WO2013102576A1 - Intermediate reflection structure in thin film solar cells - Google Patents

Intermediate reflection structure in thin film solar cells Download PDF

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Publication number
WO2013102576A1
WO2013102576A1 PCT/EP2012/076425 EP2012076425W WO2013102576A1 WO 2013102576 A1 WO2013102576 A1 WO 2013102576A1 EP 2012076425 W EP2012076425 W EP 2012076425W WO 2013102576 A1 WO2013102576 A1 WO 2013102576A1
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layer
doped
silicon
microcrystalline
solar cell
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PCT/EP2012/076425
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French (fr)
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Marian Fecioru-Morariu
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Tel Solar Ag
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Priority to CN201280065857.0A priority Critical patent/CN104025307A/en
Priority to ARP130102283 priority patent/AR091593A1/en
Publication of WO2013102576A1 publication Critical patent/WO2013102576A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L31/00—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof
    • H01L31/0248—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies
    • H01L31/036—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes
    • H01L31/0368—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including polycrystalline semiconductors
    • H01L31/03682—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including polycrystalline semiconductors including only elements of Group IV of the Periodic Table
    • H01L31/03685—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including polycrystalline semiconductors including only elements of Group IV of the Periodic Table including microcrystalline silicon, uc-Si
    • H—ELECTRICITY
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    • H01L31/0376—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including amorphous semiconductors
    • H01L31/03762—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof characterised by their semiconductor bodies characterised by their crystalline structure or particular orientation of the crystalline planes including amorphous semiconductors including only elements of Group IV of the Periodic Table
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    • H01L31/054—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means
    • H01L31/056—Optical elements directly associated or integrated with the PV cell, e.g. light-reflecting means or light-concentrating means the light-reflecting means being of the back surface reflector [BSR] type
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    • H01L31/06—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers
    • H01L31/075—Semiconductor devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation; Processes or apparatus specially adapted for the manufacture or treatment thereof or of parts thereof; Details thereof adapted as photovoltaic [PV] conversion devices characterised by potential barriers the potential barriers being only of the PIN type, e.g. amorphous silicon PIN solar cells
    • H01L31/076—Multiple junction or tandem solar cells
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    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1804—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof comprising only elements of Group IV of the Periodic Table
    • H01L31/182—Special manufacturing methods for polycrystalline Si, e.g. Si ribbon, poly Si ingots, thin films of polycrystalline Si
    • H01L31/1824—Special manufacturing methods for microcrystalline Si, uc-Si
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    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/20—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof such devices or parts thereof comprising amorphous semiconductor materials
    • H01L31/202—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof such devices or parts thereof comprising amorphous semiconductor materials including only elements of Group IV of the Periodic Table
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
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    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/52—PV systems with concentrators
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/545—Microcrystalline silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/548—Amorphous silicon PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • Photovoltaic devices or solar cells are devices which convert light into electrical power.
  • Thin film solar cells nowadays are of a particular importance since they have a huge potential for mass production at low cost.
  • amorphous and micro- or nanocrystalline silicon in multijunction solar cells offer the perspective of achieving energy conversion efficiencies exceeding 10% due to the better use of the solar irradiation compared to, for example, an amorphous silicon single junction solar cell. Due to the use of 2 or more photovoltaic junctions of different band gap the impinging light with a broad spectral distribution (such as solar irradiation) can be used more efficiently.
  • amorphous-microcrystalline silicon multijunction solar cell shows a smaller degradation of its initial conversion efficiency compared to a "pure" amorphous silicon single junction solar cell.
  • Processing in the sense of this invention includes any chemical, physical or mechanical effect acting on substrates.
  • Substrates in the sense of this invention are components, parts or workpieces to be treated in a processing apparatus.
  • Substrates include but are not limited to flat, plate shaped parts having rectangular, square or circular shape.
  • this invention addresses essentially planar substrates of a size >lm 2 , such as thin glass plates.
  • a vacuum processing or vacuum treatment system or apparatus comprises at least an enclosure for substrates to be treated under pressures lower than ambient atmospheric pressure.
  • Chemical Vapour Deposition is a well known technology allowing the deposition of layers on heated substrates.
  • a usually liquid or gaseous precursor material is being fed to a process system where a thermal reaction of said precursor results in deposition of said layer.
  • LPCVD is a common term for low pressure CVD.
  • TCO stands for transparent conductive oxide, TCO layers consequently are transparent conductive layers.
  • layer, coating, deposit and film are interchangeably used in this disclosure for a film deposited in vacuum processing equip- ment, be it CVD, LPCVD (low-pressure CVD) , plasma enhanced CVD
  • PECVD PECVD
  • PVD physical vapour deposition
  • a solar cell or photovoltaic cell is an electrical component capable of transforming light (essentially sun light) directly into electrical energy by means of the photoelectric effect.
  • a thin-film solar cell in a generic sense includes, on a supporting substrate, at least one p-i-n junction established by a thin film deposition of semiconductor compounds, sandwiched between two electrodes or electrode layers.
  • a p-i-n junction or thin-film photoelec- trie conversion unit includes an intrinsic semiconductor compound layer sandwiched between a p-doped and an n-doped semiconductor compound layer.
  • the term thin-film indicates that the layers mentioned are being deposited as thin layers or films by processes like, PEVCD, CVD, PVD or alike.
  • Thin layers essentially mean layers with a thick- ness of ⁇ or less, especially less than 2 ⁇ .
  • Any layer thicknesses described in this disclosure refer to an averaged thickness measured perpendicular to the base of the respective layer; averaged over a sufficiently dimensioned number of metering points .
  • FIG. 1 A basic type of a working a-Si ⁇ c-Si tandem junction thin film solar cell is shown in Fig. 1.
  • a thin-film solar cell 50 usually in- eludes a first or front electrode 42, one or more semiconductor thin-film p-i-n junctions (52-54, 51, 44-46, 43) , and a second or back electrode 47, which are successively stacked on a substrate 41.
  • Substantially intrinsic in this context is understood as not intentionally doped or exhibiting essentially no resultant doping, Photoelectric conversion occurs primarily in this i-type layer; it is therefore also called absorber layer .
  • a-Si, 53 amorphous
  • pc-Si, 45 microcrystalline solar cells, independent of the kind of crystallinity of the adjacent p and n-layers .
  • Microcrystalline layers are being understood, as common in the art, as layers comprising of a significant fraction of crystalline silicon - so called micro-crystallites - in an amorphous matrix.
  • Stacks of p-i-n junctions are called tandem or triple junction photovoltaic cells.
  • the combination of an amorphous and microcrystalline p-i-n- junction, as shown in Fig. 1, is also called micromorph tandem cell.
  • Front- 42 and back 47 electrode layers are preferably made from ZnO:B (boron doped zinc oxide) prepared e.g. in an industrial Oer- likon TCO system by LPCVD deposition.
  • ZnO:B boron doped zinc oxide
  • other transparent conductive layers can also be used, such as Sn0 2 , ITO and others.
  • the back reflector 48 is preferably a white plastic foil laminated on the back electrode.
  • the invention described in more detail below can also be used successfully with other types of specular or diffuse reflective layers, such as metallic reflectors, white paint or alike.
  • a so called intermediate reflector can be used (a low refractive index layer generally arranged between the i-layers of top and bottom cell 53, 45) , which reflects part of the passing light back into the amorphous cell thus increasing the amount of light absorbed there and resulting in an increase of the current of the amorphous cell.
  • TCO e.g. ZnO; N. Pellaton Vaucher et al., "Light Management in tandem cells by ' an intermediate reflector layer", 2 nd World conference and exhibition on photovoltaic solar energy conversion, 1998) or of Silicon alloyed with a band gap widen- ing agent (e.g. SiOx, Tawada et al . US 4,476,346 of 1984)
  • the object of the invention is thus to overcome at least some of the above-mentioned drawbacks in the art.
  • Each of these layers may be a single layer, or a multilayer comprising a plurality of sublayers.
  • the n-layer of one of the stacked solar cells has a multilayer structure comprising, in the intended direction of impinging light (that is to say in the direction in which, in use, incident light will travel through the cell) , a sequence of an n-doped a-Si:H layer, a first n-doped ⁇ -5 ⁇ : ⁇ layer, an intermediate reflector layer, and a second n-doped ⁇ -3 ⁇ : ⁇ layer.
  • the intermediate reflector layer comprises a plurality of layer sequences, each layer sequence consisting essentially of a ⁇ - ⁇ layer and a n-doped ⁇ -3 ⁇ : ⁇ layer in the intended direction of impinging light.
  • the ⁇ - ⁇ layer is described as "essentially of ⁇ - ⁇ " or "a microcrystalline essentially silicon oxide layer" since other components may be present in smaller quantities, for instance ⁇ - ⁇ in the case that C02 is used as a source of oxygen in processing.
  • each microcrystalline essentially silicon oxide layer may have a thickness of 8-18 nm, particularly 8-16 nm, particularly 12-16 nm, 9-11 nm, and each mi- crocrystalline silicon n-layer may be approximately 2 nm thick.
  • the number of layer sequences is 2-6, particularly 2-5, which has proven to give good results in practice.
  • the intermediate reflector layer is a multilayer structure of a plurality of layer sequences, it is a single ⁇ - ⁇ layer.
  • This single intermediate reflector layer structure likewise results in a cell of high-efficiency which is extremely easy to produce, since the intermediate reflector layer is implemented as part of the n-doped layer of one of the cells, merely by adding oxidising gas such as oxygen or carbon dioxide in the same PECVD apparatus used to deposit the other cell layers.
  • This single layer may have a thickness of at least 15-50 nm, particularly 20-40 nm, and is deposited on the first microcrystalline silicon n-layer.
  • the amorphous silicon n-layer has a thickness of 3-8 nm, particularly 4-6 nm and is deposited onto the intrinsic silicon layer of the cell closer to the incoming light, said intrinsic silicon layer of the cell closer to the incoming light being amorphous;
  • the first microcrystalline silicon n-layer has a thickness of 5-50 nm, particularly 5-15 nm, particularly 6-12 nm, particularly 6-9 nm, and is deposited on to said amorphous silicon n-layer;
  • the second microcrystalline silicon n-layer has a thickness of 4-8 nm, particularly 4-6 nm, particularly essentially 5 nm and is situ- ated following the intermediate reflector layer in the direction of the impinging light.
  • the multijunction thin-film solar cell further comprises a front contact made from zinc oxide deposited by low pressure chemical vapour deposition as a transparent conductive oxide layer electrode. This allows a generally thinner intermediate reflector layer since LPCVD-deposited zinc oxide is relatively rough and thus has good light-scattering properties.
  • the intrinsic semiconductor compound layer situated closest to the impinging light is amorphous, permitting fast top-cell deposition.
  • the multijunction thin-film solar cell comprises the following layer sequence in the intended direction of impinging light: a substrate; a front electrode; a p-layer; an intrinsic layer of a-Si:H; a n- doped a-Si:H layer; a n-doped ⁇ -3 ⁇ : ⁇ layer; an intermediate reflector layer of n-doped ⁇ - ⁇ ; an n-doped c-Si:H layer; a p-layer; an intrinsic layer of c-Si:H; an n-layer; a back electrode; a back reflector .
  • the multijunction thin-film solar cell comprises the following layer sequence in the intended direction of impinging light: a substrate; a front electrode; a p-layer; an intrinsic layer of a-Si:H; a n-doped a-Si:H layer; a n-doped ⁇ -3 ⁇ : ⁇ layer; an intermediate reflector layer consisting of a plurality of layer sys- terns each consisting of a layer of n-doped ⁇ - ⁇ followed by an n- doped c-Si layer; an n-doped ⁇ -3 ⁇ : ⁇ layer; a p-layer; an intrinsic layer of ⁇ - ⁇ ; an n-layer; a back electrode; a back reflector.
  • An object of the invention is likewise attained by a method of manufacturing a n-layer stack from multijunction solar cell, comprising depositing in the intended direction of impinging light (i.e. the direction in which light will travel into the cell when the cell is in use) an a-Si:H n-layer, a first ⁇ - ⁇ : ⁇ n-layer; intermediate re- flector layer, and a second ⁇ -3 ⁇ : ⁇ n-layer.
  • the intermediate reflector layer comprises a plurality of layer sequences, deposition of each layer sequence consisting of depositing a ⁇ -3 ⁇ layer and depositing a n-doped ⁇ -3 ⁇ : ⁇ layer arranged in the intended direction of impinging light.
  • the deposition of the intermediate reflector layer comprises depositing a single ⁇ - ⁇ layer rather than a ⁇ - ⁇ / n-doped ⁇ -3 ⁇ : ⁇ multilayer structure.
  • both of these embodiments result in a cell of high-efficiency which is extremely easy to produce, since the intermediate reflector layer is implemented as part of the n-doped layer of one of the cells, merely by adding a source of oxidiser in the same PECVD apparatus used to deposit the other cell layers.
  • the specified sublayers of the n-doped layer in question result in good adhesion between the layers, and in good electrical contact between the cells.
  • the amorphous silicon layer is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m 2 substrate:
  • the first microcrystalline silicon n-layer (65) is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m 2 substrate:
  • the intermediate reflector layer is a single microcrystalline essentially silicon oxide layer and is de- posited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m 2 substrate:
  • the second microcrystalline silicon n-layer is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m 2 substrate:
  • the invention is likewise attained by a method of manufacturing a multijunction thin film solar cell in which a first cell and at least one second cell (i.e. a total of two or more cells) are deposited on a substrate, each cell comprising a p-i-n junction comprising an intrinsic semiconductor compound layer sandwiched between a n-doped semiconductor compound layer and a p-doped semiconductor compound layer.
  • the n-layer of one of the cells is deposited according to one of the methods of manufacturing a n-layer stack as described above. This applies the principle of the invention as discussed above to a multijunction thin-film solar cell.
  • the method comprises depositing on a substrate a layer sequence of a front electrode; a p-layer; an intrinsic layer of a-Si:H; a n-doped a-Si:H layer; a n-doped ⁇ - ⁇ : ⁇ layer; an intermediate reflector layer of n-doped ⁇ - ⁇ ; an n-doped ⁇ - ⁇ : ⁇ layer; a p-layer; an intrinsic layer of ⁇ -3 ⁇ : ⁇ ; an n-layer; a back electrode; a back reflector .
  • the method comprises depositing on a substrate a lay- er sequence of a front electrode; a p-layer; an intrinsic layer of a-Si:H; a n-doped a-Si:H layer; a n-doped ⁇ -3 ⁇ : ⁇ layer; an intermediate reflector layer consisting of a plurality of layer systems each consisting of a layer of n-doped ⁇ - ⁇ followed by an n-doped microcrystalline silicon layer; an n-doped ⁇ -3 ⁇ : ⁇ layer; a p-layer; an intrinsic layer of ⁇ -3 ⁇ : ⁇ ; an n-layer; a back electrode; a back reflector .
  • the method comprises depositing on a substrate a lay- er sequence of optionally, a back reflector; a back electrode; an n- layer; an intrinsic layer of ⁇ -3 ⁇ : ⁇ ; a p-layer; an n-doped ⁇ -3 ⁇ : ⁇ layer; an intermediate reflector layer of n-doped ⁇ -3 ⁇ ; a n-doped ⁇ - ⁇ layer; a n-doped a-Si:H layer; an intrinsic layer of a-Si:H; a p-layer; a front electrode.
  • the method comprises depositing on a substrate a layer sequence of optionally, a back reflector; a back electrode; an n-layer; an intrinsic layer of ⁇ -3 ⁇ : ⁇ ; a p-layer; an n-doped ⁇ -3 ⁇ : ⁇ layer; an intermediate reflector layer of a plurality of layer systems each consisting of an n-doped microcrystalline silicon layer followed by a layer of n-doped pc-SiOx; a n-doped ⁇ - Si:H layer; a n-doped a-Si:H layer; an intrinsic layer of a-Si:H; a p-layer; a front electrode.
  • Fig. 1 A tandem junction thin film solar cell according to the prior art
  • Fig. 2 A graph of light induced degradation curves of micromorph mini-modules with different n-multilayer structures in top cell; and Fig. 3: A tandem junction thin film solar cell according to the invention .
  • Amorphous top cell here means the first cell from the light incident side featuring an amorphous i-layer 63 (this can be also a graded or multiple i-layer as long as they are amorphous) .
  • the invention proposes to amend Prior Art n-layer 54 into a multilayer-structure 64-67. Two n-multilayer structures were found to be the best solutions (BKM) with respect to highest initial and stabi- lized micromorph module power.
  • BKM best solutions
  • An amorphous silicon n-layer (nl, 64) with a thickness of 3-8nm, preferably 4-6nm is deposited on the intrinsic amorphous Si-layer 63. Its main purpose is to allow acting as n-layer for the top cell 61 and also as protection of the a-Si i-layer 63 from the subsequent aggressive (high RF power and flows) microcrystalline deposition process .
  • IMR, 66 microcrystalline silicon oxide layer with a thick- ness of at least 15nm to 50nm, preferably 20-40nm is being deposited onto layer 65 (n2, ⁇ - ⁇ - ⁇ ) thus acting as intermediate reflector.
  • a multilayer can be established from a multiple of a c-SiO and a ⁇ - ⁇ - ⁇ sequence.
  • the ⁇ - ⁇ is provided with a thickness of 8-18nm, or 8-16nm, preferably 12-16nm or 9-llnm, and an essentially 2nm microcrystalline n-layer.
  • a multiple e. g. 3 times: ⁇ - ⁇ / ⁇ - ⁇ - ⁇ / ⁇ - ⁇ / ⁇ - Si-n/ ⁇ c-SiO / ⁇ c-Si-n) layer structure is possible.
  • the total thickness of the ⁇ - ⁇ layers (which shall be in the range given in 3A) , less crucial is the number of multi-layers or the thickness of the single layers.
  • a preferred range is 2-6, or 2-5 multiples of a ⁇ - ⁇ and a ⁇ - ⁇ - ⁇ sequence.
  • Forming the intermediate reflector 66 by multilayers of ⁇ - ⁇ and by very thin and strongly crystalline ⁇ - ⁇ n-layers improves the electrical properties of the IMR ⁇ - ⁇ stack compared with the single-layer bulk ⁇ - ⁇ IMR of 3A above.
  • Strongly-oxidized ⁇ -SiO n-doped layers with refraction index values at 600 nm below 2.2 are preferred for achieving good optical properties, i.e. for achieving a strong reflection of impinging light back to the top cell 61.
  • the electrical conductivity of such ⁇ - SiO layers strongly decreases with increasing layer thickness, leading to an increase in the serial resistance of the solar cells, which is then evidenced by a reduced fill factor (FF) .
  • FF reduced fill factor
  • Good optical properties of the low refraction index ⁇ - ⁇ layer can be retained while simultaneously improving the electrical properties of the IMR by adopting the above-mentioned multilayer structure of of 2-6 thinner ⁇ - ⁇ layers that are separated by very thin, nominally 2 nm thick, strongly crystalline n-doped ⁇ - ⁇ layers so as to form the above-mentioned multilayer structure.
  • the deposition conditions for the very thin n-doped ⁇ - ⁇ interlayers can be similar to those used for the first n-doped ⁇ - ⁇ layer 65 of the IMR stack (i.e. layer n2) , which simplifies processing.
  • Table 1 Comparison of single layers and Micromorph module results with the IMR formed either by the method 3A (single ⁇ - ⁇ -SiO layer 45nm thick) or 3B (3 multilayers, each ⁇ - ⁇ -SiO layer 15nm thick, for a total thickness of ⁇ - ⁇ -SiO of 45nm) .
  • the ⁇ - ⁇ layer is has a high deposition rate of 2.4 A/sec and a refraction index at 600 run of 2.04.
  • the degree of crystallinity, and thus in consequence the electrical conductivity, of the ⁇ - ⁇ of the IMR layer is important for excellent results in Micromorph tandem solar cells.
  • the crystallinity and hence the conductivity of a single, bulk ⁇ - ⁇ layer as in 3A above can be also increased by choosing PECVD process parameters that favor more crystalline growth.
  • these regimes require strongly H-diluted SiH4 plasma, leading to very low deposition rates. From a manufacturing perspective, such low deposition rates are not desirable.
  • the multilayer solution presented above permits deposition of ⁇ - ⁇ layers at a relatively high rate for the IMR, the overall crystallinity and hence the elec- trical conductivity of the IMR layer being increased by the incorporation of the very thin and strongly crystalline ⁇ -Si-n interlayers.
  • a microcrystalline silicon n-layer (n3, 67) with a thickness of 4-8nm, preferably 4-6nm, essentially 5nm follows the layer described in section 3A or 3B respectively.
  • the main purpose is to act as a contact between intermediate reflector and subsequent p-layer 44 of bottom cell 43 and acting as a nucleation layer for the p-layer 44. It also allows for a better tunnel recombination junction between top and bottom cell.
  • the detailed deposition conditions for an embodiment of the invention can be seen in Table 2. After the last layer N3 the bottom cell 43 is starting with its p-layer 44.
  • the gas fluxes and RF power are given based on a PECVD parallel plate reactor designed for deposition on 1.4m 2 substrate (1100x1300mm 2 ) .
  • the phosphine PH 3 is, as usually in the art, delivered in a dilution of 0.5:100 in hydrogen.
  • the underlying teaching of the invention can be up- or downscaled to other substrate or deposition system sizes accordingly. Deposition rates may be varied (and thus the fluxes and the RF power density values given in table 1) , however the absolute values of Phosphorus atomic concentration of the various n-doped layers needs to be kept essentially constant.
  • the absolute Phosphorus atomic concentrations should be understood with an accuracy of +/-50%, preferably +/-20%.
  • Table 3 Comparison of top and bottom cell currents with and without intermediate reflector derived from EQE (external quantum efficiency) measurements .
  • top and bottom cells are 220 nm / 900 nm in ref- erence (without IMR) and 200 nm / 1000 nm with IMR.
  • the thickness of IMR was 33 nm in this example.
  • the top cell current increases by more than 0.7 mA/cm2 (i.e. ⁇ 7%) when using an IMR, and this although the top cell used with IMR is thinner by 20 nm than in the reference.
  • the bottom cell current decreases only slightly indicating an additional beneficial effect of the intermediate reflector due to the higher band gap energy (IMR is a more transparent layer for the light passing to bottom cell than the ⁇ - ⁇ n-layers) .
  • the EQE data also reveal a stronger bottom cell current limitation of the module with IMR with respect to the reference.
  • the thinner top cell together with the stronger current limitation due to IMR can result in a lower light induced degradation of the modules with IMR.
  • Table 4 shows the IV data of micromorph mini-modules that contain top cell n-multilayer with different structures. The differences in IV parameters between the best known method (BKM) and different n- layer multi-structures with missing component layers are also evidenced in table 5.
  • Table 4 IV data of best known method and of devices missing crucial sublayers. Experiments are based on multiple IMRs (case 3B) .
  • thin n2 is to be understood as l-3nm, about 1/5 of the thickness of n2 proposed for the invention.
  • Table 5 Difference in IV data induced by missing of crucial layers Experiments are based on multiple IMRs (case 3B) .
  • the necessity of the n-multilayer structure which is part of the invention can be seen from the data shown in Table 4 and 5.
  • Lowest serial resistance and highest FF are obtained for the BKM n-multilayer structure.
  • the n2 layer also has a significant impact on the Voc.
  • the lack of nl and n3 layers results in a loss of both FF and Voc.
  • the intermediate reflector layer reflects part of light back to top cell, it allows the use of thinner top cell absorber layer without loss in top cell current. This contributes to a reduced light induced degradation of tandem cells.
  • Figure 2 shows the light induced degradation curves of micromorph mini-modules with different n-multilayer structures in top cell. It is easy to observe that only a proper n-multilayer structure such as that described as BKM in the present invention is able to deliver a gain in efficiency both in the initial state and after light induced degradation. N-multilayer structures without nl or with very thin n2 'are not able to preserve the small gain in initial efficiency after light induced degradation with respect to the n-multilayers without IMR.
  • Table 6 shows another example that reflects the advantages of the n- multilayer structure denoted as BKM.
  • BKM the n-multilayer structure denoted as BKM has the best IV parameters and conversion efficiency as compared to other n-multilayer structures with missing nl layer or/and with very thin n2 layer.
  • a loss in FF is observed whenever the nl layer is missing, whereas thin n2 layers also contribute to a loss in FF.
  • Table 6 IV data of best known method and of devices missing crucial sublayers. Experiment is based on single IMR layer (case 3A)
  • a multijunction thin film solar cell comprises a stacked arrangement of at least two solar cells, electrically in series, wherein the n- layer of one of the stacked solar cells has a structure comprising an amorphous silicon n-layer (nl, 64) , a microcrystalline silicon n- layer (n2, 65), a microcrystalline silicon oxide layer (IMR, 66) and a microcrystalline silicon n-layer (n3, 67) .
  • the bulk IMR layer 66 can alternatively include a multilayer, established from a multiple of a c-SiO and a ⁇ - ⁇ - ⁇ sequence. The order of layer is given in the direction of the impinging (unreflected) light.
  • the amorphous silicon n-layer (nl, 64) has a thickness of 3-8nm, preferably 4-6nm and is deposited onto the intrinsic amorphous Si- layer 63 of the cell closer to the incoming light.
  • the microcrystalline silicon n-layer (n2, 65) has a thickness of 5-15nm, preferably 6-9nm and is being deposited onto amorphous Si-layer 64.
  • the micro- crystalline silicon oxide layer (IMR, 66) has a thickness of at least 15nm to 50nm, preferably 20-40nm and is being deposited onto layer 65 (n2, n- c-Si) thus acting as intermediate reflector.
  • an al- ternative multilayer may be established from a multiple of c-SiO and a ⁇ - ⁇ - ⁇ layers is characterized as follows:
  • the ⁇ - ⁇ layer has a thickness of 8-16nm, preferably 9-llnm.
  • the c-Si-n layer is essentially 2nm thick.
  • the microcrystalline silicon n-layer (n3, 67) exhibits a thickness of 4-8nm, preferably 4-6nm, essentially 5nm follows micro- crystalline silicon oxide layer (IMR, 66) or multilayer.
  • the inventive layer stack can be deposited in a state-of-the-art
  • PECVD deposition system with process gases widely known and available, such as hydrogen, silane, carbondioxide and dopants such as phosphine and boron. It has been found that a front contact made from LPCVD deposited ZnO as transparent conductive oxide layer (electrode) is beneficial, since a rough TCO allows for a generally thinner IMR layer.
  • process gases widely known and available, such as hydrogen, silane, carbondioxide and dopants such as phosphine and boron.
  • a method of manufacturing a n-layer stack as mentioned above com- prises depositing an amorphous silicon n-layer (nl, 64) , a micro- crystalline silicon n-layer (n2, 65) , a microcrystalline silicon oxide layer (IMR, 66) and a microcrystalline silicon n-layer (n3, 67) .
  • the bulk IMR layer 66 can alternatively be deposited as a multilayer stack, from a multiple of a c-SiO and a c-Si-n sequence.
  • the order of layer is given in the direction of the impinging (unreflected) light. Depending on the structure of the cell and the orientation of the base structure, the deposition order may be reversed (nip or pin junction stack) .
  • inventive layer stack can be incorporated in tandem, triple or other multijunction arrangements.
  • the examples refer to thin-film silicon layer stacks, but the invention broadly addresses the necessity to reflect light in a thin film layer stack.
  • inventive principle may be used also for other types of stacked solar cell arrangements.

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Abstract

The invention relates to a multijunction thin film solar cell (70) of high efficiency and low-cost production. The n-layer (64, 65, 66, 67) of one of the constituent cells (61) of the multijunction thin film solar cell has a structure comprising, in the intended direction of impinging light, a sequence of an amorphous hydrogenated silicon n-layer (64), a first microcrystalline hydrogenated silicon n-layer (65), an intermediate reflector layer (66) and a second microcrystalline hydrogenated silicon n-layer (67), wherein the intermediate reflector layer is one of the following: - a single microcrystalline essentially silicon oxide layer; or - a plurality of layer sequences, each layer sequence consisting of a microcrystalline essentially silicon oxide layer and a microcrystalline silicon n-layer in the intended direction of impinging light. The invention further relates to methods of manufacturing the above n-layer (64, 65, 66, 67), and to methods of manufacturing the above-mentioned multijunction thin film solar cell (70)

Description

INTERMEDIATE REFLECTION STRUCTURE IN THIN FILM SOLAR CELLS
FIELD OF THE INVENTION
Photovoltaic devices or solar cells are devices which convert light into electrical power. Thin film solar cells nowadays are of a particular importance since they have a huge potential for mass production at low cost. Especially the combination of amorphous and micro- or nanocrystalline silicon in multijunction solar cells offer the perspective of achieving energy conversion efficiencies exceeding 10% due to the better use of the solar irradiation compared to, for example, an amorphous silicon single junction solar cell. Due to the use of 2 or more photovoltaic junctions of different band gap the impinging light with a broad spectral distribution (such as solar irradiation) can be used more efficiently. Further, high-quality mi— crocrystalline silicon does not suffer from light induced degradation (Staebler-Wronski-effect) as amorphous silicon; therefore an amorphous-microcrystalline silicon multijunction solar cell shows a smaller degradation of its initial conversion efficiency compared to a "pure" amorphous silicon single junction solar cell.
DEFINITIONS
Processing in the sense of this invention includes any chemical, physical or mechanical effect acting on substrates.
Substrates in the sense of this invention are components, parts or workpieces to be treated in a processing apparatus. Substrates include but are not limited to flat, plate shaped parts having rectangular, square or circular shape. In a preferred embodiment this invention addresses essentially planar substrates of a size >lm2, such as thin glass plates.
A vacuum processing or vacuum treatment system or apparatus comprises at least an enclosure for substrates to be treated under pressures lower than ambient atmospheric pressure.
CVD Chemical Vapour Deposition is a well known technology allowing the deposition of layers on heated substrates. A usually liquid or gaseous precursor material is being fed to a process system where a thermal reaction of said precursor results in deposition of said layer. LPCVD is a common term for low pressure CVD. TCO stands for transparent conductive oxide, TCO layers consequently are transparent conductive layers.
The terms layer, coating, deposit and film are interchangeably used in this disclosure for a film deposited in vacuum processing equip- ment, be it CVD, LPCVD (low-pressure CVD) , plasma enhanced CVD
(PECVD) or PVD (physical vapour deposition)
A solar cell or photovoltaic cell (PV cell) is an electrical component capable of transforming light (essentially sun light) directly into electrical energy by means of the photoelectric effect.
A thin-film solar cell in a generic sense includes, on a supporting substrate, at least one p-i-n junction established by a thin film deposition of semiconductor compounds, sandwiched between two electrodes or electrode layers. A p-i-n junction or thin-film photoelec- trie conversion unit includes an intrinsic semiconductor compound layer sandwiched between a p-doped and an n-doped semiconductor compound layer. The term thin-film indicates that the layers mentioned are being deposited as thin layers or films by processes like, PEVCD, CVD, PVD or alike. Thin layers essentially mean layers with a thick- ness of ΙΟμηα or less, especially less than 2μπι.
Any layer thicknesses described in this disclosure refer to an averaged thickness measured perpendicular to the base of the respective layer; averaged over a sufficiently dimensioned number of metering points .
BACKGROUND OF THE INVENTION / RELATED ART
A basic type of a working a-Si^c-Si tandem junction thin film solar cell is shown in Fig. 1. Such a thin-film solar cell 50 usually in- eludes a first or front electrode 42, one or more semiconductor thin-film p-i-n junctions (52-54, 51, 44-46, 43) , and a second or back electrode 47, which are successively stacked on a substrate 41. Each p-i-n junction 51, 43 or thin-film photoelectric conversion unit includes an i-type layer 53, 45 sandwiched between a p-type layer 52, 44 and an n-type layer 54, 46 (p-type = positively doped, n-type = negatively doped) . Substantially intrinsic in this context is understood as not intentionally doped or exhibiting essentially no resultant doping, Photoelectric conversion occurs primarily in this i-type layer; it is therefore also called absorber layer .
Depending on the crystalline fraction (crystallinity) of the i-type layer 53, 45 solar cells or photoelectric (conversion) devices are characterized as amorphous (a-Si, 53) or microcrystalline (pc-Si, 45) solar cells, independent of the kind of crystallinity of the adjacent p and n-layers . Microcrystalline layers are being understood, as common in the art, as layers comprising of a significant fraction of crystalline silicon - so called micro-crystallites - in an amorphous matrix. Stacks of p-i-n junctions are called tandem or triple junction photovoltaic cells. The combination of an amorphous and microcrystalline p-i-n- junction, as shown in Fig. 1, is also called micromorph tandem cell.
Front- 42 and back 47 electrode layers are preferably made from ZnO:B (boron doped zinc oxide) prepared e.g. in an industrial Oer- likon TCO system by LPCVD deposition. However, other transparent conductive layers can also be used, such as Sn02, ITO and others. The back reflector 48 is preferably a white plastic foil laminated on the back electrode. However, the invention described in more detail below can also be used successfully with other types of specular or diffuse reflective layers, such as metallic reflectors, white paint or alike.
All Si layers mentioned above and in the invention below can be prepared on 1100mm x 1300mm glasses in an industrial KAI deposition system featuring a 1200mm x 1400mm electrode. However, the invention is not limited to this specific substrate size nor this specific deposition system. It can be realized without inventive effort on other PECVD deposition systems as well.
Since degradation of the tandem device is depending on the amorphous silicon sub-device and the degradation of the amorphous silicon sub- device is depending on the thickness of its i-layer, it is desirable to make it as thin as possible. To achieve nevertheless a high current a so called intermediate reflector can be used (a low refractive index layer generally arranged between the i-layers of top and bottom cell 53, 45) , which reflects part of the passing light back into the amorphous cell thus increasing the amount of light absorbed there and resulting in an increase of the current of the amorphous cell.
Intermediate reflectors per se are known in the art and e.g. can be realised based on e. g. a TCO (e.g. ZnO; N. Pellaton Vaucher et al., "Light Management in tandem cells by 'an intermediate reflector layer", 2nd World conference and exhibition on photovoltaic solar energy conversion, 1998) or of Silicon alloyed with a band gap widen- ing agent (e.g. SiOx, Tawada et al . US 4,476,346 of 1984)
DRAWBACKS KNOWN IN THE ART
Both mentioned approaches are well suitable for reflecting light back into the top cell. An approach based on TCO is not wanted from a production point of view, because it requires an extra step in a separate system which increases the production costs significantly. A silicon based intermediate reflector however is well suited from a production point of view. It is however a disturbing factor regarding the electrical performance of a device since it is neither suit- ed for acting as a doped layer of the devices neither facilitating a good contact between the devices. For that reason a careful optimisation of the overall structure between the intrinsic layers of top and bottom cell is necessary. SUMMARY OF THE INVENTION
The object of the invention is thus to overcome at least some of the above-mentioned drawbacks in the art.
This object is attained by a multijunction thin film solar cell com- prising a stacked arrangement of at least two solar cells electrically in series, i.e. a double, triple, or higher-order multijunction cell, each solar cell comprising a p-i-n junction comprising an intrinsic semiconductor compound layer sandwiched between an n-doped semiconductor compound layer and a p-doped semiconductor compound layer. Each of these layers may be a single layer, or a multilayer comprising a plurality of sublayers. The n-layer of one of the stacked solar cells has a multilayer structure comprising, in the intended direction of impinging light (that is to say in the direction in which, in use, incident light will travel through the cell) , a sequence of an n-doped a-Si:H layer, a first n-doped μο-5ί:Η layer, an intermediate reflector layer, and a second n-doped μο-3ί:Η layer. The intermediate reflector layer comprises a plurality of layer sequences, each layer sequence consisting essentially of a μο-είθ layer and a n-doped μο-3ί:Η layer in the intended direction of impinging light. The μο-βίθ layer is described as "essentially of μο-είθ" or "a microcrystalline essentially silicon oxide layer" since other components may be present in smaller quantities, for instance μσ-είθ in the case that C02 is used as a source of oxygen in processing.
This applies equally to the below.
This results in a cell of high-efficiency which is extremely easy to produce, since the intermediate reflector layer is implemented as part of the n-doped layer of one of the cells, merely by adding oxidising gas such as oxygen or carbon dioxide in the same PECVD apparatus used to deposit the other cell layers. Each microcrystalline essentially silicon oxide layer may have a thickness of 8-18 nm, particularly 8-16 nm, particularly 12-16 nm, 9-11 nm, and each mi- crocrystalline silicon n-layer may be approximately 2 nm thick. In a further embodiment, the number of layer sequences is 2-6, particularly 2-5, which has proven to give good results in practice.
In an alternative embodiment, instead of the intermediate reflector layer being a multilayer structure of a plurality of layer sequences, it is a single μο-ΞίιΗ layer. This single intermediate reflector layer structure likewise results in a cell of high-efficiency which is extremely easy to produce, since the intermediate reflector layer is implemented as part of the n-doped layer of one of the cells, merely by adding oxidising gas such as oxygen or carbon dioxide in the same PECVD apparatus used to deposit the other cell layers.
This single layer may have a thickness of at least 15-50 nm, particularly 20-40 nm, and is deposited on the first microcrystalline silicon n-layer.
In an embodiment which may be combined with any of the above embodiments, at least one of the following is true: - the amorphous silicon n-layer has a thickness of 3-8 nm, particularly 4-6 nm and is deposited onto the intrinsic silicon layer of the cell closer to the incoming light, said intrinsic silicon layer of the cell closer to the incoming light being amorphous;
- the first microcrystalline silicon n-layer has a thickness of 5-50 nm, particularly 5-15 nm, particularly 6-12 nm, particularly 6-9 nm, and is deposited on to said amorphous silicon n-layer;
- the second microcrystalline silicon n-layer has a thickness of 4-8 nm, particularly 4-6 nm, particularly essentially 5 nm and is situ- ated following the intermediate reflector layer in the direction of the impinging light.
These layer thicknesses have proven in practice to give good results. In an embodiment which may be combined with any of the above embodiments, the multijunction thin-film solar cell further comprises a front contact made from zinc oxide deposited by low pressure chemical vapour deposition as a transparent conductive oxide layer electrode. This allows a generally thinner intermediate reflector layer since LPCVD-deposited zinc oxide is relatively rough and thus has good light-scattering properties.
In an embodiment which may be combined with any of the above embodiments, the intrinsic semiconductor compound layer situated closest to the impinging light is amorphous, permitting fast top-cell deposition.
In an embodiment comprising a single intermediate reflector layer, the multijunction thin-film solar cell comprises the following layer sequence in the intended direction of impinging light: a substrate; a front electrode; a p-layer; an intrinsic layer of a-Si:H; a n- doped a-Si:H layer; a n-doped μο-3ί:Η layer; an intermediate reflector layer of n-doped μο-ΞίΟχ; an n-doped c-Si:H layer; a p-layer; an intrinsic layer of c-Si:H; an n-layer; a back electrode; a back reflector .
In an alternative embodiment comprising a multilayer intermediate reflector layer, the multijunction thin-film solar cell comprises the following layer sequence in the intended direction of impinging light: a substrate; a front electrode; a p-layer; an intrinsic layer of a-Si:H; a n-doped a-Si:H layer; a n-doped μΰ-3ί:Η layer; an intermediate reflector layer consisting of a plurality of layer sys- terns each consisting of a layer of n-doped μο-είθχ followed by an n- doped c-Si layer; an n-doped μσ-3ί:Η layer; a p-layer; an intrinsic layer of με-είιΗ; an n-layer; a back electrode; a back reflector. An object of the invention is likewise attained by a method of manufacturing a n-layer stack from multijunction solar cell, comprising depositing in the intended direction of impinging light (i.e. the direction in which light will travel into the cell when the cell is in use) an a-Si:H n-layer, a first μο-Ξί:Η n-layer; intermediate re- flector layer, and a second μσ-3ί:Η n-layer. The intermediate reflector layer comprises a plurality of layer sequences, deposition of each layer sequence consisting of depositing a μο-3ίΟ layer and depositing a n-doped μο-3ί:Η layer arranged in the intended direction of impinging light. In an alternative embodiment of the method, the deposition of the intermediate reflector layer comprises depositing a single μο-ΞίΟ layer rather than a μο-είθ / n-doped μο-3ί:Η multilayer structure.
When incorporated into a multijunction solar cell, both of these embodiments result in a cell of high-efficiency which is extremely easy to produce, since the intermediate reflector layer is implemented as part of the n-doped layer of one of the cells, merely by adding a source of oxidiser in the same PECVD apparatus used to deposit the other cell layers. The specified sublayers of the n-doped layer in question result in good adhesion between the layers, and in good electrical contact between the cells.
In an embodiment of the method, the amorphous silicon layer is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m2 substrate:
- SiH4 (Silane) flow rate 312 seem;
- H2 (Hydrogen) flow rate 733 seem;
- PH3 (Phosphine) flow rate 166 seem;
- RF power 415 W
- RF power density 25 mW/cm2;
the above values being linearly scaled by area for other substrate areas; and
- pressure 0.5 mbar;
- deposition rate 3.1 A/s; In an embodiment of the method, the first microcrystalline silicon n-layer (65) is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m2 substrate:
SiH4 flow rate 41 seem;
- H2 flow rate 4300 seem;
PH3 flow rate 51 seem;
- RF power 1880 W
RF power density 112 mW/cm2;
the above values being linearly scaled by area for other substrate areas; and
- pressure 2.0 mbar;
deposition rate 0.8 A/s;
In an embodiment of the method, the intermediate reflector layer is a single microcrystalline essentially silicon oxide layer and is de- posited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m2 substrate:
SiH4 flow rate 60 seem;
- H2 flow rate 9900 seem;
PH3 flow rate 300 seem;
- C02 flow rate 115 seem;
- RF power 2200 W
RF power density 131 mW/cm2;
the above values being linearly scaled by area for other substrate areas; and
- pressure 2.5 mbar;
deposition rate 0.9 A/s;
In an embodiment of the method, the second microcrystalline silicon n-layer is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m2 substrate:
- SiH4 flow rate 51 seem;
- H2 flow rate 4000 seem;
PH3 flow rate 250 seem;
- RF power 2300 W
RF power density 137 mW/cm2;
the above values being linearly scaled by area for other substrate areas; and
- pressure 2.0 mbar; deposition rate 1.1 A/s;
whereby these values are linearly scaled for other substrate areas.
The invention is likewise attained by a method of manufacturing a multijunction thin film solar cell in which a first cell and at least one second cell (i.e. a total of two or more cells) are deposited on a substrate, each cell comprising a p-i-n junction comprising an intrinsic semiconductor compound layer sandwiched between a n-doped semiconductor compound layer and a p-doped semiconductor compound layer. The n-layer of one of the cells is deposited according to one of the methods of manufacturing a n-layer stack as described above. This applies the principle of the invention as discussed above to a multijunction thin-film solar cell.
In an embodiment of the method of manufacturing a multijunction thin film solar cell with a single intermediate reflector layer, the method comprises depositing on a substrate a layer sequence of a front electrode; a p-layer; an intrinsic layer of a-Si:H; a n-doped a-Si:H layer; a n-doped μο-Ξί:Η layer; an intermediate reflector layer of n-doped μο-ΞίΟχ; an n-doped μο-Ξί:Η layer; a p-layer; an intrinsic layer of μσ-3ί:Η; an n-layer; a back electrode; a back reflector .
In an alternative embodiment of the method of manufacturing a multijunction thin film solar cell with a multilayer intermediate reflector layer, the method comprises depositing on a substrate a lay- er sequence of a front electrode; a p-layer; an intrinsic layer of a-Si:H; a n-doped a-Si:H layer; a n-doped μο-3ί:Η layer; an intermediate reflector layer consisting of a plurality of layer systems each consisting of a layer of n-doped μσ-είθχ followed by an n-doped microcrystalline silicon layer; an n-doped μσ-3ί:Η layer; a p-layer; an intrinsic layer of μο-3ί:Η; an n-layer; a back electrode; a back reflector .
In a further alternative embodiment of the method of manufacturing a multijunction thin film solar cell with a single intermediate reflector layer, the method comprises depositing on a substrate a lay- er sequence of optionally, a back reflector; a back electrode; an n- layer; an intrinsic layer of μο-3ί:Η; a p-layer; an n-doped μσ-3ί:Η layer; an intermediate reflector layer of n-doped μο-3ίΟχ; a n-doped μο-ΞίιΗ layer; a n-doped a-Si:H layer; an intrinsic layer of a-Si:H; a p-layer; a front electrode.
In a yet further alternative embodiment of the method of manufacturing a multijunction thin film solar cell with a multilayer interme- diate reflector layer, the method comprises depositing on a substrate a layer sequence of optionally, a back reflector; a back electrode; an n-layer; an intrinsic layer of μσ-3ί:Η; a p-layer; an n-doped μο-3ί:Η layer; an intermediate reflector layer of a plurality of layer systems each consisting of an n-doped microcrystalline silicon layer followed by a layer of n-doped pc-SiOx; a n-doped μο- Si:H layer; a n-doped a-Si:H layer; an intrinsic layer of a-Si:H; a p-layer; a front electrode.
BRIEF DESCRIPTION OF THE FIGURES
The invention will now be further described in terms of specific non-limiting embodiments in the following figures, which show:
Fig. 1: A tandem junction thin film solar cell according to the prior art;
Fig. 2: A graph of light induced degradation curves of micromorph mini-modules with different n-multilayer structures in top cell; and Fig. 3: A tandem junction thin film solar cell according to the invention .
DETAILED DESCRIPTION OF THE INVENTION
This invention is focusing on the design and properties of the n- layer of an amorphous top cell 61 of a micromorph tandem cell 70, as sketched in Fig. 3. It can however be applied to any thin-film- silicon multijunction cell of such type. Amorphous top cell here means the first cell from the light incident side featuring an amorphous i-layer 63 (this can be also a graded or multiple i-layer as long as they are amorphous) . The invention proposes to amend Prior Art n-layer 54 into a multilayer-structure 64-67. Two n-multilayer structures were found to be the best solutions (BKM) with respect to highest initial and stabi- lized micromorph module power. The layer deposition sequence described in the following is starting after deposition of the amorphous i-layer 63. Variant 3Ά and 3B are individually combined with the other steps to form the two BKM:
1. An amorphous silicon n-layer (nl, 64) with a thickness of 3-8nm, preferably 4-6nm is deposited on the intrinsic amorphous Si-layer 63. Its main purpose is to allow acting as n-layer for the top cell 61 and also as protection of the a-Si i-layer 63 from the subsequent aggressive (high RF power and flows) microcrystalline deposition process .
2. A microcrystalline silicon n-layer (n2, 65) with a thickness of 5-15nm, preferably 6-12nm or 6-9nm is being deposited onto amorphous Si-layer 64. This layer facilitates the contact between a-Si-n (layer 64) and the subsequent IMR. It further facilitates nucleation and microcrystalline growth of the IMR (layer 66) .
3A. A microcrystalline silicon oxide layer (IMR, 66) with a thick- ness of at least 15nm to 50nm, preferably 20-40nm is being deposited onto layer 65 (n2, η-μσ-εί) thus acting as intermediate reflector.
3B. As an alternative to a bulk IMR layer 66 a multilayer can be established from a multiple of a c-SiO and a μο-Ξί-η sequence. Pref- erably the μο-είθ is provided with a thickness of 8-18nm, or 8-16nm, preferably 12-16nm or 9-llnm, and an essentially 2nm microcrystalline n-layer. A multiple (e. g. 3 times: μσ-ΞίΟ/μο-Βί-η/μσ-είΟ/μο- Si-n/^c-SiO/^c-Si-n) layer structure is possible. Important is the total thickness of the μο-ΞίΟχ layers (which shall be in the range given in 3A) , less crucial is the number of multi-layers or the thickness of the single layers. A preferred range is 2-6, or 2-5 multiples of a μο-είθ and a μο-εί-η sequence.
Forming the intermediate reflector 66 by multilayers of μο-είθ and by very thin and strongly crystalline μο-εί n-layers improves the electrical properties of the IMR μο-είθ stack compared with the single-layer bulk μο-ΞίΟ IMR of 3A above. Strongly-oxidized μο-SiO n-doped layers with refraction index values at 600 nm below 2.2 are preferred for achieving good optical properties, i.e. for achieving a strong reflection of impinging light back to the top cell 61. However, the electrical conductivity of such μο- SiO layers strongly decreases with increasing layer thickness, leading to an increase in the serial resistance of the solar cells, which is then evidenced by a reduced fill factor (FF) .
Good optical properties of the low refraction index μο-είθ layer can be retained while simultaneously improving the electrical properties of the IMR by adopting the above-mentioned multilayer structure of of 2-6 thinner μο-βίθ layers that are separated by very thin, nominally 2 nm thick, strongly crystalline n-doped μσ-Ξί layers so as to form the above-mentioned multilayer structure. The deposition conditions for the very thin n-doped μο-εί interlayers can be similar to those used for the first n-doped μο-βί layer 65 of the IMR stack (i.e. layer n2) , which simplifies processing.
Experimental evidence is presented below of an improvement in the electrical performance of a μσ-είθ multilayer structure as compared to a single μο-ΞίΟ layer as for 3A above, with the same thickness of μο-ΞίΟ. For the IMR multilayer structure, the strongly crystalline and very highly conductive η-μο-εί interlayers (conductivity ~E+1 l/Ohm*cm) help to increase the crystallinity and the electrical conductivity of the μο-είθ layers. As a consequence, Micromorph tandem cell modules comprising this IMR multilayer stack show a reduction in the serial resistance and therefore an improvement in the FF, as shown in the Table below. η-μο-3ίΟ-45ηιη 3x [η-μο-ΞίΟ-Ιδηπι + η-μο-3ί-2ηιη]
Single layer 8.9 E-8 3.7 E-7
transversal conductivity
(1/Ohm*cm) icromorph modules 54.3 49.5
serial resistance
Rs (Ohm)
Micromorph modules 62.7 63.9
FF (%)
Table 1 : Comparison of single layers and Micromorph module results with the IMR formed either by the method 3A (single η-μσ-SiO layer 45nm thick) or 3B (3 multilayers, each η-μσ-SiO layer 15nm thick, for a total thickness of η-μσ-SiO of 45nm) . The η-μοβϊθ layer is has a high deposition rate of 2.4 A/sec and a refraction index at 600 run of 2.04.
It has been observed that the degree of crystallinity, and thus in consequence the electrical conductivity, of the μο-ΞίΟ of the IMR layer is important for excellent results in Micromorph tandem solar cells. The crystallinity and hence the conductivity of a single, bulk μσ-είθ layer as in 3A above can be also increased by choosing PECVD process parameters that favor more crystalline growth. However, these regimes require strongly H-diluted SiH4 plasma, leading to very low deposition rates. From a manufacturing perspective, such low deposition rates are not desirable. The multilayer solution presented above permits deposition of μο-είθ layers at a relatively high rate for the IMR, the overall crystallinity and hence the elec- trical conductivity of the IMR layer being increased by the incorporation of the very thin and strongly crystalline μο-Si-n interlayers.
4. A microcrystalline silicon n-layer (n3, 67) with a thickness of 4-8nm, preferably 4-6nm, essentially 5nm follows the layer described in section 3A or 3B respectively. The main purpose is to act as a contact between intermediate reflector and subsequent p-layer 44 of bottom cell 43 and acting as a nucleation layer for the p-layer 44. It also allows for a better tunnel recombination junction between top and bottom cell. The detailed deposition conditions for an embodiment of the invention can be seen in Table 2. After the last layer N3 the bottom cell 43 is starting with its p-layer 44.
Figure imgf000015_0001
Table 2 : deposition parameters for n-sublayers
In this table the gas fluxes and RF power are given based on a PECVD parallel plate reactor designed for deposition on 1.4m2 substrate (1100x1300mm2) . The phosphine PH3 is, as usually in the art, delivered in a dilution of 0.5:100 in hydrogen. The underlying teaching of the invention can be up- or downscaled to other substrate or deposition system sizes accordingly. Deposition rates may be varied (and thus the fluxes and the RF power density values given in table 1) , however the absolute values of Phosphorus atomic concentration of the various n-doped layers needs to be kept essentially constant. The absolute Phosphorus atomic concentrations should be understood with an accuracy of +/-50%, preferably +/-20%. In other words, the relationship of the Phosphorus atomic concentration of the different n-doped layers nl : n2 : n-SiOx : n3 = 0.5:0.33:3.0:1.4 within the limits and accuracies described above is part of the invention.
Figure imgf000016_0001
Table 3 : Comparison of top and bottom cell currents with and without intermediate reflector derived from EQE (external quantum efficiency) measurements .
In table 3 the currents generated in a tandem junction thin film silicon solar cell are being presented, including individual values for top and bottom cell; with and without intermediate reflector. The thicknesses of top and bottom cells are 220 nm / 900 nm in ref- erence (without IMR) and 200 nm / 1000 nm with IMR. The thickness of IMR was 33 nm in this example.
The top cell current increases by more than 0.7 mA/cm2 (i.e.~7%) when using an IMR, and this although the top cell used with IMR is thinner by 20 nm than in the reference. This indicates a well functioning intermediate reflector. The bottom cell current decreases only slightly indicating an additional beneficial effect of the intermediate reflector due to the higher band gap energy (IMR is a more transparent layer for the light passing to bottom cell than the μσ-Ξί n-layers) . The EQE data also reveal a stronger bottom cell current limitation of the module with IMR with respect to the reference. The thinner top cell together with the stronger current limitation due to IMR can result in a lower light induced degradation of the modules with IMR.
Table 4 shows the IV data of micromorph mini-modules that contain top cell n-multilayer with different structures. The differences in IV parameters between the best known method (BKM) and different n- layer multi-structures with missing component layers are also evidenced in table 5.
Figure imgf000017_0001
Table 4 : IV data of best known method and of devices missing crucial sublayers. Experiments are based on multiple IMRs (case 3B) .
"thin n2" is to be understood as l-3nm, about 1/5 of the thickness of n2 proposed for the invention.
Figure imgf000017_0002
Table 5 : Difference in IV data induced by missing of crucial layers Experiments are based on multiple IMRs (case 3B) . The necessity of the n-multilayer structure which is part of the invention can be seen from the data shown in Table 4 and 5. Lowest serial resistance and highest FF are obtained for the BKM n-multilayer structure. The n2 layer also has a significant impact on the Voc. The lack of nl and n3 layers results in a loss of both FF and Voc.
As above mentioned, since the intermediate reflector layer reflects part of light back to top cell, it allows the use of thinner top cell absorber layer without loss in top cell current. This contributes to a reduced light induced degradation of tandem cells.
Figure 2 shows the light induced degradation curves of micromorph mini-modules with different n-multilayer structures in top cell. It is easy to observe that only a proper n-multilayer structure such as that described as BKM in the present invention is able to deliver a gain in efficiency both in the initial state and after light induced degradation. N-multilayer structures without nl or with very thin n2 'are not able to preserve the small gain in initial efficiency after light induced degradation with respect to the n-multilayers without IMR.
Table 6 shows another example that reflects the advantages of the n- multilayer structure denoted as BKM. In this example a different IMR layer with a refraction index of 2.1 and a layer thickness of 40 nm has been used. Additionally the PECVD process for the microcrystal- line bottom cell was slightly different. Similar as in the example given in the table 3, the n-multilayer structure denoted as BKM has the best IV parameters and conversion efficiency as compared to other n-multilayer structures with missing nl layer or/and with very thin n2 layer. A loss in FF is observed whenever the nl layer is missing, whereas thin n2 layers also contribute to a loss in FF. A slight tendency of higher Voc for the cells with the BKM n- multilayer structure can also be recognized. Depending on the crys- tallinity and structure of the subsequent bottom cell that is depos- ited on the top cell n-multilayer structure a stronger increase in the Voc of micromorph modules can be observed when using the BKM n- multilayer structure. BKM No n1 thin n2 No n1 &
Thin n2
FF (%) 71.11 69.84 70.76 68.99
Voc (mV/segment) 1.303 1.287 1.296 1.299
Rs (Ohm) 32.88 33.51 34.14 35.59
Jsc (mA/cm2) 11.65 11.75 11.64 11.75
Eta initial state (%) 10.77 10.54 10.66 10.52
Eta after light induced 9.97 9.84 9.83 9.80 degradation (%)
Table 6: IV data of best known method and of devices missing crucial sublayers. Experiment is based on single IMR layer (case 3A)
SUMMARY
A multijunction thin film solar cell comprises a stacked arrangement of at least two solar cells, electrically in series, wherein the n- layer of one of the stacked solar cells has a structure comprising an amorphous silicon n-layer (nl, 64) , a microcrystalline silicon n- layer (n2, 65), a microcrystalline silicon oxide layer (IMR, 66) and a microcrystalline silicon n-layer (n3, 67) . The bulk IMR layer 66 can alternatively include a multilayer, established from a multiple of a c-SiO and a μο-εί-η sequence. The order of layer is given in the direction of the impinging (unreflected) light.
The amorphous silicon n-layer (nl, 64) has a thickness of 3-8nm, preferably 4-6nm and is deposited onto the intrinsic amorphous Si- layer 63 of the cell closer to the incoming light. The microcrystalline silicon n-layer (n2, 65) has a thickness of 5-15nm, preferably 6-9nm and is being deposited onto amorphous Si-layer 64. The micro- crystalline silicon oxide layer (IMR, 66) has a thickness of at least 15nm to 50nm, preferably 20-40nm and is being deposited onto layer 65 (n2, n- c-Si) thus acting as intermediate reflector.
Instead of the microcrystalline silicon oxide layer (IMR, 66) an al- ternative multilayer may be established from a multiple of c-SiO and a μσ-Ξί-η layers is characterized as follows: The μο-ΞίΟ layer has a thickness of 8-16nm, preferably 9-llnm. The c-Si-n layer is essentially 2nm thick. Preferably a 2-5 multiple of a c-SiO and a μο-Ξί-η sequence is being used.. Finally the microcrystalline silicon n-layer (n3, 67) exhibits a thickness of 4-8nm, preferably 4-6nm, essentially 5nm follows micro- crystalline silicon oxide layer (IMR, 66) or multilayer.
The inventive layer stack can be deposited in a state-of-the-art
PECVD deposition system with process gases widely known and available, such as hydrogen, silane, carbondioxide and dopants such as phosphine and boron. It has been found that a front contact made from LPCVD deposited ZnO as transparent conductive oxide layer (electrode) is beneficial, since a rough TCO allows for a generally thinner IMR layer.
A method of manufacturing a n-layer stack as mentioned above com- prises depositing an amorphous silicon n-layer (nl, 64) , a micro- crystalline silicon n-layer (n2, 65) , a microcrystalline silicon oxide layer (IMR, 66) and a microcrystalline silicon n-layer (n3, 67) . The bulk IMR layer 66 can alternatively be deposited as a multilayer stack, from a multiple of a c-SiO and a c-Si-n sequence. The order of layer is given in the direction of the impinging (unreflected) light. Depending on the structure of the cell and the orientation of the base structure, the deposition order may be reversed (nip or pin junction stack) . The inventive layer stack can be incorporated in tandem, triple or other multijunction arrangements. The examples refer to thin-film silicon layer stacks, but the invention broadly addresses the necessity to reflect light in a thin film layer stack. Thus, the inventive principle may be used also for other types of stacked solar cell arrangements.
Although the invention has been described in terms of specific embodiments, variation therefrom is possible within the scope of the invention as defined by the appended claims.

Claims

1. Multijunction thin film solar cell (70) comprising a stacked arrangement of at least two solar cells (61; 43), electrically in series, each solar cell (61; 43) comprising a p-i-n junction comprising an intrinsic semiconductor compound layer (63; 45) sandwiched between an n-doped semiconductor compound layer (64, 56, 66, 67; 46) and a p-doped semiconductor compound layer (62; 44) , wherein the n-layer (64, 56, 66, 67) of one of the stacked solar cells (61; 43) has a structure comprising, in the intended direction of impinging light, a sequence of an amorphous hydrogenated silicon n-layer (64), a first microcrystalline hydrogenated silicon n-layer (65), an intermediate reflector layer (66) and a second microcrystalline hydrogenated silicon n-layer (67), wherein the intermediate reflector layer comprises a plurality of layer sequences, each layer sequence consisting of a microcrystalline essentially silicon oxide layer and a microcrystalline silicon n-layer in the intended direction of impinging light.
2. Multijunction thin film solar cell (70) according to claim 1, wherein each layer sequence consists of a microcrystalline essentially silicon oxide layer and of a microcrystalline silicon n-layer in the intended direction of impinging light, each microcrystalline essentially silicon oxide layer having a thickness of 8-18 nm, particularly 8-16nm, further particularly 12-16nm, further particularly 9-11 nm, and each microcrystalline silicon n-layer being essentially 2 nm thick, which are deposited on said first microcrystalline silicon n-layer (65) .
3. Multijunction thin-film solar cell (70) according to claim 2, wherein said plurality is 2-6, or 2-5.
4. Multijunction thin film solar cell (70) comprising a stacked arrangement of at least two solar cells (61; 43), electrically in series, each solar cell (61; 43) comprising a p-i-n junction comprising an intrinsic semiconductor compound layer (63; 45) sand- wiched between an n-doped semiconductor compound layer (64, 56, 66, 67; 46) and a p-doped semiconductor compound layer (62; 44) , wherein the n-layer (64, 56, 66, 67) of one of the stacked solar cells (61; 43) has a structure comprising, in the intended direction of impinging light, a sequence of an amorphous hydrogenated silicon n-layer
(64) , a first microcrystalline hydrogenated silicon n-layer (65), an intermediate reflector layer (66) and a second microcrystalline hy- drogenated silicon n-layer (67) , wherein the intermediate reflector layer comprises a single microcrystalline essentially silicon oxide layer .
5. Multijunction thin-film solar cell (70) according to claim 4, wherein the single microcrystalline essentially silicon oxide layer has a thickness of at least 15 nm to 50 nm, particularly 20-40 nm which is deposited on said first microcrystalline silicon n-layer
(65) .
6. Multijunction thin-film solar cell (70) according to any of claims 1-5, wherein at least one of:
- the amorphous silicon n-layer (64) has a thickness of 3-8 nm, particularly 4-6 nm and is deposited onto the intrinsic silicon layer (63) of the cell closer to the incoming light, said intrinsic silicon layer (63) of the cell (61) closer to the incoming light being amorphous ;
- the first microcrystalline silicon n-layer (65) has a thickness of 5-50 nm, particularly 5-15 nm, particularly 6-12nm, further particu- larly 6-9 nm, and is deposited on to said amorphous silicon n-layer (64) ;
- the second microcrystalline silicon n-layer (67) has a thickness of 4-8 nm, particularly 4-6 nm, particularly essentially 5 nm and is situated following the intermediate reflector layer (66) in the di- rection of the impinging light.
7. Multijunction thin-film solar cell (70) according to any preceding claim, wherein the multijunction thin-film solar cell (70) further comprises a front contact (42) made from LPCVD deposited ZnO as a transparent conductive oxide layer electrode.
8. Multijunction thin-film solar cell (70) according to any preceding claim, wherein the intrinsic semiconductor compound layer (63) situated closest to the impinging light is amorphous.
9. Multijunction thin film solar cell (70) according to any of claims 4-8, comprising, in the direction of impinging light:
- a substrate (41) ; thereon
- a front electrode (42) ; thereon
- a p-layer (62) ; thereon
- an intrinsic layer (63) of a-Si:H; thereon
- a n-doped a-Si:H layer (64); thereon
- a n-doped μο-3ί:Η layer (65); thereon
- an intermediate reflector layer (66) of n-doped c-SiOx; thereon
- an n-doped μο-3ί:Η layer (67); thereon
- a p-layer (44) ; thereon
- an intrinsic layer (45) of μο-ΞίιΗ; thereon
- an n-layer (46) ; thereon
- a back electrode (47) ; thereon
- a back reflector (48) .
10. Multij unction thin film solar cell according to any of claims 1- 3 or 6-8 as dependent on 1-3, comprising, in the direction of impinging light:
- a substrate (41) ; thereon
- a front electrode (42) ; thereon
- a p-layer (62); thereon
- an intrinsic layer (63) of a-Si:H; thereon
- a n-doped a-Si:H layer (64); thereon
- a n-doped μο-3ί:Η layer (65); thereon
- an intermediate reflector layer (66) consisting of a plurality of layer systems each consisting of a layer of n-doped c-SiOx followed by an n-doped microcrystalline silicon layer; thereon
- an n-doped μο-3ί:Η layer (67); thereon
- a p-layer (44) ; thereon
- an intrinsic layer (45) of c-Si:H; thereon
- an n-layer (46) ; thereon
- a back electrode (47) ; thereon - a back reflector (48) .
11. Method of manufacturing a n-layer stack for a multijunction solar cell (70), comprising depositing in the intended direction of impinging light an amorphous hydrogenated silicon n-layer (64), a first microcrystalline hydrogenated silicon n-layer (65), an intermediate reflector layer (66) , and a second microcrystalline hydrogenated silicon n-layer (67) , wherein the deposition of the intermediate reflector layer (66) comprises depositing a plurality of layer sequences, deposition of each layer sequence consisting of depositing a microcrystalline silicon oxide layer and depositing a micro- crystalline silicon n-layer.
12. Method of manufacturing a n-layer stack for a multijunction so- lar cell (70) , comprising depositing in the intended direction of impinging light an amorphous hydrogenated silicon n-layer (64) , a first microcrystalline hydrogenated silicon n-layer (65), an intermediate reflector layer (66) , and a second microcrystalline hydrogenated silicon n-layer (67) , wherein the deposition of the interme- diate reflector layer (66) comprises depositing a single microcrystalline silicon oxide layer.
13. Method of manufacturing an n-layer stack for a multijunction solar cell (70) according to claim 11 or 12, wherein the amorphous silicon layer (64) is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m2 substrate:
- SiH4 flow rate 312 seem;
- H2 flow rate 733 seem;
- PH3 flow rate 166 seem;
- RF power 415 W
- RF power density 25 mW/cm2;
these values being linearly scaled for other substrate areas, and
- pressure 0.5 mbar;
deposition rate 3.1 A/s.
14. Method of manufacturing a n-layer stack for a multijunction solar cell (70) according to one of claims 11-13, wherein the first micro-crystalline silicon n-layer (65) is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m2 substrate:
- SiH4 flow rate 41 seem;
- H2 flow rate 4300 seem;
- PH3 flow rate 51 seem;
- RF power 1880 W
- RF power density 112 mW/cm2;
these values being linearly scaled for other substrate areas. - deposition rate 0.8 A/s;
- pressure 2.0 mbar
15. Method of manufacturing a n-layer stack for a multijunction so- lar cell (70) according to one of claims 11-14, wherein the intermediate reflector layer (66) is a single microcrystalline essentially silicon oxide layer and is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m2 substrate :
- SiH4 flow rate 60 seem;
- H2 flow rate 9900 seem;
- PH3 flow rate 300 seem;
- C02 flow rate 115 seem;
- RF power 2200
- RF power density 131 mW/cm2;
whereby these values are linearly scaled for other substrate areas, and
- deposition rate 0.9 A/s;
- pressure 2.5 mbar.
16. Method of manufacturing a n-layer stack for a multijunction solar cell (70) according to one of claims 11-15, wherein the second microcrystalline silicon n-layer (67) is deposited by plasma enhanced chemical vapour deposition under the following conditions for a 1.4 m2 substrate:
- SiH4 flow rate 51 seem;
- H2 flow rate 4000 seem; - PH3 flow rate 250 seem;
- RF power 2300 W
- RF power density 137 mW/cm2;
these values being linearly scaled for other substrate areas, and - deposition rate 1.1 A/s;
- pressure 2.0 mbar.
17. Method of manufacturing a multijunction thin-film solar cell (70) comprising depositing on a substrate a first cell (61) and at least one second cell (43) , each cell comprising a p-i-n junction comprising an intrinsic semiconductor compound layer (63; 45) sandwiched between an n-doped semiconductor compound layer (64, 65, 66, 67; 46) and a p-doped semiconductor compound layer (62; 44), wherein the n-layer (64, 65, 66, 67) of one of the said cells (61) is depos- ited according to the method of manufacturing a n-layer stack of one of claims 11-16.
18. Method of manufacturing a multijunction thin-film solar cell (70) according to claim 17 as dependent on claim 12, the method com- prising depositing on a substrate the following layer sequence:
- a front electrode (42) ; thereon
- a p-layer (62) ; thereon
- an intrinsic layer (63) of a-Si:H; thereon
- a n-doped a-Si:H layer (64); thereon
- a n-doped c-Si:H layer (65); thereon
- an intermediate reflector layer (66) of n-doped μο-ΞίΟχ; thereon
- an n-doped μο-Ξί:Η layer (67); thereon
- a p-layer (44) ; thereon
- an intrinsic layer (45) of c-Si:H; thereon
- an n-layer (46) ; thereon
- a back electrode (47); thereon
- a back reflector (48) .
19. Method of manufacturing a multijunction thin-film solar cell according to claim 17 as dependent on claim 11, the method comprising depositing on a substrate the following layer sequence:
- a front electrode (42) ; thereon - a p-layer (62); thereon
- an intrinsic layer (63) of a-Si:H; thereon
- a n-doped a-Si:H layer (64); thereon
- a n-doped μο-3ί:Η layer (65); thereon
- an intermediate reflector layer (66) consisting of a plurality of layer systems each consisting of a layer of n-doped c-SiOx followed by an n-doped microcrystalline silicon layer; thereon
- an n-doped c-Si:H layer (67); thereon
- a p-layer (44); thereon
- an intrinsic layer (45) of μο-ΞίιΗ; thereon
- an n-layer (46) ; thereon
- a back electrode (47); thereon
- a back reflector (48) .
20. Method of manufacturing a multijunction thin-film solar cell according to claim 17 as dependent on claim 12, the method comprising depositing on a substrate the following layer sequence:
- optionally, a back reflector (48) ; thereon
- a back electrode (47) ; thereon
- an n-layer (46) ; thereon
- an intrinsic layer (45) of μσ-ΞίιΗ; thereon
- a p-layer (44) ; thereon
- an n-doped μσ-ΞίιΗ layer (67); thereon
- an intermediate reflector layer (66) of n-doped μο-είθχ; thereon - a n-doped μσ-3ί:Η layer (65); thereon
- a n-doped a-Si:H layer (64); thereon
- an intrinsic layer of a-Si:H (63); thereon
- a p-layer (62) ; thereon
- a front electrode (42) .
21. Method of manufacturing a multijunction thin-film solar cell according to claim 17 as dependent on claim 11, the method comprising depositing on a substrate the following layer sequence:
- optionally, a back reflector (48) ; thereon
- a back electrode (47); thereon
- an n-layer (46); thereon
- an intrinsic layer (45) of μο-3ί:Η; thereon - a p-layer (44) ; thereon
- an n-doped c-Si:H layer (67); thereon
- an intermediate reflector layer (66) of a plurality of layer systems each consisting of an n-doped microcrystalline silicon layer followed by a layer of n-doped c-SiOx; thereon
- a n-doped c-Si:H layer (65); thereon
- a n-doped a-Si:H layer (64); thereon
- an intrinsic layer of a-Si:H (63); thereon
- a p-layer (62); thereon
- a front electrode (42) .
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RU2648341C2 (en) * 2016-05-23 2018-03-23 Общество с ограниченной ответственностью "НТЦ тонкопленочных технологий в энергетике", ООО "НТЦ ТПТ" Construction of thin-film solar module and method of its manufacture

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