A new zwitterionic small molecule
4-(dimethyl(pyridin-2-yl)ammonio)butane-1-sulfonate
(PAS), synthesized from 2-dimethylaminopyrindine (2-DMAP), was developed
for the ITO cathode modifier. PAS and 2-DMAP dissolved in methanol
can form a thin layer on ITO cathode by a simple spin-coating process.
The heteroatom moieties in 2-DMAP (sp2 and sp3 nitrogen) and PAS (sp2 nitrogen and sulfonate ion) can
coordinate to the ITO surface and decrease the ITO work function by
the induced surface dipoles. The fullerene-based (PBDTT-FTTE:PC71BM) inverted OSCs using PAS and 2-DMAP interlayer can achieve
PCEs of 8.95 and 8.26%, respectively, which are superior to the devices
without a modifier (PCE = 3.25%) and comparable to the corresponding
ZnO-based device (PCE = 8.57%). Nevertheless, 2-DMAP, like other nitrogen-containing
polymer interlayer materials, turns out to be not applicable to inverted
organic solar cells (I-OSCs) with IT-4F as the n-type electron acceptor
because the amino group of 2-DMAP can act as a nucleophile to attack
the end-group of IT-4F at the interface. The decomposition of IT-4F
by 2-DMAP was carefully proved to be via retro-aldol condensation.
As a result, the device (PBDBT-F:IT-4F) modified with 2-DMAP displayed
a low PCE of 7.34%. The zwitterionic PAS with reduced nucleophilicity
and basicity can modify the ITO surface without decomposing IT-4F.
The PBDBT-F:IT-4F-based device modified with PAS maintained a high
PCE of 11.41%. Most importantly, the PAS-based device using the well-known
Y6 acceptor (PBDBT-F:Y6) can achieve a PCE of 13.82%. This new interfacial
material can be universally applied to I-OSCs employing various A–D-A-type
acceptors installed with the electrophilic 1,1-dicyanamethylene-5,6-difluoro-3-indanone
(FIC) end-group.
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