**4. MEHs based on OIHPs**

A mechanical energy harvester that can produce electricity from mechanical vibrations is a very promising tool to realize sustainable energy generation in remote/ indoor environs and even through human body movements. MEHs generally operate based on either the piezoelectric effect, triboelectric effect, or electromagnetic induction effects [13]. Amongst them, PENGs that operate based on piezoelectric effect and TENGs that operate based on triboelectric effect have attracted intense attention because of their direct power conversion ability and relatively easier fabrication processes. Intense research efforts have been conducted with regard to material and structural design, functionality, operation mechanism, and performance optimization of PENGs and TENGs [8, 13, 18]. More recently, the unique properties, low-temperature solution processing, and tunability of properties via compositionaltuning of OIHPs have highlighted their superior potential in the field of mechanical energy harvesters.
