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2 Birgit Meinel 1, Prof. Dr. Jörg Acker 1, Dr. Burcu Kantowski 2 1 University of Lausitz, institute for natural sciences, physical chemistry Großenhainer Straße 57, Senftenberg; Joerg.Acker@hs-lausitz.de 2 Analytik Jena AG, Konrad-Zuse-Strasse 1, Jena Introduction Increasing international competition and steadily increasing cost pressure result in the ever rising need for cheaper solar cell production and increased cell efficiency. A more efficient sawing process can make a significant contribution in this context. The current research work has the aim of combining the removal of saw damage depending on the saw process used with the ideal texturing of the wafer surface which creates a significant reduction of reflectivity. Production of solar cells Solar cells are manufactured from polycrystalline or monocrystalline silicon. The crystallization process has a strong influence on the potential performance and the costs of a cell. The production of monocrystalline silicon is very time-consuming. During the crystallization process, the so-called Czochralski process, a round monocrystalline rod is slowly pulled out of a silicon melt under constant rotation. To create solar cells from the crystal, it is sawn into thin slices, or wafers. This procedure is time-consuming and very expensive. This is due to the fact that the production process involves the cutting of square wafers from round crystals, which creates a lot of excess material. Polycrystalline silicon is crystallized in square blocks of several 100 kg. The resulting material consists of many small crystals with partly visible grain boundaries. That is why it is referred to as polycrystalline. The blocks are split into square bricks and processed further. Polycrystalline silicon wafers are cheaper and more energy-efficient during production. Therefore polycrystalline solar modules are also cheaper to buy. With an appropriate sawing method, the square bricks and the Czochralski monochrystalline rod are cut into slices less than 200 µm thick to produce solar cells. Until now, the only method used for this purpose was the wire saw method. This method uses an approx µm thin steel wire that is brought across the block at a speed of approx m/s. A glycole/silicon carbide mixture (slurry) is used as an abrasive. The necessary material abrasion for the separation is effected by the silicon carbide grains that produce local deformations and breakage until the silicon particles chip off. The high degree of wear of the sawing wire, the abrasive and the complex preparation of the silicon carbide brought on the development of the diamond wire sawing method in recent years. With this method the material is separated using a steel wire that is covered with small diamond crystals. An abrasive is not needed. Sawing times are reduced by half which significantly increases productivity. 2/ 7

3 The surface of the finished wafers (as cut) exhibits a heavily damaged crystalline structure, the socalled sawing damage (see figure 2a and 3c), which needs to be removed. The enormous damage of the crystals results in electron-hole pairs, which are formed while the sunlight is being absorbed. These recombine so quickly that it is practically impossible that a thus manufactured solar cell could produce electricity. The damaged crystal surface of silicon wafers is removed with etching mixtures that consist of nitric acid and hydrofluoric acid. In addition, the surface of the wafer develops a particular spatial structure during the etching process, the so-called texture (figure 4). The texture is decisive in the effective use of the incident sunlight. It is aimed to create a structure like a concave mirror or a pyramid-like structure. This structure does not throw the light rays reflected by the wafer surface directly back into the room, but instead directs them to other areas of the surface structure. The interflection of the sunlight in the silicon surface increases the efficiency of the solar cell because more of the incident sunlight is absorbed and converted to energy. Reflectance measurement with the spectrophotometer SPECORD PLUS The removal of the sawing damage and the surface texturing are effected by etchings with different material mixtures. The SPECORD PLUS (figure 1a) is used together with an integrating sphere, the so-called Ulbricht's sphere (figure 1b), as an accessory for measuring the reflectivity. The diffuse reflectance of the silicon wafers can thus be determined. The SPECORD PLUS is a double beam spectrophotometer for the wavelength range of nm, with a variable spectral resolution and two tempered photodiodes (CDD-Cooled Double Detection). It performs transmission and reflectance measurements with maximum sensitivity. The integrating sphere is inserted into the path of the beam of the sample chamber of the SPECORD PLUS. It is suited for diffuse transmission and reflectance measurements of solid, liquid and powder samples. Its interior is made of high purity Spectralon, which has very good reflectance characteristics. The optical characteristics of solids with a rough surface can be determined by means of radiance factor measurements with the integrating sphere on the SPECORD PLUS. The radiance factor of a sample is the quotient of the radiation remitted from its surface and the radiation remitted from a completely matt white surface of a standard sample under the same optical conditions. Here, the position of the sample for the transmission measurement is in front of the sphere. The position of the sample for the remission measurement is on the opposite edge of the sphere. In the case of the remission measurement with the integrating sphere, the sample is part of the sphere itself. The sample surface is irradiated with a directed light beam under a fixed angle to its surface normal. The radiation reflected from the sample surface into the integrating sphere is bundled and falls diffusely on the radiation receiver of the spectral photometer. 3/ 7

4 The integrated sphere is particularly suited for reflectance measurements of powders and samples with structured surfaces, such as cellulose, leather, textile fabrics or as in this case for silicon wafer surfaces and samples with azimuthal gloss, i.e. a gloss that changes by the rotation of the sample around its surface normal. Figure 1a: Double beam spectrophotometer SPECORD PLUS Figure 1b: Integrating sphere Performance of the measurement The diffuse/8 measuring geometry is used for measurements with the integrating sphere. This means that the sample surface is irradiated directed at an angle of 8 to its surface normal. The radiation is reflected from the sample surface into the integrating sphere and falls diffusely on the receiver. The reflectance of the sample is determined in dependency of the wavelength relative to a reference, the so-called reflectance standard, which ideally creates a 100 % diffuse reflectance and does not absorb any light. The Spectralon standard sample (reflectance standard) is used as a reference. The silicon wafers are scanned at the sample position for reflectance measurements and measured in the SPECORD PLUS after the reference spectrum has been recorded. 4/ 7

5 Results The surface reflectivity is illustrated using the example of two differently cut wafers and the resulting sawing damage. As the images from the scanning electron microscope in figure 2 (a and b) show, the surface of a slurry-cut (as cut) wafer has an evenly matt and rough sawtooth structure, which exhibits a multitude of wide fracture surfaces. The resulting reflectance value is R=24 %. The surface of a wafer cut with a diamond wire, on the other hand, has a silver shimmer and even to the naked eye grooves and striped structures are visible. At R=26 % the wafer cut with a diamond wire has a much higher reflectance. The images from the scanning electron microscope in figure 2 (c and d) show two different characteristics: There are both smooth and rough areas with grooves and fracture surfaces. a) b) c) d) Figure 2: Comparison of sawing damage; a) as cut wafer slurry-cut, 500x magnified b) as cut wafer slurry-cut, 3500x magnified c) as cut wafer cut with diamond wire, 500x magnified d) as cut wafer cut with diamond wire, 2500x magnified These structures can have a significant influence on the measurement result. The measurement beam does not hit the sample vertically. This means that the measured reflectance is additionally determined by the alignment of the grooves and stripes of the wafer cut with the diamond wire compared to the direction of the incoming light beam, as represented in table 1. The alignment of slurry-cut wafers, which have a more regular surface structure, does not affect the measurement results. 5/ 7

6 horizontal vertical angular light beam light beam light beam R=27.7±0.6 % R=28.1±0.7 % R=28.7±0.7 % Table 1: Possible sample alignment of the wafers cut with diamond wire in the integration sphere and measured reflectance values Slurry-cut wafers and wafers cut with a diamond wire also react slightly different to acidic etching. Figure 3 shows the example of a slurry-cut wafer. Even a low etching abrasion in the area of the saw damage (< 4 µm) results in a lower reflectance. This is probably due to the successive removal of the sawtooth structure (figure 2b). The sawtooth structure acts like micro mirrors and causes the high reflectance of the cut wafers. With higher abrasion and the resulting texture the R values increase steadily, without, however, reaching the high value from the beginning. After an abrasion of 8 µm a typical texture with many large and small hollows can be observed (figure 3). Figure 3: Development of the reflectance values of differently cut monocrystalline wafers during etching Wafers cut with a diamond wire show a similar development, as shown in figure 3. The reflectivity is much lower in the area of the saw damage and increases again along with the abrasion. However, it is clearly above the values of the slurry-cut wafers. As can be seen in figure 4 the etching preferably takes effect at the cracks and fracture surfaces caused by the cutting. They soon exhibit the typical texture with hollows. The smooth areas of the surface grooves initially 6/ 7

7 remain widely unchanged. Only after comparatively long etching times the wafer surface can be textured completely. It thus becomes clear that the previous etching methods cannot be applied to wafers cut with diamond wire. The new efficient cutting method with diamond wires requires an optimized process for wet chemical etching to reduce etching times for achieving a homogeneous texture across the whole surface with reflectance values that are at least as low as those currently achieved for the textured, slurry-cut wafers. Figure 4: Images from the scanning electron microscope of textured monocrystalline wafers, magnified by a factor of 1000, abrasion between 8 and 9 µm. Discussion The reduction of the reflectance of solar wafer surfaces with adjusted etching processes is an important strategy for increasing the efficiency of solar modules. Current research activities aim on a better a better understanding of the connections between the selected cutting method and the texture and reflectance after etching. With the SPECORD PLUS in combination with an integrating sphere the reflectance could be determined quickly, precisely and in a reproducible manner as part of the current analyses. The results presented here are an example of the effect that the new cutting method has on other process steps during solar cell production. Printout and further use permitted with reference to the source Analytik Jena AG Publisher: Analytik Jena AG Konrad-Zuse-Straße Jena Phone +49 (0) / Fax info@analytik-jena.com 7/ 7