Reliable characterization techniques to accurately quantify the metallization-induced recombination losses as well as contact resistivity losses of screen-printed cells are crucial for successful optimization of the contact grid design. Previously, the dark saturation current density at the contact ( ${J}_{\rm 0c}$ ) is often assumed to be constant for different finger width. Similarly, impact of finger width on contact resistivity ( ${\rho }_{\rm c}$ ) is rarely reported. Therefore, we performed a comprehensive evaluation of ${J}_{\rm 0c}$ and ${\rho }_{\rm c}$ as a function of finger width, spacing as well as firing temperature. We found out that ${J}_{\rm 0c}$ increases from $\approx$ 2000 to $\approx$ 8100 $\text{fA/cm}^{2}$ , when the finger width increases from 60 to 400 $\mu$ m; and ${\rho }_{\rm c}$ decrease from 7.2 to 2.2 $\text{m}{\Omega }\cdot \text{cm}^{2}$ when using a wide-TLM rather than a narrow-TLM structure, for samples fired at 840 $^{\circ }$ C. Based on our cross-sectional and top-down scanning electron microscopy images, we believe that the physical root cause can be explained by the difference in the microstructure formed at the metal–silicon interface during the firing process for the screen-printed contacts.

The ultraviolet fluorescence (UVF) imaging method has been widely used as a rapid and economic field inspection tool for investigating encapsulant discoloration of field-aged photovoltaic (PV) modules. In field-aged PV modules, encapsulant discoloration can result in a pronounced decrease in the short-circuit current. The spatial distribution of environmental stressors and therefore discoloration pattern over each cell of a module depends on the climate. In this article, we investigated the discoloration patterns (and their correlation with electrical parameters of modules and the extracted-cells from the modules) of ten modules produced at the same manufacturing facility and retrieved respectively from two distinct climates of Arizona (hot-dry; seven modules; 18 years of exposure) and Florida (hot-humid; three modules; ten years of exposure). In the Arizona modules, three distinct spatial patterns were observed in the UVF images: cell center region (intense browning); cell edge region (no browning); region between cell center; and cell edge region (light browning). However, in the Florida modules, only two distinct spatial distributions were observed: nonedge cell region (medium browning) and cell edge region (no browning). Furthermore, we employed a high-contrast UVF imaging technique to correlate the interdependence between the encapsulant discoloration pattern and the metallization/contact degradation pattern. For this correlative study, various experimental characterizations were carried out on both field-aged (Arizona and Florida) and unexposed modules and the cells extracted from the modules of the same model and they include: processed UVF and EL images, and I-V, TLM, optical microscopy, and XPS measurements.

A system of 180 monocrystalline aluminum back-surface field modules were installed in Cocoa, Florida, for 10 years. In total, 156 modules are characterized and compared to 3 controls. Power degradation rates vary between $-$ 0.14% to $-$ 3.22% per year, with median and average rates of −0.92% and −1.05% per year, respectively. The losses are primarily resistive with minor optical and recombination loss contributions. Electroluminescence imaging shows a characteristic pattern, which is shown to be resistive in nature when compared to photoluminescence. Resistive losses are due to corrosion of the rear contact Ag/solder interface and, to a much lesser degree, gridline Ag oxidation. Moisture ingress through the backsheet is likely responsible for mediating corrosion. Optical losses are due mostly to a combination of antireflection coating degradation, minor encapsulant browning, and delamination. Minor front contact corrosion may contribute to recombination. This study expands upon previous work on this vintage of the module by examining a large sample set, comprehensive characterization including techniques not previously used on these modules, and a comparison between two other systems of different climates.

The current popularity of photovoltaic (PV) systems is due in large part to their exceptional reliability and significantly lower cost than other energy sources. Studying cell and module degradation is key to promote further development in the state of the art. Fielded oraccelerated aged modules exhibit different failure modes, of which metallization degradation (contacts and interconnections) is prevalent. In this work, we discuss how multiscale characterization methods can be applied to a variety of module technologies that have been field exposed and have undergone accelerated age testing. These methods include performing characterization on the module level, cell level, and finally the materials level. The observed performance losses from the module- and cell-level characterization can be correlated with materials properties to find out the root cause of degradation. We recommend an initial nondestructive characterization suite, including module- and cell-level current–voltage ( $I\text{--}V$ ), Suns– V _OC , photoluminescence and electroluminescence imaging, quantum efficiency, ultraviolet fluorescence imaging, and thermal infrared imaging. Samples are then extracted from particularly degraded regions of the module and prepared for materials characterization techniques, such as top-down and cross-sectional scanning electron microscopy, energy-dispersive X-ray spectroscopy, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, Raman spectroscopy, and transmission electron microscopy, allowing a deeper look into the mechanism behind the metallization degradation. This article serves as an instructional review to introduce the different multiscale characterization methods and how they can be effectively applied to perform PV degradation studies. We also share some of our examples and discuss the strengths, limitations, and best practices for each of the characterization techniques.