Patent Application
20120256559
This invention relates to the field of photovoltaic cells. More particularly, this invention relates to a light source for testing photovoltaic cells.
One of the key areas of solar cell manufacturing is the final test and sort procedure. The purpose of final test and sort is to evaluate the current-voltage (I-V) characteristics of the solar cells, and to sort acceptable solar cells according to desired metrics, such as peak power, efficiency, fill factor, and so forth, and to detect and remove defective solar cells. The three basic components of an I-V tester are the solar illuminator, the contact probe unit, and the electrical tester unit.
The purpose of the solar illuminator is to provide light to the surface of the solar cell under test. The intensity and spectral characteristics of this light are preferably as close as possible to those of the sun under predetermined standard conditions. Solar illuminators usually operate in the spectral range of from about three hundred nanometers to about eleven hundred nanometers in pulse or continuous modes.
Current solar illuminators use gas-discharge xenon lamps, tungsten lamps, halogen lamps, or some combination of these lamps. Xenon-based illuminators tend to generate a substantial amount of heat, and require heavy direct current power sources and optical components such as infrared filters in order to operate. Another problem with xenon illuminators is their spatial and temporal non-uniformities. Using xenon illuminators, it is difficult to create a light flux that is homogeneous across the surface of the entire solar cell, which is typically about two hundred millimeters square. The spatial distribution of light intensity from these lamp-based illuminators is also not stable, in that it changes from pulse to pulse.
A light pulse in such illuminators usually requires some amount of time to reach its peak intensity value, typically between about ten microseconds and about one hundred microseconds. During this time, the junction temperature increases and the test of the solar cell is inaccurate. It is also difficult to modify the temporal profile of the pulse.
Finally, these illuminators have high operational and maintenance costs, mainly due to the short lifetime of the lamp (usually about one to two thousand hours) and the frequent downtime needed to replace them.
Light emitting diode (LED) illuminators have also been investigated. However, despite several advantages, LED illuminators have significant drawbacks. Most notably, the spatial and spectral uniformities of the light produced by such illuminators are poor and fail to meet the desired characteristics. In addition, LED illuminators require special test solar panels for periodic calibration and control of intensity, homogeneity, and spectral content.
There is a need, therefore, for an illuminator that reduces problems such as those described above, at least in part.
The above and other needs are met by an apparatus for illuminating a target surface, the apparatus having a plurality of LED arrays, where each of the arrays has a plurality of individually addressable LEDs, and where at least one of the arrays is disposed at an angle of between about forty-five degrees and about ninety degrees relative to the target surface, where all of the arrays supply light into a light pipe, the light pipe having interior walls made of a reflective material, where light exiting the light pipe illuminates the target surface, and a controller for adjusting an intensity of the individually addressable light sources.
In various embodiments, each of the arrays is monochromatic. In some embodiments, each of the arrays is monochromatic, and all of the arrays exhibit a different peak wavelength. In some embodiments a monochromatic filter is associated with each of the arrays, where each array contributes only a monochromatic light to the light pipe. In other embodiments a monochromatic filter is associated with each of the arrays, where each array contributes only a different monochromatic light to the light pipe. Dichroic beam splitters disposed within the light pipe in some embodiments, for receiving the light from the arrays and directing the light down the light pipe toward the target surface. In some embodiments the light pipe has extensions disposed on the sides thereof, with the arrays disposed at distal ends of the extensions. In some embodiments the arrays are optically coupled to the light pipe and provide light thereto via fiber optic assemblies. In some embodiments the controller selectively and individually controls an intensity of the arrays, and thereby produces a light at the target surface having predetermined characteristics. Some embodiments include a reference system having a collector for sampling the light produced by the illuminator, the collector providing the light to a spectrometer for analyzing characteristics of the light.
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:
With reference now to
At least one LED array 102 is attached to the light pipe 104, where the arrays 102 are disposed at angles of from about forty-five degrees to about ninety degrees with respect to the test surface 106. In some embodiments the illuminator 100 has from about three to about seven arrays 102. In one embodiment, one or more of the arrays 102 are formed of monochromatic LEDs 112. A combination of arrays 102 and laser diodes may be used to accurately simulate the radiation spectrum of the sun. In some embodiments, the electrical power driving each array 102 and laser diode is controlled independently. To achieve a specific mix of the light intensities from the arrays 102, the electrical current supplied to each array 102 is optimized according to the respective intensity of the corresponding spectral line in the sun radiation, such as by a controller 124.
In some embodiments, the light from one or more array 102 passes through a narrow band-pass filter 108, which passes a desired wavelength of the LEDs 112 behind the filter 108, but reflects other wavelengths, such as those emitted by other LEDs 112 In some embodiments a dichroic beam splitter 110 is used to direct the light emitted by the arrays 102 toward the test surface 106. In some embodiments, each dichroic beam splitter 110 is optimized for a predetermined wavelength or range of wavelengths, such as the wavelength emitted by an associated array 102. In this manner, the light that is emitted by several arrays 102 within the illuminator 100 is mixed in the light pipe 100 before reaching the test surface 106.
The illuminator 100 according to the present invention provides illumination to the test surface 106 with intensity uniformity, spatial uniformity, and spectral uniformity across the test surface 106 meeting all Class A specifications. The illuminator 100 of the present invention reduces thermal effects on the spatial uniformity and spectral content of the light. It is relatively easy to calibrate, maintain, and repair. Unlike prior art LED illuminators where a failure of one or more individual LEDs affects the spatial and spectral uniformities of the illumination, the illuminator 100 of the present invention compensates for such failure by adjusting the electrical drive current to the corresponding array 102.
In some embodiments, as depicted in
In another embodiment, more than one of the illuminators 100 are placed adjacent one another in a group and used to illuminate a large test surface 106, such as thin film solar modules. The intensity and spectral content of the separate illuminators 100 in the group is independently adjusted, according to the position of the given illuminator 100 in the group.
In various embodiments, the illuminator 100 includes a self-referencing system 114, as depicted in
The foregoing description of 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 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.
