Squaric Acid-Containing Hole-Collecting Monolayer Materials for p–i–n Perovskite Solar Cells

The development of hole-collecting materials is indispensable
to
improving the performance of perovskite solar cells (PSCs). To date,
several anchorable molecules have been reported as effective hole-collecting
monolayer (HCM) materials for p–i–n PSCs. However, their
structures are limited to well-known electron-donating skeletons,
such as carbazole, triarylamine, etc. In this work, we developed a
series of squaraine derivatives that have a π-conjugated core
composed of a squaric acid moiety connected to an indoline moiety.
Thanks to the polar carbonyl group of squaric acid, all of the molecules
were found to form hydrophilic monolayers after being chemisorbed
on transparent conducting oxide surfaces, which is beneficial for
the subsequent deposition of the perovskite layer. The effect of the
substituents on the squaric acid moiety and the anchoring groups connected
to the indoline moiety on the molecular electronic structure as well
as the solar cell device’s performance was elucidated. The
p–i–n PSC devices fabricated by using these squaraine
derivatives as hole-collecting monolayer materials exhibited high
power conversion efficiencies of up to 22.1%, together with good stability.
This work highlights the potential of a simple squaric acid skeleton
as the building block for hole-collecting monolayer materials to realize
high-efficiency and cost-effective PSCs.