Development of Dopant-Free Organic Hole Transporting Materials for Perovskite Solar Cells

There has been considerable progress over the last decade in development of the perovskite solar cells (PSCs), with reported performances now surpassing 25.2% power conversion efficiency. Both long-term stability and component costs of PSCs remain to be addressed by the research community, using hole transporting materials (HTMs) such as 2,2 ',7,7 '-tetrakis(N,N '-di-pmethoxyphenylamino)-9,9 '-spirbiuorene(Spiro-OMeTAD) and poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA). HTMs are essential for high-performance PSC devices. Although effective, these materials require a relatively high degree of doping with additives to improve charge mobility and interlayer/substrate compatibility, introducing doping-induced stability issues with these HTMs, and further, additional costs and experimental complexity associated with using these doped materials. This article reviews dopant-free organic HTMs for PSCs, outlining reports of structures with promising properties toward achieving low-cost, effective, and scalable materials for devices with long-term stability. It summarizes recent literature reports on non-doped, alternative, and more stable HTMs used in PSCs as essential components for high-efficiency cells, categorizing HTMs as reported for different PSC architectures in addition to use of dopant-free small molecular and polymeric HTMs. Finally, an outlook and critical assessment of dopant-free organic HTMs toward commercial application and insight into the development of stable PSC devices is provided.