WO2018119680A1

WO2018119680A1 - Method and system for monitoring laser scribing process for forming isolation trenches in solar module

Info

Publication number: WO2018119680A1
Application number: PCT/CN2016/112416
Authority: WO
Inventors: Harr MICHAEL; Frauenstein SVEN; Peng SHOU
Original assignee: China Triumph International Engineering Co., Ltd.; Ctf Solar Gmbh
Priority date: 2016-12-27
Filing date: 2016-12-27
Publication date: 2018-07-05
Also published as: JP6708756B2; JP2019517157A; CN108604618A; DE112016006757T5; CN108604618B

Abstract

A method and a system (200) for monitoring a laser scribing process used for forming isolation trenches (21, 22, 23) in a solar module are described. The method comprises the steps of providing a semi-finished solar module (10), forming an isolation trench (21, 22, 23), illuminating a region (4) of the semi-finished solar module (10) at a first surface (101) of the semi-finished solar module (10), detecting an amount of light transmitted through the illuminated region (4) of the semi-finished solar module at a second surface (102) of the semi-finished solar module (10) and evaluating the formed isolation trench (21, 22, 23) in the illuminated region (4). The system (200) comprises a device for producing a laser beam (300), a device for illuminating a region of the semi-finished solar module (400), a device for detecting an amount of light (500), and a device for evaluating the formed isolation trench (600). The method and the system (200) provide a simple, fast and effective way for continuous inspecting isolation trenches (21, 22, 23).

Description

Method and system for monitoring a laser scribing process for forming isolation trenches in a solar module

Field of Invention

The subject of the present application is a method and a system for monitoring a laser scribing process used for forming isolation trenchesin a solar module.

Description of Related Arts

Photovoltaic devices or solar modules comprise a plurality of solar cells which convert sunlight into an electric current. The solar cells of one solar module are electrically connected in series. Thin-filmsolar cells are typically formed on a large area substrate in common processing steps, whereineach solar cell comprises a first contact layer, a second contact layer, and a photovoltaic layer formed between the first contact layer and the second contact layer and converting the sunlight. In order to form separated and electrically insulated solar cells and to electrically connect adjacent ones of them in series, isolation trenches are formed within at least one of the first contact layer, the photovoltaic layer and the second contact layer during the production process of the solar cells. Thus, a superstrate production process typically comprises the following process steps in the mentioned process sequence: On a transparent substrate, a transparent first contact layer is formedand a photovoltaic layer is deposited onto the first contact layer. Subsequently, first isolation trenches are formed withinthe photovoltaic layer and the first contact layer. Then, the first isolation trenches are filled with a first isolating material. Second isolation trenches are formed within the photovoltaic layer, but not in the first contact layer, wherein the second isolation trenches are formed adjacent to the first isolating material within the first isolation trenches or are formed spaced apart from the first isolation trenches in a first lateral direction. Instead of second trenches, other second structures, like isolated holes may be formed within the photovoltaic layer, wherein the second structures may be formed adjacent to the first isolating material or spaced apart from it. Then, a second contact layer, which is usually not transparent, is deposited within the second isolation trenches or within thesecond structures and onto the photovoltaic layerand onto the first isolating material within the first isolation trenches. Last in this sequence, third isolation trenches are formed within the second contact layer and within the photovoltaic layer, but not in the first contact layer, wherein the third isolation trenches are formed adjacent to the second isolation trenches on a lateral side opposite to the first isolation trenches or are formed spaced apart fromthe second isolation trenches on a lateral side opposite to the first isolation trenches in the first lateral direction. Further steps, like filling the third isolation trenches with a second isolating material and applying a second substrate to the second contact layer, may be performed. The first lateral direction is a direction in a plane orthogonal to a direction along a line extending from the transparent substrate to the second contact layer, i.e. a thickness direction of the solar cell. In a substrate production process, the individual process steps are performed in a reverse sequence.

The isolation trenches should be narrow, but should also provide a safe isolation of individual solar cells, in particular for the first isolation trenches and the third isolation trenches, or a safe electrical contact of the second contact layer to the first contact layer within the second isolation trenches. Such isolation trenches often have a width in the range between 20 μm to 80 μm and a distance to neighbouring isolation trenches in the range between 40 μm to 80 μm. Therefore, the isolation trenches are usually formed by laser scribing, wherein material is removed by evaporation due to the inserted energy. Furthermore, the different isolation trenches should have the smallest possible distance from each other in order to reduce the area of the substrate which does not contribute to the photovoltaic conversion. On the other side, all isolation trenches have to be clearly distinguished from each other.

During laser scribing, a laser beam is focused on the layer which has to be treated. The laser beam has a specific shape on the treated layer with a specific lateral extension, for instance an oval or round shape with at least a first diameter. In order to form a trench in form of a long line, the laser beam and the semi-finished solar module are moved relative to each other. Since the laser beam usually is pulsed and due to the relative movement, the formed trench in reality is a sequence of holes having essentially the shape of the laser beam in a plan view and overlapping at least to some extent.

However, two typical defects may occur during laser scribing: First, the scribed trench may be too wide and, therefore, different trenches may not be clearly distinguished from each other. Second, some holes of the scribed trench may be too narrow and may not overlap with adjacent holes of the same trench and, therefore, the trench does not really isolate and insulate adjacent solar cells. Such defects may occur randomly, i.e. caused by a temporary defect in a laser beam producing system, or may be caused by a drift (change with time) of parameters of the laser beam producing system.

Usually off-line tests for identifying defects, the testscomprising optical inspection or electrical measurements, are conducted outside an apparatus for performing the laser scribing process. However, these tests are performed only at random, are laborious and complex, for instance if images taken by a camera have to be evaluated, and only rarely provide the possibility for rework in order to correct the defect.

Some in-line monitoring methods and systems are known which are used to control the distance of a second trench formed by a second laser scribing process from a first trench formed by a first laser scribing process performed previous to the second laser scribing process. This in-line monitoring is performed directly within the system for performing the laser scribing process and is based on taking images of previous formed trenches. For instance, US 2015/0185162 A1 describesa method and a system identifying a first trench formed by a first laser scribing process bytaking infrared images of the regions in close proximity to the first trench, wherein detected infrared rays are irradiated from the semi-finished solar module, and controlling a second laser scribing process, in particular the distance of the scribed second trench from the first trench, according to the evaluated infrared images. However, these methods are not used to identify defects in a scribed trench and also use evaluation of images, what is laborious.

Summary of the Present Invention

The object of the present invention is to provide a method and a system for monitoring a laser scribing processused for forming isolation trenchesin a solar module which method is simple and fast and gives the opportunity to correct a defect and to identify a drift of parameters within the laser beam producing system.

This object is achieved by the method according to claim 1 and a system according to claim 8. Advantageous embodiments are disclosed in the corresponding dependent sub-claims.

The method according to the application comprises the steps of providing a semi-finished solar module, forming an isolation trench, illuminating a region of the semi-finished solar module at a first surface of the semi-finished solar module, detecting an amount of light transmitted through the illuminated region of the semi-finished solar module at a second surface of the semi-finished solar module and evaluating the formed isolation trench in the illuminated region. The semi-finished solar module comprises a functional layer stack on a transparent substrate, wherein the functional layer stack comprises at least one of a first contact layer, a photovoltaic layer and a second contact layer. That is: The functional layer stack may contain only the first contact layer, or it may contain the first contact layer on the transparent substrate and the photovoltaic layer on the first contact layer, or it may contain the first contact layer on the transparent substrate, the photovoltaic layer on the first contact layerand the second contact layer on the photovoltaic layer.

The isolation trench is formed within at least one layer of the functional layer stack using a laser beam which removes the at least one layer of the functional layer stack in the region of the isolation trench. At least the transparent substrate is left essentially untreated. The formed isolation trench may be a continuous trench reaching from one lateral end of the semi-finished solar module to the opposite lateral end of the semi-finished solar module. Thus the isolation trench has a first extension along a first lateral direction, wherein the first lateral direction extends from one lateral end of the semi-finished solar module to the opposite lateral end of the semi-finished solar module, and a second extension in a second lateral direction extending orthogonal to the first lateral direction. The first lateral direction and the second lateral direction define a plane extending parallel to the first surface of the semi-finished solar module.

The first extension is also called the length of the isolation trench, whereas the second extension is also called the width of the isolation trench. The width of the isolation trench may vary over the length of the isolation trench due to variation of the laser beam characteristic or to variations within the removed layer of the functional layer stack, e.g. variation in the crystal structure. A third extension of the isolation trench is also called the depth of the isolation trench and is measured in a third direction orthogonal to the first lateral direction and to the second lateral direction. Usually the isolation trench completely reaches through the at least one layer of the functional layer stack being removed, i.e. the depth of the isolation trench is equal to the thickness of the at least one layer. However, the depth of the isolation trench may also be smaller or greater than the thickness of the at least one layer and may also vary over the length of the isolation trench due to similar reasons as mentioned for variations of the width. For instance, residues of the at least one layer may be left, if the laser beam did not completely remove the at least one layer as it should do. The isolation trench according to the present application may also be a structure which has a first extension being smaller than the extension of the semi-finished solar module from one lateral end to the opposite lateral end. That is, the isolation trench may also be a small structure isolated from other structures arranged in a line extending along the first lateral direction. In any way, the isolation trench extends from a surface of the functional layer stack in the direction towards the transparent substrate.

After forming the isolation trench, a region of the semi-finished solar module at a first surface of the semi-finished solar module is illuminated. “After forming the isolation trench” means that the isolation trench is at least formed in the illuminated region before illuminating. That is, the step of forming the isolation trench and the step of illuminating are performed subsequently with respect to a specific region of the semi-finished solar module. However, while a first region is illuminated, the isolation trench may simultaneously be formed in a second region of the semi-finished solar module. The illuminated region covers at least a part of the isolation trench in the first lateral direction and the whole isolation trench and a surrounding region in the second lateral direction. Thus, the length of the illuminated region measured along the first lateral direction may be smaller than, equal to or larger than the length of the isolation trench, whereas the width of the illuminated region measured along the second lateral direction is larger than the width of the isolation trench. For instance, the width of the illuminated region is larger by 36％than the width of the isolation trench. The light used for illuminating preferably has a small angle of beam spread and is particularly preferred a beam of parallel light rays.

The first surface of the semi-finished solar module may be that surface onto which the laser beam impinges or might be the opposite surface of the semi-finished solar module. The laser beam may impinge onto the semi-finished solar module at the side where the transparent substrate forms the surface of the semi-finished solar module or may impinge at the side where the functional layer stack lies at the surface.

At a second surface of the semi-finished solar module being the opposite surface of the semi-finished solar module with respect to the third direction, an amount of light transmitted through the illuminated region of the semi-finished solar module is detected. Adevice for detecting the light is arranged at the same optical axis as a device for illuminating. That is, the semi-finished solar module is orthogonally rayed. The detected amount of light is one value representing the quantity of light or the intensity of light which transmits through the semi-finished solar module, in contrast to an image containing a lateral distribution of transmitted light. Therefore, only this one value has to be evaluated according to the present application instead of a whole image as in the prior art.

As known from the prior art, the light for illuminating may be focused or otherwise treated before reaching the first surface of the semi-finished solar module and the light transmitted through the semi-finished solar module may also be focused or otherwise treated before reaching the device for detecting.

Subsequently, the detected amount of light is compared with at least one reference value in order to evaluate the formed isolation trench in the illuminated region. The reference value represents a width of the isolation trench or a width and a length of the isolation trench in the illuminated region, for which the isolation trench complies with the requirements for good functionality. For instance, the reference value may correspond to a lower threshold value of the width of the isolation trench. Thus, if the detected amount is smaller than this reference value, the isolation trench may be evaluated as to be too narrow, i.e. not wide enough, in the illuminated region. On the other hand, the reference value may correspond to an upper threshold value of the width of the isolation trench. Thus, if the detected amount is larger than this reference value, the isolation trench may be evaluated as to be too wide, i.e. not narrow enough, in the illuminated region. As it is apparent, the detected amount of light is preferably compared with two reference values, in particular with a lower threshold value and an upper threshold value, which gives a good evaluation of the isolation trench in the illuminated region with respect to a suitable width of the isolation trench.

The reference values are obtained, for instance, by illuminating and detecting light for isolation trenches which are evaluated by analysing taken images of the isolation trench or by measuring the width of the isolation trench with other techniques according to the state of the art.

If the isolation trench is evaluated as not complying with the requirements in the illuminated region, the isolation trench may be reworked in this region. For instance, the step of forming the isolation trench may be repeated if the firstly formed isolation trench is too narrow.

The light used for illuminating comprises a first wavelength for which the transparent substrate and all layers of the functional layer stack which are not removed are transparent and at least one layer of the functional layer stack which is removed is opaque. The terms “transparent” and “opaque” do not refer to absolute transparency or opacity, but mean that the transparent substrate and all layers of the functional layer stack which are not removed have a transmittance for light with the first wavelength which is distinguishably higher than that of the at least one layer of the functional layer stack which is removed. Preferably the difference between these two transmittances is higher than 0.2. For a solar cell, the first wavelength lies preferably in the range of 300 nm to 900 nm. Therefore, any light source producing light including at least one wavelength in this range may be used for illuminating the semi-finished solar module, for instance “white” light of a normal lamp. However, a light source providing only light of a specific first wavelength, such as a light emitting diode (LED) , may also be used.

In a specific embodiment, the semi-finished solar module is illuminated with light of two different wavelengths in separate sub-steps, in order to obtain further information about different layers of the functional layer stack using the spectral information gathered by detecting amounts of transmitted light for both wavelengths. The semi-finished solar module is illuminated with light in two sub-steps, wherein the light used for illuminating in a second sub-step should not include light with a first wavelength of the light used for illuminating in a first sub-step or the light used for illuminating in a first sub-step should not include light with a second wavelength of the light used for illuminating in a second sub-step. Preferably, light of two different LEDs is used. The functional layer comprises at least two different layers, wherein at least one layer of the functional layer stack is removed in the region of the isolation trench, and wherein at least one layer of the functional layer stack which is removed or at least one layer of the functional layer stack which is not removed has different transmittance for light with the first wavelength and light with the second wavelength. Different transmittance means that the respective layer changes from transparent to opaque or vice versa.

If at least one layer of the functional layer stack which is not removed changes its transmittance between the first wavelength and the second wavelength, information about damages, like holes or scratches, within the at least one layer which is not removed caused by removing the at least one other layer may be obtained. This might, for instance, be performed when inspecting an isolation trench formed within the photovoltaic layer, but not in the first contact layer.

In another example, at least two layers of the functional layer stack are removed, wherein at least one first layer of the functional layer stack which is removed changes its transmittance between the first wavelength and the second wavelength, and at least one second layer of these at least two layers does not change its transmittance. That is, for the first wavelength, all layers of the functional layer stack which are not removed are transparent and the at least one first layer and the at least one second layer are opaque. For the second wavelength, the transparent substrate and the at least one first layerare transparent and the at least one second layer is opaque. Thus, residuals of the first layer left in the isolation trench may be detected. This might, for instance, be performed when inspecting an isolation trench formed within the photovoltaic layer and within the first contact layer.

The first wavelength may, for instance, lie in the range of the visible light, i.e. between 400 nm and 700 nm, whereas the second wavelength may, for instance, lie in the range of infrared light, i.e. between 780 nm and 1 mm, or in the range of ultra violet light, i.e. between 10 nm and 380 nm.

A similar technique may be used for evaluating a material which is filled into a formed isolation trench. This might, for instance, be a photoresist filled into a first isolation trench according to the state of the art. The material has a first transmittance for light having a third wavelength for whichat least one layer of the functional layer stack which is removed is opaque and the transparent substrate and all layers of the functional layer stack which are not removed are transparent. The third wavelength may be equal to the first wavelength or to the second wavelength described above or may be different from them. The first transmittance of the material is different from a second transmittance characterising a layer stack containing the transparent substrate and all layers of the functional layer stack which are not removed. Preferably the second transmittance is larger than the first transmittance, such that the amount of transmitted light essentially corresponds to the first transmittance of the material, which depends from the kind of material, the quality of the material and the thickness of the material. The region of the semi-finished solar module is illuminated subsequently to the step of filling the isolation trench with the material, wherein light with the third wavelength is used for illuminating. Afterwards the detected amount of light is used for evaluating the characteristics of the material.

In a preferred embodiment, the laser beam used for forming the isolation trench is moved relatively over the first or the second surface of the semi-finished solar module and the device for illuminating and the device for detecting the light are moved in the same way as the laser beamrelatively over the first or the second surface of the semi-finished solar module, respectively. That is, the laser beam as well as the device for illuminating and the device for detecting may be moved, while the semi-finished solar module is stationary, or the semi-finished solar module is moved while the laser beam and the device for illuminating and the device for detecting are stationary. Nevertheless, it is also possible that the semi-finished solar module on the one hand and the laser beam and the device for illuminating and the device for detecting on the other hand are moved, wherein a relative movement is given. In any case, the steps of illuminating, detecting and evaluating are performed for a plurality of regions arranged along the extension of the isolation trench along the first lateral direction. In other words: A long isolation trench formed by a relatively moving laser beam is inspected and evaluated by repeating illuminating and detecting and evaluating for a plurality of regions, wherein each of these regions preferably adjoins another region or partially overlaps with adjoining regions. Thus, the whole isolation trench may be monitored.

Preferably, the device for illuminating and the device for detecting the light directly follow the laser beam at a fixed distance. That is, the light beam is impinged onto the semi-finished solar module at a fixed distance to the laser beam such that, for each time interval, the fixed distance is given between a region of the semi-finished solar module where the isolation trench is formed during the time interval and a region of the semi-finished solar module where the light for illuminating impinges. This distance may be in the range of 5mm to20 mm.

While the device for illuminating and the device for detecting are moved, a continuous sequence of detected amounts of light, i.e. a signal curve representing the change of the detected amount of light over the position of the named devices, may be acquired. The continuous sequence of detected amounts of light allows for monitoring of the isolation trench over time and therefore offers the possibility todetect a total failure of the laser beam, an increased appearance of defects in the isolation trenchor a drift in parameters of the laser beam and to counteract. To this end, a further statistical evaluation routineis performed, like determining a mean value, a smallest value or a largest value of the detected amount of light for a given time period, within the step of evaluating. The method further comprises a step of changing the parameters of a device for producing the laser beam, if one of the named events occurs and a need for changing the parameters is detected by the statistical evaluation routine.

A system for monitoring a laser scribing processused for forming an isolation trench in a solar module according to the present application comprises a device for producing a laser beam, a device for illuminating a region of the semi-finished solar module, a device for detecting an amount of light, and a device for evaluating the formed isolation trench. At least some of these devices may be combined in one apparatus, i.e. they are physically fixed to each other or form a closed unit. Nevertheless, all devices may be realized in separated apparatuses, wherein at least a data connection for transmitting data from the device for detecting an amount of light to the device for evaluating is present.

The device for producing a laser beam is suited for forming an isolation trench within at least one layer of a functional layer stack ofa semi-finished solar module by removing the at least one layer of the functional layer stack in the region of the isolation trench. The functional layer stack comprises at least one of a first contact layer, a photovoltaic layer and a second contact layer and is arranged on a transparent substrate. The device for producing a laser beam may produce at least one laser beam of a first wavelength, but may also produce two or more beams of the same or of different wavelengths.

The device for illuminating is suited for illuminating a region of the semi-finished solar module at a first surface of the semi-finished solar module. The illuminated region covers at least a part of the isolation trench in a first lateral direction and the whole isolation trench and a surrounding region in a second lateral direction orthogonal to the first lateral direction. The lateral directions are described above.

The device for detecting an amount of light is suited for detecting an amount of light transmitted through the illuminated region of the semi-finished solar module at a second surface of the semi-finished solar module. The second surface is that surface of the semi-finished solar module which is opposite to the first surface. The first surface and the second surface extend in a plane, respectively, which is orthogonal to the thickness of the solar module. The device for detecting the light is arranged at the same optical axis as the device for illuminating and may, for instance, comprise a photometer or one photo sensor or a plurality of photo sensors for detecting light.

The device for evaluating the formed isolation trench is suited for evaluating the formed isolation trench in the illuminated region by comparing the detected amount of light with a reference value. To this end, the device for evaluating may containone or more threshold values, for instance a lower threshold value and an upper threshold value, recorded in a memory unit, a comparative unit and an output unit for outputting a signal, at least if a violation of one of the threshold values had occurred.

The device for illuminating is suited for emitting light comprising a first wavelength for which the transparent substrate and all layers of the functional layer stack which are not removed are transparent and at least one layer of the functional layer stack which is removed is opaque. For instance, the device for illuminating may be a normal lamp emitting “white” light or may comprise a light emitting diode emitting light of one specific wavelength. Preferably, the device for illuminating produces light with a small angle of beam spread and particularly preferred a beam of parallel light rays. To this end, the device for illuminating may comprise also optical elements, e.g. lenses.

In one embodiment, the device for illuminating is suited for emitting light having a first wavelength and for emitting light having a second wavelength being different from the first wavelength. Preferably, at least one layer of the functional layer stack has a completely other transmittance for the second wavelength than for the first wavelength, i.e. changes from being transparent to being opaque or vice versa. In one example, the first wavelength lies in the visible light region and the second wavelength lies in the infrared region or in the UV region.

In a preferred embodiment, the system comprises a transport device for realizing a movement of the device for producing a laser beam relatively along the first lateral direction over the first or the second surface of the semi-finished solar module and for realizing a movement of the device for illuminating and of the device for detecting the light in the same way as the device for producing a laser beamrelatively over the first or the second surface of the semi-finished solar module, respectively. This transport system may, for instance, be a transport belt or a plurality of roller or shafts, wherein the transport system moves a semi-finished solar module along the first lateral direction within the system such that the semi-finished solar module moves over a laser beam produced by the device for producing a laser beam and through a light beam emitted from the device for illuminating. The device for producing a laser beam, the device for illuminating and the device for detecting an amount of light may be stationary within the system or may also move.

The device for illuminating is preferably suited for emitting light continuously over time, and the device for detecting an amount of light is also preferably suited for detecting light continuously over time. Further, the device for evaluating is suited for evaluating a continuous sequence of detected amounts of light. Thus, if the semi-finished solar module at the one hand and the device for illuminating and the device for detecting an amount of light are moved relatively to each other, the isolation trench may be evaluated in a large number of illuminated regions, i.e. over a large length, continuously, wherein defects of the isolation trench and/or a drift of at least one of the evaluated parameters of the isolation trench, i.e. a slow change, may be detected reliably and fast.

In a specific embodiment, the device for producing a laser beam and the device for illuminating are arranged within an apparatus for performing a laser scribing process such that they have a first distance to each other along the first lateral direction. This first distance preferably lies in the range of 5 mm to 20 mm, and more preferably in the range of 5 mm to 15mm, and is, for instance, 10 mm. This distance is fixed for a specific laser scribing process. Thus, a defect in the isolation trench is recognized very fast after its formation.

In a preferred embodiment, the semi-finished solar module is transported within an apparatus for performing a laser scribing beam in a first direction along the first lateral direction in a first time period and in a second direction along the first lateral direction in a second time period, wherein the second direction is opposite to the first direction. The first direction and the second direction may also be referred to as the positive and the negative direction, respectively, along the first lateral direction. That is, in order to form a plurality of isolation trenches within the semi-finished solar module, the semi-finished solar module moves first into the first direction over the laser beam and then into the second direction, wherein each time an isolation trench is formed. Between these two laser scribing steps, the semi-finished solar module is moved along a second lateral direction being orthogonal to the first lateral direction and defining a plane being parallel to the first surface of the semi-finished solar module. In order to evaluate formed isolation trenches in both directions, a first device for illuminating and a first device for detecting light are arranged on a first side of the device for producing a laser beam along the first lateral direction and a second device for illuminating and a second device for detecting light are arranged on a second side of the device for producing a laser beam along the first lateral direction, wherein the second side is opposite to the first side.

The system may further comprise a memory device for storing a position of a region of the semi-finished solar module, for which a defect of the isolation trench is identified by the device for evaluating. The stored information may later be used for reworking the isolation trench at this position. The position is relative to a reference point on the semi-finished solar module, e.g. a corner of the semi-finished solar module.

The system may further comprise a control device for controlling the device for producing a laser beam in accordance to a result provided by the device for evaluating. Thus, at least one of the parameters of the laser beam may be changed or corrected, if a need for changing the parameters is detected by the device for evaluating. Further, the control device may also control the transport device.

The system may further comprise a suction device suited for sucking gases and particles resulting from a scribing process off the ambient of the semi-finished solar module. The suction device is preferably arranged on a second side of the semi-finished solar module, wherein the transparent substrate forms the surface of the semi-finished solar module at the first side and the second side is opposite to the first side.

Brief Description of the Drawings

The accompanying drawings are included to provide a further understanding of the invention and are incorporated in and constitute a part of this specification. The drawings illustrate an embodiment of the present invention and together with the description serve to explain the principles. Other embodiments of the invention are possible and lie within the scope of the invention. The elements of the drawings are not necessarily to scale relative to each other. Like reference numbers designate corresponding similar parts.

Figure 1 schematically shows a cross section through a solar module having three different types of isolation trenches.

Figure 2 schematically shows a plan view on a semi-finished solar module.

Figure 3 schematically shows a cross section through the semi-finished solar module along line A-A’ .

Figure 4 schematically shows a cross section through the semi-finished solar module along line B-B’ .

Figure 5A schematically shows a plan view on a formed isolation trench in an enlarged viewing.

Figure 5B schematically shows a curve corresponding to the detected amount of light for a continuous monitoring of the isolation trench.

Figure6 schematically shows a system for monitoring a laser scribing process.

Figure 7 schematically shows an embodiment of a system for monitoring a laser scribing process according to the invention in a cross sectional view.

Detailed Description of the Preferred Embodiments

Fig. 1 is a schematic cross section through a solar module (1) comprising a transparent substrate (11) , a functional layer stack (12) and a back substrate (13) . The solar module is illuminated from the side of the transparent substrate (11) which is indicated by the arrows. The transparent substrate (11) is, for instance, made of glass, whereas the back substrate (13) may be made of any suitable material and may be transparent or opaque. The functional layer stack (12) comprises a first contact layer (121) made of a transparent material, for instance a transparent conducting oxide (TCO) , a photovoltaic layer (122) absorbing light and converting it to electrical current and a second contact layer (123) which may be made of an opaque material, for instance a metal. All layers of the functional layer stack (12) may comprise different layers of different materials as known from the state of the art. Such the photovoltaic layer may, for instance, comprise a CdS layer and a CdTe layer.

“Transparent” and “opaque” mentioned in the previous paragraph mean transparent or opaque with respect to light having a wavelength which is absorbed and converted to electric current by the photovoltaic layer (122) of the functional layer stack (12) .

In order to form a solar cell (100) connected to neighbouring solar cells in series, three isolation trenches (21, 22, 23) are formed within the functional layer stack (12) between two adjoining solar cells (100) . A first isolation trench (21) is formed at least within the first contact layer (121) and may also be formed within the photovoltaic layer (122) , as shown in Fig. 1. It is filled with an electrically insulating material (2) , for instance with a photoresist, and electrically insulates first contact layer regions of adjoining solar cells. A second isolation trench (22) is formed only within the photovoltaic layer (122) and is filled with an electrically conducting material, for instance with the material of the second contact layer (123) . It provides an electrically conducting connection between a second contact layer region of a solar cell and a first contact layer region of an adjoining solar cell. A third isolation trench (23) is formed at least within the second contact layer (123) , as shown in Fig. 1, and may also be formed within the photovoltaic layer (122) . It is filled with an electrically insulating material and electrically insulates second contact layer regions of adjoining solar cells.

Fig. 2 shows a plan view on a semi-finished solar module (10) during an exemplary embodiment of the method for monitoring a laser scribing process according to the present application. Figs. 3 and 4 schematic show cross sections through the semi-finished solar module (10) along line A-A’ (Fig. 3) and along line B-B’ (Fig. 4) . The figures explain the method exemplary for monitoring the laser scribing of a second portion (22b) of the second isolation trench. However, the principal of the method may be used for any laser scribed isolation trench, as long as at least one layer which is removed by forming the isolation trench is opaque und all other layers, which are not removed and are therefore present above and beneath the formed isolation trench, are transparent for an illuminating light.

The semi-finished solar module (10) comprises the transparent substrate (11) and a functional layer stack (12) containing only the first contact layer (121) and the photovoltaic layer (122) . The semi-finished solar module (10) has a first surface (101) which is a surface of the photovoltaic layer (122) in the explained example. The first surface is opposite to a second surface (102) of the semi-finished solar module, which is a surface of the transparent substrate (11) . A first isolation trench (21) is already formed within the present layers of the functional layer stack (12) , i.e. within the first contact layer (121) and the photovoltaic layer (122) and is filled with an insulating material (2) . The first isolation trench (21) reaches from a first lateral end (103) of the semi-finished solar module (10) to a second lateral end (104) of the semi-finished solar module (10) along a first lateral direction (x) . The first lateral end (103) and the second lateral end (104) lie opposite to each other along the first lateral direction (x) . The second isolation trench is formed as isolated portions (22a –22c) in this exemplary embodiment, but may also extend from the first lateral end (103) to the second lateral end (104) . Each portion (22a –22c) has a length (l₁) measured along the first lateral direction (x) , wherein the length (l₁) may be different or equal for different portions (22a –22c) . Further, each portion (22a –22c) has a width (w₁) measured along a second lateral direction (y) . The second lateral direction (y) is orthogonal to the first lateral direction (x) and defines a plane with it which is parallel to the first surface (101) . The width (w₁) may be different or equal for different portions (22a –22c) . The width (w₁) may lie in the range between 20 μm and 80 μm, for instance 50 μm. The first isolation trench (21) reaches from the first surface (101) down to the transparent substrate (11) along a third direction (z) , which is orthogonal to the first lateral direction (x) and to the second lateral direction (y) and is a thickness direction. The portions (22a –22c) of the second isolation trench reach from the first surface (101) down to the first contact layer (121) along the third direction (z) .

A first portion (22a) and a second portion (22b) are already formed, wherein the process of forming is still in progress for a third portion (22c) . This is indicated by laser beam (30) impinging on the first surface (101) in a laser beam region (3) . The laser beam (30) is moved along the first lateral direction (x) over the first surface (101) , thereby scribing portions of the second isolation trench. The movement of the laser beam (30) is realized by moving the semi-finished solar module (10) in a transport direction (V) which is in the negative direction of the first lateral direction (x) .

In a region of the first surface (101) where the second isolation trench is already formed, e.g. in the region of the second portion (22b) , an illuminating light beam (40) impinges on the semi-finished solar module (10) , thereby forming an illuminated region (4) . The photovoltaic layer (122) removed within the portions (22a –22c) of the second isolation trench is opaque for at least one wavelength of the illuminating light beam (40) , whereas the transparent substrate (11) and the first contact layer (121) are transparent. Preferably, the illuminating light beam contains only light of one wavelength or the photovoltaic layer (122) is opaque and the transparent substrate (11) and the first contact layer (121) are transparentfor most of the wavelengths contained in the illuminating light beam (40) .

The illuminated region (4) has a length (l₂) measured in the first lateral direction (x) and a width (w₂) measured in the second lateral direction (y) . The length (l₂) of the illuminated region (4) is smaller than the length (l₁) of the second portion (22b) . However, it may also be equal to or even larger than it. The width (w₂) of the illuminated region (4) is every time larger than the width (w₁) of the second portion (22b) . Therefore, only a part of the illuminating light beam (40) -at least along the second lateral direction–can pass through the semi-finished solar module (10) , i.e. in that regions, where the photovoltaic layer (122) is not present, and forms a transmitted light beam (41) , as shown in Fig. 3. An amount of the transmitted light may then be detected, for instance with photometer, as explained later.

Although the method according to the application is explained with respect to Figs. 2 to 4 such, that the laser beam and the illuminating light beam impinge on the same surface of the semi-finished solar module, this is not necessary. They may also impinge on different surfaces of the semi-finished solar module. Furthermore, it is not important whether the illuminating light beam impinges on a surface of the semi-finished solar module defined by a layer of the functional layer stack, as shown in Figs. 2 to 4, or on a surface of the semi-finished solar module defined by the transparent substrate.

Furthermore, also the first isolation trench (21) and/or the insulating material (2) in it may be inspected with the illuminating light beam (40) . It might be necessary, that the illuminating light beam then contains light with another wavelength as in the case of inspecting portions (22a –22c) of the second isolation trench which are not filled with any material.

Further, different illuminating light beams containing light having different wavelengths may be used for obtaining informationfrom the differences in the detected amount of transmitted light for different wavelengths. Nevertheless, also one illuminating light beam containing different wavelengths may be used, wherein however the transmitted light beam has to be split and the amount of transmitted light for different wavelengths has to be detected by different detecting devices. At least one layer of the functional layer stack has to change its transmittance from transparent to opaque or vice versa for the different wavelengths.

With respect to Figs. 5A and 5B, the shape of the formed isolation trench resulting from the process of laser scribing, a possible defect of the formed isolation trench and a signal corresponding to the detected amount of transmitted light and allowing an evaluation of the formed isolation trench will be explained. Fig. 5A shows a formed second isolation trench (22) in an enlarged plan view. The second isolation trench (22) is formed by a pulsed laser moved over the first surface (101) of the semi-finished solar module, wherein single, essentially round laser beam shots (3a) are formed within the first surface (101) . Nevertheless, other shapes of the laser beam shots are also possible. A device for producing a laser beam usually forms 10 to 50 laser beam shots (3a) per mm over the length of the isolation trench. Neighbouring single laser beam shots (3a) overlap with each other such that they form a continuous trench having waved edges. If the illuminating light beam is also moved over the first surface (101) along the extension of the second isolation trench (22) , the detected amount of transmitted light also has a waved course, as can be seen in Fig. 5B. By way of example, there is shown an electric signal generated by a device for detecting an amount of light and corresponding to the detected amount of light. That is, the detected amount is not a constant value, but oscillates around a mean value.

However, if there is a defect within the isolation trench, for instance a region (3b) , where a laser beam shot is missing, the detected amount largely deviates from this mean value. This is shown by the negative spike in the signal corresponding to the region (3b) of the missing laser beam shot. If the isolation trench would be too wide, a positive spike in the signal would result. A lower threshold value (T_L) and an upper threshold value (T_U) may be defined corresponding to deviations from the mean value which are not tolerable. If the signal violates one of these threshold values, a defect is detected. Furthermore, since each detected amount of light corresponds to a specific region of the second isolation trench (22) , the position of defects may be determined and later used for rework if applicable.

Further threshold values may be defined in order to control the stability of the laser beam and/or the width of theisolation trench. In order to detect a driftin parameters of the isolation trench or an increased number of failures of the laser beam, also other parameters of a generated signal, like a mean value or an amplitude (difference between a lowest and a highest value within a period of the signal) , may be evaluated for a given period of time using statistical routines.

The method according to the application provides a simple, fast and very effective way for continuous inspecting isolation trenches and detecting defects or a drift of device parameters. There is no need for taking pictures for some random regions of an isolation trench, and no need for gathering large quantities of data. The obtained signal may be evaluated automatically by a machine using suited evaluation methods and which also may control a device for producing a laser beam in accordance to the evaluation result.

Furthermore, not only defects may be found, but also distances between isolated structures may be measured, if the structures are isolated by regions having another transmittance than the structure itself.

Fig. 6 shows a system (200) for monitoring a laser scribing process used for forming an isolation trench according to the application in a schematic way. The system (200) comprises a device for producing a laser beam (300) , a device for illuminating (400) , a device for detecting an amount of light (500) and a device for evaluating the formed isolation trench (600) . The system (200) may further comprise a transport device (700) , a memory device (800) , a control device (900) and/or a suction device (950) . Some of these devices, preferably the device for producing a laser beam (300) , the device for illuminating (400) , the device for detecting an amount of light (500) , the transport device (700) and thesuction device (950) are arranged in one common apparatus for performing a laser scribing process (250) . The device for producing a laser beam (300) is suited for forming an isolation trench within a functional layer stack with the formed laser beam. The device for illuminating (400) and the device for detecting an amount of light (500) are suited for generating a signal corresponding to at least one characteristic parameter of the formed isolation trench, for instance of the width, by illuminating a region of the isolation trench and detecting the transmitted amount of light as described above. The generated signal is then evaluated by the device for evaluating the formed isolation trench (600) . The evaluation results may then be recorded in the memory device (800) or transmitted to the control device (900) which may control the device for producing a laser beam (300) and the transport device (700) according to the evaluation results. The suction device (950) is suited for removing gases or particles resulting from the process of forming the isolation trench, in particular such that these gases and particles do not disturb the illumination or detection of light.

Fig. 7 shows an example of such a system. In an apparatus for performing a laser scribing process (250) , a device for producing a laser beam (300) , two devices for illuminating (400a, 400b) , two devices for detecting an amount of light (500a, 500b) , rollers (710) of a transport device and asuction device (950) are arranged. A semi-finished solar module (10) is moved through or within the apparatus for performing a laser scribing process (250) by the rollers (710) in a transport direction (V) along a first lateral direction (x) . However, also a moving table or any other suitable device may be used as the transport device. On a first side of the semi-finished solar module (10) , e.g. below it, the device for producing a laser beam (300) and the devices for detecting an amount of light (500a, 500b) are mounted on a common first mechanical holder (261) , wherein a first device fordetecting an amount of light (500a) is mounted on a first side of the device for producing a laser beam (300) along the first lateral direction (x) and a second device fordetecting an amount of light (500b) is mounted on a second side of the device for producing a laser beam (300) along the first lateral direction (x) . The second side is the opposite side to the first side. The device for producing a laser beam (300) contains a laser device (310) for generating laser light and a laser optics (320) for forming a laser beam (30) The laser beam (30) removes a functional layer stack (12) of the semi-finished solar module (10) from a transparent substrate (11) in a laser beam region (3) thereby forming an isolation trench. This is, for instance, given for forming a first isolation trench as explained with respect to Fig. 1.

The suction device (950) mounted on a second side of the semi-finished solar module (10) , e.g. above it and on the same axis as a laser beam axis, removes gases and particles resulting from forming the isolation trench. The suction device (950) and the devices forilluminating (400a, 400b) are mounted on a common second mechanical holder (262) , wherein a first device forilluminating (400a) is mounted on a first side of the suction device (950) along the first lateral direction (x) and a second device forilluminating (400b) is mounted on a second side of the suction device (950) along the first lateral direction (x) . The first device for illuminating (400a) is mounted such that it is on the same axis as the first device for detecting an amount of light (500a) , and the second device for illuminating (400b) is mounted such that is on the same axis as the second device for detecting an amount of light (500b) . Preferably, a distance (a₁) between the axis of the laser beam and the axis of the first device for illuminating (400a) equals a distance (a₂) between the axis of the laser beam and the axis of the second device for illuminating (400b) . However, the distances a₁ and a₂ may also differ. The first holder (261) and the second holder (262) assure fixed values of distances a₁ and a₂ and the paraxial arrangement of the devices for illuminating (400a, 400b) with the corresponding respective devices for detecting an amount of light (500a, 500b) . Furthermore, the illuminating light beam (40) is automatically aligned with the formed isolation trench.

Each device for illuminating (400a, 400b) comprises a lamp (410) , for instance an LED lamp, and an illuminating optics (420) forming an illuminating light beam (40) . Each device for detecting an amount of light (500a, 500b) comprises a detecting optics (510) and a detector (520) . The detecting optics (510) leads a transmitted light beam (41) to the detector (520) which generates a corresponding signal.

This signal is transmitted to a device for evaluating (600) over a data line (610) , which may be a data wire or a wireless data line. The device for evaluating (600) may be a computer, for instance, which may be arranged outside the apparatus for performing a laser scribing process (250) .

If a laser scribing process is performed, a semi-finished solar module (10) may first be moved in a first direction along the first lateral direction (x) , e.g. negative x-direction. This is shown in Fig. 7 by the arrow with the reference sign V. In this case, the first device for illuminating (400a) and the first device for detecting an amount of light (500a) work and realize the inspection of the formed isolation trench. When the laser beam (30) and also the illuminating light beam (40) reached a lateral end of the semi-finished solar module (10) , i.e. the right end in Fig. 7, the semi-finished solar module (10) is subsequently moved into a second direction along the first lateral direction (x) . The second direction is opposite to the first direction. That is, if the first direction is the negative x-direction, then the second direction is the positive x-direction. While the semi-finished solar module (10) is moved into the second direction, the laser beam (30) forms another isolation trench In this case, the second device for illuminating (400b) and the second device for detecting an amount of light (500b) work and realize the inspection of the formed isolation trench. Providing one device for illuminating (400a, 400b) and one device for detecting an amount of light (500a, 500b) on each side of the device for producing a laser beam (300) allows for inspecting the formed isolation trench during movement of the semi-finished solar module (10) in both directions. This saves time and an alignment step for aligning the formed isolation trench and the illuminating light beam (40) with each other.

The embodiments of the invention described in the foregoing description are examples given by way of illustration and the invention is nowise limited thereto. Any modification, variation and equivalent arrangement as well as combinations of embodiments should be considered as being included within the scope of the invention.

Reference signs

1 Solar module

2 Insulating material

3 Laser beam region

3a Single laser beam shot

3b Missing laser beam shot

4 Illuminated region

10 Semi-finished solar module

11 Transparent substrate

12 Functionallayerstack

13 Back substrate

21 First isolationtrench

22 Second isolationtrench

22a -22c Portion of the second isolation trench

23 Third isolationtrench

30 Laser beam

40 Illuminating light beam

41 Transmitted light beam

100 Solar cell

101 First surface ofthe semi-finished solar module

102 Second surface ofthe semi-finished solar module

103 First lateral end ofthe semi-finished solar module

104 Second lateral end ofthe semi-finished solar module

121 First contact layer

122 Photovoltaic layer

123 Second contact layer

200 System for monitoring a laser scribing process

250 Apparatus for performing a laser scribing process

261 First holder

262 Second holder

300 Device for producing a laser beam

310 Laser device

320 Laser optics

400 Device for illuminating

400a First device for illuminating

400b Second device for illuminating

410 Lamp

420 Illuminating optics

500 Device for detecting an amount of light

500a First device for detecting an amount of light

500b Second device for detecting an amount of light

510 Detecting optics

520 Light detector

600 Device for evaluating

610 Data line

700 Transport device

710 roller

800 Memory device

900 Control device

950 Suction device

a₁ Distance between the axis of the laser beam and the axis of the first device for illuminating

a₂ Distance between the axis of the laser beam and the axis of the second device for illuminating

l₁ Length of isolation trench

l₂ Length of illumination region

w₁ Width of isolation trench

w₂ Width of illuminated region

T_L Lower threshold value

T_U Upper threshold value

V Direction of transport of the semi-finished solar module

x First lateral direction

y Second lateral direction

z Third direction

Claims

Method for monitoring a laser scribing processused for forming isolation trenchesin a solar module, the method comprising the steps:

· providing a semi-finished solar module comprising a functional layer stack on a transparent substrate, the functional layer stack comprising at least one of a first contact layer, a photovoltaic layerand a second contact layer,

· forming an isolation trench within at least one layer of the functional layer stack using a laser beamwhich removesthe at least one layer of the functional layer stack in the region of the isolation trench,

· illuminating a region of the semi-finished solar module at a first surface of the semi-finished solar module, wherein the illuminated region covers at least a part of the isolation trench in a first lateral direction and the whole isolation trench and a surrounding region in a second lateral direction orthogonal to the first lateral direction,

· detecting an amount of light transmitted through the illuminated region of the semi-finished solar moduleat a second surface of the semi-finished solar module, wherein a device for detecting the light is arranged at the same optical axis as a device for illuminating, and

· evaluatingthe formed isolation trench in the illuminated regionby comparing the detected amount of light with at least one reference value.
Method according to claim 1, characterized in that the region of the semi-finished solar module is illuminated with light comprising a first wavelength for which the transparent substrate and all layers of the functional layer stack which are not removed are transparent and at least one layer of the functional layer stack which is removed is opaque.
Method according to claim 1, characterized in that

- the functional layer stack comprises at least two layers,

- at least one layer of the functional layer stack is removed in the region of the isolation trench,

- thestep of illuminating comprises a first sub-step of illuminating the semi-finished solar module with light having a first wavelength for which the transparent substrate and all layers of the functional layer stack which are not removed are transparent and at least one layer of the functional layer stack which is removed is opaque and a second sub-step of illuminating the semi-finished solar module with light having a second wavelength for which the transparent substrate is transparent and at least one layer of the functional layer stack which is removed is opaque and at least another layer of the functional layer stack which is removed and which is opaque for the first wavelength is transparent or at least one layer of the functional layer stack which is not removed is opaque,

- the step of detecting comprises a first sub-step of detecting a first amount of light having the first wavelength and transmitted through the semi-finished solar module and a second sub-step of detecting a second amount of light having the second wavelength and transmitted through the semi-finished solar module, and

- evaluating the formed isolation trench by comparing the detected first amount of light and the detected second amount of light with respective reference values.
Method according to claim 1, further comprising a step of filling the isolation trench with a material having a first transmittance for lighthaving a third wavelength for which at least one layer of the functional layer stack which is removed is opaque and the transparent substrate and all layers of the functional layer stack which are not removed are transparent with a second transmittance larger than the first transmittance, the method characterized in that the region of the semi-finished solar module is illuminated subsequently to the step of filling the isolation trench with a material, wherein light with the third wavelength is used for illuminating, and in that characteristics of the material filled into the isolation trench are evaluated in the step of evaluating by comparing the detected amount of light with a respective reference value.
Method according to claim 1, characterized in that the laser beam used for forming the isolation trench is moved relatively over the first or the second surface of the semi-finished solar module and the device for illuminating and the device for detecting the light are moved in the same way as the laser beamrelatively over the first or the second surface of the semi-finished solar module, respectively, and in that the steps of illuminating, detecting and evaluating are performed for a plurality of regions arranged along the extension of the isolation trench along the first lateral direction.
Method according to claim 5, characterized in that the device for illuminating and the device for detecting the light directly follow the laser beam at a fixed distance.
Method according to claim 5, characterized in that a continuous sequence of detected amounts of light is acquiredduring the movement of the device for illuminating and of the device for detecting the light, in that a statistical evaluation routine is performedin the step for evaluating, and in that the method further comprises a step of changing the parameters of a device for producing the laser beam if a need for changing the parameters is detected by the statistical evaluation routine.
System for monitoring a laser scribing processused for forming an isolation trench in a solar module, the system comprising:

· a device for producing a laser beam suited for forming an isolation trench within at least one layer of a functional layer stack ofa semi-finished solar moduleby removing the at least one layer of the functional layer stack in the region of the isolation trench, the functional layer stack comprising at least one of a first contact layer, a photovoltaic layer and a second contact layer and is arranged on a transparent substrate,

· a device for illuminating a region of the semi-finished solar module at a first surface of the semi-finished solar module, wherein the illuminated region covers at least a part of the isolation trench in a first lateral direction and the whole isolation trench and a surrounding region in a second lateral direction orthogonal to the first lateral direction,

· a device for detecting an amount of light transmitted through the illuminated region of the semi-finished solar module at a second surface of the semi-finished solar module, wherein the device for detecting the light is arranged at the same optical axis as the device for, and

· a device for evaluating the formed isolation trench in the illuminated region by comparing the detected amount of light with a reference value.
System according to claim 8, characterized in that the device for illuminating is suited for emitting light having a first wavelength for which the transparent substrate and all layers of the functional layer stack which are not removed are transparent and at least one layer of the functional layer stack which is removed is opaque.
System according to claim 8, characterized in that the device for illuminating is suited for emitting light having a first wavelength and for emitting light having a second wavelength being different from the first wavelength.
System according to claim 10, characterized in that the first wavelength lies in the visible light region and the second wavelength lies in the infrared region or in the UV region.
System according to claim 8, characterized in that the system comprises a transport device for realising a movement of the device for producing a laser beam relatively along the first lateral direction over the first or the second surface of the semi-finished solar module and for realizing a movement of the device for illuminating and of the device for detecting the light in the same way as the device for producing a laser beamrelatively over the first or the second surface of the semi-finished solar module, respectively.
System according to claim 12, characterized in that the device for producing a laser beam and the device for illuminating are arrangedwithin an apparatus for performing a laser scribing process such that they have a first distance to each other along the first lateral direction.
System according to claim 12, characterized in that a first device for illuminating and a first device for detecting an amount of light are arranged on a first side of the device for producing a laser beam along the first lateral direction and a second device for illuminating and a second device for detecting an amount of light are arranged on a second side of the device for producing a laser beam along the first direction, wherein the second side is opposite the first side, and in that the transport device is suited for moving the device for producing a laser beam and the first and the second devices for illuminating relatively along the first lateral direction over the first or the second surface of the semi-finished solar module both in a first direction and a second direction, wherein the second direction is opposite to the first direction.
System according to claim 8, further comprising a memory device for storing a position of a region of the semi-finished solar module, for which a defect of the isolation trench is identified by the device for evaluating.
System according to claim 8, further comprising a control device for controlling the device for producing a laser beam in accordance to a result provided by the device for evaluating.
System according to claim 8, characterized in that the transparent substrate forms a surface of the semi-finished solar module at a first side, and in that the system further comprises a suction device arranged on a second side of the semi-finished solar module and suited for sucking gases and particles resulting from a scribing process off the ambient of the semi-finished solar module, wherein the second side is opposite to the first side.