Transport limiting mechanisms of amorphous (a-Si:H) and microcrystalline(mc-Si:H) thin film heterostructures for photovoltaic application

Direct conversion of sunlight into electricity employing amorphous (a-Si:H) and microcrystalline (mc-Si) silicon solar cell structures provides cheap, continuous, large-area device capability. However, the inherent instability due to light induced degradation of such cells poses a challenge for practical photovoltaic applications. To work around this problem, we have made use of a-Si:H/c-Si (crystalline) and mc-Si/c-Si heterostructures. Plasma decomposition of 2% silane gas (SiH₄/He) in an ECR (electron cyclotron resonance)-CVD configuration forms the basis of the deposition technique. Electro-optical properties exhibit photoconductivity (σ_p) of 6.5×10^-6 S/cm and a low dark conductivity (σ_d) of 1.4×10^-9 S/cm for a-Si:H films and a relatively high σ_p of 2.1×10^-4 S/cm and a high σ_d of 1.2×10^-7 S/cm for mc-Si films. A possible multi-tunneling transport mechanism is evident for a low forward bias with the current conduction becoming space charge limited with increasing bias, The domination of this latter mechanism may be responsible for non-ideal junction behavior.