Influence of the absorber doping for p-type polycrystalline silicon thin-film solar cells on glass prepared by electron beam evaporation and solid-phase crystallization

Abstract

A systematic investigation of the influence of the absorber doping on the performance of planar, p-type, evaporated, solid-phase crystallized polycrystalline silicon thin-film solar cells on glass is presented. It is found that the optimum Suns-Voc parameters (open-circuit voltage and pseudo fill factor) are achieved at intermediate absorber doping of Nabs∼1–2 × 1017 cm−3, while high short-circuit currents are achieved at the lowest absorber doping of Nabs ≤ 6 × 1015 cm−3. Since the short-circuit current is the dominating factor to achieve high conversion efficiencies for evaporated polycrystalline silicon cells, the maximum pseudo efficiencies are achieved at very low absorber doping. The Suns-Voc characteristics of lightly doped cells can be adequately described by a modified two-diode model with n1=1 and n2≈1.5, which is in contrast to the value of 2 for n2 commonly quoted in the literature. PC1D modeling demonstrates that such a low ideality factor for space charge region recombination can be modeled by a single trap energy level located at ∼0.18 eV away from midgap. Although the achievable short-circuit current densities and the conversion efficiencies can be higher for textured cells, planar cells are chosen intentionally to allow accurate modeling and extraction of relevant material parameters, such as minority carrier diffusion length.