Effect of dual-function nano-structured silicon oxide thin film on multi-junction solar cells

We present our recent study of using nano-structured hydrogenated silicon oxide films (nc-SiO_x:H) as a dual-function layer in multi-junction solar cells. The nc-SiO_x:H films were deposited using very high frequency glow discharge of a SiH₄ (or Si₂H₆), CO₂, PH₃, and H₂ gas mixture. By optimizing deposition parameters, we obtained "dual function" nc-SiO_x:H material characterized by a conductivity suitable for use as an n layer and optical properties suitable for use as an inter-reflection layer. We tested the nc-SiO_x:H by replacing the normal n-type material in the tunnel junction of a multi-junction structure. The advantage of the dual-function nc-SiO_x:H layer is twofold; one is to simplify the cell structure, and the other is to reduce any optical loss associated with the inter-reflection layer. Quantum efficiency measurements show the gain in top cell current is equal to or greater than the loss in bottom cell current for a-Si:H/nc-Si:H structures. In addition, a thinner a-Si:H top cell with the nc-SiO_x:H n layer improves the top-cell stability, thereby providing higher stabilized solar cell efficiency. We also used the dual-function layer between the middle and the bottom cells in a-Si:H/a-SiGe:H/nc-Si:H triple-junction structures. The gain in the middle cell current is ~1.0 mA/cm², leading to an initial active-area efficiency of 14.8%.