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

摘要

In this work, the valence band offset amp; 916;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 amp; 916;EV starting from amp; 8776; 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 amp; 916;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