Abstract:
An apparatus for inducing a current in a solar cell substrate. A substrate receiving surface receives the substrate, and an array of a plurality of individually addressable light sources illuminates the substrate in a sequenced manner. A sequencer controls the sequenced manner of illumination of the substrate by the array. A front side electrical contact makes electrical contact to a front side of the substrate, and a back side electrical contact makes electrical contact to a back side of the substrate. A meter is electrically connected to the front side electrical contact and the back side electrical contact, and senses the current induced in the substrate during the sequenced illumination of the substrate.

Description:
FIELD 
     This invention relates to the field of photovoltaic cells. More particularly, this invention relates to testing for the defects that tend to reduce the efficiency of photovoltaic cells. 
     BACKGROUND 
     Improving photovoltaic conversion efficiency and reducing manufacturing costs have been the main drivers of the solar energy industry. A majority of commercial solar cells are made from multicrystalline or single crystal silicon wafers. Other types of solar cells based on thin film technologies such as Cu(In,Ga)Se 2 , CdTe, and amorphous silicon have shown great growth potential due to their lower manufacturing cost. Regardless of the technology used, there is generally a gap between the efficiency of devices produced in the lab and devices produced by mass production, mainly because of various imperfections introduced during the fabrication process. Inspecting the solar cells for defects during the fabrication processes, and finding the root causes of defects can improve the production yield and reduce manufacturing costs. 
     Laser beam induced current has been used to investigate solar cell defects. The method can detect various types of defects that affect the solar efficiency of a solar cell. A schematic of a laser beam induced current system  10  is shown in  FIG. 1 . A laser beam  12  from a laser source  14  is focused to a small spot onto the surface of a substrate  16 , and a scanning device, typically an XY scanning stage or chuck  18  for moving the substrate  16  and an XY scanning mirror  20 , scan the laser spot across the surface of the substrate  16  in a raster scanning scheme. The current is measured by an external circuit  22 , such as a current amplifier connected to the solar cell  16 . The spot scanning image is a spatial map of the efficiency of the solar cell  16  in converting light into electrical current. Dark spots in the spatial map indicate that a lower current was collected by the external measurement circuit  22 , which can be caused by various types of defects in the solar cell  16 . These defect types typically include light blockage at the surface of the solar cell  16 , low absorption of light, low quantum efficiency, and current leakage defects (shunts). 
     Solar cells  16  have large capacitances that slow their response time when electric measurements are taken. In a production environment where the speed of inspection and testing are more important than the raw optical resolution, these slow-downs tend to be rather expensive. The speed of high resolution spot scanning laser beam induced current measurement is limited by the solar response time, and therefore is not generally suitable for inspecting very large substrates at a high resolution. It is also not generally suitable for integration with solar current-voltage testing, which can have an advantage in overall inspection and testing throughput and cost of ownership. Other disadvantages of spot scanning laser beam induced current include the higher cost associated with lasers, spot scanning hardware, and the complexity of the mechanical moving device. 
     What is needed, therefore, is a system that overcomes problems such as those described above, at least in part. 
     SUMMARY 
     The above and other needs are met by an apparatus for inducing a current in a solar cell substrate. A substrate receiving surface receives the substrate, and an array of a plurality of individually addressable light sources illuminates the substrate in a sequenced manner. A sequencer controls the sequenced manner of illumination of the substrate by the array. A front side electrical contact makes electrical contact to a front side of the substrate, and a back side electrical contact makes electrical contact to a back side of the substrate. A meter is electrically connected to the front side electrical contact and the back side electrical contact, and senses the current induced in the substrate during the sequenced illumination of the substrate. 
     In this manner, an apparatus according to the present invention can induce a current in a solar cell substrate very quickly and relatively inexpensively, because no laser and scanning hardware are required. The sequenced operation of the illuminating array provides the targeted illumination of the substrate for detection of defects, based on the current that is measured as a given portion of the substrate is illuminated. The illumination array can be constructed with no moving parts, and thus can be extremely durable and reliable with little or no maintenance required. 
     In various embodiments according to this aspect of the invention, the array is a two dimensional array of individually addressable light sources. In other embodiments the array is a linear array of individually addressable light sources. In some embodiments the array includes two linear arrays of individually addressable light sources, where the pixels of each array are offset one from another. In some embodiments the individually addressable light sources are monochromatic light emitting diodes, and in other embodiments the individually addressable light sources are multi-chromatic light emitting diodes. The array of individually addressable light sources in some embodiments is a diffuse light source adjacent a liquid crystal display, where individual pixels of the liquid crystal display are individually addressable and operable to permit or prevent transmission of light from the diffuse light source in the sequenced manner. 
     Some embodiments include a lens for focusing the sequenced illumination on the substrate, or an array of lenses, where one each of the lenses in the array of lenses is associated with one of each of the pixels in the array of individually addressable light sources. In some embodiments a gradient index lens array is disposed between the array of individually addressable light sources and the substrate receiving surface. 
     In one embodiment a detector receives reflected light from the substrate and determines an intensity of the reflected light. Some embodiments include a voltage sensing meter and a current providing instrument that are electrically connected to the substrate, and means for constructing a current-voltage profile of the substrate during the sequenced illumination of the substrate. The front side electrical contacts of some embodiments include electrical probes that are disposed on the array of individually addressable light sources, where the electrical probes make electrical contact to the front side of the substrate as the array is brought toward the substrate receiving surface. In some embodiments the array of individually addressable light sources is disposed in a gantry that is movable along a length of the substrate receiving surface. The substrate receiving surface in some embodiments is disposed on a chuck that is movable underneath the array of individually addressable light sources. 
     According to another aspect of the invention there is described a method for inducing a current in a solar cell substrate. The substrate is placed on a substrate receiving surface and illuminated in a sequenced manner with an array of a plurality of individually addressable light sources. Electrical contact is made to both the front side of the substrate and the back side of the substrate, and a current induced in the substrate during the sequenced illumination of the substrate is sensed with a meter that is electrically connected to the front side electrical contact and the back side electrical contact. 
     In various embodiments according to this aspect of the invention, a substrate voltage is sensed with a voltage sensing meter that is electrically connected to the substrate, a current is provided to the substrate with a current providing instrument that is electrically connected to the substrate, and a current-voltage profile of the substrate during the sequenced illumination of the substrate is constructed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
         FIG. 1  is prior art functional block diagram of a prior art laser beam induced current system. 
         FIG. 2  is a functional block diagram of a light induced current system according to an embodiment of the present invention. 
         FIG. 3  is a timing diagram for energizing the individually addressable pixels of an illumination source for a light induced current system according to an embodiment of the present invention. 
         FIG. 4  is a functional block diagram of a light induced current system having two offset one-dimensional light source arrays according to another embodiment of the present invention. 
         FIG. 5  is a functional block diagram of a light induced current system having a two-dimensional light source array according to an embodiment of the present invention. 
         FIG. 6  is a functional block diagram of a light induced current system according to an embodiment of the present invention, depicting the light induced current measurement module and the current-voltage measurement modules. 
         FIG. 7  is a representational diagram of light element spacing according to an embodiment of the present invention. 
         FIG. 8  is a graph of individual and overall light profiles in a light induced current system according to an embodiment of the present invention. 
         FIG. 9  is a functional block diagram of a light induced current system according to yet another embodiment of the present invention. 
         FIG. 10  is a functional block diagram of a light induced current system having lenses for the light source according to an embodiment of the present invention. 
         FIG. 11  is a functional block diagram of a light induced current system having gradient index lenses for the light source according to an embodiment of the present invention. 
         FIG. 12  is a functional block diagram of a light induced current system according to still another embodiment of the present invention. 
         FIG. 13  is a functional block diagram of a light induced current system having a single focal lens according to an embodiment of the present invention. 
         FIG. 14  is a functional block diagram of a light induced current system having a gated, diffuse light source according to an embodiment of the present invention. 
         FIG. 15  is a functional block diagram of a light induced current system having a digital mirror according to an embodiment of the present invention. 
         FIG. 16  is a functional block diagram of a light induced current system having a reflection monitor according to an embodiment of the present invention. 
         FIG. 17  is a functional block diagram of spring loaded probes on the light array of a light induced current system according to an embodiment of the present invention. 
         FIG. 18  is a functional block diagram of an in-process light induced current system according to an embodiment of the present invention. 
         FIG. 19  is a flow chart of the use of a light induced current system according to an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     According to the various embodiments of the present invention, an array of light sources other than a laser beam is used to induce a current in the substrate  16 . The light source is directed to specific known locations on the substrate  16 , eventually covering all or all of a desired portion of the substrate  16 , so that a diagram of the current profile at different positions on the substrate  16  can be created, which diagram is generally referred to as a light induced current image. In this manner, a light induced current image of the substrate  16  can be generated very quickly, with no moving parts (in some embodiments), and very inexpensively (compared to a laser induced current image). This method is also very easily combined with current-voltage testing. Further, the method can be applied to very large substrates  16 . 
       FIG. 2  shows the general construction of one embodiment of a system  100  according to the present invention. An array  102  of light emitting diodes  104  illuminates the substrate  16  at a relatively close distance. Each light emitting diode  104  in the array  102  is switched on and off in a sequence that is controlled by a timing circuit  106 . The solar cell  16  is connected to an external measurement circuit  108 , such as a current amplifier, to measure the short circuit current generated by the sequenced illumination. When the light emitting diodes  104  in the array  102  are sequenced as indicated in  FIG. 3 , the current measurement is synchronized with the sequence of illumination, and generates a spatial map of current generated by the localized illumination. Dark areas in the image indicate a low generation of current in the portion of the solar cell  16  that was illuminated at that point in time. A two-dimensional image can be generated by moving the substrate  16  or the array  102  along the direction that is perpendicular to a line of light emitting diodes  104  in one or more arrays  102   a  and  102   b , as shown in  FIG. 4 . 
     The light emitting diodes  104  in the array  102  may all be of the same wavelength, or of different wavelengths. Longer wavelength light penetrates deeper into the substrate  16  than shorter wavelengths do. Within its spectral response, a solar cell  16  tends to have a lower efficiency at shorter wavelengths. Light beam induced current images at different illumination wavelengths can be used to further classify defects—for example, optical defects versus electrical defects. Light beam induced current images at different wavelengths may be processed to derived spatial variations of junction depth, carrier diffusion length, and surface recombination. 
     A two-dimensional, addressable array  102  can be used to generate light induced current images with stationary illumination instead of with a laser beam, as shown in  FIG. 5 . The on/off sequencing of the light emitting diodes  104  can be controlled to achieve a raster scan of the full substrate  16 . A two dimensional array  102  is compatible with illuminated current-voltage testing and solar efficiency measurements, which are typically performed with a dedicated solar cell sorter/tester. When all of the light emitting diodes  104  are on, the array  102  produces a uniform illumination of the full surface of the substrate  16 . 
     As depicted in  FIG. 6 , a separate current-voltage testing circuit  110  is connected to the solar cell  16 , and a number of parameters such as short circuit current, open circuit voltage, shunt resistance, serial resistance, and solar cell  16  efficiency can be measured using separate voltage measurement circuit  112  and current providing circuit  114 . 
     For current-voltage testing, the output intensity of each light emitting diode  104  in the array  102  needs to reach the standard testing condition of one Sun of illumination, which is one milliwatt per square millimeter, with about half of the light being outside of the solar response spectrum. High intensity light emitting diodes  104  that are currently commercially available can output more than about one hundred times this amount of light. Therefore taking into account the light lost due to numerical aperture coupling, a closely packed array  102  of light emitting diodes  104  can generate one Sun of illumination intensity. The range of the solar spectrum that is within the spectral response of solar cells  16  can be simulated with multiple wavelengths of light emitting diodes  104 , where either a given light emitting diode  104  emits multiple wavelengths, or a series of light emitting diodes  104  is used, where each of the light emitting diodes  104  in the series emits a different wavelength of light. 
     For proximity illumination, the optical resolution is determined from the divergent angle theta of each diode  104  and the distance H between the array  102  and the substrate  16  according to the equation: 
     
       
         
           
             D 
             = 
             
               2 
               ⁢ 
               H 
               × 
               
                 Tan 
                 ⁡ 
                 
                   ( 
                   
                     θ 
                     2 
                   
                   ) 
                 
               
             
           
         
       
     
     as depicted in  FIG. 7 . The pixel size is given by the spacing P between the diodes  104  (for uniform spacing). The pixel size and optical resolution can be optimized for a good balance between throughput and sensitivity. Typically the optical resolution can be two to four times the pixel size. 
     For current-voltage testing, it is important to generate a uniform illumination. Therefore, the spacing between diodes  104  needs to be small enough so that the overlapping illumination profiles  116  from each of the diodes  104  add together to generate a uniform illumination profile  118 , as depicted in  FIG. 8 . Typically, if the illumination profile  116  of each diode  104  can be approximated by a Gaussian function, then the spacing between diodes  104  needs to be less than e −2  times the width of the Gaussian profile. Alternately, a diffuser  120  can be inserted between the array  102  and the substrate  16  when a current-voltage test is taken (as depicted in  FIG. 9 ). The diffuser  120  is then removed for a light induced current scan. 
     The array  102  can have a squared grid layout of the diodes  104 , or other layouts of the diodes  104  that provide for a more efficient illumination of the substrate  16  when multiple wavelengths of diodes  104  are used for illumination. To have a more flexible control of optical resolution and working distance, a lens array  122  can be used to image the array  102  onto the substrate  16 , as depicted in  FIG. 10 . A baffle tube  124  between the lens array  122  and the array  102  can block stray light, such as that from the large divergent angle of each diode  104 . A gradient index lens array  124 , such as with 1:1 erected imaging, can also be used to improve the optical resolution of the array  102 , as depicted in  FIG. 11 . For a one-dimensional array  102  with a gradient index lens  124 , two linear arrays  102   a  and  102   b  may be used, with an offset of one-half of a pixel (diode  104 ) between them, to achieve a 100% fill factor, as depicted in  FIG. 4 . 
       FIG. 12  depicts an alternate embodiment that uses a diffusive light source  126  and a liquid crystal display light modulator  128 . Each pixel of the modulator  128  switches the light passing through it on and off, thus producing a raster sequence of on/off switching of each pixel, and generating a light induced current image of the substrate  16 . 
     In another embodiment, the array  102  can be imaged onto the substrate  16  through a lens  130 , as shown in  FIG. 13 . This has the advantage of using a smaller array  102  that is magnified to cover a larger substrate  16 . The array  102  can be a chip-scale array  102  (instead of using discrete diodes  104  that are mounted on a circuit board), which can provide better resolution. However, the total illumination power may be relatively lower than the discrete array  102  of diodes  104 . The array  102  can be replaced with a diffusive light source  126  combined with a light modulator  128  such as a liquid crystal display modulator or transmission spatial light modulator, as described above and as depicted in  FIG. 14 . 
       FIG. 15  depicts an embodiment that uses a light source  144  that is focused onto a beam splitter  140  through a lens  142 . The beam splitter  140  reflects a portion of the light onto a digital mirror  138 , which is then focused by the imaging lens  130  onto the substrate  16 . 
     A detector array  146  can be used to measure the intensity of light that is reflected from an illumination source  148 , as depicted in  FIG. 16 . The illumination source  148  can be a linear array of diodes  104  with individual lenses, for example. The uniformity and thickness of the anti-reflective coating can be derived from the reflectance measurement, and the absorption image can be calculated by subtracting the reflected intensity from the illumination intensity. A quantum efficiency map can also be derived from the light induced current image and the absorption image. The illumination intensity can be calibrated by using a uniform reflective surface such as a mirror in place of the substrate  16 . 
       FIG. 17  depicts an array  102  with integrated electrical probes  152  for current-voltage testing. This apparatus reduces the steps required for loading and unloading substrates  16  when electrical testing is involved, since the illuminator  102  and the probes  152  can be raised as a single piece above the substrate  16  for loading/unloading operations, and then lowered for testing operations. Back side contact to the substrate  16  can be made automatically through a conductive surface on the substrate receiving surface of the system  100 , such as a chuck  18 , as generally depicted in various ones of the figures. Thus, electrical connections can be made to the substrate  16  without any additional steps. A heat sink  150  is preferably disposed on the back side of the array  102 , so as to remove heat from the array  102 . 
       FIG. 18  depicts yet another embodiment of the apparatus, in which the array  102  is disposed in a gantry  152  that can scan across the substrate  16  to acquire a two-dimensional light induced current image. One embodiment integrates the light induced current system  100  with other inspection and repair modules on a factory conveyor, and replaces a section of the conveyor with a simple linear motion mechanism that moves the solar cell  16  at a more controlled speed, and at the same time provides electrical contact for current measurements, such as generally depicted in  FIG. 18 . In such configurations, the typical resolution is around 0.5 mm±1 mm, and the inspection speed is around one second per solar cell  16  at a dimension of 150 mm×150 mm. Illuminated current-voltage testing can also be included in this embodiment. The embodiment of  FIG. 18  can also be scaled to work with large thin film solar cells that can be larger than two meters square. 
       FIG. 19  is a flow chart depicting one series of steps for using the system  100  in a solar cell  16  fabrication process. According to this embodiment of the method, the process  200  produces a solar cell substrate  16 , which is tested on the apparatus  100  as described above, and given in step  202 . Information from the testing process  202  is fed back to the process  200 , to improve the process  200 . Information from the testing on the apparatus  100 , as provided in step  202 , is used to determine whether the substrate  16  should be passed directly to yield/efficiency binning  206 , or processed through shunt detection  204  and optional shunt removal  208 . 
     The foregoing description of preferred embodiments for this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide the best illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.