WO2022075884A1

WO2022075884A1 - Polymeric electrically conductive paste for solar cells with heterojunctions

Info

Publication number: WO2022075884A1
Application number: PCT/RU2021/050132
Authority: WO
Inventors: Максим Михайлович ВЛАСЕНКО; Александра Александровна БУЛГАКОВА; Иван Сергеевич МИТЧЕНКО; Нина Ивановна ШАЛЬКО; Илья Алексеевич АГАРКОВ; Алена Геннадиевна ВАРАВИНА; Антон Андреевич ОСТРЫЙ; Мария Александровна ПОНОМАРЕНКО
Original assignee: Общество с ограниченной ответственностью "Научное Предприятие Монокристалл Пасты"
Priority date: 2020-10-06
Filing date: 2021-05-17
Publication date: 2022-04-14
Also published as: RU2746270C1; CN116325026A

Abstract

A polymeric electrically conductive paste for solar cells with heterojunctions comprises silver powder, an organic binder and a functional additive. A halogen-containing polymer with a softening point below 200˚C is used as a structuring component, and a polymeric organosilicon compound with fewer than 3000 siloxane units is used as the functional additive, with the following ratio of components: 80-95 wt% silver powder, 4-18 wt% organic binder and 0.1-2.0 wt% functional additive. The silver powder can have a mean particle size D50 of 0.5-5.0 μm. The quantity of the structuring component included in the organic binder can constitute 5-10 wt%. The technical result is to increase the efficiency of the solar converter, decrease the degree of surface tension of the paste, improve the screen printing process and the topology of the conductive contact, decrease the linear and specific resistance of the conductor, and increase conductor conductivity.

Description

POLYMERIC CONDUCTIVE PASTE FOR SOLAR CELLS WITH HETERJUNCTIONS

Technical field

The present invention relates to thick-film microelectronics, namely to materials for the manufacture of electrically conductive contact layers by screen printing, and can be used to form a contact grid of solar cells.

Prior Art

Semiconductor solar cells are made from a semiconductor material such as silicon. Contacts on the surface of a silicon substrate can be obtained by applying a conductive liquid suspension (hereinafter paste) by screen printing.

The composition of the paste includes, as a rule, a conductive component, which is a finely dispersed metal powder, a structure-forming component, an organic solvent, and functional additives for various purposes to ensure the specified printing and technical characteristics of a thick-film material.

One of the main trends in the production of thick-film solar cells at present is to reduce the width of the conductive contact grid based on conductive paste and, as a result, increase the efficiency of the solar converter by reducing the linear resistance of the conductor. This result is achieved by introducing an integrated approach in the development of the composition of the conductive paste, namely, the use finely dispersed metal powder with specified granulometric parameters that ensure stable printing characteristics of the paste; a solvent or a mixture of solvents that provide the required level of wettability of the solid phase and the substrate and, as a consequence, the relative monodispersity of the final product; a polymer component with a given range of molecular weight and the presence of functional groups; one or more functional additives.

The composition of the conductive paste may contain the following components:

(A) at least one conductive filler,

(B) structure-forming component,

(B) at least one functional additive.

The term "conductive" means the property of thermal conductivity and/or electrical conductivity.

As the conductive filler (A), a powder of silver, platinum, palladium, nickel, copper and/or various compositions of the above is used. The particle shape of the metal powder may be spherical, flake, or a mixture of the above.

As a structure-forming component (B), a component based on a thermosetting and/or thermoplastic resin is used. The structure-forming component can be polyester resins, phenolic resins, phenol-formaldehyde resins, epoxy resins, polyacrylates and various compositions of the above.

As a functional additive, dispersants, emulsifiers, slip additives, adhesive additives, thixotropic additives, defoamers, rheological additives and various compositions can act. of the above, providing the paste with the required complex of printing technical characteristics.

The composition of the conductive paste may also contain a solvent (D).

As solvents, glycols such as diethylene glycol monomethyl ether, diethylene glycol monoethyl ether, diethylene glycol monobutyl ether, ethylene glycol monomethyl ether, ethylene glycol monoethyl ether, ethylene glycol monobutyl ether, propylene glycol monomethyl ether and propylene glycol monoethyl ether, esters such as 2,2 ,4-trimethyl-1,3-pentanediol monoisobutyrate, 2,2,4-trimethyl-1,3-pentanedioldiisobutyrate and the like, ketones such as cyclohexanone and isophorone, terpineol. It is preferable to use high boiling solvents to avoid problems during the screen printing and drying process. Preferred high boiling solvents include diethylene glycol monoethyl ether acetate, diethylene glycol monobutyl ether acetate, tripropylene glycol monomethyl ether, 2,2,4-trimethyl-1,3-pentanediol monoisobutyrate, terpineol, and the like.

A conductive paste is obtained by mixing and then homogenizing components (A), (B) and (C) in a certain proportion using a mixer, a three-roller, a gyromixer.

Brief description of drawings and other materials

In FIG. Figure 1 shows a schematic representation of a section of the front surface of a solar cell (SC) with a part of a conductive contact grid in the form of two parallel conductors, the geometric characteristics are indicated: L - conductor length, h - conductor height, d - conductor width. In FIG. 2 shows the dependence of the absolute value of the efficiency on the relative reduction in the width of the conductor d at a constant cross-sectional area of the conductor Seen.

In FIG. 3 shows the dependence of the absolute value of the efficiency on the relative reduction in the conductor width d at a constant conductor height h.

In FIG. Figure 4 shows the distribution profile of the velocities of thin layers of metallization paste in its volume along the cross section of the screen opening in the case of turbulent flow: a) when the paste is punched; b) when separating the substrate and stencil.

In FIG. Figure 5 shows the distribution profile of the velocities of thin layers of metallization paste in its volume along the cross section of the stencil opening in the case of laminar flow: a) when the paste is punched through; b) when separating the substrate and stencil.

In FIG. Figure 6 shows the topology of conductors made using stencils with different opening widths: a - opening width 60 µm; b - opening width 40 µm.

In FIG. 7 shows the dependence of the linear resistance of the conductor Rline, [Ohm / cm], on the width of the stencil opening, [Ohm].

In FIG. Figure 8 shows the flow curves of samples of metallization pastes with different nature of interaction with the elements of equipment for screen printing: sample A is highlighted in red, sample B is highlighted in green.

In FIG. 9 shows photographs of the traces for the samples from Examples 1-4.

In FIG. 10 shows the results of the track width measurements for the samples from Examples 5-7. In FIG. 11 shows the results of measurements of the CVC for the samples from Examples 5-7.

In FIG. 12 shows micrographs of the SC section with samples from Examples 8-12 after the drying stage (a-8, b-9, c-10, d-11, e-12) and after the sintering stage (al, 61, c1, d1, e1 ).

In FIG. 13 shows the resistivity measurements for the samples of Examples 8-12 after the drying step (pl) and the sintering step (p2).

In FIG. 14-15 show the results of measurements of the I-V characteristics for the samples from Examples 8-12.

Disclosure of invention

The design of solar-to-electrical energy converters (solar cells, solar cells, solar cells) includes one or more conductive contacts. Most types of modern solar cells (mono/multi/amorphous Si, OPV, thin-film) are characterized by the placement of conductive contacts on a surface that absorbs solar radiation (front surface, front side); in this case, the collection and transfer of electric charge carriers from the generation region to the external electric circuit is ensured by making a conductive contact in the form of a grid of thin parallel conductors (Fig. 1). This design is most widespread, since it provides a significant reduction in costs in the mass production of solar cells, despite the partial shading of the front side by the contact located on it.

With the development of technology, the optimization of the geometric dimensions of the current-carrying contact has become an increasingly significant factor in the growth of the efficiency of the solar cell (COP). In FIG. 1 shows a schematic representation of a section of the front surface of the solar cell with a part of the conductive contact grid in the form of two parallel conductors.

The fraction of the solar cell surface, unoccupied by the contact conductor, is characterized by an effective area of 8 _E ff. For example, in the case of a solar cell with a front contact grid containing 5 busbars and about 100 parallel conductors, the value of 8 _eff is at least 95% of the total front surface area. An increase in 8 _eff helps to improve the efficiency of the solar cell due to a larger number of photons absorbed by the front surface and, accordingly, a larger number of generated charge carriers. As follows from FIG. 1, an increase in 8 _E ff is possible by reducing the width of the conductors d. In this case, depending on the ratio of the final width d and the height of the conductor h, the value of the linear electrical resistance of the conductor Rune changes due to the well-known relation

Inline ~ P " L / S _ce4 , (1) where p is the specific resistance of the conductor material, [Ohm cm];

L - conductor length, [cm];

S _ce4 ~ dh - cross-sectional area of the conductor, [cm ² ] .

The Riine parameter, which characterizes the contact conductor, is one of the components of the series resistance of the solar cell, and, along with other "parasitic" resistances, has a negative impact on the final efficiency value. The contribution of the width d to the efficiency value achieved to date, provided that the cross-sectional area of the conductor S _ce4 is unchanged and, accordingly, R _line is unchanged, is shown in the graph in Figure 2, according to which a decrease in d by 5% leads to an increase in the absolute value of the efficiency by 0.04% per due to the above-described effect of increasing the number of generated charge carriers. With a decrease in d, the condition for maintaining the values of S _ce4 and, accordingly, Riine constant is achieved due to the proportional increasing the height of the conductor h. If, as d decreases, the conductor height h remains unchanged (or decreases), then due to an increase in R _line (according to formula 1), the dependence AEff(d) shows a negative trend, fully compensating the effect of increased carrier generation (Fig. 3). Therefore, in the development and manufacture of high-performance solar cells, it is necessary to balance as accurately as possible between the values of the cross-sectional area and the linear resistance of the conductor S _ce4 and Rii _ne - This circumstance imposes serious requirements on the stability and reproducibility of the method of manufacturing a conductive contact.

Currently, screen printing is the most common metallization technology, as it most fully meets the criteria for mass industrial production. In the screen printing process, the geometric dimensions of the conductor are determined by the opening width in the emulsion layer of the stencil. During the process, the metallization paste is pressed through the opening of the stencil due to the pressure applied from the side of the elastic squeegee, thus forming a conductor. In the general case, the velocity distribution profile of thin layers of paste in its volume along the cross section of the stencil opening has the form shown schematically in Fig. 4, while the nature of the flow of the paste along the walls of the opening is largely turbulent. Turbulence in this context means the presence of strong interactions between the wall of the opening and the thin layer of paste directly adjacent to the wall. Such an interaction leads to an uneven distribution of velocities in the volume of the paste, both when it is punched through and when the substrate and stencil are separated, as a result of which defects in the form of splashes, drops and halo; the presence of such defects leads to a significant variation in the local values S _ce4 of the conductor. Stepwise narrowing of the opening width in the course of optimizing the geometry of the conductive contact enhances the contribution of the turbulent flow of the paste to the variability of the conductor manufacturing process. For stencil openings with a width of 50 μm or less, a metallization paste is preferred, which is characterized by a predominantly laminar flow near the opening walls (Fig. 5). In laminar flow, the force of interaction between the opening wall and the thin layer of paste adjacent to it is much less pronounced; in this case, a thin layer of paste slides along the opening wall, which leads to a smoothing of the velocity distribution profile. As a result, defects in the form of splashes, drops and halo are not formed, the shape of the conductor more fully reproduces the shape of the screen opening, and the conductor is characterized by a stable and reproducible cross-sectional area S _ce4 .

In addition to the interaction between the paste and the opening walls of the stencil, which affects the linear resistance of the conductor Ri _ine through local variations in its cross-sectional area S _ce4 , a number of technological factors also affect the R _line value of the conductor during screen printing. The basis for the design of the most common type of equipment for screen printing - a mesh stencil - is a grid in the form of interlaced threads that form a strictly regular array of cells of a fixed volume. The paste enters the opening area directly through the cells separated from each other by a distance equal to the thickness of the thread. In the course of optimizing the geometric parameters of the current-carrying contact, when the opening width is narrowed to 50 μm or less, the structure of the manufactured conductor largely begins to reproduce the periodic structure of the stencil grid, as can be seen in the photographs below. Here in FIG. 6. a shows a photograph of a section of the conductor made using a stencil opening 60 µm wide. This conductor is characterized by a pronounced linear structure, while the shape of the conductor made using a screen opening 40 μm wide (Fig. 6.6) is not linear, but is characterized by alternating local thickenings and narrowings (constrictions), which makes an additional contribution to the linear conductor resistance Rii _ne - The dependence of conductor Rime on the width of the stencil opening used in its manufacture is shown in FIG. 7. As follows from the presented graph, when the opening width is reduced to less than 50 µm, the corresponding increase in the linear resistance of the manufactured conductor becomes exponential. To eliminate the effect of the formation of local thickening/narrowing, the metallization paste should be characterized by the property of rapidly passing through cells of small and ultra-small volume, followed by uniform filling of the screen opening area, which is also facilitated by the laminar flow mechanism described above.

The metallization paste is a liquid highly filled suspension, which, in addition to the electrically conductive filler, includes organic solvents and polymer compounds of various spatial structures with a wide range of molecular weights. Rheological methods are widely used to study the properties of liquid systems whose macroscopic properties are determined by the forces of intermolecular and interparticle interaction. Qualitative determination of the nature of the interaction of the metallization paste with the elements of the technological screen printing tooling is possible with a standard CR rheometer. Constructed in the coordinates “shear rate y [c' ¹ ] - shear stress t [Pa]”, the flow curves of samples of metallization pastes A and B (Fig. 8) are characterized by areas of a linear increase in shear stress I, due to the gradual destruction during testing of the structural frame made of intertwined polymer chains in the bulk of the samples. The change in the slope of the curves after reaching a shear rate of 40 s' ¹ indicates the completion of the process of disentangling the polymer molecules and their predominant orientation in the direction of rotation of the rotor. The location of section II of the flow curve of sample A is practically parallel to the abscissa axis indicating a weakly pronounced dependence of the shear stress on the shear rate after 40 s' ¹ . At the micro level, this means that the mechanical energy of the rotor rotation with a further increase in the shear rate is completely transferred to the paste layer immediately adjacent to it due to the forces of interparticle interaction and is dissipated in the sample volume during collisions of the particles of the conductive filler. Thus, the nature of the flow curve of sample A with section II parallel to the abscissa axis makes it possible to judge the manifestation of strongly pronounced interactions of this sample of metallization paste with the structural elements of the rheometer. In the case of sample B, the nature of the flow curve, which after 40 s' ¹ demonstrates a negative slope, indicates the presence of specific interactions responsible for weakening the response of the sample to the force applied from the rotor after reaching a certain threshold value of the shear rate. At the micro level, this effect is explained by the delamination of the metallization paste sample, in which, as the shear rate increases further, in a thin layer between the paste and the tooling surface rheometer, a layer of polymer molecules is released, which acts as a lubricant.

Thus, a metallization paste for high-performance solar cells, suitable for applying narrow conductive conductors with stable local S _ce4 and low Ri _ine by screen printing, when measuring its rheological characteristics in the coordinates "shear rate y [s' ¹ ] - shear stress t [Pa ]» is characterized by a negative slope of the flow curve when the threshold value of the shear rate equal to 40 s' ¹ is exceeded.

To implement the described slip mechanism, which contributes to the laminar flow of the metallization paste along the elements of technological equipment in the process of screen printing, functional additives are introduced into the paste, which can be siloxanes.

Applications W02009035453 (published on March 19, 2009) and US20180163063 (published on June 14, 2018) are known for an electrically conductive composition with polysiloxane in the composition, where this material can act as one of the components of an organic binder along with other thermoplastic and thermosetting polymers. But these patent applications do not disclose the mechanism by which the polysiloxane gives advantages to electrically conductive compositions based on it.

In the composition of electrically conductive materials according to applications US20030107026 (published on June 12, 2003) and SA2461011 (published on April 3, 2003), polyorganosiloxane is the main component of the organic binder, but the described materials themselves are not intended for the manufacture of an electrically conductive contact similar to this invention. To minimize resistivity, it is necessary to use a submicron powder; it sinters better at low temperatures of the order of 200°C. But with a decrease in the particle size, the forces increase, leading to agglomeration due to an increase in the surface area of 8 _B et-

With an increase in the surface area of the powder 8 _Sbt , the processes leading to the formation of an oxide film on the surface of silver particles are more active and the role of subsequent surface treatment of the powders to prevent oxidation and agglomeration increases.

As a surface treatment, fatty acids with boiling points of 350-400°C are used, which also prevent powder particles from sintering at a temperature of 200°C (the highest possible temperature of the HJT process) and thereby increase the resistivity of the annealed conductor.

When minimizing the specific resistance for more efficient sintering of particles, it is necessary to find chemical elements that will clean the surface of oxide and fatty acid residues and chemically activate the surface by reducing the energy of particle fusion. These chemicals during the annealing process should easily leave the conductor layer and not disturb the ITO structure.

The technical solution in this case is the use of polymers stabilized by halides. Namely, halogen-containing polymers with a softening point below 200 °C. In particular, chlorine-containing polymers, in which the chlorine atom has increased mobility. A chlorine-containing polymer is known, the melting point of which is 165-170 ° C, however, when heated above 35 ° C, degradation processes begin in it, accompanied by the elimination of atomic chlorine, followed by the formation of hydrogen chloride, causing intense destruction of macrochains.

The objective of the present invention is to provide the required printing, technical and thermal characteristics of the paste, meeting the current trends in the field of "fine" printing. Namely, the preservation of the complex of rheological properties of the paste while reducing the width of the stencil openings due to the optimal ratio of all components and by introducing additives that provide the above-described flow pattern in the screen printing process. As well as the use of halogen-containing polymers with a low temperature of the onset of decomposition, which ensure efficient sintering of silver powder already at the stage of drying the conductive contacts.

The technical result consists in increasing the efficiency of the solar converter by reducing the linear resistance of the conductor by reducing the track width on the one hand, and by reducing the resistivity of the conductor by more efficient sintering of fine silver powder on the other.

This technical result is achieved by the fact that the polymer conductive paste for solar cells with heterojunctions includes silver powder, an organic binder containing a structure-forming component in the solvent composition, and a functional additive, and a halogen-containing polymer with a softening temperature below 200 ° C is used as a structure-forming component in the composition. , and as a functional additive, a polymeric organosilicon compound is used with the number of siloxane units less than 3000, in the following ratio of components, in wt.%: silver powder - 80-95; organic binder - 4-18; functional additive - 0.1 -2.0. The silver powder may have an average particle size D50 of 0.5-5.0 µm.

The amount of structure-forming component in the composition of the organic binder may be 5-10 wt.%.

The functional additive may contain additional components.

When measuring the rheological characteristics of the conductive paste in the coordinates "shear rate y [s-1] - shear stress t [Pa]", the flow curve is characterized by a negative slope when the threshold shear rate of 40 s-1 is exceeded.

The conductive paste includes finely dispersed silver powder, a structure-forming component in a solvent, and a rheological additive that ensures the optimal topology of the printed track. The fine silver powder has an average particle size D50=0.5-5.0 µm, more preferably D50=l, 0-3.0 µm. The content of the silver powder can vary within the range of 80-95 wt.%, preferably the content of silver 85-92 wt.%. The builder may be a polyvalent epoxy resin with two or more epoxy groups per molecule. For example, epichlorohydrin and phenol novolak, cresol novolac, polyhydric phenols, ethylene glycol and bisphenol A, hydrogenated bisphenol A, bisphenol F, bisphenol AD, resorcinol. These epoxy resins can be used singly or in combination. As a rheological additive, a polyamide can be used, namely a non-polar polyamide modified with urea; a polyolefin, in particular an emulsion of polyethylene with a molecular weight of 1000-3000; esters of fatty acids, preferably with 26-30 carbon atoms. Also as additives that improve printing properties, polymeric organosilicon compounds can be used, preferably silicone fluids with a number of siloxane units of less than 3000. It is the additive from the above series of organosilicon compounds that provides the required rheological behavior by reducing the surface tension at the paste-stencil interface and giving the paste "slip ” (reducing the friction of the conductive paste on the walls of the stencil openings), which results in a uniform print of the paste during the screen printing process without the “sticking off” effect, which is especially important for modern printing of tracks up to 30 microns wide. Said rheological additive is essentially inert, i.e. does not significantly affect the electrophysical and mechanical parameters of the solar converter, but significantly improves the process of passing the paste through the stencil, minimizing possible defects. As the structure-forming component, it is preferable to use a halogen-containing polymer with a low degradation temperature, namely, a chlorine-containing polymer. A macromolecule of this type of polymer undergoes degradation at a relatively low temperature, as a result of which additional energy is released, which contributes to the thermal catalysis of the sintering of silver particles in the conductor layer and, as a result, a decrease in the resistivity level of the conductor. Thus, the technical result is provided by the presence of two key factors of the present invention: a rheological additive from a number of siloxanes and a halogen-containing polymer with a low decomposition temperature.

In the present invention, fine silver powder is used, the average particle size of which is D50 0.5-5.0 µm. If the average particle size D50 of the silver powder exceeds 5.0 μm, then there is a tendency to reduce the dynamic viscosity of the paste, which, in turn, leads to a deterioration in printing properties, spreading of the conductor track. Otherwise, when the average particle size D50 decreases below 0.5 µm, dynamic viscosity increases, which can also lead to a decrease in print quality due to uneven distribution of the paste, the appearance of gaps in the conductor tracks. By reducing the dynamic viscosity by reducing the polymer content, there may be a tendency for the adhesive strength to deteriorate.

The organic binder contains a mixture of high-boiling solvents and a structure-forming component. As a structure-forming component in this invention, a halogen-containing thermoplastic polymer with a softening point below 200 ° C is used. If the content of the structure-forming component exceeds 10% of the composition of the organic binder, then the viscosity of the paste increases and, accordingly, its printing characteristics deteriorate. In addition, with an increased content of the structure-forming component, there is a tendency to increase the linear resistance in the composition of the solar cell. When the content of the structure-forming component is below 5% in the composition of the organic binder, there is a deterioration in the printing properties of the conductive paste due to the loss of plasticity.

In addition, the conductive paste according to the present invention contains a functional rheological additive that reduces the surface tension of the paste and thereby improves the passage of the paste through the stencil during printing to obtain an optimal topology of conductive contacts. Specified rheological additive refers to a series of polymeric organosilicon compounds with less than 3000 siloxane units. In the present invention, it is preferable to use 0.1-2.0% of the above additive. With the introduction of the additive at a concentration below 0.1%, the required printing characteristics are not provided. Exceeding 2.0% siloxane additive leads to an increase in resistivity.

The main characteristic of the composition of the conductive paste, in accordance with the present invention, is the use of a halogen-containing polymer as a structure-forming component, which ensures efficient sintering of silver particles, leading to an increase in the conductivity of the conductive layer, and the introduction of a functional additive based on polymeric siloxane organic compounds, which improves the printing and technical characteristics of the paste .

The optimal quantitative composition of the paste is confirmed by the fact that with the introduction of its components above or below the claimed limits, the required operational and rheological properties are not provided.

The analysis of the prior art showed that the claimed set of essential features set forth in the claims is unknown. This allows us to conclude that the claimed technical solution complies with the condition of patentability "novelty".

Comparative analysis showed that the prior art did not identify solutions that have features that coincide with the features of the claimed invention, and the knowledge of the influence of these features on the technical result has not been confirmed. Thus, the declared technical the solution satisfies the patentability condition "inventive step".

Implementation of the invention

Conductive paste is prepared as follows. A predetermined amount of all the above components is weighed in the mixer tank, mixed using a planetary mixer. Next, the paste is homogenized by means of a three-roll pasto grater until the required degree of grinding is obtained.

The measurement of the degree of grinding is carried out using a Hegman grindometer. The device consists of a measuring plate with a wedge-shaped groove and a scraper. A sample of paste in an amount sufficient to fill the entire groove is placed beyond the upper limit of the scale. The scraper is set perpendicular to the measuring surface and moved at an angle of 90° within a few seconds from the maximum value of the scale to zero.

Dynamic viscosity is measured on a rotational viscometer of the "plate-cone" system. The principle of operation is based on the dependence of torque on viscosity, which causes the sample to resist displacement.

Example 1

For the preparation of a conductive paste, the following were used: silver powder, the average particle size of which is 0.5–5.0 μm, in an amount of 80–95 wt.%, organic binder, including a structure-forming component and a solvent, functional additive A based on a polyacrylate solution in an amount 0.1 -2.0 wt.%.

Example 2

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in an amount of 80-95 wt.%, an organic binder, including a structure-forming component and a solvent, a functional additive B based on polydimethylsiloxane modified with polyester in an amount of 0.1 -2.0 wt.%

Example 3

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in the amount of 80-95 wt.%, an organic binder, including a structure-forming component and a solvent, a functional additive C based on polydimethylsiloxane modified with polyester in amount of 0.1 -2.0 wt.%

Example 4

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in an amount of 80-95 wt.%, an organic binder, including a structure-forming component and a solvent, a functional additive D based on polydimethylsiloxane in an amount of 0, 1 -2.0 wt.%.

The study of the properties of the above samples was carried out in the composition of solar cells with heterojunctions. The basis of the ST technology is an n-type crystalline silicon substrate, on which thin films of amorphous silicon are deposited by means of plasma-chemical vapor deposition, followed by the formation of heterojunctions, the next step is the deposition of an ITO film, on which current-collecting contacts with silver-containing paste are printed.

Current-collecting contacts with the above samples were applied to the structures by screen printing on an EKRA E2 semi-automatic printer. Drying was carried out in a conveyor dryer "JRT" DT-040-Rk-X, baking was carried out in a drying oven "Heraeus" D-6450 at a temperature of 210°C for 15 minutes. The width of the conductor tracks was measured using an Olympus microscope. The measurement of the height of the conductor tracks was carried out on a microscope "Leitz" 512761-20

Printing-technical and electrophysical parameters of the obtained conductive pastes from examples 1-4 are shown in table 1.

As can be seen from table 1, samples of conductive pastes from Examples 1-4 have a satisfactory degree of grinding and dynamic viscosity, thus providing the required printing performance properties. In addition, there is a direct dependence of the width of the conductor track on the conditional coefficient of surface tension. Thus, the sample from Example 4 with a functional additive D based on polydimethylsiloxane has the highest conditional coefficient of surface tension, thereby reducing the degree of friction of the paste against the walls of the openings of the stencil and providing the topologically most optimal contact grid. In addition, the sample from Example 4 has the lowest value of line resistance, which confirms the above theoretical part.

In FIG. 9 shows photographs of the conductor tracks from the examples described above. The improvement in the profile of the conductive track is clearly visible, both from the side of reducing its width, and from the side of reducing the edge "blots" (Fig. 9).

Example 5

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in the amount of 80-95 wt.%, an organic binder, including a structure-forming component and a solvent.

Example 6

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in the amount of 80-95 wt.%, an organic binder, including a structure-forming component and a solvent, a functional additive based on polymeric siloxane organic compounds in the amount of 0.25 wt.%.

Example 7

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in an amount of 80-95 wt.%, an organic binder, including a structure-forming component and a solvent, a functional additive based on polymeric siloxane organic compounds in an amount of 0.50 wt%. The study of the properties of the samples from Examples 5-7 was carried out in the same way as for the samples from Examples 1-4.

In FIG. 10 shows the results of measurements of the width of the conductor track for the samples from Examples 5-7.

In FIG. 11 shows the results of measurements of current-voltage characteristics for samples from Examples 5-7.

As can be seen from the results presented in Fig. 6, the sample from Example 5 without functional additive has the widest conductive path. With an increase in the content of functional additives, there is a tendency to reduce the width of the track (Example 6 and Example 7). The obtained results correlate with the measurement data of the CVC (Fig. 11). The introduction of a functional additive that reduces paste spreading makes it possible to increase the short circuit current and reduce the value of the series resistance in the composition of solar cells.

Example 8

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in the amount of 80-95 wt.%, an organic binder, including a structure-forming component based on a halogen-containing thermoplastic polymer in the amount of 5-10 wt.% of the composition of the organic binder and solvent.

Example 9

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in an amount of 80-95 wt.%, an organic binder, including a structure-forming component based on phenoxy resin in an amount of 5-10 wt.% of the composition organic binder and solvent.

Example 10

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in an amount of 80-95 wt.%, an organic binder, including a structure-forming component based on acrylate resin in an amount of 5-10 wt.% of composition of the organic binder and solvent.

Example 11

To prepare a conductive paste, silver powder was used, the average particle size of which is 0.5-5.0 μm, in an amount of 80-95 wt.%, an organic binder, including a structure-forming component based on epoxy resin in an amount of 5-10 wt.% of composition of the organic binder and solvent.

Example 12

The study of the properties of the samples from Examples 8-12 was carried out in the same way as for the samples from the above Examples.

Samples from Examples 8 and 12 based on halogen-containing thermoplastic polymers have a reduced resistivity already after the drying stage (Fig. 13), which confirms the theory of catalytic sintering of silver particles through thermal degradation of the halogen-containing polymer at low temperature. This theory is also indirectly confirmed by the presented microphotographs (Fig. 12), in which samples a) and d) (Example 8 and 12, respectively) show partial sintering of silver powder already at the drying stage.

The samples from Examples 8, 10, 11, 12 showed a comparable level of efficiency in the composition of solar cells, while the samples from Examples 10 and 11 did not have sufficient adhesion to the substrate, which also affected the measurement of the short circuit current for these samples. The sample from Example 9 has a high level of series resistance and, as a result, low efficiency (Fig. 14-15).

Industrial Applicability

The polymer conductive paste for solar cells with heterojunctions according to the present invention contains a silver powder with an average particle size of D50 of 0.5-5.0 μm with a content of 80-95 wt.% in the composition of the paste, an organosilicon polymer additive that reduces the degree of surface tension of the paste and, thereby improving the screen printing process and the topology of the conductive contact and, as a result, reducing the linear resistance conductor, a structure-forming component based on a halogen-containing polymer, which makes it possible to reduce the resistivity of the conductor and increase its conductivity through the process of thermal catalytic sintering of silver particles. This composition allows us to conclude that its composition is unique in comparison with existing analogues.

The conductive paste according to the present invention can be used in solar cells with a bake-in temperature below 210°C.

Claims

Claim

1. Polymer conductive paste for solar cells with heterojunctions, including silver powder, an organic binder containing a structure-forming component in the solvent composition, and a functional additive, moreover, a halogen-containing polymer with a softening temperature below 200 ° C is used as a structure-forming component in the composition, and functional additive, a polymeric organosilicon compound is used with the number of siloxane units less than 3000, in the following ratio of components, in wt.%:

Silver powder - 80-95

Organic binder - 4-18

Functional additive - 0.1 -2.0

2. Conductive paste according to claim 1, characterized in that the silver powder has an average particle size D50 of 0.5 -5.0 µm.

3. Conductive paste on and. 1, characterized in that the amount of the structure-forming component in the composition of the organic binder is 5-10 wt.%.

4. Conductive paste on and. 1, characterized in that the functional additive contains additional components.

5. Conductive paste on and. 1, characterized in that, when measuring its rheological characteristics in the coordinates "shear rate y [s' ¹ ] - shear stress t [Pa]", the flow curve is characterized by a negative slope when the threshold value of the shear rate equal to 40 s' ¹ is exceeded.

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