IBC cells are an attractive concept for a future cell generation since optical shading losses due to front-side metallization are avoided and the rear-side metallization can be completely silver-free. By application of novel and smart module interconnection concepts the prominent advantages of an IBC-cell are fully exploited: high voltage, extraordinarily high current density and nice appearance.
With conventional techniques, the process sequence for IBC cells is quite complex, since patterning steps and several furnace steps are required to form the locally doped regions on the rear side and the full area doping of the front side. Ion implantation offers a huge process simplification potential here, because implantation is per se single-sided and patterning is realized in-situ by inserting shadow masks into the ion beam. Moreover, only one high temperature step, the co-annealing of all irradiation defects, is required. Due to this drastic simplification of the process flow, ion implantation technology is disruptive to the IBC cell cost structure.
The focus of the program was to develop industrial-like IBC cells without the usage of lab-type techniques. For example, an industrial evaporation tool was used for PVD (physical vapor deposition) metallization and laser-based structuring rather than photolithography was applied.
Some alternative processes, especially passivation of the doped regions with aluminum oxide, their insulation after evaluation of various insulation layer stacks and PVD metallization using sputtering instead of evaporation were developed in the parallel publicly funded project "x?-Zellen Phase 2" in the frame of the Cluster "Solarvalley Mitteldeutschland."
The best energy conversion efficiency achieved for the pseudosquare 156mm×156mm solar cells on n-type Czochralski-grown Silicon wafers, supplied by Bosch SE, was measured to be 22.1 percent where the open circuit voltage (Voc) was 676.2mV, the short circuit current density (jsc) 41.6mA/cm2, and the Fill Factor (FF): 78.5 percent. This corresponds to a cell power of 5.32 Wpeak which is the highest value reported for a homojunction silicon solar cell.
Moreover, potential is identified for further improvements of Voc and FF.
The project results have shown the high efficiency potential of this solar cell concept and the patterned ion implantation as an enabling and economical doping technology.
