Experimental Fatigue Investigation of Solar Cell Interconnectors in Module Laminates

© Fraunhofer CSP
A) measuring arrangement for fatigue tests on minimodules, b) load profile, c) current course during the test, d) EL recordings after defined cycles.

In a crystalline solar module the cells are connected in series with solar cell interconnectors (solder coated copper ribbons). Ribbon fatigue is critical for solar modules because the interconnection has direct influence on series resistance. Thus ribbon breakage leads to power loss, locally increased temperatures and even arcing. Loading of the ribbons results from relative displacement of the solar cells due to temperature changes or mechanical loading. The amount of displacement and frequency depends among other things on climatic region as well as installation conditions and therefore influences the loading of the ribbon and long-term module performance. The opening of new markets in extreme climates (e.g. desert) and the associated different loading conditions requires an enhancement of the only sporadic empirical knowledge on ribbon fatigue in solar industry. Fraunhofer CSP deals with characterization of ribbon materials and ribbon fatigue in solar module laminates with experimental methods and numerical simulation.

The group Module Reliability has developed an assembly for cyclic mechanical testing of small modules in a four point bending test setup. The test setup allows an in-situ current measurement and regular electroluminescence (EL) images during different mechanical load steps. In this manner broken interconnectors are identified and cell breakage can be investigated. With help of finite element simulations the loading profile is designed to achieve the same cell displacement in the cell gaps as the maximum amplitude found in a full size module simulation under ±1000 Pa cyclic loading, which corresponds to the current IEC proposal.

This way ribbon fatigue based on the IEC proposal can be tested in a much faster way, which enables a much more economic comparison of different ribbon materials, geometries and their dependence on module production steps and other module components.