DE3804069A1 - Process for producing solar silicon - Google Patents

Abstract

A process is proposed in which a silicon raw material still containing boron and oxygen after a first purification process is converted into a solar silicon by at least partial compensation of the boron content by addition of phosphorus-containing materials and a subsequent crystallisation or recrystallisation process. By means of the compensation, a greater diffusion length of the minority charge carriers is achieved in the solar silicon. As a result of this, solar cells produced from this material achieve higher performance. As raw material, even silicon oxide materials containing relatively high amounts of boron can be used. <IMAGE>

Description

The invention relates to a method for producing for Silicon made of pure silicon suitable for solar cells Boron also contains oxygen due to the process.

The conversion of sunlight into electricity with the help of solar cells made of crystalline silicon has so far been special and restricted to smaller applications. A wider one Use of solar energy to generate electricity through this environmentally friendly Technology can only be achieved by lowering the high Cost of the electronic currently used for solar cells Reach silicon (EG-Si). This is only possible if that expensive electronic silicon through a suitable cheaper Silicon is being replaced, and the cost is currently around 20, - DM per watt peak performance to approx. 3, - to 4, - DM per Watts can be reduced.

The base material for the production of crystalline silicon solar cells is now obtained exclusively by the so-called Siemens-C process, in which high-purity silicon is obtained by reducing trichlorosilane with hydrogen at approx. This process produces a silicon of extraordinarily high purity, which is well suited for the semiconductor industry, but the high costs of which preclude the material from being widely used for the production of solar cells. So-called off-grade silicon is currently used for the photovoltaic industry, which is produced as low-quality silicon in the Siemens C process or as waste material in the Czochralski pulling process and does not meet the high requirements of the semiconductor industry. Given the currently small market volume for solar cells, this waste material can meet the demand for solar silicon. But as the market continues to expand, a new process for producing suitable silicon is required. This material must also meet high purity requirements. For example, Davies, in an article in IEEE Trans. ED 27 (1980) on pages 677 to 687, was able to show that in single-crystalline silicon, concentrations of around 10 12 cm ^-3 at transition elements already have a drastic influence on the efficiency of solar cells made therefrom.

A promising way to manufacture inexpensive silicon provides the implementation of high-purity silicon dioxide high-purity carbon in the arc furnace. Aulich et al describe for example in J. Mater. Sci. 19 (1984) on pages 339 to 345 the use of quartz sand and soot, which in a special process to be cleaned as starting materials for this process. The pure silicon thus produced is then for the removal of carbon and dissolved residual impurities subject to directional solidification. Thereby is achieved that the impurities in the surface layers of the silicon blocks obtained are concentrated means in the floor and ceiling layer as well as the side walls, which can be removed by sawing and / or sandblasting. The p-type silicon obtained in this way contains experience has shown that less than 1 ppmw metallic impurities and has a boron content depending on the degree of purity of the starting substances from 0.5 to 1 ppmw.

Studies have now shown that the efficiency of solar cells is particularly dependent on the boron content of the silicon. Fig. 1 illustrates this, with the efficiency η of crystalline dislocation-free solar cells plotted against the resistivity of the silicon material. As can be seen in the figure, the efficiency η decreases significantly as soon as the specific resistance ρ drops below 0.4 Ohm × cm by adding boron. Furthermore, it is found that the diffusion length L _{D of} the minority charge carriers also decreases sharply with increased on-board doping, for example from approx. 90 μm at ρ = 1 ohm × cm to approx. 5 μm at ρ = 0.1 ohm × cm. The reasons for the sharp decrease in diffusion length at these relatively low boron concentrations have not yet been clarified. The high oxygen concentration in the Czochralski-grown silicon may lead to the formation of electrically active boron-oxygen complexes, which lead to a reduction in L _D. Diffusion lengths of approx. 170 µm were measured on dislocation-free single-crystal disks made of solar silicon (boron content approx. 1 ppmw), which were obtained by crucible-free zone pulling and therefore have a low oxygen concentration of only approx. 10¹⁵ cm ^-3 . This strong influence of boron doping with a high oxygen concentration on the efficiency of solar cells, as well as the fact that boron with its high distribution coefficient of approx. 0.8 is very difficult to remove from the silicon, has made it necessary to choose the starting materials to limit to extremely low boron SiO₂ and soot materials. This causes high costs because most inexpensive starting materials such as natural quartz have a higher boron content.

The object of the present invention is therefore a method for the production of silicon suitable for solar cells, which also involves the use of boron-containing starting materials allowed and to solar cells with an efficiency of greater leads 10 percent.

This task is accomplished by a method of the type mentioned at the beginning Art solved according to the invention in that the boron content of the pure silicon at least by adding phosphorus-containing materials is partially compensated, and the compensated silicon then by a crystallization or recrystallization process is converted into solar silicon.

Further embodiments of the invention are the subclaims refer to.

Studies have shown that, for example, when using Brazilian quartz with a natural boron content of approx. 10¹⁷ cm ^-3 in a process sequence consisting of a cleaning procedure (e.g. leaching glass fibers with acid), a reduction process (e.g. carbothermal reduction in an electric arc furnace with high-purity carbon) and a subsequent recrystallization (for example Czochralski pulling) using the method according to the invention, a solar silicon can be produced which has a specific conductivity ρ of approx. 1 ohm cm (p-type), and which is good for the production of Solar cells is suitable. The boron compensation significantly increases the diffusion length L _{D of} this material, which also guarantees a higher efficiency of solar cells made from it. The higher the L _{D of} a silicon material, the greater the efficiency of the corresponding solar cell. It proves to be particularly advantageous if the specific resistance ρ is increased to a value of approximately 1 ohm cm (p-silicon) by the partial compensation of the boron contained in the pure silicon by phosphorus for the solar silicon. However, the optimal value for each crystallization process must be determined individually.

The increase in the diffusion length and thus the solar cell efficiency P compensation of boron generally applies to silicon with a relatively high oxygen content. Such results necessarily from the respective representation process for Silicon as long as it has not been used in the absence of oxygen becomes.

The actual compensation is done by adding phosphorus Materials for pure silicon or for a melt thereof before the last crystallization process like a process is described for example in DE-OS 36 11 950. Is the compensation basically also before the reduction process possible, but can then the degree of compensation because of control the depletion of phosphorus that occurs and not set exactly. As a source of phosphorus silicon that contains a lot of phosphorus, for example in a CVD process by deposition from the gas phase in Presence of gases containing phosphorus (for example phosphine, PH₃) may have been produced. But are also inorganic phosphorus Compounds or salts suitable as a source of phosphorus. To Calcium phosphate and phosphorus pentoxide are ideal.

The amount of phosphorus to be added is calculated taking into account the content of pure silicon and the distribution  coefficients of boron and phosphorus, which in turn depends on the applied Crystallization process and the rate of crystallization are dependent. To calculate the compensation required Equivalents of phosphorus can use the following rule of thumb be used

in which mean:
P = number of phosphorus equivalents to be added
(B) = boron concentration in pure silicon in equivalents per volume
VK _B = distribution coefficient of boron dependent on the crystallization process
VK _P = corresponding distribution coefficient for phosphorus
S = empirical factor to avoid overcompensation, which allows the setting of a given conductivity or a given specific resistance
Vol = volume of pure silicon.

With the invention it is therefore possible to also contain more boron Silicon oxide raw materials for the manufacture of solar silicon and thus through cheaper raw materials the cost of the manufacturing process of solar silicon to reduce consistent quality. By compensation the specific resistance and the diffusion length of the Solar silicon, as well as the efficiency of those made from it Solar cells.

The invention is described below using an exemplary embodiment explained in more detail.

The starting material is quartz sand, which is converted into a homogeneous glass phase by melting using suitable aggregates of carbonates of the alkaline earth metals and aluminum oxide. Glass bodies with a large surface area are produced from this melt and leached with hot mineral acid. Further details can be found in the article cited by Aulich et al. The resulting high-purity silicon dioxide is reduced to pure silicon with high-purity carbon in an arc furnace. (See also German patent application P 37 32 073.4). The boron content of the silicon melt obtained is approximately 10¹⁷ cm ^-3 . Using a rule of thumb described above, the amount of high phosphorus-containing CVD silicon is compensated for, taking into account a boron distribution coefficient of approximately 0.8, a large part of the boron. Dislocation-free single crystal bodies are then obtained from the melt in a Czochralski train and are produced from this crystalline solar cells with pn junctions. The table compares the physical data of these solar cells with the data of solar cells manufactured in parallel, which were produced in an identical process, but without phosphorus compensation. With an 85% phosphorus compensation of the boron content, significant improvements in the solar cell characteristics can be observed. A clear increase can be seen, for example, in the specific resistance from 0.2 to 1 ohm cm, in the fill factor FF from 63 to 75 and above all in the efficiency η from 6.8 to 9.2%. Since otherwise matching measurement conditions were used, the table shows the advantages of the method according to the invention.

table

Claims (7)

1. A process for the production of silicon suitable for solar cells from pure silicon, which in addition to boron also contains oxygen due to the process, characterized in that the boron content of the pure silicon is at least partially compensated for by the addition of phosphorus-containing materials, and the compensated silicon then by a crystallization or recrystallization process is converted into solar silicon. 2. The method according to claim 1, characterized in that the amount of added phosphorus-containing material required for compensation is calculated according to the following rule of thumb: where mean:
P = number of phosphorus equivalents to be added
(B) = boron concentration in pure silicon in equivalents per volume
VK _B = distribution coefficient of boron dependent on the crystallization process
VK _P = corresponding distribution coefficient for phosphorus
Vol = volume of pure silicon
S = empirical factor to avoid overcompensation, which allows the setting of a given conductivity 3. The method according to at least one of claims 1 or 2, characterized characterized in that as a phosphorus Material is a silicon with a high phosphorus concentration is used. 4. The method according to claim 3, characterized in  that the silicon with high phosphorus concentration is a silicon deposited by a CVD process. 5. The method according to at least one of claims 1 or 2, characterized in that as phosphorus-containing Material uses inorganic phosphorus compounds will. 6. The method according to claim 5, characterized in that the inorganic phosphorus compounds are selected from the group P₂O₅, Ca₃ (PO₄) ₂. 7. The method according to at least one of claims 1 to 6, characterized in that so much boron is compensated for that in solar silicon the specific resistance is increased to approx. 1 ohm cm.