DE19950187A1 - Production of crystalline silicon thin layer solar cells comprises melting molten highly pure and high doped silicon waste in a crucible, pouring the melt into a mold and solidifying - Google Patents

Abstract

Production of crystalline silicon thin layer solar cells comprises melting molten highly pure and high doped silicon waste in a crucible, pouring the melt into a mold and solidifying. Additional dopants are preferably added to adjust the required conductivity type and the required high conductivity. The surface of the substrate is roughened using a mechanical or chemical-mechanical process.

Description

The invention relates to a method for producing crystalline silicon Thin-film solar cells based on a silicon-compatible and temperature-resistant Substrate.

Renewable energies are becoming increasingly important. Here comes the photovoltaic play a key role as it is possible to use sunlight directly in electrical Convert electricity that is versatile. The current production of Solar cells are extremely complex, which contributes to the fact that solar power has so far only is of minor importance. In the usual manufacturing processes, it becomes rod-shaped Semiconductor material, mostly monocrystalline or polycrystalline silicon in a thickness of approx. 300 µm Cut slices, which are the base of the manufactured in further process steps Form solar cell. As an alternative, octagonal Si tubes can also be used the EFG (edge-defined-silicon-film-fed-growth) process are then cut into approx. 10 × 10 cm slices by laser and turned into solar cells are processed.

Common to these processes is that they are expensive to produce the rods, blocks or tubes Silicon starting material is used.

To counteract the high costs, use is made in the manufacture of semiconductor Single crystals for further processing into crystal wafers for electronic components emerging crystal bodies but not suitable for their intended purpose. These are primarily the initial and final cones of the crystals and Crystal bodies with crystal dislocations or other causes of failure. As of today In technology, these crystal bodies are used to manufacture crystal wafers for solar cells crushed, melted and recrystallized. The available quantities and prices depend essentially on the production of the semiconductor industry and on demand the ever-growing photovoltaic industry.

There is no question that with widespread use of photovoltaics this is not enough Significantly reduce costs for the production of solar modules.

In the field of crystalline "thick-film technology", the thickness of the Substrates significantly reduced and the efficiency can be significantly improved. But that too  has its limits, since in general the breakage rate when using thin panes increases and the increase in efficiency requires expensive technologies.

In parallel to the "thick film technology" described, work has been carried out on the so-called "thin film technology" for over 15 years. Direct semiconductor connections such as e.g. B. amorphous Si: H, CuInSe ₂ , CdTe etc. These are on inexpensive substrates, for. B. glass, evaporated and then processed into modules according to known technology.

Even with "thin film technology" it has so far not been possible to manufacture modules on a production scale at a cost that is lower than with the established "thick film technology". For example, the a-Si: H material limits the efficiency to approx. 8% for large areas. When the sun is shining, a drop in the degree of efficiency can also be observed, so that a significant reduction in production costs appears questionable for this material. This also applies to the two other promising thin-film materials, CuInSe ₂ and CdTe, which require complex and expensive encapsulation and sealing techniques due to the small layer thicknesses in order to achieve the required module life of <20 years.

In addition, crystalline silicon thin-film technology has become increasingly important in recent years. With this technology, high-purity silicon is deposited on the temperature-resistant and silicon-compatible substrates, such as graphite or ceramic, by thermal decomposition of SiHCI ₃ at approx. 850 ° C from the gas phase in layer thicknesses of approx. 20 µm. The conversion into solar cells is then carried out using suitable technologies.

Alternatively, amorphous or at low temperatures of 300-400 ° C microcrystalline silicon can be deposited on glass substrates and after appropriate High temperature conversion to the desired crystal structure processing Solar cells are made.

This technology has the advantage that silicon is available almost indefinitely can be deposited inexpensively, has no aging problems in the crystalline state and ultimately high efficiency.

A disadvantage of this technology, however, is always based on the total costs of the Solar cell, the high cost of silicon-compatible substrates such as graphite or ceramic.  

The invention has for its object the high cost of producing Significantly reduce solar cells and find a substrate that meets the requirements The crystalline silicon thin film technology does justice and is inexpensive can be made.

According to the invention this is achieved in that the substrate is made of melted high-purity and highly doped silicon waste.

This has the advantage that this waste generated in the semiconductor industry is too general low prices and none other than the desired basic funding have other impurities.

The silicon waste is melted in a reusable crucible and the Melt, poured into molds, with a high temperature gradient of a short-term subject to directional solidification. To set the desired high conductivity If necessary, additional dopants are added to the melt. The one after solidification high purity and highly conductive multicrystalline Si block is used for further processing in saws large, multicrystalline thin slices and cuts through the substrate surface mechanical or chemical-mechanical processes roughened. This has the advantage that the then deposited Si layer is structured so that when light falls through Total reflection at the two interfaces epi-layer-substrate and epi-layer-air one optically longer path length of the incident photons is reached and higher Solar cell efficiencies can be realized.

After cleaning, the large-area, multi-crystalline disks reach 850 ° C heated and by introducing volatile Si compounds of high purity of the deposition subjected to the active Si layer. After the active Si layer has been deposited, the Large-area panes are surface passivated, contacted and made using known technology provided with an anti-reflective coating. When applying the front and back contacts The later geometry of the solar cell is already printed on the large panes and after electrical measurement of the subunits, by mechanical separation Subdivision into the desired solar cell formats made.

The solution according to the invention has the advantage that the consistently large-format processing - block, disc, Si layer, cell process - and the provision of suitable Si substrates according to the invention allows a substantial reduction in production costs. The Si substrate according to the invention fulfills all the main conditions, such as

• - High temperature resistance under the conditions of Si deposition from the gas phase as well as compatibility between substrate and Si layer,
• - High purity of the electrically well conductive substrate, so in the High temperature separation no disturbing impurities (transition metals etc.) can diffuse into the high-purity and active Si layer,
• - good adaptation of the thermal expansion coefficient between substrate and Si Layer so that there is no cracking or detachment of the Si layer,
• - Good compatibility and processability of the substrate / Si layer composite in the following cell and module process steps.

The invention will be explained in more detail below using an exemplary embodiment.

Highly pure and highly doped silicon waste from the Semiconductor industry used that cannot be used for conventional photovoltaic products are. This material has no other than the basic doping (As, Sb, P, B, Ga) Impurities. This silicon waste, in the form of rod sections, Crucible residues and broken panes are present in a quartz glass crucible melted and poured into quartz glass molds, which are made with a protective layer for example SiN are lined. The molten silicon is high Temperature gradient subjected to directional solidification in order at high Crystallization rate to obtain mechanically stable multicrystalline Si blocks. For Setting the desired line type - preferably p-type - and the desired one high conductivity, As, Sb, P, B or Ga is added to the melt if necessary. If necessary, the melt can also be refined, for example to add particles remove.

In this way you get a highly pure and after the directional solidification highly conductive multicrystalline Si block, which for further processing in 30 × Blocks 30 × 30 cm in size and cut into approx. 200 µm-400 µm thin slices are sawn. The multicrystalline slices will be chosen as large as possible so that in each of the subsequent cell and Module process steps a high power / slice processed and thus low cost / watt Top performance of the later solar cell can be realized.  

After the sawing process, the panes are cleaned and subjected to the subsequent CVD process of silicon deposition. For this purpose, the wafers are preferably heated to temperatures of approximately 850 ° C. in a reactor and then the active Si layer is deposited by introducing SiHCl ₃ , SiH ₄ , SiH ₂ CI ₂ or similar volatile Si compounds of high purity. During the deposition in the reactor, the Si substrates are guided through the coating zone on tool carriers (quartz, graphite, etc.) or without tool carriers.

The deposition of the active Si layer is controlled so that in the last approx. 0.5 µm a highly doped P layer is deposited to achieve the required p-n junction realize. Alternatively, the active Si layer can also be n-conducting; in this case the last 0.5 µm deposited, which is the boron doping required for the n-p junction exhibit.

In the case of a very thin active Si layer (≦ 10 µm) it is advantageous to use mechanical or chemical-mechanical processes to roughen the substrate surface and thus the to structure the deposited Si layer, so that when light falls through total reflection a optically longer path length of the incident photons arises and a longer one Solar cell efficiency is realized.

After the active Si layer has been deposited, the large-format wafers are surface passivated with known technology, contacted (e.g. screen printing technology for the front and back) and provided with an anti-reflective layer (e.g. SiN, TiO _x ).

It is crucial in the method proposed here that all steps up to the measurement the solar cell parameters are carried out with the large format so that the Move manufacturing costs to a low level. To achieve this, the Applying the front and rear contacts already the later geometry of the solar cell printed and after the electrical measurement of the subunits - e.g. B. 10 cm x 10 cm, 15 cm × 15 cm, etc. - through mechanical separation such as scratching / breaking, laser cutting, sawing the division into the desired solar cell formats.

The division of the large, approximately 30 cm × 30 cm large Si substrates into the pre-structured, contacted subunits of the desired solar cells is off physical reasons necessary, because with a 30 cm × 30 cm large solar cell current modules with approx. 14 V open circuit voltage cannot be implemented.  

After cutting the, for example, 30 cm × 30 cm large substrates for the corresponding solar cells, these are connected using known technology and to form modules processed.

Claims (8)

1. A method for producing crystalline thin-film silicon solar cells based on a silicon-compatible and temperature-resistant substrate, characterized in that the substrate consists of molten, highly pure and highly doped silicon waste. 2. Process for the production of crystalline silicon thin-film solar cells Claim 1, characterized in that the silicon waste in a rever meltable crucible and melted, poured into molds, with high temperature gradient is subjected to brief solidification. 3. The method according to claim 2, characterized in that for adjusting the desired line type and the desired high conductivity if required Melt additional dopants can be added. 4. The method according to claim 1-3, characterized in that the directed Solidification of highly pure and highly conductive multicrystalline Si blocks for further processing is sawn into large, multicrystalline thin slices. 5. The method according to claim 1-4, characterized in that by mechanical or chemical-mechanical process the substrate surface is roughened. 6. The method according to claim 1-5, characterized in that the large-area multi crystalline disks are heated to approx. 850 ° C and cleaned after cleaning Introduction of volatile Si compounds of high purity, the deposition of the active ones Si layer is carried out. 7. The method according to claim 1-6, characterized in that after separating the active Si layer, the large-area panes according to known technology surfaces passivated, contacted and provided with an anti-reflective coating.   8. The method according to claim 1-7, characterized in that when applying the front and rear contacts on the large panes the later geometry of the Printed solar cell and after electrical measurement of the subunits by mecha separation into the desired solar cell formats becomes.