Rational Design of Donor Acceptor Based Semiconducting Copolymers with High Dielectric Constant
semanticscholar(2020)
摘要
An efficient photogeneration of free charge carriers has long been recognized as the
paramount challenge in organic photovoltaic (OPV) devices. The low dielectric constant
organic semiconductors fall short to reduce strong Coulombic interaction of tightly bound
exciton and hence lead to a loss mechanism in OPVs due to charge-carrier recombination. To
circumvent this problem, we adopt a strategy to enhance the dielectric constant of organic
semiconductors by incorporating tetraethyleneglycol (TEG) side-chains. We report synthesis
of three new semiconducting copolymers by combining thiophene substituted
diketopyrrolopyrrole (TDPP) monomer with three other monomeric units with varying
electron donating strength: benzodithiophene (BBT-3TEG-TDPP), TDPP (TDPP-3TEG-TDPP)
and naphthalene diimide (PNDITEG-TDPP). BBT-3TEG-TDPP and PNDITEG-TDPP showed
highest dielectric constants (~ 5) at 1MHz frequency suggesting efficient contribution of
dipolar polarization from TEG side-chains. To understand the electronic contribution of the
polymer backbone and the polarity of TEG side-chains, and the resulting enhancement of the
dielectric constant, we further performed first-principles density functional theory
calculations. Single-component organic solar cells (OSC) fabricated utilizing these polymers
resulted in poor performance which is attributed to the absence of free charge generation.
Furthermore, transient absorption spectroscopy studies show low exciton diffusion length as
observed in donor-acceptor type conjugated polymers. Our results suggest that, the strategy
of enhancing dielectric constant with polar side-chains is not sufficient to reduce Coulombic
interaction between hole and electron in OSCs.
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