**5.2. Vapor deposition methods**

One of the vapor deposition methods consists of a dual‐source evaporation process. The vapor deposition method through using dual evaporation sources of PbCl2 and CH3NH3PbI3 was first used by Snaith et al. These were realized for CH3NH3PbI3‐xClx‐based planar SCs. The manufactured PSCs attained an efficiency level of 15% [84]. In this process, the dual sources contain PbCl2 and CH3NH3PbI3. Here the two sources are simultaneously heated to about 120 and 325°C, respectively. Thereafter, the evaporated materials are codeposited onto the TiO2/FTO substrate in a high vacuum chamber. The pinhole‐free and extremely uniform perovskite films can be produced by using this method. However, since it is crucial to use high vacuum for this method, thermal evaporation method is limited because of high cost, low thermal stability of sources, and unsuitability to mass production. Hence, only a few research groups have published high efficiency PSCs using this method [102, 103].

The other vapor deposition method is called vapor‐assisted solution process. The vapor‐ assisted solution process is almost same as the two‐step solution process except an additional modification. In this method, the first step of the two‐step solution process is applied initially. Then, CH3NH3X is coated onto substrate by vapor deposition. This method provides better grain size and surface morphology control through gas‐solid crystallization. In contrast, liquid‐ solid interactions, which occur in the two‐step coating process, may result in delamination of the film. On the other hand, this deposition method allows uniform surface, large grains, and full conversion to the perovskite structure. Nevertheless, this method is restricted by its long processing time passed during the manufacture. For example, the gas‐solid reaction takes tens of hours for the full conversion. With all that, the PSCs produced using this method could only attain device efficiencies of up to 12% [104, 105].
