Method for producing a photovoltaic module

p-0002This invention relates to a method for producing a photovoltaic module according to the preamble of claim <b>1</b>.

p-0003In the production of a photovoltaic module there are deposited over a large area on a transparent, electrically insulating substrate, for example a glass plate, three functional layers, namely, a transparent front electrode layer, a semiconductor thin-film layer and a back electrode layer.

p-0004To form series-connected cells from these monolithic layers, the layers are patterned by separating lines e.g. with a laser, by mechanical methods or by chemical means.

p-0005For patterning the front electrode layer, which consists for example of a transparent, electrically conductive metal oxide, for example tin oxide, it is customary to use laser technology. The laser used is typically a neodymium doped yttrium aluminum garnet (Nd:YAG) solid-state laser or yttrium vanadate (Nd:YVO₄) solid-state laser which emits infrared radiation with a wavelength of 1064 nm. At this wavelength the transparent tin oxide front electrode layer has an optical absorption of several percent.

p-0006The patterning of the semiconductor layer, for example a silicon thin film, is likewise typically done with laser technology. The laser used is a double-frequency Nd:YAG or Nd:YVO₄solid-state laser which thus emits laser light with a wavelength of 532 nm in the visible range. At this wavelength the silicon thin film possesses high optical absorbance in comparison with the transparent front electrode layer. This permits selective ablation of the semiconductor layer without impairing the front electrode layer.

p-0007The patterning of the back electrode layer is done using mechanical methods, such as the “lift-off” technique wherein an adhesive paste is applied to the semiconductor layer in the areas where the back electrode layer is to be provided with separating lines and then stripped to remove the back electrode layer in said areas, but also using laser processes with a double-frequency Nd:YVO₄or Nd:YAG laser with a wavelength of 532 nm.

p-0008When the separating lines have been formed with the laser in the semiconductor layer, the front electrode layer located therebelow is exposed. Upon subsequent deposition of the back electrode layer, the back electrode layer is thereby contacted with the front electrode layer, causing the cells of the photovoltaic module to be series-connected.

p-0009The production of the photovoltaic module requires a plurality of costly machines. The machine for semiconductor coating usually involves the highest investment costs. Also, the laser machines are important cost factors.

p-0010The capacity of the semiconductor coating machine and the capacity of the laser machines are usually different. Being the costliest machine, the semiconductor coating machine is generally utilized at a rate of 100% in the industrial production of photovoltaic modules.

p-0011If the semiconductor coating machine can for example perform at the most a coating of 210,000 modules a year, but each laser machine with a wavelength of 1064 nm or 532 nm is designed e.g. only for at most 90,000 modules a year, this has the consequence that altogether six costly laser machines must be procured because the capacity of two laser machines per stated wavelength and process for the front electrode and back electrode, at 180,000, does not suffice. The number of laser machines thus leads to a considerable cost problem in the industrial production of photovoltaic modules.

p-0012It is therefore the object of the invention to considerably reduce costs for the industrial production of photovoltaic modules.

p-0013This is achieved according to the invention by using the laser used for patterning the front electrode layer for patterning the back electrode layer as well.

p-0014For it has been ascertained that the back electrode layer can be removed with the semiconductor layer located therebelow, for example a silicon layer, by a laser even when the laser emits an infrared radiation that is absorbed by the transparent front electrode layer but not by the semiconductor layer.

p-0015For the absorption of the laser radiation by the front electrode layer has the consequence that the front electrode layer heats up through the laser radiation such that not only the semiconductor layer located thereabove is thermally removed and thus a patterning of the semiconductor layer by separating lines can be carried out, but when the semiconductor layer is coated with the back electrode layer, the semiconductor layer is thermally removed together with the back electrode layer and thus a patterning of the back electrode layer by separating lines is also possible. The process might also require a beam expander and a diaphragm or an optical beam conversion system, e.g. refractive or diffractive optic, which gives the laser beam a top-hat profile.

p-0016This does cause additional separating lines to be formed in the semiconductor layer upon patterning of the back electrode layer. However, said additional separating lines in the semiconductor layer have practically no effect on the power of the photovoltaic module.

p-0017It is essential, however, that when the laser is used for patterning the semiconductor layer and the semiconductor layer located therebelow, its power is so reduced that the front electrode layer is ideally not damaged, i.e. broken or otherwise impaired. This means that while the laser is used possibly at full power, but at least at high power, for patterning the front electrode layer, a relatively lower laser power is adjusted for patterning the back electrode layer with the semiconductor layer located therebelow.

p-0018Since the invention permits one and the same laser machine to be used both for patterning the front electrode layer and for patterning the back electrode layer with the semiconductor layer located therebelow, the invention permits the number of laser machines to be reduced in the industrial production of photovoltaic modules. This means that if, according to the example described at the outset, the semiconductor coating machine has a maximum capacity of 210,000 modules a year, it suffices to use five lasers with a maximum capacity of 90,000 modules a year each, so that one laser machine can be saved compared to the prior art.

p-0019If the laser patterning of the semiconductor layer is now also carried out at the wavelength 1064 nm with the Nd:YVO₄or Nd:YAG solid-state laser, this being done at the wavelength 532 nm according to the prior art, and said lasers likewise have a maximum capacity of 90,000 modules a year, then two laser machines can be saved compared to the prior art. Thus, seven laser machines are then necessary for patterning front electrode, semiconductor and back electrode layers. According to the prior art, nine machines would be necessary in this example.

p-0020Likewise, according to the invention the redundancy of the laser patterning processes ensures substantially higher flexibility and total plant availability in production. In case of maintenance or malfunction of one laser patterning machine (for example for a front electrode layer) it is possible, due to the redundancy, for laser patterning machines for the back electrode (extended also for the semiconductor layer) to perform the processes for patterning the front electrode layer, or also additionally for patterning the semiconductor layer.

p-0021The front electrode layer can consist of an electrically conductive metal oxide, for example tin oxide (SnO₂), in particular fluorine doped tin oxide or another material. It is only essential that it is transparent and electrically conductive, absorbs at least part, preferably at least 0.5%, in particular at least 2%, of the emitted infrared radiation of the laser, and is thermally more stable than the back electrode layer with the semiconductor layer located therebelow.

p-0022Thus the separating lines are produced with the laser preferably at its full power for patterning the front electrode layer which was previously applied to the transparent substrate, for example a glass plate, e.g. by chemical vapor deposition.

p-0023On the patterned front electrode layer there is subsequently applied the semiconductor layer for example by chemical vapor deposition.

p-0024The semiconductor layer can consist of silicon, for example amorphous, nano-, micro- or polycrystalline silicon, but also of semiconductor, for example cadmium tellurium. The semiconductor layer can comprise silicon, for example amorphous, nano-, micro-or polycrystalline silicon, but also of other semiconductors, for example cadmium tellurium.

p-0025The back electrode layer is preferably a metal layer, consisting for example of aluminum, copper, silver or the like. It can be applied by sputtering. It is also possible to apply between the metal layer and the semiconductor layer, optionally also by sputtering, an interlayer which consists for example of a doped semiconductor, such as indium oxide, or aluminum doped zinc oxide, and which prevents metal atoms from diffusing out of the metal layer into the semiconductor layer of the module.

p-0026The laser used according to the invention emits infrared radiation, that is, radiation with a wavelength of at least 800 nm, preferably 1000 nm and more, in particular a solid-state laser emitting in the near infrared range being used. The solid-state laser can also be a fiber laser or a disk laser.

p-0027The solid-state laser is preferably a Nd:YVO₄laser, i.e. it has yttrium vanadate as the host crystal. Instead, the host crystal can also be yttrium aluminum garnet (YAG) for example. For doping it is preferable to employ neodymium, i.e. to use a solid-state laser with a wavelength of 1064 nm. It is also possible to use erbium, ytterbium or another element for doping the laser. A neodymium doped yttrium vanadate laser (Nd:YVO₄laser) or neodymium doped YAG laser (Nd:YAG laser) is particularly preferred.

p-0028For patterning the semiconductor layer the laser beam can be focused on the front electrode layer and semiconductor layer through the transparent substrate at the place where the separating lines are to be produced in the semiconductor layer. Instead it is also possible to perform the patterning of the semiconductor layer such that the laser beam is directed from the other side, i.e. directly onto the semiconductor layer.

p-0029It is thereby possible to produce in the semiconductor layer a separating line with a width of for example 10 to 100 μm which exposes the front electrode layer located therebelow. The semiconductor layer is subsequently coated with the back electrode layer for example by sputtering.

p-0030For patterning the back electrode layer the laser beam is focused through the transparent substrate onto the functional layers at the place where the separating lines are to be produced in the back electrode layer. It is thereby possible to produce in the back electrode layer a separating line with a width of for example 10 to 100 μm, which obtains the electrical isolation of the cells of the photovoltaic module that is necessary for the integrated series connection. As a side-effect, but not necessarily, the semiconductor thin-film layer is also removed completely or partly during this process.

p-0031The patterning of the back electrode layer is preferably done in pulsed mode, i.e. in particular in Q switch mode, in order to obtain separating lines with sharp edges in the back electrode layer. The pulsed mode is carried out here in combination with a beam expander and a diaphragm in the laser beam in order e.g. to form a “top-hat” shaped beam profile causing only the center of the laser beam to take effect. The optical devices, for example for beam expansion and the diaphragm or an optical beam conversion system (e.g. refractive or diffractive optic), are preferably adapted to be swiveled, or otherwise moved, out of the laser beam.

p-0032According to the invention, the energy of the laser is absorbed by the front electrode layer and transferred by heat conduction to the semiconductor thin-film layer with the back electrode layer located thereabove.

p-0033When the laser beam is coupled into the front electrode layer through the transparent substrate, e.g. the glass plate, to produce a separating line in the back electrode layer, the semiconductor layer is burned off together with the back electrode layer, i.e. removed thermomechanically.

p-0034Hereinafter the invention will be explained in more detail by way of example with reference to the enclosed drawing. Therein are shown schematically in longitudinal section:

p-0035<figref idrefs="DRAWINGS">FIG. 1</figref> a photovoltaic module; and

p-0036<figref idrefs="DRAWINGS">FIGS. 2</figref><i>a </i>to <b>2</b><i>f </i>the individual steps for producing the photovoltaic module according to <figref idrefs="DRAWINGS">FIG. 1</figref>.

p-0037According to <figref idrefs="DRAWINGS">FIG. 1</figref>, the photovoltaic module <b>1</b> has a transparent substrate <b>2</b>, e.g. a glass plate, having deposited thereon, one on the other, three functional layers, namely, a transparent front electrode layer <b>3</b>, a semiconductor thin-film layer <b>4</b> and a back electrode layer <b>5</b>.

p-0038The module <b>1</b> comprises individual strip-shaped cells C₁, C₂. . . C_n, C_n+1which are series-connected and extend perpendicular to the current flow direction F. For this purpose the front electrode layer <b>3</b> is interrupted, and thus patterned, by separating lines <b>6</b>, the semiconductor layer <b>4</b> by separating lines <b>7</b>, and the back electrode layer <b>5</b> by separating lines <b>8</b>.

p-0039As explained hereinabove and again hereinafter with reference to <figref idrefs="DRAWINGS">FIGS. 2</figref><i>e </i>and <b>2</b><i>f, </i>there also arise upon formation of the separating lines <b>8</b> in the back electrode layer <b>5</b> further separating lines <b>9</b> flush therewith in the semiconductor layer <b>4</b>. However, the separating lines <b>9</b> are irrelevant to the functioning and effectiveness of the module <b>1</b>.

p-0040According to <figref idrefs="DRAWINGS">FIG. 1</figref>, the back electrode layer <b>5</b> of one cell C₁, C_nthus contacts the front electrode layer <b>3</b> of the adjacent cell C₂, C_n+1through the separating line <b>7</b> in the semiconductor layer <b>4</b>, thereby connecting the negative pole of one cell C₁, C_nwith the positive pole of the adjacent cell C₂, C_n+1.

p-0041The current produced by the photovoltaic module is collected by the contacts <b>11</b>, <b>12</b> on the outermost cells C₁, C_n+1. On the back of the module <b>1</b> with the contacts <b>11</b>, <b>12</b> there is provided a back protection (not shown) made of plastic or another electrically insulating material.

p-0042<figref idrefs="DRAWINGS">FIG. 2</figref><i>f </i>shows the series connection of two adjacent cells by the example of the cells C₁and C₂according to <figref idrefs="DRAWINGS">FIG. 1</figref>. According to <figref idrefs="DRAWINGS">FIGS. 2</figref><i>a </i>to <b>2</b><i>f, </i>the series-connected cells C₁and C₂are produced as follows.

p-0043Starting out from a glass substrate <b>2</b> coated with the front electrode layer <b>3</b>, the separating lines <b>6</b> are produced with the focused laser beam <b>13</b> of the laser <b>14</b> which emits infrared radiation, for example a Nd:YVO₄or Nd:YAG laser with a wavelength of 1064 nm, for patterning the front electrode layer <b>3</b> consisting for example of tin oxide, this happening through melting or evaporation of the front electrode layer <b>3</b> since the front electrode layer <b>3</b> absorbs the laser radiation of this wavelength.

p-0044In <figref idrefs="DRAWINGS">FIG. 2</figref><i>a </i>the laser beam <b>13</b> is directed onto the front electrode layer <b>3</b> through the glass substrate <b>3</b>. However, the patterning of the front electrode layer <b>3</b> can also be performed from the other side, i.e. with the laser beam directed onto the front electrode layer <b>3</b> directly.

p-0045On the thus formed patterned front electrode layer <b>3</b> according to <figref idrefs="DRAWINGS">FIG. 2</figref><i>b, </i>the semiconductor thin-film layer <b>4</b>, consisting for example of silicon, is deposited e.g. by chemical vapor deposition.

p-0046As shown in <figref idrefs="DRAWINGS">FIG. 2</figref><i>c, </i>the laser <b>14</b> emitting the same infrared radiation, for example an Nd:YAG or Nd:YVO₄laser with a wavelength of 1064 nm, is used for patterning the semiconductor layer <b>4</b>, said laser being directed, i.e. focused, onto the two layers <b>3</b> and <b>4</b> in the area where the separating lines <b>7</b> are to be formed for patterning the semiconductor layer <b>4</b>, either from the semiconductor layer side, as illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref><i>c </i>by the laser <b>14</b> disposed over the semiconductor layer <b>4</b>, or through the glass substrate <b>2</b>, as illustrated in <figref idrefs="DRAWINGS">FIG. 2</figref><i>c </i>by the laser <b>13</b> disposed under the glass substrate <b>2</b>.

p-0047Although the semiconductor layer <b>4</b> absorbs no, or very little, radiation of this wavelength, there is an absorption of the laser radiation <b>13</b> of the laser <b>14</b> by the front electrode layer <b>3</b> located therebelow, which thereby heats up thermally such that the thermally less stable semiconductor layer <b>4</b> is burned off, or in any case removed, so as to form the separating lines <b>7</b>, thereby exposing the front electrode layer <b>3</b> in the area of the separating lines <b>7</b>. To ideally avoid damaging the front electrode layer <b>3</b>, the power of the laser <b>14</b> is accordingly reduced during this process.

p-0048On the glass substrate <b>2</b> coated with the patterned front electrode layer <b>3</b> and patterned semiconductor thin-film layer <b>4</b> according to <figref idrefs="DRAWINGS">FIG. 2</figref><i>d, </i>the back electrode layer <b>5</b> consisting e.g. of metal is subsequently deposited for example by sputtering.

p-0049For patterning the back electrode layer <b>5</b> the laser <b>14</b> emitting the same IR radiation, e.g. the Nd:YVO₄or Nd:YAG laser with a wavelength of 1064 nm, optionally with a swing-in optical device <b>15</b> (e.g. expander, diaphragm, refractive/diffractive optic), is used according to <figref idrefs="DRAWINGS">FIG. 2</figref><i>e, </i>said laser being directed, i.e. focused, onto the layers <b>3</b>, <b>4</b>, <b>5</b> through the glass substrate <b>2</b> in the area where the separating lines <b>8</b> are to be formed for patterning the back electrode layer <b>5</b>.

p-0050The semiconductor layer <b>4</b> absorbs no, or very little, radiation of this wavelength but there is an absorption of the laser radiation <b>13</b> of the laser <b>14</b> by the front electrode layer <b>3</b> located therebelow, which thereby heats up thermally such that the thermally less stable semiconductor layer <b>4</b> is burned off, in any case removed, so as to form the separating lines <b>7</b>, thereby exposing the front electrode layer <b>3</b> in the area of the separating lines <b>7</b>. To avoid damaging the front electrode layer <b>3</b>, the power of the laser <b>14</b> is accordingly reduced during this process.

p-0051Although the back electrode layer <b>5</b> absorbs only part of this wavelength and the semiconductor layer <b>4</b> none or very little of the radiation of this wavelength, there is an absorption of the laser radiation <b>13</b> of the laser <b>14</b> by the front electrode layer <b>3</b> under the semiconductor layer <b>4</b>, which thereby heats up thermally such that the thermally less stable semiconductor layer <b>4</b> and the back electrode layer <b>5</b> located thereabove is burned off, in any case removed, in this area so as to form the separating lines <b>8</b> in the back electrode layer <b>5</b> and the further separating lines <b>9</b> flush therewith in the semiconductor layer <b>4</b>. To avoid damaging the front electrode layer <b>3</b>, the power of the laser <b>14</b> is accordingly reduced during this process.

p-0052There thus do arise the additional separating lines <b>9</b> in the semiconductor layer <b>4</b> according to the inventive method. However, they do not lead to any noticeable impairment of the functioning or power of the photovoltaic module <b>1</b>.