Interplay of annealing temperature and doping in hole selective rear contacts based on silicon-rich silicon-carbide thin films
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We present a detailed optimization of a hole selective rear contact for p-type crystalline silicon solar cells which relies on full-area processes and provides full-area passivation. The passivating hole-contact is based on a layer stack comprising a chemically grown thin silicon oxide, an intrinsic silicon interlayer, and an in-situ boron doped non-stoichiometric silicon-rich silicon-carbide layer on top. After deposition, the structure is annealed at 775-900 degrees C to diffuse dopant impurities to the c-Si wafer and a hydrogenation step is carried out. It is shown that hydrogenation is essential to obtain high quality surface passivation. In particular, we compare the effect of annealing in forming gas and annealing with a silicon-nitride overlayer as hydrogen source. We present a systematic optimization of the hole-selective contact, for which we varied the doping concentration, annealing parameters and report the implied open circuit voltage (iV(oc)) and combined specific contact resistivity (p(c)). It is observed that for highly doped layers the optimum annealing temperature for high quality surface passivation is 800 degrees C while for lowly doped layers the optimum annealing condition shifts to 850 degrees C. Excellent surface passivation and efficient current transport is evidenced by an iV(oc) value of 718 mV which corresponds to a saturation current density (J(0)) of 11.5 fA/cm(2) and a p(c) of 17 mg Omega cm(2) on p-type wafers. Moreover, the evolution of the boron diffusion profiles with different annealing conditions is investigated. Finally, we demonstrate proof-of concept p-type hybrid solar cells employing the full-area hole-selective rear contact presented here and standard heterojunction front electron contact. The excellent efficiency potential of our passivating rear contact is highlighted by conversion efficiencies up to of 21.9%, enabling V-oc of 708 mV, FF of 79.9% and J(sc) of 38.7 mA/cm(2).
Solar Energy Materials and Solar Cells, vol. 173, pp. 18-24, Dec 2017.