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Every Atom Counts: Elucidating the Fundamental Impact of Structural Change in Conjugated Polymers for Organic Photovoltaics
journal contribution
posted on 2018-04-19, 13:21 authored by Chi Kin Lo, Bhoj R. Gautam, Philipp Selter, Zilong Zheng, Stefan D. Oosterhout, Iordania Constantinou, Robert Knitsch, Rylan M. W. Wolfe, Xueping Yi, Jean-Luc Brédas, Franky So, Michael F. Toney, Veaceslav Coropceanu, Michael Ryan Hansen, Kenan Gundogdu, John R. ReynoldsAs many conjugated polymer-based
organic photovoltaic (OPV) materials
provide substantial solar power conversion efficiencies (as high as
13%), it is important to develop a deeper understanding of how the
primary repeat unit structures impact device performance. In this
work, we have varied the group 14 atom (C, Si, Ge) at the center of
a bithiophene fused ring to elucidate the impact of a minimal repeat
unit structure change on the optical, transport, and morphological
properties, which ultimately control device performance. Careful polymerization
and polymer purification produced three “one-atom change”
donor–acceptor conjugated alternating copolymers with similar
molecular weights and dispersities. DFT calculation, absorption spectroscopy,
and high-temperature solution 1H nuclear magnetic resonance
(NMR) results indicate that poly(dithienosilole-alt-thienopyrrolodione), P(DTS-TPD), and poly(dithienogermole-alt-thienopyrrolodione), P(DTG-TPD) exhibit different rotational
conformations when compared to poly(cyclopentadithiophene-alt-thienopyrrolodione), P(DTC-TPD). Solid-state 1H MAS NMR experiments reveal that the greater probability of the
anticonformation in P(DTS-TPD) and P(DTG-TPD) prevail in the solid
phase. The conformational variation seen in solution and solid-state
NMR in turn affects the polymer stacking and intermolecular interaction.
Two-dimension 1H-1H DQ-SQ NMR correlation spectra
shows aromatic–aromatic correlations for P(DTS-TPD) and P(DTG-TPD),
which on the other hand is absent for P(DTC-TPD). In a thin-film interchain
packing study using grazing incidence wide-angle X-ray scattering
(GIWAXS), we observe the π-face of the conjugated backbones
of P(DTC-TPD) aligned edge-on to the substrate, whereas in contrast
the π-faces of P(DTS-TPD) and P(DTG-TPD) align parallel to the
surface. These differences in polymer conformations and backbone orientations
lead to variations in the OPV performance of blends with the fullerene
PC71BM, with the device containing P(DTC-TPD):PCBM having
a lower fill factor and a lower power conversion efficiency. Ultrafast
transient absorption spectroscopy shows the P(DTC-TPD):PCBM blend
to have a more pronounced triplet formation from bimolecular recombination
of initially separated charges. With a combination of sub-bandgap
external quantum efficiency measurements and DFT calculations, we
present evidence that the greater charge recombination loss is the
result of a lower lying triplet energy level for P(DTC-TPD), leading
to a higher rate of recombination and lower OPV device performance.
Importantly, this study ties ultimate photovoltaic performance to
morphological features in the active films that are induced from the
processing solution and are a result of minimal one-atom differences
in polymer repeat unit structure.
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altincidence wide-angle X-rayunit structures impact device performance1 H DQ-SQ NMR correlation spectraMASthienopyrrolodioneone-atomabsorption spectroscopypower conversion efficiencypower conversion efficienciesfullerene PC 71 BMunit structure changeconformationphotovoltaicpolyOPV device performanceTwo-dimension 1 HDFTgroup 14 atomGIWAXScontrol device performancecharge recombination lossπ-quantum efficiency measurementstriplet energy levelpolymercalculationsolution 1 Hvariationblend
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