Band Alignment and Surface Photo-Voltage Analysis of C-Si/Sio/Poly-Silicon Passivating-Contact Stacks Through X-Ray Photoelectron Spectroscopy.

Ultrathin SiO layers and c-Si/SiO interfaces find application in tunnel-oxide passivated contacts (TOPcon) for high-efficiency silicon solar cells. Here, we investigate their detailed microscopic properties, with specific attention for the case of c-Si(100) substrates, capped either by p-type or n-type poly-silicon layers [c-Si/SiO/poly-Si (p) or c-Si/SiO/poly-Si (n)]. Our focus is on the effects of the substrate preparation conditions (either by a dry-plasma or wet SiO process) and the high-temperature annealing step (as required for the poly-Si crystallization) on the SiO stoichiometry and its microscopic structure. Through advanced photoemission techniques, we find a clearly decreased valence band offset between the c-Si and SiO (from 4.5 eV to 4.15 eV) when comparing the dry SiO with the wet SiO process, independent of the SiO film thickness, but correlating with the relative fraction of sub-stochiometric Si states. We lastly examine the magnitude of band-bending of the contact structure through controlled in-situ exposure to light of the surfaces and subsequent tracking of core and valence band levels via a surface photovoltage and a junction photo-voltage (JPV) effect. By analyzing this JPV effect qualitatively, we find it to be proportional to the expected quasi fermi level splitting within the c-Si wafer.