HALOGENIDE CONTAINING GLASSES IN METALLIZATION PASTES FOR SILICON SOLAR CELLS

Abstract

In general, the invention relates to a paste comprising:

i) silver particles;
ii) a particulate lead-silicate glass comprising iia) at least one oxide of silicon; iib) at least one oxide of lead; iic) at least one chloride; iid) optionally at least one further oxide being different from components iia) and iib);
iii) an organic vehicle.

The invention also relates to a solar cell precursor, to a process for the preparation of a solar cell, to a solar cell obtainable by this process, to a module comprising such a solar cell and to the use of a particulate lead-silicate glass as a component in a silver paste that can be used for the formation of an electrode.

Claims

1. A paste (313) comprising: i) silver particles; ii) a particulate lead-silicate glass comprising iia) at least one oxide of silicon; iib) at least one oxide of lead; iic) at least one chloride; iid) optionally at least one further oxide being different from components iia) and iib); iii) an organic vehicle.

2. The paste (313) according to claim 1, wherein the oxide of silicon iia) is SiO.sub.2.

3. The paste (313) according to claim 1, wherein the at least one chloride iic) is selected from the group consisting of LiCl, NaCl, KCl, RbCl, CsCl, MgCl.sub.2, CaCl.sub.2, SrCl.sub.2, BaCl.sub.2, ZnCl.sub.2, PbCl.sub.2, AgCl and mixtures of at least two of these chlorides.

4. The paste (313) according to claim 1, wherein the at least one further oxide iid) being different from components iia) and iib) is an oxide selected from the group consisting of the oxides of aluminium, boron, phosphorus, titanium, zirconium, cerium, lanthanum, vanadium, niobium, tantalum, chromium, molybdenum, tungsten, manganese, iron, cobalt, nickel, copper, zinc, silver, lithium, sodium, potassium, rubidium, caesium, magnesium, calcium, strontium, barium, tin, bismuth, or a mixture of at least two, at least three or at least four of these oxide.

5. The paste (313) according to claim 1, wherein the particulate lead-silicate glass ii) comprises iia) at least 5 mol % of at least one oxide of silicon; iib) 25 to 80 mol % of at least one oxide of lead; iic) 0.1 to 50 mol % of at least one chloride; iid) 1 to 40 mol % of at least one further oxide being different from components iia) and iib); wherein the amounts are in each case based on the total mole number of components iia) to iid) in the glass and sum up to 100 mol %.

6. The paste (313) according to claim 1, wherein the molar ratio of chloride ions to oxygen ions (Cl.sup.−:O.sup.2−) in the particulate lead-silicate is in the range from 0.001 to 0.1.5.

7. The paste (313) according to claim 1, wherein the particulate lead-silicate glass ii) is obtainable by mixing components iia), iib), iic) and optionally iid), melting the thus obtained mixture, cooling the thus obtained glass and subjecting it to pulverization.

8. The paste (313) according to claim 1, comprising i) at least 60 wt.-% of the silver particles; ii) 0.5 to 10 wt.-% of the particulate lead-silicate glass; iii) 5 to 25 wt.-% of the organic vehicle; iv) up to 10 wt.-% of further additives being different from components i) to iii); wherein the amounts are in each case based on the total weight of the paste (313) and sum up to 100 wt.-%.

9. A solar cell precursor comprising the following solar cell precursor constituents: a) a wafer (300) having a front side and a back side; b) a paste (313) according to claim 1 superimposed on at least one side of the wafer (300), the at least one side being selected from the group consisting of the front side and the back side.

10. A process for the preparation of a solar cell (200) comprising the following preparation steps: A) provision of a solar cell precursor according to claim 9; B) firing of the solar cell precursor to obtain a solar cell.

11. The process according to claim 10, wherein the holding temperature in process step B) is a range from 660 to 760° C.

12. A solar cell (200) obtainable by the process according to claim 10.

13. The solar cell (200) according to claim 12, wherein the solar cell is an n-type solar cell or a PERC cell.

14. A module comprising at least two solar cells, at least one of which is a solar cell (200) according to claim 12.

15. Use of a particulate lead-silicate glass as defined in claim 1 as a component in a silver paste that can be used for the formation of an electrode.

Description

DESCRIPTION OF THE DRAWINGS

[0167] The invention is now explained by means of figures which are intended for illustration only and are not to be considered as limiting the scope of the invention. In brief,

[0168] FIG. 1 shows a cross sectional view of the minimum layer configuration for a solar cell;

[0169] FIG. 2 shows a cross sectional view a common layer configuration for a solar cell;

[0170] FIGS. 3a, 3b and 3c together illustrate the process of firing a front side paste.

[0171] FIG. 1 shows a cross sectional view of a solar cell 100 and represents the minimum required layer configuration for a solar cell according to the invention. Starting from the back face and continuing towards the front face the solar cell 100 comprises a back electrode 104, a back doped layer 106, a p-n junction boundary 102, a front doped layer 105 and a front electrode 103, wherein the front electrode penetrates into the front doped layer 105 enough to form a good electrical contact with it, but not so much as to shunt the p-n junction boundary 102. The back doped layer 106 and the front doped layer 105 together constitute a single doped Si wafer 101. In the case that 100 represents an n-type cell, the back electrode 104 is preferably a silver electrode, the back doped layer 106 is preferably Si lightly doped with P, the front doped layer 105 is preferably Si heavily doped with B and the front electrode 103 is preferably a mixed silver and aluminium electrode. In the case that 100 represents a p-type cell, the back electrode 104 is preferably a mixed silver and aluminium electrode, the back doped layer 106 is preferably Si lightly doped with B, the front doped layer 105 is preferably Si heavily doped with P and the front electrode 103 is preferably a silver electrode. The front electrode 103 has been represented in FIG. 1 as consisting of three bodies purely to illustrate schematically the fact that the front electrode 103 does not cover the front face in its entirety. The invention does not limit the front electrode 103 to those consisting of three bodies.

[0172] FIG. 2 shows a cross sectional view of a common layer configuration for a solar cell 200 according to the invention (excluding additional layers which serve purely for chemical and mechanical protection). Starting from the back face and continuing towards the front face the solar cell 200 comprises a back electrode 104, a back passivation layer 208, a highly doped back layer 210, a back doped layer 106, a p-n junction boundary 102, a front doped layer 105, a front passivation layer 207, an anti-reflection layer 209, front electrode fingers 214 and front electrode bus bars 215, wherein the front electrode fingers penetrate through the anti-reflection layer 209 and the front passivation layer 207 and into the front doped layer 105 far enough to form a good electrical contact with the front doped layer, but not so far as to shunt the p-n junction boundary 102. In the case that 200 represents an n-type cell, the back electrode 104 is preferably a silver electrode, the highly doped back layer 210 is preferably Si heavily doped with P, the back doped layer 106 is preferably Si lightly doped with P, the front doped layer 105 is preferably Si heavily doped with B, the anti-reflection layer 209 is preferably a layer of silicon nitride and the front electrode fingers and bus bars 214 and 215 are preferably a mixture of silver and aluminium. In the case that 200 represents a p-type cell, the back electrode 104 is preferably a mixed silver and aluminium electrode, the highly doped back layer 210 is preferably Si heavily doped with B, the back doped layer 106 is preferably Si lightly doped with B, the front doped layer 105 is preferably Si heavily doped with P, the anti-reflection layer 209 is preferably a layer of silicon nitride and the front electrode fingers and bus bars 214 and 215 are preferably silver. FIG. 2 is schematic and the invention does not limit the number of front electrode fingers to three as shown. This cross sectional view is unable to effectively show the multitude of front electrode bus bars 215 arranged in parallel lines perpendicular to the front electrode fingers 214.

[0173] FIGS. 3a, 3b and 3c together illustrate the process of firing a front side paste to yield a front side electrode. FIGS. 3a, 3b and 3c are schematic and generalised and additional layers further to those constituting the p-n junction are considered simply as optional additional layers without more detailed consideration.

[0174] FIG. 3a illustrates a wafer (300) before application of front electrode. Starting from the back face and continuing towards the front face the wafer before application of front electrode optionally comprises additional layers on the back face 311, a back doped layer 106, a p-n junction boundary 102, a front doped layer 105 and additional layers on the front face 312. The additional layers on the back face 311 can comprise any of a back electrode, a back passivation layer, a highly doped back layer or none of the above. The additional layer on the front face 312 can comprise any of a front passivation layer, an anti-reflection layer or none of the above.

[0175] FIG. 3b shows a wafer (300) with paste (313) applied to the front face before firing. In addition to the layers present in FIG. 3a described above, paste 313 is present on the surface of the front face.

[0176] FIG. 3c shows a wafer (300) with front electrode applied. In addition to the layers present in FIG. 3a described above, a front side electrode 103 is present which penetrates from the surface of the front face through the additional front layers 312 and into the front doped layer 105 and is formed from the paste 313 of FIG. 3b by firing.

[0177] In FIGS. 3b and 3c, the applied paste 313 and the front electrodes 103 are shown schematically as being present as three bodies. This is purely a schematic way of representing a non-complete coverage of the front face by the paste/electrodes and the invention does not limit the paste/electrodes to being present as three bodies.

Test Methods

[0178] The following test methods are used in the invention. In absence of a test method, the ISO test method for the feature to be measured being closest to the earliest filing date of the present application applies. In absence of distinct measuring conditions, standard ambient temperature and pressure (SATP) (temperature of 298.15 K and an absolute pressure of 100 kPa) apply.

Viscosity

[0179] Viscosity measurements were performed using the Thermo Fischer Scientific Corp. “Haake Rheostress 600” equipped with a ground plate MPC60 Ti and a cone plate C 20/0.5° Ti and software “Haake RheoWin Job Manager 4.30.0”. After setting the distance zero point, a paste sample sufficient for the measurement was placed on the ground plate. The cone was moved into the measurement positions with a gap distance of 0.026 mm and excess material was removed using a spatula. The sample was equilibrated to 25° C. for three minutes and the rotational measurement started. The shear rate was increased from 0 to 20 s.sup.−1 within 48 s and 50 equidistant measuring points and further increased to 150 s.sup.−1 within 312 s and 156 equidistant measuring points. After a waiting time of 60 s at a shear rate of 150 s.sup.−1, the shear rate was reduced from 150 s.sup.−1 to 20 s.sup.−1 within 312 s and 156 equidistant measuring points and further reduced to 0 within 48 s and 50 equidistant measuring points. The micro torque correction, micro stress control and mass inertia correction were activated. The viscosity is given as the measured value at a shear rate of 100 s.sup.−1 of the downward shear ramp.

Specific Surface Area

[0180] BET measurements to determine the specific surface area of particles are made in accordance with DIN ISO 9277:1995. A Gemini 2360 (from Micromeritics) which works according to the SMART method (Sorption Method with Adaptive dosing Rate), is used for the measurement. As reference material Alpha Aluminum oxide CRM BAM-PM-102 available from BAM (Bundesanstalt für Materialforschung und -prüfung) is used. Filler rods are added to the reference and sample cuvettes in order to reduce the dead volume. The cuvettes are mounted on the BET apparatus. The saturation vapour pressure of nitrogen gas (N.sub.2 5.0) is determined. A sample is weighed into a glass cuvette in such an amount that the cuvette with the filler rods is completely filled and a minimum of dead volume is created. The sample is kept at 80° C. for 2 hours in order to dry it. After cooling the weight of the sample is recorded. The glass cuvette containing the sample is mounted on the measuring apparatus. To degas the sample, it is evacuated at a pumping speed selected so that no material is sucked into the pump. The mass of the sample after degassing is used for the calculation. The dead volume is determined using Helium gas (He 4.6). The glass cuvettes are cooled to 77 K using a liquid nitrogen bath. For the adsorptive, N.sub.2 5.0 with a molecular crosssectional area of 0.162 nm.sup.2 at 77 K is used for the calculation. A multi-point analysis with 5 measuring points is performed and the resulting specific surface area given in m.sup.2/g.

Ag Particles Size Determination (d.sub.10, d.sub.50, d.sub.90)

[0181] Particle size determination for Ag particles is performed in accordance with ISO 13317-3: 2001. A Sedigraph 5100 with software Win 5100 V2.03.01 (from Micromeritics) which works according to X-ray gravitational technique is used for the measurement. A sample of about 400 to 600 mg is weighed into a 50 ml glass beaker and 40 ml of Sedisperse P11 (from Micromeritics, with a density of about 0.74 to 0.76 g/cm.sup.3 and a viscosity of about 1.25 to 1.9 mPa*s) are added as suspending liquid. A magnetic stirring bar is added to the suspension. The sample is dispersed using an ultrasonic probe Sonifer 250 (from Branson) operated at power level 2 for 8 minutes while the suspension is stirred with the stirring bar at the same time. This pre-treated sample is placed in the instrument and the measurement started. The temperature of the suspension is recorded (typical range 24° C. to 45° C.) and for calculation data of measured viscosity for the dispersing solution at this temperature are used. Using density and weight of the sample (density 10.5 g/cm.sup.3 for silver) the particle size distribution is determined and given as d.sub.50, d.sub.10 and d.sub.90.

Efficiency and Series Resistance

[0182] The sample solar cell is characterized using a commercial IV-tester “cetisPV-CTL1” from Halm Elektronik GmbH. All parts of the measurement equipment as well as the solar cell to be tested were maintained at 25° C. during electrical measurement. This temperature is always measured simultaneously on the cell surface during the actual measurement by a temperature probe. The Xe Arc lamp simulates the sunlight with a known AM1.5 intensity of 1000 W/m.sup.2 on the cell surface. To bring the simulator to this intensity, the lamp is flashed several times within a short period of time until it reaches a stable level monitored by the “PVCTControl 4.313.0” software of the IV-tester. The Halm IV tester uses a multi-point contact method to measure current (I) and voltage (V) to determine the cell's IV-curve. To do so, the solar cell is placed between the multi-point contact probes in such a way that the probe fingers are in contact with the bus bars of the cell. The numbers of contact probe lines are adjusted to the number of bus bars on the cell surface. All electrical values were determined directly from this curve automatically by the implemented software package. As a reference standard a calibrated solar cell from ISE Freiburg consisting of the same area dimensions, same wafer material and processed using the same front side layout is tested and the data compared to the certificated values. At least 5 wafers processed in the very same way are measured and the data interpreted by calculating the average of each value. The software PVCTControl 4.313.0 provides values for efficiency and series resistance.

Temperature Profile in the Firing Furnace

[0183] The temperature profile for the firing process was measured with a Datapaq DQ 1860 A datalogger from Datapaq Ltd., Cambridge, UK connected to a Wafer Test Assembly 1-T/C 156 mm SQ from Despatch (part no. DES-300038). The data logger is protected by a shielding box TB7250 from Datapaq Ltd., Cambridge, UK and connected to the thermocouple wires of the Wafer Test Assembly. The solar cell simulator was placed onto the belt of the firing furnace directly behind the last wafer so that the measured temperature profile of the firing process was measured accurately. The shielded data logger followed the Wafer Test assembly at a distance of about 50 cm to not affect the temperature profile stability. The data was recorded by data logger and subsequently analysed using a computer with Datapaq Insight Reflow Tracker V7.05 software from Datapaq Ltd., Cambridge, UK.

EXAMPLES

[0184] The invention is now explained by means of examples which are intended for illustration only and are not to be considered as limiting the scope of the invention.

Example 1—Preparation of Particulate Lead-Silicate Glass

[0185] Three different particulate lead-silicate glasses are prepared by homogeneously mixing the components mentioned in the following table 1 in the given relative amounts. The mixture was melted at 850° C. and then quenched in water to room temperature. The quenched glass was crushed and fine-grounded using a planetary ball mill to prepare particulate lead-silicate glasses having an average particle diameter of about 1 μm.

TABLE-US-00001 TABLE 1 composition of the different articulate lead-silicate glasses (amounts in mol %) glass 1 glass 1a glass 1b PbO 59.95 56.95 56.95 SiO.sub.2 25.99 25.99 25.99 Al.sub.2O.sub.3 6.99 6.99 6.99 ZnO 0 0 0 B.sub.2O.sub.3 5.06 5.06 5.06 ZnO 2.01 2.01 2.01 PbCl.sub.2 0 3 0 (AgCl).sub.2* 0 0 3 (*(AgCl).sub.2 has been chosen to perpetuate the comparability with PbCb.sub.2-addition - same amount of substance of Cl-ions).

[0186] Glass 1 is a glass that corresponds to the teaching in WO 2013/105812 A1. In glasses 1a and 1b 3 mol % of (AgCl).sub.2 and PbCl.sub.2, respectively, were added.

Example 2—Preparation of Pastes

[0187] A paste was made by mixing, by means of a Speedmixer (Speedmixer DAC800, Hauschild &Co. KG, Hamm), the appropriate amounts of organic vehicle (table 2), Ag powder with a d.sub.50 of 1.2 μm and particulate lead-silicate glass prepared in Example 1 (table 3). The paste was passed through a 3-roll mill Exakt 80 E with stainless steel rolls with a first gap of 120 μm and a second gap of 30 μm with progressively decreasing gaps to 20 μm for the first gap and 10 μm for the second gap several times until homogeneity. The viscosity was measured as mentioned above and appropriate amounts of organic vehicle with the composition given in Table 2 were added to adjust the paste viscosity toward a target in a range from about 16 to about 20 Pas. The wt. %-amounts of the constituents of the paste are given in Table 3.

TABLE-US-00002 TABLE 2 composition of the organic vehicle. Proportion of Organic Vehicle Component component [wt %] 2-(2-butoxyethoxy)ethanol) 84 [solvent] ethyl cellulose (DOW Ethocel 6 4) [binder] Thixcin ® E [thixotropic 10 agent]

TABLE-US-00003 TABLE 3 composition of the pastes Proportion of Component component [wt %] Ag powder 88.0 paniculate lead-silicate glass 2.5 organic vehicle 9.5

Example 3—Solar Cell Preparation and Efficiency, Contact Resistance and Series Resistance Measurement

[0188] Pastes were applied to mono-crystalline Cz p-type Silicon wafers with a phosphor doped front face and boron doped back face. The wafers had dimensions of 156×156 mm.sup.2 and a full-square shape. The wafers had an anti-reflect/passivation layer of SiN.sub.x with a thickness of about 75 nm on the front face. The solar cells used were textured by alkaline etching. The pastes prepared in Example 2 were screen-printed onto the n-doped face of the wafer using a semi-automatic screen printer E2 (from Asys Group, EKRA Automatisierungssysteme) set with the following screen parameters: 360 mesh, 16 μm wire thickness, 15 μm emulsion over mesh, 100 fingers, 40 μm finger opening, 3 bus bars, 1.5 mm bus bar width. A commercially available aluminum paste was printed on the full back face of the device using the same printer and the following screen parameters: 200 mesh, 40 μm wire thickness, 10 μm emulsion over mesh. The device with the printed patterns was dried for 10 minutes at 150° C. in an oven after printing each side. The substrates were then fired with the n-doped side up in a Centrotherm DO-FF 8600-300 oven for less than 1 min. For each example, firing was carried out with maximum firing temperature of 745° C. The fully processed samples were then tested for IV performance using a HALM IV-Tester. Table 4 shows the resulting efficiency, contact resistance and series resistance.

TABLE-US-00004 TABLE 4 electrical properties of solar cells. paste 1 paste 2 paste 3 (glass 1) (glass 1a) (glass 1b) not according to according to according to the invention the invention the invention Efficiency (eta) −− ++ ++ Series resistance −− ++ ++ [Ohm × cm.sup.2] Results displayed as −− very unfavourable, ++ very favourable

[0189] By adding PbCl.sub.2 in glass 1a the total lead content of the glasses was kept constant while the addition of AgCl in glass 1b will provide the same amount of chloride but replaces lead by silver ions. The results in table 4 therefore show that only is the active ingredient and the cation has no influence on the performance.

[0190] The results in table 4 show that metallization pastes containing the particulate-lead-silicate glasses as described in the present application, i. e. glass that, in addition to SiO.sub.2 and PbO also comprise chlorides, are suitable to improve the cell efficiency of silicon solar cells. Being able to reduce the required peak firing temperature allows cell manufactures to improve their cells and processes in order to achieve the best possible efficiency. For n-type cells it would be possible to apply one paste on both sides of the cell and replacing the currently used combination of Al containing and standard frontside pastes that do not match in optimum firing temperature.

REFERENCE LIST

[0191] 100 Solar cell [0192] 101 Doped Si wafer [0193] 102 p-n junction boundary [0194] 103 Front electrode [0195] 104 Back electrode [0196] 105 Front doped layer [0197] 106 Back doped layer [0198] 200 Solar cell [0199] 207 Front passivation layer [0200] 208 Back passivation layer [0201] 209 Anti-reflection layer [0202] 210 Highly doped back layer [0203] 214 Front electrode fingers [0204] 215 Front electrode bus bars [0205] 300 Wafer [0206] 311 Additional layers on back face [0207] 312 Additional layers on front face [0208] 313 Paste