Recent progress in organic hole transport materials for energy applications

The ever-increasing demand for energy that necessitates fabrication of optoelectronic devices for efficient solar harvesting systems has driven scientists towards the development of the device components. One of the main components is the organic hole transport materials (HTMs) as a p-type semiconductor that has the advantages over inorganic materials of being biodegradable, low cost, and easily processed. Also, dopant free HTMs fulfilling easy film formation, thermal and light stability as well as good hole mobility are presented. In this review, more than 500 HTMs, small molecules and some oligomers together with their synthetic approaches and how the chemical structure affects their properties as HTMs are discussed. The vast majority of HTMs were synthesized through metal-catalyzed cross-coupling reactions such as Ullmann reaction, Suzuki reaction, Buchwald-Hartwig reaction and Stille reaction. Therefore, the scope of this review comprises the importance of organic HTMs in energy applications, the synthetic approaches, and their classification according to the chemical structure. In addition, the review will present the structural dependent HOMO-LUMO energies, as these energy levels are of vital importance for the suitability of HTMs in different energy applications.