WO2012027857A2 - Method for manufacturing a tandem solar cell with microcrystalline absorber layer - Google Patents

Method for manufacturing a tandem solar cell with microcrystalline absorber layer Download PDF

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Publication number
WO2012027857A2
WO2012027857A2 PCT/CH2011/000201 CH2011000201W WO2012027857A2 WO 2012027857 A2 WO2012027857 A2 WO 2012027857A2 CH 2011000201 W CH2011000201 W CH 2011000201W WO 2012027857 A2 WO2012027857 A2 WO 2012027857A2
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layer
precursor
solar cell
diluent
crystallinity
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PCT/CH2011/000201
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French (fr)
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WO2012027857A3 (en
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Markus Kupich
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Oerlikon Solar Ag, Trübbach
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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02521—Materials
    • H01L21/02524—Group 14 semiconducting materials
    • H01L21/02532—Silicon, silicon germanium, germanium
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/22—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the deposition of inorganic material, other than metallic material
    • C23C16/24—Deposition of silicon only
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C16/00—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes
    • C23C16/44—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating
    • C23C16/50—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating using electric discharges
    • C23C16/505—Chemical coating by decomposition of gaseous compounds, without leaving reaction products of surface material in the coating, i.e. chemical vapour deposition [CVD] processes characterised by the method of coating using electric discharges using radio frequency discharges
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02518—Deposited layers
    • H01L21/02587—Structure
    • H01L21/0259—Microstructure
    • H01L21/02595—Microstructure polycrystalline
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/02—Manufacture or treatment of semiconductor devices or of parts thereof
    • H01L21/02104—Forming layers
    • H01L21/02365—Forming inorganic semiconducting materials on a substrate
    • H01L21/02612—Formation types
    • H01L21/02617—Deposition types
    • H01L21/0262—Reduction or decomposition of gaseous compounds, e.g. CVD
    • 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
    • H01L31/04—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
    • 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
    • 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
    • H01L31/18—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
    • 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 solar energy conversion offers the perspective to pro- vide for an environmentally friendly means to generate electricity.
  • electric energy provided by photovoltaic energy conversion units is still significantly more expensive than electricity provided by conventional power stations. Therefore, the development of more cost-effective means of producing photovol- taic energy conversion units attracted attention in the recent years.
  • thin film silicon solar cells combine several advantageous aspects:
  • thin-film silicon solar cells can be prepared by known thin-film deposition techniques such as plasma enhanced chemical va- por deposition (PECVD) and thus offer the perspective of synergies to reduce manufacturing cost by using experiences achieved in the past for example on the field of other thin film deposition technologies such as the displays sector.
  • PECVD plasma enhanced chemical va- por deposition
  • thin-film silicon solar cells can achieve high energy conversion efficiencies striving to- wards 10% and beyond.
  • the main raw materials for the production of thin-film silicon based solar cells are abundant and nontoxic .
  • FIG. 1 shows a tandem-junction silicon thin film solar cell as known in the art.
  • a thin-film solar cell 50 generally includes 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.
  • the i-type layer 53, 45 which is a substantially intrinsic semiconductor layer, occupies the most part of the thick- ness of the thin-film p-i-n junction. Substantially intrinsic in this context is understood as "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 solar cells or photoelectric (conversion) devices
  • uc-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.
  • amorphous-microcrystalline silicon mul- ti-junction 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.
  • two or more sub-cells can be stacked by depositing the corresponding layers subsequently. If materials of different band gap are used as absorbing layers, the material with the largest band gap will be on the side of the device, which is oriented to the incident direction of the light.
  • Such a solar cell structure offers several possible advantages: firstly, due to the use of two or more photovoltaic junctions of different band gap, the light with a broad spectral distribution as for example solar irradiation can be used more efficiently due to the reduction of thermalization losses. Secondly, due to the fact that high-quality microcrystalline silicon does not suffer from light induced degradation, as known for amorphous silicon due to the so-called Staebler-Wronski-effect , an amorphous- microcrystalline silicon multi-junction solar cell shows a smaller degradation of its initial conversion efficiency compared to an amorphous silicon single junction solar cell.
  • the solar cell For optimum conversion efficiency of an amorphous-microcrystalline multi-junction thin film solar cell, the solar cell needs to have both good Voc as well as a high current density (Isc) , both at good fill factor.
  • One important material property influencing these parameters is for example the crystallinity of the microcrystalline absorber layer as measured, for example by Raman scattering, giving a value defined as Raman crystallinity (RC) .
  • RC Raman crystallinity
  • Fig. 1 shows a tandem junction thin film solar cell as known in Prior Art .
  • Fig. 2 shows a tandem junction thin film solar cell according to a combination of two embodiments according to the invention.
  • a first interface layer is prepared using the "interface regime I" under the following conditions :
  • the "bulk" deposition regime then will be used to deposit the remaining thickness of the intrinsic absorber layer of the solar cell, the thickness of which may vary over a wide range from several hundred nm to some micrometers, depending of the type of top cell chosen.
  • Method according to a)-g) with the thickness of the layers deposited using the interface regime is not greater than 200nm, preferably in the range of 10 - 70nm. In the present example the thickness of the first interface layer is chosen to 50nm.
  • Method according to a)-h) wherein the layers are deposited in a continuous deposition mode without interruption.
  • Method according to a)-h) wherein the layers are deposited in a layer-by-layer deposition sequence with interruption of the deposition process.
  • a schematic sketch of an amorphous-microcrystalline tandem solar cell 60 using interface layers 61, 62 at both interfaces between the interfaces of doped and non-doped layers of the microcrystalline bottom cell is shown in Fig. 2.
  • a method for the plasma deposition of a layer of microcrystalline semiconductor material on a substrate comprising the steps of injecting at least a process gas including a precursor of a semiconductor material and a diluent into a process chamber with the sub- strate arranged therein, creating a RF powered plasma from said process gas, plasma-assisted-depositing a layer of microcrystalline semiconductor material onto said substrate, wherein said step of plasma-assisted depositing is conducted at least in two steps, with the RF power density per flow unit of precursor gas in the second step is being chosen to be lower than in the first step whilst the flow ratios of diluent and precursor are being kept constant .
  • said precursor of semiconductor material is a silicon containing component, such as silane or alike.
  • said diluent is hydrogen or hydrogen plus an inert gas .
  • the difference between the RF power density per flow unit of precursor gas of the second compared to the first step is between -12% to -8%, preferably around -10%.
  • the gas flows and RF power are being increased between the first and the second step, still obeying a.m. power density per flow unit of precursor gas rule.
  • a further embodiment according to the invention is using n steps of plasma assisted depositing, wherein for all n steps is valid: with the RF power density per flow unit of precursor gas in a subsequent step is being lower than in the previous step whilst the flow ratios of diluent and precursor are being kept constant (for a given total process pressure) .
  • An embodiment of the inventive method comprises choosing the n steps of plasma assisted depositing such that for all steps is valid: the power density per flow unit of precursor gas decreases and the flow ratio of diluent and precursor gas is being kept constant.
  • a method for manufacturing a thin film solar cell comprising steps of depositing, on a substrate, a transparent conductive oxide, at least one photovoltaically active layer stack comprising a p-i-n layer stack of a semiconductor component, wherein the i-layer portion of said p-i-n layer stack is a substantially intrinsic micro- crystalline silicon layer and is being deposited incorporating a. m. method (s) for the plasma deposition of a layer of microcrystalline semiconductor material on a substrate.
  • a substantially intrinsic semiconductor layer stack comprises at least two sub-layers with different Raman-crystallinity, wherein said at least two sub layers have been manufactured according to one of the method steps described above.

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Abstract

A tandem solar cell comprising an amorphous top cell and a microcrystalline bottom cell is described, wherein the plasma deposition of the microcrystalline absorber of the bottom cell is being performed in at least two steps, with the RF power density per flow unit of precursor gas in the second step being chosen to be lower than in the first step whilst the flow ratios of diluent and precursor are being kept constant.

Description

METHOD FOR MANUFACTURING A TANDEM SOLAR CELL WITH MICROCRYSTALLINE
ABSORBER LAYER
Photovoltaic solar energy conversion offers the perspective to pro- vide for an environmentally friendly means to generate electricity. However, at the present state, electric energy provided by photovoltaic energy conversion units is still significantly more expensive than electricity provided by conventional power stations. Therefore, the development of more cost-effective means of producing photovol- taic energy conversion units attracted attention in the recent years. Amongst different approaches of producing low-cost solar cells, thin film silicon solar cells combine several advantageous aspects:
firstly, thin-film silicon solar cells can be prepared by known thin-film deposition techniques such as plasma enhanced chemical va- por deposition (PECVD) and thus offer the perspective of synergies to reduce manufacturing cost by using experiences achieved in the past for example on the field of other thin film deposition technologies such as the displays sector. Secondly, thin-film silicon solar cells can achieve high energy conversion efficiencies striving to- wards 10% and beyond. Thirdly, the main raw materials for the production of thin-film silicon based solar cells are abundant and nontoxic .
FIELD OF THE INVENTION
Figure 1 shows a tandem-junction silicon thin film solar cell as known in the art. Such a thin-film solar cell 50 generally includes 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). The i-type layer 53, 45, which is a substantially intrinsic semiconductor layer, occupies the most part of the thick- ness of the thin-film p-i-n junction. Substantially intrinsic in this context is understood as "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 (uc-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.
BACKGROUND OF THE INVENTION
Amongst various approaches to prepare thin film silicon solar cells particularly the concept of amorphous-microcrystalline silicon mul- ti-junction 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. In such a multi-junction solar cell two or more sub-cells can be stacked by depositing the corresponding layers subsequently. If materials of different band gap are used as absorbing layers, the material with the largest band gap will be on the side of the device, which is oriented to the incident direction of the light. Such a solar cell structure offers several possible advantages: firstly, due to the use of two or more photovoltaic junctions of different band gap, the light with a broad spectral distribution as for example solar irradiation can be used more efficiently due to the reduction of thermalization losses. Secondly, due to the fact that high-quality microcrystalline silicon does not suffer from light induced degradation, as known for amorphous silicon due to the so-called Staebler-Wronski-effect , an amorphous- microcrystalline silicon multi-junction solar cell shows a smaller degradation of its initial conversion efficiency compared to an amorphous silicon single junction solar cell.
STATE OF THE ART
For optimum conversion efficiency of an amorphous-microcrystalline multi-junction thin film solar cell, the solar cell needs to have both good Voc as well as a high current density (Isc) , both at good fill factor. One important material property influencing these parameters is for example the crystallinity of the microcrystalline absorber layer as measured, for example by Raman scattering, giving a value defined as Raman crystallinity (RC) . In literature, it was repeatedly shown that highest open circuit voltages Voc of micro- crystalline solar cells are obtained for a material with low crystallinity below RC<50%, whereas highest current densities are usual- ly obtained at higher crystalline fraction due to a lower infrared response of low-crystallinity materials. Particularly for amorphous- microcrystalline multi-junction thin film solar cells, where the microcrystalline subcells need to provide for a good response in the infrared part of the spectrum, the material optimization has to find a compromise between Voc and Isc, which has the consequence that compared to single junction microcrystalline solar cells, the sub- cells of a multi-junction device would not use the full range of obtainable Voc . BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a tandem junction thin film solar cell as known in Prior Art .
Fig. 2 shows a tandem junction thin film solar cell according to a combination of two embodiments according to the invention.
DETAILED DESCRIPTION OF THE INVENTION
Due to fact that the built-in field of a thin film silicon solar cell is strongly affected by the interface properties at the p/i interface and, to a lower extent, at the i/n interface, the defined variation of this interfaces offers the possibility to achieve both good Voc and current density by intentionally modifying these interfaces and choosing different deposition parameters for the interfac- _
4
es and the "bulk" i-layer. For amorphous silicon solar cells, in prior art EP 0 949 688 Al a control of the interface quality by controlling the hydrogen content of the material close to the interface is described.
For microcrystalline silicon p-i-n structures, the requirements for the material quality are more complex compared to amorphous silicon, due to the fact that besides hydrogen content and density other material properties have an impact on the cell performance, such as crystallinity or crystal sizes. Besides that, microcrystalline sili- con thin films exhibit a pronounced anisotropy of the transport properties. Prior art WO 2005/072302 A2 and US 6,274,461 describe a method, in which the dilution of the process gases is varied during the entire deposition of a microcrystalline layer as a function of the thickness of the layer which has been deposited. We found, that this is not necessary and still specific modifications of the layer structure and in particular, the interfaces between several layers, e.g. of doped and non-doped material stacked to a pin- or nip-device, are possible in more simple ways. In addition to this, the more simple requirements for the quality of the "bulk" i-layer compared to the interface regions allow to optimize the deposition parameters for the "bulk" i-layer with respect to other criteria, e.g. deposition rate, which has a strong impact on the throughput of a industrial deposition system and therefore on the manufacturing cost. In the setup of the plasma discharge reactor of an Oerlikon Solar KAI 1200 plasma deposition system suitable for substrates of 1.4m2 size, such modifications might be realized for example according to the following processing parameters for the gas flows F, the process pressure p and the RF power P (RF) :
After depositing a p-type microcrystalline silicon layer, a first interface layer is prepared using the "interface regime I" under the following conditions :
F(SiH4) =200sccm, F(H2)=3130 seem, p = 2.5 mbar, R (RF) = 2100W. Here, the concentration of the precursor gas is c(SiH4)=6.0% and the RF power density per flow of precursor gas is 10.5 W /seem.
Under theses conditions, intrinsic microcrystalline silicon with a low defect density and a moderately high crystallinity as measured by Raman scattering in the range of RC = 60% +/- 7% can be deposited. Such deposition regime will be used to deposit the first 50nm of the absorbing i-layer of the solar cell.
Hereafter, the deposition regime will be transferred to the "bulk deposition regime II", which in the present example is run at
F(SiH4)=400sccm, F(H2)=6260 seem, p = 2.5 mbar, R (RF) = 3830W. Here, the concentration of the precursor gas is kept constant at
c (SiH ) =6.0%, allowing to use the same gas mixture, and the RF power density per precursor gas is 9.575 W/sccm, leading to an intrinsic microcrystalline silicon material grown at higher deposition rate at a moderate crystallinity in the range of RC=50% +/- 7%. The "bulk" deposition regime then will be used to deposit the remaining thickness of the intrinsic absorber layer of the solar cell, the thickness of which may vary over a wide range from several hundred nm to some micrometers, depending of the type of top cell chosen.
The incorporation of the interface layer will, in comparison to a cell prepared of identical total i-layer thickness using the "bulk deposition regime II" alone, improve the cell performance of a tan- dem solar cell as follows:
Samples using interface layer: AVoc = +1.2 %, AFF = +0.45%, AJsc = +0.2%, Δ(η) = +1.9%.
Depending on the specific requirements of a certain solar cell de- sign this process may be further modified. However, in the setup of a plasma discharge deposition system like the Oerlikon Solar KAI 1200 plasma deposition system, such modifications can generally be described as : a) using a specific growth regime ("interface regime") at the
first interface between doped and non-doped material, leading to a material with specifically chosen properties, particularly low defect density and a specific crystallinity RC, thereafter going to a specific deposition regime for the bulk i-layer
("bulk regime"), leading to a material deposited with higher deposition rate and different material properties than the ma- terial prepared with the interface regime, independently from the thickness of the layer deposited with the "bulk regime" . b) using a specific regime for the bulk i-layer, and thereafter preparing an intrinsic interface layer at the second interface between doped and non-doped material using a second "interface regime", leading to a material deposited with specifically chosen crystallinity RC and low defect density, independently from the thickness of the layer deposited with the "bulk regime" . c) Combining both approaches a) and b)
d) Method according to a)-c) where the "interface regimes" are characterized by a lower precursor gas flow than the "bulk regime", irrespective of the concentration of the precursor gas, leaving the concentration constant within a range of +/-2% for the entire i-layer deposition. In the present example the con- centration of the process gas is kept constant at c (SiH4) =6.0% . e) Method according to a)-d) where the "interface regimes" are characterized by a different plasma discharge power density per precursor gas flow unit than the "bulk regime", which is adjusted in a way to achieve the desired crystallinity RC. In the present example the plasma discharge power density per precursor gas flow unit for the first interface layer is 9.7% greater than for the bulk regime.
f) Method according to a)-c) where the "interface regimes" are characterized by a higher process pressure than the "bulk re- gime"
g) Method according to a)-c), where the "interface layers" themselves are divided into 2 or more sub-layers, which again are prepared by modifying the deposition parameters as described in d)-e)
h) Method according to a)-g), with the thickness of the layers deposited using the interface regime is not greater than 200nm, preferably in the range of 10 - 70nm. In the present example the thickness of the first interface layer is chosen to 50nm. i) Method according to a)-h), wherein the layers are deposited in a continuous deposition mode without interruption. j) Method according to a)-h), wherein the layers are deposited in a layer-by-layer deposition sequence with interruption of the deposition process. A schematic sketch of an amorphous-microcrystalline tandem solar cell 60 using interface layers 61, 62 at both interfaces between the interfaces of doped and non-doped layers of the microcrystalline bottom cell is shown in Fig. 2. SUMMARY:
A method for the plasma deposition of a layer of microcrystalline semiconductor material on a substrate, comprising the steps of injecting at least a process gas including a precursor of a semiconductor material and a diluent into a process chamber with the sub- strate arranged therein, creating a RF powered plasma from said process gas, plasma-assisted-depositing a layer of microcrystalline semiconductor material onto said substrate, wherein said step of plasma-assisted depositing is conducted at least in two steps, with the RF power density per flow unit of precursor gas in the second step is being chosen to be lower than in the first step whilst the flow ratios of diluent and precursor are being kept constant .
In an embodiment of the invention, said precursor of semiconductor material is a silicon containing component, such as silane or alike. In an embodiment of the invention said diluent is hydrogen or hydrogen plus an inert gas .
In an embodiment of the invention the difference between the RF power density per flow unit of precursor gas of the second compared to the first step is between -12% to -8%, preferably around -10%.
In an embodiment of the invention the gas flows and RF power are being increased between the first and the second step, still obeying a.m. power density per flow unit of precursor gas rule. A further embodiment according to the invention is using n steps of plasma assisted depositing, wherein for all n steps is valid: with the RF power density per flow unit of precursor gas in a subsequent step is being lower than in the previous step whilst the flow ratios of diluent and precursor are being kept constant (for a given total process pressure) . An embodiment of the inventive method comprises choosing the n steps of plasma assisted depositing such that for all steps is valid: the power density per flow unit of precursor gas decreases and the flow ratio of diluent and precursor gas is being kept constant. A method for manufacturing a thin film solar cell comprising steps of depositing, on a substrate, a transparent conductive oxide, at least one photovoltaically active layer stack comprising a p-i-n layer stack of a semiconductor component, wherein the i-layer portion of said p-i-n layer stack is a substantially intrinsic micro- crystalline silicon layer and is being deposited incorporating a. m. method (s) for the plasma deposition of a layer of microcrystalline semiconductor material on a substrate.
A substantially intrinsic semiconductor layer stack comprises at least two sub-layers with different Raman-crystallinity, wherein said at least two sub layers have been manufactured according to one of the method steps described above.

Claims

CLAIMS :
1) A method for the plasma deposition of a layer of microcrystal- line semiconductor material on a substrate, comprising the steps of injecting at least a process gas including a precursor of a semiconductor material and a diluent into a process chamber with the substrate arranged therein, creating a RF powered plasma from said process gas, plasma-assisted-depositing a layer of mi- crocrystalline semiconductor material onto said substrate,
wherein said step of plasma-assisted depositing is conducted at least in two sub-steps, with the RF power density per flow unit of precursor gas in the second step is being chosen to be lower than in the first step whilst the flow ratios of diluent and precursor are being kept constant.
2) A method according to claim 1, wherein said precursor of semiconductor material is a silicon containing component, such as silane or alike. 3) A method according to claims 1 and 2, wherein said diluent is
hydrogen or hydrogen plus an inert gas .
4) A method according to claims 1-3, wherein the precursor is
silane and the ratio of diluent hydrogen and silane is about 6%.
5) A method according to claims 1-4, wherein the thickness of the layer deposited in said first sub-step is not greater than 200nm, preferably in the range of 10-70nm. 6) A method according to claims 1-5, wherein the difference between the RF power density per flow unit of precursor gas of the second compared to the first step is between -12% to -8%, preferably around -10%. 7) A method according to claims 1-6, wherein the gas flow and RF
power are being increased between the first and the second step while still the RF power density per flow unit of precursor gas in the second step is being chosen to be lower than in the first step and the flow ratios of diluent and precursor are being kept constant .
8) A method according to claims 1-7, wherein the first sub-step of plasma assisted depositing is further divided in n sub-steps, wherein for all n steps is valid: the RF power density per flow unit of precursor gas in a subsequent step is being lower than in the previous step whilst the flow ratios of diluent and precursor are being kept constant for a given total process pres- sure.
9) A method for manufacturing a thin film solar cell comprising depositing, on a substrate, a transparent conductive oxide, further depositing at least one photovoltaically active layer stack comprising a p-i-n layer stack of a semiconductor component, wherein the i-layer portion of said p-i-n layer stack is a substantially intrinsic microcrystalline silicon layer and is being deposited applying a method according to claims 1-7. 10) A method according to claim 9, wherein said substantially intrinsic semiconductor layer stack comprises at least two sublayers with different Raman-crystallinity .
11) A method according to claim 10, wherein said first sub-layer has a Raman crystallinity of 60% +/-7% and said second sub-layer a
Raman crystallinity of 50% +/-7%.
12) A tandem solar cell (60) comprising a top cell (51) with an essentially intrinsic amorphous Si:H absorber layer (53) and a bottom cell (43) with an essentially microcrystalline Si:H absorber layer (45) , wherein a microcrystalline silicon interface layer (61) is arranged adjacent absorber layer (45) facing the direction of impinging light, said interface layer (61) exhibiting a Raman crystallinity of 60% +/-7% compared to absorber lay- er's (45) Raman crystallinity of 50% +/-7%.
13) A tandem solar cell according to claim 12, wherein the thickness of the interface layer (61) is not greater than 200nm, prefera- bly in the range of 10-70nm.
A tandem solar cell according to claims 12 and 13, wherein a further interface layer (62) is arranged adjacent absorber layer (45) facing away from the direction of impinging light with a Raman crystallinity different from the Raman crystallinity of absorber layer (45) .
PCT/CH2011/000201 2010-09-02 2011-09-01 Method for manufacturing a tandem solar cell with microcrystalline absorber layer WO2012027857A2 (en)

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US6274461B1 (en) 1998-08-20 2001-08-14 United Solar Systems Corporation Method for depositing layers of high quality semiconductor material
WO2005072302A2 (en) 2004-01-27 2005-08-11 United Solar Systems Corporation Method for depositing high-quality microcrystalline semiconductor materials

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US7875486B2 (en) * 2007-07-10 2011-01-25 Applied Materials, Inc. Solar cells and methods and apparatuses for forming the same including I-layer and N-layer chamber cleaning
US20090104733A1 (en) * 2007-10-22 2009-04-23 Yong Kee Chae Microcrystalline silicon deposition for thin film solar applications
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EP0949688A1 (en) 1998-03-31 1999-10-13 Phototronics Solartechnik GmbH Thin film solar cell, method of manufacturing the same, and apparatus for carrying out the method of manufacturing
US6274461B1 (en) 1998-08-20 2001-08-14 United Solar Systems Corporation Method for depositing layers of high quality semiconductor material
WO2005072302A2 (en) 2004-01-27 2005-08-11 United Solar Systems Corporation Method for depositing high-quality microcrystalline semiconductor materials

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