Apparatus and method for multispectral determination of spectral irradiance sensitivity and voltage-dependent current behavior of photovoltaic units

Abstract

The invention relates to a measuring apparatus (1) and a method for determining a spectral irradiance sensitivity SR(λ) (17) and a voltage-dependent current behavior I(U) under standard sun conditions of a photovoltaic unit (3). The measuring apparatus comprises an illumination source (5), a contacting device (7), a controller (9) and an evaluation device (11). The illumination source can alternatively emit light (13) with variable spectral distributions (15) or with a standardized solar spectrum. The controller can control the illumination source for emission of the solar spectrum in a first operating mode and for progressive emission of light with a plurality of N different spectral distributions in a second operating mode. The evaluation device is configured, in the first operating mode, to determine the voltage-dependent current behavior I(U) of the photovoltaic unit (3) under the action of the standardized solar spectrum from a measured PV current I meas and, in the second operating mode, to determine the spectral irradiance sensitivity SR(λ) of the photovoltaic unit from a plurality of currents I meas,n (n = 1…N) respectively measured during illumination with the N different spectral distributions (15), and taking account of a spectral irradiance Eλ,n(λ) (n = 1…N) already known for each of the N spectral distributions.

Description

[0001] Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 1 of 46 ------------------------------------------------------------------------------------------------- Institut für Solarenergieforschung GmbH Am Ohrberg 1, 31860 Emmerthal, Germany ------------------------------------------------------------------------------------------------- DEVICE AND METHOD FOR THE MULTISPECTRAL DETERMINATION OF SPECTRAL IRRADIANCE SENSITIVITY AND VOLTAGE-DEPENDENT CURRENT BEHAVIOR OF PHOTOVOLTAIC UNITS ------------------------------------------------------------------------------------------------- FIELD OF THE INVENTION The present invention relates to a device and a method for characterizing properties of photovoltaic units. Furthermore, the invention relates to a corresponding computer program product and a computer-readable medium.TECHNICAL BACKGROUND Photovoltaic units are semiconductor devices that utilize the internal photoelectric effect to generate an electrical signal upon absorption of light. Photovoltaic units can generally be described as radiation sensors. In particular, photovoltaic units can be designed as solar cells. Solar cells serve to convert light, particularly that radiated by the sun, into electrical energy. Such solar cells are also called photovoltaic cells. Several solar cells can be interconnected to form a solar module.Both individual solar cells and entire solar modules, as well as other types of radiation sensors, are generally referred to herein as photovoltaic units, with photovoltaics sometimes being abbreviated to PV. Various physical properties of photovoltaic units are conventionally determined using different measuring devices and methods, usually developed specifically for a specific application. For example, to assess the behavior of a photovoltaic unit, and in particular its efficiency, it may be necessary to know the voltage-dependent current behavior I(U) of the photovoltaic unit.This current behavior indicates how the electric current I flowing through the photovoltaic unit behaves as a function of the electric voltage U applied to the photovoltaic unit. In particular, information about this current behavior when the photovoltaic unit is illuminated may be desired. The aim is generally to illuminate the photovoltaic unit in a way that corresponds as closely as possible to real-life use when illuminated with light radiated by the sun. In order to be able to characterize photovoltaic units as precisely and standardized as possible and to avoid the need for illumination with real sunlight, the conditions that prevail when illuminated with real sunlight have been standardized with regard to light intensity and spectral distribution. A standardized solar spectrum, for example, conforms to the AM1 standard.5G corresponds to an intensity of 1000 W / m² and a spectral distribution that averages the radiation at the Earth's surface, i.e., after passing through 1.5 times the Earth's atmosphere. Such a standardized AM1.5G solar spectrum is therefore preferred for characterizing terrestrial photovoltaic units. A standardized AM0 solar spectrum corresponds in intensity and spectral distribution to solar radiation as it exists in space and is therefore used primarily for characterizing photovoltaic units for satellites and similar devices.Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 3 of 46 Technical devices designed to characterize the voltage-dependent current behavior of photovoltaic units under radiation conditions that are as realistic as possible are also referred to as IV measuring stations with a solar simulator as the light source. This generally involves a measuring station at which a photovoltaic unit can be electrically contacted using a contacting device. An evaluation device can then be used to measure the current flowing through the photovoltaic unit as a function of the applied electrical voltage. From the current-voltage characteristic curve determined in this way, characteristic data such as the short-circuit current I can be derived. SC , the open-terminal voltage V OC , the maximum power Pmax, the fill factor FF and / or the efficiency eta can be determined, whereby the photovoltaic unit can be evaluated and / or characterized. The IV measuring station is usually set up so that the photovoltaic unit can be measured under standard test conditions (sample temperature, irradiance and spectrum of the solar simulator according to the applicable standard). For this purpose, the IV measuring station has an illumination source which is designed to reproduce the applicable standard spectrum (solar spectrum) as accurately as possible. For this purpose, the solar simulator can, for example, be equipped with a thermal light source whose radiation is then modified using various filters in order to replicate the standardized solar spectrum.Alternatively, modern solar simulators can incorporate multiple light-emitting diodes (LEDs), whose emitted light can collectively reproduce the standardized solar spectrum, possibly with the use of filters. LEDs can achieve significantly reduced energy consumption, lower maintenance requirements, a longer service life, faster switching of light emission, and / or more stable light emission behavior over time. Furthermore, LEDs make it easier to restrict a spectrum to a spectral range relevant for measurement.In order to be able to influence the spectral distribution of the light spectrum prevailing in the solar simulator, various LEDs can be provided in the solar simulator which differ in terms of the spectral distribution of the light they emit. If necessary, these various LEDs can be controlled individually or in several groups. Another way of characterizing photovoltaic units is by determining the so-called spectral irradiance sensitivity SR(λ) (English: spectral responsivity, SR). The spectral irradiance sensitivity thus indicates the current the photovoltaic unit emits when illuminated with light of a certain wavelength ora narrowband wavelength range and is usually specified in the unit mA / (W / m²) or, independent of the area, in the unit A / W. The spectral irradiance sensitivity curves (SR curves) are used, among other things, during the development and characterization of photovoltaic devices such as solar cells or solar modules and for material analysis, since the SR curve spectrally resolves the current behavior, in particular a short-circuit current ^sc, of the photovoltaic device. For example, wavelength ranges that increase or decrease the short-circuit current can be identified for material analysis. Furthermore, with physical modeling, it is possible to attribute the shape of the SR curve to cell properties such as recombination or absorption in order to determine the cell properties.For the calibration of photovoltaic units and the determination of their characteristic parameters under standard test conditions, SR curves are generally essential for determining a spectral mismatch factor and thus for precisely adjusting the irradiance of the solar simulator. Only with the help of this spectral mismatch factor can a characteristic curve of the photovoltaic unit be determined precisely and in compliance with standards in IV measurement systems. This applies to both offline analyses and inline measurements. Institute for Solar Energy Research (ISFH) Multispectral Method for Determining Spectral Irradiance Response and Current Behavior of Solar Cells / Modules Page 5 of 46 The spectral irradiance sensitivity ^^(^) of solar cells and solar modules is usually determined at special SR measuring stations (so-called DSR / SR measuring stations). This approach is referred to herein as the "SR standard method."One operating principle of these existing DSR / SR measuring stations consists of irradiating the test object with two radiation sources: (1) a temporally constant white (bias) light and (2) a quasi-monochromatic light as a small signal with a significantly lower intensity. The white light is generated, for example, using an array of halogen lamps. Ideally, the white light can be the same or largely similar to the standard spectrum under which the current-voltage characteristic is to be measured later, usually the AM1.5G or AM0 spectrum. The quasi-monochromatic light can be generated, for example, using a combination of a lamp with a broadband spectrum and a monochromator and / or various filters. The quasi-monochromatic light can be temporally modulated, for example, using a chopper wheel.Before measuring the test object, the radiant contribution of the quasi-monochromatic light is calibrated with a reference solar cell. During the measurement process, the test object is kept under short-circuit conditions. The current response of the test object is then a direct current with a small pulsating component. A transimpedance converter separates the pulsating current component from the direct current component and provides it as a proportional voltage signal, which is measured and evaluated using a lock-in amplifier. The result is the differential SR of the test object, which is integrated over various bias irradiances and thus provides the desired SR of the test object. This method has been established for many decades for PV applications, but requires complex measurement technology and is time-consuming.For a few years now, there have been attempts to determine the SR curve using LEDs of different wavelengths (herein referred to as the "SR-LED reference solar cell method"). This is a variant or further development of the previously described SR standard method. The test object is individually irradiated with the LEDs, and the resulting short-circuit current is measured. The irradiance of the LEDs is previously measured using a calibrated solar cell. It has been shown that this method can be used to determine the SR curve of the test object. A disadvantage of this method is that the LEDs are often broadband, and therefore the SR cannot be determined sufficiently accurately in some wavelength ranges.Mathematically complex methods have been proposed to prevent this problem. An additional challenge is stabilizing the light intensity of each individual LED, since the monitor diodes installed in solar simulators typically collect broadband light and are optimized for different operating points. To date, this method has not been widely adopted for PV applications. SUMMARY OF THE INVENTION AND EMBODIMENTS There may be a need for an alternative measuring device and an alternative method for characterizing photovoltaic units. In particular, there may be a need to be able to characterize photovoltaic units quickly, cost-effectively, and / or reliably with little effort.Furthermore, there may be a need for a computer program product by means of which corresponding methods can be implemented, as well as for a computer-readable medium with such a computer program product stored thereon. These needs can be met at least in part by the subject matter of one of the independent claims of the present application. Advantageous embodiments are specified in the dependent claims as well as the following description and the figures. Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 7 of 46 According to a first aspect of the present invention, a measuring device for determining a spectral irradiance sensitivity SR(λ) and a voltage-dependent current behavior I(U) of a photovoltaic unit under standard solar conditions is described.The measuring device comprises at least one illumination source, a contacting device, a controller, and an evaluation device. The illumination source is configured to selectively emit light with variable spectral distributions or with an intensity and spectral distribution corresponding to a standardized solar spectrum. The contacting device is configured to electrically contact the photovoltaic unit. The controller and the evaluation device can be selectively operated in a first or a second operating mode. The controller is configured to (i) in the first operating mode, control the illumination source to emit light with an intensity and spectral distribution corresponding to the standardized solar spectrum, and (ii) in the second operating mode, control the illumination source to successively emit light with a plurality N of different spectral distributions.Furthermore, the evaluation device is configured to apply an electrical voltage to the photovoltaic unit via the contacting device and to measure a current flowing through the photovoltaic unit as a function of the applied voltage and in doing so (i) in the first operating mode, to determine the voltage-dependent current behavior I(U) of the photovoltaic unit under the influence of the standardized solar spectrum from the measured current, and (ii) in the second operating mode, from a plurality of currents Imeas,n (n = 1…N) measured when illuminated with the N different spectral distributions and taking into account a spectral irradiance E known in advance for each of the N spectral distributions. λ,n(λ) (n = 1…N) to determine the spectral irradiance sensitivity SR(λ) of the photovoltaic unit. According to a second aspect of the present invention, a method for determining a spectral irradiance sensitivity SR(λ) of a photovoltaic unit using a measuring device for determining a voltage-dependent current behavior I(U) of a photovoltaic unit under standard solar conditions is described. The measuring device comprises an illumination source, a contacting device, a controller, and an evaluation device. The illumination source is configured to selectively emit light with variable spectral distributions or with an intensity and spectral distribution that correspond to a standardized solar spectrum.The contacting device is configured to electrically contact the photovoltaic unit. The controller is configured to drive the illumination source in the first operating mode to emit light with an intensity and spectral distribution corresponding to the standardized solar spectrum. The evaluation device is configured to apply an electrical voltage to the photovoltaic unit via the contacting device and, depending on the applied voltage, to measure a current flowing through the photovoltaic unit. In the first operating mode, the evaluation device is configured to determine the voltage-dependent current behavior I(U) of the photovoltaic unit under the influence of the standardized solar spectrum from the measured current.The method comprises at least the following steps, possibly but not necessarily in the specified order: controlling the illumination source by means of the controller in a second operating mode for successively emitting light with a plurality N of different spectral distributions, measuring, by means of the evaluation device, in the second operating mode, currents I flowing through the photovoltaic unit when illuminated with the N different spectral distributions. meas,n (n = 1…N), and determining the spectral irradiance sensitivity SR(λ) of the photovoltaic unit taking into account a previously known spectral irradiance E for each of the spectral distributions λ,n(λ) (n = 1…N). Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 9 of 46 According to a third aspect of the invention, a computer program product is described which has machine-readable instructions which, when executed by a processor of a measuring device for determining a current behavior under standard solar conditions of a photovoltaic unit, instruct the measuring device to carry out or control the method of an embodiment of the second aspect of the invention. According to a fourth aspect of the invention, a computer-readable medium is described which has machine-readable instructions of a computer program product according to an embodiment of the third aspect of the invention stored thereon.By way of introduction, a basic idea and possible advantages relating to embodiments of the invention described herein will be briefly explained. This explanation is to be interpreted as merely a rough summary and not as limiting the invention. The present invention relates in particular to a measuring device with which both the voltage-dependent current behavior of a photovoltaic unit when illuminated with a standardized solar spectrum and the spectral irradiance sensitivity of the photovoltaic unit can be determined. The measuring device comprises components such as, in particular, an illumination source, a contacting device, a controller, and an evaluation device, as are frequently used in modern solar simulators.However, in addition to a functionality implemented in solar simulators within a first operating mode, in which the illumination source is controlled to emit the standardized solar spectrum and the evaluation device is configured to measure a resulting voltage-dependent current of the photovoltaic unit, the control and evaluation device are configured to implement a second operating mode. In this second operating mode, the measuring device can then determine the spectral irradiance sensitivity of the photovoltaic unit.For the second operating mode, a different approach is used than is the case with the conventional methods explained in the introduction to the description, in particular the "SR standard method" or the "SR-LED reference solar cell method". In particular, an approach is chosen in which the spectral irradiance sensitivity can be determined largely using components that are often used in a solar simulator anyway, i.e. in which a conventional solar simulator does not need to be modified in terms of hardware, or only slightly (for example, as explained below, by adding a spectrometer or a spectroradiometer). In particular, this takes advantage of the fact that modern solar simulators often have an illumination source with, for example, several differently colored LEDs and are thus able to emit light with a variable spectral distribution.As described in detail below, this can be used to illuminate the photovoltaic unit several times in the second operating mode, each time with different spectral distributions, and to measure the resulting current in each case. If the corresponding spectral irradiance is known for each spectral distribution, for example, based on an additional measurement performed with a spectrometer or spectroradiometer, the spectral irradiance sensitivity can be determined, for example, using a relatively easy-to-implement calculation method. The "multi-spectral determination method" described here can offer various advantages over the conventional methods for determining spectral irradiance sensitivity described above, particularly when implemented in combination with the resources or components of a solar simulator or IV measuring station.Institute for Solar Energy Research (ISFH) Multispectral Method for Determining Spectral Irradiance Response and Current Behavior of Solar Cells / Modules Page 11 of 46 The following advantages over the standard SR method can be mentioned: - The SR of the photovoltaic unit can be determined directly within an IV measuring station using a spectrally adjustable light source. A separate SR measuring station is not necessary. This saves costs, effort, and time. - Mechanical stress on the photovoltaic unit can be reduced, particularly since it only needs to be contacted once to determine both an IV curve and an SR curve. - No complex lock-in technology or transimpedance converter is required.This can be particularly advantageous for characterizing large solar cells (M6 - M12), since up to now, no transimpedance converter for applying the standard method has been commercially available. - The measurement time can be reduced from approximately 20-60 minutes with the SR standard method to a few minutes or even less than a minute. Compared to the "SR-LED reference solar cell method", the following advantages can be mentioned, among others: - The SR curve can be determined directly at the 1-sun operating point of the solar cell or photovoltaic unit. - The proposed method is mathematically simple to implement. - No corrections are required for broadband LEDs. - In particular, a spectrometer or a spectroradiometer (calibrated spectrometer) can be used as a reference instead of a calibrated reference solar cell. Such a spectrometer orA spectroradiometer of sufficiently good quality can be available, particularly commercially. - The light intensity of the LEDs can be stabilized using commonly used broadband monitor diodes. Stabilization of each individual LED channel is not necessary. Institute for Solar Energy Research (ISFH) Multispectral Method for Determining Spectral Irradiance Sensitivity and Current Behavior of Solar Cells / Modules Page 12 of 46 Furthermore, different variants of radiation sensors can be characterized using embodiments of the measuring device presented here. A first variant behaves like a solar cell. A second variant of a radiation sensor has an internal shunt resistor. In such radiation sensors, a voltage drop of an internally flowing current across a resistor is measured. Such sensors can also be characterized using the method or device presented here.the presented measuring device. Furthermore, the approach presented here can also be used to characterize digital sensors that output a value proportional to the irradiance. Overall, an advantageous integration of the methodology presented here into a multi-channel LED solar simulator is conceivable. This makes it possible, for example, to determine the spectral irradiance sensitivity SR(λ) of a solar cell to be measured in addition to a voltage-dependent current behavior of the solar cell determined with the solar simulator, and thus to directly determine the factor of spectral mismatch. In particular, no additional measuring station is required, which means that, among other things, considerable costs and / or space requirements, for example for equipping a solar cell production line with an SR measurement capability, can be saved.Possible configurations and advantages of embodiments of the measuring device and of a method are described in more detail below: The measuring device presented here is designed, among other things, for implementing the functionality of a solar simulator. Accordingly, the measuring device can determine the voltage-dependent current behavior of an individual solar cell or an entire solar module under standard solar conditions. Institute for Solar Energy Research (ISFH) Multispectral Method for Determining Spectral Irradiance Sensitivity and Current Behavior of Solar Cells / Modules Page 13 of 46 The illumination source of the measuring device is capable of emitting light whose intensity corresponds to that of natural sunlight, i.e., approximately 1000 W / m², and which also has a spectral distribution that largely resembles natural sunlight.For example, the illumination source is configured to emit an AM1.5G solar spectrum. The illumination source can illuminate a measurement area as homogeneously as possible, covering at least the surface of the photovoltaic unit to be characterized. This means, for measuring individual solar cells, a measurement area of ​​typically between 0.1 cm² and 1000 cm², usually between 1 cm² and 500 cm², or for measuring entire solar modules, a significantly larger measurement area of, for example, at least 0.3 m² up to several square meters. Operation and, in particular, power supply of the illumination source are controlled by the control system of the measuring device. The measuring device has a contacting device with which the photovoltaic unit to be characterized can be electrically contacted and, if necessary, suitably positioned and / or mechanically held.Via the contacting device, the evaluation unit of the measuring device can apply an electrical voltage to the photovoltaic unit and measure the resulting current (hereinafter sometimes referred to as "PV current"). The voltage can, for example, be varied over a range that is at least dependent on a short-circuit voltage U. SC (short-circuit) up to an open-terminal voltage U OC(open-circuit, sometimes also referred to as open-circuit voltage) of the photovoltaic unit, ie, for example, in the case of a single silicon solar cell, over a voltage range from 0 V to approximately 0.7 V. In addition to the solar simulator functionality, the measuring device presented here is configured to determine the spectral irradiance sensitivity SR(λ). Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 14 of 46 For this purpose, the illumination source is configured in such a way that it can be used to emit light with different spectral distributions.The light can be emitted successively with a plurality N of different spectral distributions, where N > 1, N > 2, N > 5, preferably N > 10 or N > 20, N > 50, N > 100, N > 200, N > 500 or N > 1000 and / or N < 100000, N < 50000, N < 10000, preferably N < 5000 or N < 1000, N < 500 or N < 1000. An emitted spectral distribution can be controlled and varied via the control of the measuring device. The spectral distributions differ in particular with regard to the radiant power emitted at different wavelengths.Differences between the spectral distributions can be at least 1% relative, at least 2% relative, at least 5% relative, at least 10% relative or at least 20% relative over at least one wavelength sub-range, wherein the wavelength sub-range can extend, for example, over at least one wavelength interval of at least 5 nm, at least 10 nm, at least 20 nm or at least 30 nm. The illumination source can emit light with a spectral distribution over at least one illumination duration that is sufficiently long to be able to measure the current subsequently occurring in the photovoltaic unit. For example, the illumination duration can be longer than 10 ms, longer than 100 ms, longer than 1 s or longer than 10 s and / or shorter than 100 s, shorter than 10 s, shorter than 1 s or shorter than 100 ms.The evaluation device is additionally configured to determine the electrical PV current I resulting in the photovoltaic unit in the second operating mode for each of the N different spectral distributions radiated by the illumination source. meas,n (with n = 1…N). Based on this multitude of current measurements, the irradiance sensitivity of the photovoltaic unit can then be determined, for example, by analytical and / or numerical calculation, at least to an approximation sufficiently good for practical applications, as explained in detail below for an example. Institute for Solar Energy Research (ISFH) Multispectral Method for Determining Spectral Irradiance Sensitivity and Current Behavior of Solar Cells / Modules Page 15 of 46 In addition to the information on the measured PV currents, a corresponding spectral irradiance E must also be determined for each of the spectral distributions. λ,n(λ) (n = 1…N) must be known in advance with sufficient quality and taken into account when determining the irradiance sensitivity. According to one embodiment, the spectral irradiance E λ,n(λ) for each of the spectral distributions must be known in advance by measurement using the spectrometer or spectroradiometer. In other words, a spectrometer or spectroradiometer can be used to measure the spectral irradiances of the spectral distributions to be emitted in the measuring device described herein with an accuracy sufficient for the subsequent determination of the spectral irradiance sensitivity SR(λ). A spectrometer is designed to measure a light spectrum as intensity as a function of wavelength. Simple spectrometers usually record the light intensity at the various wavelengths as a relative quantity and typically output measured values ​​as [counts].A spectroradiometer, sometimes also called a spectroradiometer, is a light measuring instrument in the form of a calibrated spectrometer that can measure both the wavelength and the spectral irradiance of the light emitted by a light source. Unlike a simple spectrometer, which can generally only measure a spectrum of incident light in terms of its relative intensity differences at different wavelength ranges, a spectroradiometer, as a calibrated spectrometer, can determine an absolute irradiance for each wavelength range. Spectrometers in the form of so-called array spectrometers distinguish the wavelength based on the position at which the light strikes a detector array, allowing the entire spectrum to be captured with a single detection.If the light source is stable over time, the spectrum can alternatively be measured using a scanning system, whereby the wavelength is usually selected using dispersive optics such as an optical grating, a prism, or similar. In this case, the dispersive optics can be rotated step by step, and for each rotation step, the (spectral) irradiance can be measured using a small, broadband sensor, for example, based on silicon. Most spectrometers have a base measurement of counts, which is an uncalibrated measured value and is thus influenced by the sensitivity of the detector for each wavelength.By applying calibration, the spectrometer is then able to function as a spectroradiometer, providing measurements of spectral irradiance [W / (m²*nm)], spectral radiance, and / or spectral flux. This data can then be used to obtain measurements of, for example, irradiance (W / cm²), illuminance (lux or fc), radiance (W / sr), luminance (cd), luminous flux (lumens or watts), chromaticity, color temperature, peak value, and / or dominant wavelength. Spectrometers can cover many wavelength ranges. The effective wavelength range (spectral range) of a spectrometer is determined not only by the dispersiveness of a grating used within it, but also by the sensitivity range of the detectors. For example, limited by a band gap of the semiconductor, a silicon-based detector responds to 200-1100 nm, while an InGaAs-based detector is usually sensitive to 900-1700 nm.To be based on the spectral irradiance E. λ,nTo subsequently determine the spectral irradiance sensitivity SR(λ) with sufficient quality, the spectral irradiance should be known in advance with sufficient accuracy. For example, the spectral irradiance should be known in advance with a wavelength resolution of at least 100 nm, preferably at least 50 nm, at least 20 nm, or at least 10 nm. In addition, the spectral irradiance should be known in advance with a spectral bandwidth of at most 100 nm, preferably at most 50 nm, at most 20 nm, or at most 10 nm. Institute for Solar Energy Research (ISFH) Multispectral Method for Determining Spectral Irradiance Sensitivity and Current Behavior of Solar Cells / Modules Page 17 of 46.Furthermore, the spectral irradiance should be known in advance over a wavelength range of at least 400 nm to 800 nm, with a lower limit of the wavelength range preferably being 380 nm, 350 nm, 330 nm, or 300 nm or even lower, and an upper limit of the wavelength range preferably being 850 nm, 900 nm, 950 nm, 1000 nm, 1050 nm, or 1100 nm or even higher. Furthermore, the spectral irradiance should be known in advance with a nonlinearity with respect to intensity of better than 20%, preferably better than 10%, or better than 5%. Furthermore, a relative intensity calibration should be better than 20%, preferably better than 10%, or better than 5%. Finally, a relative wavelength calibration should be better than 40 nm, preferably better than 10 nm, or even better than 3 nm.In particular, the spectral irradiance can be measured using a high-quality spectroradiometer designed for measuring spectral irradiance with the aforementioned properties. Furthermore, such a spectroradiometer should, if possible, have a computer interface to communicate the measured spectral irradiance and, if applicable, other data to other components of the measuring device described herein. By using the spectrometer or spectroradiometer, the spectral irradiance E for each of the spectral distributions irradiated by the illumination source onto the photovoltaic unit can be determined. λ,n (λ) can be measured with high accuracy and individually for the spectral distribution actually irradiated onto the photovoltaic unit. Based on such measured values ​​E λ,n (λ) can then be calculated together with the information about the measured currents I meas,nThe spectral irradiance sensitivity SR(λ) can be calculated with high accuracy and sufficient resolution. If the spectral irradiance was measured with an uncalibrated spectrometer, information about the relative spectral irradiance sensitivity can be derived based on the measured values. If, however, the spectral irradiance was measured with a calibrated spectroradiometer, information about the absolute spectral irradiance sensitivity can be derived based on the measured values. From this information about the absolute spectral irradiance sensitivity, an absolute value for the short-circuit current I SCof the solar cell. According to one embodiment, the spectrometer or spectroradiometer can be integrated into the measuring device to measure the spectral irradiance E λ,n(λ) for each of the spectral distributions emitted within the measuring device. However, it should be noted that although it appears advantageous to measure the spectral irradiance for each irradiated spectral distribution individually using a spectrometer or spectroradiometer integrated directly into the measuring device, this is not absolutely necessary for determining the spectral irradiance sensitivity as long as the spectral irradiance is at least previously known from other information sources. For example, the spectral irradiance can also be calculated or determined from other measurement data and / or simulations. In this case, it is generally necessary to measure the emission properties of the illumination source used in the measuring device at least once using a spectrometer or spectroradiometer, i.e.either simultaneously with the measurement of the respective currents I. meas,n or at a different time, ie, an earlier time or a later time. For example, the spectrometer or spectroradiometer can be kept externally or only temporarily integrated into the measuring device. In this case, the spectrometer or spectroradiometer can be used to measure the spectral irradiance E once in advance or at certain intervals. λ,n (λ) are measured and stored for each of the spectral distributions to be emitted within the measuring device. Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 19 of 46, so that at a time when the spectral distributions are actually emitted in the measuring device, their spectral irradiance E λ,n(λ) no longer needs to be measured up-to-date, but the stored values ​​can be retrieved. It may be possible to measure the emission spectra of individual light sources, such as individual LEDs, which together form the illumination source of the measuring device, in advance using the spectrometer or spectroradiometer. The emission spectra of each individual light source should be measured under different control conditions, i.e. at different emission intensities. From the measurement data thus obtained, the total light emitted by the illumination source, including its spectral distribution and its spectral irradiance, can then be determined, i.e. calculated, for example, later during operation of the measuring device. With such an approach, it is assumed that the illumination source of the measuring device generates the various spectral distributions, e.g.by specifically controlling their individual light sources can be reproduced very precisely and with temporal stability. If such an exact reproduction of different spectral distributions is not possible or cannot be guaranteed, a spectrometer or spectroradiometer should preferably be provided directly in the measuring device and each controlled spectral distribution should be measured with the help of this spectrometer or spectroradiometer with regard to its spectral irradiance. According to a further specified embodiment, the spectrometer or spectroradiometer is configured and arranged such that it receives light from a position adjacent to and / or in a plane with the photovoltaic unit to be contacted in the contacting device. In other words, the spectrometer orThe spectroradiometer should be arranged in the measuring device or optically coupled to the measuring device in such a way that, like the photovoltaic unit to be measured, it is irradiated with light emitted by the illumination source, and the light reaching the photovoltaic unit differs only insignificantly in terms of quality and / or quantity from the light reaching the spectrometer or spectroradiometer. Preferably, the spectrometer or spectroradiometer should be arranged in such a way that the spectral irradiance of the light reaching the photovoltaic unit is, overall and / or at each wavelength, within the effective spectral range of the spectrometer orspectroradiometer by less than 10%, less than 5%, less than 2%, less than one percent, less than 0.5% or even less than 0.1% from the light reaching the spectroradiometer. The spectrometer or spectroradiometer can, for example, be arranged at a distance of less than 20 cm, less than 10 cm, less than 5 cm or even less than 2 cm from an edge of the contacting device or an edge of the photovoltaic unit to be fixed in the contacting device. A detection surface of the spectrometer or spectroradiometer can extend in or parallel to a surface of the photovoltaic unit to be held in the contacting device or at an angle of less than 30°, less than 20°, less than 10°, less than 5° or even less than 2° to this. Alternatively, the spectrometer orThe spectroradiometer can be arranged remotely from the photovoltaic unit to be fixed in the contacting device, but can be optically coupled into a beam path reaching the photovoltaic unit at a position close to the thus fixed photovoltaic unit. For example, a beam splitter, such as a glass plate, can be arranged in the beam path, which couples out a portion of the light directed onto the photovoltaic element and directs it onto the spectrometer or spectroradiometer.According to one embodiment, the illumination source has a plurality M of light sources, which can be controlled independently of one another by the controller to emit different emission spectra and / or different emission intensities for different light sources. In other words, the illumination source can be provided not as a single light source but as a composite of multiple light sources, wherein each of the light sources or groups of multiple light sources can be controlled independently of one another by the controller and, due to such control, then emit light with different emission spectra and / or different emission intensities.The total light emitted by the illumination source thus results, in terms of its intensity and spectral distribution, from a superposition of the light components emitted by the individual light sources. In particular, the spectral distribution of this total emitted light can therefore be easily varied by varying the individual light sources in terms of their emitted light intensity, i.e., in particular, their power. In other words, the spectral distribution of the light emitted by the illumination source can be varied in a technically simple manner by supplying the individual light sources with different power levels, controlled by the controller.In particular, according to one embodiment, the illumination source can comprise a plurality M of light sources in the form of LEDs with different emission spectra, which can be controlled independently of one another by the controller. LEDs typically have low energy requirements compared to thermal light sources. Accordingly, significant energy savings can be achieved through the use of LEDs, particularly during long irradiation times, as are regularly required to operate a solar simulator. Furthermore, LEDs with widely differing emission spectra are now available. For example, there are LEDs that emit in the near UV spectral range, in various sub-ranges of the visible spectral range, and / or in the infrared spectral range.By using different LEDs in the illumination source, a spectral range can be covered that is sufficient for illumination within the scope of an irradiance sensitivity measurement. Such a spectral range extends, for example, from at least 400 nm, from at least 350 nm, preferably from at least 280 nm, from at least 250 nm, or from at least 200 nm and / or to at least 1100 nm, to at least 1150 nm, to at least 1180 nm, to at least 1300 nm, to at least 1500 nm, or even preferably to 2500 nm. It is considered advantageous to select the light sources and their emission spectra such that, within the entire spectral range, at least one of the light sources emits light with significant intensity.The LEDs can differ with regard to the semiconductor materials used in them, since in particular, for example, a band gap of such semiconductor materials significantly influences the emission spectrum emitted by an LED. The LEDs can also be equipped with optical filters to influence the emission spectrum. The emission spectra of different LEDs can differ from one another with regard to a central emission wavelength and / or with regard to a wavelength-dependent distribution of the emission intensity. According to one embodiment, the plurality N of different spectral distributions is greater than the plurality M of light sources. In other words, it is considered advantageous to select the plurality of illuminations of the photovoltaic unit with different spectral distributions to be greater than the plurality of individually controllable light sources provided in the illumination source.In other words, it is generally considered advantageous to determine spectral irradiance sensitivity by measuring PV currents under illumination with the largest possible number of N different spectral distributions, for example, N > 10, N > 20, N > 50, or N > 100. However, this does not necessarily require a very high number of light sources to be provided in the illumination source. Theoretically, any number of different spectral distributions can be generated with just two light sources with different emission spectra by varying their respective emission intensities.For use in the presented measuring device, an illumination source with a plurality M of light sources with different emission spectra with M > 2, M > 5, M > 10 or M > 20 and / or M < 100, M < 50, M < 20 or M < 10 appears advantageous. In this case, there can be groups of several light sources with the same emission spectrum. According to one embodiment, the controller and the illumination source are configured to control the illumination source in the second operating mode to successively emit light such that each of the plurality of different spectral distributions has an emission power other than zero at each wavelength within an emission band of at least 400 nm to 1100 nm. In other words, the light emitted by the illumination source should have a significant emission power for each of the successively emitted spectral distributions at each wavelength within an entire emission band.The phrase "a non-zero emission power" can be understood to mean that at each wavelength, a minimum emission power is emitted that allows a sufficiently precise measurement of the PV current generated. In other words, the spectral distribution within the emission band should not contain any gaps where no light intensity is emitted within wavelength sub-intervals, or where the light intensity is insufficient to evaluate the resulting PV current. It is assumed that the presence of such gaps in one or more spectral distributions is detrimental to the determination of the spectral irradiance sensitivity, i.e.This can lead, for example, to an inaccurate determination of the irradiance sensitivity in the wavelength range corresponding to the gap. Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 24 of 46 Accordingly, it can be advantageous to select the light sources used in the illumination source such that their emission spectra extend over the entire spectral range to be detected and / or at least partially overlap, thus preventing any gaps. According to one embodiment, the controller and the evaluation device are configured, in the second operating mode, to control the illumination source to emit light successively with at least ten different spectral distributions and, for each of the spectral distributions, to determine the current I flowing through the photovoltaic unit. meas,nto measure. By selecting a sufficiently large number N of different spectral distributions with which the photovoltaic unit is successively irradiated by the illumination source, i.e., with N ≥ 10, N ≥ 50, or even N ≥ 100, the accuracy with which the spectral irradiance sensitivity can ultimately be determined can be increased. It is assumed to be advantageous if at least some of the different spectral distributions differ from one another in such a way that, ultimately, for each wavelength range within the entire spectral range to be detected, two or more spectral distributions exist that differ from one another within this wavelength range.According to one embodiment, the evaluation device is configured to create and solve a system of equations for determining the spectral irradiance sensitivity SR(λ) of the photovoltaic unit, wherein - an entire wavelength range, within which the spectral irradiance sensitivity SR(λ) is to be determined, is divided into a plurality of K wavelength intervals [λ. k , λ k+1 ] with (k = 1…K), - for each of the N different spectral distributions and each of the K Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 25 of 46 wavelength intervals an irradiance P related to the respective wavelength interval k,n by integrating the spectral irradiance E λ,n (λ) over the respective wavelength interval [λ k , λ k+1], and - a proportional spectral irradiance sensitivity SR k with respect to each of the K wavelength intervals is approximately determined by solving the following system of equations: Details of a possible embodiment of such an approximate and analytical calculation are set out below with reference to an exemplary embodiment. According to a further specific embodiment, the proportional spectral irradiance sensitivity SR kfor each of the K wavelength intervals is assumed to be constant within the wavelength interval. As explained in more detail below with reference to the exemplary embodiment, the assumption that the fractional spectral irradiance sensitivity can be regarded as approximately constant within each of a plurality of wavelength intervals can enable or simplify an analytical and / or numerical calculation of the overall spectral irradiance sensitivity. According to one embodiment, the spectral irradiance sensitivity SR(λ) is determined taking into account a relative spectral irradiance E known in advance for each of the N spectral distributions. λ,n(λ) or taking into account an absolute spectral irradiance E known for each of the N spectral distributions. Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 26 of 46 λ,n(λ). For some applications, it may be sufficient to determine only a relative spectral irradiance sensitivity. For example, such a relative spectral irradiance sensitivity may be sufficient for determining a spectral mismatch factor, but not for determining a short-circuit current. To determine the relative spectral irradiance sensitivity, it may generally be sufficient to know the relative spectral irradiance for each of the N spectral distributions. Measuring such relative spectral irradiances can be simpler than measuring corresponding absolute values. However, if absolute spectral irradiances can be measured for all spectral distributions, for example using a sufficiently precise spectroradiometer, these values ​​can be used to determine the absolute spectral irradiance sensitivity.According to one embodiment, the controller is configured to control the illumination source in the second operating mode to emit light with an intensity of between 60% and 140%, preferably between 80% and 120% of the standardized solar spectrum, or with an intensity of less than 30% of the standardized solar spectrum. In other words, the controller of the measuring device in a first variant of the second operating mode can be designed to let the illumination source emit light, as is essentially also used during operation within the first operating mode to implement the sun simulator functionality. The light can here have an intensity equal to that of the standardized solar spectrum, e.g., with a standardized AM1.5G spectrum with 1000 W / m², directed onto the photovoltaic unit, whereby a tolerance range with regard to the emitted light intensity should be a maximum of 20%, a maximum of 10%, or a maximum of 5%. Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 27 of 46 The various spectral distributions successively controlled in the second operating mode can resemble the spectral distribution of the standardized solar spectrum, at least within a tolerance range. For example, the spectral distributions of the light successively varying from the lighting device during the second operating mode can differ from the standardized solar spectrum by less than 30%, preferably less than 20% or less than 10%, for each wavelength or each wavelength sub-range within the entire spectral range.Accordingly, the spectral irradiance sensitivity can then be determined under lighting conditions that correspond to those prevailing in real-world applications. In an alternative second variant of the second operating mode, the illumination source can be controlled by the controller to emit light at an intensity significantly reduced compared to the standardized solar spectrum. For example, the emitted light intensity can be less than 30%, less than 20%, less than 10%, or even less than 5% of that of the standardized solar spectrum. Due to the reduced incident light intensity, excessive heating of the photovoltaic unit during the irradiance sensitivity measurement can be avoided, for example.This can prevent or at least reduce effects such as measurement distortion due to changing temperatures of the photovoltaic unit. A relative spectral distribution of the light emitted during the second operating mode can be the same as the standardized solar spectrum within a tolerance range of, for example, less than 20%, less than 10%, or less than 5%. This means that the spectral distributions emitted during the second operating mode can be considered a downscaled solar spectrum within the tolerance range.Institute for Solar Energy Research (ISFH) Multispectral Method for Determining Spectral Irradiance Sensitivity and Current Behavior of Solar Cells / Modules Page 28 of 46 Embodiments of the method according to the second aspect of the invention can be configured such that a measuring device in the form of a solar simulator, in addition to its original function in a first operating mode for determining a voltage-dependent current behavior of a photovoltaic unit when illuminated with a standardized solar spectrum, can also be operated in a second operating mode. In the second operating mode, the spectral irradiance sensitivity of the photovoltaic unit can be determined using the components of the solar simulator with the aid of a cleverly adapted process control and suitably designed signal and data processing specifications, without the need for a separate measuring station.The method is designed to implement the properties of embodiments of the measuring device described above and below for implementing the second operating mode. Accordingly, the features of embodiments of the measuring device described above and below can be transferred analogously to the method. The method can be implemented, in particular, using a programmable measuring device. In particular, the controller and / or the evaluation unit of the measuring device can be programmable. The measuring device can have one or more processors and one or more data memories. A computer program product according to the third aspect of the invention can be stored in the data memory.The machine-readable instructions of the computer program product can be formulated in any computer language and instruct the processor to execute or control embodiments of the method described herein. The computer program product, as software, can enable the hardware of a measuring device, such as a solar simulator, to implement a functionality for determining the spectral irradiance sensitivity in addition to its original functionality of measuring IV curves under standard solar illumination. The computer program product can be stored in any computer-readable medium according to the fourth aspect of the invention.The computer-readable medium can store data reproducing the computer program product in any physical manner, for example magnetically, optically, electrostatically, etc. For example, the computer-readable medium can be a portable data storage device such as a CD, DVD, flash memory, RAM memory, ROM memory, PROM memory, EPROM memory, etc. Alternatively, the computer-readable medium can be a data storage device stored in a separate computer, server, or a data cloud, from which the computer program product can be downloaded, for example, via a network, in particular the Internet. It is pointed out that possible advantages and configurations of embodiments of the invention are described herein partly with reference to a measuring device according to the invention and partly with reference to a method according to the invention.A person skilled in the art will recognize that the described features can be suitably transferred, adapted, exchanged, or modified to obtain further embodiments of the invention. BRIEF DESCRIPTION OF THE DRAWINGS Embodiments of the invention are described below with reference to the accompanying drawings, wherein neither the drawings nor the description are to be interpreted as limiting the invention. Fig. 1 shows a measuring device according to an embodiment of the present invention. Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 30 of 46 Fig. 2 shows an exemplary spectral distribution. Fig. 3 shows an exemplary irradiance sensitivity. The figures are merely schematic and not to scale. The same reference numerals denote the same or equivalent features.DESCRIPTION OF PREFERRED EMBODIMENTS Fig. 1 shows an embodiment of a measuring device 1 according to the invention. The measuring device 1 is configured to be able to determine both a spectral irradiance sensitivity SR(λ) and a voltage-dependent current behavior I(U) for a photovoltaic unit 3 upon irradiation with a standardized solar spectrum. For this purpose, the measuring device 1 comprises an illumination source 5. In the example shown, the illumination source 5 has a plurality of light sources 21 in the form of LEDs 23. At least some of the LEDs 23 differ in terms of their emission spectrum 25 (see Fig. 2). The illumination source 5 is therefore capable of selectively emitting light 13 with variable spectral distributions 15.Furthermore, the illumination source 5 is designed to emit light 13, as in a conventional solar simulator, with an intensity and spectral distribution corresponding to a standardized solar spectrum. The measuring device 1 further comprises a contacting device 7, by means of which the photovoltaic unit 3 can be electrically contacted. For this purpose, the contacting device 7 has, for example, two or more reversibly attachable contacts 31, with which contact structures such as busbars or fingers on the photovoltaic unit 3 can be contacted. Institute for Solar Energy Research (ISFH) Multispectral Method for Determining Spectral Irradiance Sensitivity and Current Behavior of Solar Cells / Modules Page 31 of 46 Furthermore, the measuring device 1 comprises a controller 9 and an evaluation device 11.The controller 9 is connected to the illumination source 5 and is designed to individually control the illumination source 5 and in particular its various light sources 21 such that light 13 is either emitted approximately with the standardized solar spectrum in a first operating mode or, in a second operating mode, light 13 is successively emitted with a plurality of different spectral distributions 15. Fig. 2 shows an exemplary spectral distribution 15, which results from a superposition of several different emission spectra 25 of the different light sources 21 of the illumination source 5. A spectral range of the entire spectral distribution 15 extends, for example, from approximately 340 nm to approximately 1180 nm, wherein the achievable spectral range depends in particular on the available light sources 21 and their emission spectra.The evaluation device 11 is connected to the contacting device 7 and can use it to tap and measure the PV current Imeas flowing or generated in the photovoltaic unit 3. For this purpose, the evaluation device 11 has, among other things, a processor 29. In the first operating mode, the evaluation device 11 can use the measured current to determine the voltage-dependent current behavior I(U) of the photovoltaic unit 3 under the influence of the standardized solar spectrum. In the second operating mode, the evaluation device 11 can alternatively measure the various PV currents I. meas,n , as they are generated when the photovoltaic unit 3 is illuminated with each of the N different spectral distributions 15 in the photovoltaic unit 3. A spectral irradiance E λ,n(λ) can be known in advance for each of the spectral distributions 15, for example by being measured using a spectrometer, preferably in the form of a calibrated spectroradiometer 19. For this purpose, the spectroradiometer 19 can preferably be arranged adjacent to the position at which the photovoltaic unit 3 is held on the contacting device 7 and / or in the same plane therewith, so that the spectroradiometer 19 receives essentially the same light 13 as the photovoltaic unit 3. Alternatively, the spectroradiometer 19 can be arranged at a remote position and light directed onto the photovoltaic unit 3 can be proportionally coupled out, for example by means of a beam splitter (not shown), and directed onto the spectroradiometer 19.Details of a possible embodiment of the measuring device described herein and of the method for multi-spectral determination of the spectral irradiance sensitivity that can be carried out therewith are described with reference to an exemplary embodiment. To apply the multi-spectral determination method of the spectral irradiance sensitivity ^^(^) of a test object (solar cell or solar module), at least three components are required: - A spectrally adjustable light source. This can be realized, for example, using LEDs with different colors. Lasers, lamps with a broadband spectrum in combination with filters, mirrors and similar optical components or light sources whose spectral distribution can be changed are also conceivable. - A spectrometer or spectroradiometer for measuring the spectral irradiance in the plane of the test object. As an alternative to the spectrometer orSpectroradiometers can also be used to calculate / determine the spectral irradiance in the plane of the test object from other measurement data or simulations. - A current measurement option for the test object under investigation. This usually consists of a contact unit and a current sink. Institute for Solar Energy Research (ISFH) Multispectral Method for Determining Spectral Irradiance Response and Current Behavior of Solar Cells / Modules Page 33 of 46 To implement the method, the spectrally adjustable light source is configured to generate ^ different spectra. For each light source configuration, (a) the spectrum. and (b) the current response ^ meas,n of the test object (for example, the short-circuit current I SC or the current I MPP at the point of maximum power). Here n = 1… ^. The spectrum and the current response ^ meas,ncan be recorded both simultaneously and sequentially. It is also possible to first measure (or determine / calculate) all spectra and then determine the current contribution of the test object at a different time, provided the spectral irradiance in the plane of the test object is sufficiently stable over time. The relationship between the current response of the test object, spectral irradiance, and SR of the test object is expressed by the formula which is valid for each of the ^ spectra and current responses (n = 1… ^). To solve this equation, the integral is represented by a sum, This equation can be written as follows, assuming a value in the respective interval [^ k , ^ k+1 ] constant spectral irradiance sensitivity to ^ ^^^^,^ ≈ ^^ ^ ^ ^,^ + ^^ ^ ^ ^,^ + ⋯ + ^^ ^ ^ ^,^Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 34 of 46 simplify. P 1,n , P 2,n , … , P^, n correspond to the irradiances in each wavelength interval k, with ^ = 1 … ^. This results in a system of equations with ^ equations and ^ unknowns ^^1 to ^^ K In matrix notation, this system of equations can be written This system of equations can be solved using known numerical and / or analytical methods (regression methods, inverse matrices, etc.). The resulting values ​​are ^^1 to ^^ K. Each of these values ​​is assigned a wavelength value within the interval [^k, ^k+1], typically the mean value. For example, the value SR1 is assigned the wavelength (^1+^2) / 2. The result is the SR curve of the test object. As described above, there are various applications for the evaluation and use of SR curves, including the determination of the spectral mismatch factor ^^. As an alternative to applying the MM factor (and thus using a reference solar cell), the short-circuit current ^ sc directly from the SR curve using Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 35 of 46. Here, ^^ ,ref(^) the tabulated target reference solar spectrum, for example, the AM1.5G spectrum. It should be noted that for the application of the multi-spectral determination method for determining a relative spectral irradiance sensitivity (sufficient for determining the spectral mismatch factor, but not for determining a short-circuit current), knowledge of the relative spectral irradiance E is in principle required. λ,rel (^) is sufficient. This relative spectral irradiance is related to the absolute spectral irradiance ^^(^) via a constant scaling factor ^, ^ ^ (^) = ^ ⋅ ^ ^,^^^ (^). If only the relative spectral irradiance is known, the application of the method yields the relative spectral irradiance sensitivity ^^ rel(^) of the test object. This in turn depends on the absolute spectral irradiance sensitivity ^^(^) and the constant scaling factor ^ via ^^(^) = ^ ⋅ ^^ ^^^ (^). With knowledge of the absolute spectral irradiance ^^(^) one can directly calculate the absolute spectral irradiance sensitivity ^^(^) and, using the above equation, the short-circuit current ^ scUnder standard test conditions. Exemplary experiments for implementing the approach described herein are explained below. A first test of the methodology was carried out on an IVmj measurement system with an LED lamp system (Wavelabs SinusGUI 360 Plus). This lamp system consists of 27 LEDs of different wavelengths, each of which can be controlled separately. A WPVS reference solar cell (active cell area 4 cm²) served as the test object. The spectrometer and the solar cell were irradiated by the lamp system with ^ = 108 different spectra. These 108 spectra are based on channel settings for an approximation of an AM1.5G spectrum with classification A++.For each spectrum, the channel control of two channels (channel spacing 10, for example, channels 1 and 11) is multiplied by a factor of 0, 1, or 2 (combinations chosen here: 0-0, 0-1, 2-0, and 2-1). First, the 108 spectra were measured with a spectroradiometer, and in a second step, 108 short-circuit currents were measured. To analyze the measured data, the wavelength range from 340 nm to 1180 nm is divided into ^ = 27 equidistant subintervals (interval width of 31.1 nm), and the irradiances P are calculated. k,n for each wavelength interval k. This results in a matrix ^ of dimension 108x27 (108 rows, 27 columns). Together with the measured currents ^ meas,n(vector with 108 entries) results in a system of 108 equations and 27 unknowns. This system of equations is solved using a least-square regression method. The result of the regression method produces an SR curve similar to that shown in Fig. 3. It should be emphasized that the resulting SR curve of the multi-spectral determination method does not necessarily require scaling, since the spectroradiometer can be used directly to determine the absolute spectral irradiance ^ ^(^) was determined. The required calibration procedures for spectroradiometers are available at various calibration institutions, including the ISFH CalTeC operated by the applicant of the present patent application. In contrast to the method described here, the SR standard procedure for classic DSR / SR measuring stations generally requires additional subsequent scaling of the data, especially for large solar cells. It should be noted that terms such as "comprising", "comprising", etc. do not exclude other elements or steps, and terms such as "a" or "an" do not exclude a plurality.Furthermore, it should be noted that features or steps described with reference to one of the above embodiments may also be used in combination with other features or steps of other embodiments described above. Reference symbols in the claims are not to be considered as limiting.

[0002] Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 38 of 46 LIST OF REFERENCE SYMBOLS 1 Measuring device 3 Photovoltaic unit 5 Illumination source 7 Contacting device 9 Control system 11 Evaluation device 13 Light 15 Spectral distribution 17 Spectral irradiance sensitivity 19 Spectroradiometer 21 Light sources 23 LED 25 Emission spectrum 27 Wavelength interval 29 Processor 31 Reversibly attachable contact

Claims

Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 39 of 46 Claims:

1. Measuring device (1) for determining a spectral irradiance sensitivity SR(λ) (17) and a voltage-dependent current behavior I(U) under standard solar conditions of a photovoltaic unit (3), wherein the measuring device (1) comprises: - an illumination source (5), - a contacting device (7), - a controller (9), and - an evaluation device (11), wherein the illumination source (5) is configured to selectively emit light (13) with variable spectral distributions (15) or with an intensity and spectral distribution that correspond to a standardized solar spectrum, wherein the contacting device (7) is configured to electrically contact the photovoltaic unit (3),wherein the controller (9) and the evaluation device (11) are selectively operable in a first or a second operating mode, wherein the controller (9) is configured to - in the first operating mode, control the illumination source (5) to emit light (13) with an intensity and spectral distribution corresponding to the standardized solar spectrum, and - in the second operating mode, control the illumination source (5) to successively emit light (13) with a plurality N of different spectral distributions (15), and wherein the evaluation device (11) is configured to, Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 40 of 46 Contacting device (7) to apply an electrical voltage U to the photovoltaic unit (3) and, depending on the applied voltage, to measure a current I flowing through the photovoltaic unit meas to measure and thereby - in the first operating mode from the measured current I meas to determine the voltage-dependent current behavior I(U) of the photovoltaic unit (3) under the influence of the standardized solar spectrum, and - in the second operating mode from several currents I measured during illumination with the N different spectral distributions (15) meas,n (n = 1…N) and taking into account a spectral irradiance E known for each of the N spectral distributions (15) λ,n(λ) (n = 1…N) to determine the spectral irradiance sensitivity SR(λ) (17) of the photovoltaic unit (3).

2. Measuring device according to claim 1, wherein the spectral irradiance E λ,n (λ) for each of the spectral distributions (15) by measurement using a spectrometer, preferably a spectroradiometer (19).

3. Measuring device according to claim 2, wherein the measuring device further comprises an integrated spectrometer or spectroradiometer (19) for determining the spectral irradiance E λ,n (λ) for each of the spectral distributions emitted within the measuring device.

4. Measuring device according to claim 3, wherein the spectrometer or spectroradiometer (19) is configured and arranged such that it detects light from a position adjacent to and / or in a plane with the light source to be contacted in the contacting device (7). Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 41 of 46 Photovoltaic unit (3).

5. Measuring device according to one of the preceding claims, wherein the illumination source (5) has a plurality M of light sources (21) which can be controlled independently of one another by the controller (9) to emit different emission spectra (25) and / or different emission intensities for different light sources (21).

6. Measuring device according to one of the preceding claims, wherein the illumination source (5) has a plurality M of light sources (21) in the form of LEDs (23) with different emission spectra (25), which can be controlled independently of one another by the controller (9). 7.Measuring device according to one of claims 5 and 6, wherein the plurality N of different spectral distributions (15) is greater than the plurality M of light sources (21).

8. Measuring device according to one of the preceding claims, wherein the controller (9) and the illumination source (5) are configured to control the illumination source (5) in the second operating mode to successively emit light (13) such that each of the plurality of different spectral distributions (15) has an emission power different from zero at each wavelength within an emission band of at least 400 nm to 1100 nm.

9. Measuring device according to one of the preceding claims, wherein the controller (9) and the evaluation device (11) are configured to control the illumination source (5) in the second operating mode to emit light. Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 42 of 46 (13) successively with at least ten different spectral distributions (15) and for each of the spectral distributions (15) the current I flowing through the photovoltaic unit (3) meas,n to measure.

10. Measuring device according to one of the preceding claims, wherein the evaluation device (11) is configured to create and solve a system of equations for determining the spectral irradiance sensitivity SR(λ) (17) of the photovoltaic unit (3), wherein - an entire wavelength range, within which the spectral irradiance sensitivity SR(λ) (17) is to be determined, is divided into a plurality of K wavelength intervals (27) [λ k , λ k+1] with (k = 1…K), - for each of the N different spectral distributions (15) and each of the K wavelength intervals (27) an irradiance P related to the respective wavelength interval (27) k,n by integrating the spectral irradiance E λ,n (λ) is determined over the respective wavelength interval (27), and - a proportional spectral irradiance sensitivity SR k with respect to each of the K wavelength intervals (27) is approximately determined by solving the following system of equations:

11. Measuring device according to claim 10, wherein the proportional spectral irradiance sensitivity SR k for each of the K wavelength intervals (27) is assumed to be constant within the wavelength interval (27). Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 43 of 46 12. Measuring device according to one of the preceding claims, wherein the spectral irradiance sensitivity SR(λ) (17) is determined taking into account a relative spectral irradiance E known in advance for each of the N spectral distributions (15). λ,n (λ) or taking into account a known absolute spectral irradiance E for each of the N spectral distributions λ,n(λ) is determined.

13. Measuring device according to one of the preceding claims, wherein the controller (9) is configured to control the illumination source (5) in the second operating mode to emit light (13) with an intensity of between 60% and 140%, preferably between 80% and 120%, of the standardized solar spectrum, or with an intensity of less than 30% of the standardized solar spectrum. 14.Method for determining a spectral irradiance sensitivity SR(λ) (17) of a photovoltaic unit (3) by means of a measuring device (1) for determining a voltage-dependent current behavior I(U) of the photovoltaic unit (3) under standard solar conditions, wherein the measuring device has: - an illumination source (5), - a contacting device (7), - a controller (9), and - an evaluation device (11), wherein the illumination source (5) is configured to selectively emit light (13) with variable spectral distributions (15) or with an intensity and spectral distribution which correspond to a standardized solar spectrum, wherein the contacting device (7) is configured to. Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 44 of 46 Photovoltaic unit (3) to be electrically contacted, wherein the controller (9) is configured to control the illumination source (5) in the first operating mode to emit light (13) with an intensity and spectral distribution corresponding to the standardized solar spectrum, wherein the evaluation device (11) is configured to apply an electrical voltage U to the photovoltaic unit (3) via the contacting device (7) and to determine a current I flowing through the photovoltaic unit (3) depending on the applied voltage U meas to measure and thereby in the first operating mode from the measured current I measto determine the voltage-dependent current behavior I(U) of the photovoltaic unit (3) under the influence of the standardized solar spectrum, the method comprising: - controlling the illumination source (5) by means of the controller (9) in a second operating mode for successive emission of light (13) with a plurality N of different spectral distributions (15), - measuring, by means of the evaluation device (11), in the second operating mode, currents I flowing through the photovoltaic unit (3) when illuminated with the N different spectral distributions (15) meas,n (n = 1…N), and - determining the spectral irradiance sensitivity SR(λ) (17) of the photovoltaic unit (3) taking into account a spectral irradiance E known in advance for each of the spectral distributions (15) λ,n (λ) (n = 1…N).

15. The method according to claim 14, wherein the spectral irradiance E λ,n(λ) is previously known for each of the spectral distributions (15) by measurement using a spectrometer or a spectroradiometer (19). Institute for Solar Energy Research (ISFH) Multispectral method for determining spectral irradiance sensitivity and current behavior of solar cells / modules Page 45 of 46 16. A computer program product comprising machine-readable instructions which, when executed by a processor (29) of a measuring device (1) for determining a current behavior under standard solar conditions of a photovoltaic unit (3), instruct the measuring device (1) to execute or control the method according to claim 14.

17. A computer-readable medium comprising machine-readable instructions of a computer program product according to claim 16 stored thereon.

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