WO2016087820A1

WO2016087820A1 - Conductive paste, electrode, solar cell and method

Info

Publication number: WO2016087820A1
Application number: PCT/GB2015/053597
Authority: WO
Inventors: Luca MARETTI
Original assignee: Johnson Matthey Public Limited Company
Priority date: 2014-12-05
Filing date: 2015-11-25
Publication date: 2016-06-09
Also published as: TW201631056A; GB201421658D0

Abstract

The present invention relates to conductive pastes, suitable for use in solar cells, a process for preparing a conductive paste, a light receiving electrode for a solar cell and a solar cell. The paste comprises a solids portion dispersed in an organic medium, wherein the organic medium comprises an ammonium salt of a phosphate ester free acid. The invention further relates to a method of printing.

Description

CONDUCTIVE PASTE, ELECTRODE, SOLAR CELL AND METHOD

Field of the Invention

The present invention relates to conductive pastes suitable for use in solar cells, to a method of manufacturing a light receiving surface electrode of a solar cell, to a light receiving surface for a solar cell, to an organic medium for inclusion in a conductive paste suitable for use in solar cells and to a method of printing.

Background of the Invention

Screen printed metal (e.g. silver) pastes are routinely used to form conductive tracks in electronics and semiconductor applications, for example in solar cells (e.g. silicon solar cells). The pastes typically comprise metal (e.g. silver) powder, mixed oxide (e.g. glass frit), and sometimes one or more additional additives, all dispersed in an organic medium. The organic medium plays an important role in the conductive pastes. For example, the properties of the organic medium are important for enabling reliable and large scale paste production, for providing pastes with good printability and providing the paste with good green strength. After printing and drying (which is typically carried out at about 200^eC), the paste should have good green strength allowing, if necessary, transportation and handling of the solar cell without damaging the lines.

Additionally, the ability to print very fine lines of the conductive paste is important for solar cell efficiency. Narrow, highly defined lines maximise the exposed area of the light receiving surface of a solar cell, enhancing its efficiency. However, the shape of the printed line is also important: a larger cross section reduces resistivity. Therefore, the preferred line shape is one with a narrow, tall cross section as this exposes as much light receiving surface as possible while minimising resistivity.

The ability to print fine lines with the desired cross sectional shape depends strongly on the rheology of the paste, and the nature of the organic medium can be important in providing pastes exhibiting suitable rheological behaviour. Particularly important is a highly structured paste, which retains its structure under the high strain conditions observed in screen printing. The present inventors have found that pastes can lose their structure at the point of screen printing, for example by solid / liquid phase separations during printing. This leads to an accumulation of paste or paste components on the underside of the screen, which causes the formation of dirt at the edges of the printed lines. This dirt is undesirable as it covers part of the light receiving surface of the cell without contributing to the conductivity of the printed lines. It also increases paste wastage.

Summary of the Invention

There remains a need for improved conductive pastes which exhibit good screen printability, such as pastes which are suitable for fine line printing with minimal dirt at the line edges. There remains a need for pastes with improved rheological behaviour.

As demonstrated in the Examples below, the present inventors have surprisingly found that the inclusion of an ammonium salt of a phosphate ester free acid in a screen printable paste provides good rheological behaviour and good screen printability. Accordingly, in a general aspect the present invention relates to the inclusion of an ammonium salt of a phosphate ester free acid in a screen printable paste.

In a first preferred aspect, the present invention provides a conductive paste, the paste comprising a solids portion dispersed in an organic medium, the solids portion comprising electrically conductive metal and mixed oxide, wherein the organic medium comprises an ammonium salt of a phosphate ester free acid. Typically, the ammonium salt of a phosphate ester free acid is the reaction product of a phosphate ester free acid and an amine.

Typically, the conductive paste is prepared by mixing together electrically conductive metal, mixed oxide and the components of the organic medium, in any order. In a second preferred aspect, the present invention provides a process for preparing a conductive paste, wherein the process comprises mixing together the electrically conductive metal, the mixed oxide and the components of the organic medium, in any order, and wherein the organic medium comprises an ammonium salt of a phosphate ester free acid. In a third preferred aspect, the present invention provides a light receiving electrode for a solar cell, the light receiving electrode comprising a conductive track on a semiconductor substrate, wherein the conductive track is obtained or obtainable by firing a paste according to the first aspect on the semiconductor substrate. In a fourth preferred aspect, the present invention provides a solar cell, comprising a light receiving electrode according to the third aspect. In a further preferred aspect the present invention provides use of a conductive paste according to the first aspect in the manufacture of a light receiving surface electrode of a solar cell. In a still further preferred aspect the present invention provides use of a conductive paste according to the first aspect in the manufacture of a solar cell.

In a further preferred aspect, the present invention provides an organic medium for use in a screen-printable paste, the organic medium comprising an ammonium salt of a phosphate ester free acid dispersed or dissolved in a solvent. Screen printable pastes comprising the organic medium of the present invention may be conductive pastes. Alternatively, the organic medium of the present invention may be employed in screen printable pastes suitable for use in other applications, for example, the printing of enamel onto a glass surface, such as automotive glass. Thus, in a further aspect, the present invention provides a method of printing comprising applying a paste composition to a substrate by screen-printing, wherein the paste

composition comprises a solids portion dispersed in an organic medium and wherein the organic medium comprises an ammonium salt of a phosphate ester free acid. The substrate may be a semiconductor substrate and the paste composition may be a conductive paste wherein the solids portion comprises electrically conductive metal and mixed oxide.

Alternatively, the substrate may be a glass surface, such as an automotive windscreen, and the conductive paste may be an enamel composition wherein the solids portion comprises pigment and mixed oxide. Brief Description of the Drawings

Figure 1 shows a screen printed line using the paste of Comparative Example 1 .

Figures 2 to 6 show screen printed lines using the pastes of Examples 1 to 5, respectively.

Figure 7 shows the results of rheological testing carried out as described in the Examples section below.

Figure 8 shows a screen printed line using the paste of Comparative Example 2.

Figure 9 shows a screen printed line using the paste of Example 6. Detailed Description

Preferred and/or optional features of the invention will now be set out. Any aspect of the invention may be combined with any other aspect of the invention unless the context demands otherwise. Any of the preferred and/or optional features of any aspect may be combined, either singly or in combination, with any aspect of the invention unless the context demands otherwise.

Conductive Paste

Typically, the conductive paste is a conductive paste for a solar cell. Typically, it is a front side conductive paste. Typically, it is a screen printable conductive paste.

(As the skilled person will understand, the conductive paste is not necessarily conductive in its paste form. Instead, the paste is understood to contain conductive metal, and be fireable in order to form a conductive track on a substrate.)

The conductive paste comprises a solids portion dispersed in an organic medium. The organic medium may constitute, for example, at least 2 wt%, at least 5 wt%, at least 7wt%, at least 9 wt%, at least 10 wt% or at least 1 1 wt% of the conductive paste. The organic medium may constitute 25wt% or less, 20 wt% or less, 15 wt% or less, 13 wt % or less, 12 wt% or less or 10 wt % or less of the conductive paste.

Accordingly, it will be understood that the solids portion may constitute at least 75 wt%, at least 80 wt%, at least 85 wt%, at least 87 wt%, at least 88 wt% or at least 90 wt% of the conductive paste. The solids portion may constitute 98 wt% or less, 95 wt% or less, 91 wt% or less, 90 wt% or less or 89 wt% or less of the conductive paste.

Organic Medium

The organic medium comprises an ammonium salt of a phosphate ester free acid. Typically, it further comprises an organic solvent optionally with one or more additives dissolved or dispersed therein. As the skilled person will readily understand, the components of the organic medium are typically chosen to provide suitable consistency and rheology properties to permit the conductive paste to be printed onto a semiconductor substrate, and to render the paste stable during transport and storage. As discussed above and demonstrated in the Examples below, the inclusion of an ammonium salt of a phosphate ester free acid provides a paste with rheological properties which make it particularly suitable for screen printing. Typically, the ammonium salt of a phosphate ester free acid is the reaction product of a phosphate ester free acid and an amine. The skilled person will understand the term phosphate ester free acid. As the skilled person understands, a phosphate moiety has the following structure:

0 The phosphate ester free acid may include a single phosphate moiety, or may include two or more phosphate moieties, linked to form a polyphosphate moiety (wherein a polyphosphate moiety is understood to include moieties comprising two or more, e.g. three or more, phosphate moieties linked in a chain (e.g. covalently linked)). A phosphate ester comprises a phosphate moiety in which at least one of the oxygen atoms is attached to an organic moiety. In a phosphate ester free acid, at least one of the oxygen atoms of a phosphate moiety forms an -OH group. This -OH group is the "free acid" group.

The phosphate ester free acid useful in the present invention may be a phosphate ester free acid according to Formula I below:

Formula I wherein:

each is independently selected from H and an organic moiety;

n is an integer of 1 or more; and

at least one R group is an organic moiety and at least one R group is H. The organic moiety is typically an optionally substituted hydrocarbon moiety, such as an optionally substituted C₅ to C₄₀ hydrocarbon moiety. For example, the hydrocarbon moiety may include at least 5, a least 10 or at least 15 carbon atoms. It may include 40 or fewer, 35 or fewer, or 30 or fewer carbon atoms.

Where R is an optionally substituted hydrocarbon moiety, it may be selected from optionally substituted alkyl (including cycloalkyl), alkenyl (including cycloalkenyl), alkynyl, aryl, alkaryl, aralkyl and alkaralkyl. The hydrocarbon moiety may be linear or branched. It is preferable that the hydrocarbon moiety is optionally substituted alkyl, aryl, alkaryl, aralkyl or alkaralkyl, which may be linear or branched.

Where R is a substituted hydrocarbon moiety which is substituted, typically one, two, three, four or more hydrogen atoms of the hydrocarbon moiety are replaced with other functional groups. Suitable functional groups include -OH, -SH, -OR₂, -SR₂, -Hal, -NR₂R₂, C(0)COR₂, - OC(0)R₂, -NR₂C(0)R₂ and C(0)NR₂R₂, wherein each R₂ is independently H or Ci to Ci₀ alkyl or alkenyl and wherein each -Hal is independently selected from -F, -CI and -Br, e.g. -CI. For example, suitable substituent functional groups include -OH, -OR₂, -Hal, C(0)COR₂, - OC(0)R₂, -NR₂C(0)R₂ and C(0)NR₂R₂, wherein each R₂ is independently H or Ci to Ci₀ alkyl or alkenyl and wherein each -Hal is independently selected from -F, -CI and -Br, e.g. -CI. Particularly suitable substituent functional groups include -OH and -OR₂, wherein each R₂ is independently H or d to Ci₀ or Ci to C₆ alkyl or alkenyl (e.g. alkyl).

In some embodiments, it is preferable that the hydrocarbon moiety is unsubstituted. The phosphate ester free acid typically has a molecular weight of at least 1000, for example at least 1500 or at least 2000. It may have a molecular weight of 10000 or less, 7000 or less, 5000 or less or 3000 or less.

As provided above, n is an integer of 1 or more. For example, n may be at least 2, at least 3, at least 4 or at least 5. It may be 50 or less, 30 or less, 20 or less, 15 or less or 10 or less.

The phosphate ester free acid typically has an acid number of about 38. For example, it may have an acid number of at least 5, at least 10, at least 15, at least 20, at least 25 or at least 30. It may have an acid number of less than 70, less than 60, less than 55, less than 50, less than 45, or less than 40. As the skilled person will understand, the acid number is the mass of potassium hydroxide (KOH) in milligrams that is required to neutralise one gram of the substance in question (in this case, the phosphate ester free acid).

A particularly suitable phosphate ester free acid is Lubrizol® 2062, available from Lubrizol®, which has a molecular weight of at least 2000, an acid number of about 38 and mixed alkyl/aryl (e.g. alkaralkyl) side chains (corresponding to the R groups which are not H in Formula I above).

A particularly suitable Ri group has a structure according to Formula II below:

where ^* indicates the point of attachment to the phosphate moiety and each m is

independently an integer from 1 to 20, more preferably from 5 to 10.

As noted above, the ammonium salt of a phosphate ester free acid is typically the reaction product of a phosphate ester free acid and an amine. The nature of the amine is not particularly limited in the present invention. The amine is typically a hydrocarbon substituted with one or more amine groups, which hydrocarbon is optionally further substituted. The amine may comprise two or more amine groups (e.g. it may be a diamine or a triamine etc). There is no particular upper limit on the number on amine groups, however the amine may include 10 or fewer, 5 or fewer, or 3 or fewer, amine groups. The amine group(s) may be primary, secondary or tertiary amine group(s), preferably primary or secondary, most preferably primary amine group(s).

Where the amine is a hydrocarbon substituted with one or more amine groups, which hydrocarbon is optionally further substituted, the hydrocarbon group may be selected from alkyl (including cycloalkyi), alkenyl (including cycloalkenyl), alkynyl, aryl, alkaryl, aralkyi and alkaralkyl. The hydrocarbon moiety may be linear or branched. It is preferable that the hydrocarbon moiety is optionally substituted alkyl (including cycloalkyi), aryl, alkaryl, aralkyi or alkaralkyl, which may be linear or branched.

Where the amine is a hydrocarbon substituted with one or more amine groups, which hydrocarbon is optionally further substituted, the hydrocarbon group is typically a C₂ to C₃₀ hydrocarbon. For example, the hydrocarbon may include at least 3, at least 5, at least 7 or at least 10 carbon atoms. It may include 30 or fewer, 25 or fewer, 20 or fewer, or 15 or fewer carbon atoms. Where the amine is a hydrocarbon substituted with one or more amine groups, which hydrocarbon is optionally further substituted, typically one, two, three, four or more hydrogen atoms of the hydrocarbon are replaced with other functional groups. Suitable functional groups include -OH, -SH, -OR₃, -SR₃, -Hal, C(0)COR₃, -OC(0)R₃, -NR₃C(0)R₃ and

C(0)NR₃R₃, wherein each R₃ is independently H or d to C₁₀ alkyl or alkenyl and wherein each -Hal is independently selected from -F, -CI and -Br, e.g. -CI. For example, suitable substituent functional groups include -OH, -OR₃, -Hal, C(0)COR₃, -OC(0)R₃, -NR₃C(0)R₃ and C(0)NR₃R₃, wherein each R₃ is independently H or d to C₁₀ alkyl or alkenyl and wherein each -Hal is independently selected from -F, -CI and -Br, e.g. -CI. Particularly suitable substituent functional groups include -OH and -OR₃, wherein each R₃ is

independently H or d to Cio or Ci to C₆ alkyl or alkenyl (e.g. alkyl).

In some embodiments, it is preferred that the hydrocarbon is not further substituted.

A particularly suitable amine is tetrahydrodicyclopentadiene diamine (TCD diamine), which has the following structure:

For the avoidance of doubt, the terms cycloalkyl and cycloalkenyl as used herein include alkyl and alkenyl groups including at least one ring structure, including fused ring systems and bridged fused ring systems. For the avoidance of doubt, the term aryl as used herein is intended to include fused aromatic ring systems, and the terms alkaryl, aralkyi and alkaralkyi should be interpreted accordingly.

Alternatively, the amine may be a polymeric amine. Particularly suitable polymeric amines include polyetheramines and their co-polymers, such as polyether monoamines, diamines and triamines. Suitable polyetheramines include Jeffamine® M-600® or XTJ-505®, Jeffamine M-1000 ® or XTJ-506®, Jeffamine® M-2005® and Jeffamine® M-2070®

(available from Huntsman).

Other suitable amines include, Armac C^®, Armeen HT^®, Armeen HT/97^®, ArmeenM2HT^®, Duomeen TDO^®, Athomeen^® and Tetrameen T^® (available from Akzo Nobel).

As noted above, the ammonium salt of a phosphate ester free acid is typically the reaction product of a phosphate ester free acid and an amine. As the skilled person will understand, ammonium salt formation typically occurs by deprotonation of the free acid group to form a phosphate ester anion, and protonation of the amine to form an ammonium salt.

The ammonium salt of phosphate ester free acid may be prepared before it is added to the organic medium, by combining a phosphate ester free acid and an amine in a suitable solvent and permitting them to react to form the ammonium salt. Suitable solvents include polar aprotic solvents such as a glycol ether solvents, e.g. butyl diglycol. Where the ammonium salt of phosphate ester free acid is prepared before it is added to the organic medium, it is typically in the form of a gel (along with the solvent in which it is prepared).

Alternatively, the phosphate ester free acid and the amine may be added directly to the other components of the organic medium (or directly to the paste) without prior reaction to form the ammonium salt. The present inventors have found that the rheological behaviour of pastes prepared by these two methods are very similar.

Typically, the ratio of phosphate ester free acid to amine is about 3:1 by weight. The ratio may be at least 0.5:1 , at least 1 :1 , or at least 2:1 by weight. It may be 10:1 or less, 5:1 or less, or 4:1 or less by weight.

The conductive paste may include at least 0.1 wt% of ammonium salt of phosphate ester free acid. It may include at least 0.2 wt%, at least 0.3 wt%, at least 0.4 wt%, at least 0.5 wt%, at least 0.6 wt%, at least 0.7 wt% or at least 1 wt%. It may include 10 wt% or less, 8 wt% or less, 6 wt% or less, 5 wt% or less, 3 wt% or less, 2 wt% or less, 1 .5 wt% or less or 1 .2 wt% or less. The quantity of ammonium salt of phosphate ester free acid is calculated by combining the quantity of amine and the quantity of phosphate ester free acid provided to the paste. The organic medium may comprise at least 0.5 wt% of ammonium salt of phosphate ester free acid. It may include at least 1 wt%, 2 wt%, at least 3 wt%, at least 4 wt%, at least 5 wt%, at least 6 wt%, at least 7 wt% or at least 10 wt% of ammonium salt of phosphate ester free acid. It may include 75 wt% or less, 50 wt% or less, 30 wt% or less, 20 wt% or less, 15 wt% or less or 12 wt% or less of ammonium salt of phosphate ester free acid. The quantity of ammonium salt of phosphate ester free acid is calculated by combining the quantity of amine and the quantity of phosphate ester free acid provided to the paste.

As noted above, in addition to the ammonium salt of the a phosphate ester free acid, the organic medium typically comprises an organic solvent optionally with one or more additives dissolved or dispersed therein. Examples of suitable solvents for the organic medium include one or more solvents selected from the group consisting of butyl diglycol, butyldiglycol acetate, terpineol, diakylene glycol alkyl ethers (such as diethylene glycol dibutyl ether and tripropyleneglycol monomethylether), ester alcohol (such as Texanol ®), 2- (2-methoxypropoxy)-1 -propanol and mixtures thereof.

Examples of suitable additives include those dispersants to assist dispersion of the solids portion in the paste, viscosity/rheology modifiers, thixotropy modifiers, wetting agents, thickeners, stabilisers and surfactants.

For example, the organic medium may comprise one or more selected from the group consisting of rosin (kollophonium resin), acrylic resin (e.g. Neocryl ®, such as Neocryl®723 or 725 or mixtures thereof), alkylamaonium salt of a polycarboxylic acid polymer (e.g.

Dysperbik ® 1 10 or 1 1 1 ), polyamide wax (such as Thixatrol Plus ® or Thixatrol Max ®), nitrocellulose, ethylcellulose, hydroxypropyl cellulose, rosin and lecithin.

In some embodiments, it may be preferred that the organic medium consists essentially of solvent, ammonium salt of phosphate ester free acid, any residual (unreacted) amine and/or phosphate ester free acid and incidental impurities.

Mixed Oxide

The solids portion of the conductive paste of the present invention may include 0.1 to 15 wt% of mixed oxide (e.g. glass frit) with respect to the total mass of the solids portion. The solids portion of the conductive paste may include at least 0.5 wt% or at least 1 wt% of mixed oxide (e.g. glass frit). The solids portion of the conductive paste may include 10 wt% or less, 7 wt% or less or 5 wt% or less of mixed oxide (e.g. glass frit). Typically, the mixed oxide (e.g. glass frit) will have a softening point in the range from 200^eC to 400^eC. For example, the mixed oxide may have a softening point in the range from 250^eC to 350^eC. The softening point may be determined e.g. using DSC measurement according to the standard ASTM E1356 "Standard Test Method for Assignment of the Glass Transition Temperature by Differential Scanning Calorimetry".

The particle size of the mixed oxide powder (e.g. glass frit) is not particularly limited in the present invention. Typically, the D50 particle size may be at least 0.1 μηι, at least 0.5 μηι, or at least 1 μηι. The D50 particle size may be 15 μηι or less, 10 μηι or less, 5 μηι or less, 4 μηι or less, 3 μηι or less or 2 μηι or less or 1 μηι or less. The particle size may be determined using a laser diffraction method (e.g. using a Malvern Mastersizer 2000).

Typically, the mixed oxide is a glass frit. Typically, the glass frit is prepared by mixing together raw materials and melting them to form a molten glass mixture, then quenching to form the frit. The process may further comprise milling the frit to provide the desired particle size.

The composition of the mixed oxide (e.g. glass frit) is not particularly limited in the present invention. Typically, the mixed oxide (e.g. glass frit) comprises tellurium, and may further comprise lead. It may be a lead and tellurium based glass frit. The skilled person is well aware of glass frits suitable for use in conductive pastes for photovoltaic applications.

Electrically Conductive Metal

The solids portion of the conductive paste of the present invention may include 75 wt% to 99.9 wt% of electrically conductive metal, with respect to the total mass of the solids portion. For example, the solids portion may include at least 75 wt%, at least 80 wt%, at least 85 wt%, at least 90 wt%, at least 93 wt% or at least 95 wt% of electrically conductive metal. The solids portion may include 99.9 wt% or less, 99.5 wt% or less or 99 wt% or less of electrically conductive metal.

The conductive paste may include 75 wt% to 95 wt% of electrically conductive metal, with respect to the total mass of the conductive paste. For example, the conductive paste may include at least 75 wt%, at least 80 wt%, at least 82 wt%, at least 84 wt% or at least 85 wt% of electrically conductive metal. The conductive paste may include 99 wt% or less, 95 wt% or less, 90 wt% or less, 87 wt% or less, or 86 wt% or less of electrically conductive metal. As demonstrated in the Examples below, particularly with reference to Example 5, excellent performance is surprisingly observed even for pastes including a reduced quantity (e.g.

about 85 wt%) of electrically conductive metal. The electrically conductive metal may comprise one or more metals selected from silver, copper, nickel and aluminium. Preferably, the electrically conductive metal comprises or consists of silver.

The electrically conductive metal may be provided in the form of metal particles. The form of the metal particles is not particularly limited, but may be in the form of flakes, spherical particles, granules, crystals, powder or other irregular particles, or mixtures thereof.

The particle size of the electrically conductive metal is not particularly limited in the present invention. Typically, the D50 particle size may be at least 0.1 μηι, at least 0.5 μηι, or at least 1 μηι. The D50 particle size may be 15 μηι or less, 10 μηι or less, 5 μηι or less, 4 μηι or less, 3 μηι or less or 2 μηι or less. The particle size may be determined using a laser diffraction method (e.g. using a Malvern Mastersizer 2000).

The solids portion may include one or more additional additive materials, e.g. 0 to 10 wt% or 0 to 5 wt% of additional additive material with respect to the total mass of the solids portion.

Manufacture of a Light Receiving Surface Electrode and Solar Cell

The skilled person is familiar with suitable methods for the manufacture of a light receiving surface electrode of a solar cell. Similarly, the skilled person is familiar with suitable methods for the manufacture of a solar cell.

The method for the manufacture of a light receiving surface electrode of a solar cell typically comprises applying a conductive paste onto the surface of a semiconductor substrate, and firing the applied conductive paste.

The conductive paste may be applied by any suitable method. For example, the conductive paste may be applied by printing, such as by screen printing.

The skilled person is aware of suitable techniques for firing the applied conductive paste. A typical firing process lasts approximately 30 seconds, with the surface of the light receiving surface electrode reaching a peak temperature of about 800^eC. Typically the furnace temperature will be higher to achieve this surface temperature. The firing may for example last for 1 hour or less, 30 minutes or less, 10 minutes or less or 5 minutes or less. The firing may last at least 10 seconds. For example, the peak surface temperature of the light receiving surface electrode may be 1200^eC or less, 1 100^eC or less, 1000^eC or less, 950^eC or less or 900^eC or less. The peak surface temperature of the light receiving surface electrode may be at least 600^eC.

The semiconductor substrate of the light receiving surface electrode may be a silicon substrate. For example, it may be a single crystal semiconductor substrate, or a multi crystal semiconductor substrate. Alternative substrates include CdTe. The semiconductor may for example be a p-type semiconductor or an n-type semiconductor.

The semiconductor substrate may comprise an insulating layer on a surface thereof.

Typically the conductive paste of the present invention is applied on top of the insulating layer to form the light receiving surface electrode. Typically, the insulating layer will be non-reflective. A suitable insulating layer is SiNx (e.g. SiN). Other suitable insulating layers include Si₃N₄, Si0₂, Al₂0₃ and Ti0₂.

Methods for the manufacture of a solar cell typically comprise applying a back side conductive paste (e.g. comprising aluminium) to a surface of the semiconductor substrate, and firing the back side conductive paste to form a back side electrode. The back side conductive paste is typically applied to the opposite face of the semiconductor substrate from the light receiving surface electrode. Typically, the back side conductive paste is applied to the back side (non-light receiving side) of the semiconductor substrate and dried on the substrate, after which the front side conductive paste is applied to the front side (light-receiving side) of the semiconductor substrate and dried on the substrate. Alternatively, the front side paste may be applied first, followed by application of the back side paste. The conductive pastes are typically co-fired (i.e. the substrate having both front- and back-side pastes applied thereto is fired, to form a solar cell comprising front- and back-side conductive tracks).

The efficiency of the solar cell may be improved by providing a passivation layer on the back side of the substrate. Suitable materials include SiNx (e.g. SiN), Si₃N₄, Si0₂, Al₂0₃ and Ti0₂. Typically, regions of the passivation layer are locally removed (e.g. by laser ablation) to permit contact between the semiconductor substrate and the back side conductive track. Where ranges are specified herein it is intended that each endpoint of the range is independent. Accordingly, it is expressly contemplated that each recited upper endpoint of a range is independently combinable with each recited lower endpoint, and vice versa.

Examples

Paste Preparation

Pastes were prepared having the compositions specified in Table 1 below:

Table 1

(All compositions given in wt%; CE means Comparative Example.)

Dispersant: Tallicin®

Rheology Modifier: Thixatrol Max®

Acrylic Resin: Mixture of Neocryl® 725 and Neocryl® 723

Glass Frit: Lead and tellurium based glass frit.

The ammonium salt of phosphate ester free acid was prepared by combining phosphate ester free acid (Lubrizol® 2062) with tetrahydrodicyclopentadiene diamine (TCD diamine) in butyldiglycol solvent, and allowing a reaction to occur between the phosphate ester free acid and the diamine, to produce the ammonium salt in butyldiglycol sovent. The reaction product has the form of a gel. The components were combined in the proportions shown in Table 2 below, in order to produce the gel:

Table 2

This gel was then combined with the other components of the paste. In the compositions of Table 1 above, the butyldiglycol of the gel additive is included in the total quantity of butyldiglycol specified. The quantity of ammonium salt of phosphate ester free acid is specified with reference to the amount of Lubrizol® 2062 and TCD diamine starting materials which react to form the ammonium salt of phosphate ester free acid.

(Note that the present inventors have found that an alternative method of preparing the paste, where the phosphate ester free acid and the amine are added directly to the paste, without first reacting them to form the ammonium salt, provides a paste with very similar printability and rheological properties to those reported herein.)

Pastes were prepared having the compositions specified in Table 3 below:

Table 3

(All compositions given in wt%; CE means Comparative Example)

Rheology Modifier: CLiQFLOW® 709

Glass Frit: Tellurium based glass frit

In Comparative Example 2 and Example 6, the phosphate ester free acid and the amine were added directly to the paste, without first reacting them to form the ammonium salt.

Paste Printing

The pastes were screen printed onto silicon wafer. Figure 1 shows a printed line using the paste of Comparative Example 1 . Figures 2 to 5 show a printed line using the pastes of Examples 1 to 5, respectively. Figures 8 and 9 show a printed line using the pastes of Comparative Example 2 and Example 6, respectively. The results show that less dirt is formed at the edge of the lines of the Examples, on account of the inclusion of the

ammonium salt of phosphate acid free ester. The results also show that less dirt is formed at the edge of the lines where a larger amount of the ammonium salt of phosphate acid free ester is included. Rheoloqical Testing

Oscillating rheological measurements were carried out on the pastes of Comparative Example 1 , Example 4 and Example 5. Oscillation experiments were carried out with rheometer from TA Instruments (model Discovery HR-2), under the following conditions:

Spindle 20mm flat plate .

Method: 3 steps measurement

1 ^st step : Conditioning - sample at 25C

2^nd step : Frequency 1 Hz, Strain 4.0e-3% to 100%

3^rd step : Conditioning - End of Test

Figure 7 reports sweep strain measurements on the pastes, plotting strain (x axis) against tan5 (y axis). A value of tan5 of less than 1 is preferred. Figure 7 clearly indicates that the pastes of Examples 4 (609lm) and 5 (6191m) have a higher structure than the paste of the Comparative Example 1 (jmfs005), because breaking of the paste structure occurs at higher oscillation strains (Figure 7). In other words, the tan5 value exceeds 1 at higher oscillation strain. This is believed to be the reason that improved screen printing behaviour is observed for the pastes of the present invention.

Solar Cell Performance Testing

Multicrystalline wafers with sheet resistance of 90 Ohm/sq, 6 inches size, were screen printed on their back side with commercially available aluminum paste, dried in an IR Mass belt dryer and randomized in 3 groups of 15 cells each. Each of these groups was screen printed with a front side silver paste prepared as described above. The first group of 15 cells was screen printed with the front side silver paste of Comparative Example 1 , the second group was screen printed with the front side silver paste of Example 4 and the third group was screen printed with the front side silver paste of Example 5. Multicrystalline wafers with sheet resistance of 90 Ohm/sq, 6 inches size, were screen printed on their back side with commercially available aluminum paste, dried in an IR Mass belt dryer and randomized in 2 groups of 10 cells each. The first group of 10 cells was screen printed with the front side silver paste of Comparative Example 2, and the second group of 10 cells was screen printed with the front side silver paste of Example 6. The screen used for the front side pastes had finger opening of 40 μηι. After printing the front side the cells were dried in the IR Mass belt dryer and fired in a Despatch belt furnace. The Despatch furnace had six firing zones with upper and lower heaters. The first three zones are programmed around 500 °C for burning of the binder from the paste, the fourth and fifth zone are at a higher temperature, with a maximum temperature of 945 °C in the final zone

(furnace temperature). The furnace belt speed for this experiment was 610 cm/min. The recorded temperature was determined by measuring the temperature a the surface of the solar cell during the firing process, using a thermocouple. After cooling the fired solar cells were tested in an l-V curve tracer from Halm, model cetisPV-CTL1 . The results are shown in Table 4 below. The results shown in Table 4 are provided by the l-V curve tracer, either by direct measurement or calculation using its internal software. The results shown for Comparative Example 1 and Examples 4 and 5 are median values from the measurement of 15 cells of each paste. The results shown for Comparative Example 2 and Example 6 are median values from the measurement of 10 cells of each paste.

The results indicate that the paste of Example 4 gives finer lines than the paste of

Comparative Example 1 , due to the higher J_sc value. The paste of Example 5, which has a considerably lower conductive metal content, surprisingly gives comparable results to the paste of Comparative Example 1 . The results further indicate that the paste of Example 6 gives finer lines than the paste of Comparative Example 2, due to the higher J_sc value.

Table 4

Fill factor indicates the performance of the solar cell relative to a theoretical ideal

(0 resistance) system. The fill factor correlates with the contact resistance - the lower the contact resistance the higher the fill factor will be. But if the glass frit of the conductive paste is too aggressive it could damage the pn junction of the semiconductor. In this case the contact resistance would be low but due to the damage of the pn junction (recombination effects and lower shunt resistance) a lower fill factor would occur. A high fill factor therefore indicates that there is a low contact resistance between silicon wafer and the conductive track, and that firing of the paste on the semiconductor has not negatively affected the pn junction of the semiconductor (i.e. the shunt resistance is high). A fill factor of 77% or above is desirable.

The quality of the pn junction can be determined by measuring the pseudo fill factor

(SunsVocFF). This is the fill factor independent of losses due to resistance in the cell.

Accordingly, the lower the contact resistance and the higher the SunsVoc FF, the higher the resulting fill factor will be. The skilled person is familiar with methods for determining

SunsVoc FF, for example as described in Reference 1 . SunsVoc FF is measured under open circuit conditions, and is independent of series resistance effects. Eta represents the efficiency of the solar cell, comparing solar energy in to electrical energy out. Efficiency of a high quality cell is typically in the range from 17% to 18%. Small changes in efficiency can be very valuable in commercial solar cells.

The open-circuit voltage, U₀c, is the maximum voltage available from a solar cell, and this occurs at zero current. The open-circuit voltage corresponds to the amount of forward bias on the solar cell due to the bias of the solar cell junction with the light-generated current. The Uoc value may be reduced if the paste damages the p-n junction.

Jsc is the short circuit current density. The short circuit current is the current through the solar cell when the voltage across the solar cell is zero (i.e. when it is short circuited). The short circuit current is dependent on the area of the solar cell, which is why it is provided as a current density, in mA/cm². For an ideal solar cell at most moderate resistive loss

mechanisms, the short-circuit current and the light-generated current are identical. Therefore, the short-circuit current is the largest current which may be drawn from the solar cell. A higher J_sc can indicate that finer lines have been printed, as it indicates that the cell can generate a larger current.

Significant power losses caused by the presence of a shunt resistance, R_SH, are typically due to manufacturing defects, rather than poor solar cell design. Low shunt resistance causes power losses in solar cells by providing an alternate current path for the light-generated current. Such a diversion reduces the amount of current flowing through the solar cell junction and reduces the voltage from the solar cell. The effect of a shunt resistance is particularly severe at low light levels, since there will be less light-generated current. The loss of this current to the shunt therefore has a larger impact. In addition, at lower voltages where the effective resistance of the solar cell is high, the impact of a resistance in parallel is large. GridResFront is the grid resistance of the front side of the solar cell. It is a part of the series resistance (Rs) in Ω. The resistance is measured from busbar to busbar on the front of the solar cell. The higher the grid resistance is, the higher the series resistance will be. The series resistance has an direct impact on the fill factor, and therefore on the efficiency of the cell.

Claims

1 . A conductive paste, the paste comprising a solids portion dispersed in an organic medium, the solids portion comprising electrically conductive metal and mixed oxide, wherein the organic medium comprises an ammonium salt of a phosphate ester free acid.

2. A conductive paste according to claim 1 , wherein the ammonium salt of a phosphate ester free acid is the reaction product of a phosphate ester free acid and an amine.

3. A conductive paste according to claim 2, wherein the phosphate ester free acid has a molecular weight of at least 2000.

4. A conductive paste according to claim 2 or claim 3 wherein the phosphate ester free acid has an acid number in the range from 20 to 40.

5. A conductive paste according to any one of claims 2 to 4 wherein the phosphate ester free acid comprises (i) a phosphate moiety in which at least one of the oxygen atoms is attached to an organic moiety, and (ii) a phosphate moiety in which at least one of the oxygen atoms of a phosphate moiety forms an -OH group, wherein phosphate moieties (i) and (ii) may be the same or different phosphate moieties.

6. A conductive paste according to claim 5 wherein the organic moiety is an optionally substituted C₅ to C₄₀ hydrocarbon moiety.

7. A conductive paste according to any one of claims 2 to 6 wherein the amine is a C₂-C₃₀ hydrocarbon substituted with one or more amine groups, which is optionally further substituted.

8. A conductive paste according to claim 7 wherein the hydrocarbon is substituted with 2 to 5 amine groups.

9. A conductive paste according to claims 2 to 6 wherein the amine is a polyetheramine.

10. A process for preparing a conductive paste as defined in any one of the preceding claims, wherein the process comprises mixing together the electrically conductive metal, the mixed oxide and the components of the organic medium, in any order, wherein the organic medium comprises ammonium salt of a phosphate ester free acid.

1 1 . A process according to claim 10 wherein the ammonium salt of a phosphate ester free acid is prepared by reacting a phosphate ester free acid and an amine before it is added to the organic medium.

12. A process according to claim 10 wherein a phosphate ester free acid and an amine are provided to the organic medium and then allowed to react to form an ammonium salt of a phosphate ester free acid.

13. A light receiving electrode for a solar cell, the light receiving electrode comprising a conductive track on a semiconductor substrate, wherein the conductive track is obtained or obtainable by firing a paste as defined in any one of claims 1 to 9 on the semiconductor substrate.

14. A solar cell, comprising a light receiving electrode as defined in claim 13.

15. A method of printing comprising applying a paste composition to a substrate by screen-printing, wherein the paste composition comprises a solids portion dispersed in an organic medium and wherein the organic medium comprises an ammonium salt of a phosphate ester free acid.

16. A method as claimed in claim 15 wherein the substrate is a semiconductor substrate and the paste composition is a conductive paste wherein the solids portion comprises electrically conductive metal and mixed oxide.

17. A method as claimed in claim 15 wherein the substrate is a glass surface and the paste is an enamel composition wherein the solids portion comprises pigment and mixed oxide.

18. Use of a conductive paste as defined in any one of claims 1 to 9 in the manufacture of a light receiving electrode for a solar cell.

19. Use of a conductive paste as defined in any one of claims 1 to 9 in the manufacture of a solar cell.