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Abstract
Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. However, the widely reported low efficiency of doping remains a crucial limitation to obtain high performance. Here we investigate how charge transfer between dopant and donor-acceptor copolymers is affected by the spatial arrangement of the dopant molecule with respect to the copolymer repeat unit. We p-dope a donor-acceptor copolymer and probe its charge-sensitive molecular vibrations in films by infrared spectroscopy. We find that, compared with a related homopolymer, a four times higher dopant/polymer molar ratio is needed to observe signatures of charges. By DFT methods, we simulate the vibrational spectra, moving the dopant along the copolymer backbone and finding that efficient charge transfer occurs only when the dopant is close to the donor moiety. Our results show that the donor-acceptor structure poses an obstacle to efficient doping, with the acceptor moiety being inactive for p-type doping.
Highlights
Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices
We provide a microscopic description of doping in the D–A copolymer (poly[2,6-(4,4-bis(2-ethylhexyl)-4H-cyclopenta[2,1-b;3,4-b00]dithiophene)-alt-4,7-(2,1,3-benzothiadiazole)] (PCPDT-BT) mixed at different concentrations with the dopant 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ)
To calculate the % molar ratio, we use the number of moles of F4-TCNQ divided by the number of moles of polymer repeat unit
Summary
Molecular doping of conjugated polymers represents an important strategy for improving organic electronic devices. In contrast to previous studies looking at the vibrational absorption bands of the dopant[21,32], our work is concentrating on the modes of the conjugated polymer backbone This distinctive approach is motivated by the following two reasons: (i) the much higher sensitivity of these bands in witnessing charge carriers and the possibility to study the low doping concentration regime of 1% molar ratio, and (ii) a more detailed understanding of the role of polymer-dopant geometrical structure on doping, which we achieve by ab initio simulations despite the demanding computational effort in reproducing the experimental spectra. As expected in the case of more efficient doping, this homopolymer exhibits the characteristic spectroscopic signals of charges on the conjugated chain at lower doping concentrations
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