Showing papers by "Peter Hacke published in 2017"

Potential-induced degradation in photovoltaic modules: a critical review

Abstract: Potential-induced degradation (PID) has received considerable attention in recent years due to its detrimental impact on photovoltaic (PV) module performance under field conditions. Both crystalline silicon (c-Si) and thin-film PV modules are susceptible to PID. While extensive studies have already been conducted in this area, the understanding of the PID phenomena is still incomplete and it remains a major problem in the PV industry. Herein, a critical review of the available literature is given to serve as a one-stop source for understanding the current status of PID research. This paper also aims to provide an overview of future research paths to address PID-related issues. This paper consists of three parts. In the first part, the modelling of leakage current paths in the module package is discussed. The PID mechanisms in both c-Si and thin-film PV modules are also comprehensively reviewed. The second part summarizes various test methods to evaluate PV modules for PID. The last part focuses on studies related to PID in the omnipresent p-type c-Si PV modules. The dependence of temperature, humidity and voltage on the progression of PID is examined. Preventive measures against PID at the cell, module and system levels are illustrated. Moreover, PID recovery in standard p-type c-Si PV modules is also studied. Most of the findings from p-type c-Si PV modules are also applicable to other PV module technologies.

In-Situ Characterization of Potential-Induced Degradation in Crystalline Silicon Photovoltaic Modules Through Dark I–V Measurements

Abstract: A temperature correction methodology for in-situ dark I–V (DIV) characterization of conventional p-type crystalline silicon photovoltaic (PV) modules undergoing potential-induced degradation (PID) is proposed. We observe that the DIV-derived module power temperature coefficient $(\gamma _{{\rm{dark}}})$ varies as a function of the extent of PID. To investigate the relationship between $\gamma _{{\rm{dark}}}$ and DIV-derived module power $(P_{{\rm{dark}}}(T_{s})$ , measured in situ and at the stress temperature) two parameters are defined: change in the DIV-derived module temperature coefficient $(\Delta\gamma _{{\rm{dark}}})$ and DIV-derived module power degradation at the PID stress temperature $(\Delta P_{{\rm{dark}}}(T_{s}))$ . It is determined that there is a linear relationship between $\Delta\gamma _{{\rm{dark}}}$ and $\Delta P_{{\rm{dark}}}(T_{s})$ . Based on this finding, we can easily determine the module $\gamma _{{\rm{dark}}}$ at various stages of PID by monitoring $P_{{\rm{dark}}}(T_{s})$ in situ . We then further develop a mathematical model to translate $P_{{\rm{dark}}}(T_{s})$ to that at 25 °C $(P_{{\rm{dark}}}({\text{25}}\, ^\circ {\text{C}}))$ , which is correlated with the module power measured at the standard testing conditions ( P STC). Our experiments demonstrate that, for various degrees of PID, the temperature correction methodology offers a relative accuracy of ±3% for predicting $P_{{\rm{STC}}}$ . Furthermore, it reduces the root-mean-square error (RMSE) by around 70%, compared with the $P_{{\rm{STC}}}$ estimation without the temperature correction.

Notice of Removal Sodium accumulation at potential-induced degradation shunted areas in polycrystalline silicon modules

Abstract: We investigated potential-induced degradation (PID) in silicon mini-modules that were subjected to accelerated stressing to induce PID conditions. Shunted areas on the cells were identified with photoluminescence and dark lock-in thermography (DLIT) imaging. The identical shunted areas were then analyzed via time-of-flight secondary-ion mass spectrometry (TOFSIMS) imaging, 3-D tomography, and high-resolution transmission electron microscopy. The TOF-SIMS imaging indicates a high concentration of sodium in the shunted areas, and 3-D tomography reveals that the sodium extends more than 2 μm from the surface below shunted regions. Transmission electron microscopy investigation reveals that a stacking fault is present at an area identified as shunted by DLIT imaging. After the removal of surface sodium, tomography reveals persistent sodium present around the junction depth of 300 nm and a drastic difference in sodium content at the junction when comparing shunted and nonshunted regions.

Qualification Testing versus Quantitative Reliability Testing of PV – Gaining Confidence in a Rapidly Changing Technology

Abstract: Continued growth of PV system deployment would be enhanced by quantitative, low-uncertainty predictions of the degradation and failure rates of PV modules and systems. The intended product lifetime (decades) far exceeds the product development cycle (months), limiting our ability to reduce the uncertainty of the predictions for this rapidly changing technology. Yet, business decisions (setting insurance rates, analyzing return on investment, etc.) require quantitative risk assessment. Moving toward more quantitative assessments requires consideration of many factors, including the intended application, consequence of a possible failure, variability in the manufacturing, installation, and operation, as well as uncertainty in the measured acceleration factors, which provide the basis for predictions based on accelerated tests. As the industry matures, it is useful to periodically assess the overall strategy for standards development and prioritization of research to provide a technical basis both for the standards and the analysis related to the application of those. To this end, this paper suggests a tiered approach to creating risk assessments. Recent and planned potential improvements in international standards are also summarized.

Further Studies on the Effect of SiNx Refractive Index and Emitter Sheet Resistance on Potential-Induced Degradation

Abstract: We present the impacts of silicon nitride (SiN x ) antireflection coating refractive index and emitter sheet resistance on potential-induced degradation of the shunting type (PID-s). Previously, it has been shown that the cell becomes more PID-s-susceptible as the refractive index decreases or the emitter sheet resistance increases. To verify the effect of refractive index on PID-s, we fabricated cells with varying SiN x refractive index (1.87, 1.94, 2.05) on typical p-type base solar cells with ∼60 Ω/sq emitters. However, none of these cells showed output power degradation, regardless of the refractive index. Further investigation of the emitter showed that the PID-s was suppressed at ∼60 Ω/sq due to the extremely high surface phosphorus concentration (6 × 1021 cm−3), as measured by secondary ion mass spectrometry. Furthermore, PID-s was observed on cells possessing a high emitter sheet resistance (∼80 Ω/sq). The emitter surface phosphorus concentration plays an important role in determining PID-s susceptibility.

Quantification of solar cell failure signatures based on statistical analysis of electroluminescence images

Abstract: We demonstrate a method to quantify the extent of solar cell cracks, shunting, or damaged cell interconnects, present in crystalline silicon photovoltaic (PV) modules by statistical analysis of the electroluminescence (EL) intensity distributions of individual cells within the module. From the EL intensity distributions (ELID) of each cell, we calculated summary statistics such as standard deviation, median, skewness and kurtosis, and analyzed how they correlate with the magnitude of the solar cell degradation. We found that the dispersion of the ELID increases with the size and severity of the solar cell cracks, correlating with an increase in standard deviation and decrease in kurtosis. For shunted cells, we found that the ELID median is strongly correlated with the extent of cell shunting. Last, cells with damaged interconnect ribbons show current crowding and increased series resistance regions, characterized by increased dispersion and skewness of the ELID. These cell-level diagnostic parameters can be used quantify the level of mismatch between the solar cells in the module, which can represent the extent of the module degradation, due to transportation, installation, or field operation. The method can be easily automated for quality control by module manufacturers or installers, or as a diagnostic tool by plant operators and diagnostic service providers.

Process Induced Deflection and Stress on Encapsulated Solar Cells

Abstract: Cell cracking is one of the most common factors that limit the lifetime of PV modules. Until now electroluminescence (EL) has been the tool of choice to inspect cracks in finished modules. However, there are intrinsic limitations to the size of the cracks that this technique can resolve making it complicated to study the origins of crack formation. We also argue that the process of module assembly today is optimized from the point of view of the inactive materials (e.g. encapsulant cross-linking) offering no insights into the solar cell status. To this regard, even the correlation of module degradation to cell cracking in flat modules is only evident when cracks are of detectable size and detrimental to the electrical performance as measured by EL or PL. We have shown that in-house X-ray topography (XRT) is a unique technology that provides a nondestructive way of assessing the mechanical state of encapsulated solar cells, not only the evaluation of cracks and microdefects developed during handling and outdoor operation, but also the analysis of intrinsic deflection and stress induced by materials and processes (e.g. soldering, lamination). In this contribution, we present results on the deflection of the solar cells caused by different processing temperatures, various lamination materials, accelerated testing as well as metal ribbon.

Correction for Metastability in the Quantification of PID in Thin-Film Module Testing

Abstract: A fundamental change in the analysis for the accelerated stress testing of thin-film modules is proposed, whereby power changes due to metastability and other effects that may occur due to the thermal history are removed from the power measurement that we obtain as a function of the applied stress factor. In this work, initial thermal treatment of the module is performed before application of the independent variable stress of system voltage so that any temperature-dependent processes (e.g., diffusion) that affect the module power are largely activated beforehand. Secondly, the power of reference modules normalized to an initial state-undergoing the same thermal and light exposure history but without the applied stress factor such as humidity or voltage bias-is subtracted from that of the stressed modules. For better understanding and appropriate application in standardized tests, the method is demonstrated and discussed for potential-induced degradation testing in view of the parallel-occurring but unrelated physical mechanisms that can lead to confounding power changes in the module.

Investigating PID Shunting in Polycrystalline Silicon Modules via Multi-Scale, Multi-Technique Characterization

Abstract: Here we present a method for calculating the TCO sheet resistance of complete thin film modules using EL imaging. The method uses the characteristic decay of the junction voltage over the width of a cell stripe to derive four simple analytic equations for the TCO sheet resistance and the internal and contact resistances. Experimentally, only the module dark JV curve, Jsc-Voc curve, and electroluminescence imaging is required. Unlike previous methods, the internal and contact resistances of the modules are accounted for, and the method does not require curve fitting. The TCO sheet resistances of four 10cm × 10cm amorphous silicon modules are calculated and show excellent agreement with the experimentally measured values.

Automatic Detection of Inactive Solar Cell Cracks in Electroluminescence Images

Abstract: Inactive solar cell regions resulted from their disconnection from the electrical circuit of the cell are considered to most severe type of solar cell cracks, causing the most power loss. In this work, we propose an algorithm for automatic determination of the electroluminescence (EL) signal threshold level corresponding these inactive solar cell regions. The resulting threshold enables automatic quantification of the cracked region size and estimation of the risk of power loss in the module. We tested the algorithm for detecting inactive cell areas in standard mono and mc-Si, showing the influence of current bias level and camera exposure time on the detection. Last, we examined the correlation between the size of the detected solar cell cracks and the power loss of the module.

Photoluminescence-imaging-based Evaluation of Non-uniform CdTe Degradation

Abstract: Photoluminescence (PL), electroluminescence (EL), and dark lock-in thermography are collected during stressing of a CdTe module under one-Sun light at an elevated temperature of 100°C. The PL imaging system is simple and economical. The PL images show differing degrees of degradation across the module and are less sensitive to effects of shunting and resistance that appear on the EL images. Regions of varying degradation are evaluated using time-of-flight secondary ion-mass spectrometry, and there is a correlation between PL intensity and Cu concentration at the front interface.

Large-Area Junction Damage in Potential-Induced Degradation of c-Si Solar Modules

Abstract: We report a large area of millimeter-scale p-n junction damage caused by potential-induced degradation (PID) of lab-stressed crystalline-Si modules. Kelvin probe force microscopy results show electrical potential change across the junction, and a recovery was observed after heat treatment. Electron-beam induced current results support the large-area damage instead of local shunts and a much lower collected current for the affected junction area. Furthermore, secondary-ion mass spectrometry indicates that the large-area damage correlates with sodium contamination. The consistent results shed new light on PID mechanisms to investigate that are essentially different than the widely reported local junction shunts.

Field Investigations of Potential-Induced Degradation (PID) for Crystalline Silicon PV Panels in Different Climates

Abstract: The relationship between field performance and indoor chamber measurement for potential-induced degradation in crystalline silicon PV modules is still unclear. Many acceleration models have been proposed but without strong evidence from field experiments. Our approach focuses on building field testing systems under three different climates: tropical, subtropical, and mountain climates in Singapore, Changzhou (China) and Golden (CO, USA) respectively, in a collaborative testing campaign involving Trina, SERIS and NREL. Each testing system has six groups of modules developed by Trina Solar with different grades of PID resistance from weak to strong. The objective of this 3-year project is to build up the correlation between the outdoors behaviors to chamber tests, and deduce a climate-specific degradation model. A degradation of up to 23% was observed after 18 weeks for the most PIDsusceptible module was found in Singapore. Activation energy of 0.94-1.1eV and 0.55-0.63eV were determined for wet and dry conditions, respectively, by fitting the leakage current data in Singapore and in Golden, and an exponential decay of degradation was shown as a function of accumulated charge.