transparent conductive substrate, an electron transport layer, a perovskite layer, a hole transport layer, and a metal electrode. Among these parts, the perovskite layer plays a vital role in determining the device performance since it closely relates to light absorption and photocarrier transport. The quality of the perovskite film can be assessed in terms of morphology and electronic properties. A dense perovskite film without pinholes would reduce the shunting paths and carrier recombination between electron selective layer and hole transport layer, while the smooth surface with low roughness facilitates a conformal and full coverage coating of the carrier selective layer to avoid direct contact between perovskite and metal electrode. [17,18] Perovskite film with a small defect density can suppress the ion diffusion and charging effect, which have been considered as the major cause of the hysteretic behavior in perovskite-based photovoltaic devices. [19,20] According to the fabrication process, the perovskite film can be obtained through a one-step method or sequential deposition. [9,12] As for the one-step method, stoichiometric precursors are mixed in a solvent and transformed into perovskite film after solvent evaporation and post-treatment, which stands for the simplest processing. In spite of advantages in cost and upscaling fabrication, crystal growth in the onestep method is relatively hard to control, and the crystallization process is sensitive to the atmosphere condition. Alternatively, the perovskite thin film can also be prepared through sequential deposition, a process with easily controlled nucleus formation and crystal growth. [21,22] However, the PCE of the device based on sequential deposition processed perovskite thin film still lags behind and needs further improvement.The stability of perovskite solar cell is another key issue which is closely related to its practical application. The current organic-inorganic halide perovskite solar cells typically show poor stability in ambient environment, where the moisture is regarded as the main cause of performance deterioration. [23][24][25][26] Compared to the constant pursuit of PCE, less efforts have been paid on the understanding of degradation mechanism and enhancement of device stability. Various strategies have been demonstrated to improve the tolerance to moisture or The power conversion efficiency and the stability are the most important figures of merit for perovskite solar cells which attract increasing attention in recent years. Herein, the growth of high-quality perovskite films with micrometer-scale grain size through sequential deposition is first reported using PbI 2 /CH 3 NH 3 Cl mixed precursors during the first step of deposition. The amount of CH 3 NH 3 Cl in the precursor has a great influence on the morphology of perovskite films and the device performance. Compared with traditional sequential deposition with pure PbI 2 in the first step, this method renders the perovskite solar cells with an improved power conversion efficienc...