Decoupling of a Perowskit solar cell in darkness

Description

Technical area

[0001] The invention relates to a method for operating a photovoltaic module comprising at least one perovskite solar cell. State of the art

[0002] Photovoltaic modules in a photovoltaic system are typically operated using maximum power point tracking (MPPT). This is an electrical engineering process in which the electrical load on the photovoltaic module is regulated so that the maximum power is drawn from the photovoltaic module. The optimal operating point of the photovoltaic module is not constant over time, but depends on parameters such as the irradiance, the irradiation spectrum, and the temperature of the solar cells in the photovoltaic module. The type of solar cell used (e.g., monocrystalline / multicrystalline / amorphous silicon) also influences the optimal operating point. The optimal operating point is usually continuously adjusted.

[0003] Maximum power point tracking is an established method for maximizing the power output of photovoltaic modules with crystalline silicon solar cells.

[0004] To implement the maximum power point tracking method for a photovoltaic module, a control device is typically connected to the photovoltaic module (or a string of photovoltaic modules). The control device specifies a resistance, also known as the load resistance. The load resistance is set to correspond to the internal resistance of the photovoltaic module at which the photovoltaic module's power is maximum. The photovoltaic module is operated with power adjustment with respect to the control device.

[0005] One challenge of the MPPT method is to reliably determine the optimal operating point with the least possible energy consumption. This is not trivial, for example, when the photovoltaic system is partially shaded, since in such cases the control system may only determine a local rather than the global power maximum. Approaches for determining the global power maximum of a photovoltaic module or photovoltaic system as reliably as possible are disclosed, for example, in patents EP 1 750 193 B1 and EP 2 360 546 B1.

[0006] In any case, the photovoltaic module must be continuously connected to the control system to ensure maximum electrical power can be drawn even when the solar radiation is diffused, for example, during cloudy conditions or at dusk. At night, the photovoltaic module remains connected to the control system, even if no power can be drawn from the photovoltaic module in complete darkness.

[0007] For several years now, perovskites, such as CH3NH3PbI3, have been investigated for their potential use in solar cells. Due to their optoelectronic properties, they essentially allow highly efficient conversion of electromagnetic radiation energy into electrical energy. Perovskite-based solar cells, also referred to as perovskite solar cells, are characterized by their comparatively low cost of production. Furthermore, perovskite solar cells represent an attractive and serious alternative to conventional silicon-based solar cells. The rapid progress made in recent years in terms of efficiency, which has increased from a few percent to over 25% today, indicates that efficiencies significantly exceeding those of conventional silicon solar cells can be achieved.It is conceivable to operate a perovskite solar cell alone or in a tandem photovoltaic module, for example in combination with a crystalline silicon solar cell.

[0008] Photovoltaic modules with perovskite solar cells are not yet commercially available. Accordingly, there is still no information available regarding the optimal operation of a photovoltaic module with perovskite solar cells.

[0009] WO 2016 / 164718 describes a control system for PV systems that determines a radiation value. A threshold is also defined. If the radiation falls below this threshold, the PV system can be disconnected from the electrical load. This involves monitoring / predicting the PV system's output power, i.e., monitoring changes (in series resistance). For this measurement, the cell must deliver power and requires corresponding irradiance.

[0010] US 2018 / 259990 A1 also describes a control system for a perovskite PV cell.

[0011] Against this background, the present invention has set itself the task of developing a concept for an efficient operating method for the operation of a photovoltaic module containing one or more perovskite solar cells. General description of the invention

[0012] In its own investigations, the applicant surprisingly discovered that perovskite solar cells degrade significantly when subjected to MPPT exposure without lighting, e.g., at night, for extended periods. Even with low-intensity lighting, degradation could be measured under certain conditions.

[0013] The reasons for the degradation of a photovoltaic module containing one or more perovskite solar cells operated using maximum power point tracking in low or no light conditions are currently unclear. The physical / chemical effects leading to the observed degradation have not yet been conclusively clarified.

[0014] However, according to the invention, a significantly improved method for operating a photovoltaic module containing one or more perovskite solar cells is represented by the following method: The method comprises at least temporarily operating the photovoltaic module at its maximum power point by means of a control device connected to the photovoltaic module. The extraction of electrical energy is interrupted when the irradiance of electromagnetic radiation incident on the photovoltaic module falls below a predetermined threshold.

[0015] Of course, the perovskite photovoltaic module typically comprises more than one perovskite solar cell, namely a plurality of perovskite solar cells. A perovskite solar cell can also be part of a tandem cell, which consists of a perovskite solar cell and another solar cell, for example, a conventional silicon solar cell. An impressive 28% efficiency has recently been achieved for such tandem solar cells on a laboratory scale.

[0016] In particular, the photovoltaic module is operated at the point of maximum power by means of the control device essentially during the entire period in which the irradiance exceeds the predetermined threshold.

[0017] This means that the photovoltaic module is advantageously operated at the point of maximum power by means of the control device when the irradiance exceeds the predetermined threshold, and the energy extraction is interrupted when the irradiance falls below the predetermined threshold.

[0018] The control device is, in particular, an inverter assigned to the photovoltaic module, which is equipped, in particular, with an adjustable load resistance. The inverter can be assigned to a single photovoltaic module (such an inverter is also referred to in technical terms as a microinverter) or to an entire string of photovoltaic modules. Theoretically, an individual inverter per solar cell is also conceivable.

[0019] Since the load of the photovoltaic module is adjusted based on the load voltage, but the output voltage of the control device should be virtually constant, a DC-DC converter is typically used to adjust the voltage differences and thus the load on the solar cell. The continuous adjustment of the optimal operating point of the photovoltaic module (namely, operating the photovoltaic module at the point of maximum power) is advantageously performed by a microcontroller or a digital signal processor. The required measurement data can be provided to the processor by an analog-to-digital converter. The analog-to-digital converter can then perform the necessary calculations and pass the results on to the DC-DC converter using pulse-width modulation.

[0020] The control device can also consist of a so-called "power optimizer" assigned to the photovoltaic module.

[0021] According to the invention, the extraction of electrical energy is interrupted when the irradiance of the electromagnetic radiation incident on the photovoltaic module falls below a predetermined threshold. Extraction of electrical energy refers to the extraction of electrical energy from the photovoltaic module. This extraction is controlled by the control device, if a control device is connected, in such a way that the amount of energy extracted is ideally always as high as possible.

[0022] The photovoltaic module is electrically disconnected from the control system when the irradiance falls below a predetermined threshold. The circuit to which the photovoltaic module is connected is therefore open, and the open-circuit voltage is present at the photovoltaic module.

[0023] In practice, the decoupling or separation of the photovoltaic module from the control device can be achieved simply by breaking the electrical contact (in other words, electrically isolating it) between the control device and the photovoltaic module. For this purpose, the temporary separation of the photovoltaic module from the control device is achieved electronically, using a transistor controlled with high impedance. If, for example, a metal-oxide-semiconductor field-effect transistor (MOSFET) is used, no significant energy would be required to control the transistor. The advantage of using a transistor over a mechanical switch is that no moving parts are involved, making it maintenance-free and durable.Since no electrical voltage can be drawn from the photovoltaic module when the transistor is activated, the electrical power that can be drawn from the photovoltaic module (or, accordingly, the electrical energy that can be drawn) is also zero.

[0024] According to the invention, the extraction of electrical energy is interrupted when the irradiance is less than a predetermined threshold. This threshold lies in the range between 2 W / m 2 and 20 W / m 2 , preferably between 5 W / m 2 and 15 W / m 2 and particularly preferably at 10 W / m 2 . Our own measurements specifically on perovskite solar cells have surprisingly shown that these function very well and do not exhibit any degradation even at a few W / m 2 . A threshold in one of these ranges corresponds to weak or very weak illumination of the perovskite solar cell(s). Setting the threshold in one of these ranges is advantageous because degradation of the perovskite solar cell, in particular the reduction of its efficiency, has been observed precisely with no or (very) weak illumination in conjunction with a connected MPPT control device.

[0025] Preferably, if the irradiance falls below the predetermined threshold, the photovoltaic module is electrically short-circuited. This can also be achieved using a suitable transistor, which in this case electrically connects the two electrical contacts (front and rear contacts or p- / n contacts) of the photovoltaic module. Since no electrical current can be drawn from the photovoltaic module in this case, the available electrical power (or, accordingly, the electrical energy) is zero.

[0026] Irradiance is the total power of incoming electromagnetic energy hitting a surface, relative to the area. Irradiance, in other words, is the radiant flux through the irradiated area. It is also referred to as irradiance or radiant flux density.

[0027] The photocurrent generated by the photovoltaic module, for example, can be used as a measure of the irradiance of the electromagnetic radiation striking the photovoltaic module. This has the advantage that the photocurrent is usually measured anyway and thus appears suitable as a control variable for the disconnection or electrical connection of the control device to the photovoltaic module. Under certain circumstances (assuming the module efficiency is known and stable over time), the output power of the photovoltaic module could also be used as a measure of the irradiance instead of the photocurrent.

[0028] Alternatively, the irradiance can also be determined using impedance spectroscopy on one or, if present, several perovskite solar cells of the photovoltaic module.

[0029] In impedance spectroscopy, particularly electrochemical impedance spectroscopy, an alternating voltage is applied to the perovskite solar cell, and the resulting current flow is measured for various frequencies of the alternating voltage. Based on a suitable plot of the measurement results, for example, in a so-called Bode or Nyquist diagram, conclusions can then be drawn about the internal parameters of the perovskite solar cell, such as charge carrier transport or the recombination rate of the charge carriers. Using the values ​​of suitable internal parameters, the irradiance of the solar cell under investigation can also be determined.

[0030] Another way to determine the irradiance of the electromagnetic radiation incident on the photovoltaic module is to provide an additional photocell to measure the irradiance. Specifically, in this case, the irradiance can be determined, for example, using the photocurrent generated by the additional photocell.

[0031] The additional photocell can be relatively small (e.g., 1 × 1 cm2) and is advantageously integrated into the photovoltaic module. It can, for example, be integrated into the frame, i.e., at the edge of the photovoltaic module.

[0032] Another way to estimate the irradiance of the solar radiation hitting the photovoltaic module is to use meteorological data. Sunrise and sunset for a specific location on Earth throughout the year (i.e., for each day) are known and can be programmed into the control system, for example, once the location of the photovoltaic module is known. This has the advantage that no measurement data needs to be collected and processed to control the disconnection or connection of the control system. However, such a method is also less precise, since cloud cover or shading of the photovoltaic module or part of it cannot be detected and taken into account. Short description of the drawings

[0033] The invention is illustrated below with reference to the attached figures.

[0034] The pictures show: Fig. 1: a photovoltaic module that is electrically decoupled from a control device so that the extraction of electrical energy is interrupted, Fig. 2: the same photovoltaic module that is now electrically connected to the control device so that the photovoltaic module can be operated at the point of maximum power, Fig. 3: the same photovoltaic module that is now short-circuited and Fig. 4: the temporal developments of the efficiencies of two perovskite solar cells. Detailed description of the drawings

[0035] Figures 1 to 3 (also known as Fig. 1 to 3 denoted) show an embodiment of a photovoltaic device 1 according to the invention. The photovoltaic device 1 has a photovoltaic module 10 with a plurality of perovskite solar cells 11, of which in Figure 1For the sake of clarity, only one is shown. Each of the perovskite solar cells 11 has an electron-conducting layer, a light-absorbing layer (also called an "absorber"), and a hole-conducting layer. The absorber contains a perovskite with the general structural formula ABX 3 , where, for example, A=CH 3 NH 3 , B=Pb, and X=I 3 . Furthermore, the solar cell has a front-side contact 111 and a back-side contact 112. The front-side contact 111 is preferably transparent to electromagnetic radiation from a specific wavelength range for which the solar cell is designed. The back-side contact can be designed to be full-surface and non-transparent (e.g., comprising an aluminum alloy). The perovskite solar cells 11 are embedded in glass panes, which are intended to protect them, for example, from contamination and damage.

[0036] Figures 1 to 3 also show electromagnetic radiation 2 from the sun, also referred to as "solar radiation", which strikes the front side of the photovoltaic module 10 and thus also the front side of the perovskite solar cell 11.

[0037] The front and rear contacts 111, 112 of the Figure 1 The solar cell 11 shown can be electrically connected to a control device 20. The photovoltaic module 10 can be electrically connected to or electrically decoupled from the control device 20 by means of a switch 30. In addition, the photovoltaic module 10 can also be short-circuited by means of the switch 30.

[0038] In Figure 1A first position 301 of switch 30 is shown. In the first position 301, the switch is open, and the photovoltaic module 10 is electrically isolated from the control device 20. Therefore, an open-circuit voltage is present at the photovoltaic module 10. No energy is drawn from the photovoltaic module 10.

[0039] In Figure 2 A second position 302 of switch 30 is shown. In the second position 302, the switch is closed, and the electrical connection between photovoltaic module 10 and control device 20 is established. Control device 20 is capable of extracting the maximum power (or, accordingly, the maximum energy) from the photovoltaic module by adjusting its load resistance to the current internal resistance of the photovoltaic module.

[0040] In Figure 3A third position 303 of switch 30 is shown. In the third position 303, the switch connects the front contact 111 and the rear contact 112 of the photovoltaic module 10, thus short-circuiting it. A corresponding short-circuit current flows through the photovoltaic module 10, and no energy can be drawn from the photovoltaic module 10.

[0041] In particular, the control device 20 itself can move the switch 30 to the appropriate position 301, 302, 303. If the irradiance is sufficiently high, the switch is closed (position 302), and maximum power point tracking of the photovoltaic module 10 is performed. However, if the irradiance falls below a predetermined threshold, the photovoltaic module 10 is either disconnected from the control device (position 301) or short-circuited. In both cases, the result is that maximum power point tracking of the photovoltaic module by means of the control device 20 is no longer possible, thus preventing or at least reducing degradation of the perovskite solar cell(s) 11 of the photovoltaic module 10.

[0042] The Figure 4 (also known as Fig. 4) shows the time course of the efficiency of a first perovskite solar cell 53 and the time course of the efficiency of a second perovskite solar cell 54. The time in hours is plotted on the abscissa 51, and the standardized efficiency is plotted on the ordinate 52. The efficiency is standardized, which means that it was set to the relative value 1.0 for both solar cells at the start of the measurements. Both solar cells are comparable in terms of structure, material, and performance. Both solar cells were artificially illuminated for six hours each, each interrupted by a six-hour period in the dark. This was intended to simulate the day / night rhythm in a time-condensed manner. The illumination corresponded to real sunlight in terms of intensity and spectrum.

[0043] The first solar cell was connected to a conventional control device for operating the solar cell at its maximum power point, a so-called "maximum power point tracking control device," for the entire measurement period of 100 hours (approximately 8 day / night cycles). In particular, the solar cell was also connected to, or "loaded," this control device during the dark periods.

[0044] This resulted in a significant degradation of the efficiency of the solar cell under investigation. At the end of the 100-hour measurement period, the solar cell's efficiency was only approximately a quarter of its initial value.

[0045] The second solar cell was disconnected from the "Maximum Power Point Tracking Control System" during the periods in which it was exposed to darkness (8 x 6 hours). Here, too, a certain degradation of the cell efficiency was observed during the light phases and during the dark phases. However, the degradation was significantly less than in the first solar cell, which was permanently connected to the control system. At the end of the 100-hour measurement period, the efficiency of the solar cell was still approximately three-quarters of the value at the beginning of the test.

[0046] Disconnecting a photovoltaic module with a perovskite solar cell from the MPPT control system during darkness (or low light) can therefore at least mitigate, possibly even completely eliminate, or at least limit the degradation of the perovskite solar cell. List of reference symbols

[0047] 1Photovoltaic device 2Electromagnetic radiation 10Photovoltaic module 11Perovskite solar cell 111Front contact 112Rear contact 20Control device 30Switch 301First position 302Second position 303Third position 51Abscissa 52Ordinate 53Efficiency of a first perovskite solar cell 54Efficiency of a second perovskite solar cell

Claims

1. Method for operating a photovoltaic module (10) having at least one perovskite solar cell (11), the method comprising the at least temporary operation of the photovoltaic module (10) at the maximum power point by means of a regulation device (20) connected to the photovoltaic module (10), wherein the withdrawal of electrical energy is interrupted when the irradiance of an electromagnetic radiation (2) impinging on the photovoltaic module (10) falls below a predetermined threshold value, characterized in that, when the irradiance falls below the predetermined threshold value, the photovoltaic module (10) is electrically decoupled from the regulation device (20) by means of a transistor and in that the predetermined threshold value is in the range of between 2 W / m2 and 20 W / m2, in particular between 5 W / m2 and 15 W / m2, particularly preferably 10 W / m2.

2. Method according to Claim 1, wherein the photovoltaic module (10) is operated at the maximum power point by means of the regulation device (20) substantially throughout the time period in which the irradiance exceeds the predetermined threshold value.

3. Method according to either of Claims 1 or 2, wherein, when the irradiance falls below the predetermined threshold value, the photovoltaic module (10) is shortcircuited.

4. Method according to one of the preceding claims, wherein the photocurrent generated by the photovoltaic module (10) is used as a measure of the irradiance of the electromagnetic radiation (2) impinging on the photovoltaic module (10).

5. Method according to one of the preceding claims, wherein the irradiance of the electromagnetic radiation (2) impinging on the photovoltaic module (10) is determined by means of impedance spectroscopy at one or more perovskite solar cells (11) of the photovoltaic module (10).

6. Method according to one of the preceding claims, wherein the photovoltaic module (10) has an additional photoelectric cell, and the irradiance of the electromagnetic radiation (2) impinging on the photovoltaic module (10) is determined by means of the additional photoelectric cell.

7. Method according to Claim 6, wherein the irradiance is determined by means of the photocurrent generated by the additional photoelectric cell.

8. Method according to one of the preceding claims, wherein the irradiance of the electromagnetic radiation (2) impinging on the photovoltaic module (10) is estimated on the basis of meteorological data.

9. Method according to one of the preceding claims, wherein the regulation device (20) is an inverter, in particular a microinverter, assigned to the photovoltaic module (10).

10. Method according to one of the preceding claims, wherein the regulation device (20) is a power optimizer assigned to the photovoltaic module (10).