Silicon germanium active layer with graded band gap and µc-Si:H buffer layer for high efficiency thin film solar cells

We investigated solar cells with graded band gap hydrogenated amorphous silicon germanium active layer and hydrogenated microcrystalline silicon buffer layer at the interface of intrinsic and n-type doped layer. A significantly improved, 10.4% device efficiency was observed in this type of single junction solar cell. The intrinsic type microcrystalline silicon buffer layer is thought to play dual roles in the device; as a crystalline seed-layer for growth of n-type hydrogenated microcrystalline silicon layer and helping efficient electron collection across the i/n interface. Based on these, an enhancement in cell parameters such as the open-circuit voltage (V_oc), and fill factor (FF) was observed, where the FF and V_oc reaches up to 69% and 0.85 V respectively. Our investigation shows a simple way to improve device performance with narrow-gap silicon germanium active layer in solar cells in comparison to the conventionally constant band gap device structure.