Solar cell test method, solar cell test device, and computer-readable medium

Abstract

The invention relates to a solar cell test method, to a solar cell test device, and to a computer-readable medium for implementing the solar cell test method. The solar cell test method is used to characterize a contacted solar cell or a contacted solar module and has the following steps: carrying out a first measuring process in which the solar cell is irradiated with light with an intensity which changes over time, and during the irradiation, the time curve of the cell voltage in the solar cell is measured in order to obtain a first sequence of time-resolved first light intensity values and corresponding time-resolved first voltage values; carrying out a second measuring process in a corresponding manner in order to obtain a second sequence of time-resolved second light intensity values and corresponding time-resolved second voltage values; and carrying out a calculation for each first voltage value in order to ascertain a respective stationary light intensity value (Ls), wherein the total quantity of stationary light intensity values (Ls) together with respective first voltage values (V) forms a stationary curve.

Description

[0001] Solar cell test method, solar cell test device and computer-readable medium

[0002] Description:

[0003] The invention relates to a solar cell testing method, a solar cell testing device and a computer-readable medium for implementing the solar cell testing method.

[0004] The doping concentration of the solar cell substrate of silicon-based solar cells is a component of the solar cell series resistance and thus an important parameter in describing the solar cell properties, for example, using the two-diode equivalent circuit. The series resistance is therefore very important for the theoretical analysis of a solar cell's performance and for identifying possible causes of process problems. In a solar cell, it basically has three causes: first, the current flow through the emitter and the base of the solar cell, which can be referred to as the base resistance; second, the contact resistance between the metal contact and the solar cell material, in particular the silicon; and finally, the conduction resistance of the front and back metal contacts themselves.The main effect of series resistance is to reduce the fill factor and thus the solar cell efficiency, although very high resistance values ​​can also reduce the short-circuit current.

[0005] The doping concentration or conductivity of the base material of the solar cell is usually measured in solar cell production at the beginning of the production chain during incoming goods inspection. However, the conductivity changes primarily due to thermal processes during production, so it would be advantageous to measure it only or even at the end of production. However, due to the emitter diffusion and contacting using printed metal that then occurs, the measurement method used during incoming goods is no longer usable. In alternative processes, a current-voltage curve (a so-called IV curve) is determined for the electrical characterization of the solar cell or solar module, from which the doping concentration or conductivity can then be derived. In this process, the solar cell is exposed to a light pulse, and the current flow is measured at varying voltage.The measuring devices used for this purpose are therefore also called flashers.

[0006] US 10,027,278 B2 (Sinton) describes a method for determining substrate doping concentration using a light pulse. A constant voltage is applied to the solar cell contacts while the cell is exposed to the light pulse. The applied contact voltage is then increased, and the change in charge density resulting from the increase in voltage is determined. The substrate doping concentration is then calculated from the change in charge density. This method uses two voltage points on the IV characteristic curve, which are influenced by noise and measurement inaccuracies. The doping determined in this way is accordingly inaccurate.

[0007] In “Determination Of Base Doping Concentration Of Silicon Solar Cells From Light IV-curves,” AIP Conference Proceedings 2487, 030008 (2022),

[0008] K. Ramspeck et al. describe an alternative method in which the charge stored in the cell is calculated from a hysteresis-affected IV curve and an IV curve without hysteresis by first subtracting the two curves and then integrating the charge curve piecewise. From the stored charge of the solar cell calculated in this way, the minority charge carrier density An can be calculated. The thickness of the solar cell is required for this calculation. Due to

[0009] Ndop = ni AThe doping concentration can then be plotted against the voltage Vj using the formula 2 / An * exp (J / kT) - An. However, the resulting curve does not show a constant doping concentration, but rather a voltage-dependent curve. The authors therefore propose calculating the doping concentration by averaging over a so-called evaluation range. It is unclear how this evaluation range is defined, and therefore the doping concentration depends on the choice of evaluation range. Furthermore, measurement errors, e.g., due to noise, affect all subsequently calculated charges due to the integration of the difference curve for the stored charge.

[0010] Instead of scanning an IV characteristic curve with current and voltage, the illumination intensity can be varied and the corresponding open-circuit voltages (and, with some methods, the short-circuit currents) measured. In addition, corresponding currents can be measured or determined in other ways, for example via the illumination intensity and a measurement of the IV characteristic curve under standard test conditions (STC conditions). The solar cell should be in a stable state (steady state) to deliver correct measurement results. Since the measurement method should be fast, it is also referred to as quasi-steady state (Qss), and the rate of change of the illumination is changed sufficiently slowly. Since no current flows at open circuit (Voc), the values ​​obtained can now be used with various methods to generate an IV characteristic curve at different illumination intensities that is free from the influence of the solar cell's series resistance.

[0011] Since the illumination intensity is usually expressed in suns, measurements in which the illumination intensities are varied and the open circuit voltage (Voc) is measured are also referred to as SunsVoc measurement methods for short. What such a SunsVoc measurement method, carried out using a flash lamp, might look like can be found in the publication Kerr et. Al, “Generalized analysis of quasi-steady-state and transient decay open circuit voltage measurements”, Journal of Applied Physics, Vol. 91, No. 1, pages 399-404 (January 1, 2002). However, the aim of this publication is not to generate (SunsVoc) characteristics, but to determine injection-dependent lifetimes of the minority charge carriers using a long-established QssPC method.The method is then further developed by measuring the Voc (QssVoc) of the solar cell at different illumination intensities instead of measuring the photoconductivity (QssPC) of the sample. The light flash is started and then continuously decreases in intensity. The measured characteristic curve (fallingSunsVoc) is then still subject to error because the generated charge carriers have a specific charge carrier lifetime (Kerr et al. mentions 20 ps). In order to determine a SunsVoc characteristic curve from this, the method according to Kerr et al. also requires the doping concentration (base doping) of the solar cell as an input variable. This doping concentration can, for example, come from another measurement. Using the doping concentration, a correction function must be determined in order to then calculate the SunsVoc curve under steady-state conditions using both curves.The latter is referred to as the stationary curve, which shows an open circuit voltage as a function of an illumination intensity.

[0012] The method known from Kerr et al. has the disadvantage that the base doping of the solar cell is required to calculate the stationary curve (i.e., the SunsVoc curve under steady-state conditions) from the measured characteristic curve.

[0013] It is an object of the invention to provide a method and a device with which a solar cell or a solar module can be characterized quickly and reliably.

[0014] The object is achieved according to the invention by a solar cell testing method having the features of claim 1, by a solar cell testing device having the features of claim 12, and by a computer-readable medium having the features of claim 13. Advantageous developments of the invention are listed in the subclaims.

[0015] According to one aspect of the invention, a solar cell test method for characterizing a contacted solar cell or a contacted solar module is proposed. Similar to the previously described prior art, the solar cell is exposed to varying light irradiation, and the open-circuit voltage is measured during this time. However, unlike what is known in the prior art, two different measurement processes are carried out, which differ in that the temporal change in light intensity differs in the first measurement process from that in the second measurement process. In particular, the rate of change in light intensity can be different in the two measurement processes, or the light intensity can increase in one measurement process while decreasing in the other.

[0016] During a first measurement process, the solar cell is irradiated with light of a temporally varying intensity. During the irradiation, a temporal progression of the cell voltage across the solar cell, in particular the open-circuit voltage, is measured to obtain a first sequence of time-resolved initial light intensity values ​​and corresponding time-resolved initial voltage values. The first sequence of initial light intensity values ​​and corresponding initial voltage values ​​can also be referred to as the first measurement curve. "Time-resolved" or time-dependent values ​​means that the time of measurement of the respective value is also stored, which can be an absolute or relative time.

[0017] Afterwards or beforehand, during a second measuring process, the solar cell is irradiated with light of a temporally changing intensity, whereby the temporal change in intensity during the second measuring process differs from the temporal change in intensity during the first measuring process. Here, too, the temporal course of the cell voltage at the solar cell, in particular the open circuit voltage, is measured during the irradiation in order to obtain a second sequence of time-resolved second light intensity values ​​and corresponding time-resolved second voltage values. The second sequence of second light intensity values ​​and corresponding second voltage values ​​can also be referred to as a second measurement curve. While the first measuring process can be carried out before the second measuring process, this is not mandatory. Instead, the second measuring process can be carried out before the first measuring process.

[0018] With the silicon-based solar cells currently produced, it is not possible to carry out measurements slowly enough during ongoing solar cell production to ensure that each measurement point reaches a steady state. Such a measurement would sometimes require several seconds, but only a fraction of a second is available. As a result, the measurement is carried out in a shorter period of time than required for a steady-state measurement. Due to the rapid change in light intensity, the electrical capacitance of the solar cell distorts the open-circuit voltages measured during the two measurement processes. In order to calculate out the error and obtain a steady-state curve, for each measurement point on the first measurement curve, i.e. for each first voltage value, the point on the second measurement curve at which the second measurement curve has an equivalent second voltage value is determined.The equivalent secondary voltage values ​​determined in this way are not necessarily available as direct measured values, but may be in close proximity or, for example, obtained by interpolation. Using a correction formula, a steady-state light intensity value is then determined for each of the primary voltage values.

[0019] Accordingly, the following steps are carried out for each initial voltage value: a) Determine the initial light intensity value (L u ) from the first sequence; b) Determine a time derivative (dV / dt up) of the voltage in the first sequence at the first voltage value; c) determining a second voltage value equivalent to the first voltage value; d) determining the second light intensity value (Ld) associated with the second voltage value from the second sequence; e) determining a time derivative (dV / dtdown) of the voltage in the second sequence at the second voltage value; and f) determining a stationary light intensity value (Ls) using the following formula: L s = (L u * dV / dtdown + Ld * dV / dt up ) / (dV / dtdown + dV / dt up ).

[0020] The total set of stationary light intensity values ​​(Ls) with the respective corresponding initial voltage values ​​(V) are the value pairs of the desired stationary curve.

[0021] The fact that the first voltage value is equivalent to the second voltage value means, in particular, that the two voltage values ​​are equal or nearly equal. Since both voltage values ​​are measured values, they will not be identical in practice, so a closest value or an interpolated second voltage value can be used as the equivalent voltage value. When using an interpolated second voltage value, interpolated values ​​for the time and light intensity value must also be used accordingly.

[0022] The two sequences are each a function of the open circuit voltage as a function of the light intensity, V(L U ) and V(Ld), whose time derivative is determined numerically. The subscripts u and d indicate that the first sequence (u) or the second sequence (d) is used for the calculation. In particular, dV / dt uby the time derivative of the open circuit voltage along the first sequence and dV / dtd by the time derivative of the open circuit voltage along the second sequence.

[0023] According to a preferred embodiment, the solar cell is irradiated with light of temporally increasing intensity during the first measurement process and with light of temporally decreasing intensity during the second measurement process. Alternatively, the reverse sequence can be used, so that the solar cell is irradiated with light of temporally decreasing intensity during the first measurement process and with light of temporally increasing intensity during the second measurement process. Each increasing or decreasing intensity profile can preferably be configured as a monotonically increasing or monotonically decreasing profile.

[0024] According to an advantageous embodiment, the solar cell is irradiated with light of increasing intensity over time during both the first and second measurement processes, with the intensity profiles during irradiation differing from each other in the first and second measurement processes. In particular, the change in light intensity can occur more rapidly in one measurement process than in the other, for example, at least twice, three times, five times, or ten times as fast. This means that the intensity increases more rapidly over time.

[0025] Alternatively, the solar cell can be irradiated with light of decreasing intensity over time during both the first and second measurement processes, with the intensity profiles during irradiation differing in the first and second measurement processes. In particular, the change in light intensity can occur more rapidly in one measurement process than in the other, for example, at least twice, three times, five times, or ten times faster. This means that the intensity decreases more rapidly over time.

[0026] In other words, the temporal course of the intensity preferably has a gradient whose absolute value is on average significantly greater or significantly smaller during the first measuring process than during the second measuring process, in particular at least by a factor of 2, 5 or 10.

[0027] In particular, the measurement can be performed more quickly with increasing light intensity than with a preceding or subsequent measurement with decreasing intensity, since the charge carrier lifetime is limited in the latter case, but this is irrelevant with increasing light intensity. In this way, the entire process can be accelerated. Solar cell testing method according to one of the preceding claims, characterized in that before performing the first measurement process and / or before performing the second measurement process, the solar cell is short-circuited or subjected to a reverse voltage for a short-circuit period. Using such a reverse voltage or blocking voltage, the solar cell can be discharged before the respective measurement process.

[0028] Discharge of the solar cell is preferably used when the intensity initially increases in the first measurement process and then decreases in the second measurement process. This allows, for example, the transition from a conventional one-sun measurement to the SunsVoc measurement method described here without having to wait for the solar cell to discharge in the meantime. This can save about 10 ms per solar cell when using conventional flashers. Conversely, i.e. in the case of a light intensity that initially decreases and then increases, a prior discharge of the solar cell is not crucial because at the beginning of the first measurement process with decreasing intensity, the charge carriers are immediately generated at an initially high intensity the moment the light is switched on, and thus the measurement can begin immediately.

[0029] If an increasing light intensity is used during the first measurement process, then according to an advantageous embodiment, at the end of the first measurement process, the light intensity is maintained at a maximum intensity during a holding period. If, however, a decreasing light intensity is used during the first measurement process, then according to an advantageous embodiment, at the end of the first measurement process, the light intensity is maintained at a minimum intensity during a holding period. During the holding period, the solar cell has the opportunity to discharge. The holding period is preferably in the range of several milliseconds.Solar cell testing method according to one of the preceding claims, characterized in that during the first measurement process, the light intensity is adjusted to the respective first light intensity values ​​and / or that during the second measurement process, the light intensity is adjusted to the respective second light intensity values. A controlled adjustment of the light intensity can be achieved, for example, by means of a control loop. The control loop can comprise a light sensor that measures the actual value of the light intensity as a controlled variable. Alternatively, a calibrated light source can be used, thus eliminating the need for a light sensor.

[0030] Alternatively, it is also possible to control the light source in such a way that rising or falling light intensity values ​​are set, without the exact light intensity values ​​being specified by the controller. Instead, the exact light intensity values ​​can be recorded using a light sensor and used for the calculations. In this case, "set" means controlled setting, in which the time intervals between the light intensity values ​​and the rate of rise or fall of the light intensity can be controlled. The controlled setting of the light intensity differs particularly from the use of a light flash in a conventional flasher, in which the course of the light flash is recorded by a sensor. In the latter case, the rate of fall of the light flash can only be influenced to a limited extent.Furthermore, during a flash, the light intensity initially begins at a maximum level and then decreases to zero at a rate that depends on the light source. An increasing intensity gradient is not possible with such a flash source.

[0031] Preferably, an LED light source is used to generate the light with which the solar cell is irradiated during the first measurement process and / or during the second measurement process. As explained above, flash lamps have a high intensity immediately after ignition, which then decays, with the decay rate being controllable within certain limits. In contrast, an LED light source can be better controlled, in particular so that an increasing light intensity can be generated. In addition, flash lamps have the problem that the spectrum changes during the light flash, which, in conjunction with the spectral sensitivity of a solar cell, leads to measurement errors. LED light sources do not usually have such a spectral shift. Should a spectral shift be present, the LED spectrum can be adjusted accordingly during the illumination process.For this purpose, the LED light source preferably has LEDs for light of different wavelengths.

[0032] In recent years, solar cells have been improved to such an extent that the lifetime of minority charge carriers has increased from a few tens of ps to a few hundred ps to a few milliseconds. This poses a problem for measuring solar cells using flashers, as the illumination duration must be extended to maintain quasi-steady-state conditions. The illumination duration, and thus the flash, must now be much longer. This requires special flash lamps and corresponding flash controllers, which feature correspondingly larger spectral shifts. Such disadvantages can be avoided with LED light sources, as illumination durations of several milliseconds can be set.

[0033] Preferably, light intensities of less than 1.5 suns, preferably between 0.02 and 1.3 suns, are used for the first and / or second measuring process, whereby one sun corresponds to a radiation intensity of 1 kW / m 2 corresponds.

[0034] Each pair of values ​​from a stationary light intensity value (L s) and an associated initial voltage value (V) forms a point on the stationary curve. Without the calculation described here, the stationary curve could be determined by irradiating the solar cell with the associated light intensity value for each point on the stationary curve for long enough to achieve a stationary open circuit voltage. However, this would take too much time and the solar cell would heat up and its properties would change if it was not sufficiently cooled during the measurement. Instead, according to the invention, two hysteresis-affected measurement curves are determined, and the stationary curve is calculated from these.

[0035] From the pairs of values ​​forming the stationary curve, each consisting of a light intensity value and a value for the open-circuit voltage Voc measured at the solar cell, a stationary current-voltage curve (i.e., a hysteresis-free IV curve or, in short, stationary IV curve or IV characteristic) can be calculated by replacing the light intensity values ​​with corresponding short-circuit current values. This allows IV characteristics to be constructed. However, these IV characteristics are corrected for series resistance, or freed from the effects of series resistance. This means that the IV curve calculated from the stationary curve corresponds to a theoretical IV characteristic that would have been measured on a solar cell without series resistance. This is the result of the series resistance having no influence on the open-circuit voltage because no current flows at open-circuit voltage Voc and the entire IV characteristic calculated in this way is composed of open-circuit voltage values.It is therefore different from the conventionally measured IV characteristic. Comparing the two IV characteristics, i.e., the one calculated from the steady-state curve and the one measured directly on the solar cell, may provide some insight into the losses in the solar cell caused by series resistance.

[0036] In other words, the series resistance of the solar cell or the losses caused by this series resistance can be determined by combining the stationary curve and an IV characteristic curve of the solar cell.

[0037] Preferably, when the solar cell is irradiated at at least one light intensity value, a short-circuit current value is measured, whereby a current-voltage curve of the solar cell or solar module is determined from the determined steady-state curve and the measured short-circuit current value. In order to obtain the corresponding short-circuit current value for each light intensity value, the short-circuit current (Jsc - current density or Isc - current value) and the open-circuit voltage can be measured for each light intensity value. For this purpose, the solar cell is charged and discharged respectively. However, due to the long minority charge carrier lifetimes of modern solar cells, this takes a very long time. Instead, the short-circuit current can be known, for example, from a previous IV characteristic curve measurement, e.g., from a sun (STC). All light intensity values ​​can then be assigned to a current value, and the IV characteristic curve (without series resistance) can thus be calculated.

[0038] The short-circuit current value Isc(Ls), which depends on the light intensity value Ls, can be determined using a conventional flasher by illuminating the solar cell with a flashlight and measuring an IV curve. This can be performed, for example, as a calibration with a solar cell before applying the method to the same or other solar cells. It may be sufficient to determine a single Isc(Ls) value at a single light intensity value Ls, because Isc depends essentially linearly on Ls.

[0039] The calculated IV characteristic curve without the series resistance effect also has, like a conventional IV characteristic curve, a short-circuit current Isc, an open-circuit voltage Voc, a fill factor FF, and a maximum power point (MPP). The short-circuit current Isc and the open-circuit voltage Voc of the series resistance-adjusted IV characteristic curves are essentially identical to those of the conventional IV characteristic curve at the same illumination intensity. However, the fill factor FF (the so-called pseudo fill factor) and the maximum power point MPP (the pseudo MPP) differ. They are higher without the series resistance than with the series resistance. The power loss of the solar cell caused by the series resistance can be calculated from the difference between the two MPP values.

[0040] From the stationary curve, the effective

[0041] Calculate the minority carrier lifetime and the effective lifetime depending on the charge carrier concentration in the solar cell base material (injection level dependent minority carrier lifetime). Current minority carrier lifetimes of very good solar cells reach 2 ms. This would limit the minimum measurement time for the method described here to 7 ms, with the minority carrier lifetime being the limiting factor. As explained previously, the measurement process could be accelerated by performing a measurement process with light of increasing intensity more quickly than a subsequent measurement process with light of decreasing intensity. In contrast to measurements with decreasing intensity, the minority carrier lifetime plays no role in measurements with increasing intensity.

[0042] Thus, the determined steady-state curve can be used to determine, preferably, an effective minority carrier lifetime, a fill factor value, a series resistance value, an injection-level-dependent minority carrier lifetime, and / or a doping concentration. The doping concentration can be used, for example, to check whether the base resistance is still within the target range at the end of a solar cell manufacturing process. Furthermore, the doping concentration is an important simulation parameter in solar cell simulation.

[0043] According to a preferred embodiment, the measured values ​​are fitted with a model in order to improve the signal-to-noise ratio.

[0044] The methods and devices described here can also be applied to solar modules instead of individual or multiple solar cells. For this purpose, instead of a single solar cell, a solar cell string or a solar module is contacted and illuminated either partially or completely. A further aspect of the invention provides a solar cell testing device. It comprises a contacting device for contacting a solar cell or a solar module. Furthermore, it comprises a light source and a control device for controlling the light source and for characterizing the contacted solar cell or the contacted solar module. The light source is preferably an LED light source.

[0045] The solar cell testing device is preferably part of a solar cell production system and is arranged, for example, at the end of the system for so-called end-of-line testing. The embodiments and advantages listed above and below in connection with the solar cell testing method also apply accordingly to the solar cell testing device. This also applies to the computer-readable medium, which forms a further aspect of the invention.

[0046] The invention is explained below using exemplary embodiments with reference to the figures. Herein:

[0047] Fig. 1 is a flowchart of the process steps of a preferred solar cell testing method;

[0048] Fig. 2 is a diagram showing two sequences of light intensity and open circuit voltages measured during them; and

[0049] Fig. 3 shows another diagram showing two sequences and a stationary curve calculated from them.

[0050] Fig. 1 shows a flow diagram of a solar cell testing method. In a first method step 101, a solar cell is first contacted, i.e., provided with electrodes such that at least an open-circuit voltage can be measured on it. The solar cell is then irradiated with light of varying intensity in two different measuring processes 102, 103. In the specific case described here, in a first measuring process 102, the cell is irradiated with light of increasing intensity over time. Subsequently, in a second measuring process 103, the solar cell is irradiated with light of decreasing intensity over time. During both measuring processes 102, 103, the open-circuit voltage occurring at the solar cell is measured at specific light intensity values. The measuring points can, for example, be equally spaced along the light intensity. Alternatively or additionally, the measuring points can be determined equidistant in time.

[0051] The value pairs determined in the first measurement process from initial light intensity values ​​and corresponding initial voltage values ​​form the points on a first sequence. Correspondingly, value pairs determined in the second measurement process from initial light intensity values ​​and corresponding initial voltage values ​​form the points on a second sequence. An example of the first sequence 11 and the second sequence 12 is shown in Fig. 2. Since the solar cell is charged at increasing intensity during the first measurement, the initial voltage values ​​along the first sequence 11 are smaller than the secondary voltage values ​​along the second sequence 12, which was recorded at decreasing intensity. The voltage values ​​are plotted in volts along the x-axis, and the light intensity values ​​in suns along the y-axis.

[0052] After both sequences 11, 12 have been determined, a program loop 104 is executed in which a conversion of the light intensity value takes place for each measured initial voltage value in order to obtain a stationary light intensity value L s The pairs of values ​​from the calculated stationary light intensity value L sand the associated first voltage values ​​V then form a stationary curve. An example of a calculated stationary curve 23 is shown in the diagram in Fig. 3 as a dashed line. The two curves with solid lines form a further first sequence 21 and a further second sequence 22, from which the stationary curve 23 was calculated using the method described here. As in Fig. 2, in Fig. 3 the voltage values ​​in volts are plotted along the x-axis and the light intensity values ​​in suns (suns) are plotted along the y-axis. Here, however, during the measurements to determine the two sequences 22, 21, the intensity was not completely reduced to absolute darkness.

[0053] Finally, in a further method step 105 shown in Fig. 1, an IV curve is determined from the stationary curve 23. Using this IV curve or directly from the stationary curve 23, other properties of the solar cell can then be calculated, for example, an effective minority charge carrier lifetime, a fill factor value, a series resistance value, an injection level-dependent minority charge carrier lifetime, and / or a doping concentration.

Claims

Patent claims: 1 . Solar cell test method for characterizing a contacted solar cell or a contacted solar module, comprising the following process steps: - Carrying out a first measuring process in which the solar cell is irradiated with light of a time-varying intensity and, during the irradiation, a time course of a cell voltage on the solar cell is measured in order to obtain a first sequence of time-resolved first light intensity values ​​and associated time-resolved first voltage values; - Carrying out a second measuring process in which the solar cell is irradiated with light of a time-varying intensity, wherein the time-varying change in intensity during the second measuring process differs from the time-varying change in intensity during the first measuring process, and during the irradiation the time course of the cell voltage at the solar cell is measured in order to obtain a second sequence of time-resolved second light intensity values ​​and associated time-resolved second voltage values; - for each initial voltage value: a) Determine the initial light intensity value (L u ) from the first sequence; b) Determine a time derivative (dV / dt u) of the voltage in the first sequence at the first voltage value; c) determining a second voltage value equivalent to the first voltage value, d) determining the second light intensity value (Ld) associated with the second voltage value from the second sequence; e) determining a time derivative (dV / dtd) of the voltage in the second sequence at the second voltage value, f) determining a stationary light intensity value (L s ) using the following formula: L s = (L u * dV / dtd + Ld * dV / dt u ) / (dV / dtd + dV / dt u ), where the total amount of stationary light intensity values ​​(L s ) with the respective initial voltage values ​​(V) forms a stationary curve.

2. Solar cell testing method according to claim 1, characterized in that the solar cell is irradiated with light of temporally increasing intensity during the first measuring process and with light of temporally decreasing intensity during the second measuring process, or that the solar cell is irradiated with light of temporally decreasing intensity during the first measuring process and with light of temporally increasing intensity during the second measuring process.

3. Solar cell testing method according to claim 1, characterized in that in the first measuring process the solar cell is irradiated with light of a temporally increasing intensity and in the second measuring process the solar cell is irradiated with light of a temporally increasing intensity, wherein the intensity profiles during irradiation in the first measuring process and in the second measuring process differ from one another.

4. Solar cell testing method according to claim 1, characterized in that in the first measuring process the solar cell is irradiated with light of a temporally decreasing intensity and in the second measuring process the solar cell is irradiated with light of a temporally decreasing intensity, wherein the intensity profiles during irradiation in the first measuring process and in the second measuring process differ from one another.

5. Solar cell test method according to one of the preceding claims, characterized in that the temporal course of the intensity has a gradient whose absolute value is on average significantly greater or significantly smaller during the first measuring process than during the second measuring process, in particular at least by a factor of 2, 5 or 10.

6. Solar cell testing method according to one of the preceding claims, characterized in that before carrying out the first measuring process and / or before carrying out the second measuring process, the solar cell is short-circuited or subjected to a reverse voltage for a short-circuit period.

7. Solar cell testing method according to one of the preceding claims, characterized in that at the end of the first measuring process, the intensity of the light is maintained at a minimum intensity or at a maximum intensity during a holding period.

8. Solar cell testing method according to one of the preceding claims, characterized in that during the first measuring process the intensity of the light is adjusted to the respective first light intensity values ​​and / or that during the second measuring process the intensity of the light is adjusted to the respective second light intensity values.

9. Solar cell testing method according to claim 8, characterized in that the solar cell is irradiated with light from an LED light source during the first measuring process and / or during the second measuring process.

10. Solar cell test method according to one of the preceding claims, characterized in that during irradiation at at least one light intensity value a short-circuit current value is measured, wherein a current-voltage curve of the solar cell or solar module is determined from the determined stationary curve and the measured short-circuit current value. 11 . Solar cell test method according to one of the preceding claims, characterized in that from the determined stationary curve an effective minority charge carrier lifetime, a fill factor value, a series resistance value, injection level-dependent Minority carrier lifetime and / or a doping concentration are / will be determined.

12. A solar cell testing device comprising a contacting device for contacting a solar cell or a solar module, a light source, and a control device for controlling the light source and for characterizing the contacted solar cell or the contacted solar module, wherein the control device is designed to carry out the following method steps: - Carrying out a first measuring process in which the solar cell is irradiated with light of a time-varying intensity and, during the irradiation, a time course of a cell voltage on the solar cell is measured in order to obtain a first sequence of time-resolved first light intensity values ​​and associated time-resolved first voltage values; - Carrying out a second measuring process in which the solar cell is irradiated with light of a time-varying intensity, wherein the time-varying change in intensity during the second measuring process differs from the time-varying change in intensity during the first measuring process, and during the irradiation the time course of the cell voltage at the solar cell is measured in order to obtain a second sequence of time-resolved second light intensity values ​​and associated time-resolved second voltage values; - for each initial voltage value: a) Determine the initial light intensity value (L u ) from the first sequence; b) Determine a time derivative (dV / dt up) the voltage in the first sequence at the first voltage value; c) determining a second voltage value equivalent to the first voltage value, d) determining the second light intensity value (Ld) corresponding to the second voltage value from the second sequence; e) Determining a time derivative (dV / dtdown) of the voltage in the second sequence at the second voltage value, f) Determining a stationary light intensity value (Ls) using the following formula: Ls = (Lu * dV / dtdown + Ld * dV / dt up ) / (dV / dtdown + dV / dtup), where the total amount of stationary light intensity values ​​(Ls) with the respective initial voltage values ​​(V) form a stationary curve.

13. A computer-readable medium having computer-executable instructions which, when executed, implement a method according to any one of claims 1 to 11.

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