Doped but Stable: Spirobisacridine Hole Transporting Materials for Hysteresis-Free and Stable Perovskite Solar Cells

Four
spirobisacridine (SBA) hole-transporting materials were synthesized
and employed in perovskite solar cells (PSCs). The molecules bear
electronically inert alkyl chains of different length and bulkiness,
attached to in-plane N atoms of nearly orthogonal spiro-connected
acridines. Di-p-methoxyphenylamine (DMPA) substituents
tailored to the central SBA-platform define electronic properties
of the materials mimicking the structure of the benchmark 2,2′,7,7′-tetrakis­(N,N-di-4-methoxyphenylamino)-9,9′-spirobifluorene
(spiro-MeOTAD), while the alkyl pending groups affect molecular packing
in thin films and affect the long-term performance of PSCs. Devices
with SBA-based hole transporting layers (HTL) attain efficiencies
on par with spiro-MeOTAD. More importantly, solar cells with the new
HTMs are hysteresis-free and demonstrate good operational stability,
despite being doped as spiro-MeOTAD. The best performing MeSBA-DMPA
retained 88% of the initial efficiency after a 1000
h aging test under constant illumination. The results
clearly demonstrate that SBA-based compounds are potent candidates
for a design of new HTMs for PSCs with improved longevity.