Description

The invention relates to a conductive film paste for forming an electrode on a p-type emitter on an n-type base semiconductor substrate. The paste comprises a conductive metal, a lead- tellurium-oxide, a tungsten-tellurium-oxide, a molybdenum-tellurium-oxide or a zinc-tellurium- oxide or any combination thereof, and an organic medium.

Conductive inks or pastes are used to form electrodes, such as silver gridlines and bus bars, on the surface of silicon solar cells or photovoltaic cells. Photovoltaic ("PV") cells convert sunlight into electricity by promoting charge carriers in the valence band of a semiconductor into the conduction band of the semiconductor. The interaction of photons from incident sunlight with doped semiconductor materials forms electron-hole charge carriers. These electron-hole pair charge carriers migrate in the electric field generated by the p-n semiconductor junction and are collected by electrodes applied to the surface of the semiconductor, through which the current flows to the external circuit. Modern crystalline silicon solar cells are typically coated with at least one thin passivation layer for the purpose of reducing electron-hole recombination caused by dangling bonds at the silicon wafer surface. Crystalline solar cells are also usually coated with anti-reflection coatings to minimize reflected light and promote light absorption. The effect of both coatings is to increase solar cell efficiency i.e. the percentage of incident sunlight that can be converted into electricity. In some cases a single coating can function both as a passivation layer and as an antireflection layer. For example amorphous, hydrogenated silicon nitride can perform both functions on n- type silicon surfaces, e.g. phosphorus doped silicon. However amorphous hydrogenated silicon nitride does not passivate p-type silicon surfaces, e.g. boron doped silicon. Thus typical passivation layers for p-type silicon surfaces include SiO _x , AIO _x , SiC, borosilicate glass and others. In the case where a p-type surface is passivated with SiO _x , AIO _x , SiC or borosilicate glass it is common to deposit a high refractive index antireflection coating on top of the passivation layer such as amorphous hydrogenated silicon nitride. Unfortunately passivation layers and antireflection coatings are typically electrical insulators and thus prevent charge carriers (electrons or holes) from transferring from the substrate to the corresponding electrode. Solar cells are typi- cally covered by the passivation layer and anti-reflection coating before a conductive paste is applied. Conductive pastes are commonly applied by screen printing, ink jet printing, laser printing or extrusion. The aforementioned passivation layers can be amorphous or crystalline. The thickness and stoichiometry of such layers can be varied in order to tune the performance. Anti- reflection coatings often comprise silicon nitride or titanium oxide. Such a nti reflective coatings can be amorphous or crystalline. The thickness and stoichiometry of such coatings can also be varied in order to tune the performance. Such antireflective coatings can also be partially hydrogenated. Amorphous hydrogenated silicon nitride coatings also act as passivation layers for n- type silicon surfaces. Some solar cell architectures use multiple layers to optimize the cell passivation and antireflective properties. Such "dielectric stacks" are commonly used in the industry and often consist of a-Si _y N _x :H layers on top of very thin (<3nm) AIO _x , SiO _x or SiC layers. In particular, such dielectric stacks are commonly used on top of p-type silicon surfaces as AIO _x , SiO _x and SiC provide excellent passivation of these types of solar cells whereas silicon nitride variants do not.

An electrode for a solar cell optimally provides low electrical resistance so that the percentage of incident sunlight converted to usable electrical energy is maximized. The amount of sunlight converted to electricity is referred to as "efficiency". Both the resistivity of the electrode as well as the contact resistance between the electrode and the silicon wafer have a strong influence on solar cell efficiency. The resistivity and contact resistance should be minimized in order to improve solar cell efficiency. The electrode can reduce the efficiency of a solar cell by introducing undesirable contaminants or defects into the silicon. Such defects are recombination sources and reduce the cell efficiency and thus reduce the amount of power that can be generated by a cell. Thus the performance of the cell is improved by using electrode compositions that do not introduce recombination sources.

Conductive pastes are used to form electrodes, conductive grids or metal contacts. Conductive pastes as described for example in US 8,497,420 typically include one or more glass frits, a conductive species, such as silver particles, and an organic medium. In some cases the glass frit may be partially crystalline. To form the electrode, conductive inks are printed onto the anti- reflective coating in a pattern of grid lines or other pattern by screen printing or another suitable process. The substrate is then fired, during which electrical contact is made between the grid lines and the substrate. Typically the firing is done in a belt furnace in air or an oxygen containing atmosphere. Performance of such electrode pastes can be optimized by adjusting the firing temperature and time. Typically peak firing temperatures are between 600°C and 900°C. Typi- cally the firing time for such cells can vary between about 30 seconds to several minutes.

As mentioned previously, the anti-reflective coating enhances light absorption but also acts as an insulator which impairs the charge carriers from flowing from the substrate to the electrode. Accordingly, during the firing cycle the conductive ink should etch at least part of the anti- reflective coating and part of any passivation layer to form electrodes having low contact resistance. To accomplish this, conductive inks incorporate at least one glass frit. The glass frit performs multiple functions. First, glass frit will aid with sintering metal particles, thus improving resistivity of the electrode and enabling solder connections to be made. Second, the glass frit will interact with antireflection coatings and passivation layers to reduce contact resistance be- tween the formed metal electrode and the substrate. Third, the glass provides the medium for development of metal colloids which can further enhance charge carrier collection. Fourth the glass provides adhesion to the substrate. Fifth the glass provides some added chemical durabil- ity to the electrode for example moisture resistance. From US 7,736,546 it is known that particularly Te02 containing glass frits can be effective for use in pastes used for making electrodes on silicon solar cells. During the firing process the glass frit liquefies, and tends to flow within the open microstructure of the electrode paste, coating the silver particles and the anti-reflective coating on the substrate. It is believed that the melted glass dissolves and/or oxidizes at least part of the anti- reflective coating and any passivation layer as well as some of the metal particles contained in the paste. As the firing process proceeds to the cooling stage, dissolved metal silver, ionic silver or silver oxide can recrystallize to metallic silver at the silicon surface. As a result, some of these silver crystallites are able to penetrate the antireflective layer and form a low contact resistance electrode with the substrate. This enables at least some direct contact between the substrate and the sintered bulk metal of the paste. If the interfacial glass layer near the substrate is thin enough and/or contains metal colloids, it is believed that the contact resistance between the electrode and the substrate can be enhanced. This process is referred to as "fire-through" and facilitates a low resistivity, low contact resistance contact with a strong bond between conductive grid or metal contact and the substrate.

Without being bound by theory, it is believed that charge carriers are transferred from the sub- strate to the silver crystallites and then transferred to the gridline through direct contact. It is also believed that electrical contact can be further enhanced if regions of the glassy layer at the interface between the substrate and the electrode are very thin. It is also believed that electrical resistance can be further enhanced if these thin layers of interfacial glass contain silver colloids. It is believed that thin interfacial glassy layers and the presence of metal colloids can provide additional pathways for charge carriers to be collected through tunneling or hopping.

The process described above applies generally to both p-type and n-type based solar cells. However, making low contact resistance electrodes to p-type surfaces using a fire-through process is difficult. P-type surfaces are used in conjunction with n-type silicon bases. This configu- ration is desirable for several reasons. For example it is well known that wafers having this orientation (n-type base with p-type surface) do not suffer from light induced degradation (LID), a phenomenon that is common in solar cells utilizing a p-type (boron doped) silicon base and n- type surface (emitter). LID results in a significant drop in solar cell efficiency after initial illumination. An additional benefit to making solar cells with an n-type base with a p-type surface is that they tend to be less sensitive to metal impurities within the wafer.

Presently, it is common that the addition of aluminum metal or aluminum containing alloys to the electrode pastes is required in order to make low contact resistance electrodes to p-type silicon surfaces. Multiple sources in literature indicate that the addition of aluminum or aluminum alloys does tend to improve contact resistance to p-type silicon surfaces, but the addition of aluminum causes problems as well. For example, the addition of Al to a silver paste will increase the electrical resistivity of the electrode. A consequence of this effect is that a thicker amount (more mass) of paste is needed to achieve the same circuit resistance. This adds cost to the cell man- ufacturer. The addition of aluminum also introduces recombination centers to the silicon which tend to lower the open circuit voltage of the solar cell and reduce the efficiency of the cell. Additionally, aluminum tends to interfere with soldering, thus making the assembly of solar cells into modules more problematic. US-A 2010/059106 discloses a composition for making contact to a p-type silicon layer that is free from aluminum and aluminum alloys. It is relied on the addition of metal particles from a group consisting of Mo, Tc, Ru, Pd, W, Re, Os, Ir and Pt. Many of the aforementioned metals are rare and expensive and are apt to increase the cost of the solar cell.

Therefore, it is an objective of the present invention to provide a conductive paste and a pro- cess for producing electrodes on a semiconductor substrate comprising at least one p-type emitter on a n-type base , which does not have the disadvantages of the known pastes and processes and which particularly are able to form low contact resistance electrodes to p-type silicon layers without the addition of aluminum, aluminum alloys or Mo, Tc, Ru, Pd, W, Re, Os, Ir and Pt.

This objective is achieved by a conductive paste for forming an electrode on a p-type emitter on an n-type base semiconductor substrate, the paste comprising

30 to 99.5% by weight of an aluminum-metal-free electrically conductive metal,

0.1 to 15 % by weight of a lead-tellurium-oxide, the lead-tellurium-oxide comprising 5 to 95 mol% of lead oxide wherein the molar ratio of lead to tellurium in the lead-tellurium-oxide is between 95 to 5 and 5 to 95, or of a tungsten-tellurium-oxide, the tungsten-tellurium- oxide comprising 5 to 95 mol% of tungsten oxide wherein the molar ratio of tungsten to tellurium in the tungsten-tellurium-oxide is between 95 to 5 and 5 to 95, or of a molybdenum-tellurium-oxide, the molybdenum-tellurium-oxide comprising 5 to 95 mol% of mo- lybdenum oxide wherein the molar ratio of molybdenum to tellurium in the molybdenum- tellurium-oxide is between 95 to 5 and 5 to 95, or of a zinc-tellurium-oxide, the zinc- tellurium-oxide comprising 5 to 95 mol% of zinc oxide wherein the molar ratio of zinc to tellurium in the zinc-tellurium-oxide is between 95 to 5 and 5 to 95, or any combination thereof

- and a solvent,

wherein the paste is free from particulate additions of Mo, Tc, Ru, Rh, Pd, W, Re, Os, Ir, and Pt.

Surprisingly it has been found that a conductive paste which comprises a lead-tellurium-oxide, a tungsten-tellurium-oxide, a molybdenum-tellurium-oxide or a zinc-tellurium-oxide or a combina- tion thereof can be used for forming electrodes on a p-type emitter on a n-type base semiconductor substrate without adding aluminum.

In the context of the present invention, aluminum-metal-free means that the amount of elemental or metallic aluminum in the paste is less than 0.2 wt%, preferably less than 0.1 wt% and that particularly preferably no elemental or metallic aluminum is comprised in the paste. Traces of aluminum may result for example from contamination with aluminum or impurities. In the following, the p-type emitter on an n-type base is also referred to as p-type area.

Te02 containing glass frits can be effective for use in pastes used for making electrodes to silicon solar cells. Te02 containing glasses typically have low glass transition temperature (Tg) and very low viscosities both of which are generally desirable. Lead tellurite glasses are particularly desired systems for their inherently low Tg, low molten viscosity and good wetting properties. Lead oxide and tellurium oxide are further advantageous because they are capable of participating in redox reactions with the various layers present on the silicon wafer for example passivation layers and antireflection coatings. Lead oxide as well as tungsten oxide, molyb- denum oxide and zinc oxide are capable of modifying tellurium oxide glasses in respect to glass viscosity and stability.

The electrically conductive metal which is used for the conductive paste can be selected from the group consisting of silver, copper, and nickel and mixtures thereof. Particularly preferred, the electrically conductive metal is silver either in the form of a fine silver powder or a silver alloy. In further embodiments, some of the silver also can be added as silver oxide (Ag20), as a silver salt, e.g. silver chloride (AgCI), silver fluoride (AgF), silver nitrate (AgNOs), silver acetate (AgC2H302), or silver carbonate (Ag2COs). Silver containing resonates or silver containing metallo-organic compounds can also be effectively introduced to the paste.

The particle size of the conductive metal powder should be consistent with the method of deposition and the desired size of the printed features. Preferably, the electrically conductive metal is in the form of particles having a particle size in the range from 20 nm to 20 μηη. For modern solar cell fabrication the particle size of the electrically conductive metal powders should be finer than 10 μηη in average diameter. The powder diameter should more preferably be less than 5 μηη and most preferably less than 1 μηη.

In one embodiment of the invention, the electrically conductive metal is in the form of particles having a bimodal distribution or of a mixture having different particle size distributions. In this embodiment it is particularly preferred when a first powder of the electrically conductive metal has a D50 particle size in the range from 0.25 to 2 μηη and a second powder of the electrically conductive metal has a D50 particle size in the range from 2 to 5 μηη. The use of a bimodal distribution can improve particle packing and enhance the conductive metal densification during the firing process.

Tap density is a commonly used parameter to describe the particle packing of a powder distribution. Tap density is known to those skilled in the art. Powders of electrically conductive metals used in this invention preferably have a tap density of greater than 2.5 g/cm ³ , more preferable tap density of greater than 3.5 g/cm ³ and most preferable tap density of greater than 4.5 g/cm ³ .

The particles of the electrically conductive metal which form the powder can have any shape. The typical particle shape of the powder of the electrically conductive metal used in this inven- tion can be spherical, platy, flake-like or irregular (amorphous). Blends of particles having different morphologies can be utilized to achieve the final desired characteristics.

Preferably, the conductive paste additionally comprises one or more polymers, a solution of polymers and at least one additive, the additive being selected from the group consisting of dis- persants, stabilizers, surfactants, and thickeners. The addition of a polymer or a solution of polymers has the advantage that the paste rheology can be influenced to achieve a viscosity which is useful to apply the paste on the semiconductor substrate without spreading or running on the substrate such that very narrow features can be printed and the resulting electrodes formed by the paste have well-defined edges. There are multiple advantages for printing narrow features. First, the electrical current obtained from the cell is substantially increased by printing narrow lines. Second, well defined, narrow electrodes will minimize shadowing on the solar cell, thus increasing the amount of cell area available to capture incident sunlight. Further, the polymer or polymer solution improves the adhesion and green-strength of the paste after being deposited on the semiconductor substrate and dried. Suitable polymers which can be added either as dispersion or as a solution are for example ethyl cellulose, methyl cellulose, nitrocellulose, ethyl hydroxyl ethyl cellulose, carboxymethyl cellulose, hydroxypropyl cellulose and other cellulose derivatives. Other examples include resins such as acrylic acid esters, methacrylic acid esters, polyvinyl alcohols, polyvinyl butyrals, polyesters and polyketones.

When the polymers are added as a solution, it is preferred, that the polymer can be solved in the solvent which is used to form the paste. Typical solvents are organic solvents or water.

When an organic solvent is used, the solvent can be selected from alcohols (including glycols), esters of such alcohols such as the acetates, propionates and phthalates, for instance dibutyl phthalate, terpenes such as pine oil, terpineol and the like. More specific the organic solvent is selected from diethylene glycol monobutyl ether, terpineol, isopropanol, tridecanol, and 2,2,4- trimethyl-1 ,3-pentanediol monoisobutyrate. Some embodiments utilize solvents that also contain volatile liquids to promote fast setting after application to the substrate.

In one specific embodiment, solutions of resins such as polymethacrylates of lower alcohols are used, while in a more specific embodiment, ethyl cellulose dissolved in solvents such as pine oil and monobutyl ether of diethylene glycol is used. The ratio of solvent to solids in the conductive paste according to one or more embodiments can vary considerably and is determined by the final desired formulation viscosity which, in turn, is determined by the printing requirements of the system. In one or more embodiments, the conductive ink can contain about 30 to about 95% by weight solids and about 5 to about 70% by weight solvent. One or more embodiments of the conductive pastes may additionally comprise further additives known in the art, such as rheology modifiers, adhesion enhancers, sintering inhibitors, green- strength modifiers, surfactants and the like. The conductive electrode paste preferably contains one or more glass frit powders. Many methods are known in the art for producing glass frits. For example, suitable raw materials such as oxides, nitrates, and/or carbonates can be mixed together and melted at suitably high temperatures to produce a fluid molten mass. This mass can then be quenched directly into water or alternatively quenched onto water cooled counter rotating steel rolls. The resulting coarse glass frit can be ball milled, bead milled or vibratory milled to the desired particle size distribution. Alternatively one can produce glass frit powder by flame synthesis, plasma synthesis and sol-gel methods. The method of producing the glass frit powder is irrelevant to this invention provided the method results in a glass having the desired composition and particle size distribution.

The particle size of the glass frit powder should be consistent with the method of deposition and the desired size of the printed features. For modern solar cell fabrication glass frit powders should be finer than 10 μηη in average diameter. The glass frit powder diameter should more preferably be less than 5 μηη and most preferably less than 1 μηη.

If the conductive paste comprises lead-tellurium oxide, suitable metal oxides which can be incorporated into the lead-tellurium oxide are selected from the group consisting of T1O2, L12O, B2O3, PbF, S1O2, Na ₂ 0, K ₂ 0, Cs ₂ 0, AI2O3, MgO, CaO, SrO, BaO, V ₂ 0 ₅ , ZrO, MoO, MnO, Mn ₂ 0 ₃ , Mn0 ₂ , Ag ₂ 0, ZnO, Ga ₂ 0 ₃ , Ge0 ₂ , ln ₂ 0 ₃ , Sn0 ₂ , Sb ₂ 0 ₃ , Bi ₂ 0 ₃ , BiF ₃ , P ₂ 0 ₅ , CuO, NiO, Cr20 ₃ , Fe20 ₃ , CoO, Co20 ₃ , and Ce02. These metal oxides can be added either alone or in any mixture of two or more of them. These metal oxides are typically incorporated into the lead- tellurium oxide by high temperature melting, though the method is not restrictive. The amount of these oxides in regard to the total amount of metal oxides is in the range from 0.01 % to 50 mol% by, preferably in the range from 0.1 % to 40 mol%.

If the conductive paste comprises tungsten-tellurium oxide, suitable metal oxides which can be incorporated into the tungsten-tellurium oxide are selected from the group consisting of T1O2, U2O, B ₂ 0 ₃ , PbF, S1O2, Na ₂ 0, K ₂ 0, Cs ₂ 0, Al ₂ 0 ₃ , MgO, CaO, SrO, BaO, V ₂ 0 ₅ , ZrO, MoO, MnO, Mn ₂ 0 ₃ , Mn0 ₂ , Ag ₂ 0, ZnO, Ga ₂ 0 ₃ , Ge0 ₂ , ln ₂ 0 ₃ , Sn0 ₂ , Sb ₂ 0 ₃ , Bi ₂ 0 ₃ , BiF ₃ , P ₂ 0 ₅ , CuO, NiO, Cr20 ₃ , Fe20 ₃ , CoO, Co20 ₃ , and Ce02. These metal oxides can be added either alone or in any mixture of two or more of them. These metal oxides are typically incorporated into the tungsten- tellurium oxide by high temperature melting, though the method is not restrictive. The amount of these oxides in regard to the total amount of metal oxides is in the range from 0.01 % to 50 mol% by, preferably in the range from 0.1 % to 40 mol%.

If the conductive paste comprises molybdenum-tellurium oxide, suitable metal oxides which can be incorporated into the molybdenum-tellurium oxide are selected from the group consisting of T1O2, U2O, B2O3, PbF, S1O2, Na ₂ 0, K ₂ 0, Cs ₂ 0, AI2O3, MgO, CaO, SrO, BaO, V ₂ 0 ₅ , ZrO, MoO, MnO, Mn ₂ 0 ₃ , Mn0 ₂ , Ag ₂ 0, ZnO, Ga ₂ 0 ₃ , Ge0 ₂ , ln ₂ 0 ₃ , Sn0 ₂ , Sb ₂ 0 ₃ , Bi ₂ 0 ₃ , BiF ₃ , P ₂ 0 ₅ , CuO, NiO, Cr20 ₃ , Fe20 ₃ , CoO, Co20 ₃ , and Ce02. These metal oxides can be added either alone or in any mixture of two or more of them. These metal oxides are typically incorporated into the mo- lybdenum-tellurium oxide by high temperature melting, though the method is not restrictive. The amount of these oxides in regard to the total amount of metal oxides is in the range from 0.01 % to 50 mol% by, preferably in the range from 0.1 % to 40 mol%.

If the conductive paste comprises zinc-tellurium oxide, suitable metal oxides which can be in- corporated into the zinc-tellurium oxide are selected from the group consisting of T1O2, L12O, B ₂ 0 ₃ , PbF, S1O2, Na ₂ 0, K ₂ 0, Cs ₂ 0, Al ₂ 0 ₃ , MgO, CaO, SrO, BaO, V ₂ 0 ₅ , ZrO, MoO, MnO, Mn ₂ 0 ₃ , Mn0 ₂ , Ag ₂ 0, Ga ₂ 0 ₃ , Ge0 ₂ , ln ₂ 0 ₃ , Sn0 ₂ , Sb ₂ 0 ₃ , Bi ₂ 0 ₃ , BiF ₃ , P ₂ 0 ₅ , CuO, NiO, Cr ₂ 0 ₃ , Fe20 ₃ , CoO, Co20 ₃ , and Ce02. These metal oxides can be added either alone or in any mixture of two or more of them. These metal oxides are typically incorporated into the zinc-tellurium oxide by high temperature melting, though the method is not restrictive. The amount of these oxides in regard to the total amount of metal oxides is in the range from 0.01 % to 50 mol% by, preferably in the range from 0.1 % to 40 mol%.

If the conductive paste comprises lead-tellurium-oxide, the lead-tellurium oxide which is com- prised in the conductive paste may further comprise one or more elements selected from the group consisting of: Si, Sn, Li, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, In, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, Bi, F, Zr, Mn, P, Cu, Ce, and Nb. In this case, a mixed oxide comprising lead, tellurium and at least one further metal of the aforementioned is formed. In the mixed oxide, the amount of lead is in a range from 5 to 95 mol% , preferably in a range from 20 to 80 mol% by weight, the amount of tellurium is in a range from 5 to 95 mol %, preferably in a range from 15 to 80 mol % and the amount of the further metals is in a range from 0 to 50 mol % by weight, preferably in a range from 5 to 40 mol%. If the conductive paste comprises tungsten-tellurium-oxide, the tungsten-tellurium oxide which is comprised in the conductive paste may further comprise one or more elements selected from the group consisting of: Si, Sn, Li, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, In, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, Bi, F, Zr, Mn, P, Cu, Ce, and Nb. In this case, a mixed oxide comprising tungsten, tellurium and at least one further metal of the aforementioned is formed. In the mixed oxide, the amount of tungsten is in a range from 5 to 95 mol% , preferably in a range from 20 to 80 mol% by weight, the amount of tellurium is in a range from 5 to 95 mol %, preferably in a range from 15 to 80 mol % and the amount of the further metals is in a range from 0 to 50 mol % by weight, preferably in a range from 5 to 40 mol%.

If the conductive paste comprises molybdenum-tellurium-oxide, the molybdenum-tellurium oxide which is comprised in the conductive paste may further comprise one or more elements select- ed from the group consisting of: Si, Sn, Li, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni, Zn, Ca, Mg, Sr, Ba, Se, Mo, In, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, Bi, F, Zr, Mn, P, Cu, Ce, and Nb. In this case, a mixed oxide comprising molybdenum, tellurium and at least one further metal of the aforementioned is formed. In the mixed oxide, the amount of molybdenum is in a range from 5 to 95 mol% , preferably in a range from 20 to 80 mol% by weight, the amount of tellurium is in a range from 5 to 95 mol %, preferably in a range from 15 to 80 mol % and the amount of the further metals is in a range from 0 to 50 mol % by weight, preferably in a range from 5 to 40 mol%. If the conductive paste comprises zinc-tellurium-oxide, the zinc-tellurium oxide which is comprised in the conductive paste may further comprise one or more elements selected from the group consisting of: Si, Sn, Li, Ti, Ag, Na, K, Rb, Cs, Ge, Ga, In, Ni, Ca, Mg, Sr, Ba, Se, Mo, In, Y, As, La, Nd, Co, Pr, Gd, Sm, Dy, Eu, Ho, Yb, Lu, Bi, Ta, V, Fe, Hf, Cr, Cd, Sb, Bi, F, Zr, Mn, P, Cu, Ce, and Nb. In this case, a mixed oxide comprising zinc, tellurium and at least one further metal of the aforementioned is formed. In the mixed oxide, the amount of zinc is in a range from 5 to 95 mol% , preferably in a range from 20 to 80 mol% by weight, the amount of tellurium is in a range from 5 to 95 mol %, preferably in a range from 15 to 80 mol % and the amount of the further metals is in a range from 0 to 50 mol % by weight, preferably in a range from 5 to 40 mol%.

In one embodiment of the invention, the paste further comprises an addition of fluorates, borates, ruthenates, tungstates and/or vanadates.

When additional metallic additives are used, these can be introduced either in the form of an oxide or in the alternative in the form of metallo-organic compounds. In this case generally the metallo-organic compounds form a metal oxide or a pure metal during firing. Suitable metallo- organic compounds are selected from compounds containing metal atoms, including metal car- boxylate such as neodecanoates, acetates and propionates, salicylate, metal alkoxide and metal complexes. Metallo-organics can also contain any aromatic or aliphatic groups and are sometimes referred to as metal resinates when the organic portion consists of groups derived from resins or other natural products. Other suitable metallo-organic precursors include metal mercaptides. The metallo-organic components used in one or more embodiments can have more than one metal atom. If the paste comprises lead-tellurium-oxide, the paste can comprise either only lead-tellurium oxide as glass frit or a mixed oxide in which the lead-tellurium oxide comprises at least one further metal or it can comprise a second glass frit being different from the lead-tellurium-oxide. The lead- tellurium oxide and subsequent oxide modified versions can be crystalline, partially crystalline or amorphous. The second glass frit being selected preferably from the aforemen- tioned oxides either alone or in any suitable mixture. If the paste comprises tungsten-tellurium-oxide, the paste can comprise either only tungsten- tellurium oxide as glass frit or a mixed oxide in which the tungsten-tellurium oxide comprises at least one further metal or it can comprise a second glass frit being different from the tungsten- tellurium-oxide. The tungsten- tellurium oxide and subsequent oxide modified versions can be crystalline, partially crystalline or amorphous. The second glass frit being selected preferably from the aforementioned oxides either alone or in any suitable mixture.

If the paste comprises molybdenum-tellurium-oxide, the paste can comprise either only molybdenum-tellurium oxide as glass frit or a mixed oxide in which the molybdenum-tellurium oxide comprises at least one further metal or it can comprise a second glass frit being different from the molybdenum-tellurium-oxide. The molybdenum- tellurium oxide and subsequent oxide modified versions can be crystalline, partially crystalline or amorphous. The second glass frit being selected preferably from the aforementioned oxides either alone or in any suitable mixture. If the paste comprises zinc-tellurium-oxide, the paste can comprise either only zinc-tellurium oxide as glass frit or a mixed oxide in which the zinc-tellurium oxide comprises at least one further metal or it can comprise a second glass frit being different from the zinc-tellurium-oxide. The zinc- tellurium oxide and subsequent oxide modified versions can be crystalline, partially crystalline or amorphous. The second glass frit being selected preferably from the aforemen- tioned oxides either alone or in any suitable mixture.

The method used to produce the conductive paste is not limited provided that the resulting paste is consistent in rheology and comprises adequately dispersed powders. Commonly used processes well known to those skilled in the art can be used for preparing such pastes. For ex- ample, planetary mixers, vacuum mixers, three-roll mills, and colloid mills can produce suitable dispersions for said invention.

The invention further relates to a process for producing electrodes on a semiconductor substrate comprising at least one p-type area, the process comprising:

(i) applying of the conductive paste to form a pattern on an area over a p-type area of the semiconductor substrate;

(ii) firing the semiconductor substrate with the conductive paste applied thereon to form an electrode in electrical contact with the semiconductor substrate.

Applying the conductive paste to form a pattern on the area over a p-type area of the semiconductor substrate means that the conductive paste is either applied directly onto the p-type area of the semiconductor substrate or preferably on one or more layers, for example onto an anti- reflective layer, which is over the p-type area. Printing the conductive paste onto a dielectric stack comprising an antireflective layer over a passivation layer over a p-type area is also illustrative of this. The semiconductor substrate preferably is made of a silicon-based semiconductor material.

P-type surfaces (emitters) generally consist of boron, aluminum or gallium doped silicon and are used on n-type silicon bases. A variety of processes can be used for preparing such substrates. The uniqueness of this invention is not dependent on the process used to prepare the wafer. For purely illustrative purposes, one method for producing n-type wafers with a p-type emitter layer is described as follows. Wafers are cut from n-type silicon ingot grown by the Czochralski method. These wafers were in turn chemically textured to produce surfaces populated by ran- dom pyramids. After cleaning, the textured wafers are introduced to a diffusion furnace with BBr3 gas at high temperature for a period of time. The resulting p-type emitter had a sheet resistance of approximately 70 ohms per square. The surface contained a borosilicate glass residue that was further etched away. The rear side was ion implanted with phosphorus. Afterwards the wafers were thermally annealed to reduce damage from the ion implantation and to grow a thin (<1 nm) SiO _x passivation layer. This was followed by deposition of a 70 nm thick hydrogen- ated silicon nitride layer to function as an antireflective coating. Subsequently the wafer is ready for screen printing the electrode pastes to each surface.

To improve the passivation of the solar cell, it is preferred that at least one insulating layer (which also is referred to as passivation layer) is deposited onto the surface of the semiconductor substrate and the conductive paste is applied on the insulating layer. The insulating layer for example comprises an aluminum oxide, silicon carbide, a silicon nitride, or a silicon oxide.

In a preferred embodiment, the semiconductor substrate is a solar cell. In this case, generally an antireflection layer is formed on a surface of the semiconductor substrate or on an insulating layer which is coated on a surface of the semiconductor substrate to minimize reflected light and promote light absorption. The antireflection layer thereby additionally can act as a passivation layer. The anti-reflective coating according to some embodiments comprises titanium oxide, silicon nitride or other coatings known in the art.

One or more embodiments employ crystalline silicon such can be either amorphous, monocrys- talline or multicrystalline as semiconductor substrate. Passivation layers and antireflective coatings may be applied to the silicon substrates, and such coatings or layers can be produced according to known processes, such as thermal processes, chemical vapor deposition, plasma vapor deposition, atomic layer deposition and the like. The anti-reflective coatings can also be applied using chemical vapor deposition techniques. In some embodiments, plasma enhanced chemical vapor deposition techniques are used to dispose the anti-reflective coating on the substrate. The conductive paste is printed onto semiconductor substrate or if one or more of an insulating layer, a passivation layer or an antireflection layer is formed on the semiconductor substrate, the conductive paste is printed onto the uppermost layer. For printing the conductive paste, any known printing process such as ink jet printing, laser printing or screen printing can be used.

After printing, the semiconductor substrate with the conductive paste printed thereon is fired. Firing generally is carried out at temperatures between 600 and 900°C for a time between about 30 seconds to several minutes. Generally, the maximum time for firing is 5 minutes.

The firing can be done for example in a belt furnace. Usually, the firing is carried out in an oxygen-comprising atmosphere or an oxygen-free atmosphere. A typical oxygen-comprising at- mosphere is air. When an oxygen-free atmosphere is used, the atmosphere may be a reducing atmosphere and comprising for example hydrogen. Further it is also possible to use an oxygen- free atmosphere which is inert. Such an atmosphere may be for example nitrogen, carbon dioxide or a noble gas as for example helium or argon. In one embodiment of the invention, the semiconductor substrate comprises n-type areas and p-type areas which are adjacent to one another. In this embodiment, it is possible to apply the conductive paste onto the p-type areas and the n-type areas to form electrodes. In contrast to systems known in the art, this has the advantage, that only one conductive paste is used to print the electrodes. In contrast, present processes use different pastes for the n-type areas and the p-type areas to produce the electrodes onto the semiconductor substrate. In such cases an intermediate drying step is often needed followed by careful realignment of the wafer to allow precise location of the second print. One advantage of this invention is the avoidance of the aforementioned intermediate steps. Finally, the invention also relates to a device comprising a semiconductor substrate comprising at least one p-type area wherein one or more insulating layers are deposited on a surface of the p-type area of the semiconductor device and electrodes being formed with a paste according as described above. In one embodiment, the substrate comprises n-type areas and p-type areas, with at least one insulating layer being deposited onto the n-type areas and p-type areas and the electrodes connecting n-type areas and p-type areas are formed by the paste as described above.

The semiconductor substrate can also be used to make bifacial solar cells, which means that the wafer can be oriented in a way to collect sunlight on both surfaces. For example, a bifacial module, comprising bifacial cells can be made by using transparent materials on both sides of the module. Thus a bifacial module can generate substantially more powder than a standard monofacial module. When the semiconductor substrate is bifacial, in one embodiment one side of the semiconductor substrate is n-type and the opposite side is p-type. In this case electrodes are formed on the n-type side as well as on the p-type side. By using the inventive conductive paste it is possible to print the electrodes on both sides of the semiconductor substrate using the same conductive paste. This allows easier production of the semiconductor substrate with electrodes thereon, particularly of the solar cells.

Reference throughout this specification to "one embodiment", "certain embodiments", "one or more embodiments" or "an embodiment" means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as "in one or more embodiments", "in certain embodiments", "in one embodiment" or "in an embodiment" in various places throughout this specification are not necessarily referring to the same embodiment of the inven- tion. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.

Although the invention herein has been described with reference to particular embodiments, it is to be understood that these embodiments are merely illustrative of the principles and applica- tions of the present invention. It will be apparent to those skilled in the art that various modifications and variations can be made to the method and apparatus of the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention include modifications and variations that are within the scope of the appended claims and their equivalents.

Examples

A solar cell has been produced by printing a conductive paste comprising 86 wt% silver powder, 3 wt% lead-tellurium oxide glass A and 1 1 wt% organic medium on to an 70 nm thick amorphous hydrogenated silicon nitride anti reflective layer which was on top of a 2nm SiOx passiv- ated layer on top of a textured monocrystalline silicon wafer having a 70 ohm per square p-type emitter and a n-type base. The lead-tellurium oxide glass had the composition as indicated below:

The cell was also printed on the reverse side with a conventional silver paste and was fired in air using a belt furnace. The firing cycle was less than 120 seconds and a peak temperature of 780°C was achieved. For comparative purposes a commercially available silver -aluminum paste was fired on an identical cell and fired under the same conditions. The following results were achieved, normalized to the aluminum containing commercially available reference paste: Fill Fac¬

Glass Efficiency tor

Commercially

available

Ag/AI paste Unknown 100.0% 100.0% 100.0%

Paste A A 99.8% 100.1 % 100.5% The "fill factor" and "efficiency" refer to measurements of the performance of a solar cell. The term "fill factor" is defined as the ratio of the maximum power (V _m ^■ Lp) divided by the product of the short-circuit current (l _sc ) and open-circuit voltage (V _oc ) of a solar cell. The open circuit voltage (Voc) is the maximum voltage obtainable at the load under open-circuit conditions. The short circuit current (l _sc ) is the maximum current through the load under short-circuit conditions. The fill factor (FF), is thus defined as (V _m ^■ lmp)/(V _0C ^■ lsc), where l _m and V _m represent the electrical current and voltage at the maximum power point. The cell efficiency, η, is given by the equation η = (lsc ^■ Voc ^■ FF)/Pin where l _sc equals the short circuit current, V _oc equals the open circuit voltage, FF equals fill factor and Ρ,η is the power of the incident solar radiation.