Patent Application
20110315210
Embodiments of the invention relate to a silicon semiconductor device, and a conductive thick film composition containing glass frit for use in a solar cell device.
A conventional solar cell structure with a p-type base has a negative electrode that may be on the front-side (also termed sun-side or illuminated side) of the cell and a positive electrode that may be on the opposite side. Radiation of an appropriate wavelength falling on a p-n junction of a semiconductor body serves as a source of external energy to generate hole-electron pairs in that body. Because of the potential difference which exists at a p-n junction, holes and electrons move across the junction in opposite directions and thereby give rise to flow of an electric current that is capable of delivering power to an external circuit. Most solar cells are in the form of a silicon wafer that has been metalized, i.e., provided with metal contacts that are electrically conductive.
There is a need for compositions, structures (for example, semiconductor, solar cell or photodiode structures), and semiconductor devices (for example, semiconductor, solar cell or photodiode devices) which have improved electrical performance, and methods of making.
An embodiment of the invention relates to composition including: (a) one or more conductive materials; (b) one or more glass frits, wherein at least one of the glass frits comprises, based on the wt % of the glass composition: SiO2 5-20 wt %, Al2O3 1-11 wt %, PbO 68-87 wt %, ZrO2 0-2.5 wt %, and P2O5 2-6 wt % and (c) organic vehicle.
A further embodiment relates to a method of manufacturing a semiconductor device including the steps of: (a) providing a semiconductor substrate, one or more insulating films, and the thick film composition described herein; (b) applying the insulating film to the semiconductor substrate; (c) applying the thick film composition to the insulating film on the semiconductor substrate, and (d) firing the semiconductor, insulating film and thick film composition. In an aspect, the insulating film may include one or more components selected from: alumina, titanium oxide, silicon nitride, SiNx:H, silicon oxide, and silicon oxide/titanium oxide.
A further embodiment relates to a semiconductor device made by the methods described herein. An aspect relates to a semiconductor device including an electrode, wherein the electrode, prior to firing, includes the composition described herein. An embodiment relates to a solar cell including the semiconductor device.
An embodiment relates to a semiconductor device including a semiconductor substrate, an insulating film, and a front-side electrode, wherein the front-side electrode comprises one or more components selected from the group consisting of zinc-silicate, willemite, and bismuth silicates.
Reference numerals shown in
As used herein, “thick film composition” refers to a composition which, upon firing on a substrate, has a thickness of 1 to 100 microns. The thick film compositions contain a conductive material, a glass composition, and organic vehicle. The thick film composition may include additional components. As used herein, the additional components are termed “additives”.
The compositions described herein include one or more electrically functional materials and one or more glass frits dispersed in an organic medium. These compositions may be thick film compositions. The compositions may also include one or more additive(s). Exemplary additives may include metals, metal oxides or any compounds that can generate these metal oxides during firing.
In an embodiment, the electrically functional powders may be conductive powders. In an embodiment, the composition(s), for example conductive compositions, may be used in a semiconductor device. In an aspect of this embodiment, the semiconductor device may be a solar cell or a photodiode. In a further aspect of this embodiment, the semiconductor device may be one of a broad range of semiconductor devices. In an embodiment, the semiconductor device may be a solar cell.
In an embodiment, the thick film compositions described herein may be used in a solar cell. In an aspect of this embodiment, the solar cell efficiency may be greater than 70% of the reference solar cell. In a further embodiment, the solar cell efficiency may be greater than 80% of the reference solar cell. the solar cell efficiency may be greater than 90 of the reference solar cell.
An aspect of the invention relates to glass frit compositions. In an embodiment, glass frit compositions (also termed glass compositions) are listed in Table I below.
Glass compositions, also termed glass frits, are described herein as including percentages of certain components (also termed the elemental constituency). Specifically, the percentages are the percentages of the components used in the starting material that was subsequently processed as described herein to form a glass composition. Such nomenclature is conventional to one of skill in the art. In other words, the composition contains certain components, and the percentages of those components are expressed as a percentage of the corresponding oxide form. A certain portion of volatile species may be released during the process of making the glass. An example of a volatile species is oxygen.
If starting with a fired glass, the percentages of starting components described herein (elemental constituency) may be calculated using methods known to one of skill in the art including, but not limited to: Inductively Coupled Plasma-Emission Spectroscopy (ICPES), Inductively Coupled Plasma-Atomic Emission Spectroscopy (ICP-AES), and the like. In addition, the following exemplary techniques may be used: X-Ray Fluorescence spectroscopy (XRF); Nuclear Magnetic Resonance spectroscopy (NMR); Electron Paramagnetic Resonance spectroscopy (EPR); Mössbauer spectroscopy; Electron microprobe Energy Dispersive Spectroscopy (EDS); Electron microprobe Wavelength Dispersive Spectroscopy (WDS); Cathodoluminescence (CL).
The glass compositions described herein, including those listed in Table I, are not limiting; minor substitutions of additional ingredients may be made and not substantially change the desired properties of the glass composition. For example, substitutions of glass formers such as P2O5 0-3, GeO2 0-3, V2O5 0-3 in weight % may be used either individually or in combination to achieve similar performance. For example, one or more intermediate oxides, such as TiO2, Ta2O5, Nb2O5, ZrO2, CeO2, and SnO2 may be substituted for other intermediate oxides (i.e., Al2O3, CeO2, SnO2) present in a glass composition.
An aspect of the current invention relates to glass frit compositions including one or more fluorine-containing components, including but not limited to: salts of fluorine, fluorides, metal oxyfluoride compounds, and the like. Such fluorine-containing components include, but are not limited to PbF2, BiF3, AlF3, NaF, LiF, KF, CsF, ZrF4, TiF4 and/or ZnF2. In an embodiment, elemental fluorine maybe substituted for the oxygen anion up to 6 weight % in the glass composition.
An exemplary method for producing the glass frits described herein is by conventional glass making techniques. Ingredients are weighed then mixed in the desired proportions and heated in a furnace to form a melt in platinum alloy crucibles. Oxides may be used as raw materials as well as fluoride or oxyfluoride salts. Alternatively, salts, such as nitrate, nitrites, carbonate, fluorides or hydrates, which decompose into oxide or oxyfluorides at temperatures below the glass melting temperature, can be used as raw materials. As well known in the art, heating is conducted to a peak temperature (800-1400° C.) and for a time such that the melt becomes entirely liquid, homogeneous, and free of any residual decomposition products of the raw materials. The molten glass is then quenched between counter rotating stainless steel rollers to form a 10-15 mil thick platelet of glass. The resulting glass platelet is then milled to form a powder with its 50% volume distribution set between to a desired target (e.g. 0.8-1.5 μm). Alternative synthesis techniques may be used such as but not limited to water quenching, sol-gel, spray pyrolysis, or others appropriate for making powder forms of glass.
The glass compositions used herein, in weight percent total glass composition, are shown in Table 1. Unless stated otherwise, as used herein, wt % means wt % of glass composition only. In another embodiment, glass frits compositions described herein may include one or more of SiO2 5-20 wt %, Al2O3 1-11 wt %, PbO 68-87 wt %, ZrO2 0-2.5 wt %, and P2O5 2-6 wt %.
In another embodiment, glass frits compositions described herein may include one or more of SiO2 6-14 wt %, Al2O3 3-11 wt %, PbO 68-83 wt %, ZrO2 1-2.5 wt %, and P2O5 3-5.5 wt %.
In another embodiment, glass frits compositions described herein may include one or more of SiO2 8-14 wt %, Al2O3 5-11 wt %, PbO 70-80 wt %, ZrO2 1-2.5 wt %, and P2O5 3-5.5 wt %.
In a further embodiment, glass frits compositions described herein may include one or more SiO2 10-14 wt %, Al2O3 8-11 wt %, PbO 70-75 wt %, ZrO2 1-2.5 wt %, and P2O5 3-4.5 wt %.
In a further embodiment, the glass frit composition(s) herein may include one or more of a third set of components: CeO2, SnO2, Ga2O3, In2O3, NiO, MoO3, WO3, Y2O3, La2O3, Nd2O3, FeO, HfO2, Cr2O3, CdO, Nb2O5, Ag2O, Sb2O3, alkaline earth oxides, alkali metal oxides and alkali metal halides (e.g. NaCl, KBr, NaI). Such additives are acceptable in the range of 0-3 wt %. In an embodiment, ZnO maybe substituted for lead bearing compounds in the glass composition up to 10 weight %.
The choice of raw materials could unintentionally include impurities that may be incorporated into the glass during processing. For example, the impurities may be present in the range of hundreds to thousands ppm.
The presence of the impurities would not alter the properties of the glass, the thick film composition, or the fired device. For example, a solar cell containing the thick film composition may have the efficiency described herein, even if the thick film composition includes impurities.
In a further aspect of this embodiment, thick film composition may include electrically functional powders and glass-ceramic frits dispersed in an organic medium. In an embodiment, these thick film conductor composition(s) may be used in a semiconductor device. In an aspect of this embodiment, the semiconductor device may be a solar cell or a photodiode.
The amount of glass frit in the total composition is in the range of 3-7 wt % of the total composition. In one embodiment, the glass composition is present in the amount of 4-6 wt % of the total composition. In a further embodiment, the glass composition is present in the range of 5-6 wt % of the total composition.
In an embodiment, the thick film composition may include a functional phase that imparts appropriate electrically functional properties to the composition. In an embodiment, the electrically functional powder may be a conductive powder. In an embodiment the electrically functional phase may include conductive materials (also termed conductive particles, herein). The conductive particles may include conductive powders, conductive flakes, or a mixture thereof, for example.
In an embodiment, the conductive particles may include Ag. In a further embodiment, the conductive particles may include silver (Ag) and aluminum (Al). In a further embodiment, the conductive particles may, for example, include one or more of the following: Cu, Au, Ag, Pd, Pt, Al, Ag—Pd, Pt—Au, etc. In an embodiment, the conductive particles may include one or more of the following: (1) Al, Cu, Au, Ag, Pd and Pt; (2) alloy of Al, Cu, Au, Ag, Pd and Pt; and (3) mixtures thereof.
In an embodiment, the functional phase of the composition may be coated or uncoated silver particles which are electrically conductive. In an embodiment in which the silver particles are coated, they are at least partially coated with a surfactant. In an embodiment, the surfactant may include one or more of the following non-limiting surfactants: stearic acid, palmitic acid, a salt of stearate, a salt of palmitate, lauric acid, palmitic acid, oleic acid, stearic acid, capric acid, myristic acid and linoleic acid, and mixtures thereof. The counter ion may be, but is not limited to, hydrogen, ammonium, sodium, potassium and mixtures thereof.
The particle size of the silver is not subject to any particular limitation. In an embodiment, the average particle size may be less than 10 microns, and, in a further embodiment, no more than 5 microns. In an aspect, the average particle size may be 0.1 to 5 microns, for example.
In an embodiment, the silver may be 60 to 90 wt % of the paste composition. In a further embodiment, the silver may be 70 to 88 wt % of the paste composition. In a further embodiment, the silver may be 75 to 88 wt % of the paste composition. In a further embodiment, the silver may be 78 to 86 wt % of the paste composition.
In an embodiment, the silver may be 90 to 99 wt % of the solids in the composition (i.e., excluding the organic vehicle). In a further embodiment, the silver may be 92 to 97 wt % of the solids in the composition. In a further embodiment, the silver may be 93 to 96 wt % of the solids in the composition.
As used herein, “particle size” is intended to mean “average particle size”; “average particle size” means the 50% volume distribution size. Volume distribution size may be determined by a number of methods understood by one of skill in the art, including but not limited to LASER diffraction and dispersion method using a Microtrac particle size analyzer.
The thick film composition may include an additive. In an embodiment, the additive may be selected from one or more of the following: (a) a metal wherein said metal is selected from Zn, Pb, Bi, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, and Cr; (b) a metal oxide of one or more of the metals selected from Zn, Pb, Bi, Gd, Ce, Zr, Ti, Mn, Sn, Ru, Co, Fe, Cu, Mg, Li and Cr; (c) any compounds that can generate the metal oxides of (b) upon firing; and (d) mixtures thereof.
In an embodiment, the additive may have an average particle size in the range of 1 nanometers to 10 microns. In a further embodiment, the additive may have an average particle size of 40 nanometers to 5 microns. In a further embodiment, the additive may have an average particle size of 60 nanometers to 3 microns. In a further embodiment the additive may have an average particle size of less than 100 nm; less than 90 nm; less than 80 nm; 1 nm to less than 100 nm; 1 nm to 95 nm; 1 nm to 90 nm; 1 nm to 80 nm; 7 nm to 30 nm; 1 nm to 7 nm; 35 nm to 90 nm; 35 nm to 80 nm, 65 nm to 90 nm, 60 nm to 80 nm, and ranges in between, for example.
In an embodiment, additive may be present in the composition in the range of 0.1-4 wt % total composition. In an embodiment, the additive may be present in the range of 0.2-3 wt % total composition. In a further embodiment, the additive may be present in the range of 0.5-1 wt % total composition.
In an embodiment, the thick film compositions described herein may include organic medium. The inorganic components may be mixed with an organic medium, for example, by mechanical mixing to form pastes. A wide variety of inert viscous materials can be used as organic medium. In an embodiment, the organic medium may be one in which the inorganic components are dispersible with an adequate degree of stability. In an embodiment, the rheological properties of the medium may lend certain application properties to the composition, including: stable dispersion of solids, appropriate viscosity and thixotropy for screen printing, appropriate wettability of the substrate and the paste solids, a good drying rate, and good firing properties. In an embodiment, the organic vehicle used in the thick film composition may be a non-aqueous inert liquid. The use of various organic vehicles, which may or may not contain thickeners, stabilizers and/or other common additives, is contemplated. The organic medium may be a solution of polymer(s) in solvent(s). In an embodiment, the organic medium may also include one or more components, such as surfactants. In an embodiment, the polymer may be ethyl cellulose. Other exemplary polymers include ethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins, polymethacrylates of lower alcohols, and monobutyl ether of ethylene glycol monoacetate, or mixtures thereof. In an embodiment. the solvents useful in thick film compositions described herein include ester alcohols and terpenes such as alpha- or beta-terpineol or mixtures thereof with other solvents such as kerosene, dibutylphthalate, butyl carbitol, butyl carbitol acetate, hexylene glycol and high boiling alcohols and alcohol esters. In a further embodiment, the organic medium may include volatile liquids for promoting rapid hardening after application on the substrate.
In an embodiment, the polymer maybe present in the organic medium in the range of 8 wt. % to 11 wt % of the total composition. The thick film silver composition may be adjusted to a predetermined, screen-printable viscosity with the organic medium.
In an embodiment, the ratio of organic medium in the thick film composition to the inorganic components in the dispersion may be dependent on the method of applying the paste and the kind of organic medium used. In an embodiment, the dispersion may include a solids embodiment range 70-95 wt % of inorganic components and medium embodiment range 5-30 wt % of organic medium (vehicle) in order to obtain good wetting.
The organic medium may be removed during the drying and firing of the semiconductor device. In an aspect, the glass frit, Ag, and additives may be sintered during firing to form an electrode. The fired electrode may include components, compositions, and the like, resulting from the firing and sintering process.
In an aspect of this embodiment, the semiconductor device may be a solar cell or a photodiode.
An embodiment relates to methods of making a semiconductor device. In an embodiment, the semiconductor device may be used in a solar cell device. The semiconductor device may include a front-side electrode, wherein, prior to firing, the front-side (illuminated-side) electrode may include composition(s) described herein.
In an embodiment, the method of making a semiconductor device includes the steps of: (a) providing a semiconductor substrate; (b) applying an insulating film to the semiconductor substrate; (c) applying a composition described herein to the insulating film; and (d) firing the device.
Exemplary semiconductor substrates useful in the methods and devices described herein include, but are not limited to: single-crystal silicon, multicrystalline silicon, ribbon silicon, and the like. The semiconductor substrate may be junction bearing. The semiconductor substrate may be doped with phosphorus and boron to form a p/n junction. Methods of doping semiconductor substrates are understood by one of skill in the art.
The semiconductor substrates may vary in size (length×width) and thickness. In a non-limiting example, the thickness of the semiconductor substrate may be 50 to 500 microns; 100 to 300 microns; or 140 to 200 microns. In a non-limiting example, the length and width of the semiconductor substrate may both equally be 100 to 250 mm; 125 to 200 mm; or 125 to 156 mm.
Exemplary insulating films useful in the methods and devices described herein include, but are not limited to: silicon nitride, silicon oxide, aluminum oxide, titanium oxide, SiNx:H, hydrogenated amorphous silicon nitride, and silicon oxide/titanium oxide film. The insulating film may be formed by PECVD, CVD, and/or other techniques known to one of skill in the art. In an embodiment in which the insulating film is silicon nitride, the silicon nitride film may be formed by a plasma enhanced chemical vapor deposition (PECVD), thermal CVD process, or physical vapor deposition (PVD). In an embodiment in which the insulating film is silicon oxide, the silicon oxide film may be formed by thermal oxidation, thermal CVD, plasma CVD, or PVD. The insulating film (or layer) may also be termed the anti-reflective coating (ARC).
Compositions described herein may be applied to the ARC-coated semiconductor substrate by a variety of methods including, but not limited to, screen-printing, ink-jet, co-extrusion, syringe dispense, direct writing, and aerosol ink jet. The composition may be applied in a pattern. The composition may be applied in a predetermined shape and at a predetermined position. In an embodiment, the composition may be used to form both the conductive fingers and bus bars of the front-side electrode. In an embodiment, the width of the lines of the conductive fingers may be 20 to 200 microns; 40 to 150 microns; or 60 to 100 microns. In an embodiment, the thickness of the lines of the conductive fingers may be 5 to 50 microns; 10 to 35 microns; or 15 to 30 microns.
In a further embodiment, the composition may be used to form the conductive, Si contacting fingers.
The composition coated on the ARC-coated semiconductor substrate may be dried, for example, for 0.5 to 10 minutes, and then fired. In an embodiment, volatile solvents and organics may be removed during the drying process. In exemplary, non-limiting, firing conditions the silicon wafer substrate is heated to maximum temperature of between 600 and 900° C. for the duration of 1 second to 2 minutes. In an embodiment, the maximum silicon wafer temperature reached during firing ranges from 650 to 800 C for the duration of 1 to 10 seconds. In a further embodiment, the electrode formed from the conductive thick film composition(s) may be fired in an atmosphere composed of a mixed gas of oxygen and nitrogen. This firing process removes the organic medium and sinters the glass frit with the Ag powder in the conductive thick film composition. In a further embodiment, the electrode formed from the conductive thick film composition(s) may be fired above the organic medium removal temperature in an inert atmosphere not containing oxygen. This firing process sinters or melts base metal conductive materials such as copper in the thick film composition.
In an embodiment, during firing, the fired electrode (preferably the fingers) may react with and penetrate the insulating film, forming electrical contact with the silicon substrate.
In a further embodiment, prior to firing, other conductive and device enhancing materials are applied to the opposite type region of the semiconductor device and co-fired or sequentially fired with the compositions described herein. The opposite type region of the device may be on the opposite side of the device. The materials serve as electrical contacts, passivating layers, and solderable tabbing areas. In an embodiment, the opposite type region may be on the non-illuminated (back) side of the device. In an aspect of this embodiment, the back-side conductive material may contain aluminum. Exemplary back-side aluminum-containing compositions and methods of applying are described, for example, in US 2006/0272700, which is hereby incorporated herein by reference.
In a further aspect, the solderable tabbing material may contain aluminum and silver. Exemplary tabbing compositions containing aluminum and silver are described, for example in US 2006/0231803, which is hereby incorporated herein by reference.
In a further embodiment the materials applied to the opposite type region of the device are adjacent to the materials described herein due to the p and n region being formed side by side. Such devices place all metal contact materials on the non illuminated (back) side of the device to maximize incident light on the illuminated (front) side.
The semiconductor device may be manufactured by the following method from a structural element composed of a junction-bearing semiconductor substrate and a silicon nitride insulating film formed on a main surface thereof. The method of manufacture of a semiconductor device includes the steps of applying (such as coating and printing) onto the insulating film, in a predetermined shape and at a predetermined position, the conductive thick film composition having the ability to penetrate the insulating film, then firing so that the conductive thick film composition melts and passes through the insulating film, effecting electrical contact with the silicon substrate. The electrically conductive thick film composition is a thick-film paste composition, as described herein, which is made of a silver powder, a glass or glass powder mixture having a softening point of 300 to 600° C., dispersed in an organic vehicle and optionally, additional metal/metal oxide additive(s).
An embodiment of the invention relates to a semiconductor device manufactured from the methods described herein. Devices containing the compositions described herein may contain zinc-silicates, as described above.
An embodiment of the invention relates to a semiconductor device manufactured from the method described above.
Additional substrates, devices, methods of manufacture, and the like, which may be utilized with the thick film compositions described herein are described in US patent application publication numbers US 2006/0231801, US 2006/0231804, and US 2006/0231800, which are hereby incorporated herein by reference in their entireties.
Table I provides a list of selected exemplary glass compositions; currently the testing has provided test data for Glass #5. Additional testing of glasses of a type similar to those listed in Table I will be performed. The glass compositions listed in Table II are example compositions that may be blended glasses with glasses of Table I. Additional examples of the amounts of glass, types of silver, and sizes of silver conductive used in formulation will be obtained.
The glass frit compositions outlined in Table I will be characterized to determine density, softening point, TMA shrinkage, diaphaneity, and crystallinity.
Paste preparations, in general, were prepared using the following procedure: The appropriate amount of solvent, medium and surfactant were weighed and mixed in a planetary mixer for 15 minutes, then glass frits were added progressively and mixed for another 15 minutes. Ag was added incrementally to ensure better wetting. When well mixed, the paste was repeatedly passed through a 3-roll mill at progressively increasing pressures from 0 to 300 psi. The gap of the rolls was set to 1 mil. The degree of dispersion was measured by fineness of grind (FOG), as determined by using a fineness of grind block (sometimes known as a Hegman gauge). Roll milling continued until the 4th scratch was below 15 μm and the 50% point was below 8 μm.
The paste was formulated to a solids and viscosity target suitable for screen printing by making additions of solvent or medium to lower or raise the viscosity. Viscosity was measured using a Brookfield HBT viscometer with a utility cup and spindle #14.
Table III describes the silver pastes that were made. The amounts of each component are in wt % of the total paste composition.
The compositions were tested by printing on 3″ square acid textured multicrystalline n-type silicon wafers. The sheet resistance of the phosphorous diffused layer was approximately 65 Ohm/sq. The back side of these wafers was printed with a commercial aluminum thick film paste (PV381, DuPont). An Ekra printer with a 15″ 200 mesh stainless steel screen was used to print the aluminum, after which the parts were dried in a belt oven with a peak temperature set point of approximately 220° C.
The front electrode was printed in an H pattern consisting of two 1.5 mm wide bus bars at the edges of the wafer connected by approximately 32 fingers nominally 100 μm wide. An Ekra printer with a 15″ 325 mesh stainless steel screen with 25 μm thick emulsion was used to print the samples. The printed samples were dried in belt oven with a peak temperature set point of approximately 220° C.
After printing and drying, the parts were fired in a Centrotherm 4-zone IR furnace. The set point of the spike firing zone was between 875 and 950° C. Total profile length was around 1 minute. After firing, the performance of the parts was assessed using a Halm CellTest2 IV tester.
The solar cells built according to the method described herein were tested for conversion efficiency.
The solar cells built according to the method described herein were placed in a commercial I-V tester for measuring efficiencies (ST-1000). The Xe Arc lamp in the I-V tester simulated the sunlight with a known intensity and irradiated the front surface of the cell. The tester used a multi-point contact method to measure current (I) and voltage (V) at approximately 400 load resistance settings to determine the cell's I-V curve. Both fill factor (FF) and efficiency (Eff) were calculated from the I-V curve.
Paste efficiency and fill factor values were normalized to corresponding values obtained with cells contacted with industry standards.
Table IV provides photovoltaic conductor performance data with several formulations showing very good performance. The performance is benchmarked against an industry standard.
Table IV illustrates the electrical properties of the silver pastes. Tested pastes contained 81.6 to 85.1% silver powder.
| Number | Date | Country | |
|---|---|---|---|
| 61287820 | Dec 2009 | US |
