High-Speed characterization of electrically conductive adhesives for industrial SHJ solar cell interconnection

Electrically conductive adhesives (ECAs) are an alternative to soldering technology for solar cell interconnection, offering low-temperature and lead-free processing. ECAs are especially relevant for novel, often temperature sensitive high-efficiency cell types such as silicon heterojunction technology (SHJ). ECA suppliers offer a large number of products with different characteristics. In the current work most important ECA properties and their thresholds relevant for industrial string production are identified. They are compiled into an efficient testing scheme, which allows quick pre-selection of products suitable for further tests in the production environment. The proposed scheme includes determination of curing time with differential scanning calorimetry (DSC), investigation of the mechanical stability of the bond with a 90° peel test, one-cell modules production and their I-V and EL measurements to assess initial ECA performance, damp heat (DH) and accelerated thermocycling (aTC) testing for early determination of severe material or design flaws. In order to illustrate the approach, four (I to IV) commercially available ECAs were tested and compared. DSC revealed that ECA-II and ECA-III cure quicker and at lower temperatures than ECA-I and ECA-IV. Peel test showed peel forces ranging between 0.2 to 1.2 N/mm. After production of one-cell modules no performance differences between the ECAs have been detected. Damp heat testing showed mostly uniform degradation throughout the variation. Accelerated thermocycling testing proved to subject modules to greater degradation than conventional TC, most likely due to 6 six times quicker temperature change in the aTC chamber. Cell breakage was abundant in glass-glass (GG) samples after both aTC and TC testing and did not occur after encapsulation material was changed and the aTC50 test was repeated. Results of the aTC50 test on modules with another encapsulation material suggest different reliability of the interconnection with different ECAs as well as generally better performance of glass-backsheet (GBS) modules in comparison to GG. After all tests and corresponding measurements had been finished, results were incorporated in a simple color-coded matrix to optimally compare values. In the current case, the product for further tests would be ECA-III, which is capable of curing at 140°C within 30 seconds and provides a mechanically stable joint. One-cell modules produced with it lost <1 %rel PMPP after DH500, and 1.5 (GBS) to 3.6 (GG) %rel PMPP after aTC50. The whole described testing procedure can be done within one month. However, in case no DH500 is required, testing and comparison scheme requires one week.