**4.3. Inorganic and polymer HTM**

*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 HTMs which makes them disadvantageous against others, as well.

*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 more. Besides PTAA and P3HT, PANI is also known to satisfy a long‐term stable device, which is tested through characterizations.
