posted on 2023-11-28, 09:20authored byYulong Zheng, Rahul Venkatesh, Connor P. Callaway, Campbell Viersen, Kehinde H. Fagbohungbe, Aaron L. Liu, Chad Risko, Elsa Reichmanis, Carlos Silva-Acuña
Linear and nonlinear optical line shapes reveal details
of excitonic
structure in polymer semiconductors. We implement absorption, photoluminescence,
and transient absorption spectroscopies in DPP-DTT, an electron push–pull
copolymer, to explore the relationship between their spectral line
shapes and chain conformation, deduced from resonance Raman spectroscopy
and from ab initio calculations. The viscosity of
precursor polymer solutions before film casting displays a transition
that suggests gel formation above a critical concentration. Upon crossing
this viscosity deflection concentration, the line shape analysis of
the absorption spectra within a photophysical aggregate model reveals
a gradual increase in interchain excitonic coupling. We also observe
a red-shifted and line-narrowed steady-state photoluminescence spectrum
along with increasing resonance Raman intensity in the stretching
and torsional modes of the dithienothiophene unit, which suggests
a longer exciton coherence length along the polymer-chain backbone.
Furthermore, we observe a change of line shape in the photoinduced
absorption component of the transient absorption spectrum. The derivative-like
line shape may originate from two possibilities: a new excited-state
absorption or Stark effect, both of which are consistent with the
emergence of a high-energy shoulder as seen in both photoluminescence
and absorption spectra. Therefore, we conclude that the exciton is
more dispersed along the polymer chain backbone with increasing concentrations,
leading to the hypothesis that polymer chain order is enhanced when
the push–pull polymers are processed at higher concentrations.
Thus, tuning the microscopic chain conformation by concentration would
be another factor of interest when considering the polymer assembly
pathways for pursuing large-area and high-performance organic optoelectronic
devices.