ABSTRACT
As the power of photovoltaic modules continues to increase due to advances in commercial crystalline silicon solar cell technology, power losses in the tabbing ribbon used for series interconnection of cells become increasingly significant. The shading loss is directly proportional to the surface area of the tabs, while the resistive (I2R) loss is inversely proportional to the tab cross section. The use of a thick but narrow tabbing ribbon is therefore highly desirable to minimise both the I2R and shading loss components. In the present work, the losses in a typical high power (85Wp) module were modelled as a function of tabbing ribbon dimensions and the results were confirmed by experiment. A comparison of continuous and spot soldering of the tabs and the effect on the I2R loss component was also made. The suitability of commercially available copper tabbing ribbons was evaluated for use in a manufacturing process. The yield in the module assembly process was assessed and a number of modules were fabricated and tested. In an 85 Wp module of 36 cells, a reduction in the combined resistive and shading losses of 1.4 W was shown to be achievable by increasing the thickness of the continuously-soldered ribbon from 125 μm to 250 μm. In practice however, it was found that continuous soldering of the thicker tab caused an unacceptable degree of curvature in the wafer, due to mismatch of the thermal expansion coefficients of copper and silicon. The use of spot soldering reduced the curvature to an acceptable level, but contributed an additional resistive loss of 0.4 W. Therefore the net gain obtainable in practice by increasing the ribbon thickness from 125 to 250 μm was 1.0 W. In external lightsoak and accelerated environmental testing, the use of 250 μm interconnect ribbon was found to have no adverse effect on the module reliability. These results indicate that by the use of a tabbing ribbon of dimensions appropriate to the module power, a power gain of 1.2 % is obtainable.
