The Future of Spirobifluorene‐Based Molecules as Hole‐Transporting Materials for Solar Cells

Organic–inorganic halide perovskite solar cells (PSCs) and organic solar cells (OSCs) attract great attention as alternative renewable photovoltaic technology. The state‐of‐the‐art spiro‐OMeTAD (2,2′,7,7′‐tetrakis‐( N,N ‐di‐ p ‐methoxyphenylamine)‐9,9′‐spirobifluorene) is the most successful hole‐transport material (HTM) employed in PSCs, whereas solution‐processed inverted OSCs generally use poly(3,4‐ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS). Recently, various types of spirobifluorene‐based organic small molecules are reported to overcome the known disadvantages of spiro‐OMeTAD, such as the complex synthetic route, high synthetic cost, and requirement for hygroscopic dopants to improve the charge‐carrier mobility and device performance. Examples of spirobifluorene‐based molecules are also reported as alternative HTMs in inverted OSCs to exceed the drawbacks of PEDOT:PSS, such as acidity and batch‐to‐batch reproducibility. These features significantly limit spiro‐OMeTAD and PEDOT:PSS for large‐scale application in the future. Herein, an overview of recent developments in spirobifluorene organic small molecules as HTM in PSCs and OSCs is provided by focusing on synthetic and electrical features. Finally, the further research directions are discussed to develop novel spirobifluorene‐based HTMs for the realization of reliable and long‐term stable photovoltaic devices.