CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claim priority under 35 U.S.C. §119(e) to U.S. Provisional Patent Application No. 61/175,109, filed May 4, 2009, the disclosure of which is hereby incorporated herein in its entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    Embodiments of the present invention generally relate to devices used to test solar cells, and particularly to a procedure for calibrating a light source used to simulate the sun in a solar cell testing apparatus. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Photovoltaic devices, such as solar cells, convert light into direct current electrical power. Thin film silicon solar cells typically are formed on a substrate and have one or more p-i-n junctions. Each p-i-n junction comprises a p-type layer, an intrinsic type layer, and an n-type layer that are amorphous, polycrystalline or microcrystalline materials. When the p-i-n junction of a solar cell is exposed to sunlight, consisting of energy from photons, the sunlight is converted to electricity. Solar cells may be tiled into larger modules or arrays. 
         [0004]    Typically, a thin film photovoltaic solar cell includes active regions and a transparent conductive oxide film disposed as a front electrode and/or as a back electrode. The photoelectric conversion unit includes a p-type silicon layer, an n-type silicon layer, and an intrinsic type (i-type) silicon layer sandwiched between the p-type and n-type silicon layers. Several types of silicon films including microcrystalline silicon film (μc-Si), amorphous silicon film (a-Si), polycrystallinesilicon film (poly-Si) and the like may be utilized to form the p-type, n-type, and/or i-type layers of the solar cell. The backside contact may contain one or more conductive layers. 
         [0005]    To assure that solar cell devices formed in a solar cell production line meet desired power generation and efficiency standards, various tests are performed on each formed solar cell. In some cases, a dedicated solar cell qualification module is placed in the solar cell production line to qualify and test the output of the formed solar cells. Typically, in these qualification modules a light emitting source and a solar cell probing device are used to measure the output of the formed solar cell. If the qualification module detects a defect in the formed device, it can take corrective actions or the solar cell can be scrapped. However, to assure that the tests performed in the testing module are the same for every tested device, the qualification module must be calibrated and recalibrated from time to time. The calibration and recalibration process requires the use of a number of devices, which may include a reference solar cell used to qualify the output of lamps and the environment of the testing module. 
         [0006]    Multiple junction tandem solar cells comprise a plurality of (typically three) distinct layers of photovoltaic devices that are electrically connected in series to one another. Each layer uses a different portion of the solar spectrum, thereby taking advantage of the fact that devices sensitive to short wavelength light can be transparent to longer wavelengths. 
         [0007]    When solar cells are manufactured, especially for space-based applications, testing of each cell is important in order to ensure adequate performance. Multiple junction solar cells are typically tested under a steady state solar simulator using relatively slow curve tracing and data acquisition equipment. The steady state solar simulator is a large, expensive device and at best has temporal instability (“flicker”) in the range of a few percent. Adjusting the spectral filtering for multiple junction solar cells requires additional equipment. 
         [0008]    For space-based applications, it is desirable to simulate sunlight that impinges an orbiting satellite, which is commonly known as Air Mass Zero sunlight, or AM0. Although the light source used for simulation of AM0 sunlight for electrical tests of solar cells need not be an exact match at all wavelengths (which would be extremely difficult), it must produce the same effect on each individual junction as would AM0 sunlight. 
         [0009]    The photoelectric conversion characteristics of photovoltaic devices, photo sensors and the like are measured by measuring the current-voltage characteristics of the photovoltaic devices under irradiance. In the measurement of the characteristics of photovoltaic devices, a graph is set up with voltage on the horizontal axis and current on the vertical axis and the acquired data is plotted to obtain a current-voltage characteristics curve. This curve is generally called an I-V curve. 
         [0010]    As the measurement methods, there are methods that use sunlight as the irradiating light and methods that use an artificial light source as the irradiating light. Of the methods that use an artificial light source, methods that use a fixed light and methods that use a flash are described in, for example, Japanese Patent No. 2886215and Laid-open Japanese Patent Application No. 2003-31825. 
         [0011]    Conventionally, with the commercialization of photovoltaic devices, and particularly with photovoltaic devices with large surface areas, the current-voltage characteristics are measured under a radiation irradiance of 1,000 W/m 2 , which is sunlight standard irradiance. Measured values are corrected mathematically by formula so as to compensate for when irradiation during measurement exceeds or falls short of 1,000 W/m 2 . 
         [0012]    In addition, measurement of the current-voltage characteristics of large-surface-area photovoltaic devices requires irradiation of light with an irradiance of 1,000 W/m 2  to a large-surface-area test plane uniformly. As a result, when using an artificial light source, for example, a high-power discharge lamp capable of providing several tens of kilowatts per square meter of radiation surface is required. However, in order for such a high-power discharge lamp to provide a fixed light it must be provided with a steady supply of high power. As a result, very large-scale equipment is required, which is impractical. 
         [0013]    Furthermore, with a solar simulator that uses a steady light, a xenon lamp, a metal halide lamp or the like for continuous lighting is used as the light source lamp. It usually takes several tens of minutes or more from the start of lighting of such lamps until irradiance stabilizes. Moreover, unless lighting is continued under the same conditions, the irradiance does not reach saturation and a great deal of time is required until measurement is started. On the other hand, as the accumulated lighting time grows by long hours of lighting, the irradiance tends to decrease gradually and thus the irradiance characteristics are not stable. In addition, the radiation of the light on the photovoltaic devices under measurement is conducted by changing shielding and irradiation of light with the opening and the closing of the shutter. Thus, the irradiation time required for the devices under testing depends on the operating speed of the shutter, and often exceeds several hundred milliseconds. As the irradiation time lengthens, the temperatures of the photovoltaic devices rise, thus making accurate measurement difficult. 
         [0014]    With a solar simulator that uses a fixed light, although it is necessary to maintain continuous lighting in order to stabilize the irradiance, doing so causes the temperature inside the housing that contains the light source to increase sharply. In addition, the components inside the housing are constantly exposed to light, which causes the optical components (minors, optical filters, etc.) to deteriorate. 
         [0015]    Once a fixed light source lamp is turned off and turned on again, it takes several tens of minutes for the irradiance to reach saturation. In order to avoid this, the fixed light source lamp is usually kept on and used as is. As a result, however, the accumulated lighting time of such fixed light lamps adds up easily and results in a tendency for the lamps to reach the end of their useful lives relatively quickly. Therefore, when using a fixed light-type solar simulator in a photovoltaic devices module production line, the number of lamps that burn out is added to the running cost, which increases not only the cost of measurement but also the cost of production. 
         [0016]    Moreover, with a fixed light solar simulator, the length of time during which light from the light source irradiates the photovoltaic devices under measurement is relatively long. As a result, when I-V curve measurements are repeated for the same photovoltaic devices, the temperatures of the photovoltaic devices increase. As the temperature of the photovoltaic devices increases, the output voltage and the maximum output power, P max , tend to decrease. 
         [0017]    In general, measurement of the photovoltaic devices current-voltage characteristics requires indicating standard test condition values. Here, the temperature of the photovoltaic devices under standard test conditions is 25° C. and the radiation irradiance is 1,000 W/m 2 . The measurement of the current-voltage characteristics of photovoltaic devices by a solar simulator is carried out with the temperature range of the photovoltaic devices in the range of 15° C.-35° C. The temperature is corrected to 25° C., the reference temperature, using a measured temperature of the photovoltaic devices. The correction formula used for this purpose is prescribed by industry standard. 
         [0018]    Thus, in order to ensure accurate output current and voltage performance measurements, there is a need for a viable and repeatable method of calibrating the light source used in testing photovoltaic devices. 
       SUMMARY OF THE INVENTION 
       [0019]    A method of calibrating a light source used as a solar simulator in solar cell testing apparatus comprising: 
         [0020]    a) using a control cell to measure the intensity of light from the light source at a first wavelength range, the intensity being measured as a function of output short circuit current of the control cell; b) comparing the measured intensity to a targeted intensity value; c) optionally adjusting power to the light source until the measured intensity is substantially equal to the targeted intensity value; d) using a calibrated monitoring module to periodically measure monitoring measured values for monitoring module output short circuit current, monitoring module output open circuit voltage and monitoring module quantum efficiency; e) repeating step d) and obtaining average values for monitoring module output short circuit current, monitoring module output open circuit voltage and monitoring module quantum efficiency; f) comparing the measured values obtained in step d) with the average values obtained in step e); and g) determining if differences in the measured values obtained in step d) and the average values obtained in step e) are within an acceptable limit 
         [0021]    In one embodiment, the method further includes, in step a), using a control cell to measure the intensity of light from the light source at a second wavelength range, the intensity being measured as a function of output short circuit current of the control cell and calculating a ratio of light intensity at the first and second wavelengths to provide a measured intensity ratio, and in step b) comparing the measured intensity ratio to a targeted intensity ratio value. 
         [0022]    In one embodiment, when the differences obtained in step g) are greater than an acceptable value, the method further comprises: h) using a calibrated reference module to obtain reference module measured values for reference module output short circuit current, reference module output open circuit voltage and reference module quantum efficiency; i) comparing the reference module measured values obtained in step h) to calibrated values for output short circuit current, open circuit voltage and quantum efficiency; and j) determining if differences in measured values obtained in step h) and calibrated values are within an acceptable limit 
         [0023]    In one embodiment, when the differences obtained in step j) are greater than an acceptable value, the method further comprises: k) adjusting power to the light source until the measured output short circuit current is within the acceptable limit In this embodiment, the method may further comprise: l) repeating step h); m) comparing the reference module measured values obtained in step l) to calibrated values for output short circuit current, open circuit voltage and quantum efficiency; and n) determining if differences in measured values obtained in step l) and calibrated values are within an acceptable limit. In a specific embodiment, when the differences obtained in step n) are less than an acceptable value, the method further comprises: o) using the calibrated reference module to obtain a reference module measured value for reference module output short circuit current; and p) determining if a difference in measured value obtained in step o) and a calibrated value is within an acceptable limit 
         [0024]    According to one embodiment, steps a) through c) are performed for each solar cell measurement, steps d) through g) are performed at least once daily, and steps g) through h) are performed on a weekly basis. 
         [0025]    In a specific embodiment, an acceptable percentage difference between the measured intensity and the targeted intensity value in b) is about 1%. 
         [0026]    In another specific embodiment, an acceptable percentage difference in monitoring module output short circuit current determined in step g) is about 2%, an acceptable percentage difference in monitoring module output open circuit voltage determined in step g) is about 2% and an acceptable percentage difference in monitoring module quantum efficiency determined in step g) is about 4%. 
         [0027]    In another specific embodiment, an acceptable percentage difference in monitoring module output short circuit current determined in step j) is about 1%, an acceptable percentage difference in monitoring module output open circuit voltage determined in step j) is about 1% and an acceptable percentage difference in monitoring module quantum efficiency determined in step j) is about 2%. 
         [0028]    In one embodiment, the targeted maximum percentage voltage difference in n) is about 1% and the targeted maximum percentage efficiency difference in n) is about 2%. In at least one embodiment, the control cell is a single crystal silicon cell having an appropriate band pass filter approximating tandem-junction spectra responses, has dimensions of about 2 cm×2 cm and is mounted in a hermetic package. In one embodiment, the monitoring module comprises a junction box which is also used to monitor intensity of light from the light source and electrical connections. 
         [0029]    According to one or more embodiments, the reference module is a filtered crystal silicon module designed to match an output short circuit current, an output open circuit voltage and a quantum efficiency of an amorphous silicon module. 
         [0030]    In one or more embodiments, the reference module is a crystal silicon solar cell module with dimensions of about 50 cm×50 cm and having a plurality of cells in series and an appropriate band pass filter. 
         [0031]    In certain embodiments, the solar cell testing apparatus is configured for measuring tandem junction solar cell modules. 
         [0032]    In one or more embodiments, the method comprises, in step a), monitoring an intensity of light from the light source by measuring an output short circuit current of the control cell on a daily basis. 
         [0033]    In a specific embodiment, the method includes, in step d), measuring an output short circuit current, an output open circuit voltage and a quantum efficiency of the monitoring module on a daily basis. 
         [0034]    In a specific embodiment, the method includes, in step h), measuring an output short circuit current, an output open circuit voltage and a quantum efficiency of the reference module on a weekly basis. 
         [0035]    In specific embodiments, the first wavelength range is in the range of about 620 nm to 750 nm, and the second wavelength range is in the range of about 440 nm to 490 nm. 
         [0036]    In specific embodiments, an acceptable percentage difference between the measured intensity and the targeted intensity value in b) is about 1% or about 3%. In other specific embodiments, an acceptable percentage difference in monitoring module output short circuit current determined in step g) is about 2%, an acceptable percentage difference in monitoring module output open circuit voltage determined in step g) is about 2% and an acceptable percentage difference in monitoring module quantum efficiency determined in step g) is about 4%. 
         [0037]    In other specific embodiments, an acceptable percentage difference in reference module output short circuit current determined in step j) is about 1%, an acceptable percentage difference in reference module output open circuit voltage determined in step j) is about 1% and an acceptable percentage difference in reference module quantum efficiency determined in step j) is about 2%. 
         [0038]    The foregoing has outlined rather broadly certain features and technical advantages of the present invention. It should be appreciated by those skilled in the art that the specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures or processes within the scope present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0039]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawing. It is to be noted, however, that the appended drawing illustrates only typical embodiments of this invention and is therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
           [0040]      FIG. 1  is an enlarged isometric view of one embodiment of a solar cell testing apparatus used in the calibration method of the present invention. 
           [0041]      FIG. 2  is an enlarged isometric view of one embodiment of a control cell used in the calibration method of the present invention. 
           [0042]      FIG. 3  is an elevated isometric view of one embodiment of a monitoring module or a reference module used in the calibration method of the present invention. 
           [0043]      FIG. 4  is a cross-sectional side view of one embodiment of a monitoring module or a reference module used in the calibration method of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0044]    Embodiments of the present invention provide a solar cell testing apparatus and methods of calibrating a light source used to simulate the sun in the solar cell testing apparatus. 
         [0045]    Equipment utilized in the inventive solar simulator calibration procedure will be discussed first.  FIG. 1  depicts an exemplary embodiment of a solar cell testing apparatus  10 , which can be of any appropriate size and configuration for testing a variety of differently sized modules. In the interior  12  of the apparatus  10 , a light source  14  such as a lamp is present. The light source  14  serves as a solar simulator; it has the capacity of providing light  16  as intense as sunlight. The light source  14  may be physically attached to a wall of the apparatus  10 , as shown, or freestanding in the interior  12  of the apparatus  10 . Power can be supplied to the light source  14  through a variety of means as well known in the art. A substrate holder  18  is positioned inside the apparatus  10  so that the light  16  focuses directly on its upper surface. The holder  18  is appropriately sized to accommodate at least one module of solar cells. 
         [0046]    A control cell  20  for use in the present invention is shown in  FIG. 2 . The control cell  20  may consist of monocrystalline silicon and have a rectangular shape. For example, the control cell  20  may be an Oriel or a Sinton cell having dimensions of 2 cm×2 cm and may be mounted in a hermetic package. A filter  22  is securely adhered to the upper surface of the control cell  20 . According to one embodiment, the filter  22  is an Oriel KG5 filter, which is a colored glass filter capable of serving as a broadband, band-pass or long-wave pass filter. In certain embodiments, the filter  22  is an appropriate band pass filter approximating tandem-junction spectra responses. The filter  22  is glued or otherwise fastened to the control cell  20 . 
         [0047]      FIG. 3  illustrates one embodiment of a monitoring module or a reference module  24  utilized to test the light source  14  used to qualify one or more solar cells formed in a solar cell production line. Generally, the monitoring module or reference module  24  contains an array of cells  26  positioned on a substrate  28  so that when the module  24  is positioned and oriented in a desired location, at least a portion of the light  16  from the light source  14  can be received by each cell  26 . The module  24  may have dimensions of, for example, 50 cm×50 cm. The substrate  28  can be formed from any desirable material capable of supporting and retaining the cells  26 . In one embodiment, the substrate  28  is made from a material such as a glass or a metal. According to other embodiments, the substrate  28  is either made from or at least partially covered with a dielectric material that will provide electrical isolation between the metal connections formed on each of the cells  26 , and between two or more cells  26 . The cells  26  in the module  24  may also be encapsulated between the substrate  28  and a cover  30  to prevent environmental attack of the cells  26  or other components in the module  24 , which may degrade the long-term performance of the module  24 . The module  24  may comprise a junction box which is also used to monitor intensity of the light  16  from the light source  14  and electrical connections. In certain embodiments, the module  24  is designed to match an output short circuit current, an output open circuit voltage and a quantum efficiency of an amorphous silicon module. 
         [0048]    A schematic cross-sectional side view of the module  24  is presented in  FIG. 4 . A layer of a polymeric material  32  has been disposed between the cover  30  and the cells  26  and substrate  28  to isolate the cells  26  and other components from the environment. In one embodiment, the polymeric material  32  is a polyvinyl butyral or ethylene vinyl acetate, which is sandwiched between the substrate  28  and the cover  30  using a process that provides heat and pressure to form a bonded and sealed structure. In general, the cover  30  and polymeric material  32  are made from a material that is optically transparent to allow the light  16  delivered from the light source  14  to reach the cells  26 . In one embodiment, the cover  30  is made of a glass, sapphire or quartz material. While not shown in  FIGS. 3 and 4 , the module  24  also generally contains a support frame that is used to retain, support and mount one or more components in the reference module. 
         [0049]    In one embodiment, as shown in  FIG. 4 , each cell  26  is attached to the substrate  28  using one or more supports  34 . In one embodiment, the supports  34  are electrically conductive and are formed and positioned in a desired pattern on the substrate  28  to electrically connect the cells  26  so that a desired power output can be achieved when a desired amount of light is delivered to the module  24 . In one aspect of the invention, all of the cells  26  are connected in series so that desired electrical output can be achieved. In cases where the cells have connections on both sides of each cell  26 , the supports  34  and/or other electrical connective elements (not shown) may be used to form a connection path to deliver a desired power output. 
         [0050]    According to one embodiment, an optical filter  36  is positioned within the module  24  to block certain wavelengths of light from reaching the cells  26 . This configuration allows more stable solar cells with different absorption spectrums to be used in the formed module  24 , rather than using a module  24  with solar cells that have similar absorption spectrums but varying electrical properties over time (e.g., silicon thin film solar cells). The more stable solar cells thus allow the module  24  to be a relatively unvarying “gold” calibration standard, which can be used in a solar cell qualification module to assure that it is functioning correctly without worrying about the module&#39;s shelf life or number of hours of light exposure. It should be noted that the addition of any filtering type device over the cells  26  will reduce the amount of energy striking the surface of the cells  26 . This effect can be compensated for by increasing the total surface area of the cells  26 , by using cells  26  that are more efficient than the solar cell devices formed in the production line, and/or by correcting the systematic error by software in the solar cell qualification module. While the filter  36  shown in  FIG. 4  is positioned within the module  24 , this configuration is not intended to be limiting as to the scope of the invention since the filter  36  could also be affixed to the cover  30 , deposited on the cover  30 , or the cover  30  could be altered by adding a doped impurity within the cover material to provide a desirable optical filtration. 
         [0051]    An exemplary embodiment of the single-junction solar cell light source calibration procedure will now be described. First, the light source  14  is placed inside the apparatus  10  as shown in  FIG. 1 . The control cell  20  is then placed in the substrate holder  18  and power is supplied to the light source  14 . The intensity of light  16  from the light source  14  is monitored by measuring an output short circuit current of the control cell  20 . This may be done on a daily basis. Next, a percentage difference between the output short circuit current and an externally calibrated short circuit current of the control cell  20  is calculated. If this percentage difference is greater than a targeted maximum percentage current difference, the power supplied to the light source  14  is adjusted and the resulting output short circuit current of the control cell  20  is measured until the percentage difference is less than the targeted maximum percentage current difference. The targeted maximum percentage current difference may be about 1%. 
         [0052]    A monitoring module  24  is then placed inside the apparatus  10  and connected to measurement circuitry. Subsequently, an output short circuit current, an output open circuit voltage and a quantum efficiency of the monitoring module  24  are measured several times. This may be done on a daily basis. A percentage difference between the output short circuit current measured on the last occasion and an average of the previously measured output short circuit currents is then computed. Next, a percentage difference between the output open circuit voltage measured on the last occasion and an average of the previously measured output open circuit voltages is calculated. Likewise, a percentage difference between the quantum efficiency measured on the last occasion and an average of the previously measured quantum efficiencies is also computed. If the percentage difference in short circuit currents is less than a targeted maximum percentage current difference, the percentage difference in open circuit voltages is less than a targeted maximum percentage voltage difference, and the percentage difference in quantum efficiencies is less than a targeted maximum percentage efficiency difference, the method ends and the light source  14  has been calibrated. However, if the percentage difference in short circuit currents is more than the targeted maximum percentage current difference, the percentage difference in open circuit voltages is more than the targeted maximum percentage voltage difference, or the percentage difference in quantum efficiencies is more than the targeted maximum percentage efficiency difference, the procedure continues with a reference module  24 , as now discussed. The targeted maximum percentage current difference may be about 2%, the targeted maximum percentage voltage difference may be about 2%, and the targeted maximum percentage efficiency difference may be about 4%. 
         [0053]    A reference module  24  is placed inside the apparatus  10  and connected to measurement circuitry. An output short circuit current, an output open circuit voltage and a quantum efficiency of the reference module  24  are then measured. This may be done on a weekly basis. Next, a percentage difference between the measured output short circuit current and an externally calibrated short circuit current is calculated. A percentage difference between the measured output open circuit voltage and an externally calibrated open circuit voltage of the reference module is also computed. Likewise, a percentage difference between the measured quantum efficiency and an externally calibrated quantum efficiency of the reference module is calculated. If the percentage difference in short circuit currents is less than a targeted maximum percentage current difference, the percentage difference in open circuit voltages is less than a targeted maximum percentage voltage difference, and the percentage difference in quantum efficiencies is less than a targeted maximum percentage efficiency difference, the procedure ends and the light source  14  has been calibrated. However, if the percentage difference in short circuit currents is more than the targeted maximum percentage current difference, the power supplied to the light source  14  is adjusted. The resulting output short circuit current of the reference module  24  is measured until the percentage difference in short circuit currents is less than the targeted maximum percentage current difference. The targeted maximum percentage current difference may be about 1%, the targeted maximum percentage voltage difference may be about 1%, and the targeted maximum percentage efficiency difference may be about 2%. 
         [0054]    Next, the output short circuit current, the output open circuit voltage and the quantum efficiency of the reference module  24  are measured. Percentage differences between the measured output short circuit current and an externally calibrated short circuit current of the reference module, the measured output open circuit voltage and an externally calibrated open circuit voltage of the reference module, and the measured quantum efficiency and an externally calibrated quantum efficiency of the reference module are then calculated. If the percentage difference in open circuit voltages is more than a targeted maximum percentage voltage difference and the percentage difference in quantum efficiencies is more than a targeted maximum percentage efficiency difference, a detailed system check of the apparatus  10  must be undertaken. If the percentage difference in short circuit currents is less than a targeted maximum percentage current difference, the method terminates. Alternatively, if the percentage difference in short circuit currents is more than the targeted maximum percentage current difference, a detailed system check of the apparatus  10  must be performed. The targeted maximum percentage voltage difference may be about 1%, and the targeted maximum percentage efficiency difference may be about 2%. 
         [0055]    An exemplary embodiment of the tandem-junction solar cell light source calibration procedure will now be described. First, the light source  14  is placed inside the apparatus  10  as shown in  FIG. 1 . A first control cell is then placed in the substrate holder  18  and power is supplied to the light source  14 . The intensity of light  16  from the light source  14  is monitored by measuring an output short circuit current of the first control cell. Next, a percentage difference between the output short circuit current and an externally calibrated short circuit current of the first control cell is calculated. If this percentage difference is greater than a targeted maximum percentage current difference, the power supplied to the light source  14  is adjusted and the resulting output short circuit current of the first control cell is measured until the percentage difference is less than the targeted maximum percentage current difference. 
         [0056]    A second control cell is then placed inside the apparatus  10  and connected to measurement circuitry. The second control cell is designed for monitoring a light intensity in a first wavelength range, which may be from about 620 nm to 750 nm. A third control cell is subsequently placed inside the apparatus  10  and connected to measurement circuitry. The third control cell is designed for monitoring a light intensity in a second wavelength range, which may be from about 440 nm to 490 nm. Output short circuit currents of both the second control cell and the third control cell are then measured. A light intensity ratio equal to the output short circuit current of the second control cell divided by the output short circuit current of the third control cell is then computed. After these steps are repeated on several occasions, a percentage difference between consecutive light intensity ratios is then calculated. If this percentage difference is more than a targeted maximum percentage ratio difference, the light source  14  is replaced and all of these steps are repeated. 
         [0057]    A monitoring module  24  is then placed inside the apparatus  10  and connected to measurement circuitry. Subsequently, an output short circuit current, an output open circuit voltage and a quantum efficiency of the monitoring module  24  are measured several times. A percentage difference between the output short circuit current measured on the last occasion and an average of the previously measured output short circuit currents is then computed. Next, a percentage difference between the output open circuit voltage measured on the last occasion and an average of the previously measured output open circuit voltages is calculated. Likewise, a percentage difference between the quantum efficiency measured on the last occasion and an average of the previously measured quantum efficiencies is also computed. If the percentage difference in short circuit currents is less than a targeted maximum percentage current difference, the percentage difference in open circuit voltages is less than a targeted maximum percentage voltage difference, and the percentage difference in quantum efficiencies is less than a targeted maximum percentage efficiency difference, the method ends and the light source  14  has been calibrated. However, if the percentage difference in short circuit currents is more than the targeted maximum percentage current difference, the percentage difference in open circuit voltages is more than the targeted maximum percentage voltage difference, or the percentage difference in quantum efficiencies is more than the targeted maximum percentage efficiency difference, the procedure continues with a reference module  24 , as now discussed. 
         [0058]    A reference module  24  is placed inside the apparatus  10  and connected to measurement circuitry. An output short circuit current, an output open circuit voltage and a quantum efficiency of the reference module  24  are then measured. Next, a percentage difference between the measured output short circuit current and an externally calibrated short circuit current is calculated. A percentage difference between the measured output open circuit voltage and an externally calibrated open circuit voltage of the reference module is also computed. Likewise, a percentage difference between the measured quantum efficiency and an externally calibrated quantum efficiency of the reference module is calculated. If the percentage difference in short circuit currents is less than a targeted maximum percentage current difference, the percentage difference in open circuit voltages is less than a targeted maximum percentage voltage difference, and the percentage difference in quantum efficiencies is less than a targeted maximum percentage efficiency difference, the procedure ends and the light source  14  has been calibrated. However, if the percentage difference in short circuit currents is more than the targeted maximum percentage current difference, the power supplied to the light source  14  is adjusted. The resulting output short circuit current of the reference module  24  is measured until the percentage difference in short circuit currents is less than the targeted maximum percentage current difference. 
         [0059]    Next, the output short circuit current, the output open circuit voltage and the quantum efficiency of the reference module  24  are measured. Percentage differences between the measured output short circuit current and an externally calibrated short circuit current of the reference module, the measured output open circuit voltage and an externally calibrated open circuit voltage of the reference module, and the measured quantum efficiency and an externally calibrated quantum efficiency of the reference module are then calculated. If the percentage difference in open circuit voltages is more than a targeted maximum percentage voltage difference and the percentage difference in quantum efficiencies is more than a targeted maximum percentage efficiency difference, a detailed system check of the apparatus  10  must be undertaken. If the percentage difference in short circuit currents is less than a targeted maximum percentage current difference, the method terminates. Alternatively, if the percentage difference in short circuit currents is more than the targeted maximum percentage current difference, a detailed system check of the apparatus  10  should be performed. 
         [0060]    Reference throughout this specification to “one embodiment,” “certain embodiments,” “one or more embodiments” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases such as “in one or more embodiments,” “in certain embodiments,” “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments. The order of description of the above method should not be considered limiting, and methods may use the described operations out of order or with omissions or additions. 
         [0061]    It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of ordinary skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.
A method of calibrating a light source used to simulate the sun in solar cell testing apparatus. The method comprises using a control cell to measure the intensity of light from the light source at a first wavelength range as a function of output short circuit current, comparing the measured intensity to a targeted intensity value, optionally adjusting power to the light source until the measured intensity is substantially equal to the targeted intensity value, repeatedly using a calibrated monitoring module to periodically measure monitoring measured values for monitoring module output short circuit current, monitoring module output open circuit voltage and monitoring module quantum efficiency, obtaining average values for monitoring module output short circuit current, monitoring module output open circuit voltage and monitoring module quantum efficiency, comparing the measured values with the average values, and determining if differences in measured values and average values are within an acceptable limit.