Valence band offset and hole transport across a-SiOx (0<x<2) passivation layers in silicon heterojunction solar cells

Abstract

In this work, the valence band offset (EV) and hole transport across the heterojunction between amorphous silicon suboxides (a-SiOx:H) and crystalline silicon (c-Si) is investigated. Thin layers ranging from pure intrinsic a-Si:H to near-stoichiometric a-SiO2 were grown by varying precursor gas mixtures during chemical vapor deposition. A continuous increase of EV starting from ≈ 0 .3 eV for the a-Si:H/c-Si to > 4 eV for the a-SiO2/c-Si heterointerface was measured by in-system photoelectron spectroscopy. Furthermore, (p)a-Si:H/(i)a-SiOx:H/(n)c- Si/(i,n+)a-Si:H heterojunction solar cells, with intrinsic a-SiOx:H passivation layers deposited using the same parameter sets, were fabricated. We report a linear decrease of the solar cell fill factor for increasing EV in the range of 0.27 – 0.85 eV. The reason is an increase of the barrier height for holes at the (i)a-SiOx:H/(n)c-Si heterojunction and a simultaneous change of the hole transport mechanism from thermionic emission to defect-assisted tunnel hopping through valence-band tail-states. It is demonstrated that as compared to a single layer, significantly larger barrier heights can be tolerated in a stack of high band gap material and a material with lower band gap, forming a staircase of band offsets. This could allow the application of these layers in silicon heterojunction solar cells.