DE102011052916A1 - Method for manufacturing back-contact solar cell e.g. wafer solar cell, involves contacting solar cell with n-type base region having doping gradient of thermal donor by emitter contact and base contact - Google Patents

Abstract

The method involves generating a doping gradient from thermal donor in n-type silicon wafer (1) and generating photovoltaic active junction (13) for solar cell. The solar cell is contacted with n-type base region (12,12td) having doping gradient of thermal donor by emitter contact (32) and base contact (31).

Description

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

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

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

Both the back surface field and the front surface field serve to reduce the recombination probability at the respective surface by increasing the doping concentration and the associated reduction of 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 heavily doped semiconductor volume. In addition, the recombination probability of the remaining minorities on the highly doped semiconductor surface also increases compared to less doped surfaces.

In contrast to surface passivating reactions which saturate free silicon bonds (which act as recombination centers), such BSF or FSF are particularly stable against external influences such as e.g. UV irradiation and can thus ensure the passivation in the long term. In addition, this reduces the lateral series resistance and associated losses 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 few micrometers depth, since the penetration depth of the rear side or front side field is limited by the low diffusion rates of the dopants and the resulting long duration of the production processes. Although the diffusion times can be reduced by raising the temperature, at the same time the danger of producing defects in the substrate material increases as a result.

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

It is known to generate a pn junction by means of a gradient of thermal donors in p-type silicon. The production of a solar cell with such a gradient is described for example in US Pat Publication "Increased Efficiencies in a-Si: H (n) / Cz-Si (p) Heterojunction Solar Cells due to Gradient Doping by Thermal Donors," MLD Scherff et al., 21st European Photovoltaic Solar Energy Conference, 4-8 September 2006 , Dresden, Germany , It explains how a pre-doped 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 decreasing from the wafer front side with the depth is created, which is referred to below as a gradient, doping gradient or concentration gradient.

As the name implies, the thermal donors are donors, which cause an n-conduction of the semiconductor. The gradient of thermal donors is adjusted so that due to the high donor density on the semiconductor surface, a conductivity inversion and thus a pn junction is formed, which serves as a photovoltaic active transition of the solar cell. Subsequently, an isotopic heterojunction is produced on the front side 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 generated by the thermal donors.

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

This known prior art has the disadvantage that due to the gradient of the thermal Donors both the photovoltaic active junction and the drift field in the semiconductor volume can be determined. These two mechanisms are not separately controllable in this case. For example, there is the danger that not enough surface area of sufficient thermal donors are generated to produce a pn junction over the whole area, which either additionally requires the amorphous n-type silicon used or, after contacting, short-circuits between the two existing p and n types. Type areas entails.

It is therefore an object of the invention to provide a solar cell manufacturing method and a wafer solar cell with which a larger parameter range for drift field and photovoltaic active junction can be detected by means of a single technology.

The object is achieved by a solar cell manufacturing method having the features of claim 1 and by a wafer solar cell with the features of claim 11. Advantageous developments of the invention are listed in the subclaims.

The invention is based on the idea of using intrinsically present oxygen in an n-type silicon wafer in order to generate a gradient of thermal donors starting from a wafer surface. For this purpose, atomic hydrogen is introduced into the wafer surface. This can take place 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, in all processes a simultaneous and / or subsequent heat treatment. With this approach it is achieved, in drift fields generated by means of thermal donors, to increase the charge carrier diffusion length in the direction of the gradient, and / or the transverse conductivity in the higher doped regions generated by the thermal donors. In this case, the thermal donors can be generated starting from a front side or light incident side wafer surface, ie a wafer surface facing the incident light in the later solar cell, or from a wafer surface facing away from the front side wafer surface in the silicon wafer.

In the heat treatment, also referred to as annealing, the semiconductor wafer is heated to a suitable temperature, for example in a temperature range between 300 and 900 ° C, more preferably 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 intrinsic oxygen under the influence of hydrogen. The hydrogen acts as a catalyst for the generation of thermal donors.

There are a number of thermal donors in silicon that are generated at different temperatures and are destroyed at higher temperatures. The most relevant forms of thermal donors for practical use are produced at a tempering temperature of about 400 ° C and become inactive again in a subsequent warming to 550 ° C and more. The latter temperature can therefore be regarded as a critical temperature for this type of thermal donors. However, short heat-up times beyond this temperature are not necessarily detrimental because when the temperature is exceeded, not all thermal donors are destroyed automatically, but the destruction takes place successively over time.

In the following, method steps in which the semiconductor wafer remains below this critical temperature are referred to as low-temperature steps. In contrast, method 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 thus also dependent on the above parameters of the method of preparation of the thermal donors. In particular, gradients of thermal donors with a critical temperature of about 550 ° C. or 600 ° C. are of high practical relevance for wafer solar cells. The temperatures mentioned are a mean temperature 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 by means of laser irradiation in the manufacture 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 the wafer is hardly warmed up on average.

The entire solar cell fabrication process includes, in addition to creating a photovoltaic active junction for the solar cell, 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 separate from the gradient of thermal donors and not due to or by the generation of the gradient of the thermal donors. In other words, either the photovoltaically active junction is created after the thermal donors have already been formed, or the doping gradient of thermal donors is formed in the semiconductor following the creation of the active junction. In certain embodiments, however, sub-steps for the generation of the thermal gradient on the one hand and for the generation of the active transition on the other hand, 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 in the inventor's publication "Increased Efficiencies ..." described in the introduction. The reverse dependence (during the generation of the pn junction, the gradient of thermal donors, or a part thereof) is, however, conceivable if, during the formation of the pn junction or in general a photovoltaically active junction, which, for example, a metal-insulator Semiconductor contact (MIS contact), in a heating step, which serves at this moment only as annealing process for the formation of the MIS contact, previously introduced hydrogen is activated.

Finally, the solar cell thus produced must be contacted at the emitter and base. Here, both exclusively back-contacted solar cells are useful in which both the emitter contact and the base contact are formed on the back of the solar cell, as well as both sides contacted solar cells in which the emitter contact on the front side, so light incidence side is formed, and the base contact back, or vice versa. In bifacial solar cells, ie in solar cells, which can also convert incident light into electrical energy, one speaks instead of a back side facing away from the light also from a rear side facing away from the sun. Further method steps may be integrated in this production process, for example etching processes, texturing or the deposition of passivation layers, antireflection layers and the like.

Since thermal donors, as discussed above, may be destroyed when the wafer is exposed to a high temperature step, in embodiments incorporating such high temperature steps, the gradient of thermal donors should be generated after performing all high temperature steps such that no further high temperature steps are involved consequences. In this case, after producing the gradient of thermal donors, only low-temperature steps are carried out, for example sputtering, plasma depositions, electrodeposition processes and the like. That is, after generating the doping gradient from thermal donors, the temperature in the region of the semiconductor wafer in which the thermal donors are located remains substantially at any point below the critical temperature discussed above. Exceptions can be laser processes and short annealing processes as mentioned above. Laser processes heat the wafer only very briefly and locally far above the critical temperature. In particular, processes such as laser chemical processing, in which the laser beam is passed in a liquid jet, are particularly suitable because of the local cooling by the liquid. Another example of tempering above critical temperature is firing the metal contacts of standard solar cells, where the cell only exceeds the critical temperature for about one minute and only gets warmer than 600 ° C for a few seconds.

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

As explained above, in certain embodiments, process steps necessary for the formation of the gradient of thermal donors may at least be superimposed with process steps intended for the formation of the photovoltaically active transition. However, this generally also applies to method steps of other processes in the manufacture of the solar cell. For example, increasing the temperature of the wafer during the deposition of a nitride may be used as an antireflection layer simultaneously for the annealing step to create the thermal donors. Further, a functional layer such as such an antireflection layer may serve as a source or pro-rata source of the hydrogen to be introduced into the wafer.

The wafer solar cell manufactured by the method described herein has an n-type base region with a base contact for collecting current from the base region, and a photovoltaic active junction. In particular, the active transition can be one of the following structures: a pn junction in which an emitter is formed by diffusion of dopants in the wafer; a hyperabrupt transition in which there is an emitter produced by deposition, for example by 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 one Silicon layer (abbreviated as a-Si: H / c-Si heterojunction describes) or in the form of a heterojunction with intrinsic amorphous intermediate layer (also referred to as HIT transition); or a metal-insulator-semiconductor junction, also referred to as MIS structure, which stands for "metal insulator semiconductor".

In this case, the pn junction does not necessarily extend over the entire surface of a wafer surface and can also be embodied locally differently. This does not only apply to back-contacted solar cells with back emitter in which emitter and base are contacted on the back, but also contacted on both sides of solar cells with local emitter areas and / or local contact, which then takes place 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 formed, for example, in the MIS junction through the metal of the junction itself. In addition, the base region has, at least in regions, a doping gradient or concentration or density gradient of thermal donors. This gradient generates a drift field in the respective regions, which accelerates the charge carriers in the semiconductor volume in the direction of the gradient and thereby positively influences the mode of operation of the solar cell. Additional layers may additionally be provided in order to improve or optimize the function of the solar cell. Examples thereof are an antireflection layer, a passivation layer and the like.

The gradient of thermal donors should extend at least over part of the wafer thickness. It thereby assumes 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, depending on whether the emitter is located on the front or on the back. In other words, the gradient should extend into the wafer at least up to a certain penetration depth, for example of 5 μm. According to an expedient embodiment, however, it is provided that the thermal donors are essentially produced in depths of 10 μm to 100 μm, but at most up to half the thickness of the semiconductor wafer, and thereby assume primarily the function of an FSF or BSF. According to another expedient embodiment, however, it is provided that the thermal donors are generated substantially over an entire wafer thickness of the semiconductor wafer. This means that the gradient of thermal donors penetrates more than half the wafer thickness, ie that there is a significant donor concentration of the thermal donors not only on the wafer front side, but also deep in the volume of the n-type semiconductor wafer, beyond the wafer center. After completion of the solar cell, this feature means that the distance of a significant concentration of thermal donors is less than half the wafer thickness, ideally equal to zero.

Preferably, it is provided that the thermal donors are generated substantially along the entire wafer surface of the semiconductor wafer. In essence, in this case, it is meant that edge regions or transition regions to certain structures on the semiconductor wafer over the entire wafer thickness may not have thermal donors. In this case, it is quite possible that the penetration depth of the gradient of thermal donors into the wafer varies along the wafer surface.

In an alternative embodiment, 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 applied protective layers. In other cases, a locally generated plasma or a local annealing, for example by means of laser radiation, ensure that the thermal donors arise only at these areas.

In an advantageous embodiment, it is provided that the generation of the doping gradient from thermal donors takes place before the photovoltaically active transition is generated. However, it is also possible and in some cases advantageous to select the reverse order and first to generate the photovoltaically active transition and only then the doping gradient of thermal donors in the semiconductor wafer. If the generation of the active transition involves the deposition of an additional layer, for example, in the formation of a heterojunction, then the gradient of thermal donors may extend to this additional layer, if provided, upon subsequent gradient formation.

Advantageously, the step of generating the doping gradient from thermal donors comprises a hydrogen-introducing process. In this case, the hydrogen can be introduced into the wafer by means of implantation and / or from a diffusion source. According to a preferred embodiment, however, it is provided that the step of generating the Dotiergradienten from thermal donors comprises a hydrogen plasma treatment of a front and / or a back side of the semiconductor wafer and a heat treatment of the semiconductor wafer simultaneously and / or following the hydrogen plasma treatment. The heat treatment may also be at least in part the process temperature for a further process, for example for a deposition or doping process. It may also be a warm-up or cool-down phase of such a further process.

Advantageously, it is 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 method or by means of a further plasma treatment. It may also be that no additional surface treatment is required.

According to a preferred embodiment it is provided that the photovoltaically active junction is generated by an emitter layer is generated in the semiconductor wafer by means of diffusion of dopant into the semiconductor wafer by an emitter layer is deposited on the semiconductor wafer and / or by means of a dielectric and a metal layer Metal-insulator-semiconductor junction is generated on the semiconductor wafer. In one embodiment, the active junction may have a homotransition with equal band gap on both sides of the junction, which is generated for example by epitaxy. In alternative embodiments, the active transition is a heterojunction with different bandgaps that can be made, for example, by plasma processes.

It is preferably provided that the emitter contact is generated on the backside of the semiconductor wafer. This may 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 wafer backside. In this case, the base contacts are formed on the front side, preferably as electrode fingers. However, back emitter contacts are present in particular in the case of back contact solar cells, that is to say in the case of solar cells in which emitter and base contacts are arranged on the back side of the solar cell. If such solar cells have no doping gradients, disadvantageous "shading effects" can occur. In these cases, what is known as electrical shading, in which the collection probability of the minorities decreases due to extended charge carrier paths above base contacts or the series resistance component of the majority increases above emitter contacts. Both loss mechanisms can be reduced by thermal donors and their gradients.

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

The silicon wafer may be cut from a silicon ingot (so-called Cz silicon) produced by a Czochralski method. Such silicon wafers are monocrystalline. That is, they have a crystal structure which has 99% or more of a single crystal orientation, with correspondingly few grain boundaries. Alternatively, so-called quasi-monocrystalline wafers may also be suitable for the process described here. These are silicon wafers, which have fewer grain boundaries than polycrystalline wafers, but are not regarded as 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, which melts a single crystal on a small seed crystal of about 1 cubic centimeter size, quasi-monocrystalline ingots are currently produced mainly by processes using polysilicon in a crucible (similar to those used to make multi-silicon) Melted much so that the monocrystalline silicon seeds also located in the crucible does not melt and then solidifies the melt directed starting at these seeds. Care is taken here that the polycrystalline semiconductor pieces melt completely, while the monocrystalline semiconductor piece only melts on its surface, or that the crystallization takes place in such a targeted manner that, for the most part, crystals grow with a single orientation. Subsequently, starting from this monocrystalline semiconductor piece (ingot), as in ingots, multisilicon is processed and quasi-monocrystalline wafers are formed therefrom.

The invention will be explained below with reference to embodiments with reference to the figures. Hereby show:

1 a solar cell with backside transition and front panel;

2 a solar cell with backside transition and doping profile of thermal donors;

3 a back-contacted solar cell with front panel;

4 a back-contacted solar cell with doping profile of thermal donors in the entire base region;

5 a rear-contacted solar cell with a base region, which partially has doping profile of thermal donors;

6 a double-sided contacted solar cell with front side transition and doping profile of thermal donors; and

7 a flowchart for illustrating 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, with a wafer front 15 and a wafer back 16 are defined as the light incidence side or side facing away from the light of a solar cell resulting from the semiconductor wafer. On the wafer front 15 indicates the semiconductor wafer 1 a texturing to increase the Lichteinfangeffizienz on. The semiconductor wafer 1 itself is predoped to an n-type semiconductor and serves as a base region 12 for the solar cell. An emitter area 2 is formed on the back side such that between it and the n-type conductive base region 12 a photovoltaic active transition 13 formed.

While the base area 12 front side with strip or finger-shaped base contacts 31 is electrically connected, serves a rear emitter contact 32 for collecting the current from the emitter region 2 , wherein between the emitter contact 32 and the emitter area 2 an apertured dielectric 4 is arranged and allows electrical contact only at the openings. Front side is an antireflection layer 6 applied.

In the case of the reference numeral 11 provided and by a dashed line from the remaining part of the base area 12 delimited area is a front field of the type described above, which is formed by means of diffusion of dopants. It is in the representation in 1 So a well-known on both sides contacted solar cell with back emitter 2 and a front end doping 11 , the FSF.

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

A back-contacted solar cell, in which therefore both the base contact 31 as well as the emitter contact 32 on the wafer back 16 are formed in the 3 shown. So that the base contacts 32 the back of the room is the emitter area 2 structured or only partially on the wafer back 16 arranged. At the other areas of the wafer back 16 is a basic contact doping 14 provided an improved electrical connection between the base area 12 and the base contacts 31 to build. This basic contact 31 may optionally also be omitted, or it may alternatively be designed as hetero, MIS, or epi contact.

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

In contrast, in the 4 a similarly constructed solar cell shown, but instead of the front side doping 11 throughout the base area 12 has thermal donors, so this is a base area with Gradients of thermal donors 12td is. It should be noted that the orientation of the gradients takes place in such a way that the minorities are accelerated in the direction of the contacted solar cell side. As with 2 may be optional in addition to the range with thermal donors 12td also an area with a front side doping 11 be present, and / or the base contact 31 may optionally be omitted or carried out as a hetero, MIS, or epi contact.

Similar to the embodiment of the 2 can also be provided here a border area, so that the base area with gradients of thermal donors 12td from the wafer front 15 from only up to a certain wafer thickness in the semiconductor wafer 1 extends and below it a base region without such a gradient 12f is present. Preferably, the base region 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 previous and subsequent embodiments that the absence of the gradient of thermal donors can be adjusted both by the absence of the thermal donors altogether and by the homogeneity of the distribution of thermal gradients.

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 region 12 Penetrates and fills, as with the solar cell from the 4 the case is. Instead, the base region is the deep gradient of thermal donors 12td to a partial area of the base area 12 limited, which is from the wafer back 16 Seen from as possible with the emitter area 2 overlaps or, as in 5 shown, flat with the emitter area 2 coincidentally covered. Only in the remaining base area 12f no gradient of thermal donors is provided. Above the base area 12f It is also possible for a region with thermal donors to be present, which, however, does not extend as deeply into the base material as in the region above the emitter region 2 , The base area 12 of the silicon wafer 1 consists of two areas, one area 12td with gradients with a large penetration depth above the emitter area 2 and an area 12f without gradients or a gradient of low penetration depth, which can be seen in the illustration in 5 immediately above the base contacts 31 extends. Above the base contacts 31 these two areas overlap 12f . 12td ,

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

Finally, in the 6 an embodiment of a solar cell shown, which front side emitter region 2 and at the emitter contacts 32 thus also formed on the front side. Although present on the wafer front 15 No texturing is shown, it may also be advantageous in this embodiment. As with the solar cell from the 4 is also here in the base area 12td a gradient of thermal donors is formed, which completely fills the n-type base region, ie, from the backside dielectric 4 until the emitter area 2 extends. In an embodiment not shown here, however, the gradient may also be in front of the emitter 2 end up. In this case, an area would 12f arise below the emitter.

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

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

On the provided silicon wafer 1 n-type silicon with front texturing is first on the front wafer facing the light 15 a dopant in the surface of the semiconductor wafer 1 diffused 101 , such as phosphorus, to form a front surface field (FSF). This front panel is not necessarily required and can therefore be omitted accordingly. The function of the FSF, namely, the reduction of the front-side recombination and the front-side sheet resistance, may alternatively be accomplished by the subsequently-produced thermal donors, which also induce n-type doping, although not so highly doped, but deeper.

Subsequently, hydrogen is introduced into the wafer on the front side, for example by means of a hydrogen plasma 102 , an antireflective layer deposited 103 and the sample tempered 104 , The step 103 can also claim only part of the annealing time and the step 104 represent the remaining time required. These three steps 102 . 103 and 104 can also in one step 111 or in two steps 112 respectively. 113 be summarized. This is just an example sequence, other process sequences are possible.

Be the steps 102 and 103 through a step 111 This means that the hydrogen used to form the thermal donors during the deposition of the antireflective layer from the hydrogen-rich deposition plasma Antireflection layer diffused into the wafer. Alternatively, to form an antireflective layer 103 also another hydrogen-rich deposition plasma as a hydrogen source in the step 103 serve, for example, a plasma for depositing a passivation layer such as amorphous silicon or amorphous silica or alumina.

Be the steps 103 and 104 through a step 112 replaced, this means that the deposition of the antireflection coating on the wafer takes place simultaneously with the annealing step.

Be the steps 102 . 103 and 104 through a step 113 replaced, this means that the hydrogen to form the thermal donors during the deposition of the layer 103 diffused from the hydrogen-rich deposition plasma of the antireflection layer in the wafer and that at the same time corresponds to the deposition temperature of the annealing temperature to form the thermal donors.

Subsequently, on the light-remote wafer back 16 of the silicon wafer 1 a photovoltaic active transition 2 generated with a low temperature process.

Finally the contacting of the emitter and the base takes place.

Hereinafter, basic mechanisms of low-temperature doping of Czochralski-produced silicon (so-called Cz-silicon) by hydrogen-assisted thermal donor (TD) formation will be described.

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

Cz-silicon contains as impurity high oxygen concentrations, 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 during the drawing process, especially in the upper region (top area) of Crystals are installed. This oxygen is usually interstitial in silicon and is then electrically inactive. Annealing processes diffuse the oxygen in the silicon and form SiO ₄ defect complexes that 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, as at these temperatures further oxygen accumulates and thus the complexes are electrically inactive again.

The formation of thermal donors is influenced by the carbon concentration, the 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, because then the complex formation between carbon and oxygen atoms is energetically more favorable than the formation of TD. From carrier concentrations above 10 ¹⁷ cm ^-3 , the formation of TD for p-type materials is enhanced or reduced for n-type materials.

In addition, atomic hydrogen accelerates the diffusion of oxygen into silicon, thereby promoting the formation of 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. With the introduction of hydrogen increases by the increase in the mobility of oxygen, the probability that the oxygen atoms meet on suitable local defect areas in the silicon lattice and form there with existing oxygen atoms TD.

Hydrogen gradients can be generated in silicon wafers by means of unilateral hydrogen treatment of a silicon surface. A simultaneous annealing or subsequent tempering at 300 ° C. to 550 ° C. then generates 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 re-doping of p-doped Cz silicon to n-type, so that deep pn junctions can be generated within minutes, for example, in 100μm depth after 20 minutes annealing. With this n-type doping, it should be noted that doping atoms (such as phosphorus) are diffused in, Instead, structural defects in the form of TD are generated in the silicon, which behave like diffused donors. The supplied hydrogen acts as a catalyst for the formation of the TD. Its inhomogeneous distribution in silicon causes 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 plasma treatment time. The maximum TD concentration can be up to 3 × 10 ¹⁶ cm ^-3 or more.

LIST OF REFERENCE NUMBERS

1

silicon wafer

11

Front side doping (phosphor)

12

base region

12f

without gradients of thermal donors

12td

Base region with gradients of thermal donors

13

photovoltaically active transition

14

Base contact doping

15

Wafer front side (light incidence side)

16

Wafer backside (light side away)

2

emitter region

31

base contact

32

emitter contact

4

Dielectric (insulator)

5

transparent conductive oxide

6

Antireflection coating

100

Manufacturing method (an embodiment)

101

Generation of a front field

102

Hydrogen plasma treatment

103

Deposition of an antireflection layer

104

Heat treatment (annealing)

105

Creation of a photovoltaic active transition

106

contact

111

Hydrogen plasma treatment on deposition of an antireflective layer

112

Heat treatment when depositing an antireflection coating

113

Hydrogen plasma treatment and simultaneous heat treatment when depositing an antireflection coating

QUOTES INCLUDE IN THE DESCRIPTION

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Cited non-patent literature

• Publication "Increased Efficiencies in a-Si: H (n) / Cz-Si (p) Heterojunction Solar Cells due to Gradient Doping by Thermal Donors," MLD Scherff et al., 21st European Photovoltaic Solar Energy Conference, 4-8 September 2006 , Dresden, Germany [0007]

Claims (15)

A solar cell manufacturing method comprising the following method steps: - providing an n-type silicon wafer ( 1 ); Generating a doping gradient of thermal donors in the semiconductor wafer ( 1 ); - generating a photovoltaic active junction ( 13 ) for the solar cell; and - contacting the solar cell by an emitter contact ( 32 ) and a basic contact ( 31 ) be generated. Manufacturing method according to claim 1, characterized in that the thermal donors are substantially over an entire wafer thickness of the silicon wafer ( 1 ) be generated. The manufacturing method according to claim 1 or 2, 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 claim 1 or 2, characterized in that the thermal donors along a wafer surface on one side of the silicon wafer partially ( 12td ) be generated. Manufacturing method according to one of the preceding claims, characterized in that the generation of the doping gradient from thermal donors before, during or after the generation of the photovoltaically active junction ( 13 ) he follows. Manufacturing method according to one of the preceding claims, characterized in that the introduction of the hydrogen by means of implantation, from a diffusion source and / or a Wasserstroff-containing plasma takes place. A manufacturing method according to any one of the preceding claims, characterized in that the step of generating the doping gradient of thermal donors comprises hydrogen plasma treatment on one side of the semiconductor wafer and heat treatment of the semiconductor wafer simultaneously and / or subsequent to the hydrogen plasma treatment. Manufacturing method according to claim 7, characterized in that the hydrogen plasma treated side of the semiconductor wafer is subsequently freed from plasma damage and / or textured. Manufacturing method according to one of the preceding claims, characterized in that the photovoltaically active junction is generated by an emitter layer is generated in the semiconductor wafer by means of diffusion of dopant into the semiconductor wafer by an emitter layer is deposited on the semiconductor wafer and / or by means of a dielectric and a metal-insulator-semiconductor junction is produced on the semiconductor wafer. Manufacturing method according to one of the preceding claims, characterized in that the emitter contact is generated on the back side of the semiconductor wafer or that the emitter contact and the base contact are generated on the back side of the semiconductor wafer. Wafer solar cell with an n-type base region ( 12 . 12td ), the base area ( 12 . 12td ) contacting base contact ( 31 ), a photovoltaic active junction ( 2 ) and an emitter contact ( 32 ), where the base area ( 12 . 12td ) has a doping gradient of thermal donors. Wafersolarzelle according to claim 11, characterized in that the base contact ( 31 ) on a light-incident side or sun-facing wafer front side ( 15 ) and the emitter contact ( 32 ) on a light-remote wafer backside ( 16 ) is arranged. Wafer solar cell according to claim 11, characterized in that the emitter contact ( 32 ) and the basic contact ( 31 ) on a light-remote wafer backside ( 16 ) are arranged. Wafersolarzelle according to any one of claims 11 to 13, characterized in that the doping density of the thermal donors to the emitter region ( 2 ) decreases. Wafersolarzelle according to one of claims 11 to 14, characterized in that the base region ( 12 ) along a wafer surface in regions ( 12td ) has a doping gradient of thermal donors.

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