HWCVD - Hot Wire Chemical Vapor Deposition. In the embodiment of vacuum chamber 101 shown in FIG. 4 for depositing layer 7, graded band gap layer 6 and n-layer 5 the latter, HWCVD - Hot Wire Chemical Vapor Deposition is applied, using hot wire 119. For deposition of first conductive layer 3, reflective layer 4 formed substantially from aluminium or silver and deposited on substrate 2, on the side opposite to light incidence direction second conductive layer 8, anti-reflective layer 9, protective laminating layer 10, as well as current pick-off grid 11, PVD method is used - Physical vapor Deposition, including Magnetron Sputtering.

[0045] As shown in FIG. 4, substrate 115 is secured on heater 113, and then thin films 117 are successively deposited thereover, which form a first conductive layer 3 with a reflective layer 4, first doped n-layer 5, graded band gap layer 6, second doped p-layer 7, a second conductive transparent layer 8, anti-reflective layer 9, protective layer 10 with a current collector grid 11.

[0046] According to the proposed method, the deposition of thin films 117 is performed by feeding into vacuum chamber 101 (FIG. 4) a mixture of silane SiH₄ with hydrogen H₂ and other gases. Graded band gap layer 6, containing pure silicon and silicon in chemical compositions with germanium, carbon, nitrogen, or nitrogen and oxygen, of the formula selected from the group consisting of: Si_x Ge_1-X , Si_xCy, Si_xN_y, and Si_xOyN₂, is formed by feeding into vacuum chamber 101 hydrogen-, chlorine- and fluorine-containing gases, and, whenever necessary, methane or another carbon-containing gas, nitrogen or ammonia and oxygen, and smooth changing of composition and volume consumption of these gases provide smooth composition change of this graded band gap layer 6. In particular, to produce silicon carbide Si_xCy, methane CH4 or other carbon-containing gases are added to aforesaid gases; to produce nitride Si_xN_y nitrogen N₂ or ammonia NH₃ are added; and to produce Si_xO₇N₂ nitrogen and oxygen - N₂ and O₂ are added. Besides, to form a first doped n-layer 5, a graded band gap layer 6 and a second doped p- layer 7 hydrogen is used, which allows to reduce the number of split atomic bonds in layer films of photovoltaic conversion device 1 being formed, to a level defined by density of the defect states within the range of 10¹⁶ - 10¹⁷ cm ^" and remove the defects of split bonds.

[0047] FIG. 3 presents a diagram of the relationship between consumption of gases being fed (cm³ /win) and composition of graded band gap layer 6. The diagram symbolizes by lines:

1.+ + + 5% SiH₄ + 95% H₂,

2. • •• O₂,

3. A A A N₂ Or NH₃,

4. Δ Δ Δ CH₄.

5. xxx GeH₄.

[0048] In more detail, n-type layer 5 is formed in vacuum chamber 101 (FIG. .4) on substrate 115 by a HWCVD method - Hot Wire Chemical Vapor Deposition, by attacking a gas mixture including silane - SiH₄ fed at a flow rate of 20 - 150 seem, and a mixture of gases - 5% phosphine - PH₃ and 95% hydrogen - H₂ fed at a flow rate of 1 - 5 seem. Therewith substrate 115 is heated to a temperature T _SUb of 150 - 300 C⁰ and the forming process is going on at a pressure of 30 - 600 mT, filament current of 30 - 100 A during deposition time - 300 - 600 sec. [0049] Graded band gap layer 6 is formed over n-type layer 5 in the same vacuum chamber 101 (FIG. 4) on substrate 115 by HWCVD method, via attacking a gas mixture including silane - SiH₄ fed at a flow rate of 0 - 150 seem, as well as a mixture of gases - germane - GeH₄ , methane - CH₄ or another carbon-containing gas, nitrogen N₂ or ammonia - NH₃ and oxygen - O₂ fed at a flow rate of 0 - 100 seem. Therewith substrate 115 is heated to a temperature T _sub = 150 - 300 C⁰ and the forming process is going on at a pressure of 30 - 600 mT, filament current 30 - 100 A deposition time - 600 - 1200 sec.

[0050] The ρ-type layer 7 is formed over graded band gap layer 6 in the same vacuum chamber 101 (FIG. 4) on substrate 115 by HWCVD method by attacking a gas mixture including silane - SiH₄ fed at a flow rate of 5 - 40 seem, as well as a mixture of gases - 1,5% diborane - B₂H₆ and 98,5% hydrogen - H₂ fed at a flow rate of 1 - 5 seem. Therewith substrate 115 is heated to a temperature T _sub = 150 - 300 C⁰ , and the forming process is going on at a pressure of 30 - 600 mT, filament current 30 - 100 A during deposition time 300 - 600 sec.

[0051] According to another embodiment of the proposed invention, the described method of manufacturing a thin-film photovoltaic conversion device 1 may be realized on the basis of several vacuum chambers 201, 203, 205, 207 (FIG. 5), which are designed like aforesaid vacuum chamber 101. Vacuum chambers 201, 203, 205, 207 further comprise nozzles 213, 215, 217, 219 with gas inlets for gas feeding and channels 221, 223, 225, 227 connected with pumps. The upper parts of vacuum chambers 201, 203, 205, 207 enclose heaters 231, 233, 235, 237 whereon substrates are secured, and over these substrates thin films are deposited. In the example shown in FIG. 5, films forming the proposed p-type layer 7, graded band gap layer 6 and n-type layer 5 are deposited in vacuum chambers 201, 203, 205, by one of the methods of reactive chemical deposition from gas stage, selected from a group including CVD - Chemical Vapor Deposition, LPCVD - Low Pressure Chemical Vapor Deposition, PECVD - Plasma Enhanced Chemical Vapor Deposition, HF PECVD - High Frequency Plasma Enhanced Chemical Vapor Deposition, VHF PECVD - Very High Frequency Plasma Enhanced Chemical Vapor Deposition (30 - 300 MHz), HWCVD - Hot Wire Chemical Vapor Deposition. In the embodiment of vacuum chambers 201, 203, 205 shown in FIG. 5 the last of aforesaid methods - Hot Wire Chemical Vapor Deposition is applied, using hot wires 241, 243, 245. Hydrogen-, chlorine- and fluorine-containing gases, and, whenever necessary, also methane or another carbon-containing gas, nitrogen or ammonia and oxygen are similarly fed into these chambers 201, 203, 205, and by smooth composition change of these gases and their volume consumption, there is provided a smooth composition change of graded band gap layer 6 and other layers of thin-film photovoltaic conversion device 1. [0052] According to a second embodiment of the proposed method using several vacuum chambers 201, 203, 205, 207 (FIG. 5), n-type layer 5 in proposed thin-film photovoltaic conversion device 1 is formed in the first of vacuum chambers - chamber 201. The substrate is inserted therein, and then, by the HWCVD method, n-type layer 5 is deposited to attack the gas mixture, including silane - SiH₄ fed at a flow rate of 20 - 150 seem, and a mixture of gases - 5% phosphine - PH₃ and 95% hydrogen - H₂ fed at a flow rate of 1 - 5 seem. Therewith the substrate is heated to a temperature T _sub = 150 - 300 C⁰, and the forming process is going on in chamber 201 at a pressure of 30 - 600 mT, filament current 30 - 100 A during deposition time - 300 - 600 sec.

[0053] Graded band gap layer 6 (FIG. 5) in proposed thin-film photovoltaic conversion device 1 is formed in second vacuum chamber 203, wherein the substrate is inserted, and then, by HWCVD method, there is deposited thereon, over n-type layer 5, a graded band gap layer 6, attacking the gas mixture, including silane - SiH₄ fed at a flow rate of 0 - 150 seem, as well as the mixture of gases - germane - GeH₄, methane - CH₄ or another carbon-containing gas, nitrogen N₂ or ammonia - NH3 and oxygen - O₂ fed at a flow rate of 0 - 100 seem. Therewith the substrate is heated to a temperature T _sub - 150 - 300 C⁰ , and the forming process is going on in chamber 203 at a pressure of 30 - 600 mT, filament current 30 - 100 A during deposition time - 600 - 1200 sec.

[0054] The p-type layer 7 in proposed thin-film photovoltaic conversion device 1 is formed in the third of vacuum chambers 205 (FIG. 5), wherein the substrate is inserted, and then, by HWCVD method, there is deposited thereon, over graded band gap layer 6, the next p-type layer 7, attacking the gas mixture, including silane - SiH₄ fed at a flow rate of 5 - 40 seem, as well as a mixture of gases - 1,5% diborane - B₂H₆ and 98,5% hydrogen - H₂ fed at a flow rate of 1 - 5 seem. Therewith the substrate is heated to a temperature T_SUb - 150 - 300 C⁰, and the forming process is going on in chamber 205 at a pressure of 30 - 600 mT, filament current 30 - 100 A during deposition time- 300 - 600 sec.

[0055] At last, in proposed thin-film photovoltaic conversion device 1 first conductive layer 3, reflective layer 4, second conductive layer 8, anti-reflective layer 9, as well as protective laminating layer 10, and collector grid 11 disposed on photovoltaic conversion device 1 on the side opposite to substrate 2 may be formed by a method of physical deposition from gas stage, and in particular, PVD - Physical Vapor Deposition, including Magnetron Sputtering, in the fourth vacuum chamber 207 (FIG. 5), having nozzle 219, gas inlet 227 for feeding gas, heater 237 and target 247. The target 247 consists of material for layers 3, 4, 8,9,10, 11 depositions, for example, aluminium, silver, etc.

EXAMPLES

[0056] The present invention will be described in further detail with examples of solar cells as photovoltaic conversion devices, but it is noted that the present invention is by no means intended to be limited to the examples. The following examples are meant to illustrate and not limit the scope of the invention. Example 1

[0057] This example demonstrates the fabrication of photovoltaic conversion devices 1 as solar cells, including a substrate 115, first conductive layer 3 together with reflective layer 4 applied on its surface, first doped n-layer 5, graded band gap layer 6, second doped p-layer 7 and second conductive transparent layer 8. Substrate 115 of this conversion device 1 is made of plastic polyimide. [0058] The solar cell is fabricated in the following manner. In the described example substrate 115 of polyimide is secured on heater 113 (FIG. 4), and then thin films 117 are successively deposited thereover to form first conductive layer 3 with reflective layer 4, first doped n-layer 5, graded band gap layer 6, second doped p- layer 7, second conductive transparent layer 8, anti-reflective layer 9 and, at last, protective layer 10 with collector grid 11. High light absorption factor (α >10⁵ CM^"1) of graded band gap layer 6 is achieved by smooth change of its composition on the light incident side: Si_xGe_y - Si - Si_x C_y - Si_xNy - Si_xOyN₂. The deposition of first conductive layer 3 from aluminium or silver in the described example is performed by the well known magnetron sputtering method under following conditions: argon pressure 5 - 10 mT, cathode voltage 3 kV, cathode current 500 niA.

[0059] First doped n-layer 5 has been grown by the HW CVD method under the following conditions: T_sub - 250° C; flow rate (5% SiH₄, 95% H₂) - 100 seem; (5%

PH₃, 95% H₂) - 3 seem; pressure - 250 mT. Second doped p-layer 7 has been grown by the HW CVD method under the following conditions: T_SUb - 250° C; flow rate (5% SiH₄, 95% H₂) - 100 scmm; (1,5 B₂H₆, 98,5% H₂) - 5 seem; pressure -

250 mT.

[0060] In fabrication of graded band gap layer 6 in this example, following parameters have been defined:

Gases used: for Si_xGe_y- mixtures (5% SiH₄+ 95% H₂) and (5% GeH₄+ 95% H₂); for Si_xC_y - mixtures (5% SiH₄ + 95%H₂), (100% CH₄ or 100 % CO₂); for Si_xNy - mixture (5% SiH₄+ 95% H₂), (100% N₂); for Si_xNyO₂- mixture (5 % Si H₄+ 95% H₂), N₂, O₂.

Gas flow rates (see FIG. 3):

-mixture of gases - 5% phosphine - PH₃ and 95% hydrogen - H₂ = 1 - 5 seem. Substrate temperature T_SUb = 250° C Pressure 20O mT. Filament current 5OA Deposition time 600 sec. Example 2

[0061] This example demonstrates fabrication of photovoltaic conversion device 1 as solar cell, including a substrate 2 made of glass, first conductive layer 3 together with reflective layer 4 applied on its surface, first doped n-layer 5, graded band gap layer 6, second doped p-layer 7 and second conductive transparent layer 8. [0062] The solar cell is fabricated in the following manner. Glass substrate 115 is secured on heater 113 (FIG. 4), and then thin films 117 are successively deposited thereover to form first conductive layer 3 with reflective layer 4, first doped n- layer 5, graded band gap layer 6, second doped p-layer 7, second conductive transparent layer 8, anti-reflective layer 9 and, at last, protective layer 10 with collector grid 11. High light absorption factor (α >10⁵ CM^"1) of the graded (varizone) semiconductor layer is achieved by smooth change of its composition on the light incident side: Si_xGe_y _ Si - Si_xCy - Si_xNy - Si_xO_yN_z. The deposition of first conductive and reflective layer 3, 4 from aluminium or silver, as well as layers 8, 9, 10, 11 was performed by the well known magnetron sputtering method under following conditions: argon pressure 5 - 10 mT, cathode voltage 3 kV, cathode current 500 mA.

[0063] First doped n-layer 5 is grown by the HW CVD method under the following conditions: T_sub - 250° C; flow rate (5% SiH₄, 95% H₂) - 100 seem; (5% PH₃, 95% H₂) - 3 seem; pressure - 250 mT. Second p-type doped layer is grown by the HW CVD method under following conditions: T_sub - 250⁰C; flow rate (5% SiH₄, 95% H₂) - 100 seem; (1.5% B₂H₆, 98.5% H₂) -5 seem; pressure - 250 mT. [0064] In fabrication of graded band gap layer 6 in this example, following parameters have been defined: Gases used: for Si_xGe_y- mixtures (5% SiH₄+ 95% H₂), (5% GeH₄+ 95% H₂); for Si_xC_y - mixtures (5% SiH₄+ 95% H₂), (100% CH₄ or 100 % CO₂); for Si_xNy - mixture (5% SiH₄+ 95% H₂), (100% N₂); for Si_xNyO₂- mixture (5 % SiH₄ + 95% H₂), N₂₅ O₂.

Gas flow rates (see FIG. 3): R= 20 - 150 seem

Substrate temperature T_SUb= 250° C

Pressure 20O mT.

Filament current 5OA

Deposition time 600 sec.

[0065] The growth is performed in the medium of silane (SiH₄) diluted with hydrogen (H₂). To produce the p-layer, a gas from three- valent chemical elements of the periodic system, such as diborane (B₂H₆), is added to the gas mixture. To produce the n-layer a gas on the basis of five-valent elements of the periodic system is added to the gas mixture, such as phoshorus, arsenic, antimony, in particular, phosphine (PH₃).

Example 3

[0066] This example demonstrates the fabrication of photocells with reverse disposition of layers. When photocell 1 is exposed to daylight on the side of transparent substrate 2, the order of layers deposition is changed. On glass substrate 2 there is applied a metal grid (comb) 11, then a transparent electroconductive and anti-reflective coating 9. Therewith protective layer 10 is excluded. Then n-doped layer 5 is deposited. Graded band gap 6 is also deposited in reverse order: Si_xOyN₂ - Si_xNy - Si_xC_y - Si - Si_xGe_y. On the surface of graded (varizone) semiconductor layer 6 there is deposited doped p-layer 7 and then a metal current collector and reflective layer 3, 4 (Back-Contact). [0067] Substrate 2 is secured on heater 113, and then, thereover thin films 117 are successively deposited. Anti-reflective coating 9, metal grid (comb) 11, current collector and reflective layers 3, 4 are deposited by the well known magnetron sputtering method under following conditions: argon pressure - 5 - 10 mT, cathode voltage - 3 kV, cathode current 500 niA. [0068] First doped n-layer 5 is grown by the HW CVD method under the following conditions: T_sub - 250° C; flow rate (5% SiH₄, 95% H₂) - 100 seem; (5% PH₃, 95%

H₂) - 3 seem; pressure - 250 mT. Second doped ρ-layer 7 is grown by the HW

CVD method under following conditions: T_sub - 250° C; flow rate (5% SiH₄, 95%

H₂) - 100 scmm; (1.5 % B₂H₆ , 98.5% H₂) - 5 seem; pressure- 250 mT.

[0069] In fabrication of graded band gap layer 6 in this example, following parameters have been defined:

Gases used: for Si_xGe_y- mixtures (5% SiH₄+ 95% H₂) and (5% GeH₄+ 95% H₂); for Si_xCy - mixtures (5% SiH₄+ 95% H₂), (100% CH₄ or 100 %CO₂); for Si_xNy - mixture (5% SiH₄+ 95% H₂), (100% N₂); for Si_xNyO₂- mixture (5 % SiH₄+ 95% H₂), N₂, 0₂.

Gas flow rates (see FIG. 3): 20 - 150 seem

Substrate temperature T_SUb= 250° C

Pressure 200 mT

Filament current 50 A

Deposition time 600 sec.

[0070] Conductive layer 3 and reflective layer 4 (FIG. 6) are fabricated from metals with high factor light reflection and high electroconductivity - aluminium or silver. They serve as upper collector electrodes. Layer 11 - the grid or comb is a lower collector electrode and is also fabricated from metals with high electroconductivity. The total surface of this layer should not exceed 5 - 10% of the cell total surface, and the distance between grid 11 lines is 5 - 50 mm. These layers are deposited by the PVD method - Physical Vapor Deposition. Conductive transparent layer 8 is transparent to light and has a low electric resistance. The layer is produced from a mixture of indium tin oxide (ITO) or zinc oxide alloyed by aluminium - ZnO/ Al. This layer is also deposited by the PVD method. Example 4

[0071] This example demonstrates the fabrication of p-type layer 7, graded band gap layer 6 and n-type layer 5 in the first three chambers 201, 203, 205 (FIG.5). The layers in these three chambers are deposited by the HVCVD method. Substrate from polyimide, with conductive layer 3, reflective layer 4 (FIG. 1) deposited thereon is inserted into chamber (chamber deposition n-layer) 201 (FIG. 5) and secured on heater 231. Via backing and diffusion pumps 221 the chamber is pumped out to a vacuum P = 210^"5 Torr.

[0072] Then heater 231 is turned on to heat the substrate to a temperature of 250° C. Filament heater 241 is turned on to heat this filament to a temperature of 1600 - 2000° C. A gas mixture is fed into chamber 201 via nozzle 213 to grow n-layer 5. The following gases are used in this case: 5% SiH₄+ 95% H₂ , flow rate R= 40 - 80 seem; 5% PH₃ + 95% H₂ , flow rate R= 1-3 seem.

[0073] Then the substrate with deposited films is displaced to chamber 203 for deposition graded band gap layer 6 and submitted to aforesaid operations. The gas composition fed into the chamber 203 is as follows: 5% SiH₄ + 95% H₂; 5% GeH₄ + 5% H₂; 100% N₂ ; 100% CH₄ ; 100% O₂. Gas consumption is changing according to the diagram shown in FIG. 3. After the deposition of graded band gap layer 6 the substrate with films is displaced to chamber 205 (FIG. 5). By a backing and diffusion pumps 225 the chamber is pumped out to a vacuum P = 2 10 ^~5 Torr. Then heater 235 is turned on to heat the substrate to a temperature of 250⁰C. Heater 245 is turned on to heat the filament to a temperature of 1600 - 2000 ⁰C. Then gas composition is fed during nozzle 217 in the chamber 205 for deposition p-layer 7. Gases used: 5% SiH₄ + 95% H₂ ; flow rate R = 40 - 80 seem, 1.5 B₂H₆ + 98.5 H₂ ; R = 1 - 3 seem.

[0074] For forming on the substrate first conductive layer 3, reflective layer 4, second conductive transparent layer 8, anti-reflective layer 9 and grid 11, the substrate with deposited films is displaced to chamber 207 wherein these layers are deposited by the method of physical deposition from gas phase - PVD - Physical Vapor Deposition, including magnetron sputtering. Chamber 207 has a nozzle 219, channel 227 for gas feeding, heater 237 and target 247. Target 247 consists of a material that must be deposited as layers 3, 4, 8, 9, 10, 11, such as aluminium, silver etc.

[0075] The advantages of proposed photovoltaic conversion device 1 and method of its fabrication are as follows. In most known photovoltaic conversion devices there is used a homogeneous i-layer or a cascade of these layers with a definite band gap layer and limited possibilities of light conversion. The proposed photovoltaic conversion device and method of its fabrication allow to set up the production of semi-conductors wherein, instead of one or several i-layers, there is formed one layer of a complex smoothly changeable structure Si_xGe_1-x - Si - Si_xC_y - Si_xNy - Si_x OyN_z . Owing to such varizone structure of this layer the band gap layer size is essentially changed, and it absorbs the solar spectrum much better, which, in its turn, enables an increase of solar energy conversion to η ~ 12 % . [0076] Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by terms of appended claims.

Claims

1. A thin-film photovoltaic conversion device, the device comprising in sequence:

(a) a substrate;

(b) a first conductive layer;

(c) a first doped layer;

(d) a graded band gap layer, having a first side face and a second side face, including pure silicon and silicon in chemical compositions with germanium, carbon, nitrogen, or nitrogen and oxygen, of the formula selected from the group consisting of: Si_x Ge_1-X , Si_xCy, Si_xNy, and Si_xOyN₂, wherein said chemical compositions being simultaneously comprised in said band gap layer and disposed successively, and smoothly transitioning, in a direction from said first side face to said second side face;

(e) a second doped layer; and

(f) a second conductive layer; wherein said substrate is made of metal, glass, or plastic and said graded band gap layer mainly comprising silicon.

2. The device of claim 1, the device further comprising in sequence:

(g) a reflective layer disposed near said substrate on a light-incident substrate-side;

(h) an anti-reflective layer disposed above said layers on light-incident side; and (i) a protective laminating layer disposed on the device on a device side opposite to said substrate.

3. The device of claim 1, wherein said substrate is made of a flexible plastic.

4. The device of claim 1, wherein said first conductive layer and said second conductive layer are electrodes.

5. The device of claim 4, wherein at least one of said electrodes, disposed on said light-incident substrate side, is transparent.

6. The device of claim 1, wherein both said doped layers are disposed on both said side faces of said graded band gap layer, and wherein one said doped layer is a p-type layer and another said doped layer is an n-type layer.

7. The device of claim 1, wherein chemical composition of said graded band gap layer is smoothly transitioning in sequence: Si_x Ge₁^_x , Si, Si_xCy, Si_xNy, Si_xOyN₂, in a light direction.

8. The device of claim 7, wherein energetic limits of said chemical compositions, comprised in said band gap layer, change from 0.9 eV to 3.5 eV.

9. A method of manufacturing a thin-film photovoltaic conversion device, the method comprising the steps of sequentially forming on a substrate:

(i) a first conductive layer;

(ii) a first doped layer;

(iii) a graded band gap layer including pure silicon and silicon in chemical compositions with germanium, carbon, nitrogen, or nitrogen and oxygen, of the formula selected from the group consisting of Si_x Ge_1-X , Si_xCy₅ Si_xNy, and

Si_xOyN₂;

(iv) a second doped layer; and

(v) a second conductive layer.

10. The method of claim 9, including additional forming in said device: (vi) a reflective layer disposed near said substrate on a light-incident substrate-side;

(vii) an anti-reflective layer disposed above said layers on light-incident side; and

(viii) a protective laminating layer disposed on the device on a device side opposite to said substrate.

11. The method of claim 9, wherein said steps of forming said first doped layer, said graded band gap layer, and said second doped layer include forming deposition films using at least one method of reactive gas-phase chemical deposition selected from the group consisting of: chemical vapor deposition, low- pressure chemical-vapor deposition, plasma-enhanced chemical-vapor deposition, high-frequency plasma-enhanced chemical-vapor deposition, very-high-frequency plasma-enhanced chemical-vapor deposition, and hot-wire chemical-vapor deposition.

12. The method of claim 9, wherein said step of forming said band gap layer includes using hydrogen-, chlorine-, and fluorine-containing gases, and wherein chemical compositions of said band gap layer are smoothly transitioned by smoothly changing a gas composition and a gas-volume consumption.

13. The method of claim 12, wherein step of forming part of said band gap layer, including Si_xCy , is performed by adding CH₄ or another carbon-containing gas.

14. The method of claim 12, wherein step of forming part of said band gap layer, including Si_xNy , is performed by adding N₂ or NH₃.

15. The method of claim 12, wherein step of forming part of said band gap layer, including Si_xOyN₂ , is performed by adding N₂ and O₂.

16. The method of claim 9, wherein said steps of forming said first doped layer, said band gap layer, and said second doped layer are accompanied by adding H₂, thereby reducing a split-atomic-bond occurrence in films forming said layers of the device.

17. The method of claim 16, wherein said split-atomic-bond occurrence is characterized by a defect-state density, and wherein said defect-state density is within a range of about 10¹⁶ - 10¹⁷ cm^"3.

18. The method of claim 9, wherein said first doped layer is an n-type layer, said second doped layer is a p-type layer, and wherein said n-type layer, said graded band gap layer and said p-type layer are formed from deposition films, using at least one method of chemical gas-phase deposition, substantially hot-wire chemical-vapor deposition.

19. The method of claim 18, wherein said n-type layer is formed, using substantially hot-wire chemical-vapor deposition, by using a primary gas mixture including SiH₄, wherein said primary gas mixture is fed at a flow rate of about 20- 150 seem, and a secondary gas mixture, including about 5% PH₃ and 95% H₂, wherein said secondary gas mixture is fed at a flow rate of about 1-5 seem, while said substrate is heated to a temperature of about 150-300 ⁰C at a pressure of about 30-600 mT using a filament current of about 30-100 A and a deposition time of about 300-600 sec.

20. The method of claim 18, wherein said graded band gap layer is formed over said n-type layer, using substantially hot-wire chemical- vapor deposition, by using a primary gas mixture including SiH₄, wherein said primary gas mixture is fed at a flow rate up to 150 seem, and a secondary gas mixture including GeH₄, CH₄ or another carbon-containing gas, N₂ or NH₃, and O₂, wherein said secondary gas mixture is fed at a flow rate of about 0-100 seem, while said substrate is heated to a temperature of about 150-300 OC at a pressure of about 30-600 mT using a filament current of about 30-100 A and a deposition time of about 600-1200 sec.

21. The method of claim 18, wherein said p-type layer is formed over said graded band gap layer, using substantially hot-wire chemical- vapor deposition, by using a primary gas mixture including SiH₄, wherein said primary gas mixture is fed at a flow rate of about 5-40 seem, and a secondary gas mixture, including about 1.5% B₂H₆ and 98.5% H₂, wherein said secondary gas mixture is fed at a flow rate of about 1-5 seem, while said substrate is heated to a temperature of about 150-300 ⁰C at a pressure of about 30-600 mT using a filament current of about 30-100 A and a deposition time of about 300-600 sec.

The method of claim 9, wherein said steps of forming are performed using at least one vacuum chamber configured to supply hydrogen-, chlorine-, and fluorine- containing gases, and CH₄ or another carbon-containing gas, N₂ or NH₃, and O₂, and wherein smoothly changing gas compositions and gas-volume consumptions results in a smooth transition of said chemical compositions of said graded band gap layer.

23. The method of claim 22, wherein said first doped layer is n-type layer, formed in a first said vacuum chamber, in which said substrate is inserted, using substantially hot-wire chemical-vapor deposition, by using a primary gas mixture including SiH₄, wherein said primary gas mixture is fed at a flow rate of about 20- 150 seem, and a secondary gas mixture, including about 5% PH₃ and 95% H₂, wherein said secondary gas mixture is fed at a flow rate of about 1-5 seem, while said substrate is heated to a temperature of about 150-300 ⁰C at a pressure of about 30-600 mT using a filament current of about 30-100 A and a deposition time of about 300-600 sec.

24. The method of claim 22, wherein said graded band gap layer is formed, over said first doped layer, in a second said vacuum chamber, in which said substrate is inserted, using substantially hot-wire chemical-vapor deposition, by using a primary gas mixture including SiH₄, wherein said primary gas mixture is fed at a flow rate up to 150 seem, and a secondary gas mixture including GeH₄ , CH₄ or another carbon-containing gas, N₂ or NH₃ , and O₂, wherein said secondary gas mixture is fed at a flow rate of about 0-100 seem, while said substrate is heated to a temperature of about 150-300 OC at a pressure of about 30-600 mT using a filament current of about 30-100 A and a deposition time of about 600-1200 sec.

25. The method of claim 22, wherein said second doped layer is p-type layer, formed over said graded band gap layer in a third said vacuum chamber, in which said substrate is inserted, using substantially hot-wire chemical-vapor deposition, by using a primary gas mixture, including SiH₄, wherein said primary gas mixture is fed at a flow rate of about 5-40 seem, and a secondary gas mixture, including about 1.5% B₂H₆ and 98.5% H₂, wherein said secondary gas mixture is fed at a flow rate of about 1-5 seem, while said substrate is heated to a temperature of about 150-300 OC at a pressure of about 30-600 mT using a filament current of about 30-100 A and a deposition time of about 300-600 sec.

26. The method of claim 10, wherein said first conductive layer, said reflective layer, said second conductive layer, said anti-reflective layer, and said protective laminating layer are formed using substantially physical-vapor deposition, in at least one additional vacuum chamber.