Organic-inorganic metal halide perovskite solar cells (PSCs) are one of the emerging technologies in photovoltaic research. The certified maximum power conversion efficiency (PCE) of PSCs has reached as high as 25%. In particular, the development of hole transport layer (HTL) materials plays a key role in increasing PCEs. Among a vast number of HTL materials developed to date, the most common HTL material is 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD), because it can provide very high performance in PSCs. Besides the high PCE, the issue of long-term stability is of paramount importance. The room-temperature operational stability of PSCs with the spiro-OMeTAD HTL has been improved significantly past few years, but it is still low, compared with Si-based technology. In addition, the instability at high temperature is the Achilles heel of PSCs with the spiro-OMeTAD HTL. Although the low operational stability of PSCs, especially at high temperature, is generally associated with instability of spiro-OMeTAD, the high-temperature stability has been improved significantly by understanding the degradation mechanisms. In this mini-review, we discuss the degradation mechanisms and suggest our perspectives to overcome the degradation. Thus, this mini-review will guide the development of stable PSCs with the spiro-OMeTAD HTL and the design of new HTL materials to replace spiro-OMeTAD toward commercialization of PSCs in the future.
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