MilestonesRegarding the wet chemical etching steps – chemical edge isolation, Si wrap-around etch, and BSG removal – in the TOPCon fabrication process, the research institute established a cluster of inline and batch-type wet chemical etch processes in collaboration with RENA. This high-throughput clustered process includes an inline, fast single-side oxide removal on the wafer, with the rear side protected by a water capping during the process. It is followed by a batch-type rear-side etching process. According to Fraunhofer ISE, this high-throughput clustered etching process offers a cost-effective wraparound solution for the industry.However, out of the 3 existing routes – ex-situ doping in LPCVD, in-situ doping for both PECVD and PVD – for TOPCon’s rear-side surface passivation layer deposition, the institute demonstrated the advantages of in-situ doping PVD tunnel oxide formation and PVD Si layer deposition in collaboration with VON ARDENNE. Moreover, Wolf stated that hydrogen management such as hydrogen concentration and distribution is essential to achieve good passivation. He also raised concern about recent trends of TOPCon cell degradation under UV light.The LECO process, which decoupled the contact resistance from the contact recombination parameters, was first applied by the German institute to TOPCon cells. According to Wolf, the LECO process produces high-quality contacts with very low recombination parameters, due to the formation of small current-fired contacts and the fact that most of the interface under the silver finger remains passivated by the passivated layers. This latest metallization process enables the usage of aluminum-free silver paste featuring much lower firing temperatures and boron diffusion parameters with lower diffusion temperatures compared to its peers for cost-effective process steps.Fraunhofer ISE’s baseline technology for TOPCon currently achieves 24.5% efficiency on M2 wafers and has recently scaled all tools to the M10 format, delivering 24% efficiency, noted the institute.Other Technological AdvancementsTo meet the demand for silver paste cost optimization in the metallization process, the German R&D lab has recently achieved a champion cell efficiency of up to 24% on the first pilot line for M10-sized TOPCon solar cells with plated Ni/Cu metallization. The lab has also developed passivated edge technology (PET), which passivates the edges of half-cut cells by depositing an Al2O3 layer followed by a subsequent annealing step.Emerging TechnologiesIn terms of emerging technologies in the TOPCon cell development route, Fraunhofer ISE has advanced the TOPCon Rear Emitter (TOPCoRE) structure by omitting the front-side p+ emitter and replacing the n-type wafer with a p-type wafer. This development has recently achieved up to 26% efficiency and up to 732 mV Voc for a 2 x 2 cm² TOPCoRE cell.Additionally, Fraunhofer ISE shared updates on progress in multi-junction tandem cells featuring the TOPCon cell structure as the bottom cell—specifically, inverted TOPCon and TOPCon². The lab has demonstrated an III-V/Silicon solar cell with a TOPCon² structure, achieving a record efficiency of up to 36.1%.
