WO2013029835A2 - Solar cell and method for producing same - Google Patents

Abstract

The invention relates to a solar cell of the crystalline silicone type, comprising an anti-reflection coating (5; 4) based on silicone nitrite on the front of said solar cell, wherein, in a surface sublayer (4), the anti-reflection coating has a silicone content that is increased in relation to the stoichiometric silicone content in Si₃N₄ in such a way that the refraction index has a value in the range between 2.1 and 2.5.

Description

 Description title

Solar cell and process for its production

The invention relates to a solar cell of the crystalline silicon type and to a method for the production thereof.

State of the art

Solar cells for the construction of photovoltaic systems, especially those of the crystalline silicon type, are usually connected to modules of 40-100 solar cells, which in turn are connected to strings of some modules. These strings are often connected to an inverter in such a way that the solar cells of some modules have a very high electrical potential difference compared to the grounded module housing. If increased temperature and / or humidity is added to these conditions, an effect commonly known as potential induced degradation (PID) occurs, see Swanson et al., 15th PVSEC, Shanghai, 2005, and Berghold et al. , 25th EU-PVSEC, Valencia, 2010.

In US 7,554,031 B2 various ways are shown to eliminate this adverse effect. At the level of the entire photovoltaic system, ie the interconnected modules and the inverter, it is possible to prevent the cells of a module from having too great potential differences with the grounded module housing either by grounding the inverters of the modules (which is only possible in the case of a transformer). Inverter), smaller string voltages are used or the system voltage is reversed by additional equipment at night. At the module level, the problem can be solved by, for example, introducing a conductive layer between the glass and the encapsulating material.

At the level of the solar cell, the problem can be solved by applying a conductive layer on the cell front side. This may, according to US 7,554,031 B2 and US 7,786,375 B2. be a conductive oxide. However, the material most commonly used as an antireflection coating on solar cells is silicon nitride with a refractive index between 1.9 and 2.1. This is not electrically conductive enough to effectively prevent the PID effect. Although it is also possible to produce silicon nitrides with higher conductivity, they absorb significantly more light and thus lead to power losses of the solar cell.

Existing system-level solutions have the disadvantages that, in the case of grounding the inverter, the transformer inferior in efficiency and heavier transformer in place of transformerless inverters must be used, and that lower string voltages associated with more losses or the disadvantage that accessories and thus additional investments are necessary.

Inserting an additional layer at the module level means a further process step and is therefore very expensive.

Disclosure of the invention

With the invention, a solar cell with the features of claim 1 is proposed. Advantageous developments of the inventive concept are the subject of the dependent claims. Furthermore, a method for producing such a solar cell is proposed.

The invention includes the idea to seek the solution of the problem at the cell level, regardless of the conflicting goals in the known solar cell design. It further includes the idea, this the To modify known silicon nitride-based antireflection layer such that it is sufficiently effective for suppressing the PID effect, without a significant deterioration of the optical properties occurs.

In particular, a layer system applied on the front side of the solar cell is proposed, which consists of silicon nitrides of different thickness and composition. Here, the uppermost layer is a silicon nitride layer with a very high silicon content. This composition makes the layer very conductive, which is why even very thin layers effectively prevent the PID effect. The remaining part of the layer stack is now to be selected in refractive index and thickness so that the optical properties of the solar cell are minimally negatively affected.

With the invention, a very cost-effective solution is provided to exclude the PID effect already during the production of the solar cell. Thus, in the design of the module and the PV system all freedoms for the construction of an optimal system can be exploited. Furthermore, the solar cells according to the invention with silicon nitrides on conventional

Standard systems of PV production technology are manufactured. According to the invention, the silicon nitride-based antireflection layer is realized, wherein the surface partial layer has an Si content that is increased in Si ₃ N _{4 in} comparison with the stoichiometric Si content in such a way that the

Refractive index has a value in the range between 2.1 and 2.5. The stoichiometric Si content in Si ₃ N ₄ is 42.9%, resulting in a

Refractive index value of 1.9 results. The value 2.1 as a refractive index corresponds to an Si content of about 50%, whereby a further increase of the refractive index value is made possible by an even larger Si content. Thus, a refractive index value of 2.5 is achieved with an Si content of about 65%, so that an Si content of about 50% -65% of a silicon nitride compound has a refractive index value of 2.1 to 2.5 equivalent. For pure silicon, a refractive index value of about 3.3 would be realized. In a preferred embodiment, the silicon content in the superficial sub-layer of the antireflection layer is set such that the refractive index is in the range between 2.2 and 2.4. In a

expedient embodiment, the remaining range of

Antireflection layer below the superficial sublayer one

Refractive index in the range between 1.9 and 2.1.

In another embodiment, the reflective layer system is configured such that the thickness of the increased silicon subfloor sublayer is less than half the thickness of the antireflective layer. More specifically, the thickness of the superficial partial layer is less than _^1/4 of the thickness of the antireflection layer. In concrete embodiments, the thickness of the superficial sub-layer is in the range between 5 and 20 nm with a total thickness of the antireflection layer in the range between 60 and 80 nm. In a technologically preferred manner, the antireflection layer is formed in a process-uniform manner, including the surface partial layer. Accordingly, the proposed methodology contemplates that the superficial sublayer having increased silicon content be prepared in a contiguous deposition process of the antireflective layer by altering the composition of the process atmosphere.

A silicon nitride-based antireflective layer is produced by suitable deposition techniques (eg, PVD, CVD, PECVD). Specifically, in PECVD, the increased silicon content in the uppermost layer is adjusted by increasing the silane content in the mixing ratio of silane to ammonia during the deposition. Depending on the process parameters during the deposition, the silicon nitride layer thus formed may also contain hydrogen. However, the amount of hydrogen in the silicon nitride layer is so small that its influence on the inventive function of the silicon nitride layer can be neglected.

drawings An advantageous embodiment of the invention is illustrated in the drawing and explained here.

The figure shows as a detail illustration (omitting components / layers of a silicon solar cell which are not essential for the explanation of the invention) an Si absorber layer 1, whose one surface - which will face the incident radiation in the position of use - is provided with an intermediate layer 2 to optimize the surface passivation is provided. This is followed by a first sub-layer 3 of a silicon nitride antireflection layer with a thickness of 70 nm and a refractive index n = 2.0, the Si content of which essentially corresponds to the proportion in the case of conventional silicon nitride-based antireflection layers. This is followed by another (superficial) sub-layer 4, a layer with a thickness of lOnm and a refractive index n = 2.35, which has a significantly increased Si content and thus is sufficiently electrically conductive to suppress the PID effect.

Within the scope of expert action, further refinements and embodiments of the product and method concept described here by way of example only arise.

Claims

claims 1. A crystalline silicon type solar cell having a silicon nitride-based front antireflection layer (5; 4), wherein the antireflection layer in a superficial sublayer (4) has an Si content which is increased in relation to the stoichiometric Si content in Si ₃ N ₄ such that the refractive index has a value in the range between 2.1 and 2.5. 2. Solar cell according to claim 1,  wherein the silicon content in the surface sub-layer (4) of the antireflection layer (3; 4) is set such that the refractive index is in the range between 2.2 and 2.4. 3. Solar cell according to claim 1 or 2,  wherein the remaining area (3) of the antireflection layer (3; 4) below the surface partial layer (4) has a refractive index in the  Range between 1.9 and 2.1 has. 4. Solar cell according to one of the preceding claims,  wherein the thickness of the increased silicon content superficial sublayer (4) is less than half the thickness of the antireflective layer. 5. Solar cell according to claim 4, wherein the thickness of the superficial partial layer (4) is less than _^1/4 of the thickness of the antireflection layer (3, 4). 6. Solar cell according to claim 5,  the thickness of the surface sub - layer (4) being in the range between 5 and 20 nm for a total thickness of the antireflection layer (3; 4) in the Range is between 60 and 80nm. 7. Solar cell according to one of the preceding claims,  wherein the antireflection layer (3; 4) is formed in a process-uniform manner, including the superficial sub-layer (4). A method of manufacturing a solar cell according to any one of the preceding claims, wherein the increased silicon content superficial sublayer (4) is produced in a contiguous deposition process of the antireflective layer (3; 4) by altering the composition of the process atmosphere or other process parameters.