VALENCE BAND OFFSET AND HOLE TRANSPORT ACROSS a-SiO_x (0＜x＜2) PASSIVATION LAYERS IN SILICON HETEROJUNCTION SOLAR CELLS

摘要

In this work, the valence band offset (ΔE_V) and hole transport across the heterojunction between amorphous silicon suboxides (a-SiO_x:H) and crystalline silicon (c-Si) is investigated. Thin layers ranging from pure intrinsic a-Si:H to near-stoichiometric a-SiO_2 were grown by varying precursor gas mixtures during chemical vapor deposition. A continuous increase of ΔE_V starting from = 0.3 eV for the a-Si:H/c-Si to ＞ 4 eV for the a-SiO_2/c-Si heterointerface was measured by in-system photoelectron spectroscopy. Furthermore, (p)a-Si:H/(i)a-SiO_x:H/(n)c-Si/(i,n~+)a-Si:H heterojunction solar cells, with intrinsic a-SiO_x:H passivation layers deposited using the same parameter sets, were fabricated. We report a linear decrease of the solar cell fill factor for increasing ΔE_V in the range of 0.27 - 0.85 eV. The reason is an increase of the barrier height for holes at the (ⅰ)a-SiO_x: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.