A comparison of nitrous oxide and hydrogen ECR plasma discharges for silicon solar cell passivation

The passivation of crystalline Si solar cells using nitrous oxide (N2O) electron cyclotron resonance (ECR) plasma discharges has been studied and compared with ECR hydrogen passivation. The cells consisted of ECRCVD grown microcrystalline Si emitter layers on single crystal Si (sc-Si) and multicrystalline Si (mc-Si) substrates, without anti-reflective coatings or surface texturing. For cells on sc-Si substrates, hydrogen passivation is more effective at a substrate temperature of 300 °C and low microwave power (300 W). With increased power (500 W) H2 is less effective than N2O due to hydrogen plasma damage leading to a significant fall in the cell fill factor. In comparison with H2, N2O discharges lead to a significantly better (by > a factor of 2) improvement in the performance of cells on mc-Si substrates for treatment times of ≤15 min at a passivation temperature of 300 °C and 300 W microwave power. XPS measurements suggest that a surface oxide layer containing N and C atoms is formed by the N2O plasma which, most likely, reduces the surface state density and, hence, carrier recombination.

► N2O versus H2 ECR plasma discharges for passivation of Si solar cells.
► N2O found effective but efficacy depends on process conditions and substrate type.
► Improvement in multicrystalline cell efficiency up to ∼2% after N2O passivation.
► Behaviour likely associated with formation of a surface oxide layer containing N.