Patent Application
20130160835
The present invention is directed to a back-side electrode formed on silicon layer of p-type solar cell, an aluminum paste used for forming such electrode and method of forming p-type silicon solar cells.
A conventional solar cell structure with a p-type base has a negative electrode that is typically on the front-side or sun side of the cell and a positive electrode on the back-side. It is well known that 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. The potential difference that exists at a p-n junction, causes holes and electrons to 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 metallized, i.e., provided with metal contacts which are electrically conductive,
During the formation of a silicon solar cell, an aluminum paste is generally screen printed and dried on the back-side of the silicon wafer. The wafer is then fired at a temperature above the melting point of the eutectic point of aluminum and silicon to form an aluminum-silicon melt, subsequently, during the cooling phase, an epitaxially grown layer of silicon is formed that is doped with aluminum. This layer is generally called the back surface field (BSF) layer, and helps to improve the energy conversion efficiency of the so a cell.
Most electric power-generating solar cells currently used are silicon solar cells. Process flow in mass production is generally aimed at achieving maximum simplification and minimizing manufacturing costs. Electrodes in particular are made by using a method such as screen printing from a metal paste.
An example of this method of production is described below in conjunction with
Consequently, as shown in
However, a problem that has surfaced in recent years is that the performance of these solar cells declines gradually due to humidity in the air during long-term use in particular. This decline is attributed partly to a gradual reaction between humidity in the air and the aluminum in the aluminum paste. Since there is strong demand for long-term stable performance, “hot water resistance” is now commonly used as an evaluation standard for long-term humidity resistance in the silicon solar cell market. A sample is dipped for 10 minutes in specific hot water (distilled water) at 80° C. for example, and the degree of deterioration in the aluminum film due to the reaction of the aluminum film with water during that time is observed and evaluated by visual observation or the like. Japanese Kokai Patent No. HEI05[1993]-160420 discloses a technology for improving corrosion resistance in severe environments by means of a back-side electrode using a paste containing an aluminum alloy as an electrode material.
Under these circumstances, there has been strong demand in recent years for solar cells that have sufficiently good electrical characteristics (for example, open circuit voltage (Voc)), while providing sufficient hot water resistance to satisfy current market demands in terms of long-term stability.
The back-side electrode of the present invention adjacently formed on silicon layer of p-type solar cell, comprising a conductive component comprising, before firing, (a) aluminum powder, (b) organic medium and (c) metal-containing component selected from the group consisting of (i) metal selected from the group consisting of Bismuth (Bi), Molybdenum (Mo), Strontium (Sr) and Stibium (Sb), (ii) carbide, oxide, nitride, boride, carbonate, hydroxide and resinate of (i) metal and (iii) Copper (Cu).
In another aspect of the present invention, a method of forming a p-type silicon solar cell comprising the steps of: (I) applying an aluminum paste comprising (a) aluminum powder, (b) organic medium and (c) metal-containing component selected from the group consisting of (i) metal selected from the group consisting of Bismuth (Bi), Molybdenum (Mo), Strontium (Sr) and Stibium (Sb), (ii) carbide, oxide, nitride, boride, carbonate, hydroxide and resinate of (i) metal and (iii) Copper (Cu), on a back-side of a silicon wafer having a p-type region, an n-type region and a p-n junction; and (ii) firing the aluminum paste on the back-side of the silicon wafer, whereby the wafer reaches a peak temperature of 600 to 900° C.
In another aspect of the present invention, a conductive component (an aluminum paste) used for forming a back-side electrode of p-type solar cell comprises (a) aluminum powder, (b) organic medium and (c) metal-containing component selected from the group consisting of Mo2C, Copper (Cu), SrCo3 and a mixture thereof.
The present invention provides a solar cell having sufficiently good electrical characteristics (for example, open circuit voltage (Voc)), while also providing sufficient hot water resistance to satisfy current market demands in terms of long-term stability.
The present invention is explained in detail below.
The back-side electrode of the present invention is formed on a silicon layer of a p-type solar cell from a conductive component (aluminum paste) comprising the following components (a) to (c).
In one embodiment, the aluminum powder comprises atomized aluminum. The atomized aluminum may be atomized in either air or inert atmosphere. In one embodiment, the average particle size distribution of the atomized aluminum powder is in the range of 0.5 to 50 μm. In one embodiment, the average particle size distribution of the aluminum powder is in the range of 1 to 20 μm. The form of the aluminum powder is not particularly limited, but a spherical or flake form or the like is preferred. The aluminum powder of the present invention is one containing aluminum metal in the amount of 85 wt % of the powder. In further embodiment, the aluminum powder may be further accompanied by other additive materials, such as, magnesium (Mg), titanium (Ti), chromium (Cr), molybdenum (Mo), tungsten (W), manganese (Mn), nickel (Ni), copper (Cu), silver (Ag), zincum (Zn), silicon (Si), bismuth (Bi), stibium (Sb), Ferrum (Fe) or a mixture thereof.
In one embodiment, the content of the (a) aluminum powder in the aluminum paste is preferably 60 to 85 wt %. In another embodiment, the content is preferably 65 to 80 wt %. In further embodiment, the content is preferably 70 to 80 wt %. If the content is less than 60 wt % the film thickness is smaller after the aluminum paste is printed, and a good BSF layer may not be formed due to an insufficient reaction phase between the silicon and aluminum or the like. If the content is over 85 wt %, on the other hand, a suitable viscosity for printing may not be obtained.
A wide variety of inert viscous materials can be used as organic medium. The rheological properties of the organic medium must be such that they lend good 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. The organic vehicle used in the thick film composition of the present invention is preferably a nonaqueous inert liquid. Use can be made of any of various organic vehicles, which may or may not contain thickeners, stabilizers and/or other common additives. The organic medium is typically a solution of polymer(s) in solvent(s). Additionally, a small amount of additives, such as surfactants, may be a part of the organic medium. The most frequently used polymer for this purpose is ethyl cellulose. Other examples of polymers include ethylhydroxyethyl cellulose, wood rosin, mixtures of ethyl cellulose and phenolic resins, polymethacrylates of lower alcohols, and monobutyl ether of ethylene glycol monoacetate can also be used. The most widely used solvents are 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 addition, volatile liquids for promoting rapid hardening after application on the substrate can be included in the vehicle. Various combinations of these and other solvents are formulated to obtain the viscosity and volatility requirements desired.
The content of polymer present in the organic medium is in the range 0.5 weight percent to 11 weight percent of the total composition. The aluminum paste of the present invention may be adjusted to a predetermined, screen-printable viscosity with the organic polymer containing medium.
In one embodiment, the content (wt %) of the (b) organic medium is preferably 17 to 70 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, the content (wt %) of (b) organic medium is preferably 25 to 40 wt % per 100 wt % of the (a) aluminum powder. If the content (wt %) is less than 17 wt % per 100 wt % of the (a) aluminum powder, a suitable viscosity for printing may not be obtained. If the content (wt %) is over 70 wt % per 100 wt % of the (a) aluminum powder, on the other hand, the film thickness is smaller after the aluminum paste is printed, and a good BSF layer may not be formed due to an insufficient reaction phase between the silicon and aluminum or the like.
In one embodiment, the metal-containing component is selected from the group consisting of (i) metal selected from the group consisting of Bismuth (Bi), Molybdenum (Mo), Strontium (Sr) and Stibium (Sb), and (ii) carbide, oxide, nitride, boride, carbonate, hydroxide and resinate of (i) metal, and (iii) Copper (Cu).
By including this (c) metal-containing component in the aluminum paste, it is possible to favorably control the deterioration in performance that occurs when the aluminum in the aluminum paste reacts with humidity and the like in the air. A solar cell is thus provided that achieves long-term stable performance while maintaining adequate open circuit voltage (Voc). One such metal-containing component may be included by itself, or two or more may be included in the aluminum paste.
As mentioned above, “hot water resistance” is a commonly-used benchmark for evaluating such long-term stable performance. This is done for example by dipping a sample with aluminum electrodes in hot water at around 80° C. for 10 minutes, and evaluating based on the visual observation of bubbles or gases which are generated from the reaction between aluminum (from the sample in the hot water) and the hot water in 10 minutes. Specific evaluation methods and evaluation standards will be explained later in the context of the examples.
In one embodiment, the content (wt %) of the metal in the (c) metal-containing component is preferably 0.04 to 30.0 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, the content is preferably 0.05 to 20.0 wt %. In further embodiment, the content is preferably 0.08 to 10.0 wt %.
If the content (wt %) of the (c) metal-containing component is less than 0.04 wt %, the effects of including the metal-containing component may not be sufficient, while the intrinsic electrical characteristics of the aluminum paste may be adversely affected if the content (wt %) exceeds 30.0 wt %. The form of the metal-containing component is not particularly limited, and examples include spherical, flake and needle forms, as well as liquid, viscous and granular forms and the like in the case of an organic metal.
The particle size of a powder of the metal-containing component is not particularly limited, but in one embodiment the average particle size (D50) is 0.05 to 30 μm for example. In another embodiment it is 0.1 to 10 μm. If the average particle size (D50) is less than 0.05 μm, there may be problems of dispersibility in the paste. If the average particle size is above 30 μm, on the other hand, the applied film may have a coarser surface, and voids and other defects may occur.
The method of manufacturing a powder of the metal-containing component is not particularly limited, and examples include wet reduction of metal salts, atomization, and in the case of oxides, dehydration of metal hydroxides and decarbonation of metal carbonates. In the case of a carbonate, a dehydration reaction of a metal hydroxide with carbon dioxide is also possible. Various other known methods can also be used favorably.
Each metal in the metal-containing component (bismuth, molybdenum, strontium, stibium and copper) has a purity of 97% or more as a simple substance.
(c-1) Bismuth (Bi)
In one embodiment, when the metal contained in the (c) metal-containing component is bismuth (Bi) as a simple substance or a carbide, oxide, nitride, boride, carbonate, hydroxide and/or resinate compound of Bismuth (Bi), the content (wt %) of bismuth (Bi) is preferably 0.2 to 3.0 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, the content is preferably 0.3 to 2.0 wt %. In further embodiment, the content is preferably 0.4 to 1.0 wt %. If the content (wt %) is less than 0.2 wt %, the effect on hot water resistance may not be sufficient. If the content (wt %) exceeds 3.0 wt %, on the other hand, the open circuit voltage (Voc) may decline. When the bismuth is a compound, the content (wt %) of the bismuth is a value calculated based on the weight of only the bismuth in the compound. Specific examples of such bismuth compounds include the oxide Bi2O3, the nitride BiN and the hydroxide Bi(OH)3. Specific examples of composite oxides and organometallic compounds are described below. One of these may be used alone, or two or more may be combined.
(c-2) Molybdenum (Mo)
In one embodiment, when metal contained in the (c) metal-containing component is molybdenum (Mo) as a simple substance or a carbide, oxide, nitride, boride, carbonate, hydroxide and/or resinate of molybdenum, the content (wt %) of molybdenum (Mo) is preferably 0.05 to 12.0 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, the content is preferably 0.2 to 8.0 wt %. In further embodiment, the content is preferably 0.5 to 5.0 wt %. If the content (wt %) is less than 0.05 wt %, the effect on hot water resistance may not be sufficient. If the content (wt %) exceeds 12.0 wt %, on the other hand, the open circuit voltage (Voc) may decline. When the molybdenum is a compound, the content (wt %) of the molybdenum is a value calculated based on the weight of only the molybdenum in the compound. Specific examples of such molybdenum compounds include the oxides MoO2 and MoO3, the borides MoB and MoB2, the carbide Mo2C, and the hydroxide H2MoO4. Other examples include composite oxides and organometallic compounds such as those described below. One of these may be used alone, or two or more may be combined.
(c-3) Strontium (Sr)
In one embodiment, when metal contained in the (c) metal-containing component is strontium (Sr) as a simple substance or a carbide, oxide, nitride, boride, carbonate, hydroxide and/or resinate of strontium, the content (wt %) of strontium (Sr) for purposes of hot water resistance is preferably 0.2 to 6.0 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, the content is preferably 0.4 to 5.0 wt %. In further embodiment, the content is preferably 0.4 to 4.0 wt %. If the content (wt %) is less than 0.2 wt %, the effect on hot water resistance may not be sufficient. If the content (wt %) exceeds 6.0 wt %, on the other hand, the open circuit voltage (Voc) may decline.
When the contained metal is strontium (Sr) as a simple substance, or a carbide, oxide, nitride, boride, carbonate, hydroxide and/or resinate thereof, containing these in the following specific amounts can have the additional effect of suppressing the occurrence of bumps and other defects on the back-side electrode surface after firing. For purposes of suppressing the occurrence of bumps and other defects, the content (wt %) of strontium (Sr) is preferably 0.05 to 8.0 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, the content is preferably 0.1 to 6.0 wt %. In further embodiment, the content is preferably 0.1 to 4.0 wt %. If the content (wt %) is less than 0.05 wt %, bumps and other defects may not be sufficiently suppressed. If the content (wt %) exceeds 8.0 wt %, on the other hand, the open circuit voltage (Voc) associated with the BSF layer may be reduced, detracting from the electrical characteristics. When the strontium is a compound, the content (wt %) of the strontium is a value calculated based on the weight of only the strontium in the compound. Specific examples of such strontium compounds include the oxide SrO, the carbonate SrCO3, and the hydroxide Sr(OH)2. Other examples include composite oxides and organometallic compounds such as those described below. One of these may be used alone, or two or more may be combined.
(c-4) Stibium (Sb)
In one embodiment, when the metal contained in the (c) metal-containing component is stibium (Sb) as a simple substance or a carbide, oxide, nitride, boride, carbonate, hydroxide and/or resinate of stibium (Sb), the content (wt %) of the stibium (Sb) is preferably 0.04 to 3.0 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, the content is preferably 0.05 to 2.0 wt %. In further embodiment, the content is preferably 0.08 to 1.0 wt %.
If the content (wt %) is less than 0.04 wt %, the effect on hot water resistance may not be sufficient. If the content (wt %) exceeds 3.0 wt %, on the other hand, the open circuit voltage (Voc) may decline.
Including this stibium (Sb) as a simple substance or a carbide, oxide, nitride, boride, carbonate, hydroxide and/or resinate of stibium (Sb) within this specific numerical range not only serves to improve hot-water resistance, but also has the desirable effect of suppressing the occurrence of bumps and other defects on the back-side electrode surface when the electrode is fired. In this case, the occurrence of defects may not be sufficiently suppressed if the content (wt %) is less than 0.04 wt %. If the content (wt %) is over 3.0 wt %, on the other hand, there is similarly a risk of decreased open circuit voltage (Voc), detracting from the electrical characteristics. When the stibium is a compound, the content (wt %) of the stibium is a value calculated based on the weight of only the stibium in the compound. Specific examples of such stibium compounds include the oxides Sb2O3, Sb2O4 and Sb2O5. Other examples include organic compounds and resinates such as those described below. One of these may be used alone, or two or more may be combined.
(c-5) Copper (Cu)
In one embodiment, when the metal contained in the (d) metal-containing component is copper (Cu), the content thereof (wt %) is preferably 0.1 to 30 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, the content is preferably 0.3 to 20 wt %. In further embodiment, the content is preferably 0.5 to 10 wt %. If the content (wt %) is less than 0.1 wt %, the effect on hot water resistance may not be sufficient. If the content (wt %) exceeds 30 wt %, on the other hand, the open circuit voltage (Voc) may decline.
(c-6) Composite Oxides
If the metal-containing component is a composite oxide as discussed above, specific examples include Bi2MoO6, Bi2Sn2O7, Bi4Ti3O12, Bi2WO6, CsMoO4, SrMoO4, BaMoO4, SrMoO4, SrNb2O6, SrSnO3, SrTa2O6, SrTiO3, SrV2O6, SrWO4 and the like. When these composite oxides are used, the content thereof is preferably the same as the content given above for each metal, according to the type of metal (Bi, Sr, Mo, etc.) contained in the composite oxide. For example, in the case of Bi2Sn2O7, the content is preferably the same as the content described above for bismuth (Bi). That is, it is preferably 0.2 to 3.0 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, it is preferably 0.3 to 2.0 wt %. In further embodiment, the content is preferably 0.4 to 1.0 wt %.
(c-7) Organometallic Compounds
When the metal-containing component is an organometallic compound as discussed above, specific examples include organometallic compounds represented by the following formulae:
Me(AR)n (I);
Me(R)n (II); and
Me(B—R)n (III),
where in formulae (I) to (III). Me is a metal selected from bismuth (Bi), molybdenum (Mo), strontium (Sr) and stibium (Sb). In Formula (I), A represents and R represents a C418 straight, branched or cyclic hydrocarbon. Examples include Me(OiC3H7)n. Me(OC2H5)n and the like. In Formula (II), R represents a C4-18 straight, branched or cyclic hydrocarbon. When R is a hydrocarbon, its bonds may include a carbonyl (—C(═O)O—) group or ether (—O—) group or the like. Examples include Me(C6H5)n, Me(C11H19O2)n and the like. In Formula (III), B represents —O—CO— and R represents a C11-18 straight or branched hydrocarbon. Examples include Me(O—CO—C7H15)n and Me(O—CO—C11H23)n.
Other examples include metal complex compounds of bismuth (Bi), molybdenum (Mo), strontium (Sr) or stibium (Sb) with acetylacetone or the like.
The conductive component may comprise glass frit, or one or more organic additives, for example, surfactants, thickeners, rheology modifiers and stabilizers.
(d-1) Glass Frit
Generally, the function of the glass frit in an aluminum paste is primarily to provide a means to increase the efficiency by which the silicon is accessed by the molten aluminum during the firing process. In addition to this function, glass frit provides some additional cohesion and adhesion properties to the substrate. The glass frit affects the bowing of the aluminum layer in the finished cell. The glass frit can also increase the alloying depth of the aluminum into the silicon, therefore enhancing or increasing the aluminum dopant concentration in the eutectically grown silicon layer.
The glass frit is, in an embodiment, chosen based on the effectiveness that they have on improving the electrical performance of the aluminum thick film paste without compromising other considerations such as environmental legislation or public desire to exclude heavy metals of potential environmental concern.
In one embodiment, the content (wt %) of glass frit is 0.1 to 5.0 wt % per 100 wt % of the (a) aluminum powder. In another embodiment, the content (wt %) of glass frit is 0.2 to 3.0 wt % per 100 wt % of the (a) aluminum powder. In further embodiment, the content (wt %) of glass frit is 0.3. to 2.0 wt % per 100 wt % of the (a) aluminum powder. If the content (wt %) of glass frit is less than 0.1 wt % per 100 wt % of the (a) aluminum powder, the electrical characteristics (open circuit voltage Voc) may decline, while if the content (wt %) exceeds 5.0 wt % per 100 wt % of the (a) aluminum powder, bumps and other defects may occur on the aluminum film after firing.
Glass frit useful for the present invention is known in the art. Some examples include borosilicate and aluminosilicate glasses. Examples further include combinations of oxides, such as: B2O3, SiO2, Al2O3, CaO, BaO, ZnO, Na2O, Li2O, SrO, TiO2, Ta2O3, Bi2O3, Sb2O3, K2O, PbO and ZrO or combinations of fluoride, such as: CaF2, BaF2, ZnF2, NaF, LiF, SrF2, TiF4, TaF6, BiF3, SbF5, KF, PbF2 and ZrF4, which may be used independently or in combination to form glass frit composition. The conventional glass frit preferably used are the borosilicate frits, such as lead borosilicate frit, bismuth, cadmium, barium, calcium, or other alkaline earth borosilicate frits. The preparation of such glass frit composition is well known and consists, for example, in melting together the constituents of the glass in the form of the oxides of the constituents and pouring such molten composition into water to form the frit. The batch ingredients may, of course, be any compounds that will yield the desired oxides under the usual conditions of frit production. For example, boric oxide will be obtained from boric acid, silicon dioxide will be produced from flint, barium oxide will be produced from barium carbonate, etc. In one embodiment, the conductive component comprises at least one glass frit composition wherein upon firing said glass frit composition undergoes recrystallization or phase separation and liberates a frit with a separated phase that has a lower softening point than the original softening point. Thus, the thick film composition comprising such a glass frit composition upon processing gives lower bowing properties. In one embodiment, the glass frit is a lead-free glass frit composition which, upon firing, undergoes recrystallization or phase separation and liberates a frit with a separated phase that has a lower transition point than the original transition point. Mixtures one or more frits are possible. The glass is preferably milled in a ball mill with water or inert low viscosity, low boiling point organic liquid to reduce the particle size of the frit and to obtain a frit of substantially uniform size. It is then settled in water or said organic liquid to separate fines and the supernatant fluid containing the fines is removed. Other methods of classification may be used as well.
The glasses are prepared by conventional glassmaking techniques, by mixing the desired components in the desired proportions and heating the mixture to form a melt. As is well known in the art, heating is conducted to a peak temperature and for a time such that the melt becomes entirely liquid and homogeneous. The desired glass transition temperature is in the range of 325 to 650° C. In one embodiment, the average particle size (D50) of the glass frit composition be 0.1-10 μm. The reason for this is that smaller particles having a high surface area tend to adsorb the organic materials and thus impede clean decomposition. On the other hand, larger size particles tend to have poorer sintering characteristics.
(d-2) Organic Additive(s)
The organic additive(s) may be part of the organic medium. However, it is also possible to add the organic additive(s) separately when preparing the aluminum pastes. The organic additive(s) may be present in the conductive component (aluminum pastes) in a total proportion of, for example, 0.5 to 10(wt %), based on total composition.
The conductive component (aluminum paste) explained above is typically conveniently manufactured by mechanically mixing, a dispersion technique that is equivalent to the traditional roll milling. Roll milling or other mixing technique can also be used. The conductive component is preferably spread on the desired part of the back face of a solar cell by screen printing; in spreading by such a method, it is preferable to have a viscosity in a prescribed range. Other application methods can be used such as silicone pad printing. The viscosity of the conductive component (aluminum paste) is preferably 20-100 Pa·S when it is measured at a spindle speed of 10 rpm and 25° C. by a utility cup using a Brookfield HBT viscometer and #14 spindle.
The silver/aluminum or silver film can be cofired with the conductive component at the same time in a process called cofiring.
Next, an example in which a solar cell is prepared using the conductive component (aluminum paste) is explained, referring to the Figure (
On the back-side of the substrate, the conductive component (aluminum paste) used for forming a back-side electrode of p-type solar cell of the present invention 106 are spread by screen printing using the pattern(
In one embodiment, the aluminum paste has a dried film thickness of 15-60 μm. In another embodiment, the thickness of the aluminum paste is preferably 15-30 μm. Also, the overlapped part of the aluminum paste and the silver/aluminum electroconductive paste is preferably about 0.5-2.5 mm. Next, the substrate obtained is fired at a temperature of 600-900° C. for about 1min -15 min, for instance, so that the desired solar cell is obtained (
The present invention will be discussed in further detail by giving practical examples. The scope of the present invention, however, is not limited in any way by these practical examples.
The examples cited here relate to aluminum pastes fired onto conventional solar cells that have a silicon nitride anti-reflection coating and front side n-type contact thick film silver conductor.
The present invention can be applied to a broad range of semiconductor devices, although it is especially effective in light-receiving elements such as photodiodes and solar cells. The discussion below describes how a solar cell is formed utilizing the paste(s) of the present invention and how it is tested for its technological properties.
A solar cell was formed as follows:
The aluminum paste was produced using the following materials.
Aluminum paste preparations were accomplished with the following procedure. Aluminum powder, metal-containing component and glass frit were dispersed in the organic medium and mixed by mixer for 120 minutes. The content of aluminum powder, glass frits, and metal-containing component paste in each aluminum paste are shown in Table 1. The degree of dispersion was measured by fineness of grind (FOG). A typical FOG value was generally equal to or less than 20/10 for a conductor.
On the front-side of a Si substrate (200 μm thick multicrystalline silicon wafer of area 14.44 cm2 p-type (boron) bulk silicon, with an n-type diffused POCl3 emitter, surface texturized with acid. SiNix anti-reflective coating (ARC) on the wafer's emitter applied by CVD), a 20 μm thick silver electrode on the front surface (PV159 Ag composition commercially available from E. I. Du Pont de Nemours and Company) was printed and dried. Then, aluminum pastes for the back-side electrode of solar cell, prepared in (i) was screen-printed at dried film thickness of 40 μm. The screen-printed aluminum pastes were dried before firing.
The printed wafers were then fired in a Despatch furnace at a belt speed of 550 cm/min. The wafers reaching a peak temperature of 740° C. After firing, the metallized wafer became a functional photovoltaic device.
a-1) Measurement of Open Circuit Voltage (Voc)
Each sample of solar cells (Examples 1-30 and Comparative Example 1) formed according to the method described above were placed in a commercial I-V tester (supplied by NPC.) for the purpose of measuring light conversion efficiencies. The lamp in the I-V tester simulated sunlight of a known intensity (approximately 1000 W/m2) and illuminated the emitter of the cell. The metallizations printed onto the fired cells were subsequently contacted by four electrical probes. The photocurrent (Voc, open circuit voltage; Isc, short circuit current) generated by the solar cells was measured over arrange of resistances to calculate the I-V response curve. Voc values were subsequently derived from the I-V response curve
a-2) Evaluation Based on Open Circuit Voltage Value (Voc)
The results obtained from the Voc value measurements for each example as described above were evaluated in comparison with the open circuit voltage value (Voc) obtained from the sample measurements of Comparative Example 1. The results are shown in Table 2. Each open circuit voltage value (Voc) was considered substantially acceptable if the reduction in Voc was within −3% in comparison with the sample of Comparative Example 1.
Hot water resistance was evaluated as follows for the sample of Comparative Example 1 and the samples of Examples 1 to 25 obtained in (1) (ii) above.
Distilled water was placed in a beaker (glass, 500 ml), and heated to 80° C. with a heater. The samples were immersed together with the temperature of the distilled water in the beaker maintained at 80° C., and left for 10 minutes. Here, hot water resistance was evaluated based on the visual observation of bubbles (gases) which were generated from the reaction between aluminum (from the sample in the hot water) and hot water in 10 minutes. The following is an evaluation standard. The results are shown in Table 2.
Excellent: Almost no bubbling (gas) observed on the aluminum electrode surface
Good: Slight bubbling (gas) observed on the aluminum electrode surface, but not at a level that has a practical effect
Bad: Considerable bubbling (gas) observed on the aluminum electrode surface.
c) Observation and Evaluation of Bumps The occurrence of bumps was observed and evaluated as follows for the samples of Comparative Example 1 and Examples 23, 24 and 27 to 31 obtained in 1(ii).
That is, the aluminum electrode surface of each sample was checked visually to confirm the presence or absence of bumps on the aluminum electrode surface after firing. This was then evaluated according to the following standard. The results are shown in Table 2.
Excellent: Slight bumps observed in some areas on the aluminum electrode surface, but at a level that presents no problems for practical use.
Good/marginal: Some bumps observed on the aluminum electrode surface, but at a level that presents no fundamental problems for practical use.
Bad: Bumps observed on the aluminum electrode surface at a level that presents problems for practical use
