**2.3 Thermal evaporation technique**

M. Era et al. [43] first used thermal evaporation method. They used the dual source vapor deposition by using ammonium iodide RNH3I and lead iodide PbI2, organic and inorganic source were co-evaporated and deposited on quartz. The pressure of evaporation chamber was about 10−6 Torr. By using this method, it is possible to precisely control the smoothness and thickness of the films. However, it is often hard to balance the organic and inorganic rates, which is important in attaining the correct composition of the resultant perovskite films. Furthermore, Mitzi et al. [40] developed another method, by using a single evaporation source to deposit perovskites thin films called single source thermal ablation (SSTA) technique. This consists of a vacuum chamber, with an electrical feed-through to a thin tantalum sheet heater. A suspension of insoluble powders in a drying solvent is placed on the heater. Under a suitable vacuum, the temperature goes to approximately 1000°C in 1–2 second, the whole starting charge ablates from the heater. After ablation, the organic and inorganic parts reassemble on the substrates to yield films of the chosen product. Liu et al. [44] in 2013, improved this technique as a dual-source vapor deposition method for pinhole-free MAPbI1−xClx perovskite films with a thickness of hundreds of nanometers for planar PSCs.

Later on, the chemical vapor deposition (CVD) method was reported by Leyden et al. [45], which precisely control the crystallization process. Vapor deposition methods are appropriate for multi-layered thin-film and a variety of substrates, though needs high vacuum [39]. However, this method has drawbacks of yield and therefore is not very effectively employed at industrial scale [46]. Though great achievements have been attained, researchers still meet some challenges, involving reproducibility and grain boundaries of perovskite films which are considered as a defect region initiates carrier recombination and accelerates device degradation. Hence, efforts to increasing grain size and reducing grain boundary of films are critical for stable and highly efficient PSCs.
