Excess/unreacted lead iodide (PbI2) has been commonly used in perovskite films for the state‐of‐the‐art solar cell applications. However, an understanding of intrinsic degradation mechanisms of perovskite solar cells (PSCs) containing unreacted PbI2 has been still insufficient and, therefore, needs to be clarified for better operational durability. Here, it is shown that degradation of PSCs is hastened by unreacted PbI2 crystals under continuous light illumination. Unreacted PbI2 undergoes photodecomposition under illumination, resulting in the formation of lead and iodine in films. Thus, this photodecomposition of PbI2 is one of the main reasons for accelerated device degradation. Therefore, this work reveals that carefully controlling the formation of unreacted PbI2 crystals in perovskite films is very important to improve device operational stability for diverse opto‐electronic applications in the future.

Organometal halide based perovskites are promising materials for solar cell applications and are rapidly developing with current devices reaching $19% efficiency. In this work we introduce a new method of perovskite synthesis by hybrid chemical vapor deposition (HCVD), and demonstrate efficiencies as high as 11.8%. These cells were found to be stable with time, and retained almost the same efficiency after approximately 1100 h storage in dry N 2 gas. This method is particularly attractive because of its ability to scale up to industrial levels and the ability to precisely control gas flow rate, temperature, and pressure with high reproducibility. This is the first demonstration of a perovskite solar cell using chemical vapor deposition and there is likely still room for significant optimization in efficiency.

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In the last few years, lead halide perovskite solar cell power conversion efficiencies have risen by using a wide variety of fabrication methods and just passed 20%. Perovskite solar cells are typically fabricated in a glove box to strictly avoid any water exposure. A dry atmosphere significantly increases equipment and operational costs for industrial processes, so ambient perovskite fabrication will be less-expensive and more attractive. In this work it is demonstrated that ambient annealing is comparable to annealing in dry N 2 . Perovskite films annealed in a standard dry N 2 environment are compared with those annealed in ambient environment with 50% relative humidity. Solar cell devices were prepared with a planar structure configuration and annealed at one of three different temperatures (105, 115 or 125 C) in either N 2 or ambient air. For all temperatures, the average efficiencies for the devices annealed in air are higher than those annealed in dry N 2 . The highest efficiency achieved for air-annealed devices is 12.7%. Thus, good efficiency cells can be fabricated in an ambient environment, which facilitates mass production.