WO2018229366A1

WO2018229366A1 - Process for laser ablating thin layers in two steps for the production of semi-transparent photovoltaic modules

Info

Publication number: WO2018229366A1
Application number: PCT/FR2018/000174
Authority: WO
Inventors: Rachida BOUBEKRI
Original assignee: Sunpartner Technologies
Priority date: 2017-06-16
Filing date: 2018-06-14
Publication date: 2018-12-20
Also published as: FR3067858A1

Abstract

The invention relates to a process for manufacturing photovoltaic cells made up of semi-transparent thin layers including: a laser ablating step consisting in removing, through the substrate and through the first electrode, with a laser beam of a fluence F1 and of main wavelength λ1, a volume V1 of cross-sectional area S1 larger than 0.25 mm2 and of height H1 within the absorber and within the second electrode; and a second laser ablating step consisting in removing, through the substrate and with a second laser beam of a fluence F2 and of main wavelength λ2 different from λ1, a volume V2 of cross-sectional area S2 smaller than (S1 - 0.035 * √S1) mm2 and of height H2 within the first electrode. The beams are of top-hat type and are centred on each other.

Description

Laser thin film ablation process in two steps for producing semi-transparent photovoltaic modules

The present invention relates to a method of manufacturing, at a high production rate, photovoltaic modules by selective and local laser ablation of the material of thin-film photovoltaic cells. More specifically, it relates to a two-step thin film laser ablation process for obtaining semi-transparent photovoltaic cells whose transparency rate is obtained by creating a more or less dense network of geometric transparency zones in the structure of said thin layers.

STATE OF THE ART

Several families of photovoltaic materials are distinguished in the literature, such as crystallized solid materials, organic small molecule materials and solid thin films. Solid thin films are particularly suitable for photovoltaic technology ^{'semitransparent} ic due to their very low thicknesses. The implementation times of the process according to said technology are considerably reduced and their lower cost due to the lesser amount of material used.

A photovoltaic module is composed of a multitude of photovoltaic cells connected in series and / or in parallel. Each cell consists of a stack of thin layers. The stacking order of the thin layers is conditioned by the thin film technology considered.

For example, in the case of a thin-film technology in which the photoactive layer (hereinafter referred to as an absorber) is amorphous silicon, the cell consists of a stack of thin layers positioned in the film. following order:

A transparent substrate (for example glass or a polymer); - An electrically conductive transparent front electrode generally consisting of a transparent oxide hereinafter referred to as "TCO" (acronym for the term "Transparent Conductive Oxide");

A photoactive layer, the absorber, made of amorphous silicon;

- An electrically conductive back electrode, generally metallic. The thickness of each thin layer varies from a few hundred nanometers to a few microns.

Making semitransparent photovoltaic modules is an interesting opportunity in the building industry, especially to include glazed surfaces capable of generating electrical energy. Transparency can be achieved by etching, lithography and / or laser ablation processes. The lithography process provides excellent results but is slow and expensive in terms of investment and consumables used. It includes several stages of insolation and etching of the different layers (dry etching or wet etching with suitable chemical solutions). The steps are successive and can encompass up to 8 steps. In addition, this process is difficult to implement on large areas (greater than one square meter) that are preferred in the building industry.

In addition, to achieve production rates of said semi-transparent photovoltaic modules compatible with the price requirements of the building market, it is necessary to be able to remove unitary surfaces of material of a few tenths of mm ² in one or two unit steps maximum , corresponding to one or two successive laser shots. This is a faster process, less expensive than lithography, and requires no use of chemicals.

Nevertheless, this process often generates thermal phenomena, especially when using the laser etching in one step to simultaneously ablack a stack of thin layers made of materials whose ablation thresholds are different. The thermal effect causes the fusion of the materials of the stack, which can be at the origin:

a local modification of the chemical structure of all or some of these materials around the ablated zone; - Re-deposition of materials on the edges of the ablated surface, which can create contacts between the upper electrode of the cell and the lower electrode at the level of the blanks of the ablated patterns;

- the formation of alloys near the ablated areas.

These thermal effects have the effect of creating short circuits (also called "shunts" in English) that will increase the leakage currents and thus reduce the performance of the semi-transparent photovoltaic cell or the module.

One solution for minimizing or even eliminating these thermal effects is to use several laser ablation steps by adapting the parameters of the laser ablation to the thin layers to be ablated.

Document US 2009/0151783 (Nexpower) proposes to realize the transparency of solar cells by ablating:

o the second electrode (upper electrode) with a Si surface;

o the absorption layer and the first electrode (lower electrode) with a surface S2 _;

such as Si≥ S2.

This technique makes it possible to minimize the shunts linked, for example, to a re-deposition of materials on the edges of the ablated surface, which can create contacts between the second electrode of the cell and the lower electrode at the level of the blanks of the ablated patterns.

However, this manufacturing method uses the same laser of main wavelength λ (the main wavelength λ of a laser is defined as the majority wavelength emitted by said laser) and therefore does not take into account the specificities of the ablation thresholds of the different layers. Such a method therefore does not make it possible to eliminate undesirable thermal effects such as the structural modification of all or part of the ablated layers or the formation of alloys in the vicinity of the ablated zones.

Furthermore, in the case of photovoltaic cells whose absorber is amorphous silicon, it would be possible to generate the semi-transparency without ablating the front electrode, since it is transparent. For example, it is known that the transmission of bare glass is about 90% while that of a TCO deposited on glass is 80%. The semi-transparent photovoltaic cell obtained by single ablation of the second electrode and the absorber therefore has a lower transparency than that obtained by ablating moreover the TCO layer. This is all the more advantageous as the TCOs used in the photovoltaic cells are textured to increase the collection of photons, and thus generate a significant light diffusion conferring a milky and sometimes even colored appearance. Therefore, to obtain semi-transparency of high optical quality, it is preferable to ablate the TCO.

Finally, document US20060196536A1 (Sharp) discloses a method of interconnecting a photovoltaic module that uses two laser ablation steps in which the two beams used have distinct diameters and distinct wavelengths. Nevertheless, these YAG laser beams are of Gaussian type and therefore do not make it possible to obtain beam diameters greater than 100 μιη. As a result, the production rates of these lasers are very low and the energy distribution on any section of the beam is Gaussian, which means inhomogeneous. Thus, the optical quality (transmittance, haze, clarity) of a transparency zone of several tenths of mm ² created by this type of laser ablation that would require the recovery of many successive laser shots would be strongly impacted. Moreover, the pulse duration of these lasers, which is less than one picosecond, does not generate the same short circuits on the module since they are in athermic laser / material interaction regimes, which means that the dimensions of the sections of the two successive laser beams would be of a completely different nature than that contemplated in this invention. It would therefore not be obvious for those skilled in the art to shape the beams differently to achieve the desired result in the manufacture of photovoltaic modules object of the invention.

The present invention aims to solve these problems and to overcome the aforementioned drawbacks of the state of the art by proposing a new process for laser ablation of photovoltaic thin layers in two steps. PURPOSE OF THE INVENTION

In general, the present invention aims at providing a method for manufacturing thin-film photovoltaic cells by selective and local laser ablation of the material of said cells to maximize the performance of semitransparent photovoltaic modules by eliminating the appearance short-circuits resulting from the thermal effects of conventional laser ablation processes and by maximizing the optical quality of said photovoltaic cells.

OBJECTS OF THE INVENTION

The subject of the invention is generally a method of manufacturing photovoltaic cells in thin, semi-transparent layers by selective and local laser ablation of the material composing said thin layers. The cells may be connected in series and / or in parallel to form semitransparent photovoltaic modules. Said cells comprise, successively, at least one first electrically conductive transparent material (generally a doped semiconductor) deposited on a transparent substrate and acting as a front contact called a first electrode, one or more photoactive layers called absorber (s) and deposited on said first electrode, and a second electrically conductive material containing at least one metal layer and acting as a rear contact called second electrode. We notice :

The thickness which is equal to the cumulative thicknesses of the absorber and the second electrode;

- HiA the thickness of the absorber layer;

- HIB the thickness of the second electrode;

- H ₂ the thickness of the first electrode.

A laser generates a beam generally of circular section. In this case, the laser section is defined by its center and its diameter. It is known to those skilled in the art to shape the laser beam to obtain a beam whose section represents any geometric form. In this case, the diameter is defined as being the maximum distance between two points of the generated section. The center of the circumcircle is defined as the middle of the diameter. So any type of shape is defined by its diameter D and its center C.

The manufacturing method according to the invention of the semitransparent photovoltaic cells comprises two unitary stages of selective and local laser ablation carried out successively, and repeated by iterations.

The first unitary step Ei consists of removing, via the substrate and the first electrode, and with the aid of a first laser beam of a fluence Fi and of main wavelength λι, a volume Vi of section Si (center Ci and diameter Di) and height Hi within the absorber and the second electrode. This first laser ablation is performed while ensuring the integrity of said first electrode, that is to say without damaging the transparent conductive material that constitutes it. The absorption of the wavelength λι of said first laser beam by the absorber causes sublimation of the material or materials constituting said absorber according to volume VIA of section Si and height HIA. The volume VIB of section Si and height HIB within the second electrode is then ejected, this material ejection phenomenon under physical constraints being better known by the term "lift-off".

We define an imaginary volume Vie of section Si and height H ₂ .

The second unitary step E ₂ consists in removing, through the transparent substrate and with the aid of a second laser beam, a fluence F ₂ and a main wavelength different from λι, perfectly or partially aligned with the first laser beam, a volume V ₂ of section S2 (diameter D ₂ ) within the first electrode. Thus, V ₂ is perfectly included in the volume Life. This step removes the conductive transparent material in contact with the substrate, and increases the optical quality of an image observed across areas where all the thin layers have been ablated. Indeed, the transmittance of the light in the visible is increased since it removes the reflection phenomena at the interfaces, the refractive index of said transparent conductive material being generally stronger than that of the substrate. Moreover, since the surface of the conductive transparent material in contact with the absorber is generally textured at the nano or micrometric scale, removing said material makes it possible to increase the clarity and to reduce the "haze", that is to say the blur of an image perceived through semi-transparent photovoltaic cells.

Due to these optical and electrical requirements, and in order to maximize its production rate, the method according to the invention is characterized by the fact that said unitary steps E ₁ , E ₂ are each carried out in a single laser shot, said laser beams being type "tophat" and centered relative to each other, and in that the area of the section Si is greater than 0.25 mm ² , the area of the section S2 being less than (S _x -

0.035 * .fsi) mm ² . The two laser beams are shaped to obtain "top hat" type beams, that is to say the end of the section has the following feature: any point of said section has the same energy. The section of the first laser beam has a diameter greater than 564 μm to limit the diffraction phenomena of visible light to the passage of the opening created by said first beam, which implies that Si is greater than 0.25 mm ² . So that the two electrodes can not have contact at the ablation blanks which avoids post-ablation the creation of short circuits of the photovoltaic diode, while maintaining a good optical transparency in the area of ablation, one must choose D ₂ such that D ₂ <Di - 20 pm. This implies that S2 is less than (S _x - 0.035 *

■ fsi) mm ² . Thus, the hole made in the first electrode is at least 10 microns smaller on each side than the hole made in the active layer and the second metal electrode.

A section S2 'is defined such that its center Ci is the same as that of the section Si and of diameter D ₂ , and V2' the volume of section S2 'and of height H ₂ . The two laser beams used during the two elementary steps are centered, which means that V ₂ is perfectly included in the volume V2 '.

Advantageously, the pulse duration of said laser beams is of the order of magnitude of the nanosecond. This allows for higher production rates than in the case of pico or femtosecond laser beams. In an advantageous embodiment, the volumes Vie and V2 satisfy the relation: Vie≥1.1 V ₂ . The first and second laser beams may be shaped into disks, oval, polygonal, hexagonal, square or any combination of said shapes. For any type of shape, the laser beam defines a section and a volume of ablation of the thin film of photovoltaic ^cells' ic. In the case of photovoltaic ^cells' ic semi-transparent thin layers whose absorber predominantly comprises a layer of amorphous silicon:

The first laser is preferably a green laser whose main wavelength is between 500 and 580 nm and whose fluence Fi is between 600 and 800 mJ / cm ² so as not to cause physico-chemical modification. substrate, usually a glass or plastic, and the first electrode, usually a transparent metal oxide. For example, the first electrode may be composed of fluorine doped tin dioxide.

- The second laser is an infrared laser whose main wavelength λ ₂ is between 900 and 1100 nm and whose fluence F ₂ is between 4 and 8 J / cm ² so as not to cause modification physico-chemical substrate while allowing the ablation of the first electrode.

According to a first variant of the method according to the invention, the laser shots of the two unitary steps (Ei, E ₂ ) are all identical and without overlap during the succession of said unitary steps by iteration. This is the set of laser shots of the first unitary step Ei followed by all the shots of the second unitary step E ₂ to obtain a functional semi-transparent module whose transparency areas are non-contiguous and have all the same geometric shape to preferentially form an ordered network. According to a second variant of the method according to the invention, the semi-transparency produced by the two laser shots of the two unitary steps (E _lr E2) have shapes or dimensions that vary during their successive iterations, to form a photovoltaic module having a semitransparency gradient, or a random network.

DETAILED DESCRIPTION

The invention is now described in more detail with reference to FIGS. 1 to 4.

FIG. 1A represents a diagram of a sectional view of a photovoltaic cell portion based on amorphous silicon.

FIG. 1B represents a diagram of a sectional view of a photovoltaic cell portion based on amorphous silicon traversed by a laser beam.

FIG. 2A represents a diagram of a sectional view of a photovoltaic cell portion based on amorphous silicon traversed by a green laser beam focusing within the absorber.

FIG. 2B represents a diagram of a sectional view of the result of the first unitary step of the laser ablation according to the invention.

FIG. 3A represents a diagram of a sectional view of a photovoltaic cell portion based on amorphous silicon traversed by an infra-red laser beam focusing within the first electrode.

FIG. 3B represents a schematic sectional view of the result of the second unitary step of the laser ablation according to the invention.

FIG. 4 is a diagram showing an example of a photovoltaic module whose transparency zones are obtained by iteration of the two unitary steps of the process, which are two isolated circular laser shots.

The photovoltaic cell portion of FIG. 1A is composed of:

- a transparent substrate (1) of glass;

a first conductive transparent electrode (2) made of fluorine-doped tin dioxide (Sn0 ₂ : F); an absorber (3) formed of at least one amorphous silicon layer (aSi) forming one or more nip junctions;

a second metal conductive electrode (4) composed of aluminum

(Al).

The thickness of each thin layer varies from a few hundred nanometers to a few microns.

1B shows a diagram of a sectional view of the photovoltaic cell portion ^'ic amorphous silicon according to Figure 1A, through which a laser beam (50) section of green If the thicknesses of the various layers.:

- HIA the thickness of the absorber layer (3);

- HIB the thickness of the second electrode (4);

The thickness of all the layers (second electrode (4), absorber (3));

- H2 the thickness of the first electrode (2).

The process for manufacturing semi-transparent photovoltaic cells comprises two steps of selective local laser ablation carried out successively. The first unitary step Ei, described with reference to FIG. 2A, consists in engraving with a first green laser beam (50) (λι = 534 nm) a volume Vi of section Si and of height Hi within the absorber (3) and the second electrode (4). The fluence Fi of the laser is 600 mJ / cm ² . The beam is of the top-hat type. The diameter Di of the section Si is 600 μιτι, hence a section area Si equal to 0.28 mm ² . The area of the section Si is therefore greater than 0.25 mm ² . This first laser ablation takes place through the transparent substrate (1) and the first transparent electrode (2) which are not impacted by the green laser beam. The absorption of the wavelength λι of the laser beam by the absorber (3) causes the sublimation of the material or materials constituting said absorber according to volume VIA (53) of section Si and height HIA. The volume VIB (54) of section Si and height HIB within the second electrode is then ejected, this material ejection phenomenon (55) under physical constraints is known as the term "lift-off". The result of the first laser ablation, shown diagrammatically in FIG. 2B, leaves a vacant zone (6) of volume Vi within the second electrode (4) and the absorber (3) without thermal deterioration of the first electrode (2). ) or the substrate (1), which are transparent for this wavelength λι.

The second step E ₂ , described with reference to FIG. 3A, consists of etching with the aid of a second infrared laser beam (70) of main wavelength λ ₂ = 1064 nm, of the same shape as the laser beam. (50) green but of smaller section S2, a volume V ₂ (72) within the first electrode such that V ₂ (72) is perfectly included in the volume V (71) of section Si of height H ₂ . The fluence of the laser is F2 = 7J / cm ² . The volume V ₂ (72) of section S2 and height H2 within the first electrode is then ejected. This second laser ablation is only through the substrate (1) which is not or very little affected thermally. The infrared laser beam (70) is aligned with the first green laser beam (50), i.e. the sections of the two beams are centered with respect to each other, and the diameter of its section is 60 microns smaller than that of the green laser beam, which means that D2 is 540 μm, and the section S2 is about 0.23 mm ² . Starting from the following calculation: (Si - 0.035 * ^ βΐ) = 0.26 mm ² , the condition is satisfied that S2 is less than (Si - 0.035 * * jsi). These two conditions make it possible to ensure the absence of re-deposition of ejected material (73) on the ablated flanks of the absorber (3) and the second electrode (4). Thus, there is no contact formation between the first electrode (2) and the second electrode (4) and thus the creation of short circuits within the photovoltaic diode is avoided. In addition, the report

= S1 / S2 is equal to 1.2 and therefore satisfies the relation Life 1.1 V2.

FIG. 4 is a diagram showing an example of a photovoltaic module whose transparency zones are obtained by iteration of the two unitary steps of the process, which are two isolated circular laser shots. The white transparency areas are distinctly separated from the black photovoltaic active areas. EXEMPLARY EMBODIMENT

The method of the invention was the subject of a practical test, carried out for the purpose of producing photovoltaic modules based on amorphous silicon having 30% transparency. Semi-transparency is achieved by local and selective ablation of 600 micron diameter discs in the absorber and in the second electrode. The laser used is a green laser (534 nm) nanosecond with a fluence of 600 mJ / cm ² . The beam is of the top-hat type and its section has a diameter of 600 μm. A measurement of the optical transmission and the performance of the module was carried out before the second step. We therefore have a first fluorine-doped tin oxide electrode on the entire substrate, and an absorber and second electrode surface of 30% of the substrate surface. The measured transmission is 52.4%.

The second step of the method of the invention which consists in ablating the first electrode was carried out from an infrared laser (1064 nm) nanosecond of fluence 7J / cm ² . The beam is also of top-hat type and the diameter of its section is equal to 540 nm. The semitransparent photovoltaic module obtained by the manufacturing method according to the invention thus has, in fine, 40% of surface area of SnO 2: F. The measured optical transmission then passes to 57.5%, which makes it possible to improve the effective transparency of the module by more than 5%. In addition, the electrical performances are identical and are not deteriorated by the second step. The manufacturing method according to the invention therefore makes it possible to manufacture photovoltaic modules in semi-transparent thin layers by selective and local laser ablation of said thin layers. This makes it possible to maximize the electrical performance of said modules by eliminating the appearance of short-circuits consecutive to the thermal effects of known laser ablation processes, while maximizing the optical quality of said semi-transparent photovoltaic modules, and with good rates of production.

Claims

1 - Method for manufacturing photovoltaic ^cells' ic semi-transparent thin film by selective and local laser ablation of the material composing said thin film, said thin film layers successively comprising at least a first transparent electrically conductive material deposited on a transparent substrate (1 ) and playing the role of front contact called first electrode (2), one or more photo-active layers (s) called (s) absorber (3) and a second electrically conductive material acting as rear contact called second electrode (4). ), the method comprising a succession of the following unitary steps, repeated by iteration:

a first unit laser ablation step (Ei) of removing, through the substrate (1) and the first electrode (2), and with the aid of a first laser beam (50) a fluence Fi and main wavelength λι, a volume Vi of section Si and height Hi within the absorber (3) and the second electrode (4) while ensuring the integrity of the first electrode (2) ;

a second unitary laser ablation step (E ₂ ) of removing, via the substrate (1) and using a second laser beam (70), a fluence F ₂ and a wavelength main λ ₂ different from λι, a volume V ₂ of section S2 and height H2 within the first electrode (2);

said method being characterized in that said unitary steps (E 1, E 2) are each performed in a single laser shot, said laser beams (50, 70) being of the "top hat" type and centered with respect to each other , and in that the Si section is greater than 0.25 mm ² , the section S 2 being less than (S ₁ - 0.035 * / Si) mm ² .

2 - Process according to any one of the preceding claims, characterized in that the volumes V2 and Vie, Vie being defined by a section Si and a height H2, satisfy the relationship Life 1,1 V2. 3 - Process according to any one of the preceding claims, characterized in that the pulse duration of said laser beams (50,70) is of the order of magnitude of the nanosecond. 4 - Process according to any one of the preceding claims, characterized in that said laser beams (50,70) are shaped according to discs, oval, polygonal, hexagonal, square or any combination of said shapes. 5 - Process according to any one of the preceding claims, characterized in that the two laser shots (50,70) of the two unitary steps (Ei, E ₂ ) are all identical and without overlap during the succession of said unit steps by iteration . 6 - Process according to any one of claims 1 to 5, characterized in that the two laser shots (50, 70) of the two unitary steps (Ei, E ₂ ) have shapes or dimensions that vary during their successive iterations to form a semitransparency gradient. 7 - Process according to any one of the preceding claims, characterized in that the first laser beam (50) has a fluence Fi between 600 and 800 mJ / cm ² so as not to cause physico-chemical modification of the substrate ( 1) and the first electrode (2). 8 - Process according to any one of the preceding claims, characterized in that the second laser beam (70) has a fluence F ₂ of between 4 and 8 J / cm ² so as not to cause physico-chemical modification of the substrate (1).

9 - Process according to any one of the preceding claims, characterized in that the first laser beam (50) has a main wavelength λi between 500 and 580 nm. 10 - Process according to any one of the preceding claims, characterized in that the first laser beam (50) has a main wavelength λ ₂ between 900 and 1100 nm.

11 - Process according to any one of the preceding claims, characterized in that the height Hi is equal to the cumulative thicknesses of the absorber (3) and the second electrode (4).

12 - Process according to any one of the preceding claims, characterized in that the height H ₂ is equal to the thickness of the first electrode (2).

13 - Process according to any one of the preceding claims, characterized in that the absorber (3) is composed mainly of amorphous silicon.

14 - Process according to any one of the preceding claims, characterized in that the first electrode (2) is composed of fluorine-doped tin dioxide.