Reduction of breakage losses in silicon-cell processing

Silicon solar cells are industrially produced from thin silicon wafers. Currently the thickness of these wafers is in the range of 300 µm. Silicon is a relatively expensive material. About the half of the production costs of a silicon-cell module is due to the silicon wafers. Silicon is also brittle and fragile. During the manufacturing of wafers, cells and modules, the silicon disks are subjected to numerous processes implying substantial mechanical or thermal loadings such as sawing, etching, doping, printing, firing and handling procedures. Therefore significant economic losses can occur due to fracturing of the brittle silicon. Savings concerning this expensive material, therefore, entail a great potential for cost reductions. A wafer breaks, when the applied tensile stress exceeds the strength of the wafer. The strength of a wafer is predominantly controlled b y the conditions regarding micro-damages in its surface whereas bulk material properties or structural parameters are of minor significance, as is shown in a previous paper. From these elementary facts it can be concluded, that for an optimisation of manufacturing processes with respect to breakage loss reduction two a spects must be considered: The prevention of formation or growth of micro-damage s as well as the decrease of wafer impacting stresses. For the optimization of processes an integral concept was developed and applied in industrial manufacturing lines of three silicon-cell producers. This concept combines different approaches: - Strength analysis to study the influence of particular process steps onto the mechanical strength of the wafers and cells - Damage analysis to investigate the formation and evolution of damages and fractures - Process analysis to determine the mechanical or thermo-mechanical loading of the wafers in fracture relevant manufacturing processes - Test methods to develop practicable methods for strength characterisation as well as for damage detection which are capable to eliminate damaged wafers early in the value-added chain.