To achieve both high efficiency and long-term stability of perovskite solar cells (PSCs), it is effective to use a perovskite layer in which a low-dimensional perovskite layer is stacked on a 3D perovskite layer. However, the guidelines for the effective structure of these perovskite layers remain unclear. Herein, the gradient structured 1D perovskite layer formed on top of a 3D perovskite layer using 4-tert-butylpyridinium iodide (TBPI) as a capping layer is introduced. It is demonstrated that the gradient structured 1D perovskite layer on the 3D perovskite improves the conversion efficiency of PSCs despite the lateral orientation of the (PbI3-)(n) linear chain of the 1D perovskite, which is responsible for electronic conduction. In addition, it is found that the hydrophobic organic unit of TBPI protects the 3D perovskite layer, which enhances its long-term stability.