Ultra-narrow bandgap (ultra-NBG) small molecule acceptors (SMAs) show
great potential in organic solar cells (OSCs) due to the extended
near-infrared (NIR) absorption. In this work, a synergetic alkoxy
side-chain and chlorine-contained end group strategy is employed to
achieve A-DA’D-A type ultra-NBG SMAs by introducing alkoxy chains with
oxygen atom at the second position into the thiophene β position as well
as replacing the F atoms with Cl atoms in the end group. As a result,
the heptacyclic BZO-4F shows a redshifted absorption onset (960 nm) than
Y11 (932 nm) without oxygen atoms in the side chains. Then, the
fluorinated end groups are substituted with the chlorinated ones to
synthesize BZO-4Cl. The absorption onset of BZO-4Cl is further
redshifted to 990 nm, corresponding to an optical ultra-NBG of 1.25 eV.
When blending with the polymer donor PBDB-T, the binary devices based on
PBDB-T: BZO-4F and PBDB-T: BZO-4Cl delivers power conversion
efficiencies (PCEs) over 12%. Furthermore, ternary devices with the
addition of BZ4F-O-1 into PBDB-T: BZO-4Cl system achieve the optimal PCE
of 15.51%. This work proposes a synergetic alkoxy side-chain and
chlorine-contained end group strategy to achieve A-DA’D-A type ultra-NBG
SMAs, which is important for future molecular design.
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