**5. Film formation and stability**

Recently, some efforts on spiro‐OMeTAD molecules used as HTM are under attention of researchers in view of their increased hole conductivities [86, 87]. For example, lithium bistrifluoromethanesulfonimide (Li‐TFSI) has been found out to increase the hole conductivity when doped to spiro‐OMeTAD, within some researches [87]. Using various techniques, spiro‐ OMeTAD has widely been used for enhancements in conductivity through increasing the hole

Within the cell, the holes are injected from the perovskite into the spiro‐OMeTAD and electrons are passed from perovskite to the mesoporous TiO2 layer. Here, therefore, the TiO2 is an active counterpart of the photovoltaic conversion event which makes it a main element of the PSC. Lately, it was fortunately shown that TiO2 scaffold has leaded a PCE of up to almost 10% [20, 74]. In some studies, spiro‐OMeTAD has also been used while coupled with other scaffolds (i.e., Al2O3). Where it is mixed with perovskites and good PCE has been attained in [30, 74]. These are famed as meso‐super‐structured SCs where the CH3NH3PbX3 is served as an electron transport material (ETM) in addition to its basic sensitizer duty. Besides with all that, the metal oxide only acts as an insulator with a wide band gap and therefore do not take part in charge transport. Hence, the metal oxide becomes an inactive scaffold here. For that, these are referred as passive scaffolds in the PSC literature. It is good to say that a more uniform perovskite layer formation is applicable with a wide thickness in both of the scaffolds which improves the light

Besides all, new kinds of small molecules as for HTM are continuously introduced to the literature. For example, a Trux‐OMeTAD small molecule is reported where an excellent hole mobility is achieved [90]. Here, the constructed PSC has given efficiency as close as to 19%.

*Inorganic HTMs:* The inorganic HTMs are rather low cost materials; on the other hand, they have progressed less in recent years due to the poor existence of such materials. The most known inorganic HTMs are listed as CuI, CuSCN, and NiO with respective PCEs of around 6, 12, and 11%. These inorganic HTMs are evaluated in mesoporous‐type PSCs. The main advantage is their stability yielding property in ambient conditions compared to the organic HTMs. On the other hand, they yield a low‐quality performance compared to the organic

*Polymer HTMs:* Compared to the inorganic counterparts, organic‐based HTMs have tunable oxidization potentials and surface morphology. In addition, high Voc values can be attained together with advanced HOMO levels [78, 91–93]. Basically, polymer HTMs have some drawbacks related to filling the pores which was tested in solid‐state DSSCs before. Some kinds of polymer HTMs are known to be PTAA, P3HT, and PANI. It has been shown that they have generally good hole mobility and satisfies a better film formation. Among them, PTAA and P3HT have yielded an efficiency level of around 12 and 6%, respectively [31]. The efficiency of PTAA has later been reported to reach more than 16% using a mixed structure with perovskite material [31, 94, 95]. There has been a study where P3HT is used together with carbon nanotube and formed a novel composite [96] in which the conductivity of the new structure is increased

mobility and charge density in HTM layers [88, 89].

290 Nanostructured Solar Cells

absorption of PSCs, thus lead an increase in efficiency [74].

HTMs which makes them disadvantageous against others, as well.

**4.3. Inorganic and polymer HTM**

The optical and electrical properties of perovskite films show varieties depending on the film formation conditions, such as atmospheric conditions and materials' ratios, which affect the device performance significantly. Up to the present time, several film techniques for fabrica‐ tions of perovskite have been reported. Generally, these are grouped as (1) solution process and (2) vapor deposition process. The solution processes can be carried out either as a (a) one‐ step coating technique, or (b) sequential two‐step coating technique. The vapor deposition processes are considered through two types of production as well, using (a) the dual‐source evaporation process and (b) vapor‐assisted solution process (see **Figure 6**).

**Figure 6.** Fabrication of perovskite cells through (a) one‐step solution‐based, (b) two‐steps solution‐based, (c) two‐step solution and deposition‐based, and (d) one‐step deposition‐based processes.

#### **5.1. Solution processed methods**

Two solution process methods have been developed in order to create perovskite film onto substrates: (1) one‐step coating and (2) sequential (two‐step) coating methods.The first deposition method is the most widespread film coating method for PSCs because of its ease of processing and low production cost. In general, the precursor solution contains the mixture of CH3NH3X (or other OMH perovskites) and PbX2 (where X: I, Br, Cl) at 1:1 or 3:1 molar ratios. 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‐ bination process [6, 97].

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 a reproducible treatment.

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 may negatively affect the device efficiency.

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