Introduction

The invention relates to a method for the fabrication of a solar cell with fired rear side contacts, and to a cell manufactured with such a method. In particular, the invention relates to a method for the fabrication of a solar cell with fired rear side contacts and with via holes perforating the cell for the contacting of a front side located emitter layer from rear side located busbars. State of the art and disadvantages

Solar cells have two sides: a front side which is directed versus the light, and a rear side which usually has an at least partially metallized surface. Between these sides, a stack of different layers is present which perform different tasks, such as anti-reflection layers, a layer which collects the light, a p- n-junction with a space-charge layer, and contact layers for establishing contact to the exterior of the solar cell. Commonly, one can distinguish between front and rear side contacts, wherein the front side contacts are connected to the emitter layer, and the rear side contacts are connected to the substrate bulk. To collect the electric charge carriers on the front side, narrow so called fingers are arranged side by side which are connected by so called busbars. The latter provide solder areas onto which external wires can be soldered. Since the efficiency of a solar cell strongly depends on the area that can be irradiated by light, it is desired to reduce the area that is covered with metal and thus blocked by the fingers and busbars on the front side. A solution to this is to provide via holes that connect the metal of a multitude of front sided fingers to a busbar that is located on the rear side. Such cells are called metal wrap through cells, or M T cells. An alternative solution does not even provide front side fingers, but a high number of closely spaced via holes that have semi-conducting side walls, thus serving as means for connecting the front with the rear side. Since these holes are doped and thus conducting, such cells are called emitter wrap through cells, or EWT cells. Again, rear side arranged busbars and fingers that provide according solder areas subsequently collect the electric charge carriers.

The fabrication of such rear side contacted, perforated solar cells requires complex process sequences. The adaption of conventional solar cell processing steps to perforated substrates is quite demanding. Since the via holes are introduced into the substrate prior to any other processing step, all subsequent processing must take into account the differing physical characteristics of such perforated cells when compared to cells without holes. This is particularly relevant with regard to the fragility of the substrate that significantly increases after perforation .

Another disadvantage lies in the fact that the conductivity of the fingers and, for T cells, of the metallized via holes, is unsatisfactory. This is because screen printing is the typical process which is being used for the application of the conducting paste which forms, amongst others, the fingers, and which is, for MWT cells, also being pressed into the via holes to achieve conductivity. The paste itself contains not only conducting, but also other, process relevant ingredients which in turn reduce conductivity. Furthermore, voids can develop during temperature induced processing of the paste. When fabricating narrow fingers, their achievable width is limited, since the viscous paste tends to deliquesce, limiting the aspect ratio of an e.g. rectangular shaped longitudinal cross section of the finger. To compensate the low conductivity, the number of via holes must be increased. Additionally, even a high aspect ratio would provide only a minor area of contact between finger and substrate. It is thus desirable to replace such screen printed front side metallization by other techniques providing a better controllable finger cross section geometry as well as a higher conductivity, which is also desirable for a possibly present via hole metallization.

Object of the invention and solution The object of the invention is therefore to provide a method for the fabrication of a solar cell with fired rear side contacts and with via holes perforating the cell avoiding the disadvantages of the state of the art. In particular, the method should reduce the effort of adapting a fabrication process from substrates without to substrates with via holes. The process should further achieve a better controllability of the geometry of possibly present narrow fingers, and improve the conductivity particularly of both front side fingers and/or metal wrap though via holes. The number of process steps, and in particular, the number of process steps with already perforated substrates, should be reduced.

Description

The method according to the invention serves for the fabrication of a solar cell with fired rear side contacts and with via holes perforating the cell. A synonymous term for such a cell is "rear side contacted solar cell with fired contacts". According to the invention, the introduction of the via holes takes place subsequent to the firing of the contacts.

Typically, some or all of the subsequently described steps are carried out for the production of a solar cell onto a solar cell substrate:

(a) Providing the solar cell substrate. This substrate typically consists of silicon; however, other materials, in particular glass, ceramics, and plastics can serve as a substrate material . (b) Texturing the front and/or the rear side. This optional, but typical step is relevant for the improvement of the absorption characteristics of the cell surface (s) .

(c) Doping by diffusion for emitter generation. This is usually done by phosphorous for achieving an n-doped layer on p-type substrates. In general, the doped layer has opposite polarity than the bulk material. The process is usually carried out in a tube furnace.

(d) Optionally removing a possibly present glass layer, e.g. PSG (phosphorous-silicate-glass) after a phosphorous diffusion.

This layer can develop during the doping step.

(e) Optionally, but virtually always depositing or growing a single- or multi-layer system on the front side for passivation and anti-reflection purposes. A reduction of reflection is important for a high efficiency. Passivation helps to reduce recombination, thus also increasing the efficiency of the solar cell. Typical layers involve silicon nitride and/or silicon dioxide.

(f) Metallizing of the rear side for the generation of contacts.

The metallization is usually performed by screen printing.

The contacts for n-type areas usually consist of silver (Ag) , the ones for the p-type solder areas of silver-aluminium (AgAl), and the p-type areas of aluminium (Al).

(g) Firing of the contacts. Firing is performed on the still wet or partially dry, but brittle paste, in order to solidify the same. Typical temperatures range between 800 to 900°C. A possibly present passivation layer is opened by ingredients of the fired paste, thus allowing for a direct electric contact between the bulk material and the metallic contacts. (h) Performing an edge isolation at least on the rear side. This step can be performed using dry or wet techniques. When using wet techniques, the step is usually carried out between the doping and the passivating step, whereas dry techniques are suitable also after subsequent processing steps, such as screen printing, or even as a finalizing step. Edge isolation is commonly performed using lasers.

According to the state of the art, the via holes are introduced into the substrate in a very early stage, i.e. prior to the aforementioned step (b) . As a result, all subsequent steps are carried out on perforated substrates, with the above described disadvantages. According to the invention, the process of introducing the via holes is now performed at a much later stage, i.e. after firing of the contacts. Thus, most, if not all, of the aforementioned steps (a) to (h) are carried out on standard, i.e. non-perforated substrates. An adaption of these process steps is not applicable any more, or of minor scale. The breakage rate of the usually fragile substrate is significantly reduced. A further advantage is that the use of a so-called selective emitter is now easily achievable. A selective emitter is characterized in that the area which is reserved for the metallic contact, e.g. to the fingers, is highly doped, thus providing a low sheet resistance (e.g. 20 Ohms/square) and a good metal- silicon contact, and the remaining area that represents the illuminated surface of the solar cell is doped less, thus providing a higher sheet resistance (e.g. 120 Ohms/square) and lower recombination. According to the invention, the selective emitter is manufactured prior to the introduction of the holes, making it possible to more easily fabricate solar cells, in particular MWT cells, with via holes and selective emitters. This in turn increases cell efficiency.

For the manufacture of selective emitter structures, the techniques of liquid jet-guided laser processing, particularly LCP (laser chemical processing), and LIP (light induced plating) are preferably being used, which are described in more detail later on. According to a preferred embodiment of the present invention, the opening (=introducing) and doping of the via holes takes place by means of laser chemical processing (LCP) which is a special form of liquid jet-guided laser processing that uses a dopant containing liquid instead of water, or it is performed by dry laser doping. Herein, both techniques are using dopant source containing liquids.

LCP, which also can be referred to and described as "liquid jet guided laser processing", uses a laser beam which is coupled into and guided by a liquid jet. The laser provides energy which primarily serves for melting and ablation of the surface it is directed onto. It might also deliver energy which is necessary for, or enhances, a chemical reaction. The liquid itself contains ingredients which chemically interact during laser irradiation with the surface it is directed onto. Typical liquids are water (oxidizing characteristics on certain materials), or dopant- containing liquids, so that e.g. a via hole can be drilled and doped at the same time. Since the laser interacts differently with different materials, it is unproblematic to e.g. stop drilling when reaching a previously fabricated metal layer, e.g. the rear side contact of the via hole. However, depending on the concrete subsequent process steps, such a via hole might even be drilled all the way through the substrate and the contact, and still provide electrical contact to the front side located emitter.

An alternative "dry" technique uses a laser beam as well which is not guided by a liquid jet, but directly hits the surface to be treated. However, in order to achieve a chemical reaction or modification of the treated surface, a thin liquid layer containing the necessary chemical ingredients such as the dopant is applied onto the surface during laser treatment or prior to the same, thus enabling drying before actual laser irradiation. Phosphoric acid (H3PO4) can advantageously be used as such a liquid . The goal of both techniques is to open a possibly present passivation layer for further processing involving metallization, and to simultaneously modify the thus exposed surface.

In the case of optionally present front side fingers, the aforementioned LCP processing and doping which is performed on the via holes is preferably used for the front side opening of a passivation layer, combined with local doping, for these front side fingers .

According to a further preferred embodiment, a metallization of the via holes takes place by means of light induced plating (LIP) or electroless plating. As for the aforementioned LCP processing and doping, this metallization method is advantageously used for optionally present front side fingers.

As mentioned before, the conductivity as well as the achievable geometry of screen printed pastes commonly used for solar cell production is insufficient. A much better conductivity can be achieved with plating techniques that result in a pure and thus very low-resistance metallic layer. Furthermore, subsequent firing becomes obsolete. Whereas generally, all possible types of plating techniques can advantageously be used for the invention, the well-known technique of light-induced plating, or the use of electroless plating are particularly preferred. When irradiating a solar cell under production, charge carriers are generated and can conduct electrical current from one side to the other. Connecting one side to a first electrode, and using a second bath electrode therefore leads to a plating of the cell surface which is in contact with the electrolyte. The same is true for cell via holes that have been drilled and doped using LCP whereas masked or non- conducting areas, such as passivated areas of a solar cell, remain unplated. Typical metal stacks resulting of such plating are e.g. Ni-Ag, Ni-Cu-Sn, or Ni-Cu-Ag. Electroless plating does not use an external current and can also be used to plate or reinforce conductive structures of a solar cell .

Since there is no high-viscous phase that the metal must go through, not only the conductivity, but also the quality and controllability of the geometry of the metallic structures is far superior to the one which is achievable using pastes. Although the use of screen printing in the production process can not be entirely omitted, the number of screen printing steps is significantly reduced. E.g., for a cell where previously four screen printing steps were necessary, i.e. (1) for the rear side emitter busbars and hole contacts, (2) for the base solder pads, (3) for the rear side aluminium regions, and (4) for the front side emitter contacts, the first and the fourth step can be omitted, not only resulting in a better product quality due to a higher conductivity, but also in a saving of partially silver- containing and thus costly paste. Furthermore, a possibly necessary flipping of the cell due to dual side screen printing process steps can be omitted as well, resulting in a further simplification of the process and reduction of the danger of breakage. In addition, the subsequent firing process can be designed to "optimize" less requirements, leading to a further reduction in process complexity.

Regarding EWT cells, the high conductivity of the plated via holes results in a very advantageous reduction of the necessary hole distribution density over the cell surface.

According to another embodiment of the invention, at least an edge isolation step to be performed on the rear side takes place by means of liquid jet guided laser or other, e.g. dry, laser processing.

Edge isolation is necessary in order to prevent shunts between conductive areas of different polarity, such as the anode and the cathode of the solar cell, i.e. the front side emitter and the back side contact. If the emitter fabrication, described in step (c) above, is carried out onto the entire surface of the cell, the conductive layer covers not only the front and the rear side, but also the edges. Therefore, edge isolation is necessary. Additionally, other isolating steps that are necessary can be performed preferably using LCP or dry laser processing, for example an edge isolation step on the front side, or a contact isolation step between rear side emitter and base areas.

Typical liquids for this process step are water (H ₂ 0) or oxidizing liquids.

According to another embodiment, the edge isolation is performed using wet chemical etching instead of LCP, and emitter stripes are left on the rear side, onto which an emitter-busbar metallization subsequently takes place. The stripes area can be accordingly masked so that only the area outside the stripes is opened, and the stripes area is left covered by the emitter diffusion .

According to a further embodiment, the metallized via holes are directly being used as solder pads for the emitter contacts for the module production. No additional emitter-busbar metallization is performed on the rear side. This means that no emitter fingers and/or emitter busbars are present on the rear side, but the metallized emitter via holes are directly soldered to the interconnection tab. The module back sheet can feature solder bumps that the cells, respectively the via holes, are aligned to. Subsequently, the solder bumps on the module back sheet are soldered to the metallized via holes and the cell interconnection is achieved. The advantage here is that less material and less production steps must be performed on the cell rear side, lowering the cost of the cell. The pads and/or the corresponding areas on the panel can be covered by a solder that liquefies during heating and solidifies in the connected position when cooling . According to a further embodiment, a controlled back etching of the emitter takes place after the step of doping by diffusion for emitter generation as described above as step (c) in order to increase the sheet resistance. This optional step helps also to improve the blue sensitivity and reduce surface recombination effects, since a highly doped surface region (dead layer) is removed. Therefore, back etching enhances the cell efficiency. Back etching can be performed by e.g. a hydrofluoric acid / nitric acid (HF-HN0 ₃ ) solution, or by reactive ion etching (RIE) techniques.

According to another embodiment, further, rear side polishing is performed before a usually present front side passivation. This step is advantageously performed directly after the optional etch back step, or after the diffusion step, if no such etch back is performed. Polishing results in a smoother surface and helps to reduce recombination losses.

According to yet another embodiment, additionally or solely, a passivating of the rear side of the cell takes place. This takes place in addition to or instead of the aforementioned step (e) which refers to only the front side so far. However, if also a passivating of the front side takes place, it is advantageous to carry out the rear side passivating step directly before or after the step (e) . It is also possible to perform both steps at once, i.e. a passivating of the entire substrate, e.g. by dipping it into a proper solution, or by using PECVD (plasma-enhanced chemical vapour deposition) . Possible layer materials can be e.g. AI2O3, AI2O3 + a layer deposited by PECVD, A1 ₂ 0 ₃ + SiN _x , Al ₂ 0 ₃ + SiO _x , SiOx, S1O2, SiN _x , or Si0 ₂ + SiN _x . Also, a sequence of several stacked materials is possible. According to yet another embodiment, an opening of the rear side passivation layer takes place by means of LCP. Since the process is preferably used for the edge isolation as described above, both steps can advantageously be performed by the same technique. According liquids are water or liquids containing doping agents. Subsequent to this step, the rear side metallization takes place. A further advantage of LCP instead of dry laser opening is the avoidance of dust that otherwise might develop during ablation. Summing up, LCP is preferred not only for edge isolation, but also for the opening of areas which are later on metallized (i.e. screen printed or plated) for the production of busbars.

Accordingly, the subsequent rear side metallization takes place by means of screen printing or LIP. Herein, "subsequent" refers to the previously carried out step of opening the passivation layer either by wet etching or LCP.

Since all processes have already been described in detail above, no further explanation are given thereto.

According to a preferred embodiment, the rear side metallization takes place by means of screen printing and is followed by through-firing of the passivation layer without separate opening of the same. In other words, the opening of the passivation layer takes place by means of the through-firing itself. As mentioned above, the temperature-triggered chemical reaction of the according ingredients of the screen printing paste results in etching away the passivation layer where it is covered by the paste, resulting in an electrical contact without further need of performing an opening step.

According to another embodiment of the present invention, a finalizing temperature treatment in an inert or reducing atmosphere takes place. As an example, temperatures between 100 to 500°C, and treatment times from 0.5 to 30 minutes are preferably used. The atmosphere advantageously consists of pure nitrogen (N ₂ ), or of a forming gas such as 4% hydrogen in nitrogen. The invention further relates to a metal wrap through (MWT) or emitter wrap through (EWT) solar cell with fired contacts and with via holes perforating the cell, wherein the cell is characterized by a void-free, pure and smooth metallic layer on at least the walls of the via holes.

In particular, the invention relates to such a cell which is obtained by and/or obtainable by a method as defined above.

Since a solar cell being fabricated according to the state of the art is manufactured using screen printing and the according paste, this paste results in conducting volumes of inhomogeneous filling. On one hand, additional ingredients, e.g. such that are necessary for proper viscosity of the paste, do not contribute to the (desired) conductivity. On the other hand, these additives can shrink or even vanish during solidification of the paste, leaving voids behind. This does not only reduce conductivity, but can also be a source of reduced mechanical stability. Furthermore, the surface of such solidified pastes is usually not flat and smooth on a micro scale, but bumpy and full of fissures. On the contrary, a solar cell which is fabricated according to the invention, i.e. by use of LCP and LIP as well as via holes that were introduced into the substrate at a late stage as described above, shows smooth and flat metallized surfaces as well as virtually void-free volumes when inspecting a cross section of e.g. a via hole, or a finger, if present. Furthermore, since no additives are deposited during metallization on or into the layer, the resulting material consists of pure metal, providing also a superior conductivity.

As set forth, the present invention solves a number of problems known from the art. In particular, the method according to the invention reduces the effort of adapting a fabrication process from substrates without to substrates with via holes, since the introduction of holes takes place at a very late stage during the production of the cell. The process achieves a better controllability of the geometry of possibly present narrow fingers, since it allows to substitute of some of the common screen printing steps with LIP. It also improves the conductivity particularly of both front side fingers and/or metal wrap though via holes. The number of process steps, and in particular, the number of process steps with already perforated substrates, is significantly reduced.