<p>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. <i>N</i>-heterocyclic hydrophobic ionic
liquid, 1-butyl-3-methylpyidinium bis(trifluoromethylsulfonyl)imide (BMP<i>y</i>TFSI) 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. </p>
Elucidating the Doping Mechanism in Fluorene-Dithiophene Based Hole Selective Layer Employing Ultra-Hydrophobic Ionic Liquid Dopant