The bulk heterojunction (BHJ) grown with nanopillar has been predicted to be high performance solar cell due to having high photon and electron collection capability. Here, we report a reproducible high efficient Si-ZnO-ZnMgO BHJ solar cell having nanopillar structure (efficiency > 20.9%). Due to its tendency to formation of single crystal orientation (002) in its structure and reduced contact and series resistance, long term stability is guaranteed as the device maintains its initial efficiency for 2400 h at 75% stability/reliability.