Analytical Review of Spiro‐OMeTAD Hole Transport Materials: Paths Toward Stable and Efficient Perovskite Solar Cells

The hole transport material (HTM) of organic–inorganic perovskite solar cells (PVSCs) plays a very important role for achieving high power conversion efficiency and long‐term stability. 2,2’,7,7’‐tetrakis[ N,N ‐di(4‐methoxyphenyl)amino]‐9‐9’‐spirobifluorene (spiro‐OMeTAD) is the first solid‐state HTM used in PVSCs and has gained tremendous attention during the last decade. Herein, the concept of spirolinkage for synthesis of spiro‐based HTMs is discussed, followed by an overview of the desirable optical and electrical properties of spiro‐OMeTAD. Recent progress in efficiency improvements of spiro‐based PVSCs is analyzed systematically, and the impacts of interface engineering, dopant‐free spiro‐OMeTAD, and novel spiro‐based HTMs are reviewed in detail. The hole mobility of spiro‐OMeTAD depends on the types of dopants and doping concentration. Commonly used lithium bis(trifluoromethylsulfonyl)imide and 4‐ tert ‐butylpyridine additives reduce the PVSC stability due to hygroscopicity and corrosiveness, respectively. The effects of additives on device stability and the techniques to improve the long‐term stability of spiro‐based PVSCs are also discussed. The review and analysis of various methods and strategies presented is useful for the perovskite research community, providing guidance and directions toward the further development of spiro‐based HTMs for PVSCs with improved efficiency and stability.