All electrochemical layer deposition for crystalline silicon solar cell manufacturing: Experiments and interpretation

Abstract

Abstract A manufacturing process for crystalline silicon solar cells is presented which consists mainly of electrochemical steps. The deposition of doping glass layers for the front side emitter as well as the back surface field is performed anodically onto the etched and cleaned wafers. The doping atoms, phosphorus or boron, are diffused into the silicon crystal in a furnace at 950 °C in an atmosphere of simply clean air. After the diffusion process the front side doping glass has a blue colour and is suitable to serve as an antireflection coating with a very low surface recombination velocity. For this reason, the doping glass is not etched away on the sun exposed regions of the solar cell. The masking technology for all electrochemical processes provides inherently an edge exclusion and, therefore, no additional processing for preventing shorts on the wafer edge is necessary. For the metallization a reusable rubber mask defines the pattern. First, the mask is used for the doping glass patterning by wet chemical etching. Then, on both sides first nickel is deposited electrolytically directly onto silicon, and in a second step copper electroplating onto the nickel barrier is performed. All three steps, etching, nickel and copper deposition are self adjusting through said rubber mask. A short forming gas anneal finishes the solar cell processing. During all electrochemical processing the wafer is electrically contacted on the opposite surface on a stainless steel plate by the force of vacuum clamping. With this low cost processing 12.5% cell efficiency has been achieved on multi-crystalline 156 mm wafers, which originally have a minority carrier lifetime of 4 μs measured after damage etch and thermal oxidation. In this paper, experiments, surface analysis and physical interpretations are presented.