From a-Si:H to a-SiOx:H: the role of CO2 and H2 in PECVD deposition process

Recently a-SiOx:H has attracted interest in heterojunction solar cells fabrication, because of its effective crystalline silicon surface passivation and larger optical bandgap than a-Si:H. Both amorphous layers are commonly deposited by PECVD from silane dissociation in hydrogen dilution. to obtain a-SiOx:H film, CO2 as source of oxygen is added to gas mixture, that heavily modifies the film growth, the composition and hydrogen inclusion, which influence the passivation properties. In this work we have analyzed the role of CO2 and H2 in the film growth and composition. In particular we have compared the effect of the two kind of amorphous layers on the effective lifetime of c-Si wafers, monitoring the lifetime stability and the effect of thermal treatment. FTIR analysis has been used to correlate the passivation properties to chemical bonding among silicon, hydrogen and oxygen. We have found that H2 dilution in the gas mixture during the film growth is the key to obtain high effective lifetime on c-Si wafers passivated with both kind of amorphous films. On samples coated with a-Si:H or a-SiOx:H different metastability has been observed on as-deposited, thermally annealed and TCO covered films. In particular the effective lifetime can improve up to 200% after thermal annealing being almost stable. Finally we have compared heterojunction solar cells with a-Si:H and a-SiOx:H buffer to remark different performances in Jsc and Voc due to film transparency and passivation capability.