Two 'Combined Stress test with in situ measurement' setups, which allow real-time monitoring of accelerated degradation of solar cells and modules, were designed and constructed. These setups allow the simultaneous use of humidity, temperature, electrical biases, and illumination as independently controlled stress factors. The setups and various experiments executed are presented.
The levelized cost of electricity (LCOE) of photovoltaic (PV) systems is determined by, among other factors, the PV module reliability. Better prediction of degradation mechanisms and prevention of module field failure can consequently decrease investment risks as well as increase the electricity yield. An improved knowledge level can for these reasons significantly decrease the total costs of PV electricity.
In order to better understand and minimize the degradation of PV modules, the occurring degradation mechanisms and conditions should be identified. This should preferably happen under combined stresses, since modules in the field are also simultaneously exposed to multiple stress factors. Therefore, two ‘Combined Stress test with in situ measurement’ setups have been designed and constructed. These setups allow the simultaneous use of humidity, temperature, illumination, and electrical biases as independently controlled stress factors on solar cells and minimodules. The setups also allow real-time monitoring of the electrical properties of these samples. This protocol presents these setups and describes the experimental possibilities. Moreover, results obtained with these setups are also presented: various examples about the influence of both deposition and degradation conditions on the stability of thin film Cu(In,Ga)Se2 (CIGS) as well as Cu2ZnSnSe4 (CZTS) solar cells are described. Results on the temperature dependency of CIGS solar cells are also presented.
PV systems are considered to be a cost-effective form of renewable energy. PV modules represent the core of these PV systems and are generally sold with a performance guarantee of over 25 years (e.g., max. 20% efficiency loss after this period)1. It is crucial for the trust of consumers and investors that these guarantees are met. The electricity yield should therefore be as stable and high as possible over at least the desired module lifetime. This should be managed by reduction of both slow but steady degradation2 and unexpected premature module failures, which, for example can occur due to production errors. Examples of observed module failures in the field are Potential Induced Degradation (PID)3 and Light Induced Degradation (LID)4 for crystalline silicon modules or water induced corrosion in CIGS modules5,6,7,8. In order to prevent a reduced field lifetime of PV modules, degradation mechanisms should therefore be identified and minimized.
Improved understanding of degradation mechanisms occurring in PV cells or modules would also help to lower PV module production costs: in many cases, protective materials against environmental stresses are introduced in modules to offer the guaranteed lifetime. This is for example true for flexible thin film modules, like CIGS, that contain an expensive barrier to prevent water ingression. All package materials in such modules can make up to 70% of the module costs. These protective materials are often over-dimensioned in order to be certain to obtain the required lifetime: more knowledge about the degradation mechanisms can therefore make solar cells more intrinsically stable and more accurately predictable. Better understanding about the long-term stability of the module and its constituents would therefore likely prevent over-dimensioning and allow reduced costs for these protective materials.
To give a general estimation of module reliability, solar cells and modules are nowadays tested and qualified by Accelerated Lifetime Tests (ALT)9. The most profound qualification tests are defined by the International Electrotechnical Commission (IEC) 61215 tests10, which give "go/no go" decisions on the stability of PV modules. However, Osterwald et al.11 revealed that a positive outcome of the IEC tests does not always indicate that the PV module can stand outdoor conditions for 25 or more years. This limited correlation between field and laboratory testing was demonstrated to be especially true for the relatively new thin film modules12.
These tests do not yield insight into the degradation mechanisms ('Which processes and/or which stresses lead to observed slow module degradation or to rapid module failure?'). Moreover, these tests, which are currently based on single or dual stress factors (for example mechanical stress, or combined temperature and humidity) can certainly not simulate field behavior in a reliable way, since PV modules in the field are subject to numerous combined stresses (for example: temperature, humidity, wind, snow, illumination, dust, sand, water). These stresses can also vary per climate zone: while in the desert, temperature and illumination are likely important stress factors; in moderate climates, the influence of for example humidity can also be very important. To simulate the degradation and consequent failures in various climates, various combinations of multiple stresses are thus required. Consequently, simultaneous exposure to multiple stresses is very important to obtain a good estimation of the module reliability in a certain climate, and combined stress tests should thus be part of laboratory tests.
It is thus proposed that qualitative and quantitative understanding of the degradation mechanisms occurring under combined stress conditions should be improved. Ideally, information about the solar cell or module should also be gathered during these tests, to allow identification of device changes during exposure. Therefore, we have designed and constructed two setups that allow simultaneous exposure to humidity, (elevated) temperatures, electrical biases, and illumination. In these setups, the severity of these stresses can also be tuned, depending on the goal of an experiment. Additionally, the illumination allows in situ monitoring of the PV devices (Figure 1)13,14,15,16,17,18,19,20. These types of tests will be named 'Combined Stress tests with in situ measurements' (CSI). In this protocol, two hybrid degradation setups, named 'CSI 1' and 'CSI 2', will be presented. Many studies, aiming at the improvement of understanding of the performance and degradation of especially thin film CIGS solar cells, were executed with these setups. A selection of stability and temperature dependency results obtained on unpackaged CIGS and CZTS solar cells are presented. More information can also be found in21,22.
Figure 1: 'Combined Stress tests with in situ measurements' setup. Left: Schematic overview of a CSI setup including the measurement system. Middle and right: Photograph of the CSI setups (climate chambers plus solar simulators, measurement systems not depicted, setups have different sizes). Middle is CSI1, right is CSI2. This figure has been modified from19,30. Please click here to view a larger version of this figure.
NOTE: Sections 1 and 3 are specific for degradation testing of CIGS and CZTS solar cells via this procedure, but all other types of solar cells (e.g., perovskites, organic PV, and crystalline silicon) are or will be tested with these setups. It should be noted that for every device type and geometry, a sample holder should be designed. These holders should have non-corroding contacts to prevent contact degradation, since this would obscure the effects of device degradation. Moreover, it is advised to contact samples in a four-point probe configuration, to prevent the measurement of the results of corroded contacts or wires in the measurement system.
1. Preparation of CIGS Solar Cells
Figure 2: CIGS sample design. (top) Schematic representation of the cross-section of a CIGS sample and (bottom) a microscope picture of a CIGS sample taken from the top. This figure has been partly modified from references14,30. Please click here to view a larger version of this figure.
2. Analysis of the Solar Cells Before Degradation
3. Placement of the Solar Cells into Sample Holders
4. Execution of the Degradation Experiment
5. Analysis of Degraded and Reference Cells
6. Definition of the Degradation Mechanisms and Modes
The CSI setups have been used for a wide range of experiments. Experiments have both focused on the influence on the cell or module composition and design, as well as on the influence of the degradation conditions. Some examples of the development of electrical parameters are displayed in the following figures. Measurements in Figure 3, Figure 5, Figure 6, and Figure 7 were taken in CSI1, while Figure 4 was obtained in CSI2. In these figures, it is chosen to depict either the device efficiency, the open circuit voltage, or the shunt resistance, but other parameters can naturally also be plotted.
Figure 3 and Figure 4 display the influence of the degradation conditions on stability of alkali-rich CIGS solar cells without a humidity barrier or any other package material. Figure 3 shows that these cells degrade when they are exposed to illumination, heat, and humidity, while they are almost stable in the absence of humidity. This indicates that these solar cells or analog modules might be completely stable when well packaged against humidity15. Potential package materials naturally include glass, but also flexible barriers, which are often based on organic-inorganic multi-stacks15. In future experiments, these possibilities will also be tested. These results also indicate that this package material might not be necessary in a hot and dry climate. Figure 4 shows the influence of a bias voltage when exposed to damp heat plus illumination: these preliminary results indicate that a low negative voltage (-0.5 V, grey curves) likely has a more negative effect on stability than short circuit, open circuit, and MPP conditions18.
Figure 3: Influence of humidity on CIGS solar cell stability. The development of the efficiency of unpackaged CIGS solar cells as a function of exposure time to illumination plus dry heat (red) and damp heat (blue) taken at elevated temperatures. Every line represents one solar cell. This figure has been modified from reference15. Please click here to view a larger version of this figure.
Figure 4: Influence of electrical loads on CIGS solar cell stability. Evolution of the efficiency of unpackaged cells as a function of time at various voltages plus damp heat and illumination. Grey, blue, green, and red curves indicate exposure to -0.5 V, 0 V, ~ VMPP, and open circuit conditions, respectively. These parameters are obtained at elevated temperatures, while the room temperature efficiencies are around 50% higher. Every line represents one solar cell. This figure has been modified from reference18. Please click here to view a larger version of this figure.
Due to the slow heating (0.1-0.3 °C/min) during the heating phase and the real-time measurements, these setups also automatically allow the determination of the temperature dependency of solar cells. Figure 5 displays the dependency of the open circuit voltages as obtained from the heating curves before degradation experiments. This graph shows that differences exist between the open circuit voltage (Voc) temperature dependency of various CIGS solar cells, while other parameters like the series resistance and the short circuit current (not depicted) display even larger differences between cells. The development of other parameters can be found in reference34.
Figure 5: Temperature dependency of CIGS solar cells. Temperature dependency of the open circuit voltage (Voc) of two unpackaged CIGS solar cells. The colors indicate different solar cell designs: the blue squares represent samples with the cell design and deposition procedure as described above. The red circles indicate a non-packaged CIGS solar cell on polyimide foil with absorbers deposited with ion-beam assisted coevaporation. Every line represents one solar cell. This figure has been modified from reference34. Please click here to view a larger version of this figure.
Figure 6 shows that small differences in the composition of solar cells can have a large influence on the device stability. This experiment demonstrated that alkali-rich samples containing large quantities of sodium and potassium had a higher initial efficiency, but they also degraded more rapidly. On the other hand, almost stable unpackaged solar cells that only contained small quantities of alkali-elements ("alkali-poor" samples) were also produced. These solar cells were thus almost intrinsically stable and did not need any protective material. Based on this information combined with ex situ analysis results, the main degradation mechanisms for these samples could be identified: it was observed that the main driver behind the efficiency-loss of the alkali-rich samples was a sharp decrease in shunt resistance16. In-depth analysis of the properties of these cells displayed that the migration of alkali-elements, more specifically sodium, seemed to cause this decrease. More information is presented in references16,20. Later stages of this study aim to develop solar cells with the stability of the alkali-poor samples, and the high initial efficiency of the alkali-rich samples.
Figure 6: Influence of the alkali-content on CIGS solar cell stability. Evolution of the efficiency (left) and shunt resistance (right) of two types of unpackaged CIGS solar cells exposed to damp heat plus illumination. The pink and purple lines represent the alkali-poor samples, while the blue lines represent the alkali-rich samples. The values were obtained at elevated temperatures, while room temperature efficiencies are 30-80% higher. Every line represents one solar cell. This figure has been modified from reference16. Please click here to view a larger version of this figure.
A last example focuses on various CZTS samples19. Figure 7 shows that different types of unpackaged solar cells demonstrate a different IV behavior under damp heat plus illumination. It should be noted that these cells are not ideal solar cells, so the increase in efficiency and voltage as displayed in this figure is likely not representative for CZTS solar cells in general and no explanation could be provided for this behavior. More studies need to be executed to give reliable statements about the stability of these cells.
Figure 7: CZTS solar cells exposed to damp heat plus illumination. Evolution of normalized open circuit voltage and efficiency of four types of non-optimized unpackaged CZTS solar cells as a function of time, exposed to damp heat plus illumination taken at elevated temperatures. Every color depicts a different type of CZTS solar cell. Every line represents one solar cell. This figure has been modified from reference19. Please click here to view a larger version of this figure.
Two CSI setups for real-time monitoring of the electrical parameters of solar cells and modules have been designed and constructed. These setups allow simultaneous exposure to damp heat, illumination, and electrical biases, while also in situ determining the IV parameters of PV devices. These setups have been used to study the influence of environmental stresses (humidity, illumination, electrical biases, and temperature) as well as cell or module composition on the long-term stability of unpackaged solar cells. Figure 3, Figure 4, Figure 5, Figure 6, and Figure 7 display a selection of results obtained with these setups.
Stability results (Figure 3, Figure 4, Figure 6, and Figure 7) from the presented studies should always be treated with care: in order to make the translation from these studies to module stability, the constraints of all accelerated lifetime tests on the stability of PV devices (including this study) should be taken into account. These constraints are caused by the fact that the conditions in the laboratory are meant to rapidly identify degradation mechanisms, while some degradation mechanisms might not be found due to the selection of the wrong (severity of) stresses. Moreover, the chosen conditions might also lead to degradation mechanisms and consequent failures that do not occur in the field or occur in the field before or after the predicted time frame. While for example for damp heat conditions (85 °C/85% RH), an acceleration factor of 219 is assumed, reference25 showed that this rate is often non-linear and can vary in CIGS modules between 10 and 1,000, and for different degradation mechanisms.
To estimate the validity of the presented results, the most important differences between the field module exposure and the presented experiments should be taken into account:
a. Used laboratory conditions are more severe than field conditions, which is an intrinsic requirement for accelerated testing. Moreover, the conditions in these experiments are mostly constant, while modules in the field will be exposed to continuously changing conditions.
b. In the presented experiments, non-packaged solar cells were used. Naturally, barrier materials and edge sealants will play an important role in the device stability (especially under humid conditions). Additionally, the influence of interconnection and encapsulation materials is also very important and should not be neglected. Certainly, experiments with packaged and interconnected mini-modules are also possible in these setups.
c. Due to the illumination, the experiments presented in Figure 3, Figure 5, Figure 6, and Figure 7 were executed under open circuit conditions when the IV curves were not recorded. However, modules should function under MPP conditions, while the cells could also be exposed to reversed bias conditions in the case of partial module shadowing. Figure 4 shows that only limited differences between MPP and open circuit conditions were observed in that specific experiment, but that might be different for other cells or conditions.
d. The composition of the CIGS solar cells has a large influence on the long-term stability. Examples of studies on the influence of the composition on the stability can for example be found in references16,20. Since the exact nature of the influence of many small modifications in the solar cell stack is not yet identified, degradation might occur faster or slower than expected.
The above factors indicate that a large number of accelerated lifetime studies with variation in degradation conditions and sample composition is required to truly predict module field performance. Moreover, these results should therefore be combined with field studies to obtain a complete picture about the long-term stability of PV modules.
However, we propose that the setups presented in this study are substantial improvements compared to the standard IEC tests, due to the combined stress exposure as well as in situ monitoring. These properties greatly improve the predictive value of accelerated lifetime experiments and increase our understanding of degradation mechanisms. The four main advantages compared to 'standard' (e.g., IEC 61215) tests are the following capabilities:
a. Testing under exposure to combined stresses (i.e., temperature, humidity, illumination, and electrical biases).
b. Tuning of combined stresses in order to simulate local climates (e.g., desert or polar conditions).
c. Tuning of electrical biases, e.g., to simulate effects of partial shading.
d. Real-time monitoring of the device performance, allowing simpler and faster testing as well as better prediction or limitation of the degradation mechanisms due to an increased knowledge level.
e. Reduced testing time, since a test can be stopped directly after a failure has occurred, instead of after the defined test period (e.g., 1,000 h).
It is therefore proposed that lifetime studies with the presented setups can greatly improve the qualitative and quantitative understanding and prediction of long-term stability of solar cells and modules. In the future, a setup offering 'Combined Stress tests with in situ measurements' (CSI) for full scale modules will be developed: the setups with illuminated areas of 40 cm x 40 cm and 100 cm x 100 cm are too small for full-size PV modules, so plans to increase the scale of this combined stress measurement concept are underway.
The authors have nothing to disclose.
The authors would like to thank Miro Zeman (Delft University of Technology) and Zeger Vroon (TNO) for the fruitful discussions. Kyo Beyeler, Vincent Hans, Ekaterina Liakopoulou, Soheyl Mortazavi, Gabriela de Amorim Soares (all TNO), Felix Daume (Solarion), and Marie Buffière (IMEC) are acknowledged for the sample deposition and analysis and the long discussions. Furthermore, we would like to thank all employees from Eternal Sun, Hielkema Testequipment, and ReRa Solutions, and more specifically Robert Jan van Vugt, Alexander Mulder and Jeroen Vink for their contribution.
These studies were carried out under project number M71.9.10401 in the framework of the Research Program of the Materials innovation institute M2i, TKI IDEEGO project TRUST, the project PV OpMaat, financed by the cross border collaboration program Interreg V Flanders-Netherlands with financial support of the European Funds for Regional Development and the TNO ‘Technologie zoekt Ondernemer’ program.
Hybrid degradation setup | Eternal Sun | Climate Chamber Solar Simulator | More information can be found here: http://www.eternalsun.com/products/climate-chamber/ |
Sample holders | ReRa Solutions | More information can be found here: https://www.rerasolutions.com/ | |
Sample rack | Demo Delft | More information can be found here: http://www.demo.tudelft.nl/ | |
Gold deposition tool | Polaron Equipment LTD | SEM coating unit E5100 | Tool for Au deposition for SEM measurements |
Tracer IV software | ReRa Solutions | More information can be found here: https://www.rerasolutions.com/product/tracer-iv-software/ | |
Solar cells | Solliance | More information can be found here: http://www.solliance.eu. Solar cells and modules can also be obtained from many other universities, research institutes and companies |
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PL mapping setup | GreatEyes | LumiSolarCell | |
ILIT mapping setup | Infratec | ImageIR camera and Sunfilm IR lens | |
Optical microscopy | Leica | Wild M400 | coupled with a Leica DFC 320 camera and Leica Application Suite software, version 4.3.0 |
IV tester | OAI | OAI TriSol Solar Simulator | coupled with a Keithley SourceMeter 2400 and controlled using IV runner software, version 1.4.0.6. |
EQE tester | Homemade |