**5.3. The impact of film composition**

This mixture is dissolved in a polar aprotic solvent, such as *N*,*N*‐dimethylformamide (DMF), *γ*‐butyrolactone (GBL), or dimethyl sulfoxide (DMSO). Nevertheless, it is still a challenging issue to form a homogeneous pinhole‐free perovskite film using the one‐step deposition process. Therefore, the device efficiency is significantly reduced by a poorly coated perovskite layer which causes decreased light absorption and also poor shunting path for charge recom‐

The second deposition method is another kind of solution‐based coating process for PSCs. This is first introduced by Mitzi et al. [98], and first used by Burschka et al. [82] for PSC fabrication. In this method, PbX2 is coated on TiO2 layer under optimal conditions that is adjusted through spin‐coating speed and solution concentration. The optimal coating conditions should yield full penetration of this material into the mesoporous layer. Subsequently, the perovskite layer is obtained either by dipping the substrate into a solution of CH3NH3X/isopropanol [82] or by spin‐coating of CH3NH3X molecules on to the substrate [99]. Here, the perovskite films coated by using the two‐step process have a cuboid‐like crystal structure, while the one‐step method provides a shapeless morphology. Among them the process can be much better controlled to form perovskite morphology by using the two‐step method. Namely, better PbX2 confinement into the nanoporous network of TiO2 is obtained using this method. Moreover, the two‐step sequential coating process provides more uniform and dense perovskite films compared to the single‐step coating process. Hence, high efficiency perovskite devices can be fabricated using this process. In addition to all, the two‐step process is both well controlled and provides

In the two‐step coating process, the perovskite grain size can be controlled by changing molar concentration of the CH3NH3X solution. Nevertheless, there are some drawbacks of this coating method related to the trade‐off between surface smoothness and perovskite grain size. The perovskite films which have larger grain size also have poor surface morphology which

The incomplete perovskite conversion is another challenging issue within the two‐step coating process. This problem can be overwhelmed using some developed device engineering techniques expressed in the study performed by Song et al. [100]. Recently, the PCE of a

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

perovskite SC has reached 20% through the second‐coating method [101].

bination process [6, 97].

292 Nanostructured Solar Cells

a reproducible treatment.

may negatively affect the device efficiency.

**5.2. Vapor deposition methods**

The surface morphology of the perovskite film has an important effect on the device perform‐ ance. As mentioned above, high coverage and uniform crystallinity is required for high power efficiencies. One of the most important effects that influence the surface morphology is the composition of the perovskite films.

In order to increase PSC efficiency, mixed‐halide perovskite films have been studied by modifying their film compositions [30]. For instance, a perovskite layer of (NH2CH = NH2PbI3)1‐x, (CH3NH3PbBr3)x was used in a SC device. Here, in order to examine the influence of the perovskite materials' composition on the device performance, the proportion of MAPbBr3 in the FAPbI3 was altered. When the composition value is kept at *x* = 0.15, an extremely smooth morphology with EQE > 80% is attained without any noticeable pinholes [106]. Another perovskite film material MAPbI3‐xClx was also studied to examine the effect of the chlorine composition on morphology and device performance. It is reported that chlorine allows lower temperature thermal annealing, therefore, it results in reduced pinholes and voids [107, 108].

#### **5.4. The stability of perovskite solar cells**

The stability of PSCs is the most critical issue to obtain high performance devices. The stability issues can be related to both perovskite material and SC devices. In order to resolve the related problems and to develop effective strategies, physicochemical processes occurred during the perovskite degradation should be understood. We note that the comparison of stability testing results taken from different studies generally become a challenging issue due to different experimental conditions, such as light intensities, humidity level, temperature, and atmos‐ pheric conditions [17].

*Air stability:* The two molecules in the air, H2O and O2, negatively affect the stability of PSCs. Perovskite film color can change from brown to yellow. In order to obtain air stable perovskite devices, the degradation process should be clearly understood where there are few reports about the effects of O2 on perovskite films. According to the study about the degradation mechanism of the perovskite under water [109], it is shown that the hydrolysis reaction of CH3NH3PbI3 arises under humid condition.

*Photo‐stability:* The mesoporous TiO2 layer in the device is preferred in order to easily transport photo‐generated electrons. On the other hand, TiO2 is inherently sensitive to ultraviolet light, which may cause degradation in PSCs. The instability of encapsulated and nonencapsulated perovskite devices was investigated through considering the device efficiency by Snaith et al. [110], and it is showed that the degradation of the first‐type device occurs faster than that of the second type. Nevertheless, the first device has given a more stable condition in the lack of UV light. In order to overcome this photo‐instability due to the TiO2 layer, the respective authors suggest some methods such as pacifying the trap states, replacing the TiO2 layer with other materials, and avoiding UV light from the TiO2 layer. For example, stability at over 1000 hours at 40°C is accomplished when mesoporous Al2O3 is used instead of TiO2 layer in the PSC device. Nonetheless, the PCE was decreased to around by half of its first value after the first 200 hours. A continuing reduction is also observed in both Voc and FF values.

*Thermal stability:* Thermal stability is an issue regarding both perovskite material and in HTM layer. The intrinsic thermal instability of a perovskite material was reported in the literature [111], and it is showed that even though the film was maintained in an inert condition, the degradation of perovskite is seen at 85°C. It means that the SCs may not be used properly in cases where the SC temperature exceeds this temperature level.
