Synthesis, Properties,
and Photovoltaic Performances
of Donor–Acceptor Copolymers Having Dioxocycloalkene-Annelated
Thiophenes As Acceptor Monomer Units
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Abstract
New donor–acceptor-type copolymers containing
dioxocycloalkene-annelated
thiophenes as electron-accepting units have been designed and synthesized
for application to p-type organic semiconducting materials in organic
photovoltaics. The investigation of their photophysical and electrochemical
properties revealed that these copolymers possessed low optical bandgaps
(from 1.63 to 1.92 eV) and low-lying HOMO energy levels (from −5.41
to −5.33 eV). Organic field-effect transistor measurements
revealed that these copolymers had hole-transporting characteristics
with mobilities on the order of 10<sup>–7</sup>–10<sup>–4</sup> cm<sup>2</sup> V<sup>–1</sup> s<sup>–1</sup>. The bulk-heterojunction photovoltaic devices fabricated from blends
of these copolymers with fullerene derivatives as acceptors showed
high power conversion efficiencies of up to 4.87%, with an open-circuit
voltage of 0.90 V, a short-circuit current of 11.46 mA cm<sup>–2</sup>, and a fill factor of 0.48 under air mass 1.5 simulated solar illumination