EP1894254A2

EP1894254A2 - Method for production of a single-sided contact solar cell and single-sided contact solar cell

Info

Publication number: EP1894254A2
Application number: EP06753203A
Authority: EP
Inventors: Rolf Stangl; Marinus Kunst; Klaus Lips; Manfred Schmidt; Jens Schneider; Frank WÜNSCH
Original assignee: Hahn Meitner Institut Berlin GmbH
Current assignee: Helmholtz Zentrum Berlin fuer Materialien und Energie GmbH
Priority date: 2005-05-29
Filing date: 2006-05-22
Publication date: 2008-03-05
Also published as: JP2008543067A; WO2006128427A2; DE102005025125A1; DE102005025125B4; US20090014063A1; WO2006128427A3

Abstract

Conventional single-sided contact solar cells with both contact systems on one side of the absorber layer have particular structures of the emitter layer and the contact system which make complex structuring steps necessary. The inventive method permits single-sided contact without having to carry out a structuring of the absorber layer or the emitter layer and is equally suitable for the production of front face or back face contacts and provides a direct arrangement of a contact grid (KG) on one side of the absorber layer (AS) (front face contact, back face contact). The entire free surface of the contact grid (KG) is then coated with an electrically non-conducting insulation layer (IS). The emitter layer (ES) is then deposited on the whole surface such that the contact grid (KG) is arranged between the absorber layer (AS) and the emitter layer (ES). The emitter layer (ES) is then provided with a contact layer (KS) on the surface. For back face contact the emitter layer (ES) is arranged on the back face (OSA) of the absorber layer (AS) such as to avoid additional absorptive losses. Wafer-based thick layer and thin layer solar cells (HKS) can be produced.

Description

Process for producing a solar cell contacted on one side and solar cell contacted on one side

description

The invention relates to a method for producing a unilaterally contacted solar cell having at least one absorber layer and a surface deposited emitter layer of semiconductor materials with opposite p- or n-type doping, wherein excess majority and minority carriers in the absorber layer generated by incidence of light, am separated pn junction between the absorber and emitter layer and the majority charge carriers of the absorber layer via a first contacting system and the minority carriers of the absorber layer from the emitter layer and a second contacting system are collected and derived, both contacting systems are on the same solar cell side, and on a unilaterally contacted solar cell.

Solar cells are devices that convert light into electrical energy. Usually, they consist of semiconductor materials which contain regions or layers of different conductivity for positive and negative charge carriers, n-type or p-type conducting regions. The areas are referred to as emitters and absorbers. Positive and negative excess charge carriers generated by incident light are separated at the pn junction between emitter and absorber and can be collected and dissipated by contacting systems electrically connected to the respective regions. Accordingly, only those excess charge carriers which reach the contacting systems and do not previously recombine with a respective opposite polarity charge carrier contribute to the usable electrical power of solar cells. Unilaterally contacted solar cells have both contacting systems for separately collecting the excess majority and minority carriers of the absorber layer on a common side. This initially has the advantage that only one side must be processed for contacting, while the other side remains unprocessed with respect to the contact. For the purposes of the present invention, the term "front side contact" is used when both contacting systems are located on the front side and thus the side of the solar cell facing the later use so that the side of the solar cell which faces away from the light in later use are arranged. Important in the arrangement of the contacting systems, however, is always primarily their efficiency in the charge carrier collection. If the absorber layer of the solar cell is of sufficiently good electronic quality, ie if the effective diffusion length of the minority charge carriers is greater than the layer thickness of the absorber layer, then the current-dissipating contacting systems should as a rule advantageously be on the side of the solar cell which faces away from light in later use (rear-side contacting). This results in particular in the advantages that firstly no shading losses occur through a contacting system, which leads to an improvement in the efficiency of the solar cell, and secondly, a good simple full-surface front-side passivation of the front side of the solar cell is possible in order to recombine the excess charges at the solar cell Front effective and easy to prevent. However, if the absorber layer is of relatively low electronic quality, ie if the effective diffusion length of the minority carriers is smaller than the layer thickness of the absorber layer, then the current-carrying contacting systems should advantageously be located on the front side of the solar cell (front-side contact). Any minority carriers of the absorber generated at a depth less than the effective diffusion length of the absorber can then be reliably collected. Compared to the then unavoidable adverse shading by at least one Contacting system is in the one-sided front side contacting a significant advantage then in a technologically very simple contacting method, which in particular does not include back contact, and thus, for example, in the thin film deposition does not require structuring of the absorber or emitter layer.

State of the art

Solar cells contacted on one side at the front have hitherto scarcely been realized in the absence of a technologically simple and efficient production process. From the prior art mainly unilateral backside contacts are known. It must be ensured that the first contacting system for collecting the majority charge carriers of the absorber layer is electrically reliably isolated from the second contacting system for collecting the minority charge carriers of the absorber layer. Various concepts for the production and construction of single-sided back contact solar cells are known from the prior art.

A concept of the back contact is the use of surface elevations, for example, known from DE 41 43 083 A1. In this case, the first and second contacting system are arranged directly or on an insulating layer on a protruding substrate surface, (for example, pyramidal, conical or cylindrical pronounced), the surveys at least partially covered in advance with Passivierungsmaterial and then sections of the attachment of the contacting systems of this have been exposed. Furthermore, an inversion layer for the derivation of the minority charge carriers of the absorber layer extends along the substrate surface between the contacting systems. From DE 41 43 084 A1, it is further known to first passivate the entire structured substrate surface and then to remove the passivation layer again in the region of the elevations. From DE 101 42 481 A1, it is finally known to arrange these elevations in the form of ribs on the underside of the active semiconductor substrate and to provide each a rib edge by directional vapor deposition with a contacting system. Part of this concept is therefore always the creation of elevations on the underside of the substrate, which are then processed in different ways.

Another concept of back contact is the point contact (PC). Here, the two contacting systems on the back are kept very small in the form of dots in order to lower the saturation blocking current and thus increase the open circuit voltage of the solar cell. However, an extremely good surface passivation plays a decisive role here. US Pat. No. 5,468,652 discloses, for example, a point contact in which a laser is used to make contact with the second contacting system on the underside of the substrate through the emitter layer, which is arranged on the front side of the absorber layer, and through the absorber layer. In this case, the second contacting system with the first contacting system for deriving the majority charge carriers of the absorber layer is arranged nested. From WO 03/019674 A1 a point contact with different sized contact hole diameters, the rectangular areas are arranged symmetrically known. From DE 198 54 269 A1, from which the present invention proceeds as the closest prior art, a point contact solar cell is further known, in which the second contacting system for collecting the minority carriers of the absorber layer formed lattice-shaped and directly on the back of the absorber layer in front of a electrically conductive substrate is arranged. The first contacting system for collecting the majority carrier from the absorber layer is formed over the entire surface and arranged on the back of an electrically conductive substrate. The second contacting system between absorber layer and substrate is electrically insulated on both sides. The connection to the emitter layer is again given by holes through emitter and absorber layer, as Contact holes are filled with a metal (see Figure 6 ibid). The electrical contacting of the second contacting system via bridges arranged laterally from the solar cell. Even in the point contact, structuring process steps are required.

The same applies to the third concept of the interdigitated back contact (IBC) with a back contact, in which the first and second contacting system are also arranged in a comb-like manner on the substrate back side and the example of US 4,927,770, US 2004/0200520 A1 DE 195 25 720 C2 or DE 100 45 246 A1 is known. In contrast to the point-contact solar cell, however, the emitter layer is not arranged continuously on the front side of the absorber layer which faces the light in use, but in small subregions on the rear side which faces away from the light in use. There is a variety of subregions of the same, but stronger doping as the absorber layer to form a minority carrier backscatter surface field (Back Surface Field BSF). The structuring measures in this concept therefore extend to the form of the emitter layer. The electrical isolation of the different sections against each other is a major problem.

Furthermore, DE 696 31 815 T2 discloses a wafer-based, back-contacted crystalline homo-solar cell in which the emitter layer is patterned by counter-doping the absorber layer. The counter doping is carried out by dopants from a contact grid. In this case, a contact system in the form of a contact grid is arranged on the emitter layer, encased with an insulating layer and covered by the other contact system. The two contact systems are thus directly superimposed and separated only by an insulating layer. The emitter is not formed as an independent functional layer, but as integrated small areas in the semiconductor material (crystalline silicon) of the absorber layer formed by counter-doping, so it is about a homo solar cell. The insulating layer on the metal grid can be formed in a self-aligning manner, for example by a selective oxide, for example aluminum oxide. The deeper-reaching emitter regions are formed on the back side in the semiconductor material of the absorber layer or in a BSF layer previously diffused in back under high temperature effect by diffusion of portions of the metal lattice and formation of an alloy in the semiconductor material (counter-doping). The contact grid is thus always arranged on the emitter areas. Due to the counter doping, the formation of a sharp p / n transition between two oppositely doped semiconductor layers is not possible. The diffusion processes for counter doping require high temperatures and are difficult to control. All of this is efficiency-limiting for the known homo solar cells.

Finally, from DE 198 19 200 A1, which, like the present invention, is fundamentally concerned with the contacting problem, one-sided front-side contacting is known, in which the emitter layer and both contacting systems are patterned finger-shaped (compare FIG. 4, ibid). Furthermore, one-side contacting is shown by etch-patterning trench or holes and applying metallizations through shadow masks. From DE 197 15 138 A1 it is further known to connect solar cells with a front-side contact by structuring both contacting systems and the emitter layer in series, in which the webs of the comb-like contacting systems are electrically conductively connected to each other. Such series or parallel circuits with back-contacted solar cells are also known from the prior art. task

The object of the invention is to provide a method for producing a unilaterally contacted solar cell, which manages in a simple manner without complex structuring measures both for the contacting systems and for the individual solar cell layers. However, a reliable solar cell with a good electrical separation of the two contacting systems and the highest possible efficiency should nevertheless be made available. The solution according to the invention for this task can be found in the method claim and the independent product claim. Advantageous modifications are shown in the respective associated subclaims and explained in more detail below in connection with the invention.

With the method according to the invention, one-side contact is achieved without the need to structure the absorber or emitter layer. For this purpose, the first contacting system is applied in the form of a contact grid directly on one side of the absorber layer, so that forms a good ohmic contact. The contact surface of the contact grid to the absorber layer is dimensioned so that it can dissipate optimally the expected current. The total surface area of the contact grid is usually less than 5% of the absorber area. Subsequently, the contact grid is electrically insulated on its entire free, not with the absorber layer in contact surface by applying an insulating layer. In this case, this insulating layer has at least such a minimum layer thickness that the tunneling through of charge carriers is reliably prevented. Below, various ways of applying the insulation layer are shown. The electrical contacting of the contact grid can by laterally arranged webs or by cutting (for example, by shadow mask) a connection area on the

Contact grid during the deposition of the emitter layer and exposing the Connection area by removing (for example by mechanical scraping) of the subsequently produced insulation layer done.

After the electrical insulation of the contact grid, the emitter layer is applied over the entire surface of the contact grid, so that the contact grid between the absorber and emitter layer is located. The layer thickness of the applied emitter layer is dimensioned such that the minority charge carriers of the absorber layer can reach the rear side of the emitter layer facing away from the absorber layer, without them suffering appreciable ohmic losses. In particular, an emitter thin film can also be applied. Depending on the layer thickness of emitter layer and contact grid, either a closed, all-surface emitter layer (layer thickness of the emitter layer is greater than the layer thickness of contact grid and insulation layer), which completely covers the contact grid, or else an interrupted emitter layer (layer thickness of the emitter layer is smaller than the layer thickness contact grid and insulation layer), which does not completely cover the contact grid. However, such an interruption of the emitter layer is not an elaborate structuring, but a consequence of a simple, full-surface emitter deposition, which inevitably results from the layer thickness selection. Furthermore, the emitter layer consists of such a material that a well-passivating pn junction is spanned to the absorber layer, wherein a maximum interfacial recombination rate of the charge carriers of 10 ⁵ recombinations / cm ² s is to be observed. However, it is desirable to have an interface recombination rate of, for example, 10 ² recombinations / cm ² s. Embodiments of the emitter layer, see below.

The second contacting system for deriving the minority charge carriers of the absorber layer from the emitter layer is then arranged in the inventive method as an unstructured contact layer on the back of the emitter layer, so that a good ohmic contact is formed. In this case, the contact layer over the entire surface or by applying masking technology may be formed over part of the surface and in a simple manner, for example by applying a metal contact or by vapor deposition applied. The electrical contacting of the contact layer can be done directly without further measures due to their direct accessibility.

The method according to the invention is equally suitable for producing a one-sided front-side or rear-side contacting of a solar cell. It has already been stated above that the choice of the one-sided contacting depends on the electronic quality of the absorber layer. If this is good, back contact is preferable because of lower shading losses. However, if the electronic quality is poor, preference should be given to front-side contacting.

A back contact is obtained when the method step II - applying the contact grid for collecting the majority carrier of the absorber layer - is performed on the back of the absorber layer. In this case, the emitter layer is arranged on the back of the absorber layer, whereby the normally occurring absorption losses are avoided by a arranged on the front side of the absorber layer emitter layer. Since no passivation of the absorber layer on the front side by the emitter layer takes place, the absorber layer is passivated there in a further method step A after process step I - providing the absorber layer - by a correspondingly transparent cover layer. In this case, the passivating top layer, which can consist of silicon oxide or silicon nitride, for example, both surface recombination reduction (by direct passivation of the surface defects or formation of a minority carrier backscattering surface field, front surface field FSF) and the reduction of the reflection incident Light in the form of an antireflective layer. Front-side contacting is achieved when method step II is carried out on the front side of the absorber layer. Accordingly, in the case of the front-side contacting, the contact layer for deriving the minority charge carriers of the absorber layer from the emitter layer, which is likewise arranged on the later light incident side, is to be transparent, for example in the form of a transparent conductive oxide layer, TCO. Whether a cover layer is to be applied to the rear side of the absorber layer (process step B) depends in turn on its electronic quality. If this is good, a passivation layer is required to avoid charge carrier recombination. In addition, if appropriate, a reflection layer is conducive to the reflection of the unabsorbed photons. On the other hand, if the electronic quality of the absorber layer is poor, the minority carriers do not reach the absorber layer back side, so that no further measures have to be taken here. Since then the backside of the absorber layer does not require a passivating cover layer, for example, very defect-rich initial layers (seed layers) for growing the absorber layer and / or reflection layers for reflecting the unabsorbed photons can be used as cover layers. To improve the charge carrier collection on the front side, a further method step C may be provided after method step V-applying the contact layer. This is the application of a contact element on the front side of the transparent contact layer. So that the shading losses are minimized, it is advantageous if contact element and contact grid are constructed congruently and are positioned directly above one another.

The application of the contact grid-which is also taken to mean finger-like or similar shapes-can be applied in prefabricated form directly to the absorber layer, for example by means of a conductive adhesive a corresponding mask by thermal evaporation made of an electrically conductive material. Application of inkjet printing or photolithography is also possible.

In order to avoid undesired recombination of the minority charge carriers under the contact grid, an additional method step F according to method step II-applying the contact grid to the absorber layer-can be provided. This involves the tempering of the conductive material, for example aluminum, from the contact grid into the underlying absorber layer, for example p-doped silicon, to form a backside passivation field (back-surface-field BSF, alneal process). In particular, this thermal step can be combined with the thermal step of forming an electrically insulated insulation layer on the contact grid (see next paragraph).

For applying the insulating layer to the free surface of the contact grid according to method step III, it is possible, for example, to apply an insulating compound selectively using screen or inkjet printing or a mask, in particular a shadow mask, sputtering, vapor deposition or photolithography. Alternatively, an oxide layer can also be grown thermally or wet-chemically or electrochemically on the entire free surface of contact grid and absorber layer rear side (method step D). In this case, forms a different oxide layer due to the differently selected for contact grid and absorber layer materials. In the case of a contact grid, for example of aluminum corresponding to aluminum oxide, in the case of an absorber layer of silicon in the case of oxygen tempering, thermal silicon oxide. In the example of an oxygen anneal, in the system aluminum contact grid on silicon absorber layer, an approximately 20 nm thick aluminum oxide can be expected on the entire free surface of the contact grid and an approximately 5 nm thick silicon oxide on the surface of the absorber layer not covered by the contact grid , In the case of thermal generation of the oxide layer, this process can be used together with the process step F - tempering of the conductive Material of the contact grid in the absorber layer to form a BSF - be carried out in a temperature-controlled heating process.

The subsequent selective etching of the oxide layer on the absorber layer (method step E) is correspondingly easy to carry out, since the different oxides generally have different etching rates in the etching process. In particular, with a suitably selected etching medium, a metal oxide is more etch-resistant than a silicon oxide. In the material example of aluminum and silicon, which is then used correspondingly also for the emitter layer, the selective etching can be implemented, for example, by simple brief immersion in dilute hydrofluoric acid HF. The hydrofluoric acid not only selectively removes the silicon oxide, but at the same time ensures good surface passivation of the silicon absorber layer by forming Si-H bonds. The etchant can thus be chosen so that after the removal of the oxide on the absorber layer, this is well passivated on its exposed surface.

In hetero solar cells, buffer layers are often used between the emitter and the absorber layer in order to better passivate the interface between emitter and absorber. Therefore, it may be advantageous if a further optional method step G is provided after method step III-generating the insulation layer on the contact grid. This is accordingly the optional, full-surface deposition of a buffer layer in the lowest possible layer thickness. For example, in the case of doped amorphous silicon as the emitter material on a crystalline silicon wafer as an absorber, the buffer layer may be an ultrathin (about 5 nm) layer of intrinsic (undoped) amorphous silicon. However, buffer layers can also be formed from a salt, for example from cesium chloride. It is then spanned a corresponding surface dipole and also suppresses the interface recombination at the pn junction. By means of the method according to the invention described above, a highly efficient solar cell can be produced both as a thick-film cell based on a wafer as absorber layer and as a thin-film cell with a layer composite grown on a substrate or superstrate with an exclusively single-sided contacting. It should be noted that in the case of a self-supporting wafer as the absorber layer, any desired processing of both sides can take place. In the case of a thin-film structure, however, there must always be a sequential processing, starting with the substrate (light incidence first through the functional layers) or superstrate (light incidence first through the superstrate), since the thin absorber layer does not carry. The order of the method steps can therefore change between the production of a wafer or a thin-film solar cell. The individual process steps themselves remain unchanged. In principle, therefore, a solar cell according to the invention is characterized in that the first contacting system is a surface-optimized with respect to the collection of majority carriers of the absorber view and with respect to the emitter layer by means of an insulating layer electrically insulated contact grid between the absorber layer and the emitter layer and as a second contacting a flat contact layer is arranged on the side facing away from the absorber layer of the emitter layer, wherein the emitter layer consists of a semiconductor material which spans the pn junction passivating to the absorber layer with a maximum interface recombination rate of the excess charge carriers of 10 ⁵ recombinations / cm. A solar cell back-contacted in this way also displays a novel layer structure geometry, since it has a continuous emitter layer on the back of the absorber layer.

Absorber and emitter layer may preferably consist of silicon. In this case, a heterocontact solar cell can be formed if crystalline silicon, in particular with n- or p-type doping (n / p c-Si), for the absorber layer and amorphous, with hydrogen-enriched silicon, corresponding to p- or n-type doping (p / n a-Si: H) are used for the emitter layer. An optional buffer layer between absorber and emitter layer may also be preferably made of amorphous silicon, but undoped. Such a material system ensures a particularly well-passivated pn junction for charge separation. All contacting systems can in this case be made of aluminum at a back contact. For front-side contacting, the contact layer must be made of a transparent conductive material. To avoid repetition, reference is made to the specific description part with regard to further embodiments of the solar cell contacted on one side according to the invention.

embodiments

The method for producing a solar cell contacted on one side and such a solar cell itself according to the invention are explained in more detail below with reference to embodiments in the schematic, not to scale figures. Showing:

FIG. 1 shows the schematic flowchart of the method,

FIG. 2 shows a cross-sectional contact of a solar cell in cross-section, FIG. 3 shows a front-side contacted solar cell in cross-section and FIG. 4 shows a module interconnection of several solar cells contacted on one side in plan view.

The method for producing a single-sided contacted solar cell can equally be used to produce a front side as well as a rear side contact. In the following, the front side OSZ of the solar cell SZ is the side intended for the incidence of light in the later operation and the back side OSA of the solar cell SZ is the side not intended for the incidence of light in the later operation Solar cell SZ defined. The same with regard to the incidence of light applies to other components.

In the figure 1 is based on a schematic flow chart (solar cell shown in cross-section), the preparation of a back-contacted solar cell SZ explained. The production of a front-contacted solar cell takes place in an analogous manner. The production of a solar cell SZ with an absorber layer AS of crystalline silicon with p-type doping (p c-Si), an emitter layer ES of amorphous, hydrogen-enriched silicon with n-type doping (n a-Si: H) and aluminum for the contact grid KG and the contact layer KS is shown by way of example. In such a choice of material, by which the solar cell SZ is formed as a heterocontact solar cell HKS (see FIG. 2), the aluminum / silicon contact can form a local charge carrier backscattering region BSF with highly p-doped silicon in a known manner by brief annealing, so that the recombination at the contact lattice KG can be minimized (alneal process).

Process step I Selection and provision of a suitable absorber layer AS. This may be a silicon wafer, but also a thin silicon film grown by thin-film technology. It may preferably be crystalline silicon in p-type doping (p c-Si). The subsequent incidence of light in the front side OSZ of the absorber layer AS, which faces the light, is shown by arrows, these are shown in Figure 1 below the absorber layer AS. The front side OSZ of the absorber layer can be textured if necessary to improve the light coupling. In the case of a wafer-based solar cell, this also applies to the backside OSA of the absorber layer. Process step A

Passivation of the front side OSZ of the absorber layer AS with a cover layer DS of silicon oxide or silicon nitride according to a standard method. In this case, the cover layer DS can have a dual function since, in addition to the passivation (passivation layer PAS), it also reduces the reflection (antireflection layer ARS) of the incident light. The application of two or more cover layers DS with separate functions is also possible.

Process step II

Applying a contact grid KG made of aluminum on the back OSA of the absorber layer AS. The contact grid KG can be applied by thermal evaporation through a mask, by simple screen printing, by inkjet printing or by photolithography.

Process step F

Annealing (indicated in the figure 1 by vertically serpentine arrows) of the aluminum of the contact grid KG in the absorber layer AS (alneal process). As a result, local charge carrier backscattering regions BSF form below the AI contact grid KG. If appropriate, the method step A can also be interchanged with the method steps II and F to produce the BSF. Although process step F is an option, the efficiency of the solar cell SZ can be increased even further. The method step F can be carried out in a common heating process with method step D.

Process step III

Generating an electrically non-conductive insulating layer IS on the contact grid KG on its entire free surface. In this case, the insulation layer IS must have at least such a layer thickness that the tunneling of charge carriers is reliably prevented. Through this isolation measure is a reliable insulation of the two contacting systems against each other guaranteed. The insulating layer IS can be produced in a simple manner by applying an insulating compound, for example by screen or inkjet printing, masking, sputtering, vapor deposition or photolithography. Alternatively, however, it is also possible to produce an insulating oxide layer OX according to method step D (aluminum oxide Al ₂ O ₃ and silicon oxide SiO ₂ ) in which the silicon oxide on the absorber layer AS according to method step E is subsequently selectively removed again on the absorber layer AS.

Process step D

Oxidation of the surface of the AI contact grid KG, for example by annealing in an oxygen atmosphere (indicated in Figure 1 by vertically serpentine arrows). This results in an approximately 30 nm thick alumina Al ₂ O ₃ and an approximately 5 nm thick silica SiO ₂ , at least if the method step F is not performed. A wet or electrochemical growth of the oxide layer OX is also possible.

Process step E Selective etching of the silicon oxide (indicated in FIG. 1 by small upward-pointing arrows) in the region of the absorber layer AS, for example by immersion in dilute hydrofluoric acid (HF dip). Hydrofluoric acid strongly etches silicon oxide, but alumina hardly, so that when immersing only selectively, the silicon oxide is removed. If step F is omitted, the silica is also thinner than the aluminum oxide. If silicon nitride is used as passivating top layer DS, then this is also inert to the etching with hydrofluoric acid. If, on the other hand, thermal silicon oxide is used as the cover layer DS, then the etching rate must be chosen such that the silicon oxide on the front side OSZ of the absorber layer AS (about 200 nm) is still retained, but the silicon oxide on the back side OSA of the absorber layer AS (FIG. approx. 5 nm) is completely removed. By the HF dip Not only is the backside silicon oxide removed, but also the silicon surface is passivated well over forming Si-H bonds.

Process step G Optional, full-surface deposition of an ultrathin buffer layer PS, in the selected embodiment intrinsic, hydrogenated, amorphous silicon i a-Si: H, for example by plasma-assisted gas phase deposition (PECVD). The buffer layer PS serves to passivate the interface (pn junction) between the absorber layer AS and the emitter layer ES, thereby reducing recombination. For this purpose, it can be applied in the lowest possible layer thickness, for example 5 nm.

Process step IV

Whole-area deposition of the thin emitter layer ES, for example, by plasma-assisted gas-phase deposition (PECVD) of a thin-film emitter of n-doped, hydrogenated amorphous silicon, n a-Si: H. Deposition by sputtering or thermal evaporation is also possible. Since the thin emitter layer ES (at least about 5 nm in order to be able to span a pn junction) is located thicker on the back side OSA of the absorber layer AS when a rear side contact is formed, it can be deposited thicker without appreciable recombination losses (eg nm instead of 5 nm), thus ensuring complete coverage of the absorber layer AS despite the comparatively large dimensions of the contact grid KG (about 1 μm high). These layer thickness ratios result in an interruption of the emitter layer ES in the region of the contact grid KG. However, this has no influence on the functioning of the solar cell SZ. In the figures, a complete coverage of the contact grid KG is represented by the emitter layer ES, the emitter layer ES is therefore thicker than the contact grid KG and the insulation layer IS together. To the absorber layer AS spans the

Emitter layer ES on a charge carrier separating pn junction. there the emitter layer ES has such a maximum layer thickness that the charge carriers can reach the absorber-layer-facing side OSE of the emitter layer ES without significant ohmic losses.

Process step V

Applying the second contacting system in the form of a sheet-like contact layer KS on the backside of the emitter layer ES facing away from the absorber layer. For example, a full-area metallic contacting by thermal evaporation of aluminum can take place.

Process step VI

Contacting the contact grid KG and the contact layer KS. Due to the free accessibility of the contact layer KS, it can easily be electrically contacted at any desired location. The contact grid KG can be contacted directly by a small area above the contact grid KG is recessed when depositing the emitter layer ES according to method step IV and the vapor deposition of the rear contact layer KS by applying a mask. In this area, the insulating layer IS is then removed (e.g., by mechanical destruction such as scratching of the 30 nm thin alumina layer) so that an electric supply can be supplied to the contact grid KG.

Alternatively, the contact grid KG may have a comb-like web ST on the outside of the solar cell, which is half covered in the generation of the emitter layer ES and the contact layer KS. This web ST can then be electrically contacted after removal of the insulating layer IS (alternatively shown in FIG. 1 in process step VI on the right). Process step H

Optionally, a further method step H may be provided after method step IV: cleaning of the surface of the absorber layer AS not covered by the contact grid KG. In practice, however, the surface of the absorber layer AS should always be cleaned or exposed shortly before the a-Si: H deposition of the emitter layer ES (short HF dip) in order to achieve good interfacial passivation of the absorber layer AS immediately before the emitter deposition and thus a good Efficiency of HeteroSolarzelle HKS to ensure. The HF dip then removes either the natural silicon oxide, which is always present for more than 30 minutes, or else the thermal / electrochemical silicon oxide formed by the insulation process of the contact grid KG.

FIG. 2 shows the finished processed solar cell SZ in cross section (transversely to the contact fingers of the contact grid KG) (incidence of light represented by parallel arrows) with back contact. On the front side OSZ of the absorber layer AS, the double-functional cover layer DS is arranged. The contact grid KG for collecting the majority charge carriers from the absorber layer AS is located on the rear side OSA of the absorber layer AS. Below the contact grid KG, charge carrier backscattering fields BSF are formed in the absorber layer AS, which reduce the recombination losses. The contact grid KG is covered with an electrical insulation layer IS, so that no short circuit to the following full-surface emitter layer ES can occur. Between the absorber layer AS and the emitter layer ES, a buffer layer PS may optionally be arranged. Between the emitter layer ES and the absorber layer AS, the charge carrier separating pn junction forms, the minority charge carriers of the absorber layer are driven into the emitter layer. The contact layer KS for collecting the charge carriers from the emitter layer ES is applied over the entire surface of the emitter layer ES. The example metallic contact layer KS also serves as a reflector layer RS for the unabsorbed photons. The electrical contact (voltage V) takes place between the freely accessible contact layer KS and an exposed location of the contact grid KG.

FIG. 3 shows a fully processed solar cell SZ in cross-section (transverse to the contact fingers of the contact grid KG) (incidence of light represented by parallel arrows) with a front-side contact. On the front side OSZ of the absorber layer AS, the planar contact layer KS is formed in the form of a transparent, conductive oxide layer TCO. In contrast, in the case of the front side contacted solar cell SZ according to FIG. 3, an optional metallic contact element KE is arranged on the front side OSK of the transparent contact layer KS. This improves the collection and dissipation of the charge carriers, since a transparent conductive oxide layer as a contact layer KS depending on their layer thickness in their electrical conductivity is not as effective as a metallic contact layer KS. By using a contact element KE, it is possible to reduce the layer thickness of the TCO contact layer KS which is necessary for current dissipation. To minimize shading, the contact element KE is constructed and arranged congruent to the contact grid KG. It can for example consist of chromium / silver and is contacted electrically together with the contact layer KS.

Furthermore, in contrast to the backside contacted solar cell SZ, a passivation layer PAS on the back side OSA of the absorber layer AS in the front side contacted solar cell SZ is not required if the material of the absorber layer AS has a lower electronic quality. On the back side OSA of the absorber layer AS, it is therefore possible to provide as backing layer DS, for example, a seed layer SS for optimized deposition of the absorber layer AS. The seed layer SS may have previously been applied to a substrate SU. In addition, in addition to a passivation layer PAS or a seed layer SS, an additional reflection layer RS reflecting the unabsorbed photons can also be provided as the cover layer DS.

FIG. 4 shows an exemplary interconnection of a plurality of single-contact solar cells SZ in a common solar cell module SZM (simplified in the rear side facing away from the subsequent incidence of light when contacting on the back, the entire contact layer KS lies over the contact grid KG, but the web ST of the contact grid KG is not covered by the contact layer KS). The proposed concept of the unilaterally contacted solar cell SZ namely allows a technologically very simple series / parallel connection of the individual solar cells SZ to a solar cell module SZM. This interconnection is particularly useful in the case of the use of crystalline silicon wafers as absorber layer AS for the solar cell SZ, since the process of series and parallel connection can be considerably simplified by the one-sided back contact. If the web ST of the contact grid KG (compare Figure 1, alternative in step VI) placed on the edge of a square c-Si wafer as the absorber layer AS, then the emitter layer ES covering contact layer KS and the web ST of the contact grid KG to simple Way by direct contacting KT, eg with the help of a copper tape KB, in series SV or in parallel PV are interconnected.

The interconnection shown in FIG. 4 is an interconnection with solar cells SZ contacted on one side at the back. With an interconnection of solar cells SZ contacted on one side on the front side, there is a small difference in the structure due to the fact that the webs ST of the contact grid KG and of the contact element KE do not lie one above the other, but face each other. Other interconnection methods, in particular adapted to thin-film technology, are naturally also possible. LIST OF REFERENCE NUMBERS

ARS antireflection coating

AS absorber layer BSF charge carrier backscattering field

DS topcoat

ES emitter layer

HKS heterocontact solar cell

IS electrically non-conductive insulation layer KB copper tape

KE contact element

KG contact grid

KS contact layer

KT contacting OSA back side of AS

OSE absorber layer facing away from ES

OSK front side of KS

OSZ front side of AS

OX electrically insulating oxide layer PAS passivation layer pn pn junction

PS buffer layer

PV parallel connection

RS reflection layer SS seed layer

ST footbridge

SU substrate

SV series connection

SZM solar cell module SZ solar cell

TCO transparent conductive oxide layer

V voltage

Claims

claims

1. A method for producing a unilaterally contacted solar cell having at least one absorber layer and a surface deposited emitter layer of semiconductor materials with opposite p- or n-type doping, wherein excess majority and minority carriers in the absorber layer generated by incidence of light, at the pn junction between Separate absorber and emitter layer and the majority charge carriers of the absorber layer via a first contacting system and the minority carriers of the absorber layer of the emitter layer and a second contacting system are collected and derived, both contacting systems are on the same solar cell side, characterized by the method steps:

I. providing an unstructured absorber layer (AS), II. Applying the first contacting system in the form of a contact grid (KG) surface-optimized with respect to the collection of the majority charge carrier on one side of the absorber layer (AS),

III. Generation of a charge carrier tunnel preventing, electrically non-conductive insulating layer (IS) on the contact grid (KG) on its entire free surface,

IV. Depositing the emitter layer (ES) in a layer thickness through which minority carriers can reach the absorber layer side facing away from the emitter layer (ES) without significant ohmic losses, and of a semiconductor material to the absorber layer (AS) with a maximum interfacial

Recombination rate of the excess charge carriers of 10 ⁵ recombinations / cm ² s passivating pn junction (pn) spans,

V. Applying the second contacting system in the form of a laminar contact layer (KS) on the absorber layer side facing away from (OSE) the emitter layer (ES) and

VI. electrically contacting (V) the contact grid (KG) and the contact layer (KS).

2. The method according to claim 1, characterized in that the method step Il performed on the back (OSA) of the absorber layer (AS) and a further method step A according to method step I, IV or V is provided:

A. Creating a transparent cover layer (DS) on the front side (OSZ) of the absorber layer (AS).

3. The method according to claim 2, characterized in that the transparent cover layer (DS) as a passivation layer (PAS) and as an antireflective layer (ARS) is formed.

4. The method according to claim 1, characterized in that the method step II carried out on the front side (OSZ) of the absorber layer (AS) transparent embodiment of the contact layer (KS) and depending on the electronic quality of the absorber layer, a further step B before or after method step I is provided: B. generating one or more outer layers (DS) on the back (OSA) of the absorber layer (AS).

5. The method according to claim 4, characterized in that the cover layer (DS) as a passivation layer (PAS) or as

Reflection layer (RS) or as a seed layer (SS) is formed.

6. The method according to claim 4 or 5, characterized in that a further method step C is provided after the method step V: C. Application of a contact element (KE) designed congruently to the contact grid (KG) and arranged on the front side (OSK) of the transparent contact layer (KS), wherein in step VI the contact element (KE) is electrically contacted together with the contact layer (KS).

7. The method according to any one of claims 1 to 6, characterized in that the method step II is carried out by selectively applying an electrically conductive material by thermal evaporation with the aid of a mask, by screen printing, by ink jet printing or by photolithography.

8. The method according to any one of claims 1 to 7, characterized in that the process step III by selectively applying an electrically insulating material on the contact grid (KG) on its entire free surface by thermal evaporation, sputtering or vapor deposition with the aid of a mask, by screen printing , by inkjet printing or by photolithography.

9. The method according to any one of claims 1 to 7, characterized in that the process step III by thermal, by wet-chemical or by electrochemical growth of an oxide layer (OX, step D) on the contact grid (KG) and the contact grid (KG) uncovered

Positions on the absorber layer (AS) and subsequent selective etching of the oxide layer (OX, step E) takes place on the contact grid (KG) uncovered points on the absorber layer (AS).

10. The method according to any one of claims 1 to 9, characterized in that the method step VI is carried out by eliminating a connection region on the contact grid (KG) during the deposition of the emitter layer (ES) in step V and exposing the connection region by removing the insulation layer (IS) produced according to method step IV.

11. The method according to any one of claims 1 to 10, characterized in that the method step VI is carried out by thermal evaporation, sputtering or vapor deposition.

12. The method according to any one of claims 1 to 11, characterized in that a further method step F is provided after the method step II: F. tempering the conductive material of the contact grid (KG) in the absorber layer (AS).

13. The method according to claim 10 and 12, characterized in that the further method step F is carried out together with the method step III for the thermal growth of an oxide layer (OX).

14. The method according to any one of claims 1 to 13, characterized in that a further method step G is provided before the process step IV: G. deposition of a buffer layer (PS) in a small layer thickness.

15. The method according to claim 14, characterized in that the further method step G is carried out by thermal evaporation, sputtering or vapor deposition.

16. The method according to any one of claims 1 to 15, characterized in that a further method step H is provided after the method step IV: H. cleaning the non-contact grid (KG) covered surface of the absorber layer (AS)

17. The method according to any one of claims 1 to 16, characterized in that mono-, multi- or polycrystalline or recrystallized silicon for the absorber layer (AS), amorphous, hydrogenated silicon for the

Buffer layer (PS) and the emitter layer (ES), aluminum for the contact grid (KG), and aluminum for the contact layer (KS) when performing the process step Il on the back (OSA) of the absorber layer (AS) or a transparent conductive oxide (TCO ) is used in a performance of the method step II on the front side (OSZ) of the absorber layer (AS).

18. The method according to any one of claims 1 to 17, characterized in that the absorber layer AS is designed as a wafer or as a thin film on a substrate (SU) or superstrate, wherein in the thin-film technology, the process steps for depositing the layers sequentially, starting at the substrate (SU) or superstrate.

19. One-side contacted solar cell with at least one absorber layer and a surface-deposited emitter layer of semiconductor materials with opposite p- or n-type doping, wherein excess majority and minority carriers in the absorber layer generated by light, at the pn junction between the absorber and emitter layer separated and the majority carriers from the absorber layer of a first contacting system and the minority carriers from the Absorber layer are collected and derived from the emitter layer and a second Kontaktierungs- system, both Kontaktierungssysteme are located on the same solar cell side, characterized in that as a first contacting system with respect to the collection of majority charge carriers area optimized and compared to the emitter layer (ES) by means of a Charge carrier tunneling insulating layer (IS) electrically insulated contact grid (KG) between the unstructured absorber layer (AS) and the emitter layer (ES) and as a second contacting a flat contact layer (KS) on the absorber layer side facing away (OSE) of the emitter layer (ES) is arranged, wherein the emitter layer (ES) consists of a semiconductor material which spans to the absorber layer (AS) a pn junction (pn) passivating at a maximum interface recombination rate of the excess charge carriers of 10 ⁵ recombinations / cm ² s.

20. Single-sided contacted solar cell according to claim 19, characterized in that the contact grid (KG) on the back (OSA) of the absorber layer (AS) and a transparent cover layer (DS) on the front side (OSZ) of the absorber layer (AS) is arranged.

21. Single-sided contacted solar cell according to claim 20, characterized in that the transparent cover layer (DS) is formed as a passivation layer (PAS) and as an antireflection layer (ARS).

22 unilaterally contacted solar cell according to claim 19, characterized in that the contact grid (KG) on the front side (OSZ) of the absorber layer (AS) is arranged, that the contact layer (KS) is transparent and in that, depending on the electronic quality of the absorber layer (AS), a cover layer (DS) is arranged on the back side (OSA) of the absorber layer (AS).

23 unilaterally contacted solar cell according to claim 21, characterized in that on the front side (OSK) of the transparent contact layer (KS) to a contact grid (KG) congruent constructed and arranged contact element (KE) is provided.

24. Single-sided contacted solar cell according to claim 22 or 23, characterized in that the cover layer (DS) as a passivation layer (PAS) or as a reflection layer (RS) or as a seed layer (SS) is formed.

25 unilaterally contacted solar cell according to one of claims 19 to 24, characterized in that on one edge side of the solar cell (SZ) a web (ST) for electrical contacting of the contact grid (KG) is arranged.

26 unilaterally contacted solar cell according to claim 25, characterized in that the web (ST) for electrical series or parallel connection (SV, PV) of a plurality of solar cells (SZ) in a solar cell module (SZM) is formed.

27 unilaterally contacted solar cell according to one of claims 19 to 24, characterized in that an opening in the emitter layer (ES) and the insulating layer layer (IS) for electrical contacting of the contact grid (KG) is arranged.

28 unilaterally contacted solar cell according to one of claims 19 to 27, characterized in that below the contact grid (KG) a minority charge carrier backscattering surface field (BSF) is arranged.

29 unilaterally contacted solar cell according to one of claims 19 to 28, characterized in that between the absorber layer (AS) and the emitter layer (ES) a buffer layer (PS) is arranged with a small layer thickness.

30 unilaterally contacted solar cell according to one of claims 19 to 29, characterized in that the absorber layer (AS) is formed as a wafer or as a thin film on a substrate (SU) or a superstrate.

31 unilaterally contacted solar cell according to one of claims 18 to 30, characterized in that the following materials are used: absorber layer (AS): crystalline (mono-, multi- or polycrystalline or recrystallized) silicon with n- or p-type doping, emitter layer (ES): amorphous silicon with hydrogen enrichment and with p- or n-type doping opposite to the absorber layer (AS),

Buffer layer (PS): amorphous silicon with hydrogen enrichment without

doping,

Insulation layer (IS): aluminum oxide, cover layer (DS): silicon oxide or silicon nitride,

Contact grid (KG): aluminum,

Contact layer (KS): aluminum or transparent conductive oxide (TCO)

Contact element (KE): chrome or silver.