Elucidating the Doping Mechanism in Fluorene-Dithiophene Based Hole Selective Layer Employing Ultra-Hydrophobic Ionic Liquid Dopant

Perovskite solar cells have set a new milestone in terms of efficiencies in the thin film photovoltaics category. Long-term stability of perovskite solar cells is of paramount importance but remains a challenging task. The lack of perovskite solar cells stability in real-time operating conditions erodes and impedes commercialization. Further improvements are essential with a view to delivering longer-lasting photovoltaic (PV) performances. An ideal path in this direction will be to identify novel dopants for boosting the conductivity and hole mobility of hole transport materials (HTMs), and by so doing the usage of hygroscopic and deliquescent additive materials can be avoided. Pyridine-based ionic liquids represent a well-known class of ultra-hydrophobic materials, which are suitable for their application in opto-electrical devices. The present work demonstrates the employment of ionic liquids into a dissymmetric fluorene-dithiophene, FDT (2’,7’ -bis(bis(4-methoxyphenyl)amino) spiro[cyclopenta[2,1-b:3,4-b’]dithiophene-4,9’-fluorene]) based HTM to understand the doping mechanisms. N-heterocyclic hydrophobic ionic liquid, 1-butyl-3-methylpyidinium bis(trifluoromethylsulfonyl)imide (BMPyTFSI) as p-type dopant for FDT was found to increase the conductivity of FDT, to higher geometrical capacitance, to facilitate homogeneous film formation, and to enhance device stability. Our findings open up a broad range of hole-transport materials to control the degradation of the underlying water-sensitive active layer by substituting hygroscopic element.