The
optical properties of two sets of donor–acceptor–donor
molecules with terminal bithiophene donor units and a central diketopyrrolopyrrole
(DPP) acceptor unit are studied. The two sets differ in the alkyl
chains on the DPP, which are either branched at the α-carbon
(3-pentyl) (
1
–
4
) or linear (
n
-hexyl) (
5
–
8
). Within
each set, the molecules differ by the absence or presence of
n
-hexyl chains on the terminal thiophene rings in the 3′,
4′, or 5′ positions. While in solution, the optical
spectra differ only subtly; they differ dramatically in the solid
state. In contrast to
5
–
8
,
1
–
4
are nonplanar as a consequence of
the sterically demanding 3-pentyl groups, which inhibit π-stacking
of the DPP units. Using the crystal structures of
2
(brick
layer stacking) and
6
(slipped stacking), we quantitatively
explain the solid state absorption spectra. By computing the molecular
transition charge density and solving the dispersion relation, the
optical absorption of the molecules in the crystal is predicted and
in agreement with experiments. For
2
, a single resonance
frequency is obtained, while for
6
two transitions are
seen, with the lower-energy transition being less intense. The results
demonstrate how subtle changes in substitution exert large effects
in optical properties.
Poly(3,4-ethylenedioxythiophene)
blended with polystyrenesulfonate
and poly(styrenesulfonic acid), PEDOT:PSS, has found widespread
use in organic electronics. Although PEDOT:PSS is commonly used in
its doped electrically conducting state, the ability to efficiently
convert PEDOT:PSS to its undoped nonconducting state is of interest
for a wide variety of applications ranging from biosensors to organic
neuromorphic devices. Exposure to aliphatic monoamines, acting as
an electron donor and Brønsted–Lowry base, has been reported
to be partly successful, but monoamines are unable to fully dedope
PEDOT:PSS. Remarkably, some—but not all—polyamines can
dedope PEDOT:PSS very efficiently to very low conductivity levels,
but the exact chemical mechanism involved is not understood. Here,
we study the dedoping efficacy of 21 different aliphatic amines. We
identify the presence of two or more primary amines, which can participate
in an intramolecular reaction, as the key structural motif that endows
polyamines with high PEDOT:PSS dedoping strength. A multistep reaction
mechanism, involving sequential electron transfer and deprotonation
steps, is proposed that consistently explains the experimental results.
Finally, we provide a simple method to convert the commonly used aqueous
PEDOT:PSS dispersion into a precursor formulation that forms fully
dedoped PEDOT:PSS films after spin coating and subsequent thermal
annealing.
Introducing solubilizing α‐branched alkyl chains on a poly(diketopyrrolopyrrole‐alt‐terthiophene) results in a dramatic change of the structural, optical, and electronic properties compared to the isomeric polymer carrying β‐branched alkyl side chains. When branched at the α‐position the alkyl substituent creates a steric hindrance that reduces the tendency of the polymer to π–π stack and endows the material with a much higher solubility in common organic solvents. The wider π–π stacking and reduced tendency to crystallize, evidenced from grazing‐incidence wide‐angle X‐ray scattering, result in a wider optical band gap in the solid state. In solar cells with a fullerene acceptor, the α‐branched isomer affords a higher open‐circuit voltage, but an overall lower power conversion efficiency as a result of a too well‐mixed nanomorphology. Due its reduced π–π stacking, the α‐branched isomer fluoresces and affords near‐infrared light‐emitting diodes emitting at 820 nm.
In developing organic semiconductor polymers for photovoltaic applications, chlorine substitution has become an effective strategy in replacing fluorine substitution to overcome the drawbacks of low yield and high cost, commonly...
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