A METHOD FOR MANUFACTURING MODIFIED ELECTRICALLY-CONDUCTIVE COPPER PARTICLES AND MODIFIED ELECTRICALLY-CONDUCTIVE COPPER PARTICLES MANUFACTURED THEREOF

Abstract

A method for manufacturing modified electrically-conductive copper particles, includes introducing copper having an average diameter of 0.5 to 20 μm into a water bath. The water bath has no complexing agents and no organic buffer agents. The water bath includes at least one water-soluble metal salt with at least one metal cation, at least one phosphoric reducing agent and at least one surfactant. The method includes heating the water bath with the copper particles to a temperature from 30 to 99° C. within a time necessary for forming, on the surface of the copper particles, an epitaxial metal layer of a thickness from 1 to 5 nm.

Claims

1. A method for manufacturing modified electrically-conductive copper particles, the method comprising the steps of: introducing copper particles having an average diameter from 0.5 to 20 μm into a water bath, in an amount of 5-200 g of the copper particles per each liter of the water bath, wherein the water bath comprises no complexing agents and no organic buffer agents, and wherein the water bath comprises: at least one water-soluble metal salt comprising at least one metal cation selected from a metal cation group comprising nickel cation on the an oxidation degree of +2 (Ni.sup.2+), wherein a concentration of metal cations in the water bath is from 5.Math.10.sup.−2 to 200.Math.10.sup.−2 mol/dm.sup.3, at least one phosphoric reducing agent comprising phosphorus on an oxidation degree from +1 to +3, wherein a molar ratio of metal cations to a phosphoric reducing agent in the water bath is from 0,1 to 2, at least one surfactant selected from a group comprising: cationic, anionic and non-ionic surfactants, and heating the water bath with the copper particles to a temperature from 30 to 99° C. within the for a time necessary for forming, on a surface of the copper particles, an epitaxial metal layer having a thickness from 1 to 5 nm.

2. The method according to claim 1, wherein the metal cation group further comprises a silver cation having an oxidation degree of +1 (Ag.sup.+).

3. The method according to claim 1, wherein the metal salt is selected from a group consisting of: sulfate (IV), sulfate (VI), nitrate (III), nitrate (V), acetate, chloride and bromide.

4. The method according to claim 1, wherein the phosphoric reducing agent is selected from a group consisting of: phosphoric acid (I) (H.sub.3PO.sub.2), phosphoric acid (III) (H.sub.2PO.sub.3) and phosphorus oxide (III) (P.sub.4O.sub.6).

5. The method according to claim 1, wherein the water bath further comprises an inorganic buffer agent for stabilizing pH of the water bath in a range from 4 to 7 selected from a group consisting of: phosphoric acid and sodium or potassium phosphate buffer solution and boric acid and sodium borate buffer solution.

6. The method according to claim 1, wherein the surfactant is selected from a group consisting of: dodecyl sodium sulfate (CAS no. 151-21-3), 2-bromo-3-methoxyphenol (CAS no. 68439-57-6), 5-chloro-2-methyl-3(2H)-isothiazolone with 2-methyl-3(2H)-isothiazolone (CAS no. 55965-84-9) and sodium laureth sulfate (CAS no. 68891-38-3), wherein a total concentration of the surfactant in the water bath is from 1.Math.10.sup.−7 to 1000.Math.10.sup.−7 mol/dm.sup.3.

7. The method according to claim 1, comprising heating the water bath with the copper particles is a temperature from 80 to 95° C.

8. Modified electrically-conductive copper particles prepared by: introducing copper particles having an average diameter from 0.5 to 20 μm into a water bath, in an amount of 5-200 g of the copper particles per each liter of the water bath, wherein the water bath comprises no complexing agents and no organic buffer agents, and wherein the water bath comprises: at least one water-soluble metal sat comprising at least one metal cation selected from a metal cation group comprising nickel cation on an oxidation degree of +2 (Ni.sup.2+), wherein a concentration of metal cations in the water bath is from 5.Math.10.sup.−2 to 200.Math.10.sup.−2 mol/dm.sup.3, at least one phosphoric reducing agent comprising phosphorus on an oxidation degree from +1 to +3. wherein a molar ratio of metal cations to a phosphoric reducing agent in the water bath is from 0.1 to 2, at least one surfactant selected from a group comprising: cationic anionic and non-ionic surfactants, and heating the water bath with the copper particles to a temperature from 30 to 99° C. for a time necessary for forming on a surface of the copper articles, an epitaxial metal layer having a thickness from 1 to 5 nm.

9. The modified electrically-conductive copper particles according to claim 8, wherein the metal cation group further comprises a silver cation having an oxidation degree of +1 (Ag.sup.+).

10. The modified electrically-conductive copper particles according to claim 8 wherein the modified copper particles comprise a coating that comprises nickel and phosphorus.

11. The modified electrically-conductive copper particles according to claim 9 wherein the modified copper particles comprise a coating that comprises nickel, silver and phosphorus.

12. The modified electrically-conductive copper particles according to claim 11 wherein the coating has a thickness from 3 to 5 nm.

13. An electrically-conductive composition comprising modified electrically-conductive copper particles prepared by: introducing copper particles having an average diameter from 0.5 to 20 μm into a water bath, in an amount of 5-200 g of the copper particles per each liter of the water bath, wherein the water bath comprises no complexing agents and no organic buffer agents and wherein the water bath comprises: at least one water-soluble metal salt comprising at least one metal cation selected from a metal cation group comprising nickel cation on an oxidation degree of +2 (Ni.sup.2+), wherein a concentration of metal cations in the water bath is from 5.Math.10.sup.−2 to 200.Math.10.sup.−2 mol/dm.sup.3, at least one phosphoric reducing agent comprising phosphorus on an oxidation degree from +1 to +3, wherein a molar ratio of metal cations to a phosphoric reducing anent in the water bath is from 0.1 to 2, at least one surfactant selected from a group comprising: cationic, anionic and non-ionic surfactants, and heating the water bath with the copper particles to a temperature from 30 to 99° C. for a time necessary for forming, on a surface of the copper particles, an epitaxial metal layer having a thickness from 1 to 5 nm.

14. A silicon-based solar cell comprising electrically-conductive paths and/or electrodes made of the electrically-conductive composition comprising modified electrically-conductive copper particles prepared by: introducing copper particles having an average diameter from 0.5 to 20 μm into a water bath, in an amount of 5-200 g of the copper particles per each liter of the water bath, wherein the water bath comprises no complexing agents and no organic buffer agents, and wherein the water bath comprises: at least one water-soluble metal salt comprising at least one metal cation selected from a metal cation group comprising nickel cation on an oxidation degree of +2 (Ni.sup.2+), wherein a concentration of metal cations in the water bath is from 5.Math.10.sup.−2 to 200.Math.10.sup.−2 mol/dm.sup.3, at least one phosphoric reducing agent comprising phosphorus on an oxidation degree from +1 to +3, wherein a molar ratio of metal cations to a phosphoric reducing agent in the water bath is from 0.1 to 2, at least one surfactant selected from a group comprising: cationic anionic and non-ionic surfactants, and heating the water bath with the copper particles to a temperature from 30 to 99° C. for a time necessary for forming, on a surface of the copper particles, an epitaxial metal layer having a thickness from 1 to 5 nm.

Description

BRIEF DESCRIPTION OF FIGURES

[0032] The present disclosure is presented by means of exemplary embodiments in a drawing in which:

[0033] FIG. 1 is a schematic representation of the chemical reaction leading to the formation of a coating on the copper particles,

[0034] FIGS. 2A, 2B show SEM pictures of different copper particles to be used for the preparation of modified electrically-conductive cooper particles according to the present disclosure,

[0035] FIG. 3 shows an analysis of contamination of copper particles surface to be used for the preparation of modified electrically-conductive cooper particles according to the present disclosure,

[0036] FIG. 4 shows a table of a conductive characteristic of silicon-based solar cells having the upper electrodes made of a paste comprising modified electrically-conductive copper particles according to the present disclosure,

[0037] FIGS. 5A shows TEM (Transmission Electron Microscopy) picture of the epitaxial metal coating on the copper particles surface obtained according to the method of the present disclosure,

[0038] FIGS. 5B and 5C show HRTEM (High-resolution Transmission electron microscopy) picture of the boundary of the epitaxial metal coating on the copper surface according to the present disclosure, depicting in enlargement a copper and epitaxial metal coating surfaces,

DETAILED DESCRIPTION

[0039] According to the present disclosure, the electrically-conductive copper (Cu) particles are provided with an epitaxial coating comprising: nickel (Ni) and phosphorus: Cu/Ni—P and optionally silver: Cu/Ni—P—Ag. Such epitaxial coating of the electrically-conductive copper particles, and especially the developed coating composition, provides a significant reduction of the diffusion of copper atoms, from copper particles, into the silicon substrate. The aforementioned effect is especially relevant in the case of thermal treatment of the electrically conductive composition applied on the silicon substrate in order to create electrical paths, wherein the treatment is conducted at the temperatures of up to 950° C., involving for example sintering or calcination. Moreover, the reduced effect of copper diffusion from the electrode into the silicon substrate is also relevant during operation of solar cells comprising such the electrode.

[0040] The method for manufacturing the electrically-conductive copper with an epitaxial coating comprising nickel and phosphorus and optionally silver involves an electroless process. The formation of the coating on the surface of copper particles involves the chemical redox reaction conducted in the presence of phosphorus, oxygen, hydrogen and metal cations to be reduced, i.e.: nickel and optionally silver. As shown in FIG. 1, the product of the redox reaction is a coating comprising uncombined nickel—in a free state)(Ni.sup.0) and bonded phosphorus, and optionally uncombined silver—in a free state)(Ag.sup.0). Preferably, the obtained coating comprises nickel to phosphorus atomic ratio (Ni:P) that equals from 1 to 6. Such ratio of nickel and phosphorus provides an efficient barrier for copper diffusion. More preferably, the coating comprises nickel to phosphorus atomic ratio (Ni:P) that equals 2.2 which provides the effect of better barrier for copper diffusion, and thus reduced copper diffusion. Furthermore, the obtained coating may comprise nickel and phosphorus to silver atomic ratio (Ni+P:Ag) that equals from 900 to 1. Such ratio provides the effect of sufficient conductivity of the modified copper particles. More preferably, the coating may comprise nickel and phosphorus to silver atomic ratio (Ni+P:Ag) that equals 70, which provides the effect of better conductivity of the modified copper particles. The atomic ratio can be determined from the analysis of X-ray photoelectron spectra (XPS) or form the analysis of X-ray fluorescence spectra (XRF).

[0041] According to the method of the present disclosure, the coating is formed on the surface of copper particles having the average diameter of 0.5 to 20 μm. The average diameter can be measured by means of a high-resolution scanning electron microscope (SEM) on at least 20 grains, whereas the average diameter is calculated as an arithmetic average. Moreover, the method is suitable for coating copper particles of various surface morphology and various shapes. FIGS. 2A and 2B show SEM images of exemplary copper particles (magnitude: ×3500) that can be coated according to the present disclosure. The copper particles to be coated may be of a spherical or ellipsoidal shape as well as the shape of wires, flakes, grains, lumps, as well as other non-geometrical shapes.

[0042] Furthermore, the method may be used for coating even in case the copper particles surface includes impurities. FIG. 3 shows the exemplary concentration of impurities on the surface of copper particles, which may be coated according to the presented method. The surface of copper particles may be for example contaminated with oxide, carbon, nitrogen, chlorine or lead. The method is also suitable for copper particles of various surface wettability: hydrophilic and hydrophobic copper particles may be coated as well.

[0043] The impurities as well as different surface wettability do not affect the properties of the coated electrically-conductive copper particles obtained according the presented method.

[0044] The method for manufacturing electrically-conductive copper with nickel, phosphorous epitaxial coating consists in the introduction of Cu particles into the water bath that comprises:

[0045] at least one water-soluble nickel salt comprising nickel cation: Ni.sup.2+, and optionally at least one water-soluble silver salt comprising silver cation: Ag.sup.+, preferably the salt is selected from the group consisting of: sulfate (IV), sulfate (VI), nitrate (III), nitrate (V), acetate, chloride, and bromide. The total concentration of metals cations: nickel and optionally silver in the water bath is of 5.Math.10.sup.−2 to 200.Math.10.sup.−2 mol/dm.sup.3. Preferred salts which can be used for bath preparation are NiSO.sub.4 and optionally AgNO.sub.3,

[0046] at least one phosphoric reducing agent that comprises phosphorus on the oxidation degree of +1 to +3. The phosphoric reducing agent may be selected from various compounds such as for example phosphoric acid (I) (H.sub.3PO.sub.2) and/or salts of this acid, for example, hydrogen phosphates, phosphoric acid (III) (H.sub.2PO.sub.3) and/or salts of this acid. Moreover, the phosphoric reducing agent may be selected from phosphoric oxides such as for example phosphorus oxide (III) (P.sub.4O.sub.6). The total concentration of phosphoric reducing agent should be selected so as to obtain the molar ratio of total metal cations sum: Ni.sup.2+and Ag.sup.+—if present, to phosphoric reducing agent (Ni.sup.2++Ag.sup.+: phosphoric reducing agent) of 0.1 to 2.

[0047] at least one surfactant selected from the group of cationic, anionic and non-ionic surfactants. Preferably, the surfactant may be selected from the group consisting of dodecyl sodium sulfate (CAS no. 151-21-3), 2-bromo-3-methoxyphenol zo (CAS no. 68439-57-6), 5-chloro-2-methyl-3(2H)-isothiazolone with 2-methyl-3(2H)-isothiazolone (CAS no. 55965-84-9) and sodium laureth sulfate (CAS no. 68891-38-3). Nonetheless, another surfactant of similar activity may be also used. Preferably, the total surfactant concentration in the water bath is of 1.Math.10.sup.−7 to 1000.Math.10.sup.−7 mol/dm.sup.3. The presence of the surfactant in the bath composition provides significant reduction of the time necessary for the forming of the epitaxial metal coating on the copper particles. The used surfactant acts as a wetting agent which allows for the control of the coating process. Thus, without the addition of the surfactant, the time necessary for coating of the copper particles with the epitaxial metal coating, e.g. is of about 40 to 120 min, whereas with the addition of the surfactant into the bath, the time necessary for coating is of about 15 minutes. The addition of the above-mentioned surfactant provides repeatability of the bathing time.

[0048] Optionally, the water bath may further comprise:

[0049] at least one inorganic buffer agent for stabilizing pH of the water bath in the range of 4 to 7. For example, the buffer may be selected from the group consisting of: phosphoric acid and sodium or potassium phosphate buffer solution, boric acid and sodium borate buffer solution as well as other buffer solutions that can maintain pH level in the range of 4 to 7.

[0050] According to the present disclosure, the water bath does not comprise a complexing agent(s). Also, the water bath does not comprise an organic buffer agent(s). Such the composition of the water bath provides the process conditions in which the epitaxial thin layer can be formed on the Cu surface. The copper particles may be introduced into the water bath in various forms, preferably in a form of a powder or a suspension. The used copper particles are of the average diameter from 0.5 to 20 μm. The mass of the copper particles and the water bath volume are selected, namely 5-200 g of Cu particles are introduced per 1 liter of the water bath. The complication of the above features, namely: the lack of complexing agent and the lack of organic buffer in the water bath composition together with the selected ratio of the copper particles mass to water bath volume, provides the process conditions in which the epitaxial ultrathin metal coating is formed on the copper particles. The obtained epitaxial coating can have the thickness below 5 nm, and preferably has the thickness from 3 to 5 nm.

[0051] The water bath with the copper particles is mixed and heated to the temperature in the range of 30 to 99° C., and preferably to the temperature in the range of 80 to 95° C.

[0052] The coating process occurs in the water bath of the aforementioned composition due to the ongoing redox reaction leading to the reaction product: modified electrically-conductive copper particles.

[0053] The coating process, depending on the needs, may be repeated, at least once, and preferably twice, or three times, or four times or more. Repeating of the coating process enables to obtain the coating of required, controlled thickness. According to the presented method, one may obtain the coating thickness in the range of 1 to 5 nm (measured from TEM cross-section of the coated copper grain). The thickness of the obtained nickel/phosphorus coating or nickel/phosphorus/silver coating on the copper particles depends on the ratio of BET specific surface of Cu particles (which can be measured by the use of BET isotherm technique based on nitrogen adsorption) present in the water bath and the total mass of metal ions, namely nickel (Ni.sup.2+) and silver (Ag.sup.+) if present in the water bath. Whereas, the higher is the metal ion mass, the thicker is the coating formed on the copper particles surface.

[0054] The coating process, according to the present disclosure may be conducted in a batch or a continuous mode, depending on the needs. The continuous process may be carried out in a flow reactor, whereas the batch process may be carried out in a batch reactor.

[0055] After the coating process is completed, the coated copper particles are separated from the water bath, washed with water and dried in the air in a temperature not exceeding 200° C.

[0056] The epitaxial metal coating obtained this manner provides considerably limited diffusion of the copper through the metal coating. Furthermore, the selected thickness of the epitaxial coating of 1 to 5 nm exhibits improved mechanical strength of the coating at elevated temperatures up to 930° C. The epitaxial coatings are less prone to cracks generation. Therefore, the modified Cu particles may be used as the component of various electrically-conductive materials, even those treated at elevated temperatures (up to 930° C.).

[0057] FIG. 5A shows a TEM picture whereas FIGS. 5B and 5C show HRTEM picture of FIG. 5A. The thickness of the epitaxial metal coating obtained according to the method of the present disclosure may be measured from the TEM and SEM images together with the measurement of the Ni to Cu content in the modified copper particles: the line Ni 2p with respect the line Cu 2p by means of XPS method (X-ray photoelectron spectroscopy). The analytical calculation for evaluating the thickness of the epitaxial metal layer can be carried out by computer program: QUASES-IMFP-TPP2M Ver2.2 according to the method described in scientific publication S. Tanuma, C. J. Powell, D. R. Penn, Surf. Interf. Anal. 21 (1993) 165. The calculations give, as a result, the concentration from the 95% of the photoelectrons emitted from the given (selected) surface.

[0058] As can be observed in FIG. 5C, the epitaxial metal coating, obtained by the method according to the present disclosure, shows the ordering of the metal lattice matching the ordering of the Cu lattice. This provides increased adhesion of the metal coating to the Cu surface as well as reduces the mechanical stress within the material of coating. In addition, the thickness of such the epitaxial metal coating of 1 to 5 nm, and preferably from 3 to 5 nm provides yet improved mechanical characteristic of the coating, at the elevated temperatures, including reduced susceptibility to cracking of the epitaxial metal coating formed on the Cu particles of average diameter of 0.5 to 20 μm, including thermal treatment up to 930° C.

[0059] The copper particles, coated with nickel, phosphorus and optionally silver, constitute a low-cost replacement of a silver powder, and may be used in electrically-conductive compositions, such as pastes, inks or paints, especially for preparation of electrodes in silicon-based solar cells, in which the coated copper particles show enhanced properties due to the developed coating composition which sufficiently reduces the diffusion of copper atoms from the copper particles into the silicon-based substrate. Preferably, the electrically-conductive composition may comprise the modified electrically-conducive copper in the range of 3 to 90% by weight.

[0060] The copper particles modified according to the method of the present disclosure may be therefore used for preparation of electrically-conductive printing pastes, where the modified copper acting as a conductive paste ingredient may replace silver in whole or in part. The same applies to other electrically-conductive compositions, such as inks or paints. The preparation of the electrically-conductive composition comprising the modified copper particles may involve for example known preparation techniques, in which the modified copper is introduced in the same way as a typical conductive ingredient. Moreover, the modified copper may be used, as an additive, in commercially available electrically conductive compositions, into which the modified copper may be introduced for example in the form of powder or suspension. Such the addition provides enhancement of certain properties of electrically-conductive composition as well as reduction of silver content in the composition.

[0061] Furthermore, the modified copper particles may be used in various electrically-conductive compositions suitable, among others, for screen printing, ink-jet printing, drop on demand printing, tampon printing, typographic printing as well as other printing techniques. Preferably, such the electrically-conductive composition comprises the modified electrically-conducive copper in the range of 3 to 90% by weight.

[0062] Moreover, the modified copper according to the present method may be used for the preparation of components for other than the electrodes purposes, such as for example heating elements, heaters, electrically-conductive paths, circuits and contacts covering.

[0063] Example 1-preparation of modified electrically-conductive copper particles:

[0064] A) preparation of electrically-conductive copper particles: Cu/Ni—P:

[0065] 30 g of copper powder of copper particles having average diameter of 5 μm was introduced into 1 liter of the water bath comprising: 28 g NiSO.sub.4.7H.sub.2O, 21 g NaH.sub.2PO.sub.2H.sub.2O, 1.Math.10.sup.−6 mol of mixture of surfactants. Next, the bath was heated to the temperature of 95° C. After the formation of Ni-P coating on the Cu particles surface, the modified electrically-conducive copper particles were removed from the bath, rinsed with water and dried in the air atmosphere. The thickness of the coating measured by TEM was 4.5 nm.

[0066] B) preparation of electrically-conductive copper particles: Cu/Ni—P—Ag:

[0067] There was prepared the water bath comprising 28 g NiSO.sub.4.7H.sub.2O, 21 g NaH.sub.2PO.sub.2.1-1.sub.2O, 1.Math.10.sup.−6 mol of mixture of surfactants. Into the bath there were introduced: 10 g of AgNO.sub.3 and 30 g of copper powder of copper particles having an average diameter of 5 μm. Next, the bath was heated to the temperature of 95° C.

[0068] After the formation of the Ni—P—Ag coating on the Cu particles surface, the modified electrically-conducive copper particles were removed from the bath, rinsed with water and dried in the air atmosphere. The thickness of the coating calculated on a basis of XPS measurements (Ag 3d/Ni 2p/Cu 2p) was 4.3 nm.

[0069] Example 2-Preparation of solar cells

[0070] A series of silicon-based plates were prepared for manufacturing solar cells, each having a surface area of 5×5 cm. The plates were prepared on the basis of silicon: Cz-Si p-type (i.e. the monocrystalline silicon obtained by the Czochralski's method). Plates parameters: resistivity: 1 Ωm, thickness: 170 μm with the surface of crystallographic orientation: (100). After cleaning the surface of the plates in the chemical baths, on the plates surface there was prepared an emitter of sheet resistivity R.sub.shee of 50 Ω/square, by means of the donor doping process—from the source of POCl.sub.3 in the atmosphere of N.sub.2 and O.sub.2, within 30 minutes, at the temperature of 845° C. After removal of the enamel silicon-phosphoric layer (PSG) in the mixture of HF: H.sub.2O (1:9 v/v), on the plates, after the oxidation with dried O.sub.2, a layer of SiO.sub.2 was formed, within 10 minutes, at the temperature of 830° C. On the SiO.sub.2 layer, by means of chemical vapor deposition from (C.sub.2H.sub.5O).sub.4Ti, an antireflective layer of TiO.sub.2 of the thickness 80 nm was applied. On the face side of the solar cells, a collecting electrode was formed from the electrically-conductive paste. The paste was prepared by introducing, into a commercially available paste: PV20A, DuPont (France), the modified copper particles: Cu/Ni—P—Ag in the amount of 100 g per 100 g of paste PV20A, and/or the modified copper particles: Cu/Ni—P in the amount of 100 g per 100 g of paste PV20A (prepared according to Example 1 a and b above). On the back side of plates, there were formed contacts made from the commercially available paste PV505 Du Pont (France) and a covering surface rear electrode made from paste PV381 Du Pont (France) based of Al powder. After drying at the temperature of 200° C. within 15 minutes, the formed structures were subjected to heat treatment in an infrared (IR) belt oven with the linear speed of 200 cm/minute at the purified natural atmosphere, at the temperature range of 905-935° C. The prepared solar cells were measured at the testing station of Solar Light Simulator SS200 of class AAA of EM Photo Emission Tech., Inc. (USA) at the conditions of STC for spectrum: AM1.5G, at the lighting intensity of 1000 W/m.sup.2 and temperature of 25° C. The measurement of the bright current-voltage characteristics (I-V) of the cells was conducted using the Solar Cells I-V Curve Tracer SS I-V CT-02 PV system. Calibration of the station was performed using the reference cell No. 005-2013 from the Fraunhofer Institute of Solar Energy (Germany). The obtained test results are presented in the table in FIG. 4, where the abbreviations used indicate: T.sub.M-metallization temperature, I.sub.SC-short-circuit current, V.sub.oc-open-circuit voltage, I.sub.m-current at the maximum power point, V.sub.m-voltage at the maximum power point, P.sub.m-power at the maximum power point, F-fill factor, E.sub.ff-photovoltaic efficiency.

[0071] As follows from the table of FIG. 4, the prepared silicon-based solar cells, comprising the electrode formed from the electrically-conductive paste comprising copper particles modified according to the present invention, show parameters which correspond with the parameters of solar cells having electrodes comprising silver—as the electrically-conductive ingredient. The parameter, which indicates the correct barrier for the diffusion of copper atoms into the p-n connection zone, is the fill factor (FF) which, for the cells prepared form the paste comprising modified copper particles according to the present disclosure, equals 0.74 and 0.75, respectively.