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 (2,20,7,70-tetrakis-(N,N-di-p-methoxyphe-nylamine)-9,90-spirobifluorene) is the most successful hole-transport material(HTM) employed in PSCs, whereas solution-processed inverted OSCs generallyuse poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS).Recently, various types of spirobifluorene-based organic small molecules arereported to overcome the known disadvantages of spiro-OMeTAD, such as thecomplex synthetic route, high synthetic cost, and requirement for hygroscopicdopants to improve the charge-carrier mobility and device performance.Examples of spirobifluorene-based molecules are also reported as alternativeHTMs in inverted OSCs to exceed the drawbacks of PEDOT:PSS, such as acidityand batch-to-batch reproducibility. These features significantly limit spiro-OMeTAD and PEDOT:PSS for large-scaleapplication in the future. Herein, anoverview of recent developments in spirobifluorene organic small molecules asHTM in PSCs and OSCs is provided by focusing on synthetic and electricalfeatures. Finally, the further research directions are discussed to develop novelspirobifluorene-based HTMs for the realization of reliable and long-term stablephotovoltaic devices.