DE102011052916B4 - Solar cell manufacturing process and wafer solar cell - Google Patents

Abstract

Solar cell manufacturing method comprising the following method steps: - providing an n-type silicon wafer (1); - generating a doping gradient from thermal donors in the silicon wafer (1); - generating a photovoltaically active transition (13) for the solar cell; and contacting the solar cell by producing an emitter contact (32) and a base contact (31), wherein hydrogen diffuses into the wafer to form the thermal donors during the deposition of an antireflection layer (6) from a hydrogen-rich deposition plasma.

Description

The invention relates to a solar cell manufacturing method and a wafer solar cell.

In order to reduce recombination losses on the surfaces of wafer solar cells and thereby increase the efficiency of the solar cells, a so-called backside field is currently being produced on the p-type silicon-based solar cells with emitter diffused in from the front, that is to say on the light-facing side of the solar cell. For this purpose, a metal paste (ie a metal-containing paste), often an aluminum-containing paste, is usually applied to the rear surface of the semiconductor. In a so-called fire step, the wafer is then subjected to a heat treatment, as a result of which aluminum diffuses into the back of the cell and the doping concentration is increased in a surface area compared to the basic doping concentration. This surface area is referred to as the back surface field or back surface field (BSF). The metal paste also forms the back metallization during the fire step.

Alternatively, in the case of solar cells with a rear emitter, a so-called front field can be generated on the front side, that is to say on the light incidence side. Such a front side field, also called front surface field (FSF), is generated in n-type-based solar cells, for example by means of front-side diffusion of dopants such as phosphorus.

Both the rear side field and the front side field serve to reduce the probability of recombination on the respective surface by increasing the doping concentration and the associated reduction in minority charge carriers. At the same time, high doping concentrations of the back / front fields also act as recombination centers, which increase the recombination losses in the highly doped semiconductor volume. In addition, the probability of recombination of the minorities still present on the highly doped semiconductor surface increases in comparison to less doped surfaces.

In contrast to surface passivating measures that saturate free silicon bonds (which act as recombination centers), such BSF or FSF are particularly stable against external influences such as e.g. UV radiation and can thus ensure the passivation effect in the long term. In addition, the lateral series resistance and the associated losses are reduced on the respective surface. However, BSF and FSF have the disadvantage that the reduction of the lateral series resistance can only be achieved to a depth of a few micrometers, since the depth of penetration of the rear or front field is limited by the low diffusion speeds of the dopants and the resulting long duration of the manufacturing processes. The diffusion times can be reduced by raising the temperature, but at the same time this increases the risk of defects in the substrate material.

Within highly doped areas of the BSF or FSF, the minority charge carriers drift out of the BSF or FSF into the base material along the doping gradient.

It is known to generate a pn junction using a gradient made of thermal donors in p-type silicon. The production of a solar cell with such a gradient is described, for example, in the publication “Increased Efficiencies in a-Si: H (n) / Cz-Si (p) Heterojunction Solar Cells due to Gradient Doping by Thermal Donors”, M.L.D. Scherff et al., 21st European Photovoltaic Solar Energy Conference, 4-8 September 2006, Dresden, Germany. It explains how a predoped p-type semiconductor wafer is treated on the front side with a hydrogen plasma and then annealed. As a result of the oxygen impurities present in the semiconductor, so-called thermal donors are produced in the semiconductor to a depth of approximately 175 μm. Because of the only one-sided plasma treatment, a density distribution of the thermal donors decreases with the depth from the front of the wafer, which is referred to below as gradient, doping gradient or concentration gradient.

As the name suggests, the thermal donors are donors, which result in an n-type conduction of the semiconductor. The gradient of thermal donors is set so that due to the high donor density on the semiconductor surface, a conduction type inversion and thus a pn junction occurs, which serves as a photovoltaically active junction of the solar cell. An isotropic heterojunction is then generated on the front and the finished wafer solar cell is contacted on both sides. For this purpose, amorphous n-type silicon is deposited on the n-type silicon produced by the thermal donors.

If a charge carrier is in a doping gradient, then in addition to the charge carrier diffusion, a charge carrier drift takes place along the doping gradient. As a result, larger charge carrier diffusion lengths can be achieved than without a gradient.

This known prior art has the disadvantage that due to the gradient of the thermal Donors both the photovoltaically active transition and the drift field in the semiconductor volume can be determined. In this case, these two mechanisms cannot be controlled separately. There is, for example, the risk that a sufficient number of thermal donors will not be generated over the entire area in order to produce a pn junction over the entire area, which either additionally requires the amorphous n-type silicon used or short-circuits between the two p- and n- Type areas.

It is therefore an object of the invention to provide a solar cell production method and a wafer solar cell with which a larger parameter range for drift field and photovoltaically active transition can be found using a single technology.

US 7 943 416 B1 discloses a method for making local contacts in solar cell structures. Here, a surface passivation layer is first created on the surface of the solar cell. This passivation layer is locally etched away in a later process step in a plasma process. For this purpose, elastic mask foils with correspondingly arranged openings are stretched on the solar cell surface. According to a particular embodiment, the solar cell surface is treated with a hydrogen plasma through the openings thus produced in the passivation layer in order to generate thermal donors in the form of doping gradients in the semiconductor material.

The object is achieved according to the invention by a solar cell production method having the features of claim 1 and by a wafer solar cell having the features of claim 8. Advantageous developments of the invention are listed in the subclaims.

The invention is based on the consideration of using intrinsically present oxygen in an n-type silicon wafer in order to generate a gradient from thermal donors starting from a wafer surface. For this purpose, atomic hydrogen is introduced into the wafer surface. This can be done by means of an implantation, by means of a hydrogen diffusion source or by means of a plasma treatment with hydrogen plasma and, if appropriate, a simultaneous and / or subsequent heat treatment in all methods. With this procedure it is achieved to increase the charge carrier diffusion length in the direction of the gradient and / or the transverse conductivity in the more highly doped regions produced by the thermal donors in drift fields which were generated with the aid of thermal donors. In this case, the thermal donors can be produced in the silicon wafer from a wafer surface on the front side or on the light incidence side, that is to say a wafer surface which faces the incident light in the later solar cell, or from a wafer surface facing away from the front wafer surface.

In the heat treatment, also referred to as tempering, the semiconductor wafer is heated to a suitable temperature, for example in a temperature range between 300 and 900 ° C, better between 350 ° C and 500 ° C, preferably between 400 ° C and 450 ° C.

The thermal donors are structures in the semiconductor crystal lattice, which are formed from the intrinsically present oxygen under the influence of hydrogen. The hydrogen acts as a catalyst for the generation of the thermal donors.

There are a number of thermal donors in silicon that are produced at different temperatures and destroyed again at higher temperatures. The most relevant forms of thermal donors for practical use arise at an annealing temperature of around 400 ° C and become inactive again after subsequent heating to 550 ° C and more. The latter temperature can therefore be regarded as a critical temperature for this type of thermal donor. However, short heating-up times above this temperature are not necessarily harmful, since if the temperature is exceeded, all thermal donors are not automatically destroyed, but the destruction takes place successively over time.

Process steps in which the semiconductor wafer remains below this critical temperature are referred to below as low-temperature steps. In contrast, process steps in which the semiconductor wafer exceeds the critical temperature are referred to as high-temperature steps. The critical temperature between low and high temperature is therefore also dependent on the above parameters of the manufacturing process of the thermal donors. Gradients from thermal donors with a critical temperature of approximately 550 ° C. or 600 ° C. are of particular practical relevance for wafer solar cells. The temperatures mentioned are a mean temperature value distributed over the silicon wafer or over the respective wafer surface. For example, in the production of through-holes through the wafer, in ablation or doping processes using laser radiation in the production of emitter wrap-through solar cells (EWT solar cells), in the removal of surface coatings or in local doping processes such as laser doping or laser chemical processing (Laser Chemical Processing, short LCP) locally much higher temperatures arise. Nevertheless, this is often also a low-temperature step, since on average the wafer is hardly warmed up.

The entire solar cell manufacturing process includes, in addition to the generation of a photovoltaically active transition for the solar cell, the generation of the doping gradient from thermal donors in the n-type semiconductor wafer. It should be noted here that the active transition can be generated in a separate process step or locally separately from the gradient of thermal donors and not because of or by means of the generation of the gradient of the thermal donors. In other words, either the photovoltaically active transition is generated after the thermal donors have already been formed, or the doping gradient from thermal donors is formed in the semiconductor following the generation of the active transition. In certain embodiments, however, partial steps for generating the thermal gradient on the one hand and for generating the active transition on the other hand can at least partially overlap in time.

However, the generation of the gradient of thermal donors is in any case not necessary for the active transition to occur, as is the case with the inventor's publication “Increased Efficiencies ...” described in the introduction. The reverse dependency (during the generation of the pn junction, the gradient of thermal donors, or a part thereof) is conceivable if, during the formation of the pn junction or generally a photovoltaically active transition, which, for example, also a metal insulator Semiconductor contact (MIS contact) can be activated during a heating step, which at this moment only serves as an annealing process for the formation of the MIS contact, previously introduced hydrogen.

Finally, the solar cell produced in this way must be contacted at the emitter and base. Here, only rear-contacted solar cells are sensible, in which both the emitter contact and the base contact are formed on the back of the solar cell, as well as solar cells contacted on both sides, in which the emitter contact is formed on the front side, i.e. on the light incidence side, and the base contact on the back, or vice versa. In the case of bifacial solar cells, that is to say solar cells which can also convert light incident on the rear side into electrical energy, one speaks instead of a rear side facing away from the light from a rear side facing away from the sun. Further process steps can be integrated in this manufacturing process, for example etching processes, texturing or the deposition of passivation layers, of anti-reflection layers and the like.

Since thermal donors, as explained above, can be destroyed or become inactive when the wafer is subjected to a high-temperature step, in embodiments in which such high-temperature steps are provided, the gradient of thermal donors should be generated after all high-temperature steps have been carried out, so that no further high-temperature steps consequences. In this case, after the gradient of thermal donors has been established, only low-temperature steps are carried out, for example sputtering, plasma deposition, galvanic deposition processes and the like. This means that after the doping gradient has been generated from thermal donors, the temperature in the region of the semiconductor wafer in which the thermal donors are located essentially remains below the critical temperature explained above at every point. Exceptions can be laser processes and short tempering processes, as mentioned above. Laser processes only heat the wafer for a very short time and locally well above the critical temperature. Processes such as laser chemical processing, in which the laser beam is guided in a liquid jet, are particularly suitable for this due to the local cooling by the liquid. Another example of tempering above the critical temperature is the firing of the metal contacts of standard solar cells, in which the cell only exceeds the critical temperature for about a minute and only warms up to 600 ° C for a few seconds.

The use of plasma deposition after establishing the gradient of thermal donors is a preferred embodiment, for example in the manufacture of solar cells comprising a heterojunction with an intrinsic thin layer (so-called HIT solar cells, HIT - heterojunction with intrinsic thin layer).

As explained above, in certain embodiments, process steps which are necessary for the formation of the gradient of thermal donors can at least overlap with process steps which are intended for the formation of the photovoltaically active transition. However, this generally also applies to process steps of other processes in the manufacture of the solar cell. For example, the increase in the temperature of the wafer during the application of a nitride can also be used as an antireflection layer for the tempering step to produce the thermal donors. Furthermore, a functional layer such as such an anti-reflection layer can serve as a source or a partial source for the hydrogen to be introduced into the wafer.

The wafer solar cell produced using the method described here has an n-type base region with a base contact for collecting current from the base region, and a photovoltaically active transition.

The active transition can in particular be one of the following structures: a pn transition, in which an emitter is formed in the wafer by means of diffusion of dopants; a hyperabrupt transition in which there is an emitter produced by means of deposition, for example by means of epitaxy, which has the same band gap as the base (epi contact); a heterojunction, for example in the form of an amorphous silicon layer with hydrogen doping in contact with a crystalline silicon layer (abbreviated as a-Si: H / c-Si heterojunction) or in the form of a heterojunction with an intrinsic amorphous intermediate layer (also referred to as a HIT transition); or a metal-insulator-semiconductor junction, also referred to as an MIS structure, which stands for “metal insulator semiconductor”.

The pn junction does not necessarily extend over the entire surface of a wafer surface and can also be configured locally differently. This not only relates to solar cells with rear emitter on the rear side in which the emitter and base are contacted on the rear side, but also solar cells contacted on both sides with local emitter regions and / or local contacting, which is then carried out, for example, by a dielectric coating. The contacting of the base can also be carried out locally and / or have base regions which are covered by a dielectric layer.

The wafer solar cell also has an emitter contact, which is formed for example in the MIS transition by the metal of the transition itself. In addition, the base region has, at least in regions, a doping gradient or concentration or density gradient made of thermal donors. This gradient generates a drift field in the respective areas, which accelerates the charge carriers in the semiconductor volume in the direction of the gradient and thereby has a positive influence on the functioning of the solar cell. Additional layers can also be provided in order to improve or optimize the function of the solar cell. Examples include an anti-reflection layer, a passivation layer and the like.

The gradient of thermal donors should extend over at least part of the wafer thickness. Depending on whether the emitter is located on the front or rear, it thereby takes over the functions of a BSF or an FSF and / or the function of a drift field for extending the charge carrier diffusion length in the direction of the gradient. In other words, the gradient should extend at least to a certain depth of penetration, for example of 5 μm, into the wafer. According to an expedient embodiment, however, it is provided that the thermal donors are produced essentially to depths of 10 μm to 100 μm, but at most up to half the thickness of the semiconductor wafer, and thereby primarily assume the function of an FSF or BSF. According to a further expedient embodiment, however, it is provided that the thermal donors are produced essentially over an entire wafer thickness of the semiconductor wafer. This means that the gradient of thermal donors penetrates more than half the wafer thickness, meaning that there is a significant donor concentration of the thermal donors not only on the front side of the wafer, but also deep in the volume of the n-type semiconductor wafer, beyond the center of the wafer. After completion of the solar cell, this feature means that the distance between a significant concentration of thermal donors is less than half the wafer thickness, ideally zero.

It is preferably provided that the thermal donors are produced essentially along the entire wafer surface of the semiconductor wafer. In this case, essentially means that edge regions or transition regions to certain structures on the semiconductor wafer may have no thermal donors over the entire wafer thickness. It may well happen that the depth of penetration of the gradient of thermal donors into the wafer varies along the wafer surface.

In an alternative embodiment to this, it is provided that the thermal donors are generated in regions along the wafer surface of the semiconductor wafer. Areas along the wafer surface which have no thermal donors can be protected by covering the wafer surface, for example by means of a mask or by means of protective layers applied. In other cases, a locally generated plasma or local annealing, for example by means of laser radiation, can also ensure that the thermal donors only occur in these areas.

In an advantageous embodiment, it is provided that the doping gradient is generated from thermal donors before the photovoltaically active transition is generated. However, it is also possible and partly advantageous to choose the reverse order and first the photovoltaically active transition and only then the doping gradient from thermal donors in the To produce semiconductor wafers. If the generation of the active transition comprises the application of an additional layer, for example in the formation of a heterojunction, the gradient of thermal donors can extend to this additional layer if a subsequent gradient is formed, if this is provided.

The step of generating the doping gradient from thermal donors advantageously comprises a hydrogen-introducing process. Here, the hydrogen can be introduced into the wafer by means of implantation and / or from a diffusion source. According to a preferred development, however, it is provided that the step of generating the doping gradient from thermal donors comprises a hydrogen plasma treatment of a front and / or a rear side of the semiconductor wafer and a heat treatment of the semiconductor wafer at the same time or after the hydrogen plasma treatment. The heat treatment can also be at least in part the process temperature for a further process, for example for a deposition or doping process. It can also be a warm-up or cool-down phase of such a further process.

It is advantageously provided that the hydrogen plasma treated front side of the semiconductor wafer is subsequently freed from plasma damage and / or textured. This can be done for example by means of a wet chemical process or by means of a further plasma treatment. However, it may also be the case that no additional surface treatment is required.

According to a preferred embodiment, it is provided that the photovoltaically active transition is generated by creating an emitter layer in the semiconductor wafer by means of diffusion of dopant into the semiconductor wafer, by depositing an emitter layer on the semiconductor wafer and / or by using a dielectric and a metal layer Metal-insulator-semiconductor transition is generated on the semiconductor wafer. In one embodiment, the active transition can have a homojunction with the same bandgap on both sides of the transition, which is generated for example by means of epitaxy. In alternative embodiments, the active transition is a heterojunction with different band gaps, which can be produced, for example, by means of plasma processes.

It is preferably provided that the emitter contact is produced on the back of the semiconductor wafer. This can be a so-called back-junction solar cell, that is to say a solar cell in which the emitter and its contacts are arranged on the back of the wafer. In this case, the base contacts are formed on the front, preferably as an electrode finger. However, rear-side emitter contacts are present in particular in the case of rear-contact solar cells, that is to say in those solar cells in which emitter and base contacts are arranged on the rear of the solar cell. If such solar cells have no doping gradients, disadvantageous “shading effects” can occur. In these cases there is a so-called electrical shading, in which the probability of collection of the minorities decreases due to extended charge carrier paths above base contacts or the series resistance share of the majority above emitter contacts increases. Both loss mechanisms can be reduced by thermal donors and their gradients.

However, the gradient of thermal donors can alternatively also be used advantageously on solar cells with a front-side emitter and a rear-side base contact.

The silicon wafer can be cut from a silicon ingot (so-called Cz silicon) produced by means of a Czochralski process. Such silicon wafers are monocrystalline. This means that they have a crystal structure which is 99% or more of a single crystal orientation with correspondingly few grain boundaries. Alternatively, so-called quasi-monocrystalline wafers can also be suitable for the method described here. These are silicon wafers, which have fewer grain boundaries than polycrystalline wafers, but are also not considered to be monocrystals or single crystals. Quasi-monocrystalline wafers in the present sense have a structure in which about 90% to 95% or 90% to 99% of the wafer has a certain crystal orientation. Unlike Cz silicon, in which a single crystal is pulled out of the melt on a small seed crystal of about 1 cubic centimeter in size, quasi-monocrystalline ingots are currently mainly produced using processes in which polysilicon is used in a crucible (similar to that used to produce multisilicon) Most of it is melted in such a way that the monocrystalline silicon seed pieces also in the crucible do not melt and then the melt solidifies in a directed manner, starting with these seed pieces. Care is taken to ensure that the polycrystalline semiconductor pieces melt completely, while the monocrystalline semiconductor piece melts only on its surface, or that the crystallization takes place in such a way that mostly crystals with a single orientation grow. Then, starting from this monocrystalline semiconductor piece (ingot), the process is the same as for ingots made of multisilicon and quasi-monocrystalline wafers are formed therefrom.

• 1 eine Solarzelle mit Rückseitenübergang und Vorderseitenfeld;
• 2 eine Solarzelle mit Rückseitenübergang und Dotierprofil aus thermischen Donatoren;
• 3 eine rückseitenkontaktierte Solarzelle mit Vorderseitenfeld;
• 4 eine rückseitenkontaktierte Solarzelle mit Dotierprofil aus thermischen Donatoren im gesamten Basisbereich;
• 5 eine rückseitenkontaktierte Solarzelle mit Basisbereich, welcher bereichsweise Dotierprofil aus thermischen Donatoren aufweist;
• 6 eine doppelseitig kontaktierte Solarzelle mit Vorderseitenübergang und Dotierprofil aus thermischen Donatoren; und
• 7 ein Flussdiagramm zur Veranschaulichung eines Herstellungsverfahrens für die Solarzelle aus der 6 gemäß einer bevorzugten Ausführungsform.

The invention is explained below using exemplary embodiments with reference to the figures. Here show:

• 1 a solar cell with rear transition and front panel;
• 2nd a solar cell with rear transition and doping profile made of thermal donors;
• 3rd a rear-contacted solar cell with a front panel;
• 4th a rear-contacted solar cell with doping profile made of thermal donors in the entire base area;
• 5 a rear-side-contacted solar cell with a base region, which in some areas has a doping profile made of thermal donors;
• 6 a double-sided contacted solar cell with front transition and doping profile made of thermal donors; and
• 7 a flowchart to illustrate a manufacturing method for the solar cell from the 6 according to a preferred embodiment.

In the 1 a wafer solar cell is shown in a schematic cross-sectional view. It is made of a semiconductor wafer 1 formed at the front of a wafer 15 and a wafer back 16 are defined as the light incidence side or the light-facing side of a solar cell emerging from the semiconductor wafer. On the front of the wafer 15 points the semiconductor wafer 1 a texturing to increase the light trapping efficiency. The semiconductor wafer 1 itself is predoped to an n-type semiconductor and serves as the base region 12 for the solar cell. An emitter area 2nd is formed on the back in such a way that between it and the n-type conductive base region 12 a photovoltaically active transition 13 trains.

During the base area 12 with stripe or finger-shaped base contacts on the front 31 is electrically connected, a rear emitter contact is used 32 to collect the current from the emitter area 2nd , being between the emitter contact 32 and the emitter area 2nd an apertured dielectric 4th is arranged and allows electrical contact only at the openings. On the front is an anti-reflection layer 6 upset.

The one with the reference symbol 11 provided and by means of a dashed line from the rest of the base area 12 delimited area is a front field of the type described in the introduction, which is formed by means of diffusion of dopants. It is represented in 1 in other words, a known solar cell contacted on both sides with an emitter on the rear 2nd and a front-end doping 11 , the FSF.

A solar cell with a gradient consisting of thermal donors, however, is in the 2nd shown. Here too are the emitter contact 32 on the back and the base contact 31 formed on the front. Unlike the solar cell from the 1 is not, however, front-side doping 11 to form a front panel. Instead is the base area 12 now divided into an area without gradients from thermal donors 12f and one from the front of the wafer 15 out in the semiconductor wafer 1 extending base area with a gradient of thermal donors 12td . These two basic areas 12td , 12f are also shown here separated for illustration by a dashed line, but in reality it is difficult to measure a sharp dividing line, and the range can be defined as free of thermal donors if the donor density falls below a certain limit. Optionally, however, in addition to the area with thermal donors 12td also an area with a front-side doping 11 to be available.

A rear-side contact solar cell, in which both the base contact 31 as well as the emitter contact 32 on the back of the wafer 16 are formed in the 3rd shown. So that the base contacts 32 Finding space on the back is the emitter area 2nd structured or only in some areas on the back of the wafer 16 arranged. On the other areas of the back of the wafer 16 is a basic contact doping 14 provided for an improved electrical connection between the base area 12 and the base contacts 31 to build. This basic contact 31 can optionally also be omitted, or it can alternatively be designed as a hetero, MIS or epi contact.

Also in the 3rd shown known solar cell is, similar to the known embodiment from the 1 , by means of a front-end doping 11 , for example containing phosphorus, a front panel formed by a dashed line from the rest of the base area 12 is separated.

In contrast, in the 4th a similarly constructed solar cell is shown, but instead of the front side doping 11 in the entire base area 12 has thermal donors, so that this is a base region with gradients of thermal donors 12td acts. It should be noted here that the gradients are oriented in such a way that the minorities are accelerated in the direction of the contacted solar cell side. As with 2nd can optionally in addition to the area with thermal donors 12td also an area with a frontal doping 11 be present, and / or the base contact 31 can optionally be omitted or designed as a hetero, MIS, or epi contact.

Similar to the embodiment from FIG 2nd a boundary area can also be provided here, so that the base area is covered by gradients of thermal donors 12td from the front of the wafer 15 from only up to a certain wafer thickness in the semiconductor wafer 1 extends and below it a base area without such a gradient 12f is present. The base region preferably extends with gradients of thermal donors 12td however, essentially over the entire wafer thickness or over the entire thickness of the base region 12 . It should be noted in all the preceding and subsequent embodiments that the absence of the gradient of thermal donors can be adjusted both by the fact that the thermal donors are completely absent and by the fact that the distribution of the thermal gradients is homogeneous.

In the 5 Another embodiment of a rear-contacted solar cell is shown, in which the base region with gradients of thermal donors 12td does not extend substantially along the entire wafer, or in other words the entire base area 12 penetrates and fills, as is the case with the solar cell from the 4th the case is. Instead, the base area with deep gradient is thermal donors 12td to a sub-area of the base area 12 limited, which is from the back of the wafer 16 viewed from the emitter area if possible 2nd overlaps or, as in 5 shown, flat with the emitter area 2nd covered concurrently. Only in the rest of the base area 12f no gradient of thermal donors is provided. Above the base area 12f there may also be an area with thermal donors, but it does not extend as deep into the base material as in the area above the emitter area 2nd . The base area 12 of the silicon wafer 1 is composed of two areas, one area 12td with gradients with a large penetration depth above the emitter area 2nd and an area 12f without gradients or a gradient of low penetration depth, which is shown in the representation in 5 immediately above the base contacts 31 extends. Above the base contacts 31 these two areas overlap 12f , 12td .

It should be noted here that the representation is only to be understood schematically and the design of the boundaries between 12f and 12td is fluid.

Finally, in the 6 an embodiment of a solar cell shown, which has an emitter region on the front 2nd has and in which the emitter contacts 32 are thus also formed on the front. Although in this case on the front of the wafer 15 no texturing is shown, it can also be advantageous in this embodiment. As with the solar cell from the 4th is also here in the basic area 12td a gradient is formed from thermal donors, which completely fills the n-conducting base region, that is to say from the dielectric on the rear 4th up to the emitter area 2nd extends. In an embodiment not shown here, the gradient can also be in front of the emitter 2nd end up. In this case, an area 12f arise below the emitter.

Unlike in the 2nd , 4th and 5 is in 6 the orientation of the gradient is reversed. In the 2nd , 4th and 5 with the rear emitter, the highest concentration of thermal donors is on the light-facing wafer surface 15 before and takes in the direction of the emitter area 2nd with the depth of the wafer towards the back. In 6 the highest concentration of thermal donors is on the light-facing wafer surface 16 before and takes in the direction of the emitter area 2nd with the depth of the wafer towards the front.

An embodiment of a manufacturing process 100 , for example for the production of the solar cell from the 5 , is in the 7 illustrated using a flowchart and explained below.

On the silicon wafer provided 1 n-type silicon with texturing on the front is first made on the light-facing wafer front 15 a dopant in the surface of the semiconductor wafer 1 diffused 101 , for example phosphorus, so as to form a front field (FSF). This front field is not absolutely necessary and can accordingly be omitted. The function of the FSF, namely the reduction of the front-side recombination and the front-side sheet resistance, can alternatively take place through the thermal donors produced subsequently, which likewise produce an n-type doping, although not as highly doped, but all the more so.

Hydrogen is then introduced into the front of the wafer, for example by means of a hydrogen plasma 102 , deposited an anti-reflective layer 103 and annealed the sample 104 . The step 103 can also take up only part of the annealing time and the step 104 represent the remaining time required. These three steps 102 , 103 and 104 can also be done in one step 111 or in two steps 112 respectively. 113 be summarized. This is only an exemplary sequence, other process sequences are possible.

Will the steps 102 and 103 by one step 111 replaced, this means that the hydrogen to form the thermal donors, as provided according to the invention, diffuses into the wafer during the deposition of the antireflection layer from the hydrogen-rich deposition plasma of the antireflection layer. Alternatively, but insofar as not part of the invention, an anti-reflective layer can be formed 103 another hydrogen-rich deposition plasma as a hydrogen source in the crotch 103 serve, for example a plasma for the deposition of a passivation layer such as amorphous silicon or amorphous silicon oxide or aluminum oxide.

Will the steps 103 and 104 by one step 112 replaced, this means that the deposition of the anti-reflective layer on the wafer takes place simultaneously with the annealing step.

Will the steps 102 , 103 and 104 by one step 113 replaced, it means that the hydrogen to form the thermal donors during the deposition of the layer 103 diffuses into the wafer from the hydrogen-rich deposition plasma of the antireflection layer and that at the same time the deposition temperature corresponds to the annealing temperature for the formation of the thermal donors.

Then the back of the wafer is turned away from the light 16 of the silicon wafer 1 a photovoltaically active transition 2nd generated with a low temperature process.

Finally, the emitter and the base are contacted.

The basic mechanisms of low-temperature doping of silicon (known as Cz silicon) produced by means of a Czochralski process by means of hydrogen-assisted formation of thermal donors (TD) are described below.

By combining a one-sided hydrogen plasma treatment with an annealing process, very deep n-type doping profiles can be generated. It is known that doping gradients in the base material extend the charge carrier diffusion length in the direction of the gradient. In the past, deep doping gradients were generated by using rapidly diffusing lithium or by means of deposition by means of liquid phase epitaxy with variation of a doping gas concentration. Such deep doping gradients, which extend through the entire wafer thickness, can be generated with the aid of TD using a hydrogen plasma treatment and an annealing process. The process periods for generating these profiles are in the range of just a few minutes to an hour (diffusion processes reach depths of around 1 µm in comparable times at temperatures around 900 ° C.

Cz silicon contains high levels of oxygen as a contaminant, typically in the concentration range of 3 to 12 × 10 ¹⁸ cm ^-3 , which originate from the walls of the quartz crucible used for crystal production and, especially during the pulling process, in the upper area (top area) of the Crystal can be installed. This oxygen is usually present interstitially in the silicon and is then electrically inactive. The oxygen diffuses in the silicon through tempering processes and SiO ₄ defect complexes are formed which are electrically active (there are nine different stable defect species of the TD family). Depending on the annealing time, donor levels with ionization energies of 55-60 meV and 120-130 meV arise. At temperatures above 550 ° C, the TD dissolve again, because at these temperatures further oxygen accumulates and the complexes become electrically inactive again.

The formation of thermal donors is influenced by the carbon concentration, the charge carrier concentration and the hydrogen concentration in the semiconductor. For a carbon concentration above 5 × 10 ¹⁶ cm ^-3 , the formation of the TD is reduced, since then the complex formation between carbon and oxygen atoms is energetically more favorable than the formation of TD. From charge carrier concentrations above 10 ¹⁷ cm ^-3 , the formation of the TD is increased for p-type materials or reduced for n-type materials. In addition, atomic hydrogen accelerates the diffusion of oxygen in silicon and thereby supports the formation of the TD.

If a hydrogen atom comes close to an interstitial oxygen atom, this leads to a reduction in the activation energy of the oxygen diffusion, which increases the mobility of the oxygen in the silicon lattice. When the hydrogen is introduced, the increase in the mobility of the oxygen increases the likelihood that the oxygen atoms will meet suitable local defect areas in the silicon lattice and form TD there with existing oxygen atoms.

Hydrogen gradients can be generated in silicon wafers by means of one-sided hydrogen treatment of a silicon surface. A simultaneous or subsequent tempering at 300 ° C to 550 ° C then creates a gradient of thermal donors corresponding to the hydrogen concentration in the silicon. This TD gradient changes the originally homogeneous doping level in the volume of the substrate. This can lead to the redoping of p-doped Cz silicon to the n-type, so that deep pn junctions can be generated within minutes, for example at a depth of 100 µm after 20 minutes of annealing. With this n-doping, doping atoms (such as phosphorus) are diffused in, of course, but rather structural defects in the form of TD are generated in the silicon, which behave like diffused donors. The hydrogen supplied acts as a catalyst for the formation of the TD. Its inhomogeneous distribution in silicon causes the TD density profiles. The maximum TD concentration depends on the oxygen concentration, the process temperature during and / or after the plasma hydrogenation as well as the type of plasma and the duration of the plasma treatment. The maximum TD concentration can be up to 3 × 10 ¹⁶ cm ^-3 or more.

Reference symbol list

1

Silicon wafer

11

Front side doping (phosphorus)

12

Base area

12f

without thermal donor gradients

12td

Base area with gradients from thermal donors

13

photovoltaically active transition

14

Basic contact doping

15

Wafer front (light incidence side)

16

Back of the wafer (side away from the light)

2nd

Emitter area

31

Basic contact

32

Emitter contact

4th

Dielectric (insulator)

5

transparent conductive oxide

6

Anti-reflective layer

100

Manufacturing process (one embodiment)

101

Generation of a front field

102

Hydrogen plasma treatment

103

Deposition of an anti-reflective layer

104

Heat treatment (tempering)

105

Creation of a photovoltaically active transition

106

Contacting

111

Hydrogen plasma treatment when an anti-reflective layer is deposited

112

Heat treatment when an anti-reflective layer is deposited

113

Hydrogen plasma treatment and simultaneous heat treatment when depositing an anti-reflective layer

Claims (11)

Solar cell manufacturing process includes the following process steps: - Providing an n-type silicon wafer (1); - Generating a doping gradient from thermal donors in the silicon wafer (1); - Generating a photovoltaically active transition (13) for the solar cell; and - Contacting the solar cell by generating an emitter contact (32) and a base contact (31) whereby hydrogen diffuses into the wafer to form the thermal donors during the deposition of an antireflection layer (6) from a hydrogen-rich deposition plasma. Manufacturing process according to Claim 1 , characterized in that the thermal donors are produced essentially over an entire wafer thickness of the silicon wafer (1). Manufacturing process according to Claim 1 or 2nd , characterized in that the thermal donors are generated substantially along an entire wafer surface on one side of the silicon wafer. Manufacturing method according to one of the preceding claims, characterized in that the doping gradient is generated from thermal donors before or after the generation of the photovoltaically active transition (13). Manufacturing method according to one of the preceding claims, characterized in that the step of generating the doping gradient from thermal donors comprises a hydrogen plasma treatment on one side of the silicon wafer and a heat treatment of the silicon wafer simultaneously or following the hydrogen plasma treatment. Manufacturing method according to one of the preceding claims, characterized in that the photovoltaically active transition is produced by an emitter layer being produced in the silicon wafer by means of diffusion of dopant into the silicon wafer, by an emitter layer being deposited on the silicon wafer and / or by means of a dielectric and a metal layer Metal-insulator-semiconductor transition is generated on the semiconductor wafer. Manufacturing method according to one of the preceding claims, characterized in that the emitter contact is generated on the back of the silicon wafer. Wafer solar cell produced in a manufacturing process according to one of the preceding claims. Wafer solar cell after Claim 8 , characterized in that the base contact (31) is arranged on a light front or sun-facing wafer front (15) and the emitter contact (32) on a light-facing wafer back (16). Wafer solar cell according to one of the Claims 8 to 9 , characterized in that the doping density of the thermal donors drops towards the emitter region (2). Wafer solar cell according to one of the Claims 8 to 10th , characterized in that the base region (12) has a doping gradient made of thermal donors along a wafer surface in regions (12td).

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