Abstract:
The invention relates to an optical method and device for the texture quantification of photovoltaic cells. The inventive method and device are suitable for texture morphologies that are characterized by the development of geometric patterns which are correlated on the surface of the substrate supporting the photovoltaic cell. The aforementioned morphologies can be formed using different methods, including chemical attack of monocrystalline Si, with both raised and inverted pyramids. The inventive method can also be used to study other degrees of texture developed in multicrystalline Si as well as those present in polycrystalline silicon cells deposited on substrates which have been pre-textured under the aforementioned conditions. The invention can also be extended to other materials having similar texture patterns.

Description:
RELATED APPLICATIONS  
       [0001]     The present application is a Continuation of co-pending PCT Application No. PCT/ES2004/070050, filed Jul. 14, 2004 which in turn, claims priority from Spanish Application Serial No. 200301666, filed on Jul. 15, 2003. Applicants claim the benefits of 35 U.S.C. §120 as to the PCT application and priority under 35 U.S.C. §119 as to said Spanish application, and the entire disclosures of both applications are incorporated herein by reference in their entireties. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     This invention is related to the production engineering industry, and more particularly to the sector concerned with the production of photovoltaic cells, and therefore it also has a bearing on the alternative energy sector. The invention relates to the control of the methods used for texturing the surface of monocrystalline silicon, although it is also applicable to the textures developed on the surface of silicon and other multicrystalline and polycrystalline semiconductors.  
         [0004]     2. Description of the Prior Art  
         [0005]     Decreasing the optical reflectance of the photosensitive side of the cell is an essential step of the manufacturing process necessary to improve the efficiency with which the photovoltaic cell converts the incident light into electric power. This is achieved by applying anti-reflectant coatings and inducing a well defined texture on the surface of the cell. Frequently, these two effects are achieved consecutively.  
         [0006]     During the texture forming process the cell develops a rough surface morphology [1,2], a porous or cavity-ridden morphology [3,4], or a faceted morphology [5] that allows multiple reflections on the surface before the light escapes irretrievably to the incident media (typically air or a transparent coating) and that additionally traps the long wave light by means of a total internal reflection (infrared light close to the Si gap) that penetrates in the active boundary area.  
         [0007]     Due to the high cost of crystalline materials, industrial production of photovoltaic cell technology is focused on silicon substrates (Si), although several crystalline quality grades are used. The highest quality cells use monocrystalline silicon (c-Si). Multicrystalline silicon (mc-Si), in which several crystalline grains have the same orientation, is also used. Currently, there is a great ongoing effort to improve the performance of thin sheets of polycrystalline silicon (p-Si). The methods used to texture the three different types of Si are also different.  
         [0008]     The usual method to develop texture in monocrystalline silicon wafers is chemical surface treatment or etching, consisting in immersing the wafers in caustic baths that attack certain silicon crystalline planes preferentially [6,7], producing a random distribution of pyramid-shaped structures with a square base ( FIGS. 1 and 2 ), where all the pyramids present mutually parallel faces. This particular shape of the pyramids is caused by silicon&#39;s cubic symmetry and the appearance of planes (111) induced by the preferential chemical etching processes. Typically, when the degree of texture is high, the side of the pyramid&#39;s base is ≈5-10 μm and the tilt angle of the pyramid&#39;s lateral sides in relation to the substrate surface (α) is close to, but smaller than the cutting angle between the planes (111) and (100), or 54.735°. The tilt α depends slightly on the details of the chemical etching process that generates the texture and the values observed range from α=49-53°. This type of texture appears independently of the chemical agent used for the etching process that can be NaOH [7], Na 3 P04:12H20 [8] or Na 2 C03 [9]. A variation of this texturizing method is the formation of inverted pyramids [10], although in this particular case the geometry of the texture is periodical and can be pre-determined by the manufacturer by means of a lithographic process.  
         [0009]     To characterize the anti-reflectant coatings, the techniques and instruments used (spectrophotometers, interferometers and ellipsometers) are based in optical methods that allow the evaluation without establishing physical contact with the cell, however, the study of the surface&#39;s texture is usually done by scanning electron microscopy (SEM) or with a profilometer and a scanning force microscope (SFM) with or without contact. Aside from the cost incurred in the utilization of these techniques, their adaptations to the industry as a quality control system of the cell&#39;s texture present certain difficulties. The scanning electron microscopy (SEM) requires a low pressure environment around the cell to work and the analyzed area may sustain damages due to the incidence of the electron beam. Also, the time required to perform the analysis considerably exceeds the speed of processing required for the production process. The profilometers, on the other hand, either work in contact mode, which may scratch the cell, or in optical mode that, while without contact, is developed to work on flat surfaces and do not adapt well to rugged and faceted surfaces because the reflectance is non-specular. The analysis time required both for scanning electron microscopy and for profilometry is in the order tens of minutes and therefore significantly longer than the time required for the individual analysis of cells in the production line (which is in the order of seconds).  
         [0010]     There is prior research work that has focused on the optical reflectance of textured photovoltaic cells [11, 12]. These papers report either an integral measurement of the total or diffuse reflectance or measures of specular reflectance even with normal incidence. The source of collimated light used is a HeNe laser with spectral coherence. For measurements concerned with non-coherent light a quartz lamp is used as source and the beam is focused (non-collimated). Ignoring some effects observed in periodical textures [11] and that are associated to the interference of the multiple beams associated to the HeNe laser coherence, the patterns of optical intensity analyzed in these works have a circular symmetry around the measurement axis [11]. The goal of these works is not to analyze the degree of texturization (which is known beforehand), but to evaluate the light-trapping efficacy of the texture.  
         [0011]     The objective of the method invented and the proposed devices resides in the development of a non-contact method for the quantitative analysis of the degree of texture in Si wafers that is suitable for implementation in the production line. It is mainly geared to the study or random texturization, although it is applicable to textures exhibiting periodicity. It is, therefore, a measuring instrument with a high level of performance that enables the user to accurately and instantaneously know the degree of texture of each individual cell immediately after the chemical etching process. The early detection of cells with a low degree of texturization allows for re-processing, avoiding having to reject them later and therefore avoiding the associated production costs of such an action. The method is based in the analysis of the reflectance of a beam of collimated light, without the light having to have a high spectral coherency, which makes the use of a laser source not essential, although the use of a laser source of light is advisable for practical purposes.  
         [0012]     The optimal formation of the aforementioned pyramids depends on several factors, such as temperature, pH of the bath solution, immersion times and the location of the wafers in the bath, as well as the initial state of the surface of the wafer amongst other factors. The degree of ageing of the bath also plays a fundamental role. Because the effect of the chemical etching process is not cumulative, but reaches a degree of maximum texture (maximum wafer area covered by pyramids) after which these pyramids are later flattened and disappear, the early detection of the decrease in texture allows us to know at every given moment, the status of the chemical baths used for their development, and therefore, we can take appropriate action before their degradation is obvious when inspecting the final product. In this manner two objectives would be achieved: one would be to lengthen the life of the baths by means of controlled addition of the necessary components, and the other would be to avoid the introduction in the production line of wafers with an intermediate degree of texturing that would result in efficiencies below the optimum expected at the end of the process.  
         [0013]     The following is a list of references that may be referred to within the present specification. The citation of such references is for purposes of information only, and is not a characterization of their relevance to the present invention.  
       REFERENCES  
       [0000]    
       
          [1] K. Fukui, Y. Okada, H. Inomata, S. Takahashi, Y. Fujii, Fukawa and K. Shirasawa:  
          “Surface and bulk-passivated large area multicrystalline silicon solar cells.” Solar Energy Materials &amp; Solar Cells: 48,219-228 (1997).  
          [2] W. A. Nositschka, C. Beneking, O. Voigt and H. Kurz: “Texturisation of multicrystalline silicon wafer for solar cells by reactive ion etching through colloidal masks.” Solar Energy Materials &amp; Solar Cells: 76, 155-166 (2003).  
          [3] Y. Yerokhov, R. Hezel, M. Lipinski, R. Ciach, H. Nagel, A. Mylyanych and P. Panek: “Cost-effective methods of texturing for silicon solar cells.” Solar Energy Materials &amp; Solar Cells: 72, 291-298 (2002).  
          [4] J. Zhao, A. Wang, M. A. Green and F. Ferraza: “19.8% efficient “honeycomb” textured multicrystalline and 24.4% monocrystalline silicon solar cells.” Applied Physics Letters: 73, 1991-1993 (1998).  
          [5] P. Campbell and M. A. Green: “High performance light trapping textures for monocrystalline silicon solar cells.” Solar Energy Materials &amp; Solar Cells: 65, 369-375 (2001).  
          [6] Holdermann, K: “Method for the wet chemical pyramidal texture etching of silicon surfaces.” U.S. Pat. No. 6,451,218.  
          [7] Bailey W. L., Coleman, M. G. Harris, C. B. Lesk and I. A. Lesk: “Texture etching of silicon: method.” U.S. Pat. No. 4,137,123.  
          [8] Z. Xi, D. Yang and D. Que: “Texturization of monocrystalline silicon with tribasic sodium phosphate.” Solar Energy Materials &amp; Solar Cells: 77, 255-263 (2003).  
          [9] Y. Nishimoto and K. Namba: “Investigation of texturization for crystalline silicon solar cells with sodium carbonate solutions.” Solar Energy Materials &amp; Solar Cells: 61, 393-402 (2000).  
          [10] W. Müller, A. Metz and R. Hezel: “A new and simple approach for fabricating inverted pyramids on crystalline silicon solar cells.” Proceedings of the 17 th  European Photovoltaic Solar Energy Conference, Munich (2001).  
          [11] A. Paretta, E. Bobeico, L. Lancellotti, P. Morvillo, A. Wang and J. Zhao: “A new approach to the analysis of light collected by textured silicon surfaces.” Paper 1P-C3-14 in the 3 rd  World Conference on Photovoltaic Energy Conversion, Osaka, May 12-16 (2003).  
          [12] A. Paretta, A. Sarno, P. Tortota, H. Yakubu, P. Maddalena, J. Zhao and A. Wang: “Angle-dependent reflectance measurements on photovoltaic materials and solar cells.” Optical Communications 172, 139-151 (1999).  
       
     
       BRIEF DESCRIPTION OF THE INVENTION  
       [0027]     The present invention relates to a method and a device to determine the degree of texturization of materials, amongst others, the silicon wafers typically used as photovoltaic cells.  
         [0028]     The technique proposed to establish the degree of texturization consists, basically, in irradiate the wafer&#39;s surface with a collimated light beam and quantify the patterns of optical reflectance. In the following text we will refer to laser light as the source for this process because of their generally good collimation properties, without intending to communicate by it that the coherent character of the laser light has a decisive influence on the present invention.  
         [0029]     In the simplest assembly, the incident beam passes through an opening made on a flat screen where the light reflected by the wafer is viewed ( FIG. 3 ). The pattern of reflectance observed when analyzing a wafer without texture is a central point with circular symmetry ( FIG. 4 ) due to the spreading of the light over the ruggedness and surface defects of the wafer. The pattern of optical reflectance of the wafers with a high degree of texture is a central point as mentioned, but appearing with less intensity and accompanied by four circular areas with order four symmetry in relation to the direction of the incoming beam ( FIG. 5 ). This last reflectance pattern is due to the reflection on the four side faces of the pyramids. Proof of this is that when the wafer is rotated on the axis established by the incoming laser beam, the reflected pattern rotates in solidarity with the wafer. It should be noted that the light reflected by the wafer does not exhibit significant polarization changes in relation to the incoming light and that the angular opening of the pattern of light reflected is not sensitive to the wavelength of the light used, which would demonstrate that it is associated to specular reflections on the microscopic facets of the sample.  
         [0030]     The method and the device of the present patent are both applicable to those texture morphologies characterized by the development of correlated geometrical patterns on the surface of the substrate that supports the photovoltaic cell. These patterns may have individual dimensions and distances between two consecutive patterns that may be random or constant, but in all cases the faces of all the polyhedral shapes that make them up must be mutually parallel. These morphologies may be formed by several procedures, such as the chemical etching of the monocrystalline Si, whether with raised or inverted pyramids. The method described may also be used for the study of other degrees of texture developed in multicrystalline Si and the texture present in polycrystalline silicone deposited over previously textured substrates with the aforementioned conditions. It can also be applied to other materials presenting similar texturing patterns. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]      FIG. 1  Image obtained by scanning electron microscopy of the squared-based pyramids after chemical etching with NaOH. The angle formed between the pyramid&#39;s side faces is calculated from the image and the tilt of the faces in relation to the wafer&#39;s plane coincides, within the experimental uncertainty range, with the expected values between planes (111)Si and the planes (111)Si -(100) Si, 70.53° and 54.73° respectively.  
         [0032]      FIG. 2  Shows the zenithal image obtained by scanning electro microscopy of the square-based pyramids resulting from the chemical etching with NaOH. As observed, all the pyramids exhibit the same orientation, which makes the reflectance pattern be formed by four points symmetrically placed around the illumination axis.  
         [0033]      FIG. 3   a  Simple outline of a reflectance pattern observation device on the textured silicon surfaces by means of a flat display.  
         [0034]      FIG. 3   b  Simple outline of a reflectance pattern observation device on the textured silicon surfaces by means of a spherical display. The image in the figure shows a cross section.  
         [0035]      FIG. 3   c  Simple outline of a reflectance pattern observation device on the textured silicon surfaces by means of an ellipsoid of revolution display that acts internally as a reflector. The image in the figure shows a cross section.  
         [0036]      FIG. 4  Optical reflectance pattern of a non-textured (100) Si surface. The circular symmetry around the most intense central point can be observed. This point corresponds to the incoming light beam.  
         [0037]      FIG. 5  Optical reflectance pattern of a highly textured (100) Si surface. The order four symmetry around the most intense central point can be observed. This point corresponds to the incoming light beam.  
         [0038]      FIG. 6  Outline of the device proposed for the optical characterization of the degree of texture in the (100) Si wafers.  
         [0039]      FIG. 7  Comparison between the angular distribution of the intensity of the light reflected by a wafer (100) c-Si before the chemical etching process that originates the surface texture (t=0 min), and that of texture obtained in a sample subject to the etching process during 25 min., where θ is the observation angle in relation to the normal of the wafer&#39;s surface, expressed in degrees.  
         [0040]      FIG. 8  Outline of the multiple reflections of a light ray with normal incidence over a textured surface of the (100) Si.  
         [0041]      FIG. 9  Evolution of the texture parameter G incorporating the time it takes to chemically process the (100) c-Si wafers in an aqueous solution of NaOH and i-C 3 H 7 OH. The wafers marked “LAB” have been textured in trial baths while the ones marked as “PL” have been textured in the production line of a solar cell manufacturing facility. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0042]     As described above, the method entails determining the pattern of reflectance associated to a textured surface (particularly those Si surfaces that have been subject to chemical etching to attain pyramidal shapes) that is originated by the light of a laser beam with normal incidence. This pattern is captured on a flat screen as shown on  FIG. 3   a . According to the laws of reflection, the expected pattern for a surface composed of equal pyramid shapes would be composed of four points symmetrically located around the axis of the incoming beam, however, the presence of defects, non-textured areas, etc., causes an increase of the normal reflectance in detriment of the intensity associated to the texturing, which is the bases of the present method.  
         [0043]     The geometry of the screen may be adapted to the circumstances of the design.  FIGS. 3   a  and  3   b , respectively, show some of the alternatives that include spherical or semi-spherical screens, in which the wafer is placed in the middle and ellipsoid of revolution type screens in which the incoming beam passes through a focus point and the wafer, appropriately oriented, is located on the other of the foci of the ellipsoid. All these designs share a common characteristic. The beam reflected on the normal overlaps the incoming beam and therefore, makes difficult the measuring of the light reflected on the normal to the wafer. A technical solution to this problem is described further into the text.  
         [0044]     The measure of the degree of texture is obtained by comparing the intensity of the reflectance pattern with order four symmetry against the intensity exhibited in normal reflectance. To do this, it is necessary to place photodiodes over the screen and in the position of the maximums to register the intensity of the light. In a more elaborate version, detection may be done by means of a CCD detector or by a photodiode matrix distributed over the screen. The light source may be a low power monomode laser (&lt;10 mW) and good collimation properties (typically 1.5 mRad or lower) emitting in the region of λ=500-800 nm, typically a HeNe laser (λ=633 nm), of a semiconductor diode (λ=750-850 nm) with collimating optics, or one of these lasers with infrared emission (λ≈1060 nm), or Nd 3+  (λ≈1060 nm), in both cases in conjunction with a frequency doubler.  
         [0045]     The reflectance patterns corresponding to intermediate texture patterns exhibit the maximum intensity with a symmetry that corresponds to the number of faces of the geometries that make up the texture, although they may appear joined by bands of lesser intensity. This situation may be appreciated in the image that corresponds to the reflection of a wafer treated during 25 minutes, as shown in  FIG. 5 , formed by a four-faced pyramid.  
         [0046]     To characterize the degree of texture of the surface, the G parameter is defined as the relation between the sum of the intensities reflected in the maximums of the pattern with four order symmetry I 4n  (n=1−4), and the intensity normally reflected to the surface of the wafer I N  
       G   =           ∑     n   =     I   -   4                 ⁢     I     4   ⁢   n           I   N       .         
 
         [0047]     This is a non-linear function that ideally has a value of zero for the wafer without texture and a value of infinite for a wafer with perfect texture, although its value is limited by the overlapping between the patterns with circular symmetry and the patterns exhibiting order four symmetry The parameter is not sensitive to the fluctuations of intensity of the laser beam.  
         [0048]     There is a second set of parameters to characterize the uniformity of the texture:  
         χ   n     =         I     4   ⁢   n           ∑     n   =     I   -   4                 ⁢     I     4   ⁢   n           .         
 
         [0049]     Deviation from parameter χ n  from its mean value, {overscore (χ n )}, indicates lack of uniformity in the texture. A uniform texture has the following relationship:  
           χ   n     ⁢     /       =     1       χ   n     _           
        for each value of n.        
 
         [0051]     The system described so far enables us to analyze the average area irradiated by the laser beam, typically &lt;0.5×0.5 mm 2 . To incorporate the system to the production line a bi-dimensional analysis of the wafer is required. Wafers typically have a round shape with a 15 cm diameter. This analysis may be done by a xy scan of the wafer, by moving the wafer, or by moving the optic system used for the projection, however, the mobile elements complicate the design and require, in the long term, re-calibration and maintenance. The device described below minimizes the mobile elements and allows for the simultaneous analysis of several points in the wafer while said wafer moves over a conveyor belt in the y direction ( FIG. 6 ).  
         [0052]     Analysis of the direction of movement is done by activating the measuring system at a constant time interval that is related to the wafer&#39;s speed of movement. The x direction analysis is done simultaneously at several points within the wafer by means of dividing the initial laser beam into several beams of equal intensity vertically aimed (z-direction) towards the wafer.  
         [0053]     In this particular instance, it is necessary that the laser beam radiation is linearly polarized. The laser beam with an initial intensity of I o  propagating in x direction is split into several secondary beams of equal intensities I o r 1  aimed towards the z-direction. To do this several N beam splitters not-sensitive to the polarization of the incoming light are used and calibrated so their reflectivity r n  fulfills the following relationship:  
         r   n     =       r     n   -   1         1   -     r     n   -   1               
 
 Where n=1 . . . N, and  
         r   1     =     1     N   +   1           
 
         [0054]     A second set of polarization-sensitive beam divisors combined with λ/4 sheets allows us to redirect the light reflected towards the detectors. The incoming beam hitting the second set of beam divisors must linearly polarized in the y-direction and in such a manner that it can be transmitted by the beam splitter. The λ/4 sheet properly oriented with its axis at a 45° angle from the xy axis converts the light into circularly polarized light, and after reflecting from the wafer and going through a new transmission in the sheet the light emerges linearly polarized in the x-direction, and therefore it is totally reflected by the polarizing beam splitter in the y-direction. Finally, a lens (previously mentioned) collimates the image in order to project it onto the detector matrix where the intensity of the reflected beams is detected by the photodiodes and analyzed according to the previously described criteria. It is recommended to isolate the optical detection system from the ambient luminosity with interferential filters optimized to the wavelength and emission range of the laser utilized in the procedure and placed the filters in front of the photodiode matrix.  
       EXAMPLE OF THE EMBODIMENT OF THE INVENTION  
       [0055]     Following the proposed method of measuring the reflectance pattern,  FIG. 7  presents a comparison of the angular distribution of the light&#39;s intensity reflected by an untreated monocrystalline Si (100) wafer and the distribution of a wafer treated in a 5 dm 3  aqueous solution of NaOH and i-C 3 H 7 OH. The light intensity measurements have been done at various θ angles. This angle is varied on the plane containing the incoming light beam and is perpendicular to the wafer&#39;s surface and parallel to one of the sides of the pyramids&#39; base. In addition to a central maximum (θ=0) that is present even in the non-textured wafers, there are two lateral maximums observed at θ=22°, that correspond to the beams reflected twice in the sides of the pyramids as shown in  FIG. 8 .  
         [0056]     According to this figure, the angle of reflectance in relation to the incoming beam (θ) is related to the inclination or tilt (α) of the pyramid&#39;s side as shown by the relation θ=4α−180°. It has been observed that the angular gap 2θ between the non-specular reflectance varies in the 40-52° range as a function of the quality of the texture induced during the chemical etching process. This variation is associated to certain dispersion in the value of α and could be related either to the presence of stagger or any other defects present on the pyramid&#39;s faces, or to the crystalline quality of the starting substrate.  
         [0057]     To implement the system it is necessary to evaluate the intensity of the reflected light beam parallel to the incoming beam, as well as the intensity of the beams reflected in the maximums when they have an approximate value of 22°. This is resolved by using a beam splitter, typically a beam-splitter cube. The minimum dimensions of said beam-splitter cube are conditioned by the separation angle between the reflected beams and the separation between the angle and the wafer. To project the light pattern over the optical detection system, it is advisable to incorporate a collimating lens with a focal distance equal to the sum of the distances between the lens axis and the center of the beam splitter, and between the beam splitter and the wafer. To avoid undesired image in the detectors&#39; plane it is necessary that all the optical elements used (beam splitters, lenses, retardant film or sheets, detectors, etc) have anti-reflectant coatings with the same wavelength as that of the laser used during the procedure.  
         [0058]      FIG. 9  shows a simplified version of the G(I =26 °/I θ     ˜·   ) parameter as a function of the time of chemical etching process for a scan done on the plane (1{overscore (10)}) of the Si wafers treated according to the method described above. The results obtained in the production line (PL) using baths of a similar chemical composition but in larger dimensions are also included. It is observed that there is a critical time period needed to reach the maximum degree of texture and that time periods that exceed said critical time period cause the flattening of the pyramids inducing the decrease of the G parameter. The results of the wafers treated at the production line in highly clean conditions reach greater G values than those obtained during the trial baths. The kinetics of the process is probably different, and this, together with the impossibility of exceeding the treatment time during the production process has prevented the observation of the maximum.  
         [0059]     Although the example just described refers to four sided pyramidal geometries, this does not imply that the method is restricted solely to that type of geometry, since it is essentially valid for other geometrical figures (n-side pyramids, cones, etc.) that can also form on the Si surface, whether by chemical etching or other type of processes.  
         [0060]     It is to be understood that the invention is not limited to the illustrations described and shown herein, which are deemed to be merely illustrative of the best modes of carrying out the invention, and which are susceptible of modification of form, size, arrangement of parts and details of operation. The invention rather is intended to encompass all such modifications which are within its spirit and scope as defined by the claims.