Photon imaging system for detecting defects in photovoltaic devices, and method thereof

A method includes supplying current to at least one photovoltaic device via a current source and detecting emitted photon radiations from the at least one photovoltaic device via a radiation detector. The method also includes outputting a signal corresponding to the detected emitted photon radiations from the radiation detector to a processor device, and processing the signal corresponding to the detected emitted photon radiations via the processor device to generate one or more two-dimensional photon images. The method further includes analyzing the one or more two-dimensional photon images to determine at least one defect in the at least one photovoltaic device.

BACKGROUND

The invention relates generally to photovoltaic devices, and more particularly, to an imaging system for detecting defects in photovoltaic cells, modules, and method thereof.

Solar energy is considered as an alternate source of energy relative to other forms of energy. Solar energy conversion systems are used to convert solar energy into electrical energy. The solar energy conversion system typically includes photovoltaic modules, photoelectric cells, or solar cells that convert solar energy into electrical energy for immediate use or for storage and subsequent use. Conversion of solar energy into electrical energy includes reception of light, such as sunlight, at a solar cell, absorption of sunlight into the solar cell, generation and separation of positive and negative charges creating a voltage in the solar cell, and collection and transfer of electrical charges through a terminal coupled to the solar cell.

In solar module manufacturing line, it is helpful to identify local defects, hot spots, or the like in the solar devices, for example, thin-film photovoltaic modules to predict whether the solar module is prone to degradation or whether the solar module would fail. The conventional diagnostic method used for a solar module involves exposing a finished solar module under a light source that is calibrated to sun intensity, and then measuring current as a function of the applied voltage. This conventional technique provides information related to the power conversion efficiency of the solar modules but does not provide any information about the reliability of the solar module. Moreover, substantial power is consumed for the operation of the light source.

It is desirable to have a more effective system and method to identify defects in the photovoltaic devices, for example in thin-film photovoltaic modules.

BRIEF DESCRIPTION

In accordance with one exemplary embodiment of the present invention, the method includes supplying current to at least one photovoltaic device via a current source and detecting emitted photon radiations from the at least one photovoltaic device via a radiation detector. The method also includes outputting a signal corresponding to the detected emitted photon radiations from the radiation detector to a processor device, and processing the signal corresponding to the detected emitted photon radiations via the processor device to generate one or more two-dimensional images. The method further includes analyzing the one or more two-dimensional photon images to determine at least one defect in the at least one photovoltaic device.

In accordance with another exemplary embodiment of the present invention, a system includes a current source coupled to at least one photovoltaic device and configured to supply current to the at least one photovoltaic device. A radiation detector is configured to detect emitted photon radiations from the at least one photovoltaic device and output a signal corresponding to the detected emitted photon radiations. A processor device is coupled to the radiation detector and configured to receive the signal corresponding to the detected emitted photon radiations, process the signal to generate one or more two-dimensional photon images, and analyze the one or more two-dimensional photon images to determine at least one defect in the at least one photovoltaic device.

In accordance with another exemplary embodiment of the present invention, a computer readable media to enable a processor device to determine at least one defect in the at least one photovoltaic device is disclosed.

DETAILED DESCRIPTION

As discussed in detail below, embodiments of the present technique provide a diagnostic method for determining at least one defect in one or more photovoltaic device, for example a thin-film photovoltaic module. The method includes supplying current to at least one photovoltaic device via a current source and detecting emitted photon radiations from the at least one photovoltaic device via a radiation detector. The method also includes outputting a signal corresponding to the detected emitted photon radiations from the radiation detector to a processor device, and processing the signal corresponding to the detected emitted photon radiations via the processor device to generate one or more two-dimensional photon images. The method further includes analyzing the one or more two-dimensional photon images to determine at least one defect in the at least one photovoltaic device. In accordance with a specific embodiment, a diagnostic system for determining at least one defect in one or more photovoltaic device is also disclosed. In accordance with the present technique, performance and the eventual reliability of photovoltaic device can be determined from a two-dimensional photon image obtained by passing an electrical current through the photovoltaic device.

Referring toFIG. 1, a system10for diagnosing performance and reliability of a photovoltaic device12, for example a thin-film photovoltaic module is disclosed. In another embodiment, the photovoltaic device12may be a photovoltaic cell. The thin-film photovoltaic module12may include cadmium telluride based photovoltaic module, copper indium gallium selenide based photovoltaic module, silicon based photovoltaic module, an amorphous silicon based thin-film photovoltaic module, or the like. It should be noted herein that other suitable photovoltaic modules, cells are also envisaged. The system10includes a current source14, a radiation detector16, and a processor device18. The current source14is coupled to the thin-film photovoltaic module12and configured to supply current to the thin-film photovoltaic module12. In one embodiment, the current source14is configured to supply current pulses to the thin-film photovoltaic module12. In another embodiment, the current source14is configured to supply current to the thin-film photovoltaic module12for joule heating the photovoltaic module.

It is known to one skilled in the art that solar modules are devices that convert light to electric current. In the illustrated embodiment, a reverse phenomenon is observed in which the thin-film photovoltaic module12emits light when an electric current source is passed through the thin-film photovoltaic module12. The reverse phenomenon may be referred to as “electroluminescence”, which is the process by which light emitting diodes (LEDs) function. In other words, the thin-film photovoltaic module12emits photon radiations when an electric current is passed through the thin-film photovoltaic module12. The radiation detector16is configured to detect photon radiations emitted from the thin-film photovoltaic module12and output a signal corresponding to the detected emitted photon radiations. The radiation detector16may be an infrared camera, charge-coupled device, or the like. It should noted herein that other suitable radiation detectors are also envisaged. The current source14is operated in synchronization with the operation of the radiation detector16. In other words, when the current source14is activated, the radiation detector16is activated in synchronization with the current source14so as to detect the emitted photon radiations from the thin-film photovoltaic module12.

The processor device18is coupled to the radiation detector16and configured to receive the signal corresponding to the detected emitted photon radiations from the radiation detector16, and process the signal to generate one or more two-dimensional photon images. In one specific embodiment, the two-dimensional photon image includes a two-dimensional electroluminescence image. In another embodiment, in addition to the two-dimensional electroluminescence image, a two-dimensional thermal image may be generated using photon radiations of relatively longer wavelength emitted by joule heating the one thin-film photovoltaic module12. The processor device18is further configured to analyze the one or more two-dimensional photon images to determine at least one defect in the at least one thin-film photovoltaic module12. The defects may include cracks, voids, shunts, weak diode, local hot spots, weak or broken electrical contacts, or combinations thereof of the thin-film photovoltaic module12. The processor device18will typically include hardware circuitry and software for performing computations indicative of at least one defect in the thin-film photovoltaic module12as described below. The processor device18may thus include a range of circuitry types, such as, a microprocessor based module, and application-specific or general purpose computer, programmable logic controller, or even a logical module or code within such a device.

In a specific embodiment, the processor device18is configured to analyze the two-dimensional photon image by correlating a two-dimensional electroluminescence image with one or more techniques including thermography, visual inspection, microscopy, or combinations thereof to determine defects in the thin-film photovoltaic module12. In another specific embodiment, the processor device18is configured to analyze the two-dimensional photon image by correlating a two-dimensional electroluminescence image with results generated by accelerated life tests (solar flux based) to determine defects in the thin-film photovoltaic module12faster. In yet another specific embodiment, the processor device18is configured to analyze the two-dimensional photon image correlating a two-dimensional electroluminescence image with one or more electrical performance measurement parameters including efficiency, open circuit voltage (VOC), short circuit current (ISC), fill factor, or combinations thereof attributed to the thin-film photovoltaic module12to determine defects in the thin-film photovoltaic module12. It should be noted herein that the term “fill factor” in the context of solar cell technology is defined as the ratio (expressed as percent) of the actual maximum obtainable power, to the theoretical obtainable power.

In the illustrated embodiment, an optical filter20is disposed between the thin-film photovoltaic module12and the radiation detector16. The optical filter20is configured to transmit only the photon radiations having energy equal to a band gap of an absorber layer (not shown inFIG. 1) of the thin-film photovoltaic module12to the radiation detector16. It should be noted herein that although a single thin-film photovoltaic module12is shown, the exemplary system and technique may be applicable for monitoring a plurality of thin-film photovoltaic modules. In other words, the exemplary system can be incorporated in any on-line manufacturing setting to monitor and control the production line. As discussed previously, in the illustrated embodiment and the subsequent embodiments, even though a thin-film photovoltaic module is discussed, the system10is also applicable to other photovoltaic devices.

Referring toFIG. 2, a more detailed representation of the system10is disclosed. As discussed above, the current source14is coupled to the thin-film photovoltaic module12and configured to supply current to the thin-film photovoltaic module12. The radiation detector16is configured to detect photon radiations emitted from the thin-film photovoltaic module12and output a signal corresponding to the detected emitted photon radiations. In the illustrated embodiment, a voltage bias waveform to the module12is modulated via a signal generator17. The signal generator17is also configured to transmit a synchronous trigger signal to the radiation detector16. The processor device18is coupled to the radiation detector16via a frame grabber device19. The processor device18is configured to receive the signal corresponding to the detected emitted photon radiations from the radiation detector16via the frame grabber device19, and process the signal to generate one or more two-dimensional photon images.

It should be noted herein that since electroluminescence generated by photovoltaic devices is generally weak and are subjected to the influence of background scattering light, an increase in integration time constants of the radiation detector16is not sufficient to generate a clean electroluminescence image. In one embodiment, in order to obtain a background free electroluminescence image (“clean electroluminescence image”), a technique referred to as “lock-in electroluminescence detection technique” is used. In such a technique, the module12is activated by periodically modulated bias, and synchronous electroluminescence image detection is enabled. The electroluminescence image is processed digitally using low-pass filters13, for enabling detection of weak signals. The lock-in electroluminescence intensity is determined by the following relation:

S⁡(x,y)=∑i=1N⁢⁢I⁡(x,y,ti)⁢sin⁡(ωv⁢ti-φs)(1)
where “S” is the lock-in electroluminescence intensity, (x,y) are the location coordinates, I(t) is the electroluminescence intensity per image, “t” is the time, ωvis the bias angular frequency (modulation frequency), φSis the system phase shift, “N” is the total number of electroluminescence images.

As discussed previously, the modulated electroluminescence image stream is filtered using low-pass filters13to reject the broadband noise except electroluminescence signal near to the modulation frequency ωv. The detection frame rate (ωf=½πΔt) is set higher than the modulation frequency ωvso as to increase the detection accuracy. The system phase shift φSmay be determined by measuring a light emitting diode instead of the module12, or tuned in real time to maximize the electroluminescence intensity.

Referring toFIG. 3, a technique referred to as “Dual rate electroluminescence detection technique” for obtaining a background free electroluminescence image (“clean electroluminescence image”) is disclosed. Reference numeral21is representative of the module bias signal having peak regions23and low regions25. The peak regions23indicate “ON state” of the module bias cycle and low regions25indicate “OFF state” of the module bias cycle. Reference numeral27is representative of the electroluminescence signal having peak regions29and low regions31. The peak regions29indicate collection of the electroluminescence image signal and the background image signal during “ON state” of the module bias cycle and the low regions31indicate collection of background image signal during “OFF state” of the module bias cycle. Reference numeral33is representative of the detection frame rate.

In such a technique, the module is activated by periodically modulated bias, and synchronous electroluminescence image detection is enabled. In the dual rate electroluminescence detection technique, the detection frame rate is set twice that of the module voltage bias. The background image signal is subtracted from the electroluminescence signal to obtain a clean electroluminescence image. The background free images may be averaged over a plurality of frames. The module bias voltage rate depends upon the optimized detector integration time. Such a technique enables faster and real time detection of electroluminescence image even in the presence of varying background light. The dual rate electroluminescence detection technique is also immune to the variations in background images since the background images are subtracted dynamically.

With reference to embodiments discussed with respect toFIGS. 2 and 3, signal-to-noise ratio is enhanced based on the detection frame rate and measurement time. The processing of the electroluminescence may be done using digital signal process (DSP) hardware for enhancing the speed of image processing. Alternatively, the image subtraction process in dual-rate electroluminescence detection technique may be performed using an on-camera memory.

Referring toFIG. 4, a thin-film photovoltaic module12in accordance with an exemplary embodiment of the present invention is illustrated. The module12includes a glass layer22, a transparent conductive oxide layer24, an n-type cadmium sulfide layer26, a p-type cadmium telluride layer28(generally referred to as “absorber layer”), and a back-contact layer30. The p-type cadmium telluride layer28absorbs the light photons and generates free electrons. The n-type cadmium sulfide layer26emits these free electrons through the layers22,24into an outer circuit (not shown). The electrons return into the p-type cadmium telluride layer28to recombine with holes through the back contact layer30. In other words, an electron32in a conduction band34recombines with a hole36in a valence band38generating energy as a photon. As discussed above, in the illustrated embodiment, a reverse phenomenon is observed in which the thin-film photovoltaic module12emits light when an electric current source is passed through the thin-film photovoltaic module12. In other words, the thin-film photovoltaic module12emits photon radiations when an electric current source is passed through the thin-film photovoltaic module12. It should be noted herein that the illustrated module12is an exemplary embodiment and the configuration of the thin-film photovoltaic module should not be construed as limiting.

The conventional diagnostic method used for a solar module involves exposing a finished solar module under a light source that is calibrated to sun intensity, and then measuring current as a function of the applied voltage. This conventional technique provides information related to the power conversion efficiency of the solar modules but does not provide any information about the reliability of the solar module. Moreover, substantial power is consumed for the operation of the light source.

In accordance with an exemplary embodiment of the present invention, the processor device18(shown inFIG. 1) processes the signal corresponding to the detected emitted photon radiations from the radiation detector to generate one or more two-dimensional photon images representative of the thin-film photovoltaic module12. For example, the generated two-dimensional electroluminescence image is representative of an entire surface of the thin-film photovoltaic module12and can be analyzed to identify at least one defect of the thin-film photovoltaic module12. Detection of local defects, hot spots, or the like of the thin-film photovoltaic module may be indicative of potential failures of the module. A list of defects may be identified by studying the two-dimensional photon images. As a result, it is possible to predict whether the module is prone to failure or degradation.

Referring toFIG. 5, a two-dimensional electroluminescence image40representative of an entire surface of a thin-film photovoltaic module is illustrated. As discussed above, the processor device is configured to receive the signal corresponding to the detected emitted photon radiations from the radiation detector, and process the signal to generate one or more two-dimensional photon images. The processor device18is further configured to analyze the one or more two-dimensional photon images to determine at least one defect in the thin-film photovoltaic module.

In the illustrated embodiment, the two-dimensional electroluminescence image40shows variation in performance of the module across a surface of the module. A plurality of brighter regions represented by reference numeral42of the electroluminescence image40is representative of regions of relatively higher efficiency in the module. In other words, the regions42are indicative of regions of the module that convert solar light into electricity at higher efficiency. A plurality of dark lines represented by reference numeral44of the electroluminescence image40is indicative of a plurality of grid lines of the thin-film photovoltaic module. The grid lines of the module are current collection lines typically including screen-printed silver epoxy coated on the module. It should be noted herein that grid lines of the module block sunlight and electroluminescence. Hence grid lines are represented by the plurality of dark lines44. A plurality of darker regions represented by reference numeral46of the electroluminescence image40is representative of regions of defect in the module. In other words, the regions46are indicative of regions that convert solar light into electricity at lower efficiency. The defects may include cracks, voids, shunts, weak diode, local hot spots, weak or broken electrical contacts, or combinations thereof of the thin-film photovoltaic module.

Referring toFIG. 6, a two-dimensional thermal image48representative of an entire surface of a thin-film photovoltaic module is illustrated. As discussed above, in addition to a two-dimensional electroluminescence image, the two-dimensional thermal image48may be generated using photon radiations of relatively longer wavelength emitted by joule heating the one thin-film photovoltaic module. Thus different regions in the photon radiation spectrum could provide complementary information pertaining to the thin-film photovoltaic module. In the illustrated embodiment, the two-dimensional thermal image40provides complementary information indicative of variation in performance of the module across a surface of the module. A plurality of brighter regions represented by reference numeral50of the thermal image48is representative of regions of defect in the module.

Referring toFIG. 7, a graph52illustrating variation in electroluminescence intensity from a particular region of a two-dimensional electroluminescence image with respect to applied current or voltage is illustrated. In one embodiment, the logarithm of electroluminescence intensity (log EL) is represented by the Y-axis and applied voltage (V) is represented by the X-axis. In one such embodiment, the current source is configured to forward bias the thin-film photovoltaic module with higher current density. For example, the higher current density may be in the range of one to hundred mA/cm2(milliamps per centimeter square). In the illustrated embodiment, electroluminescence intensity from a reference region of the thin-film photovoltaic module is represented by a solid line54, electroluminescence intensity from a region with increased series resistance (Rs) of the module is represented by a dashed line56, and electroluminescence intensity from a region with decreased shunt resistance (Rsh) of the module is represented by a dashed line58.

When a forward bias is applied to the module with higher current density, the electroluminescence intensity distribution of the two-dimensional electroluminescence image is modulated by the series resistance and the shunt resistance. Hence by analyzing variation in electroluminescence intensity distribution of the two-dimensional electroluminescence image with respect to applied voltage, the two-dimensional electroluminescence image can be converted to two-dimensional images of series resistance and the shunt resistance. It should be noted herein that the electroluminescence intensity depends on a local diode voltage exponentially. Hence, shunt resistance decrease58shifts downward the electroluminescence-voltage curve with respect to the reference electroluminescence54, and series resistance increase56changes the slope of electroluminescence-voltage curve with respect to the reference electroluminescence54. Such series resistance change56and the shunt resistance change58maps may be generated by measuring electroluminescence intensity at a plurality of voltage values. In another embodiment, similar maps may be generated by plotting variation of the logarithm of electroluminescence intensity (log EL) with respect to logarithm of current.

Referring toFIG. 8, a two-dimensional electroluminescence image60is illustrated. In one such embodiment, the current source is configured to forward bias a thin-film photovoltaic module with lower current density. For example, the lower current density may in the range of 0.1 to 10 mA/cm2 (milliamps per centimeter square). When forward bias is applied to the module with lower current density, a series resistance of the module can be neglected since the voltage drop is relatively small. In such an embodiment, it is possible to detect defects such as non-uniformity in an absorber layer of the module. It should be noted herein that any non-uniformity in the absorber layer of the module results in a variation in diode “turn-on voltage” (above which substantial current flow into the diode) of the module. Regions of the module having lower diode “turn-on voltage” allow higher current to flow. In other words, such regions have relatively higher local current injection levels. As discussed previously, since the electroluminescence intensity depends on a current density locally, variation in local diode turn-on voltage results in a two-dimensional electroluminescence image having higher contrast. In the illustrated embodiment, one or more brighter regions62of the image60corresponds to regions with lower diode “turn-on voltage”/weak diode.

In accordance with a specific embodiment, “a micro electroluminescence technique” is employed to determine at least one defect in the at least one photovoltaic cell. In such an embodiment, the photovoltaic cell is a light emitting diode. In one such embodiment, the current source is configured to forward bias the photovoltaic cell at a predetermined voltage. The radiation detector detects photon radiations emitted from the junction regions of the photovoltaic cell and output a signal corresponding to the detected emitted photon radiations. The radiation detector has spatial resolution compatible with the features required to examine junction regions of the photovoltaic cell. In one embodiment, the radiation detector may have a resolution of the order of grain size (for example, micron) required to examine grain boundaries at the junction regions of the photovoltaic cell. The radiation detector may be an infrared microscope. The microscopic portions of the junction regions of the photovoltaic cell important for the electroluminescence emission may be determined by examining the magnitude of electroluminescence photon emission with fine spatial resolution. Such microscopic portions of the junction regions are important for photovoltaic action. The performance of the photovoltaic cell may be enhanced by increasing portions of the junction regions of the photovoltaic cell found to be most efficacious in electroluminescence and decreasing those junction regions found not to contribute to electroluminescence.

Referring toFIG. 9, a flow chart illustrating exemplary steps involved in the method of detecting one or more defects in a photovoltaic device, for example a thin-film photovoltaic module is disclosed. The method includes supplying current to a thin-film photovoltaic module via a current source as represented by the step64. In one embodiment, current pulses are supplied to the thin-film photovoltaic module. In another embodiment, the current is supplied to the thin-film photovoltaic module for joule heating the photovoltaic module.

In the illustrated embodiment, a reverse phenomenon is observed in which the thin-film photovoltaic module emits light when an electric current is passed through the thin-film photovoltaic module. The reverse phenomenon may be referred to as “electroluminescence”. The thin-film photovoltaic module emits photon radiations when an electric current source is passed through the thin-film photovoltaic module. Photon radiations emitted from the thin-film photovoltaic module are detected via a radiation detector as represented by the step66. The radiation detector may be an infrared camera, charge-coupled device, or the like. The detector outputs a signal corresponding to the detected emitted photon radiations to a processor device as represented by the step68. The current source is operated in synchronization with the operation of the radiation detector. In other words, when the current source is activated, the radiation detector is activated in synchronization with the current source so as to detect the emitted photon radiations from the thin-film photovoltaic module. In the illustrated embodiment, only the photon radiations having energy equal to a band gap of an absorber layer of the thin-film photovoltaic module is transmitted to the radiation detector via an optical filter disposed between the thin-film photovoltaic module and the radiation detector.

The method further includes receiving the signal corresponding to the detected emitted photon radiations from the radiation detector, and processing the signal to generate one or more two-dimensional photon images as represented by the step70. In one specific embodiment, the two-dimensional photon image includes a two-dimensional electroluminescence image. In another embodiment, in addition to the two-dimensional electroluminescence image, a two-dimensional thermal image may be generated using photon radiations of relatively longer wavelength emitted by joule heating the one thin-film photovoltaic module. The method further includes analyzing the one or more two-dimensional photon images to determine at least one defect in the thin-film photovoltaic module as represented by the step72. The defects may include cracks, voids, shunts, weak diode, local hot spots, weak or broken electrical contacts, or combinations thereof of the thin-film photovoltaic module.

In one specific embodiment, the thin film photovoltaic module is forward biased with higher current density and relationship between a two-dimensional electroluminescence image intensity with respect to applied voltage is analyzed to generate a series resistance map, a shunt resistance map, or combinations thereof to determine defects in the thin-film photovoltaic module. In another specific embodiment, relationship between two-dimensional electroluminescence image intensity with respect to applied current is similarly analyzed to determine defects in the thin-film photovoltaic module. In another embodiment, the thin-film photovoltaic module is forward biased with lower current density and contrast in intensity of the generated two-dimensional electroluminescence image is analyzed to determine defects in the thin-film photovoltaic module.

It should be noted herein that the exemplary electroluminescence technique facilitates capturing a two-dimensional electroluminescence image corresponding to the entire module via an infrared camera or CCD device thus eliminating the conventional need of moving a probe on a surface of the thin-film photovoltaic module as in the case of using a Light Beam Induced Current (LBIC) instrument. Also, the exemplary electroluminescence technique is more energy efficient than a conventional solar simulator tester, which consumes a lot of power for operation of a light source. The exemplary electroluminescence technique may be applicable for monitoring photovoltaic device manufacturing process, handling of the photovoltaic devices during shipping and installation, and effect of one or more elements on installed photovoltaic devices.