WO2020046204A1

WO2020046204A1 - A method of forming electrical contact in a photovoltaic device

Info

Publication number: WO2020046204A1
Application number: PCT/SG2019/050423
Authority: WO
Inventors: Jiancheng Donny LAI; Ying Huang
Original assignee: National University of Singapore
Current assignee: National University of Singapore
Priority date: 2018-08-27
Filing date: 2019-08-27
Publication date: 2020-03-05
Anticipated expiration: 2021-02-27

Abstract

There is provided a method of forming an electrical contact between a semiconductor layer and a metal layer in the manufacture of a photovoltaic (PV) device, the semiconductor layer and the metal layer being separated by a dielectric layer, the method comprising: ablating the dielectric layer to locally form a crater and to create a concentric array of recess in the dielectric layer, thereby exposing the semiconductor layer; depositing a layer of metal on the dielectric layer comprising the crater to form a metal layer; and heating the metal layer at a predetermined temperature to form the electrical contact. There is also provided a PV device comprising an electrical contact formed from the method.

Description

A method of forming electrical contact in a photovoltaic device

Technical Field

The present invention relates to a method of forming an electrical contact in a photovoltaic (PV) device.

Background

Photovoltaic (PV) devices such as passivated emitter and rear contact (PERC) silicon wafer solar cells have the potential of achieving high energy conversion efficiency by minimal modification of existing silicon wafer solar cell industrial manufacturing facilities. By inserting a good surface passivation dielectric layer (e.g. vacuum based plasma enhanced chemical vapour deposition (PECVD), atomic layer deposition (ALD) or plasma vapour deposition (PVD) AIO_x or thermally grown SiO_x) capped by PECVD/PVD SiN_x layer/layers, it is possible to reduce rear surface recombination velocity of minority carriers and therefore improve the open circuit voltage of the solar cell. Meanwhile, the mirror-like rear dielectrics/dielectric stack will also reflect the photons back into the absorber (silicon wafer) to increase the short circuit current and hence the efficiency of the solar cell. Such dielectrics/dielectric stack on the rear surface will be locally opened by laser scribing/chemical etching, allowing a metal paste to form a good contact and back surface field in the silicon wafer to extract the light induced current efficiently.

In current methods of laser scribing, the passivation dielectric layer may be locally opened by means of a laser with short pulses. Then, an aluminium layer is applied over the entire area. An electrical contact may be created by heating the silicon wafer up to 400°C or more. At a sufficiently high peak temperature, Si-AI alloying takes place. A local back surface field (LBSF) may eventually be formed around the local contacts during recrystallization of the melt that occurs in the cooling phase. However, silicon migrates during the heating step into the aluminium whilst the latter is molten, leading to less efficient light trapping resulting from a lower internal reflectance of the final solar cell. Silicon migration out of the local contacts may induce an LBSF of insufficient thickness. In addition, if screen printing of the metal layer is done after laser ablation, voids are frequently observed in the contacts, leading to excessively thin LBSF (which may even be absent) and poor electrical performances. There is therefore a need for an improved method for forming electrical contacts. Summary of the invention

The present invention seeks to address these problems, and/or to provide an improved method of forming electrical contacts in a photovoltaic (PV) device. In general terms, the invention relates to a method of forming electrical contacts during the manufacture of PV devices such as solar cells, particularly silicon wafer solar cells. The method of the present invention takes a shorter period of time to form the contacts, as well as increases the cell efficiency of the PV device.

According to a first aspect, the present invention provides a method of forming an electrical contact between a semiconductor layer and a metal layer in the manufacture of a PV device, the semiconductor layer and the metal layer being separated by a dielectric layer, the method comprising: ablating the dielectric layer to locally form a crater and to create a concentric array of recess in the dielectric layer, thereby exposing the semiconductor layer;

depositing a layer of metal on the dielectric layer comprising the crater to form a metal layer; and

heating the metal layer at a pre-determined temperature to form the electrical contact.

The semiconductor layer may be any suitable semiconductor layer. For example, the semiconductor layer may comprise silicon.

According to a particular aspect, the ablating may comprise laser ablation. The ablating may be in pulsed mode or continuous wave mode. The ablating may be under suitable conditions. For example, the ablating may comprise ablating in standard atmosphere, inert atmosphere or atmosphere of reducing gas.

According to a particular aspect, the depositing may comprise, but is not limited to: screen-printing, evaporation, sputtering, laser transfer, chemical vapour deposition, inkjet printing, electroplating, or a combination thereof. According to a particular aspect, the layer of metal may comprise aluminium or an aluminium-containing metal. In particular, the aluminium-containing metal may comprise aluminium alloy.

The metal layer formed from the depositing may have a thickness of 1-30 mhi. According to a particular aspect, the pre-determined temperature during the heating may be a temperature greater than the melting temperature of the metal comprised in the metal layer. In particular, the pre-determined temperature may be ³ 540°C. Even more in particular, the pre-determined temperature may be 540-950°C.

According to a second aspect, the present invention provides a photovoltaic (PV) device comprising an electrical contact formed from the method above. For example, the PV device may be, but not limited to, a passivated emitter and rear contact (PERC) solar cell, passivated emitter rear locally diffused (PERL) cell, passivated emitter rear totally diffused (PERT) cell, passivated emitter rear floating p-n junction (PERF) cell, n- type front and back contact solar cell, passivated contact solar cell, heterojunction solar cell, or a combination thereof.

Brief Description of the Drawings

In order that the invention may be fully understood and readily put into practical effect there shall now be described by way of non-limitative example only exemplary embodiments, the description being with reference to the accompanying illustrative drawings. In the drawings:

Figure 1 shows a schematic representation of manufacture of a PV device including electrical contacts formed from the method according to one embodiment of the present invention;

Figure 2A shows a top view of the rear-side metallization schematic of a multicrystalline silicon passivated emitted rear contact (PERC) solar cell with concentric array, Figures 2B and 2C respectively show a top view of an aluminium rear-side metallization scheme without rear busbars of a PERC solar cell with concentric array and with line array; Figure 3 shows the various measured electrical parameters of the solar cells using aluminium paste with silicon doping (laser pitch = 900 pm), with concentric array and line array, using A.M.1.5G illumination at standard conditions;

Figure 4 shows the various measured electrical parameters of the solar cells using aluminium paste without silicon doping (laser pitch = 900 pm), with concentric array and line array, using A.M.1.5G illumination at standard conditions; and

Figure 5 shows the various measured electrical parameters of the solar cells using aluminium paste without silicon doping (laser pitch = 1300 pm), with concentric array and line array, using A.M.1.5G illumination at standard conditions. Detailed Description

As explained above, there is a need for an improved method of forming electrical contacts in a photovoltaic (PV) device.

The method of the present invention enables a more time- efficient geometry to partially ablate passivation dielectric layers of a PV device. Accordingly, the method of the present invention results in the production of PV devices of a higher efficiency and in a shorter ablation processing time.

In general terms, the present invention describes a method to form electrical contacts using ablation, such as laser ablation, for a shorter period of time as compared to conventional laser ablation methods comprising straight line ablation. In particular, the shorter period of time is as a result of the ablation of the dielectric layer and thereby creating concentric arrays of recess in the semiconductor layer for formation of the electrical contact.

When a non-illuminated surface of a PV device, such as a solar cell, is covered with a layer of dielectric material separating it from a metal layer covering the dielectric layer, an electrical contact must be established locally between a non-illuminated surface and the metal layer.

The PV device may be any suitable PV device. For example, the PV device may be, but not limited to, a passivated emitter and rear contact (PERC) solar cell, passivated emitter rear locally diffused (PERL) cell, passivated emitter rear totally diffused (PERT) cell, passivated emitter rear floating p-n junction (PERF) cell, n-type front and back contact solar cell, passivated contact solar cell, heterojunction solar cell, or a combination thereof. In particular, the PV device may be a PERC solar cell. The method may be applicable to a PV device with a suitable substrate. The substrate may be any suitable substrate for manufacturing the PV device. For example, the substrate may be a wafer of a solar cell. The substrate may be a hybrid substrate. The substrate may be, but is not limited to, a silicon substrate, lll-V substrate, germanium substrate, sapphire substrate, silicon-germanium substrate, and the like. The substrate may be a n-type or p-type substrate.

According to a particular aspect, the method may be suitable for contacting p- type silicon, either as base of PV devices based on p- type silicon wafers, or as front- or rear-emitter of PV devices based on n-type silicon wafers. Even more in particular, the substrate may be a p- type silicon-based substrate. For example, the substrate may comprise, but is not limited to, multicrystalline silicon, quasi-monocrystalline silicon, or monocrystalline silicon. The substrate may be of any suitable shape and size. The substrate may have a suitable thickness. According to a particular aspect, the thickness of the substrate may be ³ 45 mhi.

For the purposes of the present invention, the semiconductor layer may be considered to be at least a part of the substrate of the PV device. Accordingly, the semiconductor layer may be any suitable semiconductor layer as described above in relation to the substrate. For example, the semiconductor layer may comprise silicon. The dielectric layer may be any suitable dielectric layer. For example, the dielectric layer may comprise at least one layer of dielectric material. The dielectric material may be, but not limited to, SiN_x, SiO_x, AIO_x, SiC_x, or a combination thereof. The dielectric layer may be formed on the semiconductor layer by any suitable method. For example, the dielectric layer may be formed by, but not limited to, plasma enhanced chemical vapour deposition (PECVD), atomic layer deposition (ALD), atmospheric pressure chemical vapour deposition (APCVD), thermal oxidation (dry or wet), chemical oxidation, or a combination thereof.

The electrical contact may be formed on any suitable surface of the PV device. According to a particular aspect, the electrical contact may be formed on the non- illuminated surface of the PV device.

The ablating may be by any suitable means. In particular, the ablating may comprise ablating with laser. The laser may be any suitable laser. According to a particular aspect, the ablating may comprise using a laser beam to create a concentric array of recess in the semiconductor layer. In view of the creation of concentric array of recess, the time taken during the ablating according to the method of the present invention is faster than the time take for creating recess using conventional straight line laser ablation. For example, the ablating according to the method of the present invention may be 25.0-33.3% faster as compared to ablating by conventional means using straight line ablation.

The ablating enables the formation of a crater and a concentric array of recess in the dielectric layer to expose the underlying semiconductor layer. For the purposes of the present invention, concentric array may comprise uniform or non-uniform pitches and/or a segmented or non-segmented array. The ablating may result in the creation of a pattern of contact points. The pattern may be any suitable pattern formed from the ablating. For example, the pattern may be segmented or non-segmented. The pattern may have a suitable pitch. For example, the pitch may be uniform or non-uniform.

According to a particular aspect, the pattern may comprise a concentric array, synergistically intercalating concentric circular lines with concentric circular dash or dot patterns, or a combination thereof. The synergistic intercalation of concentric circular lines with concentric dash or dot patterns may be formed by using a Fresnel collimation design. In particular, the ablating may comprise using a laser beam to create alternating concentric circular and dot recesses in the semiconductor layer by employing a Fresnel lens design to collimate and enhance absorption of near infrared light that is reflected from a rear-side of a PV device, such as a solar cell, and particularly a PERC solar cell. By employing a dot pattern for the concentric array of recess, throughput of the ablating may be further reduced.

According to a particular aspect, in the manufacture of half-cut solar cells, triple-cut solar cells or quadruple-cut solar cells, the pattern may be a segmented, continuous concentric array design. Similar to a Fresnel collimation design, the pitch between the concentric circles may be non-uniform.

The ablating may be in pulsed mode, continuous wave mode or a combination thereof. In particular, the ablating may be in pulsed mode. Even more in particular, the ablating may comprise ablating with laser in pulsed mode. According to a particular aspect, when many electrical contacts need to be made, the ablating may be in pulsed mode and the laser beam used in the ablating may be displaced at a speed adapted to the frequency of the laser in order to obtain the desired pattern on the wafer. This may result in craters having a slightly oval opening.

The ablating may be at sufficient pulse energy. For the purposes of the present invention, sufficient pulse energy is one which enables simultaneous ablation of the dielectric layer, while at the same time creating a recess in the semiconductor layer. For example, the pulse energy may be dependent on the thickness and type of the dielectric layer that may be used for passivation of the dielectric layer. According to a particular aspect, after the ablating, the walls of the crater should preferably by covered at least partially with molten material from the semiconductor layer projected from the recess. In particular, when the semiconductor layer comprises silicon, the molten material may be molten silicon.

The ablating may be performed under suitable conditions. For example, the ablating may comprise ablating in standard atmosphere, inert atmosphere or reducing gas atmosphere. The inert atmosphere or reducing gas atmosphere may comprise any suitable gas such as, but not limited to, N₂, H₂, forming gas (mixture of N₂ and H₂), or noble gases. The ablating may be using any suitable laser. For example, the laser pulse length may be microsecond, nanosecond, picosecond, femtosecond laser, or a combination thereof. In particular, the laser may operate in the nanosecond range. Even more in particular, the laser may operate at a wavelength of about 532 nm. Following the ablating, the method comprises depositing a layer of metal on the dielectric layer comprising the crater to form a metal layer. The depositing may be by any suitable method. For example, the depositing may comprise, but is not limited to: screen-printing, evaporation, sputtering, laser transfer, chemical vapour deposition, inkjet printing, electroplating, or a combination thereof. The chemical vapour deposition may comprise metallic chemical vapour deposition. In particular, the depositing may comprise screen-printing.

According to a particular aspect, the depositing may comprise sputtering, chemical vapour deposition or electroplating a layer of metal on the semiconductor layer to form a metal layer. In particular, the depositing may comprise depositing the layer of metal on a concentric array of recess in the dielectric layer by using a shadow mask.

According to a particular aspect, the depositing may comprise screen-printing a layer of metal on the dielectric layer.

The layer of metal deposited on the dielectric layer may comprise any suitable metal. The metal may be the metal from which the contact is formed. According to a particular aspect, the metal deposited on the dielectric layer during the depositing may be in the form of a metal paste. The layer of metal may comprise a metal or metal alloy. For example, the layer of metal may comprise aluminium or an aluminium-containing metal. In particular, the aluminium-containing metal may comprise aluminium alloy.

The metal layer formed from the depositing may have a suitable thickness. For example, the thickness of the metal layer formed may be 1-30 mhi. In particular, the thickness of the metal layer may be 1-28 mhi, 5-25 mhi, 7-20 mhi, 10-18 mhi, 12-15 mhi. Even more in particular, the thickness may be 15-30 mhi, preferably about 25 mhi.

According to a particular aspect, following the depositing, the walls of the crater formed from the ablating may be covered at least partially with material from the semiconductor layer up to and including the metal layer. The material from the semiconductor layer may be silicon.

The method of the present invention may further comprise drying the metal layer. The drying is optional. The drying enables the metal layer to be dried, free of detrimental contaminants and to obtain a good adhesion to the dielectric layer on which it is deposited. The drying should preferably be performed before the formation of any other electrical contacts to avoid any degradation of the contacts.

The heating of the metal layer may follow the drying. The heating of the metal layer may comprise heating of the metal layer and the semiconductor layer. The heating may be simultaneous as any thermal treatment required for forming any other electrical contacts of the PV device. According to a particular aspect, the heating may comprise firing the metal layer. The heating may result in the metal layer penetrating the dielectric layer to contact the semiconductor layer to form the electrical contact.

In particular, the heating may comprise etching the metal comprised in the metal layer through the dielectric layer to form an electrical contact with the underlying semiconductor layer in a concentric array pattern, wherein the metal layer is formed by screen-printing a layer of metal on the dielectric layer.

During the heating, the diffusion of material from the semiconductor layer to the metal layer should be kept to a minimum. In particular, diffusion of silicon into the aluminium metal layer should be kept to a minimum. Accordingly, the heating may be under suitable conditions. For example, the heating may be at a pre-determined temperature for a pre-determined period of time. The heating may comprise firing the metal layer at a pre-determined temperature for a pre-determined period of time sufficient to allow the alloying process between the metal layer and the semiconductor layer to occur locally. In this way, the thickness of the LBSF layer formed around the contacts may be maximised.

The pre-determined temperature may be any suitable temperature for the purposes of the present invention. In particular, the pre-determined temperature during the heating may be a temperature greater than the melting temperature of the metal comprised in the metal layer. For example, the pre-determined temperature may be ³ 540°C. In particular, the pre-determined temperature may be 540-950°C, 550-900°C, 575-875°C, 600-850°C, 650-825°C, 660-800°C, 700-775°C, 725-750°C. Even more in particular, the pre-deter ined temperature may be 720-760°C. The temperature profile for the heating should provide a fast firing profile, preferably a very sharp, spike-like profile. When the metal layer comprises aluminium with silicon doping, the pre-determined temperature may be lower.

According to a particular aspect, the pre-determined temperature may be a temperature greater than the melting temperature of aluminium. In particular, the pre determined temperature may be ³ 540°C. Even more in particular, the pre-determined temperature may be ³ 660°C. The pre-determined period of time may be any suitable time for the purposes of the present invention. For example, the pre-determined period of time for the heating may be ³ 1 second. In particular, the pre-determined period of time may be 2-30 seconds, 3-25 seconds, 5-20 seconds, 7-18 seconds, 10-15 seconds. Even more in particular, the pre-determined period of time may be 2-10 seconds. According to a particular aspect, the heating provides a continuous local p+ layer that surrounds the inner side of the concentric circular LBSF contacts formed. The high quality LBSF obtained with the method of the present invention may be related to a minimal escape of semiconductor layer material, such as silicon, from the contact to the metal layer. This may be in view of the limited physical volume assigned for metal (such as Al) and semiconductor layer material (such as Si) mass transport within the inner side of the concentric circular crater.

When the semiconductor layer comprises silicon and the metal layer comprises aluminium, during cooling following the heating, the silicon content of Al-Si melt formed during the heating gradually decreases down to the eutectic composition of about 12 mol % of Si, before solidifying at 577°C. To allow for this gradual concentration evolution, the excess Si continuously crystallizes epitaxially on the recess surfaces with an Al content below 1 mol %, i.e. the solubility limit of Al in solid Si, to form the p+ phase (LBSF). The eutectic Al-Si layer that covers the outer side on the concentric circular walls of the electrical contacts formed may contain voids resulting from a non- continuous eutectic layer. The presence and size of these voids may be minimised by conducting the heating as disclosed above. The voids may be associated with interruptions in the LBSF layer that should surround the recess but have a negligible influence on the PV device properties. The voids may appear if silicon escapes from the contact during the firing step. This effect may eventually be minimized by selecting a shorter pitch between the ablation lines, dashes or dots, by diminishing the heating peak temperature, by controlling the cooling profile during the heating, or by using specially designed aluminium conductive pastes, such as Si-containing aluminium paste.

According to a particular aspect, the heating may comprise increasing the pre determined temperature to above 540°C for about 5 seconds, with a peak temperature of about 720°C, followed by cooling down to 600°C after a total of about 8 seconds. As an example, Figure 1 provides a schematic representation of manufacturing a PV device including the method of the present invention.

The method of the present invention results in the formation of visually identifiable laser ablation pattern following the heating. The pattern may be a concentric array or synergistically intercalating concentric circular lines with concentric circular dash or dot patterns. Examples of patterns formed are shown in Figures 2A and 2B, while Figure 2C shows a typical pattern formed on a PERC cell following straight line ablation.

In particular, when monofacial PERC cells are placed in modules, the laser ablation pattern formed from the method of the present invention may be discerned by electroluminescence or photoluminescence imaging. The imaging may be by using drones for inspection.

The present method provides advantages in that the ablation processing time is sped up, the average cell efficiency of the PV device with contacts formed from the method of the present invention is improved by way of reduced series resistance, thereby enhancing the short-circuit current density, and the formation of voids is limited to the concentric array pattern only.

According to a second aspect, the present invention provides a photovoltaic (PV) device comprising an electrical contact formed from the method above. For example, the PV device may be, but not limited to, a passivated emitter and rear contact (PERC) solar cell, passivated emitter rear locally diffused (PERL) cell, passivated emitter rear totally diffused (PERT) cell, passivated emitter rear floating p-n junction (PERF) cell, n- type front and back contact solar cell, passivated contact solar cell, heterojunction solar cell, or a combination thereof. In particular, the PV device may be a PERC solar cell.

The PV device may have an improved short-circuit current (l_sc), an improved fill factor, and a lower series resistance. Accordingly, the PV device of the present invention has an improved global efficiency with an improved processing throughput.

In particular, a PV device having contacts formed from the method of the present invention may have improved cell efficiency. For example, the improvement in PERC cell efficiency may be at least 0.06-0.10% using a metal layer comprising normal aluminium paste or aluminium paste with silicon doping, respectively. The cell efficiency enhancement may be due to reduction of the series resistance, achieved through two-dimensional concentric geometry to relieving the current crowding effect and improve minority carrier collection at the rear contact.

Having now generally described the invention, the same will be more readily understood through reference to the following embodiment which is provided by way of illustration, and is not intended to be limiting.

Examples

Cell properties

The electrical parameters of cells prepared have been measured using a LED-based solar simulator, in which a flash lamp simulates the AM 1.5 sunlight spectrum (Air Mass 1.5 is defined as the light passing an atmosphere of 1.5 x normal atmosphere thickness) to illuminate a cell with a standard irradiance of 1000 W/m² (“one sun”) at 25°C.

The electrical parameters of the cell were measured (l_sc and V_oc) and computed (l_mpp and V_mpp) during the light flash by drawing its current-voltage curve (l-V) whilst applying multiple light illumination through variable resistance to an externally connected“load”. From these values, J_sc (normalized l_sc), the fill factor (FF) and cell efficiency (h or eta) were computed. The fill factor reflects the series and shunt resistance losses.

Example 1 : Aluminium Paste with Silicon Doping (Laser Pitch = 900 um) The measured electrical parameters are shown in Figure 3.

The ablation line pitch was constant for both concentric array pattern and line array pattern, at 900 pm, and the metal contact fraction was about 4.0%.

The processing time to form the concentric array took 25% less time (about 27 s) as compared to the processing time to form the line array (about 37 s).

As can be seen from Figure 3, PERC cells with concentric array yielded higher J_sc as compared to PERC cells with line array.

PERC cells with concentric array yielded the highest average cell efficiency at 18.83%, as compared to the PERC cells with line array with highest cell efficiency at 18.69%. T-test with a = 0.05 indicated that PERC cells with concentric array and PERC cells with line array were significantly different from each other in terms of the average cell efficiency, J_sc and series resistance.

PERC cells with concentric array alleviate the current crowding effect. Series resistance derived from photoluminensce (PL) mapping of PERC cells with concentric array was 0.406 Q.cm², while series resistance derived from PL mapping of PERC cells with line array was 0.407 Q.cm². The lower series resistance in PERC cells with concentric array indicated that the concentric array design improves the current collection at the rear side of the multicrystalline silicon PERC solar cells. The PL results corroborate with the measured series resistance results from the IV measurement. Example 2: Aluminium Paste without Silicon Doping (Laser Pitch = 900 urn)

The measured electrical parameters are shown in Figure 4.

PERC cells with concentric array yielded higher J_sc as compared to PERC cells with line array.

PERC cells with concentric array yielded the highest average cell efficiency at 19.13%, as compared to the PERC cells with line array with highest cell efficiency at 19.07%. T-test with a = 0.05 indicated that PERC cells with concentric array and PERC cells with line array were significantly different from each other in terms of the average cell efficiency, J_sc and series resistance.

Example 3: Aluminium Paste without Silicon Doping (Laser Pitch = 1300 urn) The measured electrical parameters are shown in Figure 5.

When the pitch was increased to 1300 pm, PERC cells with concentric array yielded better V_oc.

PERC cells with concentric array yielded the highest average cell efficiency at 19.63%, as compared to the PERC cells with line array with highest cell efficiency at 19.56%. T-test with a = 0.05 indicated that PERC cells with concentric array and PERC cells with line array are significantly different from each other in terms of the average cell efficiency, J_sc and series resistance.

Thus, it can be seen from the above Examples that PERC cells with concentric array required a shorter laser ablation processing time. PERC cells with concentric array improved the V_oc at longer laser pitch and series resistance, thus leading to an overall improvement of the average cell efficiency of the multicrystalline silicon PERC solar cells while maintaining comparable J_sc.

Whilst the foregoing description has described exemplary embodiments, it will be understood by those skilled in the technology concerned that many variations may be made without departing from the present invention.

Claims

1. A method of forming an electrical contact between a semiconductor layer and a metal layer in the manufacture of a photovoltaic (PV) device, the semiconductor layer and the metal layer being separated by a dielectric layer, the method comprising:

- ablating the dielectric layer to locally form a crater and to create a concentric array of recess in the dielectric layer, thereby exposing the semiconductor layer;

- heating the metal layer at a pre-determined temperature to form the electrical contact.

2. The method according to claim 1 , wherein the ablating comprises laser ablation.

3. The method according to claim 1 or 2, wherein the ablating is in pulsed mode or continuous wave mode.

4. The method according to any preceding claim, wherein the ablating comprises ablating in standard atmosphere, inert atmosphere or atmosphere of reducing gas.

5. The method according to any preceding claim, wherein the depositing comprises: screen-printing, evaporation, sputtering, laser transfer, chemical vapour deposition, inkjet printing, electroplating, or a combination thereof.

6. The method according to any preceding claim, wherein the layer of metal comprises aluminium or an aluminium-containing metal.

7. The method according to claim, wherein the aluminium-containing metal comprises aluminium alloy.

8. The method according to any preceding claim, wherein the metal layer has a thickness of 1-30 mhi.

9. The method according to any preceding claim, wherein the pre-determined temperature is a temperature greater than the melting temperature of the metal comprised in the metal layer.

10. The method according to any preceding claim, wherein the pre-determined temperature is ³ 660°C.

11. The method according to any preceding claim, wherein the pre-determined temperature is 540-950°C.

12. The method according to any preceding claim, wherein the semiconductor layer comprises silicon.

13. A photovoltaic (PV) device comprising an electrical contact formed from the method of any preceding claim.

14. The PV device according to claim 13, wherein the PV device is a passivated emitter and rear contact (PERC) solar cell, passivated emitter rear locally diffused (PERL) cell, passivated emitter rear totally diffused (PERT) cell, passivated emitter rear floating p-n junction (PERF) cell, n-type front and back contact solar cell, passivated contact solar cell, heterojunction solar cell, or a combination thereof.