Cell-Module Integration Concept Compatible with c-Si Epitaxial Thin Foils and with Efficiencies over 18%

27th European Photovoltaic Solar Energy Conference and Exhibition; 2207-2211
There are several reasons why key actors in the PV industry target thinner Si cells. It is an unresolved question whether stand-alone wafer processing will ever become a yielding production process on wafers thinner than ~80 μm. The approach proposed in this paper, termed i2-module, is targeted to back contact cells with a thickness down to 40 μm. The i2- module concept, aims at interconnecting very thin (~ 40 μm) back-contact cells. This is a disruptive approach starting from (epitaxial) Si foils which are silicone-glued to the module. Subsequent process steps like patterning, BSF formation and metallization, which would be critical to execute on stand-alone wafers of this thickness, are executed on bonded tiles of wafers, at the “module level”. We report here experimental results both related to cell processing and to thin wafer supply. The process development has been initiated using 170 μm thick FZ Si wafers as test platform. Front side of the wafers were processed according to imec’s i2BC baseline including a back side emitter, afterwards they were subsequently silicone- ’glued’ to the module. Further rear-side processing is done while the substrates are bonded to the module glass and should therefore have to be compatible with the glass and silicone glue. Epitaxial foils are investigated as a “supply” of very thin Si wafers. Processes have been developed for growing these 40 μm thick foils on top of a weak porous silicon layer. Those foils are transferred to the module glass after front side processing is finished, using a silicone-based glue. Afterwards the parent substrate can be removed and the rear-side processes developed at this moment for reference FZ bulk wafers will be applied on these bonded foils.