Impact of backbone fluorination on nanoscale morphology and excitonic coupling in polythiophenes

Abstract

Fluorination represents an important strategy in developing highperformance conjugated polymers for photovoltaic applications. Here, we use regioregular poly(3-ethylhexylthiophene) (P3EHT) and poly(3-ethylhexyl-4-fluorothiophene) (F-P3EHT) as simplified model materials, using single-molecule/aggregate spectroscopy and molecular dynamic simulations, to elucidate the impacts of backbone fluorination on morphology and excitonic coupling on the molecular scale. Despite its high regioregularity, regioregular P3EHT exhibits a rather broad distribution in polymer chain conformation due to the strong steric hindrance of bulky ethylhexyl side chains. This conformational variability results in disordered interchain morphology even between a few chains, prohibiting long-range effective interchain coupling. In stark contrast, the experimental and molecular dynamic calculations reveal that backbone fluorination of F-P3EHT leads to an extended rod-like single-chain conformation and hence highly ordered interchain packing in aggregates. Surprisingly, the ordered and close interchain packing in F-P3EHT does not lead to strong excitonic coupling between the chains but rather to dominant intrachain excitonic coupling that greatly reduces the molecular energetic heterogeneity.fluorinated polythiophenes | photophysics | single-molecule spectroscopy | morphology | organic electronics M orphology and excitonic coupling are among the most important factors dictating the functions and performance of conjugated polymers (CPs) in a variety of optoelectronic applications (1-4). To tune and optimize the morphological and optoelectronic properties of CPs, chemical structure modification of the backbone and side chains is one of the main and most effective approaches (5-7). It has been revealed that the position, size, and length of side chains dramatically affect the morphology and optoelectronic properties of CPs. In the efforts to develop high-performance photovoltaic CPs, the introduction of fluorine atoms onto aromatic comonomers, i.e., the fluorination of polymer backbone, offers a very appealing strategy to tune the electronic properties, morphology, and photochemical stability (6-8). It is proposed that the inherent electronwithdrawing nature of fluorine atoms lowers the highest energy occupied molecular orbital energy levels, thereby increasing the open-circuit voltage in photovoltaics. In addition, the strong electronegativity might induce F-S and F-H interactions and potentially modifies the molecular conformation and intermolecular organization (6). Furthermore, it has been shown that fluorination often leads to enhanced thermal and oxidative stability (9-11). Given a combination of these desirable properties, fluorine-containing CPs have led to most of the best-performing polymer-based solar cells to date (12-15).For typical CPs, the primarily created exciton is Coulombically bound upon photoexcitation mainly due to the low dielectric constant and strong electron-phonon interaction in polymers. The interchain and intrachain morp...

The Influence of Backbone Fluorination on the Dielectric Constant of Conjugated Polythiophenes

“…19 F NMR only afforded a single peak, again suggestive of very high degrees of backbone regioregularity. In agreement with our earlier studies, [38] all fluorinated poly mers were less soluble than their nonfluorinated analogues and required processing from hot chlorinated solvents like chlo robenzene or 1,2dichlorobenzene. The molecular weights, as measured by gel permeation chromatography (GPC) in either hot chlorobenzene (P3OTbFP3OT and P3OTaltFP3OT or hot 1,2,4trichlorobenzene (FP3OT), of all fluorinated poly mers were similar ( Table 1).…”

Ordered Solid‐State Microstructures of Conjugated Polymers Arising from Solution‐State Aggregation

“…The large blueshift up to 35 nm in absorption features might result from the molecular conformation, as absorption spectra have been shown to be depended on the molecular conformation. [27,28] Thus, we confirmed that the temperature effect can induce molecular conformation variations of conjugated polymers. The conformation variations can affect the solution-state aggregation and polymer crystallization in the film-formation process, [13,29,30] which are well supported by the following experiments and simulations.…”

Development of PolyParGen Software to Facilitate the Determination of Molecular Dynamics Simulation Parameters for Polymers

“…were conducted in order to evaluate the proposed program. F-P3EHT is a fluorinated P3EHT that is reported to have an extended structure in chloroform [8]. As another example, MD simulations of polystyrene in THF and water were also conducted.…”

“…The obtained files were implemented in the 2018 version of Gromacs to perform the 20-ns MD simulation. The simulation conditions were set as suggested by Hua et al [8]. That is, P3EHT, and F-P3EHT were set in a 14.4-nm cubic box filled with chloroform, and the periodic boundary condition was applied.…”