1 research outputs found
Breaking Down the Problem: Optical Transitions, Electronic Structure, and Photoconductivity in Conjugated Polymer PCDTBT and in Its Separate Building Blocks
Conjugated polymers with alternating electron-withdrawing
and electron-donating
groups along their backbone (donor–acceptor copolymers) have
recently attracted attention due to high power conversion efficiency
in bulk heterojunction solar cells. In an effort to understand how
the bandgap in a typical donor–acceptor copolymer is reduced
by internal charge transfer character and what the implications of
this charge transfer are, we have synthesized the isolated repeat
unit (CDTBT) of the photovoltaically highly successful PCDTBT polymer.
We compare here the spectroscopic and electrochemical properties of
the polymer, the repeat unit, and the separate carbazole donor and
dithienylbenzothiadiazole acceptor moieties (CB and dTBT, respectively)
in the solid state and in solutions of various polarity. The results
are interpreted with the help of time-dependent density functional
theory (TD-DFT) calculations. We identify the dominant electronic
transitions responsible for the first two absorption bands in the
“camel back” spectrum of PCDTBT as partial charge transfer
transitions with significant delocalization in the directly excited
states. The low bandgap, overall shape, and partial charge transfer
character of the PCDTBT absorption spectrum originate from transitions
in the dTBT unit. The attached CB moiety extends the conjugation length
in CDTBT, rather than acting as a localized donor. Further electronic
delocalization, leading to a relatively small reduction in bandgap,
occurs upon polymerization. We use our finding of higher delocalization
following excitation in the second absorption band to explain the
increased yield of photogenerated charges from this band in PCDTBT
solid thin films. Moreover, we point out the importance of initial
delocalization in the functioning of bulk heterojunction solar cells.
The results presented here are therefore not only highly important
for a better understanding of donor–acceptor copolymers in
general but can also potentially guide the strategic development of
future photovoltaic materials