Improving the stability of thin polycrystalline silicon passivated contacts using titanium dioxide interlayers

Polycrystalline silicon (poly-Si) passivated contacts are one of the key technologies in the push towards silicon's theoretical efficiency limit of 29.4 %. However, degradation of silicon surface passivation during metallisation remains an issue, necessitating thick poly-Si layers which negatively impact transparency and deposition time. In this work, we introduce titanium dioxide (TiO₂) based protective interlayers between the thin poly-Si layer (<40 nm) and metal electrodes. Thicker TiO₂ interlayers are generally found to provide better protection, however, even thin TiO₂ interlayers (∼7 nm) provide significant thermal stability enhancement over unprotected poly-Si films. Greater thermal stability is afforded when utilising a higher temperature TiO₂ deposition step (250 °C), or a pre-metallisation anneal step (450 °C). These improvements in surface passivation thermal stability come at the expense of higher contact resistivity, ρ_c, however, the final ρ_c values are still acceptable for full area contacts. The best TiO₂ films were deposited at 250 °C using titanium tetraisopropoxide (TTIP) and Tetrakis (dimethylamido) titanium (TDMAT) precursors, which permitted the preservation of implied open circuit voltages, iV_oc > 700 mV, and ρ_c values < 47 mΩ-cm² after post-metallisation annealing at ≥ 500 °C. The protective effects of this interlayer structure may allow the thinning of poly-Si layers, reducing their parasitic absorption, and permitting their usage on both sides of silicon solar cells.