Abstract:
The invention provides an improved optical system for determining the physical characteristics of a solar cell. The system comprises a lamp means for projecting light in a wide solid-angle onto the surface of the cell; a chamber for receiving the light through an entrance port, the chamber having an interior light absorbing spherical surface, an exit port for receiving a beam of light reflected substantially normal to the cell, a cell support, and an lower aperture for releasing light into a light absorbing baffle; a means for dispersing the reflection into monochromatic components; a means for detecting an intensity of the components; and a means for reporting the determination.

Description:
CONTRACTUAL ORIGIN OF THE INVENTION 
     The United States Government has rights in this invention pursuant to Contract No. DE-AC36-98G0-10337 between the United States Department of Energy and the Midwest Research Institute. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to an optical system for monitoring the optical quality of solar cells, and more particularly to a reflectance measuring system for use in monitoring the surface texture, metallization, and anti-reflective coating of solar cells in commercial production. 
     2. Description of the Prior Art 
     In the fabrication of photovoltaic cells it is necessary to precisely control the sawing, cleaning, texturing, dielectric-film-coating, and metallization process steps. Texture-etching is used to improve the light trapping ability of a solar cell, by reducing the surface reflectance over a broad light spectrum. Texture-etching is also used to remove any saw damage to the surface of a cell. Deposition of a dielectric-film layer, over the textured surface, is used to further reduce reflectance. Metallization of the cell includes alloying an aluminum back contact, and screen printing a front metal contact to the cell. Any failure to tightly control these process steps lends itself to the fabrication of devices, which exhibit a variance in the light-trapping ability, photo-current, fill-factor, and the open-circuit voltage of the cell. Accordingly, there is a need for an optical system useful in the quality control of these process steps. 
     Various optical systems are available for monitoring the physical characteristics of a photocell. In the prior art, many of these systems measure sample reflectance in a light integrating sphere. The light reflected from the sample is measured spectroscopically. For example, as described in U.S. Pat. Nos. 4,932,779, and 5,406,367, for color measurement, an integrating sphere is provided to receive light, from a light source, through an entrance port. The diffusely reflecting interior walls, of the sphere, reflect the light in multiple reflections, such that a uniform diffuse illumination is provided over the interior surface of the integrating sphere. The integrating sphere is provided with a port designed to receive a sample, the color of which is to be measured. When a sample is positioned over the sample port, the surface of the sample is illuminated with uniform diffuse illumination, reflected from the walls of the integrating sphere. An exit port is located on the sphere, opposite the sample port, for receiving diffusely reflected light from the sample, and the light passing through the exit port is separated into monochromatic components. The intensities of the components are measured, to determine the reflectance of the sample, for each monochromatic component. However, as demonstrated in the foregoing patents, an integrating sphere is used to analyze a small sample area, because the sample, itself, disrupts integration of the illumination. Because commercially sized samples have a large, 4 by 4 inch, surface, it would be necessary to provide an unreasonably large integrating sphere, to rapidly monitor the surface area of a solar cell. 
     In Sopori, B. L.,  Principle of a New Reflectometer for Measuring Dielectric Film Thickness on Substrates of Arbitrary Surface Characteristics , Rev, Sci. Instru. Vol. 59 no. 5 (May, 1988), pp. 725-727, a reciprocal optical principle, and a relative small light-absorbing sphere, has been used to determine the thickness of an antireflection film, layered over a silicon cell. The reciprocal principle is based on the projection of incident light, at a wide solid-angle of direction, at the sample surface and detecting the intensity of a reflection, normal to the sample surface. The reflectometer comprises a metallic, spherical, dome having openings for two ELH-tungsten-halogen lamps and elliptical reflectors. One lamp is located on each side of the dome. An exit aperture and lens assembly is located at the top of the dome for emitting the reflection. At the base of the dome, diametrically opposed to the exit aperture is a highly absorbing sample support. The support is covered with small-grain polycrystalline sheets, etched and layered with a Si 3 N 4  deposit, in order to reduce reflectance. Located at the top of the dome is a monochromator and detector, connected to a display device. The display generates a reflection intensity distribution curve for the reflection. The reflectance of a textured sample, having an antireflection coating, exhibits a minimum intensity, on the curve, which is useful in determining the thickness of the film, according to the equation: t=λ 0 /4 n , where λ 0  is the wavelength having a least reflectance, t is the thickness, and n is the refractive index of the film. 
     The absorbing and light integrating spheres are similar in construction, but the absorbing sphere must function to eliminate all, extraneous, scattered light. In doing so, the normal reflection is the only light detected. The major extraneous light-scattering source, in an absorbing sphere, is the cell support. While etching a fine grain polycrystalline silicon wafer and depositing a layer of Si 3 N 4  has produced a non-reflecting support, the monitoring system, according to this invention, provides a significantly different light absorbing baffle and a non-reflecting chuck in lieu of the silicon wafer support. Other significant differences are also included. This invention provides for an increase in the spectrum of projected light in order to generate a reflection from the back-side-contact, is able to monitor the area, thickness, and symmetry of a front-contact, and, because some venders produce cells having a specular surface, is able to monitor the characteristics of a cell having a polished or specular finish. These improvements are desirable in a system, which is useful, to monitor the texture, antireflective film, and metallization properties of solar cells in commercial production. 
     Thus, in view of the foregoing considerations, there is an apparent need for an optical system which is cost efficient, versatile in use, and capable of arriving at a rapid precise determination of the texture, metallization, and dielectric film optical properties of solar cells in commercial production. 
     SUMMARY 
     In view of the foregoing, it is a general object of the present invention to provide an improved optical system which is cost efficient, versatile in use, and capable of arriving at a rapid precise determination of the texture, metallization, and dielectric film optical properties of solar cells in commercial production. 
     Another object of the invention is to provide an optical system for precisely comparing the optical properties of solar cells after sawing, texturing, cleaning, antireflective coating, and metallization fabrication steps with a predetermined value. 
     Another object of the invention is to provide an improved system to determine dielectric film thickness. 
     Another object of the invention is to provide an improved system to determine the surface texture of a solar cell. 
     It is yet another object of the invention to provide a system to determine the area, thickness, and symmetry of a front-metal-contact, and the optical quality of a back-contact to a solar cell. 
     The foregoing specific objects and advantages of the invention are illustrative of those which can be achieved by the present invention and are not intended to be exhaustive of limiting of the possible advantages which can be realized. Thus, those and other objects and advantages of the invention will be apparent from the description herein or can be learned from practicing the invention, both as embodied herein or as modified in view of any variations which may be apparent to those skilled in the art. 
     Briefly, the invention provides an improved optical system for determining the physical characteristics of a solar cell. The system comprises a lamp means for projecting light in a wide solid-angle onto the surface of the cell; a chamber for receiving the light through an entrance port, the chamber having an interior light absorbing spherical surface, an exit port for receiving a beam of light reflected substantially normal to the cell, a cell support, and an lower aperture for releasing light into a light absorbing baffle; a means for dispersing the reflection into monochromatic components; a means for detecting an intensity of the components; and a means for reporting the determination. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings, which are incorporated in and form a part of the specification, illustrate the preferred embodiments of the present invention, and together with the descriptions serve to explain the principles of the invention. 
     FIG. 1 is a sectional view of one embodiment of the optical system according to the present invention. 
     FIG. 2 is a sectional view of the optical system illustrated in FIG. 1 showing the mode of operation for imaging a sample. 
     FIG. 3 is a sectional view of the optical system illustrated in FIG. 1 showing a variant of the lamp arrangement together with a diffuser for determining specular reflection. 
     FIG. 4 is a reflection intensity distribution curve showing a comparison of the reflection intensity of four samples, during the sawed/cleaned, texture/etched, and film-coating fabrication steps. 
     FIG. 5 a  is a sectional view of a silicon solar cell having an aluminum back-contact, and a front metal-contact. 
     FIG. 5 b  is a three dimensional view of the solar cell illustrated in FIG. 5 a  showing the bus-bar and finger elements of the front metal-contact. 
     FIG. 6 is a reflection intensity distribution curve showing a comparison of the reflection intensity resulting from a high, and low, quality backside-contact. 
     FIG. 7 is a reflection intensity distribution curve showing a comparison of three solar cells using asymmetric illumination to determine front-metallization. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Unless specifically defined otherwise, all technical or scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present invention, the preferred methods and materials are now described. 
     Referring now to the drawing figures, in which like numerals refer to like components, there is illustrated in FIG. 1, a preferred embodiment of the invention. Reflectometer  10  includes a spherical chamber  22 , which is preferably an 18 inch diameter dome. Chamber  22  includes an exit aperture  24  and a diametrically opposed lower aperture  26 , which serves as a sample port. Lamps  33 ( 1 - 4 ) project incident light  32  at sample  14  in a wide solid-angle of direction. A solid angle is a measure of the angle subtended at the vertex of a cone. Exit aperture  24  is included for emitting a beam of light  40  which is reflected normal to the sample  14 . The interior surfaces of the absorbing chamber  22 , baffle  11 , and post  15  are roughened, and coated with a non-reflective light absorbing coating, such as flat-black paint. This treatment is used to insure that stray light  40 - 1 ,  32 - 1  (e.g. light not reflecting normal to the sample  14  the surface) is substantially absorbed, without an inadvertent reflection passing through the exit port  24 . The lower aperture  26  is 8 inches in diameter and circumferentially disposed about the silicon sample  14 . A vacuum chuck  12  is positioned below the lower aperture  26  to secure the sample  14  for analysis. The chuck  12  is smaller in size than the sample  14  to be characterized prevent the scattering of light  32 , by the chuck  12 . The chuck  12  is supported on a movable post  15 , which aids in the positioning of sample  14  within aperture  26  for illumination. The light  32 , is projected into the chamber  22  via an entrance port  13 , and is generated with lamp sources  33 - 1 , 2 , such as two ENH tungsten-halogen lamps, one on each side of the chamber  22 . The lamps have an elliptical metallic beck reflector  35  formed by lining the inside of an existing dichroic reflector with an aluminum foil such that the projected light  32  is in a 400-1200 nm spectral range. The lower aperture  26  is positioned in diametric alignment with the exit port  24 . The light absorbing baffle  11  traps light  32 - 1  passing, sample  14 , through aperture  26  in an outward direction from the chamber. 
     In the practice of the invention, sample  14  is positioned on the vacuum chuck  12 , a vacuum is applied, and post  15  is raised such that the sample  14  is circumferentially disposed within the lower aperture  26 , of the chamber  22 . The sample  14 , is illuminated thereby causing the emission of reflected beam  40 , outwardly, through the exit aperture  24 . The exit port  24  is associated with a lens assembly  49  for convergence of the reflected beam  40 . A fiber-optic-cable  42  transmits light beam  40  for dispersion, detection, determination, and reporting  115 . 
     As seen in FIG. 1, fiber optic cable  42  is connected to a monochromator  120 , or a filter wheel  130  and servo motor  133  assembly. The fiber optic cable  42  yields a high signal to noise ratio. A strong signal is necessary to determine the precise optical differences among samples. Monochromator  120  or filter wheel  130 , disperses reflection  40  into its monochromatic components  125 . The components  125  are detected by photo-detectors  122  which generate a, computer-receptive, signal  128  relative to the intensity of the detected component. 
     The respective intensity for each monochromic component is stored in the memory of a computer  150 . The computer  150  determines the relationship between the intensity of the detected reflection and wavelength for each monochromatic component, a thickness of the dielectric film, and the metallization of the sample cell  14 . A computerized report illustrates a reflection intensity distribution curve. The report is useful in making a comparison of the surface texture, film thickness, and metallization properties of a sample with a predetermined result. The result maybe, a standard reflection intensity distribution curve, film thickness, or metallization value, which is stored in the RAM or ROM of the computer  150 . Display  152  illustrates the determination, and is used to sequentially monitor the physical quality of solar cells throughout the fabrication process. 
     Referring now to FIG. 2, it is generally shown therein a sectional view of the optical system illustrated in FIG. 1, as modified to display an image of the sample. To view an image of the cell  14 , slidable mirror or prism  50  is located into the path of reflected beam  40 . Beam  40  is, thereby, deflected in the direction of a digital camera  52  and its zoom lens  54 . Those rays, which fall in the path of the field-of-view of the camera  52 , become the object image from, which a real image of sample  14  is created. The real image may be viewed on a control monitor (not shown), associated with the video camera  52 . An optical filter (not shown) may be located in the optical path of the camera, to provide a variety of useful information. For example, an image of any variations in the thickness of a film, over the sample surface, is shown when using a filter passing waves at λ 0 , in a system calibrated according to the equation: (λ 0 )=4nt; where λ 0  is the monochromatic component having the least reflectance; t is the thickness, and n the refractive index of the film. One skilled in the art will also appreciate that camera  52  may also include a provision for a video tape recording, or input into the memory of computer  150 , for permanent documentation of the image. 
     Reflectometer  10 , of FIG. 1, is the basic embodiment of the invention. However, the invention may include a modification which is useful to characterize a sample having a specular sample. A specular sample is one having a smooth or polished surface. Referring now to FIG. 3, a sectional view of the optical system illustrated in FIG. 1, is shown, together with the modification for determining the normal reflection to a specular sample. Spherical chamber  22  further includes a diffuse reflector  28 . The reflector  28  is circumferentially disposed about the exit port  24  of the chamber  22 . Here, lamps  33 - 5 , 6  are provided, with elliptical reflectors  35 , below the horizontal axis  60 , of the chamber  22 . Projected light rays  32 , from lamps  33 - 5 , 6 , pass through converging lenses  30  in the direction of diffuser  28 , such that rays  32  fall upon a specular sample  14 - 1 , in a direction substantially normal to the surface of the cell. Reflected beam  40  is then emitted through exit port  24  for imaging  52 , or dispersion, detection, determination and reporting  115 , as described above. 
     Turning now to FIG. 4, a reflection intensity distribution curve is shown for comparison, of the optical quality, of four commercially sized, 4.5×4.5 inch, photovoltaic silicon wafers (shown as variations in line). The curve represents the determined reflection intensity of the normal reflection, plotted versus the wavelength, for each monochromatic component. The samples were monitored using the optical system of FIG. 1, Symmetric illumination was provided with lamps  33 - 1  and  33 - 2 , of FIG.  1 . These samples were monitored after the following three different stages in the fabrication process: (1) curves  4 - 1  illustrate the reflectance of the samples after texture-etching; (2) curves  4 - 2  illustrate reflectance of the samples after sawing and cleaning; and (3) curves  4 - 3  illustrate reflectance of the sample results after deposition of a TiO 2  antireflective-film coating. 
     As shown in FIG. 4, the sawing/cleaning fabrication step  4 - 2  demonstrates a high precision, sample to sample. In comparison, the texture/etching step  4 - 1  is less precise. Moreover, the antireflective-film coating step  4 - 3  has partially mitigated the variance attributable to the texture/etching step. The TiO 2  film thickness is 794-858 angstroms. This determination is made, as above, by finding the wavelength having the least reflectance  4 - 4  and then solving for the equation: λ 0 =4nt. As shown in the Figure, the reflectance minimum  4 - 4 , λ 0 , is well-defined and, as a result of the strong signal for detection, is easily detected. The curves to the right of line  4 - 5  illustrate reflectance of the back-side contact. The graph was obtained over a 15 second interval. This interval included mounting and dismounting the sample, dispersion, detection, determination, and display of the results. The test measured reflectance over the full surface of the cell. 
     With system  10 ,  115  of FIG. 1, calibrated and adjusted to a predetermined sensitivity, the invention is useful to determine the physical quality of the front or back metal-contacts of a sample cell  14 . A cross section of a typical solar cell  14  is shown in FIG. 5 a . In the Figure, the aluminum back-contact  14   a  supports a silicon layer  14   b . Dielectric film  14   c  overlays silicon layer  14   b . The front contact  14   d  is screen printed over the silicon layer  14   b . The front contact includes a bus bar  14   d - 1  and finger  14   d - 2  configuration, in an asymmetric pattern. Thus, the optical characteristics of the front contact  14   d  change with the rotational orientation of the sample  14 , on the sample support  12 , of FIG.  1 . 
     A good quality silicon-aluminum back-contact exhibits a very high reflectance. Referring briefly to FIG. 1, a determination of the quality of the back-contact is made by illuminating sample  14  with lamps  33 - 1 , 2 , and comparing the reported distribution curve obtained with that of a predetermined value, such as 90% reflectance. Reference is now made to FIG. 6, which is a reflectance intensity distribution curve, showing the difference in the back-side reflectance of three sample cells: A silicon control  6 - 2 , a high-quality aluminum-contact  6 - 1 , and a low-quality aluminum-contact  6 - 3 . As the wavelength increases, there is greater penetration of the projected light into the silicon. As shown in FIG. 6, at about 1200 nm, more light is transmitted, which results in a reflectance from the backside-contact. The difference in reflectance between the high-quality contact  6 - 1 , and the low-quality contact  6 - 3  is useful to monitor the precision of the contact fabrication process. 
     Referring once again to FIG. 5 a , the configuration, and reflectance, of the front metal contact allows for a determination of the metal surface area, thickness, and symmetry. Symmetry is used to define the orientation of the printed metal pattern  14   d , as the sample  14  is rotated 90 degrees on the sample support. Reflection intensity of the front contact  14   d , is equal to the sum of the reflectance from bus bar  14   d - 1  and finger  14   d - 2  elements. In the Figure, Ra is the reflectance from the metal, upper, surfaces (area) of elements  14   d - 1  and  14   d - 2 . Rb is the reflectance from light scattered by the step (thickness) surfaces of elements  14   d - 1  and  14   d - 2 . 
     Referring now to FIG. 5 b , it can be seen a three dimensional view of the solar cell illustrated in FIG. 5 a . In the Figure, it is shown how the above configuration, taken in conjunction with directional illumination, can be used to determine the area, thickness, and symmetry of the front metal-contact  14   d . When the sample surface  14   c ,  14   d ( 1 - 2 ) is illuminated with lamps  33 -( 1 - 4 ), from all angles (symmetric illumination, not shown), a 90-degree rotation of the sample, on the support, will not cause a change in the total reflectance (Rt). However, as shown in the Figure, with directional illumination (asymmetric) parallel to the bus bar, from lamps  33 -( 2 , 4 ), applied, the reflectance Rb is primarily due to the upper surface of the bus bar (metal area). By changing the direction of the illumination (asymmetric illumination, not shown), from lamps  33 -( 2 , 4 ), to lamps  33 ( 1 , 3 ) the reflectance Rb is now primarily due to the upper surface of the finger elements  14   d - 2 . 
     Turning now to FIG. 7, it is shown, therein, a reflection intensity distribution curve for three solar cells in order to determine the symmetry of the front metal-contact. In the Figure, solid-line  7 - 3  is the reflectance (in arbitrary units) from a cell without front metallization; line  7 - 1  shows the reflectance from symmetric illumination, parallel to the bus-bar; and line  7 - 2  shows the reflectance from asymmetric illumination, perpendicular to the bus-bar. These differences in reflectance, from symmetric  7 - 1  to asymmetric  7 - 2 , allow the computer to determine metallization of the front-contact, according to the following principles: 
     (1) Rt=(Rm+Rnm) 
     (2) (Rm)=(Ra+Rb) 
     (3) (Rnm)=Rsi or Rar/si 
     where Rt is the total reflectance, Rm is the reflectance of the metal, Rnm is the reflectance of the non-metal, Ra and Rb is the reflectance related to the area and thickness of the metal (step), respectively, Rsi is the reflectance of the silicon, and Rar/si is the reflectance of the silicon and antireflective coating  14   c , if any. Thus, where the cell has (1−x%) metal then x% is the area covered by the silicon Rsi, or coated  14 - c  silicon fraction Rar/si, of the cell. In most cases Rm is equal to Ra. Ra is a predictable constant, based on a predetermined result, and Rb varies with the direction of illumination. 
     The foregoing description is considered as illustrative only of the principles of the invention. For example, it is contemplated, that the optical system herein finds equal utility in the analysis of solar cells comprised of amorphous-silicon, cadmium-telluride, and copper-indium diselenide, and in the determination of a film thickness over these, and other, substrates. Furthermore, since numerous modifications and changes will readily occur to those skilled in the art, it is not desired to limit the invention to the exact construction and process shown as described above. Accordingly, all suitable modifications and equivalents may be resorted to falling within the scope of the invention as defined by the claims which follow.