Black silicon with tunnel oxide passivated contacts

We investigate surfaces of black silicon (bSi) fabricated by reactive ion etch (RIE) on n-type Si substrates and passivated by in-situ doped polycrystalline Si (poly-Si) deposited by low pressure chemical vapor deposition (LPCVD). We achieved full surface coverage of bSi surfaces for poly-Si thickness of 20 nm.

We determined sheet resistance on p-type and n-type poly-Si by means of micro four-point probe measurements. Effective lifetime mapping on symmetrically passivated samples shows that n-type poly-Si offers excellent surface passivation after hydrogenation, reaching effective lifetime values of almost 4 ms on non-textured substrates.

P-type poly-Si shows values of effective lifetime lower than 800 μs and requires further improvement. We calculated i-Voc of up to 711 mV and 609 mV for asymmetrically passivated lifetime samples without texturing and with bSi, respectively. The effective lifetime is limited by the less-than-optimal passivation of the p-type poly-Si.

Preliminary solar cell measurements indicate that both cell voltage and fill factor require major improvement on textured surfaces. Current work is directed towards the following goals: improving the quality of the p-type poly-Si; testing possible replacement of wet chemistry to grow the tunnel oxide using dry furnace oxidation; fabricating and comparing cells with p-n junction at the front or at the bottom of the cell.