Boundary conditions for the industrial production of LFC cells
The 'Laser-Fired Contact' (LFC) process scheme developed at Fraunhofer ISE is a fast, easy applicable and industrially feasible way of introducing the `Passivated Emitter and Rear Cell' (PERC) structure into solar cell production. Because the standard industrial production scheme needs to be adapted to the LFC process several further aspects have to be taken into account. It is already well known that thermal influence like an annealing step is beneficial for some dielectric layers as well as for LFC. Therefore experiments have been carried out to investigate the influence of temperature, duration and ambient atmosphere of a final annealing step on the performance of LFC rear sides. Results showed that in contrast to the standard ISE clean room process the solar cell efficiencies did not deteriorate by using a low-cost and contaminated metal belt furnace instead of a clean quartz tube. Additionally not even the forming gas ambient was necessary enabling the use of cheaper gases like nitrogen or cleaned pressure air. The duration and temperature can be adapted to the requirements of other process steps, since short and hot conditions result in the same efficiencies as cool and long ones. Furthermore the standard LFC process was adapted to the use of thicker aluminum layers as well as aluminum/silver stack layer systems. Pure aluminum does not enhance the LFC quality but enables lower conduction losses, the stack system is necessary for standard soldering processes for module integration. Both layers can be used with no loss in solar cell performance.