Shielded hydrogen plasma: a new hydrogenation method for silicon solar cells

Improving the efficiency of solar cells will play a key role in ensuring that solar generated power reaches grid-parity across the globe. One of the main challenges in achieving high-efficiency silicon cells is the recombination of minority carriers at defects, both at the silicon surface and throughout the bulk. Introducing hydrogen into silicon is a well-recognised method to achieve excellent chemical passivation of surface and bulk defects. In this thesis, a novel method for the introduction of hydrogen into silicon is demonstrated, called Shielded Hydrogen Plasma (SHP). SHP is shown to effectively introduce neutral atomic hydrogen into silicon without the drawbacks of other hydrogenation techniques such as plasma damage or high-temperature steps. A palladium membrane is placed between the silicon and the PECVD- generated plasma to avoid plasma-related damage whilst still allowing the release of atomic hydrogen. The potential of converting SHP from a batch process to an in-line process by using cheaper and more resistant metallic shields is also considered, to encourage industry uptake. Palladium-silver and aluminium foils were shown to be effective alternatives to palladium leaf shields. SHP passivates silicon-oxide/silicon interfaces extremely well, as demonstrated using thermally-oxidised 1W-cm 200 μm n-type silicon where the effective lifetime was found to increase from 12 μs to 1.05 ms after SHP processing and to 6.3 ms upon application of corona charge. This corresponded to a surface recombination velocity of ≤ 0.17 cm/s. SHP is also shown to be a useful tool for the study of hydrogen-related defects in silicon. These defects are shown to be introduced into n-type silicon in the form of a damaged region with or without a dielectric layer. A new hydrogen-transport model across the silicon-oxide silicon interface is presented, showing that the interface is the main barrier to hydrogen incorporation into silicon. It is observed that the introduction of charged ions at the interface generates a strong field which enables an increased incorporation flux of hydrogen into bulk silicon. This higher concentration of hydrogen in the near-surface silicon is observed to be responsible for the damage layer.