Low gap (∼1.6 eV) a-Si : H films have been prepared by using helium diluted silane as source gas mixture in the usual radiofrequency plasma enhanced chemical vapour deposition method (13.56 MHz). The films have characteristics suitable for use as active layer of the bottom cell of a dual gap double junction a-Si solar cell. The films have been prepared at different rf-power densities and chamber pressures and characterized in detail. The role played by helium in improving the structure of a-Si : H and lowering of the band gap has been analysed. The cells fabricated with the said material show higher short circuit current density (J_sc8.9 mA cm⁻²) and lower light induced degradation (9–10) before stabilization.