Simulation and characterization of epitaxial n- and p-type emitters on silicon wafer solar cells

Epitaxial emitters deposited by atmospheric pressure CVD have been studied using different characterization methods such as spreading resistance profiling (SRP) or electrochemical capacitance voltage profiling (ECV). Comparing simulations with measured data, very low minority carrier lifetimes of τeff = 20 μs have been determined for FZ-material after a standard epitaxy process. LBIC measurements showed very low and inhomogeneous distributed diffusion lengths in the range of 100-250 μm before process optimization. Therefore an emitter epitaxy process working at lower temperatures was developed [1] leading to diffusion lengths of 750 μm. The effective minority carrier lifetimes and emitter saturation currents were determined using QSSPC measurements. For samples with an emitter, deposited at optimized temperature, a best value for the effective minority carrier lifetime of τeff = 212 μs was measured and an emitter saturation current could be determined to J0e= 46 fA/cm2. In order to investigate the stress in the epitaxial layers μRaman and μPL measurements were performed showing no increased values at the interface. The doping density in boron doped emitters was determined with μPL and in good agreement with measured ECV-profiles. The Shockley-Read-Hall lifetime in the emitter bulk showed no increased values at the interface between substrate and epitaxial layer.