Effect of Surface Texture on Poly-Si/SiOx Contacts for Silicon Solar Cells
Abhijit S Kale1, 2, William Nemeth2, Harvey Guthrey2, Vincenzo LaSalvia2, San Theingi2, Dawn Findley2, Matthew Page2, Mowafak Al-Jassim2, David L Young2, Sumit Agarwal1, Pauls Stradins2
1Colorado School of Mines, Golden, CO, United States
/2National Renewable Energy Laboratory, Golden, CO, United States

Monocrystalline Si (c-Si) solar cells with passivated contacts based on the ultrathin SiOx and doped polycrystalline Si (poly-Si) layers in a poly-Si/SiOx/c‑Si structure show high solar cell efficiencies that are ~26%. Excellent surface passivation using these contacts is achieved on planar surfaces via the combined effects of chemical passivation of the SiOx/c-Si interface by the SiOx layer and field-effect passivation from the heavily doped poly-Si layer. However, the passivation on textured surfaces is significantly poorer. Previous studies have related this to the poorer SiOx/Si(111) interface quality and/or stress in the SiOx layer when grown on a pyramidical morphology of a textured surface. Here, we investigate the role of different oxidation rates on front/back poly-Si device performance made on polished Si(100), polished Si(111) and textured wafers. Our results show that contrary to current understanding, thermal oxidation of Si(111) is slower than Si(100) for SiOx layers thinner than 3 nm. Interestingly, the primarily Si(111) faceted textured surface oxidizes faster than a polished Si(111) surface, and it is the pyramidal morphology which is more difficult to passivate than a polished Si(111) surface. We then report >21% efficient and ≥80% fill-factor front/back poly-Si solar cells on both single-side and double-side textured n-Cz c-Si without using TCO layers. Finally, we perform plan-view and cross-sectional electron beam induced current (EBIC) measurements on tunneling and pinhole type poly-Si/SiOx contacts on a textured surface. While EBIC of a tunneling contact is similar to that of a diffused junction, we show that for a pinhole contact, conduction channels form preferentially in the valleys between pyramids.