ABSTRACT
This chapter provides an overview of the research in small molecule and conducting polymer hole transport materials. Development of new generation of hole or electron-conducting polymers with high charge carrier mobility and long diffusion length has been an important research area for the advancement in organic photovoltaic field. Various existing types of low-cost polymer materials and new types of small-molecule organic materials, with compatible highest occupied molecular orbital/lowest unoccupied molecular orbital energy levels relative to perovskite materials and high hole carrier mobility, have been selected to replace spiro-OMeTAD in solar cell devices. M. Qin et al. developed new type of fused quinolizino acridine core-based small molecule with low bandgap values, effective hole mobility in the same range of the commonly used spiro-OMeTAD and an oxidation potential of 5.23 eV versus the vacuum level. The hole-transporting materials that had been commonly used in solid-state dyesensitized solar cells, such as spiro-OMeTAD and poly (3-hexylthiophene), were first used in the perovskite solar cells.
