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Solar panels generally contact the mounting structure only along the surfaces of the aluminum frame surrounding the perimeter of the panel. With no support in the middle of the panel, front side mechanical loads from snow, wind, and human factors can cause significant deflection of the panel, resulting in tensile stress in the solar cells and cell cracking that can degrade system performance. This work examines an alternative approach whereby the mounting scheme is modified to place spacer elements between the rails of the support structure and the rear side of the panel. These spacers significantly reduce the panel deflection under load, and we have demonstrated a dramatic reduction in cell cracking at high load levels and in crack opening after cyclic loading. Such spacers can be applied to either the rear of the module or to the rails on the mounting structure and could be introduced for both new installations or as protective retrofits to existing systems. The spacers can also be of sufficient thickness to cause positive deflection of the panels to introduce some protective/restorative compressive stress into the cells.

Some crystalline silicon PV system underperformance is due to the presence of cracked solar cells, and this represents a financial risk for both already installed and future systems. This work describes compressive stress strategies and rear side pressure strategies that can be employed in new panel construction to prevent crack formation as front side mechanical loads from handling, wind, and snow are applied during shipping, installation and in the field. These strategies can also slow the opening of cracks and the related power loss for any cracks that do form. Furthermore, we present concepts for the retrofitting of older installed systems to close already open cracks and regain lost power, or to slow the future degradation of systems with panels that are sensitive to cracked cells.

Cell cracking presents a serious risk for the long term reliability of c-Si photovoltaic modules. Cracks may not initially result in performance loss, but over time performance may degrade as the module experiences stresses in the field such as temperature cycling and snow/ wind loading. This performance loss is due to the formation of new cracks with front side loading, propagation of existing cracks, and the opening of existing cracks in which regions of the cell become more electrically isolated. This work utilizes a new tool, the LoadSpot, that allows for I-V performance characterization and electroluminescence imaging of PV modules while under mechanical load. We explore a variety of cell technologies to understand the magnitude of mechanical stress required to induce cell fracture, and assess the impact these cracks have on performance. In addition, we study the use of cyclic loading to open existing cracks. The tests used in this work have potential applications in product development, factory quality control, product evaluations, and optimization of mounting hardware and methods.

Manufacturing, shipping & handling, installation, and in-field loading of photovoltaic solar panels are common contributors to the creation of cracks within the cells of a panel. Many cracks initially cause little or no power loss in the panel, but such tightly closed cracks may open over time due to environmental forces, and cause significant power loss and even failure of the module. We developed a method, using the LoadSpot tool, to apply a mechanical load to a panel to temporarily open pre-existing cracks while also allowing for electroluminescence (EL) imaging and flash IV testing. The change in the IV and EL measurements upon loading provides a quantifiable metric that can be used to evaluate reliability and durability. Such Predictive Crack Opening (PCO) tests have value in assessing preexisting damage as well as in the correlation with the degradation due to cracked cells opening upon environmental chamber and cyclic loading. We performed finite element modeling and simulation to illustrate the stresses applied at different load and mounting conditions. We demonstrate a wide range of mechanical loading and stress testing with accompanying EL and IV measurements which not only show the narrative of damage and power loss through static mechanical load, environmental chamber testing, and cyclic loading, but also suggests potential improvements which can be made to the order of chamber and cyclic load testing within the IEC 61215 standard. next.

Several methods have been determined for measuring the contact resistivity of solar cell devices, but these methods are all either destructive in nature or require the fabrication of special metal contacts. In this paper, we present a non-destructive method for measuring the contact resistivity of commercial grade solar cells using the circular transmission line method. We first determine an optimal method for probing the total resistance of these structures on a solar cell and investigate the importance of measuring the exact dimensions of each contact. Then, by comparing the results of the measurement to traditional TLM results, we select a proper geometry for the circular patterns so that they can be hidden within the busbars of finished cells and not affect cell efficiency or aesthetic. Good correlation is demonstrated between automated circular TLM measurements performed on a new tool, the ContactSpot-PRO, and traditional TLM measurements performed manually. The implementation of this high-speed tool can allow contact resistance to be measured on every R&D or production cell with only a minor change in front silver paste screen artwork.

The measurement of contact resistivity between the grid metallization of a solar cell and the underlying silicon wafer is most conveniently performed by cutting strips from solar cells rather than fabricating dedicated structures with variable spaced contacts. We studied the effect of strip width on the measurements and found the lowest values in the range of 10–15 mm. We found laser scribing conditions whereby strips could be successfully isolated on the emitter side without snapping strips from the rest of the cell.

Mechanical load tests are a commonly-performed stress test where pressure is applied to the front and back sides of solar panels. In this paper we review the motivation for load tests and the different ways of performing them. We then discuss emerging durability concerns and ways in which the load tests can be modified and/or enhanced by combining them with other characterization methods. In particular, we present data from a new tool where the loads are applied by using vacuum and air pressure from the rear side of the panels, thus leaving the front side available for EL and IV characterization with the panels in the bent state. Tightly closed cracks in the cells can be temporarily opened by such a test, thus enabling a prediction of panel degradation in the field were these cracks to open up over time. Based on this predictive crack opening test, we introduce the concept of using a quick load test on each panel in the factory as a quality control tool and potentially as a type of burn-in test to initiate cracks that would certainly form early on during a panel's field life. We examine the stresses seen by the cells under panel load through Finite Element Modeling and demonstrate the importance of constraining the panel motion during testing as it will be constrained when mounted in the field.

During testing of a new contact resistance measurement system (ContactSpot), we observed several unexpected results. TLM theory predicts a linear dependence of the resistance measurements vs probe spacing, and from this data set the contact resistance can be extracted. However, we found a non-linearity at wider probe spacing as well as sensitivities to the magnitude of the current, the direction of current flow, the ambient light level, and the choice of using contact pitch or spacing distances in the algorithm. Little appears in the literature concerning these effects and sensitivities. We found acceptable conditions for performing the TLM method in the dark, but we found more consistent results for contact resistance and emitter sheet resistivity values when using a previously developed algorithm that does not depend upon calculating slope and intercept values.

To reduce the cost of its String Ribbon™ solar panels, Evergreen Solar has developed automated laser cutting equipment that enables cutting silicon ribbon into wafers as it emerges from the growth furnaces. The laser source was chosen based on its ability to produce micro crack free cuts that result in high yield wafers. The system functional requirements are reviewed, and the design philosophy and design features are discussed. System implementation issues and its current status are presented.To reduce the cost of its String Ribbon™ solar panels, Evergreen Solar has developed automated laser cutting equipment that enables cutting silicon ribbon into wafers as it emerges from the growth furnaces. The laser source was chosen based on its ability to produce micro crack free cuts that result in high yield wafers. The system functional requirements are reviewed, and the design philosophy and design features are discussed. System implementation issues and its current status are presented.