WO2012098051A1 - Method for manufacturing a multilayer of a transparent conductive oxide - Google Patents

Method for manufacturing a multilayer of a transparent conductive oxide Download PDF

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Publication number
WO2012098051A1
WO2012098051A1 PCT/EP2012/050477 EP2012050477W WO2012098051A1 WO 2012098051 A1 WO2012098051 A1 WO 2012098051A1 EP 2012050477 W EP2012050477 W EP 2012050477W WO 2012098051 A1 WO2012098051 A1 WO 2012098051A1
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Prior art keywords
zinc oxide
boron
transparent conductive
layer
conductive zinc
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PCT/EP2012/050477
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French (fr)
Inventor
Paolo Losio
Onur Caglar
Peter RECHTSTEINER
Perrine CARROY
Holger Christ
Laura DING
Nicolay SYLVAIN
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Oerlikon Solar Ag, Truebbach
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Publication of WO2012098051A1 publication Critical patent/WO2012098051A1/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
    • H01L31/18—Processes or apparatus specially adapted for the manufacture or treatment of these devices or of parts thereof
    • H01L31/1884—Manufacture of transparent electrodes, e.g. TCO, ITO
    • 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
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022466—Electrodes made of transparent conductive layers, e.g. TCO, ITO layers
    • 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
    • H01L31/02—Details
    • H01L31/0224—Electrodes
    • H01L31/022466—Electrodes made of transparent conductive layers, e.g. TCO, ITO layers
    • H01L31/022483—Electrodes made of transparent conductive layers, e.g. TCO, ITO layers composed of zinc oxide [ZnO]
    • 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 at least one potential-jump barrier or surface barrier
    • 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 at least one potential-jump barrier or surface barrier the potential barriers being only of the PIN type
    • H01L31/076—Multiple junction or tandem solar 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

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.
  • This disclosure addresses issues in the production of ZnO front contacts to enhance Edge Isolation by Laser (EIL) proc- esses and improve module power.
  • EIL Edge Isolation by Laser
  • 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.
  • DEZ - diethyl zinc is a precursor material for the production of TCO layers in vacuum processing equipment.
  • TCO stands for transparent conductive oxide
  • TCO layers consequently are transparent conductive layers.
  • 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 photoelectric conversion unit includes an intrinsic semiconductor compound layer sandwiched between a p-doped and an n-doped semiconductor compound layer. The term intrinsic is to be understood as not intentionally doped.
  • 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 thickness of ⁇ or less, especially less than 2 ⁇ .
  • Fig. 1 shows a tandem-junction silicon thin film solar cell as known in the art.
  • a thin-film solar cell 50 usually 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.
  • 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 or microcrystalline ( ⁇ -Si, 45) solar cells, independent of the kind of crystallinity of the ad- jacent 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 mi- crocrystalline p-i-n- junction, as shown in Fig. 1, is also called micromorph tandem cell.
  • Processes used in the production of commercial thin film silicon photovoltaic modules should maximize module power and at the same time minimize production costs.
  • a TCO layer is applied as front electrode 42 and subsequently silicon layers (52-54) on a glass substrate 41 (or comparable materials) .
  • This coating step affects the whole surface of a panel 61 (Fig. 2) .
  • This panel 61 however includes an active area 62 with the photovoltaically active layers with cells 63 electrically connected in series and/or parallel.
  • the edge area 64 of each module or panel 61 needs to be cleaned of all TCO and Silicon layers.
  • modules can be laminated to protect them from weathering. The edge area thus provides a barrier for environmental influences to negatively affect the sensitive active cells 63 in the active area 62.
  • edge isolation process plays a key role to assure compliance with safety rules and to reduce the penetration of moisture into the active layers after lamination.
  • edge isolation involves mechanical removal of the layers in the edge area 64 by using abrasives, e.g. by sandblasting or similar techniques.
  • the main disadvantage is a damage of the substrate surface (micro cracks, roughening).
  • TCO and Silicon layers can be removed by using a laser beam.
  • a process based on laser application has several advantages : • No damaging or weakening of the substrate surface, (processing of surface is more gentle)
  • EIL Edge Isolation by Laser
  • a laser beam TTG will not be disturbed by ablated particles and plasma phenomena.
  • abrasive materials e.g. corundum
  • the EIL process works by removing (ablation and/or vaporization) the silicon and ZnO layers due to absorption of Laser energy in the layers .
  • the performance of thin film silicon modules is strongly influenced by the properties of the first TCO layer (s) (front contact 42, Fig. 1) .
  • Relevant properties of the TCO to be considered are total trans- mission, haze and conductivity.
  • Best module performance is obtained by increasing total transmission, increasing haze and increasing conductivity: obviously it is not possible to achieve all these goals in a single layer system.
  • a common tradeoff to improve module performance is therefore to reduce the doping level of TCO to improve total transmission and haze by accepting a certain loss of conductivity. If the doping is reduced too much, module performance will drop due to ohmic losses in the TCO layer. However, the EIL process requires a minimal amount of doping to work properly. A higher doping of TCO front contact improves the removal of thin film layers and allows enhancing the EIL process. Again, if the doping is too high, module performance drops due to high absorption of light (VIS, NIR) and low haze in the TCO layer .
  • VIS absorption of light
  • the present invention thus seeks to overcome the drawbacks in the prior art, and thereby provide a TCO-ZnO front electrode providing good module performance while also allowing enhanced removal of the front electrode by the EIL process.
  • This is achieved by the characteristics of the independent claims 1 and 5.
  • a front electrode for a photoelectric conversion device comprising at least one basic layer sequence with varying boron dopant concentration, said basic layer sequence comprising a thin transparent conductive zinc oxide higher-boron- doped layer and a thicker transparent conductive zinc oxide lower- boron-doped layer wherein the doping density through each individual conductive zinc oxide layer is substantially constant.
  • Such a multipart, bilayer structure enables the thinner, high-doped layer to be sufficiently doped to absorb laser light in the EIL process and thus be easily ablated while not adversely affecting the optical and/or electrical performance of the photoelectric conversion device, and the electrical and optical properties of the thicker, low-doped lay- er to be optimised for light transmission and electrical conductivity without negatively affecting the performance of the EIL process.
  • "thinner”, “thicker”, “higher” and “lower” have their usual meanings, i.e. the "thinner” layer has a lower thickness than the "thicker” layer, and the "higher”-doped layer has a higher doping concentration than the "lower”-doped layer.
  • substantially constant signifies that the doping density is broadly constant throughout the majority of the thickness of each layer. It is perfectly known by the skilled person that, due to processing ar- tefacts, dopant diffusion and similar phenomena, there may be a doping density gradient present at the junction of the two layers in a relatively thin portion of the thickness of either or both layers, which is to be construed as falling within the scope of the invention and the claims.
  • the front electrode may comprise a plurality of said basic layer sequences, i.e. a sequence of high-doped, low-doped, high-doped, low-doped etc. This enables simple and efficient production of a front electrode having the desired properties on existing production equipment without substantial modification thereto.
  • the electrode is arranged on a preferably glass substrate so that it can be formed into a solar panel or a solar cell .
  • the higher-boron-doped layer of the basic layer sequence nearest to the substrate is in direct and intimate contact with the substrate. This ensures that the EIL process can work efficiently by absorbing laser light in the portion directly adjacent to the substrate, thus ensuring removal of the zinc oxide layer. Additionally, highly-doped zinc oxide layers have a lower refractive index than lowly-doped or intrinsic layers, thus having such a layer directly on a glass substrate will result in a smooth increase of the refractive index from the glass to the zinc oxide, reducing re- flection of incoming light at the glass/ZnO interface and making more light available to the photoelectric conversion device.
  • the aim of the invention is achieved by a method for manufacturing a front electrode for a photoelectric conversion device comprising depositing on a substrate at least one basic layer sequence with varying boron dopant concentration, the said basic layer sequence comprising a thinner transparent conductive zinc oxide higher-boron-doped layer and a thicker transparent conductive zinc oxide lower-boron-doped layer, wherein the thicker transparent conductive zinc oxide lower-boron-doped layer is intentionally doped, that is to say is actively subjected to a dopant-containing environment during its deposition rather than being doped purely by diffusion or contamination.
  • This intentional doping The present application relates to a multi-part transparent conductive zinc oxide layer for a photoelectric conversion device, and a method of producing the same.
  • the transparent conductive zinc oxide layer comprises at least one basic layer sequence with varying boron doped and concentration, said basic layer sequence comprising a thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74) and a thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) .
  • the doping density through each individual conductive zinc oxide layer (72, 73; 74, 75) is substantially constant, which is achieved by intentionally doping the thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) .
  • This has the advantage of permitting efficient Edge Isolation by Laser EIL ablation of the transparent conductive zinc oxide layers while maintaining good electrical and optical properties in said layers,
  • the substantially constant doping density of the lower-doped layer and thereby enables the optimisation of the doping levels, so as to achieve the desired electrical and optical properties of the layer and of the front electrode as a whole.
  • the basic layer sequence is formed by depositing on the substrate a thinner transparent conductive zinc oxide higher- boron-doped layer, and depositing thereupon a thicker transparent conductive zinc oxide lower-boron-doped layer. This permits the basic layer sequence to be formed with the thinner transparent conduc- tive zinc oxide higher-boron-doped layer directly upon the substrate, ensuring that the EIL process can work efficiently by absorbing laser light in the portion directly adjacent to the substrate, thus ensuring removal of the zinc oxide layer.
  • highly-doped zinc oxide layers have a lower refractive index than lowly-doped or intrinsic layers, thus having such a layer directly on a glass substrate will result in a smooth increase of the refractive index from the glass to the zinc oxide, reducing reflection of incoming light at the glass/ZnO interface and making more light available to the photoelectric conversion device.
  • the layers are deposited by means of a vacuum processing method such as Chemical Vapour Deposition (CVD) , Low Pressure Chemical Vapour Deposition (LPCVD) , Plasma Enhanced Chemical Vapour Deposition (PECVD) , or Physical Vapour Deposition (PVD) .
  • CVD Chemical Vapour Deposition
  • LPCVD Low Pressure Chemical Vapour Deposition
  • PECVD Plasma Enhanced Chemical Vapour Deposition
  • PVD Physical Vapour Deposition
  • the thinner, higher-doped layer is deposited under conditions of the first B 2 H 6 /DEZ ratio of 0.1-1, preferably 0.2-0.55. This enables the desired doping properties of the thinner layer to be attained.
  • the thicker, lower-doped layer is deposited under conditions of a second B 2 H 6 /DEZ ratio of 0.01-0.2, preferably 0.02- 0.1. This enables the desired doping properties of the thicker layer to be attained.
  • the ratio of the first to second B 2 H 6 /DEZ ratios is between 5 and 15, preferably between 7 and 10. This enables the desired doping properties of the thicker layer to be attained.
  • both transparent zinc oxide layers are deposited under conditions of a H20/DEZ ratio of 0.8 to 1.5.
  • the deposition is carried out on a substrate with a temperature of 150-220°C, preferably 180-195°C, which enables good adhesion of the layers to the preferably glass substrate.
  • the deposition is carried out on a substrate with a temperature of 150-260°C, preferably 205-250°C, which enables a deposition rate up to approximately 10 nm/s, thereby enabling rapid production.
  • a plurality of basic layer sequences are deposited sequentially on the substrate, i.e.
  • the thinner, higher-boron-doped layer and the thicker, lower-boron-doped layer of the/each at least one basic lay- er sequence are deposited in two individual, separate, discrete processing steps. This enables better quality layers to be produced, particularly the thicker, lower-boron-doped layer, since by using two discrete steps there is no residual higher-concentration dopant in the deposition chamber which might affect the deposition of the lower-boron-doped layer.
  • the thinner transparent conductive zinc oxide higher-boron-doped layer and the thicker transparent conductive zinc oxide lower-boron-doped layer of the (or indeed each and every in the case of multiple layer sequences) at least one basic layer sequence are deposited sequentially by varying the dibo- rane/diethyl zinc ratio from the said first diborane/diethyl zinc ratio to the said second diborane/diethyl zinc ratio or from the said second diborane/diethyl zinc ratio to the said first diborane/diethyl zinc ratio over a time period of 30 seconds or less.
  • the ratio can be varied e.g. by varying the diborane flow as required over the desired time period. This enables faster production, while preventing any doping density gradient between the thicker and the thinner layers at their interface from becoming too pronounced.
  • Figure 1 shows a tandem junction thin-film silicon photovoltaic cell according to the prior art
  • Figure 2 shows a side view of a conventional thin-film photovoltaic panel
  • Figure 3 shows a schematic representation of the basic layer structure according to the invention.
  • Figure 4 shows a schematic representation of a more complex structure with a plurality of basic layer structures according to the invention .
  • This invention is related to using a multilayer TCO system, which can be used advantageously in combination with the EIL process.
  • a multilayer TCO system it is possible to use a stack of layers each with a specific function.
  • a highly doped layer is used which is able to absorb the laser energy during an EIL process, thus enhancing removal of unwanted material.
  • this highly doped layer will improve the conductivity of the complete TCO stack.
  • a thicker and low doped layer provides for haze and for keeping total transmission high .
  • the following solution is presented for simultaneously improving module performance and enhancing the EIL process.
  • FIG. 3 A first embodiment of a TCO Multilayer system according to the in- vention is described with a view on Fig. 3:
  • a first ZnO Layer (identified as seed layer 72) is being deposited on a substrate 71, preferably glass.
  • Said first layer is strongly doped with boron to increase the absorption in the NIR (Typically 1064 nm and 1030 nm, respectively, for an EIL system) . This layer enhances conductivity and supports the EIL process.
  • Temperature of glass 150-220°C, best range 180-195°C (for deposition rate below 4nm/s) .
  • H 2 0/DEZ ratio 0.8 to 1.5; thickness between 50nm to 300nm, preferably lOOnm to 300nm, further preferably lOOnm to 200nm.
  • the temperature of the glass may be in the range 150-260°C, best range 205-250°C (for deposition rate up to lOnm/s) .
  • a second ZnO layer (identified as 73 ZnO bulk layer in Fig. 3) is lowly doped to provide haze and to keep absorption low, thus increasing the current generated in the microcrystalline cell.
  • Process parameter for such a layer would be a B 2 H 6 /DEZ ratio from 0.01 to 0.2, best range 0.02 to 0.1.
  • the required minimal doping of the layer insures a reduced degradation of the conductivity upon exposure to moisture.
  • Temperature of the glass during deposition step 150-220°C, best range 180-195°C (for deposition rate below 4nm/s). H 2 0/DEZ ratio: 0.8 to 1.5. Thickness from 500nm to several micrometers, good range 900nm to 3um, best results with no more than 2 ⁇ total thickness.
  • the temperature of the glass may be in the range 150- 260°C, best range 205-250°C (for deposition rate up to lOnm/s) .
  • the two layers 72, 73 may be deposited in two completely separate steps, or they may be created by varying the diborane/diethyl zinc ratio in the process chamber over a time period of 30 seconds or less, e.g. by increasing or decreasing the diborane flow as required.
  • a second embodiment according to the invention includes a further developed process which may be implemented in a multiple PM deposition system.
  • a single process module PM is capable of producing a layer sequence with varying dopant addition or, in an inline system with several process modules, all PMs produce just a fraction or share of said sequence. Said fraction may be exactly the same for all PMs or varying.
  • a typical basic layer sequence is included in Figure 4 and involves a first highly doped layer 74 with a thickness of 10-120nm, preferably 10-lOOnm, further preferably 30-80nm, and a subsequently depos- ited lowly doped layer 75 with a thickness of 100-500nm, preferably 250-500nm.
  • the total thickness of a ZnO layer corresponds then to the thickness of a basic layer sequence (74/75) multiplied by the number of PM depositing it sequentially.
  • each PM may deposit two basic layer sequences; the calculation of the resulting layer stack can be easily derived.
  • Figure 4 shows such a layer stack comprising a plurality of layer sequences 74/75, wherein each layer sequence includes a first highly doped TCO-ZnO layer 74 and a subsequent second ZnO TCO layer with low dopant concentration 75.
  • the term lowly and highly doped means that the B 2 H 6 /DEZ ratio in the precursor materials is between 5 to 15 times higher than between "low” and "high” with preferred ratios of substantially 7-8 and 10. Both approaches have been shown to improve the EIL process compared to a layer consisting only of the lowly doped layer.
  • B 2 H 6 boron dopant
  • the doping ratios are based on said technical gas mixture and the term "boron" or B 2 H 6 means said technical gas mixture.
  • highly doped ZnO layers have a lower refractive index than lowly doped or intrinsic layers.
  • Adding a highly doped ZnO lay- er 72 directly on a glass substrate 71 will result in a smoother increase of the refractive index from the glass to the ZnO.
  • reflection of incoming light at the Glass/ZnO interface will be reduced and more light will be available to the PV modules.
  • an enhanced EIL process allows a safe removal of all material deposited near the substrate edge. Even material accidentally deposited on the front glass surface is removed.

Abstract

The invention relates to a multi-part transparent conductive zinc oxide layer for a photoelectric conversion device, and a method of producing the same. The transparent conductive zinc oxide layer comprises at least one basic layer sequence with varying boron dopant concentration, said basic layer sequence comprising a thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74) and a thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75). Inventively, the doping density through each individual conductive zinc oxide layer (72, 73; 74, 75) is substantially constant, which is achieved by intentionally doping the thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75). This has the advantage of permitting efficient Edge Isolation by Laser EIL ablation of the transparent conductive zinc oxide layers while maintaining good electrical and optical properties in said layers.

Description

METHOD FOR MANUFACTURING A MULTILAYER OF A TRANSPARENT CONDUCTIVE
OXIDE
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. This disclosure addresses issues in the production of ZnO front contacts to enhance Edge Isolation by Laser (EIL) proc- esses and improve module power.
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.
DEZ - diethyl zinc is a precursor material for the production of TCO layers in vacuum processing equipment.
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, 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 photoelectric conversion unit includes an intrinsic semiconductor compound layer sandwiched between a p-doped and an n-doped semiconductor compound layer. The term intrinsic is to be understood as not intentionally doped.
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 thickness of ΙΟμιη or less, especially less than 2μπι.
BACKGROUND OF THE INVENTION
Fig. 1 shows a tandem-junction silicon thin film solar cell as known in the art. Such a thin-film solar cell 50 usually 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) . 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 (μο-Si, 45) solar cells, independent of the kind of crystallinity of the ad- jacent 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 mi- crocrystalline p-i-n- junction, as shown in Fig. 1, is also called micromorph tandem cell.
DRAWBACKS KNOWN IN THE ART
Processes used in the production of commercial thin film silicon photovoltaic modules should maximize module power and at the same time minimize production costs.
The production of thin film silicon modules involves several steps. Normally, as a first step a TCO layer is applied as front electrode 42 and subsequently silicon layers (52-54) on a glass substrate 41 (or comparable materials) . This coating step affects the whole surface of a panel 61 (Fig. 2) . This panel 61 however includes an active area 62 with the photovoltaically active layers with cells 63 electrically connected in series and/or parallel. To ensure electrical insulation, the edge area 64 of each module or panel 61 needs to be cleaned of all TCO and Silicon layers. After this step modules can be laminated to protect them from weathering. The edge area thus provides a barrier for environmental influences to negatively affect the sensitive active cells 63 in the active area 62.
In other words, appropriate electrical insulation to a surrounding frame or housing of a finished solar module is necessary. Therefore, the edge isolation process plays a key role to assure compliance with safety rules and to reduce the penetration of moisture into the active layers after lamination.
One approach to edge isolation involves mechanical removal of the layers in the edge area 64 by using abrasives, e.g. by sandblasting or similar techniques. The main disadvantage is a damage of the substrate surface (micro cracks, roughening).
Alternatively, TCO and Silicon layers can be removed by using a laser beam. A process based on laser application has several advantages : • No damaging or weakening of the substrate surface, (processing of surface is more gentle)
• Edge Isolation by Laser (EIL) Process can be used through the substrate / glass (TTG) .
· A laser beam TTG will not be disturbed by ablated particles and plasma phenomena.
• No additional consumption of abrasive materials (e.g. corundum) is needed.
The EIL process works by removing (ablation and/or vaporization) the silicon and ZnO layers due to absorption of Laser energy in the layers .
Further details of an EIL process have been described in U.S. Provisional Patent Application for "METHOD AND DEVICE FOR ABALATION OF THIN FILMS FROM A SUBSTRATE", Serial No.: 61/262,691 which is in- corporated herein by reference.
The performance of thin film silicon modules is strongly influenced by the properties of the first TCO layer (s) (front contact 42, Fig. 1) . Relevant properties of the TCO to be considered are total trans- mission, haze and conductivity.
In common TCO based on LPCVD ZnO these three parameters can be varied by modifying the amount of dopant gas (usually diborane, B2H6) added to the precursor gases during growth in a LPCVD process. When the complete layer is made using one single set of gas flows and the layers thickness is kept constant, it is known in the art:
• Increasing the doping amount reduces haze, reduces total transmission of red and NIR light and increases conductivity.
• Decreasing the doping amount leads to the inverse effects.
Best module performance is obtained by increasing total transmission, increasing haze and increasing conductivity: obviously it is not possible to achieve all these goals in a single layer system.
A common tradeoff to improve module performance is therefore to reduce the doping level of TCO to improve total transmission and haze by accepting a certain loss of conductivity. If the doping is reduced too much, module performance will drop due to ohmic losses in the TCO layer. However, the EIL process requires a minimal amount of doping to work properly. A higher doping of TCO front contact improves the removal of thin film layers and allows enhancing the EIL process. Again, if the doping is too high, module performance drops due to high absorption of light (VIS, NIR) and low haze in the TCO layer .
In order to address this issue the document GROWTH OF LPCVD ZnO BI- LAYERS FOR SOLAR CELL FRONT ELECTRODES, AUTHORED BY L. Ding at al . , presented at the 25th EU-PVSEC in September 2010 in Valencia, suggests to use TCO-ZnO bilayers, consisting in the combination of a highly doped plus a non-intentionally doped part, deposited in one growth step. However, according to experiments, this particular arrangement of bilayers, specifically the non-intentionally doped por- tion, does not adequately balance the competing requirements as outlined above.
SUMMARY OF THE INVENTION
The present invention thus seeks to overcome the drawbacks in the prior art, and thereby provide a TCO-ZnO front electrode providing good module performance while also allowing enhanced removal of the front electrode by the EIL process. This is achieved by the characteristics of the independent claims 1 and 5. Specifically, this is achieved by a front electrode for a photoelectric conversion device comprising at least one basic layer sequence with varying boron dopant concentration, said basic layer sequence comprising a thin transparent conductive zinc oxide higher-boron- doped layer and a thicker transparent conductive zinc oxide lower- boron-doped layer wherein the doping density through each individual conductive zinc oxide layer is substantially constant. Such a multipart, bilayer structure enables the thinner, high-doped layer to be sufficiently doped to absorb laser light in the EIL process and thus be easily ablated while not adversely affecting the optical and/or electrical performance of the photoelectric conversion device, and the electrical and optical properties of the thicker, low-doped lay- er to be optimised for light transmission and electrical conductivity without negatively affecting the performance of the EIL process. In this case, "thinner", "thicker", "higher" and "lower" have their usual meanings, i.e. the "thinner" layer has a lower thickness than the "thicker" layer, and the "higher"-doped layer has a higher doping concentration than the "lower"-doped layer. In addition, "substantially constant" signifies that the doping density is broadly constant throughout the majority of the thickness of each layer. It is perfectly known by the skilled person that, due to processing ar- tefacts, dopant diffusion and similar phenomena, there may be a doping density gradient present at the junction of the two layers in a relatively thin portion of the thickness of either or both layers, which is to be construed as falling within the scope of the invention and the claims.
In an embodiment, the front electrode may comprise a plurality of said basic layer sequences, i.e. a sequence of high-doped, low-doped, high-doped, low-doped etc. This enables simple and efficient production of a front electrode having the desired properties on existing production equipment without substantial modification thereto.
In an embodiment, the electrode is arranged on a preferably glass substrate so that it can be formed into a solar panel or a solar cell .
In an embodiment, the higher-boron-doped layer of the basic layer sequence nearest to the substrate is in direct and intimate contact with the substrate. This ensures that the EIL process can work efficiently by absorbing laser light in the portion directly adjacent to the substrate, thus ensuring removal of the zinc oxide layer. Additionally, highly-doped zinc oxide layers have a lower refractive index than lowly-doped or intrinsic layers, thus having such a layer directly on a glass substrate will result in a smooth increase of the refractive index from the glass to the zinc oxide, reducing re- flection of incoming light at the glass/ZnO interface and making more light available to the photoelectric conversion device. Furthermore, the aim of the invention is achieved by a method for manufacturing a front electrode for a photoelectric conversion device comprising depositing on a substrate at least one basic layer sequence with varying boron dopant concentration, the said basic layer sequence comprising a thinner transparent conductive zinc oxide higher-boron-doped layer and a thicker transparent conductive zinc oxide lower-boron-doped layer, wherein the thicker transparent conductive zinc oxide lower-boron-doped layer is intentionally doped, that is to say is actively subjected to a dopant-containing environment during its deposition rather than being doped purely by diffusion or contamination. This intentional doping The present application relates to a multi-part transparent conductive zinc oxide layer for a photoelectric conversion device, and a method of producing the same. The transparent conductive zinc oxide layer comprises at least one basic layer sequence with varying boron doped and concentration, said basic layer sequence comprising a thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74) and a thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) . Inventively, the doping density through each individual conductive zinc oxide layer (72, 73; 74, 75) is substantially constant, which is achieved by intentionally doping the thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) . This has the advantage of permitting efficient Edge Isolation by Laser EIL ablation of the transparent conductive zinc oxide layers while maintaining good electrical and optical properties in said layers,
the substantially constant doping density of the lower-doped layer, and thereby enables the optimisation of the doping levels, so as to achieve the desired electrical and optical properties of the layer and of the front electrode as a whole.
In an embodiment, the basic layer sequence is formed by depositing on the substrate a thinner transparent conductive zinc oxide higher- boron-doped layer, and depositing thereupon a thicker transparent conductive zinc oxide lower-boron-doped layer. This permits the basic layer sequence to be formed with the thinner transparent conduc- tive zinc oxide higher-boron-doped layer directly upon the substrate, ensuring that the EIL process can work efficiently by absorbing laser light in the portion directly adjacent to the substrate, thus ensuring removal of the zinc oxide layer. Additionally, highly-doped zinc oxide layers have a lower refractive index than lowly-doped or intrinsic layers, thus having such a layer directly on a glass substrate will result in a smooth increase of the refractive index from the glass to the zinc oxide, reducing reflection of incoming light at the glass/ZnO interface and making more light available to the photoelectric conversion device.
In an embodiment, the layers are deposited by means of a vacuum processing method such as Chemical Vapour Deposition (CVD) , Low Pressure Chemical Vapour Deposition (LPCVD) , Plasma Enhanced Chemical Vapour Deposition (PECVD) , or Physical Vapour Deposition (PVD) . The choice of any of these processes enables efficient, economic deposition of the layers.
In an embodiment, the thinner, higher-doped layer is deposited under conditions of the first B2H6/DEZ ratio of 0.1-1, preferably 0.2-0.55. This enables the desired doping properties of the thinner layer to be attained.
In an embodiment, the thicker, lower-doped layer is deposited under conditions of a second B2H6/DEZ ratio of 0.01-0.2, preferably 0.02- 0.1. This enables the desired doping properties of the thicker layer to be attained.
In an embodiment, the ratio of the first to second B2H6/DEZ ratios is between 5 and 15, preferably between 7 and 10. This enables the desired doping properties of the thicker layer to be attained.
In an embodiment, both transparent zinc oxide layers are deposited under conditions of a H20/DEZ ratio of 0.8 to 1.5. In an embodiment, the deposition is carried out on a substrate with a temperature of 150-220°C, preferably 180-195°C, which enables good adhesion of the layers to the preferably glass substrate. In an alternative embodiment, the deposition is carried out on a substrate with a temperature of 150-260°C, preferably 205-250°C, which enables a deposition rate up to approximately 10 nm/s, thereby enabling rapid production. In an embodiment, a plurality of basic layer sequences are deposited sequentially on the substrate, i.e. forming a sequence of high-doped, low-doped, high-doped, low-doped and so on layers. This enhances the EIL process by distributing the absorption of laser light throughout the thickness of the front electrode, leading to improved ablation, while still retaining adequate electrical and optical properties for the electrode.
In an embodiment, the thinner, higher-boron-doped layer and the thicker, lower-boron-doped layer of the/each at least one basic lay- er sequence are deposited in two individual, separate, discrete processing steps. This enables better quality layers to be produced, particularly the thicker, lower-boron-doped layer, since by using two discrete steps there is no residual higher-concentration dopant in the deposition chamber which might affect the deposition of the lower-boron-doped layer. This additionally helps maintain the doping density through the lower-boron-doped layer substantially constant, enabling more precise control of the desired dopant concentration and thus electrical and optical properties of the layers, and minimises or eliminates any dopant density gradient at the interface be- tween the layers .
In an alternate embodiment, the thinner transparent conductive zinc oxide higher-boron-doped layer and the thicker transparent conductive zinc oxide lower-boron-doped layer of the (or indeed each and every in the case of multiple layer sequences) at least one basic layer sequence are deposited sequentially by varying the dibo- rane/diethyl zinc ratio from the said first diborane/diethyl zinc ratio to the said second diborane/diethyl zinc ratio or from the said second diborane/diethyl zinc ratio to the said first diborane/diethyl zinc ratio over a time period of 30 seconds or less.
The ratio can be varied e.g. by varying the diborane flow as required over the desired time period. This enables faster production, while preventing any doping density gradient between the thicker and the thinner layers at their interface from becoming too pronounced.
Further specific embodiments and advantages are described in relation to the embodiments illustrated in the figures.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows a tandem junction thin-film silicon photovoltaic cell according to the prior art;
Figure 2 shows a side view of a conventional thin-film photovoltaic panel;
Figure 3 shows a schematic representation of the basic layer structure according to the invention; and
Figure 4 shows a schematic representation of a more complex structure with a plurality of basic layer structures according to the invention .
DETAILED DESCRIPTION OF THE INVENTION
This invention is related to using a multilayer TCO system, which can be used advantageously in combination with the EIL process.
In a multilayer TCO system according to the invention it is possible to use a stack of layers each with a specific function. In this case, a highly doped layer is used which is able to absorb the laser energy during an EIL process, thus enhancing removal of unwanted material. Additionally, this highly doped layer will improve the conductivity of the complete TCO stack. Subsequently, a thicker and low doped layer provides for haze and for keeping total transmission high . The following solution is presented for simultaneously improving module performance and enhancing the EIL process.
A first embodiment of a TCO Multilayer system according to the in- vention is described with a view on Fig. 3:
A first ZnO Layer (identified as seed layer 72) is being deposited on a substrate 71, preferably glass. Said first layer is strongly doped with boron to increase the absorption in the NIR (Typically 1064 nm and 1030 nm, respectively, for an EIL system) . This layer enhances conductivity and supports the EIL process.
Process parameters for realizing such an embodiment would be a
B2H6/DEZ ratio of 0.1 to 1, best range 0.2 to 0.55. Temperature of glass: 150-220°C, best range 180-195°C (for deposition rate below 4nm/s) . H20/DEZ ratio: 0.8 to 1.5; thickness between 50nm to 300nm, preferably lOOnm to 300nm, further preferably lOOnm to 200nm. Without deviating from the inventive concept it is possible to obtain comparable results by increasing the doping ratio while reducing layer thickness or by decreasing the doping while increasing the layer thickness. Alternatively, the temperature of the glass may be in the range 150-260°C, best range 205-250°C (for deposition rate up to lOnm/s) . A second ZnO layer (identified as 73 ZnO bulk layer in Fig. 3) is lowly doped to provide haze and to keep absorption low, thus increasing the current generated in the microcrystalline cell.
Process parameter for such a layer would be a B2H6/DEZ ratio from 0.01 to 0.2, best range 0.02 to 0.1.
The required minimal doping of the layer insures a reduced degradation of the conductivity upon exposure to moisture.
Temperature of the glass during deposition step: 150-220°C, best range 180-195°C (for deposition rate below 4nm/s). H20/DEZ ratio: 0.8 to 1.5. Thickness from 500nm to several micrometers, good range 900nm to 3um, best results with no more than 2μιη total thickness. Alternatively, the temperature of the glass may be in the range 150- 260°C, best range 205-250°C (for deposition rate up to lOnm/s) .
The two layers 72, 73 may be deposited in two completely separate steps, or they may be created by varying the diborane/diethyl zinc ratio in the process chamber over a time period of 30 seconds or less, e.g. by increasing or decreasing the diborane flow as required.
A second embodiment according to the invention includes a further developed process which may be implemented in a multiple PM deposition system. Basically two approaches are possible: either a single process module PM is capable of producing a layer sequence with varying dopant addition or, in an inline system with several process modules, all PMs produce just a fraction or share of said sequence. Said fraction may be exactly the same for all PMs or varying.
A typical basic layer sequence is included in Figure 4 and involves a first highly doped layer 74 with a thickness of 10-120nm, preferably 10-lOOnm, further preferably 30-80nm, and a subsequently depos- ited lowly doped layer 75 with a thickness of 100-500nm, preferably 250-500nm. The total thickness of a ZnO layer corresponds then to the thickness of a basic layer sequence (74/75) multiplied by the number of PM depositing it sequentially. In a variant of said process, each PM may deposit two basic layer sequences; the calculation of the resulting layer stack can be easily derived.
Figure 4 shows such a layer stack comprising a plurality of layer sequences 74/75, wherein each layer sequence includes a first highly doped TCO-ZnO layer 74 and a subsequent second ZnO TCO layer with low dopant concentration 75. The term lowly and highly doped means that the B2H6/DEZ ratio in the precursor materials is between 5 to 15 times higher than between "low" and "high" with preferred ratios of substantially 7-8 and 10. Both approaches have been shown to improve the EIL process compared to a layer consisting only of the lowly doped layer. Technically B2H6 (boron dopant) is available as a gas mixture of 2% B2H6 in hydrogen. Within the context of this disclosure the doping ratios are based on said technical gas mixture and the term "boron" or B2H6 means said technical gas mixture.
FURTHER ADVANTAGES OF THE INVENTION
In general highly doped ZnO layers have a lower refractive index than lowly doped or intrinsic layers. Adding a highly doped ZnO lay- er 72 directly on a glass substrate 71 will result in a smoother increase of the refractive index from the glass to the ZnO. Thus, reflection of incoming light at the Glass/ZnO interface will be reduced and more light will be available to the PV modules. Additionally, an enhanced EIL process allows a safe removal of all material deposited near the substrate edge. Even material accidentally deposited on the front glass surface is removed.
Although the invention has been described in terms of specific em- bodiments, it is not to be construed as being limited to such, rather it encompasses all variations falling within the scope of the appended claims.
LIST OF REFERENCE SIGNS
41 - Substrate
42 - Front electrode
43 - Bottom cell
44 - p-doped Si layer (p c-Si:H)
45 - i-layer μο-3ί:Η
46 - n-doped Si layer (n a-Si:H / n μο-3ί:Η)
47 - Back electrode
48 - Back reflector
50 - Thin-film solar cell
51 - Top cell
52 - p-doped Si layer (p a-Si:H / p μο-3ί:Η)
53 - i-layer a-Si:H
54 - n-doped Si layer (n a-Si:H / n μο-3ί:Η)
61 - Solar panel
62 - Active area
63 - Cells
64 - Edge area
71 - Substrate
72 - Seed layer / higher-boron-doped layer
73 - Bulk ZnO layer / lower-boron-doped layer
74 - Hi-Doping layer / higer-boron-doped layer
75 - Low Doping layer / lower-boron-doped layer

Claims

1. Front electrode (42) for a photoelectric conversion device com- prising at least one basic layer sequence with varying boron dopant concentration, said basic layer sequence comprising a thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74) and a thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) , charachterised in that the doping density through each in- dividual conductive zinc oxide layer (72, 73; 74, 75) is substantially constant.
2. Front electrode (42) according to any previous claim wherein said front electrode comprises a plurality of said basic layer sequences.
3. Front electrode (42) according to any previous claim, wherein the said front electrode (42) is arranged on a substrate (71) , said substrate preferably comprising glass.
4. Front electrode (42) according to claim 3, wherein the thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74) of the basic layer sequence adjacent to the substrate (71) is in direct and intimate contact with said substrate (71) .
5. Method for manufacturing a front electrode (42) for a photoelectric conversion device comprising depositing on a substrate (71) at least one basic layer sequence with varying boron dopant concentration, the said basic layer sequence comprising a thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74) and a thick- er transparent conductive zinc oxide lower-boron-doped layer (73; 75) , charachterised in that the said thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) is intentionally doped.
6. Method according to claim 5, wherein the said basic layer se- quence is deposited in the following steps: - depositing on the substrate (71) a thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74),
- depositing on said thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74) a thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) .
7. Method according to claim 5 or 6, wherein the said layers (72, 73, 74, 75) are deposited by means of a vacuum processing method such as Chemical Vapour Deposition, Low Pressure Chemical Vapour Deposition, Plasma Enhanced Chemical Vapour Deposition or Physical Vapour Deposition.
8. Method according to claim 7, wherein the thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74) is deposited under conditions of a first diborane/diethyl zinc ratio of 0.1-1, preferably 0.2-0.55.
9. Method according to claim 7 or 8, wherein the thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) is deposited under conditions of a second diborane/diethyl zinc ratio of 0.01-0.2, preferably 0.02-0.1.
10. Method according to claims 8 and 9, wherein the ratio of the first diborane/diethyl zinc ratio to the second diborane/diethyl zinc ratio is between 5 and 15, preferably between 7 and 10.
11. Method according to any of claims 8-10, wherein the transparent conductive zinc oxide layers (72, 73, 74, 75) are deposited under conditions of a H20/diethyl zinc ratio of 0.8 to 1.5.
12. Method according to any of claims 5-11 wherein the temperature of the substrate during deposition is 150-220°C, preferably 180- 195°C.
13. Method according to any of claims 5-11 wherein the temperature of the substrate during deposition is 150-260°C, preferably 205- 250°C.
14. Method according to any of claims 5-13 wherein a plurality of basic layer sequences are deposited sequentially on the substrate (71) .
15. Method according to any of claims 5-14, wherein the thinner transparent conductive zinc oxide higher-boron-doped layer (72; 74) and the thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) of the/each at least one basic low sequence are deposited in two separate, discrete processing steps.
16. Method according to claims 8 and 9 or according to claim 10, wherein the thinner transparent conductive zinc oxide higher-boron- doped layer (72; 74) and the thicker transparent conductive zinc oxide lower-boron-doped layer (73; 75) of the/each at least one basic layer sequence are deposited sequentially by varying the dibo- rane/diethyl zinc ratio from the said first diborane/diethyl zinc ratio to the said second diborane/diethyl zinc ratio or from the said second diborane/diethyl zinc ratio to the said first diborane/diethyl zinc ratio over a time period of 30 seconds or less.
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