Abstract
Fluorination of conjugated polymers is an effective strategy to tune the energy levels for obtaining high power conversion efficiency (PCE) in organic solar cells. In this work, we have developed fluoro-modulated molecular geometries in diketopyrrolopyrrole based low-bandgap copolymers. In these polymers, planar conformation can be locked by intramolecular non-covalent interaction (intramolecular supramolecular interaction) between the sulfur atoms and the introduced F atoms (F···S interaction). By varying the fluorinated moieties, such a planarity can be disturbed and the molecular geometry is tuned. As a result, the polymer' properties can be modulated, including the ultraviolet-visible absorption spectrum to become broaden, charge mobility to be enhanced, open-circuit voltage (Voc) and short-circuited current (Jsc) to be elevated, and thus photovoltaic performance to be improved. The photovoltaic device based on PCFB, one of the fluorinated terpolymers, exhibited a high PCE near 8.5% with simultaneously enhanced Voc and Jsc relative to the non-fluorinated one (PCB).
Highlights
Low-band gap (LBG) polymers have attracted lots of research attention due to their possibility to extend the absorption of solar spectrum from ultraviolet-visible (UV-vis) to near-infrared (NIR) region to make a good utilization of solar energy when used in organic solar cells (OSCs)
The one located at shorter wavelength can be attributed to the localized π-π∗ transition and the strong peaks located at longer wavelength originate from the intramolecular charge transfer (ICT) (Tanaka et al, 2013)
The dominant absorption peak of PCFB (∼740 nm) is slightly red-shifted compared with that of other polymers. This result can be explained by the shorter lamellar distance existing in PCFB film as validated from Xray diffraction (XRD) data, which indicates that the polymer chains were packed more tightly (Wang Y. et al, 2017)
Summary
Low-band gap (LBG) polymers have attracted lots of research attention due to their possibility to extend the absorption of solar spectrum from ultraviolet-visible (UV-vis) to near-infrared (NIR) region to make a good utilization of solar energy when used in organic solar cells (OSCs). The photovoltaic device based on PCFB, one of the fluorinated terpolymers, exhibited a high PCE near 8.5% with simultaneously enhanced Voc and Jsc relative to the non-fluorinated one (PCB).
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