Recent Advances in Spiro-MeOTAD Hole Transport Material and Its Applications in Organic-Inorganic Halide Perovskite Solar Cells

2,2,7,7-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9-spirobifluorene (spiro-MeOTAD) hole transport material (HTM) is a milestone in the history of perovskite solar cells (PSCs). Proper choice of HTMs is key factor for efficient charge extraction and stability in solar cells. Spiro-MeOTAD is proven to be the most suitable HTM for testing PSCs due to its facile implementation and high performance. Similarly, spiro-MeOTAD is receiving attention in other applications other than in solar cells due to its desirable properties. However, spiro-MeOTAD is under debate regarding the topics of cost-performance, long-term stability, degradation issues (induced by temperature, additives, film quality, and environmental conditions), coating technologies compatibility, reliance on additives, and hysteresis. In this review, the advent of spiro-MeOTAD, and related aforementioned issues about spiro-MeOTAD are discussed. In addition, spiro-MeOTAD properties, alternative and new additives, other applications, and new HTMs that is comparable or outperforms spiro-MeOTAD in PSCs are summarized. In the outlook, the future research directions based on reported results that warrant further investigations are outlined.