The present invention relates to a method for producing a transparent solar panel. BACKGROUND The use of building integrated solar panels is increasing, giving the advantage of using available exterior wall space for the production of renewable energy. This is especially beneficial in larger cities with tall buildings having large surface areas that are well suited for holding solar panels.

In some jurisdictions, however, buildings must produce a certain amount of energy and in order to meet these demands it could be necessary to use not only walls but also other space for placing solar panels. If the solar panels could be made at least partly transparent, it would be possible to use windows for energy production and still let sufficient light through to create a desirable indoor environment.

A number of methods are known for creating transparent solar panels, for instance through US6858461 and US20110100432. Often, a part of the surface is treated by laser scribing or etching in order to remove the solar cells and expose the glass substrate beneath, rendering that part of the surface transparent. The capacity of the solar panel is lowered considerably, since the exposed area and often also regions bordering on the exposed area no longer produce energy. Since the solar cells are generally sensitive to the scribing or etching, there is also a risk of damaging the remaining cells by this process. Furthermore, the methods are expensive and time consuming, making the resulting partly transparent solar panels less attractive to the end customer.

There is therefore clearly a need for an improved method for creating transparent solar panels that can overcome these drawbacks. SUMMARY OF THE INVENTION

The object of the present invention is to eliminate or at least to minimize the problems mentioned above. This is achieved through a method according to the appended claim 1. Thanks to the invention, a cost effective and efficient method for removal of thin film solar cells from the solar panel can be used with high precision, namely abrasive blasting. By using a transparent adhesive for laminating the front glass, the roughness created on the surface of the glass substrate can be smoothed out, resulting in a solar panel with good transparency properties while at the same time leaving the remaining solar cells intact for energy production. This method is both time and cost efficient and results in high quality solar panels with transparent properties. Before abrasive blasting, a mask is applied by screen printing, thereby giving the advantages of a mask that lies directly on the solar panel without gaps where particles from the blasting could penetrate and damage the solar cells. Another significant advantage is that the screen printing enables masking the solar panel with high precision even in cases where warping of the substrate has occurred during manufacture of the solar panel. There is a prejudice in the industry that the application and removal of a screen printed mask on thin-film solar cells, particularly CIGS cells, would damage the solar cells. The present inventors have found, however, that despite this it is indeed possible to perform the steps of the inventive method without damaging the solar cells and without the abrasive blasting penetrating through the mask or through a gap between the mask and underlying solar cells. This gives the combined advantage of a safe, reliable and cost effective manufacturing process for a transparent solar panel.

According to an aspect of the invention, the abrasive blasting is performed at a pressure of less than 10 bar, preferably less than 7 bar and more preferably less than 5 bar. Thereby, the solar cells can be removed in an efficient and reliable manner, while at the same time minimizing damage to solar cells protected by the mask. It is especially beneficial to use a negative pressure where the abrasive is sucked towards the solar cell.

According to a further aspect of the invention, the abrasive blasting is performed using AI203, preferably having a grain size of 10-100μ, more preferably 44-74μ. This is a material allowing for high precision and excellent performance, and also without risking damage to surrounding solar cells.

According to yet another aspect of the invention, the abrasive blasting is performed with the nozzle moving at a speed of 10-100 mm/s. Thereby, the mask will be able to protect the remaining solar cells and is not penetrated by the grains of the abrasive blasting. By keeping the treatment time short, enough abrasion can be provided to remove solar cells according to the pattern in the mask, but the abrasion to the substrate surface can be kept low.

According to yet another aspect of the invention, the mask is removed by using NaOH at a concentration of less than 3 percent, more preferably less than 1,5 percent by weight.

Thereby, the mask can be removed without risking damage to the back contact of the solar cells. The removal of the mask may be performed using other solutions as well, and for this purpose it is beneficial to use a solution having a pH of 7 or more.

Many additional benefits and advantages of the invention will become readily apparent to the person skilled in the art in view of the detailed description below.

DRAWI NGS The invention will now be described in more detail with reference to the appended drawings, wherein

Fig. 1 discloses a first step of a method for producing a transparent solar panel according to a preferred embodiment of the invention; Fig. 2 discloses a second step of the method;

Fig. 3 discloses a third step of the method;

Fig. 4 discloses a fourth step of the method;

Fig. 5 discloses a fifth step of the method; and

Fig. 6 discloses a planar view of a resulting transparent solar panel produced through the method of Figs. 1-5

DETAILED DESCRIPTION

The manufacture of a transparent solar panel according to a preferred embodiment of the invention will now be described in detail with reference to Figs. 1-5, followed by a description of the resulting solar panel with reference to Fig. 6. In a first step, a solar panel 1 is provided. The solar panel 1 comprises a plurality of thin film solar cells 11, preferably CIGS solar cells, mounted on a substrate 10 that is transparent, preferably glass. In order to make the solar panel as a whole transparent, the solar cells 11 must be removed from at least part of the substrate 10 to allow light to penetrate the substrate 10. In a second step, a mask 12 is applied to a solar panel surface 15, the mask 12 comprising a pattern 21 designating areas where the solar cells 11 are to be removed. In this preferred embodiment, the mask 12 comprises a resist and is applied through screen printing. The mask 12 is printed directly onto the solar panel surface 15 and adheres to the surface without gaps or the like so that the intrusion of particles from the abrasive blasting can efficiently be prevented, both through the mask 12 itself and along its edges. Damage to the solar cells bordering on areas where the solar cells 11 are to be removed can thereby be prevented. Screen printing is also easy, convenient and cheap, and the mask 12 can be applied to the solar panel surface 15 by application of a material that closely follows shape and form of the solar panel surface 15. This is a significant advantage over lithographic masks which require multiple step application and are generally more expensive both in the required materials and in the time required. Another advantage is that pattern broadening can be avoided. In many manufacturing processes where the substrate is subjected to multiple process steps and treatments involving temperature changes and mechanical load, warping of the substrate emerges as a common problem during manufacture. The manufacturing steps have to be adapted to this and e.g. the application of masks must always take warping into account in order to mask the desired areas and avoid others. Thanks to the use of screen printing, the mask can follow the solar panel surface 15 closely and it can be ensured that the masking will serve the desired purpose. There has been a general prejudice in the industry of thin-film solar panels, where it has been believed that the application of a screen printed mask directly onto the front contact, generally a ZnO layer, would damage the front contact both on application and removal of the mask. However, the present inventors have realised that this is not the case and that very good results can indeed be achieved by using a screen printed mask.

In a third step, the solar panel surface 15 is treated by abrasive blasting to remove solar cells 11 according to the pattern 21. Abrasive blasting has previously not been considered suitable for use on thin film solar cells, due to the perception that it damages the substrate by turning it coarse and non-transparent, so that the resulting solar panel is not transparent even after solar cells have been removed. The present inventors have realised, however, that abrasive blasting is in fact very suitable for removing solar cells from the solar panel surface, since damage to the solar cells covered by the mask 12 can be kept to a minimum and since the surface of the substrate 10 can in fact be made transparent after blasting. By selecting suitable pressure, grain sizes and treatment times for the abrasive blasting, the benefits can be maximized while keeping possible side effects to a minimum.

Preferably, the abrasive blasting is performed using aluminium oxide, but it is also possible to use other materials such as fine sand. The grain size is preferably 10-100 μιη. The abrasive blasting is preferably may be performed at a pressure of less than 10 bar, preferably less than 7 bar and more preferably less than 5 bar, in order to achieve an efficient blasting while at the same time preventing undesired damage to the CIGS film. According to one especially beneficial embodiment of the invention, the abrasive blasting is performed at a negative pressure, i.e. where the abrasive is sucked towards the solar cell. Such negative pressure blasting is preferably performed at -0,5 to -7 bar.

During the abrasive blasting, the nozzle is preferably moved in relation to the surface to be blasted at a speed of 10-100 mm/s, in order to achieve an efficient blasting and still avoid undesired damage to the CIGS film or underlying substances.

In this preferred embodiment, aluminium oxide (AI203) is used in the form of ALOX 220 with grain sizes of 44-74μιη and at a pressure of 2-5 bar.

In a fourth step, the mask 12 is removed by subjecting the solar panel 1 to a suitable substance. It is advantageous to use a substance that allows for a removal of the mask without damaging the solar cells 11, either by affecting the CIGS film itself or the back contact, generally made from ZnO. In this preferred embodiment, a solution of NaOH at 1,5 percent by weight is used for 2-4 min. It is advantageous to use a solution having a pH of 7 or more. After removing the mask 12, the solar panel 1 comprises the substrate 10 and the solar cells 11 distributed according to the pattern 21 of the mask 12. The mask 12 has protected the solar cells 11 so that they are essentially unaffected by the abrasive blasting, even at edges of the pattern 21. This would not be possible using other known methods for removing thin film solar cells according to a pattern, such as laser scribing or etching for instance. The risk for damages through melting of solar cells when using laser scribing or dissolving under the edges of the mask when using wet etching, for instance, can be essentially eliminated through the use of abrasive blasting instead, where the angle of incidence of the grains serve to remove only solar cells not covered by the mask 12 and leaving the remaining solar cells 11 with their edges essentially intact. The abrasive blasting is preferably performed at an angle of incidence of 0-45 degrees in relation to a normal of the surface that is to be blasted, i.e. at an angle of incidence of 45-90 degrees in relation to the surface itself. However, in some embodiments other angles of incidence may also be used.

The substrate 10, on the other hand, has a surface that has been affected by the abrasive blasting in areas not covered by the mask 12 and is now non-transparent with a coarse surface, as opposed to the smooth surface previously exhibited before the blasting.

In a fifth step, a front glass 14 is added to the solar panel surface 15 and serves to protect the solar cells. The front glass 14 is generally mounted by laminating with an adhesive 13 that is applied to the solar panel surface 15 and covers the solar cells 11 as well as parts of the substrate 10 that have been exposed through the abrasive blasting. The adhesive is in itself transparent, and when applied to the coarse surface of the substrate 10 serves to smooth its unevenness and render the substrate surface transparent once again. Thus, the combination of the abrasive blasting as a very suitable method for removing solar cells 11 according to the mask 12 with the adhesive 13 for filling out the coarse surface of the substrate 10 serves to create a transparent solar panel 1 manufactured with high precision and at a low cost. The adhesive 13 is preferably EVA (ethylene-vinyl acetate), but other adhesives may also be used such as PVB (polyvinyl butyral) and Polyolefin.

With the front glass 14 has been mounted and secured, the solar panel 1 has the

appearance shown in Fig. 6, with the pattern 21 determining which areas of the solar panel 1 has had the solar cells 11 removed so that light is allowed to reach the surface of the substrate 10. It is advantageous to use a pattern 21 with longitudinal sections such as stripes 21, in order to allow a human observer to view the solar panel 1 as a window and discern shapes and structures outside.

The amount of solar cells 11 removed generally corresponds to a lowered performance of the solar panel 1, but thanks to the use of abrasive blasting rather than other methods losses due to damages to the remaining solar cells can largely be avoided. Thereby, it becomes possible to use thin stripes as the pattern 21, to trick a human observer into viewing the solar panel 21 as a striped window rather than a wall with transparent sections. Suitable proportions of the width of a stripe of exposed substrate 10 to the width of a stripe of solar cells 11 is where the former is in the range of 0,5-5 mm and the latter in the range of 1-20 mm, giving a preferred transparency of the solar panel of 20-50 percent.

According to the invention, a removal of 25% of the available solar cells 11 will also result in a performance reduction for the solar panel as a whole of 25-30 %, where additional losses that might arise through the removal of solar cells and the treatments of the solar panel can largely be avoided. The dimensions of the solar panel 1 may of course be varied. In this preferred embodiment, a substrate 10 having a thickness of about 3 mm is used, with a CIGS film of about 1-5μιη. The mask 12 is in the form of a resist and has a thickness of about 3 mm, with the patterned stripes being 1 mm wide. This is intended to serve solely as a working example of the invention, and is not to be seen as limiting to the scope of the claims. It is also to be noted that features of the embodiments described above may freely be combined with other embodiments, unless such a combination would be unsuitable, as will be readily understood by the person skilled in the art.