am7b04983_si_001.pdf (1.58 MB)
Amorphous Ternary Charge-Cascade Molecules for Bulk Heterojunction Photovoltaics
journal contribution
posted on 2017-08-10, 16:20 authored by Xavier A. Jeanbourquin, Aiman Rahmanudin, Xiaoyun Yu, Melissa Johnson, Néstor Guijarro, Liang Yao, Kevin SivulaTernary bulk heterojunctions with
cascade-type energy-level configurations are of significant interest
for further improving the power conversion efficiency (PCE) of organic
solar cells. However, controlling the self-assembly in solution-processed
ternary blends remains a key challenge. Herein, we leverage the ability
to control the crystallinity of molecular semiconductors via a spiro
linker to demonstrate a simple strategy suggested to drive the self-assembly
of an ideal charge-cascade morphology. Spirobifluorene (SF) derivatives
with optimized energy levels from diketopyrrolopyrrole (DPP) or perylenediimide
(PDI) components, coded as SF-(DPP)4 and SF-(PDI)4, are synthesized and investigated for application as ternary components
in the host blend of poly(3-hexylthiophene-2,5-diyl):[6,6]phenyl-C61-butyric acid methyl ester (P3HT:PCBM). Differential scanning
calorimetry and X-ray/electron diffraction studies suggest that at
low loadings (up to 5 wt %) the ternary component does not perturb
crystallization of the donor:acceptor host blend. In photovoltaic
devices, up to 36% improvement in the PCE (from 2.5% to 3.5%) is found
when 1 wt % of either SF-(DPP)4 or SF-(PDI)4 is added, and this is attributed to an increase in the fill factor
and open-circuit voltage, while at higher loadings, the PCE decreased
because of a lower short-circuit current density. A comparison of
the quantum efficiency measurements [where light absorption of SF-(DPP)4 was found to give up to 95% internal conversion] suggests
that improvement due to enhanced light absorption or to better exciton
harvesting via resonance energy transfer is unlikely. These data,
together with the crystallinity results, support the inference that
the SF compounds are excluded to the donor:acceptor interface by crystallization
of the host blend. This conclusion is further supported by impedance
spectroscopy and a longer measured charge-carrier lifetime in the
ternary blend.
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PDIlight absorptionpower conversion efficiencysolution-processed ternary blendsPCEresonance energy transfercomponentcascade-type energy-level configurationsoptimized energy levelsquantum efficiency measurementsbutyric acid methyl esterSFAmorphous Ternary Charge-Cascade MoleculesDPPhost blend3HTBulk Heterojunction Photovoltaics Ternary bulk heterojunctions
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