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, and to a light receiving surface of a solar cell.

Background of the Invention

Screen printed metal (e.g. silver) pastes are routinely used as conductive tracks for solar cells, such as 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 mixed oxide has several roles. During firing, it becomes a molten phase and so acts to bond the conductive track to the semiconductor wafer. However, the mixed oxide is also important in etching away the anti-reflective top layer (usually silicon nitride) provided on the surface of the semiconductor wafer, to permit direct contact between the conductive track and the semiconductor. The mixed oxide is typically also important in forming an ohmic contact with the n-type semiconductor emitter.

The quality of the contact between the conductive track and the semiconductor wafer is instrumental in determining the efficiency of the final solar cell. The best mixed oxides need to be optimised to flow at the correct temperature, and to provide the correct degree of etching of the anti reflective layer. If too little etching is provided, then there will be insufficient contact between the semiconductor wafer and the conductive track, resulting in a high contact resistance. Conversely, excessive etching may lead to deposition of large islands of silver in the semiconductor, disrupting its p-n junction and thereby reducing its ability to convert solar energy into electrical energy.

Much recent attention has focussed on improving the mixed oxide materials included in conductive pastes for photovoltaic cells, to provide a good balance of properties.

Summary of the Invention

There remains a need for mixed oxides (e.g. glass frits) which are suitable for use in conductive pastes for solar cells, which offer a good balance of properties. In particular, there remains a need for mixed oxides suitable for use in conductive pastes for solar cells which provide an excellent contact resistance without negatively influencing the p-n junction of a solar cell, and which flow at a suitable temperature for firing the conductive paste during manufacture of a solar cell. In particular, there remains a need for lead-free mixed oxides, or mixed oxides which contain a very low level of lead, which achieve these aims.

As demonstrated in the examples below, the present inventors have found that an excellent balance of properties can be achieved with a mixed oxide comprising cerium, for example comprising tellurium, bismuth and cerium. The present inventors have additionally found that pastes comprising mixed oxide according to the invention exhibit good or excellent adhesion properties, as demonstrated in the examples below. Accordingly, in a first preferred aspect, the present invention provides a conductive paste for a solar cell, the paste comprising a solids portion dispersed in an organic medium, the solids portion comprising electrically conductive metal, and mixed oxide, wherein the mixed oxide is a tellurium-bismuth-cerium mixed oxide. The mixed oxide is substantially lead-free and substantially silicon-free.

The mixed oxide may, for example, comprise 20 to 80 wt% Te0 ₂ , 10 to 60 wt% Bi ₂ 0 ₃ and 0.1 to 20 wt% CeO ₂ .

In a second preferred aspect, the present invention provides a conductive paste for a solar cell, the paste comprising a solids portion dispersed in an organic medium, the solids portion comprising electrically conductive metal, and mixed oxide, wherein the mixed oxide is substantially lead-free, substantially silicon-free and comprises Ce0 ₂ . For example, the mixed oxide may comprise 0.1 to 20 wt% Ce0 ₂ . The mixed oxide useful in the present invention is typically a powder. The mixed oxide is typically a glass frit having a composition as recited herein, or a mixture of one or more glass frits which together provide that composition.

In a third preferred aspect, the present invention provides a method for the manufacture of a light receiving surface electrode of a solar cell, the method comprising applying a conductive paste according to the first aspect or the second aspect to a semiconductor substrate, and firing the applied conductive paste.

In a fourth 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 or the second aspect on the semiconductor substrate. In a fifth preferred aspect, the present invention provides a glass frit comprising 20 to 80 wt% Te0 ₂ ;

10 to 60 wt % Bi ₂ 0 ₃ ;

0.1 to 20 wt% CeO ₂ ;

0 to 20 wt% ZnO;

0 to 10 wt% Mo0 ₃ ; and

0.5 to 10 wt% of one or more of Li ₂ 0, Na ₂ 0 and K ₂ 0

wherein the frit is substantially silicon-free.

In a sixth preferred aspect the present invention provides use of a conductive paste according to the first aspect or the second aspect in the manufacture of a light receiving surface electrode of a solar cell. In a further preferred aspect the present invention provides use of a conductive paste according to the first aspect or the second aspect in the manufacture of a solar cell.

Brief Description of the Drawings

Figure 1 shows an example firing curve for a solar cell prepared in the Examples. 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.

Mixed Oxide Composition

The mixed oxide compositions described herein are given as weight percentages. These weight percentages are with respect to the total weight of the mixed oxide. The weight percentages are the percentages of the components used as starting materials in

preparation of the mixed oxide compositions, on an oxide basis. As the skilled person will understand, starting materials such as oxides, carbonates or nitrates may be used in preparing the mixed oxides (e.g. glasses) of the present invention. Where a non-oxide starting material is used to supply a particular element to the mixed oxide, an appropriate amount of starting material is used to supply an equivalent molar quantity of the element had the oxide of that element been supplied at the recited wt%. This approach to defining mixed oxide (e.g. glass) compositions is typical in the art. As the skilled person will readily understand, volatile species (such as oxygen) may be lost during the manufacturing process of the mixed oxide, and so the composition of the resulting mixed oxide may not correspond exactly to the weight percentages of starting materials, which are given herein on an oxide basis. Analysis of a fired mixed oxide (e.g. glass) by a process known to those skilled in the art, such as Inductively Coupled Plasma Emission Spectroscopy (ICP-ES), can be used to calculate the starting components of the mixed oxide composition in question.

The mixed oxide is substantially lead-free. As used herein, the term "substantially lead-free" is intended to include mixed oxides which contain no intentionally added lead. For example, the mixed oxide may include less than 0.1 wt% PbO, for example less than 0.05 wt%, less than 0.01 wt% or less than 0.005 wt% PbO _.

It may be preferable that the mixed oxide is substantially boron-free. As used herein, the term "substantially boron-free" is intended to include mixed oxides which contain no intentionally added boron. For example, the mixed oxide may include less than 0.1 wt% B ₂ 0 ₃ , for example less than 0.05 wt%, less than 0.01 wt% or less than 0.005 wt% B ₂ 0 _3.

The mixed oxide typically includes Te0 ₂ . The mixed oxide may include at least 20 wt%, at least 25 wt%, at least 30 wt%, at least 35 wt%, at least 40 wt%, or at least 45 wt% of Te0 ₂ . The mixed oxide may include 80 wt% or less, 75 wt% or less, 70 wt% or less, 65 wt% or less or 60 wt% or less of Te0 ₂ . For example, the mixed oxide may include 35 to 65 wt% of Te0 ₂ .

The mixed oxide typically includes Bi ₂ 0 ₃ . The mixed oxide may include at least 10 wt%, at least 15 wt%, at least 18 wt%, at least 20 wt% or at least 25 wt% of Bi ₂ 0 ₃ . The mixed oxide may include 60 wt% or less, 55 wt% or less, 50 wt% or less or 45 wt% or less of Bi ₂ 0 ₃ . For example the mixed oxide may include 20 to 50 wt% of Bi ₂ 0 ₃ .

The mixed oxide includes Ce0 ₂ . The mixed oxide may comprise 0 wt% or more, e.g. at least 0 0.1 wt%, at least 0.2 wt%, at least 0.5 wt%, at least 1 wt%, at least 1 .5 wt%, at least 2 wt%, at least 2.5 wt%, at least 3 wt% Ce0 ₂ , at least 3.5 wt% Ce0 ₂ , at least 4 wt% Ce0 ₂ , at least 4.5 wt% Ce0 ₂ , at least 5 wt% Ce0 ₂ , at least 6 wt% Ce0 ₂ , or at least 7 wt% Ce0 ₂ . The mixed oxide may comprise 22 wt% or less, 20 wt% or less, 17 wt% or less, 15 wt% or less, 14 wt% or less, 13 wt% or less, 12 wt% or less, 1 1 wt% or less, 10 wt% or less, or 5 wt% or less of Ce0 ₂ . A particularly suitable Ce0 ₂ content is from 1 wt% to 15 wt%. The mixed oxide is substantially silicon-free. As used herein, the term "substantially silicon- free" is intended to include mixed oxides which contain no intentionally added silicon. For example, the mixed oxide may include less than 0.1 wt% Si0 ₂ , for example less than 0.05 wt%, less than 0.01 wt% or less than 0.005 wt% Si0 _2.

The mixed oxide may include alkali metal oxide, for example one or more selected from Li ₂ 0, Na ₂ 0, K ₂ 0, and Rb ₂ 0, preferably one or more selected from Li ₂ 0, Na ₂ 0 and K ₂ 0, more preferably one or both of Li ₂ 0 and Na ₂ 0. In some embodiments, it is preferred that the mixed oxide includes Li ₂ 0.

The mixed oxide may include 0 wt% or more, 0.1 wt% or more, 0.5 wt% or more or 1 wt% or more alkali metal oxide. The mixed oxide may include 10 wt% or less, 8 wt% or less, 7 wt% or less, 5 wt% or less, 4 wt% or less alkali metal oxide. The mixed oxide may include 0 wt% or more, 0.1 wt% or more, 0.5 wt% or more or 1 wt% or more Li ₂ 0. The mixed oxide may include 10 wt% or less, 8 wt% or less, 7 wt% or less, 6 wt% or less 5 wt% or less, 4 wt% or less Li ₂ 0. For example, the mixed oxide may include 0.5 to 6 wt% of Li ₂ 0. In some embodiments, it may be preferred that the mixed oxide includes both Li ₂ 0 and Na ₂ 0. The mixed oxide may include 0.1 wt% or more, 0.5 wt% or more, 1 wt% or more, 2 wt% or more or 3 wt% or more in total of Li ₂ 0 and Na ₂ 0. The mixed oxide may include 10 wt% or less, 8 wt% or less, 7 wt% or less, 6 wt% or less 5 wt% or less, 4 wt% or less in total of Li ₂ 0 and Na ₂ 0. The mixed oxide may include at least 0.1 wt% or at least 0.5 wt% of Li ₂ 0 and at least 0.1 wt% or at least 0.5 wt% of Na ₂ 0. The mixed oxide may include 5 wt% or less, 4 wt% or less, 3 wt% or less or 2.5 wt% or less of Li ₂ 0 and 5 wt% or less, 4 wt% or less, 3 wt% or less or 2.5 wt% or less of Na ₂ 0.

The mixed oxide may include ZnO. For example, the mixed oxide may include 0 wt% or more, 0.1 wt% or more, 0.5 wt% or more, 1 wt% or more or 1 .5 wt% or more ZnO. The mixed oxide may include 15 wt% or less, 10 wt% or less, 7 wt% or less or 5 wt% or less ZnO. For example, the mixed oxide may include 0.5 to 7 wt% of ZnO.

In some embodiments, it may be preferred that the mixed oxide is substantially zinc-free. As used herein, the term "substantially zinc-free" is intended to include mixed oxides which contain no intentionally added zinc. For example, the mixed oxide may include less than 0.1 wt% ZnO, for example less than 0.05 wt%, less than 0.01 wt% or less than 0.005 wt% ZnO _.

The mixed oxide may include Mo0 ₃ . For example, the mixed oxide may include 0 wt% or more, 0.1 wt% or more, 0.5 wt% or more of Mo0 ₃ . The mixed oxide may include 10 wt% or less, 5 wt% or less, or 3 wt% or less of Mo0 ₃ . For example, the mixed oxide may include 0.1 to 5 wt% of Mo0 ₃ .

In some embodiments, it may be preferred that the mixed oxide is substantially

molybdenum-free. As used herein, the term "substantially molybdenum -free" is intended to include mixed oxides which contain no intentionally added molybdenum. For example, the mixed oxide may include less than 0.1 wt% Mo0 ₃ , for example less than 0.05 wt%, less than 0.01 wt% or less than 0.005 wt% Mo0 _3. The mixed oxide may include W0 ₃ . For example, the mixed oxide may include 0 wt% or more, 0.1 wt% or more, 0.5 wt% or more of W0 ₃ . The mixed oxide may include 10 wt% or less, 5 wt% or less, or 3 wt% or less of W0 ₃ . For example, the mixed oxide may include 0.1 to 5 wt% of W0 ₃ . In some embodiments, it may be preferred that the mixed oxide is substantially

tungsten-free. As used herein, the term "substantially tungsten-free" is intended to include mixed oxides which contain no intentionally added tungsten. For example, the mixed oxide may include less than 0.1 wt% W0 ₃ , for example less than 0.05 wt%, less than 0.01 wt% or less than 0.005 wt% W0 _3.

The mixed oxide may include BaO. For example, the mixed oxide may include 0 wt% or more, 0.1 wt% or more, 0.5 wt% or more of BaO. The mixed oxide may include 10 wt% or less, 5 wt% or less, or 3 wt% or less of BaO. For example, the mixed oxide may include 0.1 to 5 wt% of BaO.

The mixed oxide may include P ₂ 0 ₅ . For example, the mixed oxide may include 0 wt% or more, 0.1 wt% or more, 0.5 wt% or more or 1 wt% or more P ₂ 0 ₅ . The mixed oxide may include 10 wt% or less, 7 wt% or less, 5 wt% or less or 3 wt% or less P ₂ 0 ₅ . The mixed oxide may include further components, such as further oxide components.

Typically, the mixed oxide will include 20 wt% or less, 10 wt% or less, 7 wt% or less, 5 wt% or less, 3 wt% or less, 2 wt% or less or 1 wt% or less in total of further components. The mixed oxide may include at least 0.1 wt% of further components. The further components may be one or more selected from the group consisting of Ge0 ₂ , CaO, Zr0 ₂ , CuO, AgO and Al ₂ 0 ₃ .

In a preferred embodiment, the mixed oxide may comprise

35 to 65 wt% Te0 ₂ ;

20 to 50 wt% Bi ₂ 0 ₃ ;

0.1 to 5 wt% Li ₂ 0;

0 to 5 wt% Na ₂ 0;

0.1 to 5 wt% ZnO;

0 to 3 wt% Mo0 ₃ ; and

0.5 to 15 wt% Ce0 ₂ .

The mixed oxide (e.g. glass frit) may consist essentially of a composition as described herein, and incidental impurities. In that case, as the skilled person will readily understand that the total weight % of the recited constituents will be 100 wt%, any balance being incidental impurities. Typically, any incidental impurity will be present at 0.1 wt% or less, 0.05 wt% or less, 0.01 wt% or less, 0.05 wt% or less, 0.001 wt% or less or 0.0001 wt% or less.

The mixed oxide may consist essentially of:

35 to 65 wt% Te0 ₂ ;

20 to 50 wt% Bi ₂ 0 ₃ ;

0.1 to 5 wt% Li ₂ 0;

0 to 5 wt% Na ₂ 0;

0.1 to 5 wt% ZnO;

0 to 3 wt% Mo0 ₃ ;

0.5 to 15 wt% Ce0 ₂ ;

0 to 3 wt% W03;

0 to 5 wt% BaO;

O to 10 wt% P ₂ 0 ₅ ;

0 to 10 wt% of further components, which may optionally be selected from the group consisting of Ge0 ₂ , CaO, Zr0 ₂ , CuO, AgO and Al ₂ 0 ₃ ; and

incidental impurities.

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). 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 ^e C to 400 ^e C. For example, the mixed oxide may have a softening point in the range from 250 ^e C to 350 ^e C. 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. Using X-ray diffraction techniques, the present inventors have found that some of the glass frits they have prepared in fact include a crystalline portion. Accordingly, it will be understood that the glass frits described and defined herein may include a crystalline portion in addition to an amorphous glass phase. In particular, the present inventors have found that some glass frits which include Ce0 ₂ as a component in fact include a portion of crystalline Ce0 ₂ , in addition to the amorphous glass phase. This is observed particularly where the glass frit recipe includes a large weight percent of Ce0 ₂ , (e.g. 5 wt% or more). Accordingly, it will be understood that the glass frits described herein may include crystalline Ce0 ₂ , and that the recited Ce0 ₂ content of the frit relates to the total of Ce0 ₂ in amorphous glass phase and crystalline phase in the frit.

Similarly, where another component is present in a crystalline portion, the recited content of that component in the frit relates to the total of that component in amorphous glass phase and crystalline phase in the frit. The glass frit is typically obtained or obtainable by a process as described or defined herein.

Typically, the glass frit is prepared by mixing together the raw materials and melting them to form a molten glass mixture, then quenching to form the frit. Accordingly, in a further preferred aspect, the present invention provides a process for preparing a glass frit according to the present invention, wherein the process comprises melting together starting materials for forming the frit, to provide a molten glass mixture, and quenching the molten glass mixture to form the frit. The process may further comprise milling the frit to provide the desired particle size.

The skilled person is aware of alternative suitable methods for preparing glass frit. Suitable alternative methods include water quenching, sol-gel processes and spray pyrolysis.

Conductive Paste

Typically the conductive paste is a front side conductive paste. The solids portion of the conductive paste of the present invention may include 85 to

99.9 wt% of electrically conductive metal. For example, the solids portion may include 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 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 of the conductive paste of the present invention may include 0.1 to 15 wt% of mixed oxide (e.g. glass frit). For example, the solids portion may include at least 0.2 wt%, at least 0.5 wt% or at least wt% of mixed oxide. The solids portion may include 10 wt% or less, 7 wt% or less or 5 wt% or less of mixed oxide. 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. Organic Medium

The solids portion of the conductive paste of the present invention is dispersed in organic medium. The organic medium may constitute, for example, at least 2 wt%, at least 5 wt% or at least 9 wt% of the conductive paste. The organic medium may constitute 20 wt% or less, 15 wt% or less, 13 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 80 wt%, at least 85 wt%, at least 87 wt% or at least 90 wt% of the conductive paste. The solids portion may constitute 98 wt% or less, 95 wt% or less or 91 wt% or less of the conductive paste.

The organic medium typically comprises an organic solvent 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.

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 ®), alkylammonium 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 and lecithin.

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 further preferred aspect, the present invention provides a process for preparing a conductive paste according to the first aspect, wherein the process comprises mixing together the electrically conductive metal, the mixed oxide and the components of the organic medium, in any order. 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 or inkjet printing.

The skilled person is aware of suitable techniques for firing the applied conductive paste. An example firing curve is shown in Figure 1 . A typical firing process lasts approximately 30 seconds, with the surface of the light receiving surface electrode reaching a peak

temperature of about 800 ^e C. 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 ^e C or less, 1 100 ^e C or less, 1000 ^e C or less, 950 ^e C or less or 900 ^e C or less. The peak surface temperature of the light receiving surface electrode may be at least 600 ^e C.

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 Si ₃ N ₄ , Si0 ₂ , Al ₂ 0 ₃ and Ti0 ₂ . 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), Si ₃ N ₄ , Si0 ₂ , Al ₂ 0 ₃ and Ti0 ₂ . 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 in 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

Glass Synthesis

Glass frits were prepared using commercially available raw materials. The compositions of the glass frits are given in Table 1 below. Each glass was made according to the following standard procedure.

Raw materials for the glass were mixed using a laboratory mixer. One hundred grams of the mixture was melted in ceramic crucible, in a Carbolite electrical laboratory furnace. The crucibles containing the raw material mixture was placed in the furnace while it was still cold, to avoid thermal shock and cracking of the ceramic crucible. The melting was carried out at 950-1 100 °C in air. The molten glass was quenched in water to obtain the glass frit. The frit was dried overnight in a Binder heating chamber at 120 ^e C, then wet milled in a planetary mill to provide particles having a D50 particle size less than 1 μηι (determined using a laser diffraction method using a Malvern Mastersizer 2000). Wet milling may be carried out in organic solvent or water. Table 1 - Glass Frit Compositions

(Compositions in weight % on an oxide basis)

(CE means Comparative Example)

The glass transition temperature of each of the glasses was determined, using Differential Scanning Calorimetry, according to ASTM E1356 "Standard Test Method for Assignment of the Glass Transition Temperature by Differential Scanning Calorimetry". The results are shown in Table 2 below: Table 2 - Glass Transition Temperatures

These glass transition temperatures are acceptable for silver pastes for photovoltaic applications.

Paste Preparation

Conductive silver pastes comprising each of the glass frits was prepared using 87-88 wt% of a commercial silver powder, 3-2 wt% of glass frit, the balance being standard organic medium. The paste was prepared by pre-mixing all the components and passing several times in a triple roll mill, producing a homogeneous paste.

Preparation of Solar Cells

Multicrystalline wafers with sheet resistance of 90 Ohm/sq , 6 inches size, were screen printed on their back side with commercially available aluminium paste, dried in an IR Mass belt dryer and randomized in groups. Each of these groups was screen printed with a front side silver paste prepared as described above.

The screens used for the front side pastes had finger opening 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. An example firing profile is shown in Figure 1 . The recorded temperature was determined by measuring the temperature at 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 3 below. The results are arranged in order of increasing fill factor (FF). The results shown in Table 4 are provided by the l-V curve tracer, either by direct measurement or calculation using its internal software.

(To minimize the influence of the contact area the cells were prepared using the same screen for printing, and the same viscosity paste in each individual test set. This ensures that the line widths of the compared pastes were substantially identical and had no influence on the measuring.)

The results shown in Table 3 are median values from the measurement of 5 cells of each paste. In test PV106, test PV137, test PV131 , test PV154 and test PV244 different batches of silicon wafer were used. This can affect cell performance. In tests PV106, PV137 and PV131 , a reference sample was prepared using a commercially available paste (referred to as "Reference Paste"). Note that the performance of the Reference Paste is not comparable with the Examples and Comparative Examples, due to other variables such as silver content, silver type, organics content, glass frit content. These variables were kept constant between the Examples and Comparative Examples, but the Reference Paste was used as supplied. Comparison of the Examples with the Reference Paste does not indicate the relative performance of the glass frits in the pastes, as variation in performance may be due to other variables. Table 3 - Solar Cell Test Results

Fill factor (FF) 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 ^' 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 76% or above, preferably 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, U ₀ c, 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. Further Paste Testing (XRD and Adhesion Properties)

X-Ray Diffraction (XRD)

XRD was carried out on some of the glass powder samples, for crystallographic

characterization. The analysis were made using a d iff racto meter Bruker D8, operating with CuKa radiation (1.5045 A). Data were collected using a point detector in the 2Θ range of 20- 60 ° with a step of 0.01 °. The results are shown in Table 4 below.

Soldering and pull test of solar cells

All samples were soldered and tested with the same parameters and equipment. The soldering processed and pull strength tests were compatible with industrial processes and industrial equipment. The laboratory soldering system and pull tester were produced by Somont. The samples were soldered at 320 °C for 1 .5 s. For this test, commercially available Cu-core 62Sn/36Pb/2Ag ribbons with 0.2 ^* 1 .5 mm were used, made by Kunming Sunlight Science and Tech. The pull forces were measured under an angle of 180 °. The pull force results presented in Table 5 are median results from the test of 9 busbars of each paste. All pastes were prepared with 2 wt% glass frit, 88 wt% silver powder and 10 wt% organic medium.

Table 4 - XRD and Pull Test Results

Without wishing to be bound by theory, the present inventors believe that the excellent adhesion properties observed for pastes according to the present invention are

predominantly due to the properties of the glass frit. They consider that it is a combination of (i) the excellent glass flow behaviour of the frits of the present invention, and (ii) the reinforcement of the glass matrix by the crystalline phases observed by XRD, which are believed to be a result of the presence of Ce0 ₂ in the frit.

References

1 . A. McEvoy, T. Markvart, L. Castaner. Solar cells: Materials, Manufacture and Operation. Academic Press, second edition, 2013.