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EP2586066A1 - Thin film solar cell with microcrystalline absorpber layer and passivation layer and method for manufacturing such a cell - Google Patents

Thin film solar cell with microcrystalline absorpber layer and passivation layer and method for manufacturing such a cell

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Publication number
EP2586066A1
EP2586066A1 EP11727629.5A EP11727629A EP2586066A1 EP 2586066 A1 EP2586066 A1 EP 2586066A1 EP 11727629 A EP11727629 A EP 11727629A EP 2586066 A1 EP2586066 A1 EP 2586066A1
Authority
EP
European Patent Office
Prior art keywords
layer
silicon
microcrystalline
intrinsic
passivation layer
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Withdrawn
Application number
EP11727629.5A
Other languages
German (de)
French (fr)
Inventor
Jochen Hoetzel
Evelyne Vallat-Sauvain
Stefano Benagli
Lucie Castens
Xavier Multone
Daniel Borrello
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
TEL Solar AG
Original Assignee
TEL Solar AG
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Filing date
Publication date
Application filed by TEL Solar AG filed Critical TEL Solar AG
Publication of EP2586066A1 publication Critical patent/EP2586066A1/en
Withdrawn legal-status Critical Current

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Classifications

    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F10/00—Individual photovoltaic cells, e.g. solar cells
    • H10F10/10—Individual photovoltaic cells, e.g. solar cells having potential barriers
    • H10F10/17—Photovoltaic cells having only PIN junction potential barriers
    • H10F10/172—Photovoltaic cells having only PIN junction potential barriers comprising multiple PIN junctions, e.g. tandem cells
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F10/00—Individual photovoltaic cells, e.g. solar cells
    • H10F10/10—Individual photovoltaic cells, e.g. solar cells having potential barriers
    • H10F10/16—Photovoltaic cells having only PN heterojunction potential barriers
    • H10F10/164—Photovoltaic cells having only PN heterojunction potential barriers comprising heterojunctions with Group IV materials, e.g. ITO/Si or GaAs/SiGe photovoltaic cells
    • H10F10/165—Photovoltaic cells having only PN heterojunction potential barriers comprising heterojunctions with Group IV materials, e.g. ITO/Si or GaAs/SiGe photovoltaic cells the heterojunctions being Group IV-IV heterojunctions, e.g. Si/Ge, SiGe/Si or Si/SiC photovoltaic cells
    • H10F10/166—Photovoltaic cells having only PN heterojunction potential barriers comprising heterojunctions with Group IV materials, e.g. ITO/Si or GaAs/SiGe photovoltaic cells the heterojunctions being Group IV-IV heterojunctions, e.g. Si/Ge, SiGe/Si or Si/SiC photovoltaic cells the Group IV-IV heterojunctions being heterojunctions of crystalline and amorphous materials, e.g. silicon heterojunction [SHJ] photovoltaic cells
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F10/00—Individual photovoltaic cells, e.g. solar cells
    • H10F10/10—Individual photovoltaic cells, e.g. solar cells having potential barriers
    • H10F10/17—Photovoltaic cells having only PIN junction potential barriers
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F19/00—Integrated devices, or assemblies of multiple devices, comprising at least one photovoltaic cell covered by group H10F10/00, e.g. photovoltaic modules
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F71/00—Manufacture or treatment of devices covered by this subclass
    • H10F71/10—Manufacture or treatment of devices covered by this subclass the devices comprising amorphous semiconductor material
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10F—INORGANIC SEMICONDUCTOR DEVICES SENSITIVE TO INFRARED RADIATION, LIGHT, ELECTROMAGNETIC RADIATION OF SHORTER WAVELENGTH OR CORPUSCULAR RADIATION
    • H10F77/00—Constructional details of devices covered by this subclass
    • H10F77/30—Coatings
    • 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

Definitions

  • This invention relates to solar photovoltaic conversion devices, so- lar cells, especially thin- film silicon photovoltaic devices with improved performance due to the incorporation of (a) passivation layer (s) within the photoactive microcrystalline part of the device.
  • FIG. 4A 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.
  • solar cells or photoelectric (conversion) devices are characterized as amorphous (a-Si, 53) or microcrystalline ( ⁇ - ⁇ , 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 crystallinity of the photoactive layer has to be chosen by considering that (for the standard PECVD- deposition conditions) on one hand, microcrystalline silicon layers have a better electronic quality (low defect density) when deposited close to the amorphous-microcrystalline silicon transition which results in a high open-circuit voltage (Voc) of the device.
  • high current densities (Jsc) are obtained by increasing the crystallinity well over the amorphous-microcrystalline transition.
  • the defect density in the i- ⁇ c-Si:H layer is not only related to its crystallinity. Additional defects are introduced when using rough front Transparent Conductive Oxide (TCO) layers as front electrode ("front TCO", front electrode) .
  • TCO Transparent Conductive Oxide
  • front TCO front electrode
  • Such TCOs are used primarily for in- creasing the Jsc of thin- film silicon solar cells via the increase of the optical path of light within the device.
  • the use of rough front TCOs leads usually to a decrease of the Voc and fill factor (FF) . This effect is attributed to the presence of additional morphology-related defects (zones of porous ⁇ - ⁇ -3 ⁇ : ⁇ ) which lead to a decrease in FF in Voc.
  • the chosen device crystallinity of the ⁇ - ⁇ -3 ⁇ : ⁇ layer results from a compromise between high crystallinity for high Jsc and medium crystallinity for high Voc.
  • Stand-of -the-art PECVD deposition tools and processes do not allow for an ideal ⁇ -3 ⁇ : ⁇ material with high crystallinity (high Jsc) and low defect density (high Voc) for the ⁇ - ⁇ -. ⁇ i- layer fabrication. But with a defect passivation layer, high crystallinity (high Jsc) and good Voc is possible with typical standard PECVD-deposition parameters.
  • Figure 1 I (V) characteristics of Micromorph top-limited cells with varying passivation a-Si:H i-layers (tested thicknesses: 10, 50 and 150nm, rough LPCVD-ZnO substrate) .
  • the reference cells have an average Voc of 1347 mV, Jsc of 12.2 mA/cm2 and FF of 70.2%.
  • the cells passivated with 10 nm i-a:Si:H have the following average higher electrical performances of Voc: 1356 mV, Jsc of 12.4 mA/cm2 and FF of 72.4%.
  • Figure 2 A and B Effect of the introduction of an a-Si:H passivation layer of varying thickness on the absolute values of the Voc and FF of the MM cells.
  • Figure 3 Total External Quantum Efficiency (EQE) of a Micromorph tandem cell with a 10 nm passivation layer compared to a reference cell without a passivation layer.
  • Fig. 4 A Prior Art - tandem junction thin film silicon photovoltaic cell. Thicknesses not to scale.
  • Fig. 4 B Embodiment according to the invention with passivation layer. Thicknesses not to scale.
  • the present invention comprises introducing a defect -passivation layer 55 in or adjacent to the microcrystalline i- layer 45 of a PV cell 60 (bottom cell in a Micromorph tandem cell) .
  • This additional passivation layer 55 includes an a-Si:H i-layer which is optically transparent to the light impinging on it (i.e.
  • top limited Micromorph cells were prepared on as grown rough TCO' s (LPCVD-ZnO) .
  • the reference devices 50 for comparison offers a top pin a-Si:H cell 51 with an i- layer 53 thickness of 250 nm and a bottom ⁇ -3 ⁇ : ⁇ cell 43 with a photoactive i-layer 45 of 2000 nm with a medium crystallinity (bulk Raman crystallinity measured with a 780nm laser: 50-55%) .
  • the passivated devices 60 have identical i-layer thicknesses for the top and bottom cells, except that the deposition of the ⁇ -3 ⁇ : ⁇ i-layer 45 was followed by the deposition of a fully amorphous i-layer 55 (passivation layer) of varying thickness, according to the invention.
  • the passivated devices 60 show improved electrical performances (see Figure 1) . This is the indication that the detrimental effect of some of the defects of the underlying microcrystalline silicon layer has been thus mitigated.
  • recombination centres such as dangling bonds can be efficiently passivated with a-Si:H and the corresponding decreased recombination of photocarriers leads to an increased Voc, FF and total (top plus bottom sub cells) Jsc (measured by EQE) as observed in our illustrative example.
  • the detrimental effect of the morphology- induced defects, namely growth-related defects, is as well reduced by the introduction of the passivation layer at the end of the ⁇ - ⁇ -3 ⁇ : ⁇ layer growth.
  • the relative gain in efficiency obtained by the introduction of the amorphous passivation layer at the end of the microcrystalline i- layer is about 5%.
  • the appropriate thickness of the passivation layer has to be chosen by considering its effect on the Voc and FF of the cell, as depicted in Figure 1. This figure indicates that a certain thickness of the passivation layer is needed for a simultaneously increased value of FF and Voc. However, when the passivation layer is too thick, a double diode behaviour appears in the I (V) curve which decrease considerably the device performances.
  • Figure 2 shows the limitations gain vs. layer thickness.
  • silicon-based passivation i-layers than pure a-Si:H can be used as well, alloys such as a-SiC:H, a-Si:0:H or a-SiN:H etc...
  • TCO roughness has to be chosen for each TCO roughness according to their conductivity and optical transparency in the wave- length range from 650nm-1100nm.
  • this passivation layer is not mandatory to apply this passivation layer at the end of the deposition of the intrinsic microcrystalline layer.
  • This layer can be applied at varying locations during the intrinsic mi- crocrystalline layer growth, provided that the subsequent crystalline layer has the adequate crystallinity . It is also possible to introduce more than one passivation layer (s) during the growth of the microcrystalline i- layer.
  • a passivation layer according to the invention can be prepared as follows.
  • a PECVD process chamber as known in the art (e. g. KAI-M commercially available from Oerlikon Solar) the following process parameter have been used.
  • Substrate size was about 500 x 400 mm 2 .
  • a high-quality a-Si:H passivation layer is achievable with a pressure between 0.1-2mbar, preferably 0.2-0.5mbar, a power between 5- 500W (2.5mW/cm 2 - 250m /cm 2 substrate size), preferably 30-100W
  • Process temperature was chosen between 100°-250°C, preferably around 200°C.
  • a process pressure of 1-5 mbar, a power between 100- 600W and a ratio between hydrogen and silane of 10:1 to 200:1 can be applied.
  • the deposition rate depends on the process tool used; process duration will therefore vary until the layer thickness accord- ing to the invention between 5nm-50nm has been achieved.
  • Photovoltaic cell 60 comprising a substrate 31, a front or first electrode 42 of transparent conductive oxide and at least one p-i-n junction 43 comprising microcrystalline silicon, said p-i-n junction 43 comprising a first sub-layer 44 comprising silicon and a n-dopant and a second sub-layer 46 comprising silicon and a p-dopant and a third sub-layer 45 comprising essentially intrinsic microcrystalline silicon, wherein at least one passivation layer 45 comprising essentially intrinsic amorphous silicon is arranged a) between the micro- crystalline intrinsic sub-layer 45 and n-doped silicon layer 46 or b) as a layer embedded in the microcrystalline intrinsic sub-layer 45 or c) both.
  • Passivation layer 45 has a thickness of 5nm-200nm, preferably 10-50nm.
  • Passivation layer 55 may be realized with essentially intrinsic silicon or silicon compounds/alloys such as a-SiC:H, a-Si:0:H or a- SiN:H or alike.
  • a process for depositing a passivation layer 55 in a photovoltaic thin film solar cell comprises introducing in a PECVD process chamber exhibiting a substrate to be treated a gas mixture comprising silane and hydrogen, establishing either a process pressure between 0.1-2mbar, preferably 0.2-0.5mbar, a RF power (40 MHz or more) be- tween 5-500W, preferably 30-100W, and a ratio between hydrogen and silane of 1:1;
  • a method for manufacturing a photovoltaic thin film silicon solar cell comprising:

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  • Photovoltaic Devices (AREA)

Abstract

A Photovoltaic cell 60 includes a substrate 31, a front or first electrode 42 of transparent conductive oxide and at least one p-i-n junction 43 of microcrystalline silicon, said p-i-n junction 43 comprising a first n-doped silicon sub- layer 44 and a second p-doped silicon sub-layer 46 and a third sub-layer 45 with essentially intrinsic microcrystalline silicon. A passivation layer 45 comprising essentially intrinsic amorphous silicon is arranged a) between the microcrystalline intrinsic sub-layer 45 and n-doped silicon layer 46 or b) as a layer embedded in the microcrystalline intrinsic sublayer 45 or c) both. A method for manufacturing such a photovoltaic thin film silicon solar cell includes providing a transparent substrate 41 with a TCO front electrode 42 on it; depositing a p-doped Si layer 44, a microcrystalline silicon intrinsic layer 45, a passivation layer 55 from essentially intrinsic amorphous silicon, a n-doped Si layer 46 and a back electrode layer 48.

Description

THIN FILM SOLAR CELL WITH MICROCRYSTALLINE ABSORPBER LAYER AND PASSIVATION LAYER AND METHOD FOR MANUFACTURING SUCH A CELL
This invention relates to solar photovoltaic conversion devices, so- lar cells, especially thin- film silicon photovoltaic devices with improved performance due to the incorporation of (a) passivation layer (s) within the photoactive microcrystalline part of the device.
FIELD OF THE INVENTION
Figure 4A 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 (μσ-Ξϊ, 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 microcrystal- line p-i-n- junction, as shown in Fig. 4A, is also called micromorph tandem cell. BACKGROUND OF THE INVENTION
In single-junction and multiple junction solar cells incorporating microcrystalline silicon as a photoactive (intrinsic) i- layer, two of the key physical parameters for the c-i-layer are: 1) its crys- tallinity and 2) its electronic quality (defect density) . For optimal performances of the device, the crystallinity of the photoactive layer has to be chosen by considering that (for the standard PECVD- deposition conditions) on one hand, microcrystalline silicon layers have a better electronic quality (low defect density) when deposited close to the amorphous-microcrystalline silicon transition which results in a high open-circuit voltage (Voc) of the device. On the other hand high current densities (Jsc) are obtained by increasing the crystallinity well over the amorphous-microcrystalline transition. Therefore a compromise between high Voc and high Jsc has to be found for optimal devices. The optimum is generally found for "medium" i- layer crystallinities . Known PECVD processes currently use step-wise or continuous tailoring of the deposition parameters (such as silane concentration profiling and/or power profiling) during the microcrystalline i- layer deposition for achieving optimal crystal - linity and highest electronic quality of the ϊ-μσ-3ϊ:Η layer.
The defect density in the i-^c-Si:H layer is not only related to its crystallinity. Additional defects are introduced when using rough front Transparent Conductive Oxide (TCO) layers as front electrode ("front TCO", front electrode) . Such TCOs are used primarily for in- creasing the Jsc of thin- film silicon solar cells via the increase of the optical path of light within the device. However, the use of rough front TCOs leads usually to a decrease of the Voc and fill factor (FF) . This effect is attributed to the presence of additional morphology-related defects (zones of porous ϊ-μσ-3ί:Η) which lead to a decrease in FF in Voc.
DEFICIENCIES IN THE ART
Usually, the chosen device crystallinity of the ϊ-μο-3ϊ:Η layer results from a compromise between high crystallinity for high Jsc and medium crystallinity for high Voc. Stand-of -the-art PECVD deposition tools and processes do not allow for an ideal ο-3ί:Η material with high crystallinity (high Jsc) and low defect density (high Voc) for the μο - βϊ -. Ίϊ i- layer fabrication. But with a defect passivation layer, high crystallinity (high Jsc) and good Voc is possible with typical standard PECVD-deposition parameters. BRIEF DESCRIPTION OF THE FIGURES:
Figure 1: I (V) characteristics of Micromorph top-limited cells with varying passivation a-Si:H i-layers (tested thicknesses: 10, 50 and 150nm, rough LPCVD-ZnO substrate) . The reference cells have an average Voc of 1347 mV, Jsc of 12.2 mA/cm2 and FF of 70.2%. The cells passivated with 10 nm i-a:Si:H have the following average higher electrical performances of Voc: 1356 mV, Jsc of 12.4 mA/cm2 and FF of 72.4%.
Figure 2 A and B: Effect of the introduction of an a-Si:H passivation layer of varying thickness on the absolute values of the Voc and FF of the MM cells.
Figure 3: Total External Quantum Efficiency (EQE) of a Micromorph tandem cell with a 10 nm passivation layer compared to a reference cell without a passivation layer.
Fig. 4 A: Prior Art - tandem junction thin film silicon photovoltaic cell. Thicknesses not to scale.
Fig. 4 B: Embodiment according to the invention with passivation layer. Thicknesses not to scale.
SUMMARY OF THE INVENTION
As shown in Fig. 4B the present invention comprises introducing a defect -passivation layer 55 in or adjacent to the microcrystalline i- layer 45 of a PV cell 60 (bottom cell in a Micromorph tandem cell) . This additional passivation layer 55 includes an a-Si:H i-layer which is optically transparent to the light impinging on it (i.e.
light arriving after one pass through the top + p-i σ-3ϊ:Η sub- cell) . This additional a-Si:H i-layer deposited on top of the ϊ-μσ -Si:H layer increases the full Micromorph device electrical performances (Voc, FF and Jsc measured be External Quantum Efficiency, EQE) .
DETAILED DESCRIPTION OF THE INVENTION In the following illustrative example, top limited Micromorph cells were prepared on as grown rough TCO' s (LPCVD-ZnO) . The reference devices 50 for comparison offers a top pin a-Si:H cell 51 with an i- layer 53 thickness of 250 nm and a bottom μσ-3ί:Η cell 43 with a photoactive i-layer 45 of 2000 nm with a medium crystallinity (bulk Raman crystallinity measured with a 780nm laser: 50-55%) . The passivated devices 60 have identical i-layer thicknesses for the top and bottom cells, except that the deposition of the ο-3ϊ:Η i-layer 45 was followed by the deposition of a fully amorphous i-layer 55 (passivation layer) of varying thickness, according to the invention. The passivated devices 60 show improved electrical performances (see Figure 1) . This is the indication that the detrimental effect of some of the defects of the underlying microcrystalline silicon layer has been thus mitigated. In particular, recombination centres such as dangling bonds can be efficiently passivated with a-Si:H and the corresponding decreased recombination of photocarriers leads to an increased Voc, FF and total (top plus bottom sub cells) Jsc (measured by EQE) as observed in our illustrative example.
The detrimental effect of the morphology- induced defects, namely growth-related defects, is as well reduced by the introduction of the passivation layer at the end of the ϊ-μσ-3ϊ:Η layer growth.
For a bottom cell such as used in this illustrative example, the relative gain in efficiency obtained by the introduction of the amorphous passivation layer at the end of the microcrystalline i- layer is about 5%.
The appropriate thickness of the passivation layer has to be chosen by considering its effect on the Voc and FF of the cell, as depicted in Figure 1. This figure indicates that a certain thickness of the passivation layer is needed for a simultaneously increased value of FF and Voc. However, when the passivation layer is too thick, a double diode behaviour appears in the I (V) curve which decrease considerably the device performances. Figure 2 shows the limitations gain vs. layer thickness.
In our illustrative example, the decreased defect density due to the additional layer deposited at the end of the μο-Ξϊ:Η i-layer appears as well as an increased bottom cell photocurrent (see Figure 3, about +0.5 mA/cm2 in the Jsc_total = Jsc_top_cell + Jsc_bottom_cell) . Because the Micromorph cell is top limited, this increase is not seen in the I (V) curve .
Other silicon-based passivation i-layers than pure a-Si:H can be used as well, alloys such as a-SiC:H, a-Si:0:H or a-SiN:H etc...
Their optimal thickness has to be chosen for each TCO roughness according to their conductivity and optical transparency in the wave- length range from 650nm-1100nm.
It is to be expected that the passivation a-Si:H i-layers will be even more effective when applied to μο-Ξϊ-.Ή. i-layers with higher crystallinities (i.e. more defective). Therefore, one expects that the usually observed loss in Voc due to the lower electronic grade of these layers is at least partially compensated by the additional fully amorphous passivation layer.
Finally, it is not mandatory to apply this passivation layer at the end of the deposition of the intrinsic microcrystalline layer. This layer can be applied at varying locations during the intrinsic mi- crocrystalline layer growth, provided that the subsequent crystalline layer has the adequate crystallinity . It is also possible to introduce more than one passivation layer (s) during the growth of the microcrystalline i- layer. EXPERIMENTAL :
A passivation layer according to the invention can be prepared as follows. In a PECVD process chamber as known in the art (e. g. KAI-M commercially available from Oerlikon Solar) the following process parameter have been used. Substrate size was about 500 x 400 mm2.
A high-quality a-Si:H passivation layer is achievable with a pressure between 0.1-2mbar, preferably 0.2-0.5mbar, a power between 5- 500W (2.5mW/cm2 - 250m /cm2 substrate size), preferably 30-100W
(15mW/cm2 - 50mW/cm2 substrate size) , and a ratio between hydrogen and silane of 1:1. Process temperature was chosen between 100°-250°C, preferably around 200°C. A Gas flow between 50-2000sccm, preferably 50-500sccm had been applied, will however also depend on the process tool and substrate size used. Alternatively a process pressure of 1-5 mbar, a power between 100- 600W and a ratio between hydrogen and silane of 10:1 to 200:1 can be applied. The deposition rate depends on the process tool used; process duration will therefore vary until the layer thickness accord- ing to the invention between 5nm-50nm has been achieved.
SUMMARY:
Photovoltaic cell 60 comprising a substrate 31, a front or first electrode 42 of transparent conductive oxide and at least one p-i-n junction 43 comprising microcrystalline silicon, said p-i-n junction 43 comprising a first sub-layer 44 comprising silicon and a n-dopant and a second sub-layer 46 comprising silicon and a p-dopant and a third sub-layer 45 comprising essentially intrinsic microcrystalline silicon, wherein at least one passivation layer 45 comprising essentially intrinsic amorphous silicon is arranged a) between the micro- crystalline intrinsic sub-layer 45 and n-doped silicon layer 46 or b) as a layer embedded in the microcrystalline intrinsic sub-layer 45 or c) both.
In other embodiments several embedded layers may be present. Passivation layer 45 has a thickness of 5nm-200nm, preferably 10-50nm. Passivation layer 55 may be realized with essentially intrinsic silicon or silicon compounds/alloys such as a-SiC:H, a-Si:0:H or a- SiN:H or alike. A process for depositing a passivation layer 55 in a photovoltaic thin film solar cell comprises introducing in a PECVD process chamber exhibiting a substrate to be treated a gas mixture comprising silane and hydrogen, establishing either a process pressure between 0.1-2mbar, preferably 0.2-0.5mbar, a RF power (40 MHz or more) be- tween 5-500W, preferably 30-100W, and a ratio between hydrogen and silane of 1:1;
or establishing a process pressure of 1-5 mbar, a RF power (40 MHz or more) between 100-600W and a ratio between hydrogen and silane of 10:1 to 200:1
keeping the substrate at a temperature between 100°-250°C, preferably 160 °C and depositing a layer comprising amorphous intrinsic silicon with a thickness between 5nm-100nm, preferably 20-40nm. A method for manufacturing a photovoltaic thin film silicon solar cell comprising:
Providing a transparent substrate 41 with a transparent conductive front electrode 42 on it; depositing a p-doped Si layer 44, a micro- crystalline silicon intrinsic layer 45, a passivation layer 55 corn- rising essentially intrinsic amorphous silicon, a n-doped Si layer 46 and a back electrode layer 48.

Claims

CLAIMS :
1) a Photovoltaic cell comprising
a substrate;
a front or first electrode of transparent conductive oxide;
at least one p-i-n junction comprising microcrystalline silicon, said p-i-n junction including a first n-doped silicon sublayer and a second p-doped silicon sub-layer and a third essentially intrinsic microcrystalline silicon sub- layer arranged between said first and second sub-layers
wherein at least one passivation layer comprising essentially intrinsic amorphous silicon is arranged a) between the microcrystalline intrinsic sub-layer and the n-doped silicon sub-layer or b) as a layer embedded in the microcrystalline intrinsic sub-layer or c) both.
2) A photovoltaic cell according to claim 1, wherein said passivation layer has a thickness of 5nm-200nm, preferably 10-50nm.
3) A photovoltaic cell according to claim 1-2, wherein said passivation layer includes essentially intrinsic silicon or silicon compounds/alloys such as a-SiC:H, a-Si:0:H or a-SiN:H or alike.
A method for manufacturing a photovoltaic thin film silicon solar cell comprising:
Providing a transparent substrate with a transparent conductive front electrode on it;
- depositing a p-doped Si layer,
- depositing a microcrystalline silicon intrinsic layer,
- depositing a passivation layer with essentially intrinsic amorphous silicon,
- depositing a n-doped Si layer and a back electrode layer.
A method according to claim 4, wherein the step of depositing the passivation layer includes
- introducing a gas mixture with silane and hydrogen into a PECVD process chamber exhibiting the substrate, - keeping the substrate at a temperature between 100°-250°C, preferably 160°C;
- depositing a layer comprising amorphous intrinsic silicon with a thickness between 5nm-100nm, preferably 20-40nm.
6) A method according to claims 4-5, wherein the step of depositing the passivation layer includes
establishing a process pressure between 0.1-2mbar, preferably 0.2-0.5mbar ,
- a RF power between 2.5mW-250mW per cm2 substrate size at 40 MHz or more, preferably 15mW-50mW per cm2 substrate size
- and a ratio between hydrogen and silane of 1:1; 7) A method according to claims 4-5, wherein the step of depositing the passivation layer includes
- establishing a process pressure of 1-5 mbar,
- a RF power between 50mW-300mW per cm2 substrate size at 40 MHz or more
- and a ratio between hydrogen and silane of 10:1 to 200:1
EP11727629.5A 2010-06-25 2011-06-10 Thin film solar cell with microcrystalline absorpber layer and passivation layer and method for manufacturing such a cell Withdrawn EP2586066A1 (en)

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