Patent Publication Number: US-2009238444-A1

Title: Optical imaging apparatus and method for inspecting solar cells

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to an optical imaging apparatus and method for inspecting solar cells, and more particularly to an optical imaging apparatus and method for inspecting solar cells by thermal imaging biased solar cells and performing inspections of the defects by the visible imaging technique. 
     2. Description of the Related Art 
     Photovoltaic systems, such as conventional solar cells, directly convert sunlight into electrical energy using the principles of the photovoltaic conversion. The conversion efficiency has a direct influence on the output of electrical power, and it is also one of primary factors that determine the price of the solar cell. After manufacturing, solar cells will be tested to determine their conversion efficiency. Greater conversion efficiency results in higher selling prices. So, if a solar cell manufacturer wants to attain the most economical production, the manufacturing process of solar cells must be maintained at a high level of quality. A key factor for high-quality production is a high-speed, high-throughput and high-precision inspection apparatus for solar cell testing and screening. 
     During production inspection, a manufacturer screens solar cells preliminarily, and then performs optical inspections, during which defective solar cells are screened out. This inspection process will lower the inspection time, increase the throughput, improve the process stability and, more important, lower the cost of manufacture. The current optical inspection technology for solar cells is not able to fulfill the inspection requirements for high quality manufacture in a mass-production line without increasing budget. In view of the manufacturing cost, it is essential to have an inspection with screening capability. 
     Defects or cracks in solar cells have the potential to severely limit the power output of a solar cell. Significant defects or cracks may even cause shorts or shunts. Current optical inspection apparatus may not be able to find all defects or cracks critical to solar cells in a mass production line, particularly those defects or cracks which are very small or hidden under the surface of the solar cells. 
     In view of the above-mentioned problems and requirements, a solar cell inspection apparatus that can improve inspection throughput and perform fast defect inspection is necessarily required by the solar cell industry. 
     SUMMARY OF THE INVENTION 
     The present invention proposes an optical imaging apparatus for inspecting a solar cell, which comprises a power supply, a thermal imaging device and a computing unit. The power supply is configured to apply a reverse biased voltage to the solar cell. The thermal imaging device is configured to obtain a thermal image of the solar cell. The computing unit includes a thermal image analysis module configured to identify hot spots in the thermal image, a locating module configured to locate the center positions of the hot spots and a visible image analysis module configured to identify the defect features of the hot spots. 
     In one embodiment, the reverse biased voltage is the breakdown voltage of the p-n junctions of the solar cell. The temperatures of the hot spots are higher than a temperature threshold, and the sizes of the hot spots are larger than an area threshold. 
     The method for inspection of solar cells by an optical imaging inspection apparatus comprises the steps of: applying a reverse biased voltage to a solar cell; acquiring a thermal image of the solar cell by a thermal imaging device; and identifying a hot spot with a temperature higher than a temperature threshold and a size larger than an area threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described according to the appended drawings in which: 
         FIG. 1  is a perspective view of the optical imaging inspection apparatus according to one embodiment of the present invention; 
         FIG. 2  is a side view of the laser pointer and the visible light imaging device arrangement according to one embodiment of the present invention; 
         FIG. 3  shows a side view of the optical imaging inspection apparatus according to another embodiment of the present invention; and 
         FIG. 4  is a flow chart of a method for inspecting a solar cell according to one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  and  FIG. 2  illustrate an optical imaging apparatus  100  for inspecting a solar cell  102  according to one embodiment of the present invention. During inspection, the solar cell  102  is held and electrically coupled on the stage  104  of the optical imaging apparatus  100 . A power supply  106  connected with the stage  104  and the solar cell  102  is used to provide a reverse-biased voltage to the solar cell  102  to increase its temperature. Some defects in the solar cell  102  generate heat locally under the reversed-biased voltage. A thermal imaging device  108 , used to obtain the thermal images of the solar cell  102 , includes an infrared camera  116 , which is coupled to a computing unit  110 . The computing unit  110  can extract, identify and locate the positions of the hot spots caused by defect heating effect. If there are hot spots, the computing unit  110  will also calculate the center positions of those hot spots. 
     Referring to  FIG. 1 , the stage  104  has a metal surface  134 , which is electrically coupled to the negative contact of the solar cell  102  held by the stage  104 . The power supply  106  connects and applies a positive voltage at a terminal  136  on the metal surface  134  and a negative voltage at the positive output line  138  of the solar cell  102 . Such type of connection causes the solar cell  102  to be in a reverse biased condition. Some defects in the solar cell  102  will generate heat and become hot spots after the reverse biased voltage is applied. The reverse biased voltage may be, for example, the breakdown voltage across the p-n junctions of the solar cell  102 . 
     Referring primarily to  FIG. 1  and  FIG. 2 , the optical imaging inspection apparatus  100  also includes a visible light-imaging device  114 , which is used to capture the images of defects. The visible light-imaging device  114  comprises a camera, which is coupled to the computing unit  110 . Various cameras may be used including Linescan camera, area camera, CCD or CMOS camera. The defect images captured by the visible light imaging device  114  are analyzed by a visible image analysis module. The visible image analysis module identifies the defect features, performs statistical analysis, and stores the statistical results in a statistical database. If the defects are very tiny, those images will be captured at high magnification. At high magnification, the camera  118  has a narrow field of view, and it is not easy to know the position to which the camera  118  is directed. Under this circumstance, a laser pointer  202  can be utilized for helping a user know the position at which the camera  118  is directed or where the location is on the solar cell  102  corresponding to the center of the captured visible image of the camera  118 , as illustrated in  FIG. 2 . 
     Referring again to  FIG. 1 , the computing unit  110  comprises a thermal image analysis module, a visible image analysis module, and a locating module. The thermal image analysis module identifies hot spots having temperatures higher than a temperature threshold and sizes larger than an area threshold. The locating module calculates the coordinates of the centers of hot spots, and the distances between the hot spots and the camera  118 . The temperature and area thresholds may be set by an operator or by predetermined default values. The default value of the minimum area threshold can be, for example, 1 pixel. Use of edge detection techniques or binarizing method can identify or extract hot spots. The edge detection method may be, for example, a first-order edge detection approach or a second-order edge detection approach. The binarizing method may include a fixed threshold scheme or an adaptive threshold scheme. The hot-spot temperature may be defined as, for example, the highest, median, mode, or average temperature. The computing unit  110  determines hot spots by following steps: the thermal image analysis module identifies hot spots by using an edge detection technique and then determines which hot spots exceed the thresholds, and finally the locating module calculates the centers of the hot spots by using, for example, centroid calculation method. The visible image analysis module may have, for example, the following recognition procedures: the module acquires a visible defect image, and extracts the features of the image for their templates, and compares the templates of the image with the templates stored in a database, and finally declares a match. The computing unit  110  further comprises a display  132  showing the image captured by the thermal imaging device  108  or the visible light imaging device  114 . 
     Referring again to  FIG. 1 , a drive module  112  is used to move the thermal imaging device  108  attached thereon around the stage  104  while capturing images. The drive module  112  is used to drive the visible light-imaging device  114  to the center positions of hot spots for capturing the visible defect images in sequence. The drive module  112  includes an x-motion unit  120  and a y-motion assembly  122 , and hence the drive module  112  is able to provide motion in X and Y directions. The x-motion unit  120  may be driven by a motor  124  and ball screw combination as illustrated in  FIG. 1 , or by a driver system such as linear motor, a belt of chain drive slide system, and the like. The y-motion assembly  122  comprises a y-motion unit  126  and a motion guide unit  128 . The y-motion unit  126  may be driven by a motor  130  and ball screw combination as illustrated in  FIG. 1 , or by a driver system such as linear motor, a belt of chain drive slide system, and the like. The motion guide unit  128  may be, for example, a rail guide assembly and the like. 
       FIG. 3  shows a side view of the optical imaging inspection apparatus according to another embodiment of the present invention. In this embodiment, an infrared camera  116  and an optical camera  118  are attached to a fixed frame  304 , and a moving stage  302  holding a solar cell  102  moves around to perform inspection. After the infrared camera  116  finishes capturing the thermal images of the solar cell  102  held by the moving stage  302 , the moving stage  302  will move in a direction, as illustrated by the arrow A, to the location below the optical camera  118 , and position the centers of the hot spots one after another in sequence to the optical camera  118  to capture the visible defect images. The moving stage  302  can move in X and Y directions and may include an XY moving stage or the like. Automatic driving forces or manual driving forces may be used to drive the moving stage  302 . 
       FIG. 4  is a flow chart of a method for inspecting a solar cell according to one embodiment of the present invention. In Step S 402 , the solar cell, which is undergoing inspection, is placed on the stage of the optical imaging inspection apparatus. The negative contact of the solar cell is electrically coupled to the metal surface of the stage so that electricity can pass through the metal surface to the solar cell. In Step S 404 , a power supply connects and applies a positive voltage at a terminal on the metal surface of the solar cell and a negative voltage at the positive output line of the solar cell. The output voltage of the power supply is adjusted steadily to the breakdown voltage across the p-n junctions of the solar cell. In Step S 406 , the thermal imaging device obtains the thermal images of the solar cell, which generates heat in response to the reverse biased voltage. In Step S 408 , the thermal image analysis module of the computing unit identifies hot spots having temperatures higher than a temperature threshold and sizes larger than an area threshold. In Step S 410 , if no defects are detected, the solar cell is qualified and the inspection is stopped. In Step S 412 , the locating module of the computing module calculates the center coordinates of hot spots, and then calculates the distances between the hot spots and the optical camera. In Step S 414 , the visible light-imaging device is moved to the center positions of the hot spots and captures the visible image of the defects in sequence. In Step S 416 , the visible image analysis module analyzes the defect images captured by the visible light-imaging device. The module identifies the defect features, performs statistical analysis and stores the statistical results in a statistical database. 
     The above-described embodiments of the present invention are intended to be illustrative only. Numerous alternative embodiments may be devised by persons skilled in the art without departing from the scope of the following claims.