Progress in high efficiency processing of EFG silicon solar cells

Abstract

In contrast to standard block cast multicrystalline material the performance of EFG material is strongly related to an efficient hydrogenation step. In conformance to former publications the temperature profile during firing of a hydrogen-rich PECVD SiNx layer strongly affects the effectiveness of hydrogenation. With improved firing conditions 17.9% in efficiency could be reached on 2x2 cm² EFG solar cells using a photolithography based solar cell process. Minor improvements are further reached by a better passivated emitter at the front side of the solar cells, raising the efficiency to 18.2 %. In contrast to commonly used texturing methods plasma texturing at IMEC (meanwhile also at Konstanz) is able to further increase efficiencies on EFG material. A new solar cell process is available to replace the full area Al BSF (with moderate surface passivation quality) with a SiO2/SiNx stack of good surface passivation quality using laser fired contacts. The process is applicable for multicrystalline material including a good hydrogenation together with a high temperature stable passivation for the rear side. First results are encouraging on untextured solar cells (diced out to 2x2 cm²) resulting in 19.4 % efficiency on floatzone and 18.5 % efficiency on standard block cast multicrystalline material. It is planned to apply this process on EFG material to further increase the maximum efficiency for this material in the future.