Patent Abstract:
A method of inline inspection of photovoltaic material for electrical anomalies. A first electrical connection is formed to a first surface of the photovoltaic material, and a second electrical connection is formed to an opposing second surface of the photovoltaic material. A localized current is induced in the photovoltaic material and properties of the localized current in the photovoltaic material are sensed using the first and second electrical connections. The properties of the sensed localized current are analyzed to detect the electrical anomalies in the photovoltaic material.

Full Description:
FIELD 
     This application is a divisional application of, and claims all rights and priority on prior pending U.S. patent application Ser. No. 12/631,260 filed 2009, Dec. 04, which was a divisional application of U.S. Pat. No. 7,649,365 issued 2010, Jan. 19, of which all rights and priority are also claimed. This invention relates to the field of photovoltaics. More particularly, this invention relates to the inline inspection of photovoltaic films. 
    
    
     BACKGROUND 
     Photovoltaics can be made from a variety of different materials and by way of a variety of different processes. One of the more promising fabrication methods—from a cost standpoint at least—is to form continuous webs of photovoltaic material that are sequentially processed as the web moves along a production line. Thus, various layers of the photovoltaic devices are sequentially formed, one on top of another, as the web of built-up material progresses down the moving production line. Another fabrication method is to deposit the photovoltaic film on plate glass, which is the preferred method to fabricate CdTe solar cells. 
     In the past, electrical defects in the photovoltaic film have been studied by removing a sample from the production material and inspecting the sample offline. Removing a sample often introduces serious defects near the edges of the remaining photovoltaic material where the sample was removed. 
     Further, examining the sample offline means that the information cannot be readily used in an automatic feedback loop for control of the film deposition processes. Further, such offline testing is time consuming, which results in a potential greater loss of material, in the event of a process excursion. 
     What is needed, therefore, is a system that overcomes problems such as those generally described above, at least in part. 
     SUMMARY 
     The above and other needs are met by inline inspection of the photovoltaic film for electrical anomalies without removing samples. A first electrical connection is formed to a first surface of the photovoltaic material, and a second electrical connection is formed to an opposing second surface of the photovoltaic material. A localized current is induced in the photovoltaic material, and properties of the localized current in the photovoltaic material are sensed using the first and second electrical connections. The properties of the sensed localized current are analyzed to detect the electrical anomalies in the photovoltaic material. 
     In various embodiments according to this aspect of the invention, at least one of the first electrical connection and the second electrical connection is formed using a physical contact to the photovoltaic material. In some embodiments, at least one of the first electrical connection and the second electrical connection is formed using a non-physical contact to the photovoltaic material. In yet other embodiments, the first electrical connection is formed using at least one of a laser and an electron beam. 
     According to another aspect of the invention there is described a method of inspection by applying an ultraviolet probe laser to a location of the photovoltaic material, where the probe laser is applied at a probe energy sufficient to emit photoelectrons from a conduction band of the photovoltaic material into a vacuum environment, but the probe energy is insufficient to substantially excite electrons from a valence band of the photovoltaic material into a vacuum environment, and simultaneously applying a visible pump laser to the same location of the photovoltaic material, where the pump laser is applied at a pump energy sufficient to excite photoelectrons from the valence band of the photovoltaic material to the conduction band, but the pump energy is insufficient to substantially emit photoelectrons from the conduction band of the photovoltaic material into the vacuum environment, sensing the photoelectrons that are excited into the vacuum environment to measure a current, and interpreting fluctuations in the current as electrical anomalies as a function of position on the photovoltaic surface. 
     In various embodiments according to this aspect of the invention, the photovoltaic material does not include a contact film at the location of the application of the probe laser. In some embodiments, the probe laser is applied to a first side of the photovoltaic material and the pump laser is applied to a second side of the photovoltaic material. 
     In other embodiments, both the probe laser and the pump laser are applied to a first side of the photovoltaic material through a transparent port into the vacuum environment, where an interior surface of the transparent port is coated with a transparent conductive material that is disposed in proximity to the photovoltaic material sufficient to receive and sense the photoelectrons emitted from the photovoltaic material, and the transparent conductive material is disposed in sections on the transparent port, where the sections are electrically isolated one from another, thereby enabling separate measurement of the emitted photoelectrons based on a position of the photovoltaic material from which the photoelectrons are emitted, and the probe laser further comprises multiple probe lasers, one each of the multiple probe lasers dedicated to simultaneous irradiation of the photovoltaic material through an associated section of the transparent conductive material. 
     According to yet another aspect of the invention there is described a method of inspecting continuously moving photovoltaic material for electrical anomalies without stopping the movement or removing samples, by forming electrical connections to the photovoltaic material, and inducing either via electron beam or light beam a first localized current in the photovoltaic material with a first stripe source, sensing the first localized current at a first time, and likewise inducing a second localized current either via electron or light beam on the photovoltaic material with a second stripe source, sensing the second localized current at a second time, where the first stripe source is positioned downstream along the moving photovoltaic material from the second stripe source, and the first stripe source and the second stripe source are oriented at a non-zero angle relative to one another, analyzing properties of the first and second localized currents to detect the electrical anomalies in the photovoltaic material, and determining positions of the electrical anomalies in the photovoltaic material based at least in part on a time difference between the first time and the second time and a measure of the non zero angle between the first and second stripe sources. 
    
    
     
       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 a first embodiment for a non-physical method for making electrical contact with a film, according to the present invention. 
         FIG. 2  is a first embodiment for illuminating a film, according to the present invention. 
         FIG. 3  is a second embodiment for illuminating a film, according to the present invention. 
         FIG. 4  is a third embodiment for illuminating a film, according to the present invention. 
         FIG. 5  depicts an embodiment for the flow of electrons through a first type of photovoltaic material in both the presence of a shunt and in normal operation of the photovoltaic material, when a probe and pump are applied to the material. 
         FIG. 6  depicts an embodiment for the flow of electrons through a second type of photovoltaic material in both the presence of a shunt and in normal operation of the photovoltaic material, when a probe and pump are applied to the material. 
     
    
    
     DETAILED DESCRIPTION 
     According to the several embodiments of the present invention, there is described a system  10  to perform an inline inspection for electrical defects of various kinds in a photovoltaic thin film  12 , as depicted in  FIG. 1 . The photovoltaic  12  could be one or more of a variety of different kinds, such as Cu(In,Ga)Se, Cu(In,Ga)S, or any member of this family of chalcopyrites, or CdS, CdSe, CdTe, or any member of this family of materials, or amorphous silicon. 
     In one embodiment, the inspection is performed using a beam  14 , which is either an optical beam (optical beam induced current) or, if the material  12  is under vacuum, and electron beam (electron beam induced current). Regardless of the type of beam  14  used, the beam  14  induces a current in the photovoltaic  12 . 
     If the material  12  is moving, as in the case of film deposition on a moving web of material during manufacture, the beam  14  is preferably rastered back and forth across the material  12 , normal to the direction of movement of the material  12 , to produce an x, y scan  16  of the surface of the material  12 , where x is in the direction of the motion of the material  12 . This movement of the beam  14  can be accomplished by moving either the beam  14  source, or directing the beam  14  itself back and forth, such as by a moving minor for optical beam induced current or by a changing magnetic or electrical field for electron beam induced current. Alternately, the photovoltaic material  12  could be moved relative to the beam  14 . 
     Alternately, the beam  14  remains stationary relative to the width of the moving material  12 , so as to sample a single strip along the length of the material  12 , and the data collected is used for feedback to the film deposition processes. However, this embodiment does not allow for repair of the material  12  by laser ablation, as described hereafter. In yet another embodiment, multiple beams  14  could be used to sample multiple strips along the length of the material  12 . 
     The top conductor illuminated by the beam  14  is, in one embodiment, the transparent conducting oxide that is typically formed on a photovoltaic device, such as zinc oxide or indium tin oxide. The opposite side of the material  12  would then be the conducting substrate, such as stainless steel. The variation in the current produced by the photovoltaic  12  as induced by the beam  14  is used to detect electrical non uniformities in the film  12  as a function of position. 
     Electrical contacts  18  or  22  are preferably made between an ammeter  20  or other current sensing instrumentation and both sides of the photovoltaic  12  to detect the current that is induced by the beam  14 . The electrical contacts  18  and  22  can be made using either a method that does not physically contact the material  12 , or one that does physically contact the material  12 , or a blend of both methods. 
     For example, one method of making physical contact with a moving web of material  12  is by using conducting brushes  18   a  that drag on the surfaces of the material  12  as the material  12  moves relative to the brushes  18   a.  Another method is to use a conductive roller  18   b.  Yet another method is to have a conductive physical probe  18   c  that moves with the material  12  for a period of time, is then raised, repositioned, and lowered again to make contact in a different position of the material  12 . 
     When electron beam induced current is used, the electron beam  14  itself may be used as a non-contact upper electrical connection, forming either a positive or negative contact depending on the landing energy employed. If desired, two or more electron beams could be used, one for probing the material, and the second for making positive or negative electrical contact. In this embodiment, one or more of the physical contact methods  18  could be used for the electrical connection to the bottom of the material  12 , or some non-contact method could be used. 
     An alternate method for making electrical contact without physically touching the film  12  is to use a contained plasma or corona  22 . In this case, the film  12  passes by a relatively thin plasma or corona region, which is preferably isolated from the rest of the environment, whether that be a vacuum chamber or open atmosphere. This region can be confined such as with a rectangular box  22  running across the length of the material  12 , containing the necessary field for plasma or corona generation, and having a pressure that is different from the rest of the inspection device  10 . 
     Optical beam induced current can be performed on a moving film of material with one or two light beams  14   a  and  14   b,  as depicted in  FIG. 2 . In one embodiment, these beams  14   a  and  14   b  run across the material  12  in opposite diagonal orientations, allowing for localization of any defect. The beams  14   a  and  14   b  are modulated in one embodiment, to gain additional information from the measurement.  FIG. 2  depicts an embodiment where light beams  14   a  and  14   b  simultaneously illuminate a large swath of the material  12 . Defects are imaged along the moving axis of the material  12 , and localized on the material  12  due to the different orientations of the two beams  14   a  and  14   b.  This method tends to be faster than scanning each beam  14   a  and  14   b  as a point across the width of the material  12 . Scattered light from beams  14   a  and  14   b  could also be used to detect non-electrical defects during any point of the film deposition process, including defects on the bare substrate. 
     The time difference between anomalies as produced by the two beams  14   a  and  14   b  as the material  12  proceeds along its path of motion indicates where across the width of the material  12  the defect resides. For example, a defect in the material  12  that is located at a point in the material  12  where the beams  14   a  and  14   b  are relatively close to one another would produce electrical events that are relatively closer together in time, while a defect in the material  12  that is located at a point in the material  12  where the beams  14   a  and  14   b  are relatively far apart would produce electrical events that are relatively farther apart in time. With knowledge of the angle between the beams  14   a  and  14   b  and the speed of the material  12 , the location of the anomaly in the material  12  can be determined. 
     Electrical isolation of the inspected region can be provided by scribing away a line of the transparent conducting oxide contact to effectively segment the material  12  into electrically isolated regions. Alternately, it may be possible to rely on the sheet resistance of the transparent conducting oxide. The sheet resistance is typically about ten ohms per square, but in one embodiment, a very thin layer with a much higher sheet resistance is formed, as in the deposition of a zinc oxide film on Cu(In,Ga)Se material, then the inspection is performed, and finally the remainder of the transparent conducting oxide is formed, as in the deposition of a final aluminum doped film of zinc oxide. 
     Finding electrical defects while keeping up with the speed of a moving web of material  12  is best performed using relatively high data acquisition rates. A high speed time delay and integration acquisition system is used in one embodiment to acquire the induced current data. Analysis of the data may require a measurement of the current-voltage curve at each scan point. Weak diode or shunting defects are localized in one embodiment to within an area of about two millimeters in diameter and then electrically isolated from the remainder of the surface by laser ablation. In one embodiment the beam  14  sources preferably illuminate the web  12  between two parallel strip conducting brushes  18 , not depicted in  FIG. 2 . 
     Some embodiments of the invention are especially beneficial for finding Ohmic shunts in the material  12 , which drain photocurrent from the load under any voltage bias, and also for finding weak diodes in the material  12 , which drain photocurrent from the load while under forward bias. Locating the positions of the worst shunts (the word “shunts” generally includes the concept of “weak diodes” as used hereafter) during the fabrication process allows them to be electrically isolated, which increases the efficiency of the photovoltaic material  12 . The electrical isolation of the shunts can be accomplished by laser ablation, for example, to remove the transparent conducting oxide layer or back contact material in a ring enclosing the shunt or, for the case of shunts located close to the edge of the photovoltaic material, by enclosing the shunt using both the edge of the photovoltaic material and an ablated region that intersects the edge of the material on either side of the shunt. The shunt position, and other useful information such as shunt resistance, as well as information on position and energy level of any recombination centers, and local characterization of areas of reduced carrier mobility can also be used to provide feedback for material deposition and general process control during fabrication of the photovoltaic  12 . 
     An optical beam  14  induced current scan of the film  12  that is performed after the final transparent conductive oxide contact is applied to the film  12  is generally sensitive to substantially all shunts beneath the conducting contact, not only the shunt closest to the light beam  14 . This greatly reduces the signal to noise level of the measurement. However, various embodiments of the present invention detect localized shunting defects in thin films  12  by measuring the photoelectric yield from the surface of the photovoltaic  12  before application of the final contact layer by using an ultraviolet laser  14   c,  as depicted in  FIG. 3 . This ultraviolet laser is called the probe laser  14   c.  The scan of the photovoltaic material  12  by the probe laser  14   c  may be assisted by simultaneous illumination of the probed region with a visible laser, called the pump laser  14   d.    
     The ultraviolet laser  14   c  in one embodiment is held at a high enough energy to excite electrons from the conduction band minimum to the vacuum, but at an energy that is too low to excite electrons from the top of the valence band maximum to the vacuum. The pump laser  14   d  in this embodiment is held at a high enough energy to excite electrons from the valence band to the conduction band, but at an energy that is too low to excite electrons from the conduction band to the vacuum. 
     In one embodiment, the probe laser  14   c  is a 266 nanometer, 4.66 electron Volt, frequency quadrupled YAG laser, and the pump laser  14   d  is a 532 nanometer, 2.33 electron Volt, frequency doubled YAG laser. The intensity of the pump laser  14   d  is much greater than the intensity of the probe laser  14   c  in this embodiment, so that most of the photoelectrons are excited to the conduction band by the pump laser  14   d  rather than by the probe laser  14   c.    
     This method could be applied, by way of example, to a thin (four micron) film of CdTe deposited on glass—a typical superstrate configuration CdTe solar cell—just before deposition of the final conducting film as a back contact. The ultraviolet probe laser  14   c  scans the CdTe surface  12  in a dark room. It incites the ejection of photoelectrons from the top five to ten nanometers of the CdTe material  12 . Because the energy of the probe laser  14   c  is not high enough to excite electrons from the valence band at 5.78 electron Volts, only the electrons that are initially excited by the probe  14   c  from the valence band to states in the conduction band at 4.28 electron Volts are ejected as photoelectrons. For sufficiently low intensities of the probe laser  14   c,  the count rate for these photoelectrons is relatively small. 
     A large population of electrons may be excited to the CdTe conduction band by intense illumination of the film  12  from the opposite side (through the glass substrate) by the visible light pump laser  14   d.  These electrons are excited throughout the depth of the film  12 , but relatively few photons reach the top few nanometers of the CdTe  12  surface on the opposite side from the glass substrate because of the high absorption of visible light by CdTe. Electrons in the conduction band reach the ultraviolet probe  14   c  primarily by conduction across the CdTe film  12 . In general, the action of the solar cell  12  in the presence of visible light (such as from the pump laser  14   d ) conducts the electrons away from the ultraviolet probe  14   c  and towards the glass (on the bottom of the material  12 ). However, in the presence of a shunt, the electrons conduct in the opposite direction, towards the ultraviolet probe  14   c,  where some are ejected as photoelectrons, and thereby serve to complete a circuit through an ammeter (not depicted in  FIG. 3 ) that is connected to the detector. Hence, an elevated ammeter reading during the scan indicates the presence of a shunt in the photovoltaic film  12 .  FIG. 5  depicts both the flow of electrons in the presence of a shunt, and in normal operation of the photovoltaic  12 . 
     Because the back contact of the film  12  has not yet been deposited on the CdTe surface, the shunts are generally electrically isolated from one another by the high resistivity of the CdTe film  12 , so that the probe  14   c  is only sensitive to electrical defects directly beneath it. The visible light  14   d,  besides pumping electrons to the conduction band and turning on the open circuit voltage of the solar cell  12 , also serves to induce a forward bias, and thereby turns on any weak diodes beneath the probe laser  14   c  by means of the open circuit voltage developed across the film  12  in the vicinity of the probe  14   c  during illumination. 
     For the case of a Cu(In,Ga)Se solar cell  12 , the inspection procedure is different because the Cu(In,Ga)Se solar cell  12  is grown in a substrate configuration beginning with the opaque back contact. In one embodiment of an inspection process for a Cu(In,Ga)Se solar cell  12 , both the probe laser  14   c  and the pump laser  14   d  are directed to the CdS surface opposite the substrate, as depicted in  FIG. 4 . The CdS film  12  is typically a fifty to one hundred nanometer thick layer deposited on top of the active Cu(In,Ga)Se film, and serves to complete the junction for the solar cell  12 . Substantially all of the photoelectrons ejected by the ultraviolet probe  14   c  are ejected from the CdS film  12 . The band gap of CdS is 2.4 electron volts, which exceeds the 2.33 electron volt energy of the pump laser  14   d.  Hence, electrons are only significantly pumped in the Cu(In,Ga)Se material beneath the CdS film, and predominantly conduct vertically through the film  12  to the CdS surface before being excited by the ultraviolet probe  14   c.    FIG. 6  depicts both the flow of electrons in the presence of a shunt, and in normal operation of the film  12 . 
     In general, the action of the solar cell  12  conducts electrons to the ultraviolet probe  14   c,  such that the photoelectron signal remains high. However, in the presence of a shunt the pumped electrons are conducted in the opposite direction, such that the photoelectron signal is small. Hence, a Cu(In,Ga)Se shunt is detected by a decrease in the signal—opposite that of the case for CdTe, where a shunt is detected by an increase in the signal. 
     As for the CdTe case, the inspection is predominantly sensitive to shunts beneath the probe laser  14   c,  due to the high resistivity of the CdS film  12 . The inspection is preferably performed under vacuum, to allow the photoelectrons to reach the detector. For the cases described above, the pump laser may be replaced by any source of light, such as a broad spectrum lamp, that excites electrons between the valence and conduction bands but does not have the energy to excite electrons from the conduction band to the vacuum. This could be referred to as a lower energy light source, not necessarily in the visible range of the spectrum. The intensity of this light source may be adjusted to vary the open circuit voltage across the photovoltaic film  12 , thereby allowing the inspection to selectively activate weak diodes that have different open circuit voltages. Likewise, the probe laser may be replaced by any light source, including a broad spectrum lamp, with an energy sufficient to excite electrons from the conduction band minimum to the vacuum. This could be referred to as a higher energy light source. 
     A vacuum of about one-tenth of a millitorr to about one millitorr is created in a chamber mounted to a frictionless air bearing that is passed over the film  12 . Alternately, the film  12  is passed beneath a vacuum chamber mounted to a frictionless air bearing, such as in the case of a moving conducting web on which a Cu(In,Ga)Se film is deposited. Either the probe laser  14   c  or both the probe and pump lasers  14   c  and  14   d  are directed through an ultraviolet quality fused silica window  24  that is coated with a transparent conducting oxide film on the surface that faces the photovoltaic material  12 . The separation between the window  24  and the photovoltaic film  12  is reduced to a small enough gap (such as less than about one millimeter) to allow ejected photoelectrons from the photovoltaic film  12  to reach the transparent conducting oxide film, and from there to be conducted to an ammeter (not depicted in  FIG. 4 ). 
     The opposite terminal of the ammeter is electrically connected to the single conducting contact on the solar cell  12 . For CdTe photovoltaics  12 , the conductor is the transparent conducting oxide layer that is deposited on the glass substrate. For Cu(In,Ga)Se photovoltaics  12 , the conductor is typically the steel substrate on which the Cu(In,Ga)Se is grown. Electrical contact with the substrate can be made in a variety of different ways, such as with a conducting brush in the case of a moving web of material  12 . 
     In one embodiment of this invention, the transparent conducting oxide film coating the detector is scribed in the direction of the motion of the vacuum chamber or photovoltaic material to create separate detectors  28  that are read in parallel, as depicted in  FIG. 4 . The ultraviolet probe  14   c  and the visible pump  14   d  are then focused to a streak source across the window  24 , normal to the scribe lines  26 , so that data is collected simultaneously from all of the detectors  28 . 
     Thus, use of surface contact methods that are not based on the photoelectric effect, such as by using a plasma or a brush or, in the case of electron beam induced current, the electron beam itself (as either a positive or negative contact, depending on the landing energy), can be used for direct, in-line inspection of the photovoltaic material using electron beam induced current or optical beam induced current to detect not only the areas of the shunts, but also regions of poor carrier collection due to a poorly formed p-n junction, high recombination, or low mobility. Results from this in-line inspection can be used for electrical isolation of the shunts, or feedback for control of the deposition processes or other process steps involved in the fabrication of the photovoltaic device. 
     If sufficient signal to noise levels are available, two streak sources and detectors can be deployed at, for example, nominal angles of positive forty-five degrees and negative forty-five degrees with respect to the axis normal to the direction of the moving photovoltaic material  12 . The position of the defect is then determined by the time of arrival of the signal at each source. Such a configuration can also be used with optical sources and segmented detectors to locate other defects besides electrical shunts. These defects include, for example, regions of poor carrier collection due to a poorly formed p-n junction, high recombination, or low mobility due to deviations from ideal stoichiometry or defects in the crystal structure or the size of the crystalline regions, or the presence of contaminants. In addition, the bare substrate can be inspected for scratches or surface contamination (such as organic stains). 
     Yet another embodiment uses an electron source such as a scanning electron microscope column or nanotube emitter to complete the circuit. In one embodiment the electron source is rastered across the film in a direction normal to the direction of the moving material  12 . The signal is the electron beam induced current that is collected from the contact on the opposite side of the film  12 . The landing energy is preferably varied to deposit predominantly positive or negative charge, thereby utilizing the electron beam to make electrical contact with the top surface of the photovoltaic device  12 . 
     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.

Technology Classification (CPC): 7