Interplay of annealing temperature and doping in hole selective rear contacts based on silicon-rich silicon-carbide thin films

Abstract

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°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 (iVoc) and combined specific contact resistivity (ρc). It is observed that for highly doped layers the optimum annealing temperature for high quality surface passivation is 800°C while for lowly doped layers the optimum annealing condition shifts to 850°C. Excellent surface passivation and efficient current transport is evidenced by an iVoc value of 718mV which corresponds to a saturation current density (J0) of 11.5fA/cm2 and a ρc of 17mΩcm2 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 Voc of 708mV, FF of 79.9% and Jsc of 38.7mA/cm2.