Pinhole density and contact resistivity of carrier selective junctions with polycrystalline silicon on oxide

In the pursuit of ever higher conversion efficiencies for silicon photovoltaic cells, polycrystalline silicon (poly-Si) layers on thin silicon oxide films were shown to form excellent carrier-selective junctions on crystalline silicon substrates. Investigating the pinhole formation that is induced in the thermal processing of the poly-Si on oxide (POLO) junctions is essential for optimizing their electronic performance. We observe the pinholes in the oxide layer by selective etching of the underlying crystalline silicon. The originally nm-sized pinholes are thus readily detected using simple optical and scanning electron microscopy. The resulting pinhole densities are in the range of 6.6 × 106 cm−2 to 1.6 × 108 cm−2 for POLO junctions with selectivities close to S10 = 16, i.e., saturation current density J0c below 10 fA/cm2 and contact resistivity ρc below 10 mΩcm2. The measured pinhole densities agree with values deduced by a pinhole-mediated current transport model. Thus, we conclude pinhole-mediated current transport to be the dominating transport mechanism in the POLO junctions investigated here.