Lessons learned from spiro-OMeTAD and PTAA in perovskite solar cells

Organic semiconductors have become essential parts of thin-film electronic devices, particularly as hole transport layers (HTLs) in perovskite solar cells (PSCs) where they represent one of the major bottlenecks to further enhancements in both device stability and efficiency. Small molecule 2,2 ',7,7 '-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9 '-spirobifluorene (spiro-OMeTAD) and polymer poly[bis(4-phenyl)(2,4,6-trimethylphenyl)amine] (PTAA) are two of the first successful HTLs used in PSCs, and have remained at the forefront of developing high efficiency devices for almost a decade. Since their first application, many investigations into the properties of spiro-OMeTAD and PTAA have contributed to a growing understanding of the mechanisms that enable their success as HTLs. This review summarizes and discusses the key electronic and morphological properties, doping strategies and mechanisms, and degradation pathways of both spiro-OMeTAD and PTAA. A critical comparison between the two materials is provided, highlighting both the similarities which explain their enduring popularity as well as key differences in electrical and morphological properties. From this analysis emerges an improved understanding of the fundamental properties that enable the persistent success of HTL materials, which are found to include not only hole conductivity, band gap, and morphology, but also interactions with dopants, the perovskite, and environmental stressors. The knowledge about these properties, which are critically summarized in this review, is also applicable to the many other types of organic electronic devices now employing spiro-OMeTAD and PTAA. A detailed examination of the properties of materials reveals a clear set of guiding principles for the development of future generation HTLs. Applying these design strategies to produce more advanced HTLs will be essential to further improve the stability, efficiency, and commercialization of PSCs.