Oligomer aggregation

One of the key issues to understand is how the small amount of additive in the solution changes the morphologies of the OSC during solvent drying process. DIO is a high boiling point additive, therefore, it is expected that the concentration of DIO increases as CB evaporates. To mimic this process, we made several models with FBT-biTh, CB, and DIO.

The concentration of DIO increases from 5% to 50% to 90% (w/w) to represent different states during the drying process. As shown in Figure 5.23, when we set the concentration of FBT-biTh at 5% and 10%, however, these were close to the saturation limit and due to the long alkyl chains on the backbone, the molecules started to form disordered aggregation during the equilibrium step. If we decrease the solution concentration to 2%, the individual molecule was isolated in the solution and the contact between two molecules were too rare for aggregation to occur in the MD simulation time frame (Figure 5.24).

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Figure 5.23. Representative snapshot of 100 FBT-biTh molecules in CB solution with 5%, 50% and 90% DIO. Blue: FBT-biTh, Red: DIO. The CB molecules and alkyl chains on FBT-biTh were omitted for clarity.

Figure 5.24. Radius distribution function of 2% FBT-biTh in CB solution with 1.5% DIO as additive. Grey: CB, red: DIO, blue: FBT-biTh. All alkyl chains on the FBT-biTh molecules are omitted for clarity.

Therefore, we conducted a solvent drying simulation to remove CB molecules out of the simulation box to model CB evaporation. As shown in Figure 5.25, while the solvent level dropped, the concentration of DIO increases, so we can track the dynamics of the solute molecules.

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Figure 5.25. Representative snapshots from a drying process of FBT-biTh solution with 1.5% DIO. Grey: CB, red: DIO, blue: FBT-biTh. All alkyl chains on the FBT-biTh molecules are omitted for clarity.

We extracted several intermediate snapshots during the drying process and calculated the RDF of each image, as shown in Figure 5.26. The first peak was observed at about 0.4 nm while the concentration of DIO was 5%, with the concentration increasing, the second and third peaks started to form at 0.8 nm and 1.1 nm, respectively, which indicated the formation of ordered aggregates. This is confirmed from the selected snapshots shown in Figure 5.27, where ordered aggregates start to form at 3.5% DIO and keep expending through 10% DIO systems.

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Figure 5.26. RDF of FBT-biTh molecule in the CB-DIO solution at different DIO concentrations drying solvent drying simulation.

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Figure 5.27. Representative snapshots from drying simulation shows the formation of aggregation of FBT-biTh. Grey: CB, red: DIO, blue: FBT-biTh. All alkyl chains on the FBT-biTh molecules are omitted for clarity.

We note interesting trends in terms of the position and orientation of the FBT-biTh molecules as drying proceeds. As shown in Figure 5.27, the molecular backbones in the aggregates take on a vertical orientation with respect to the liquid–vacuum interface. The position and orientation of the FBT-biTh molecules in the solution is critical to initiate the aggregation process.

To further confirm this, we selected a top region of the simulation box, which is 25% of the total depth at each step. Then we calculated the percentage of the solute and DIO molecules in this region to the total solute or DIO molecules in the system. Figure 5.28 shows the result of the percentage as a function of the DIO concentration during drying.

Clearly, the concentration of solute and DIO both increased, especially on the top region of the simulation box, presented as the blue and red dots. As we performed the drying in a short equilibration time step, this result could have been a result of the limited relaxation time after the CB was removed. When we extended the equilibration time to 10 ns, the percentage of DIO (green rectangle line) slowly dropped back, indicating the extend equilibration time allowed the DIO to diffuse through the system. However, the percentage of solute on the top region was not affected by this. As shown by the green triangle line,

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the percentage of FBT-biTh on the surface remained after the extended equilibration time.

It seems like the FBT-biTh does not want to merge into the solution and stays on the surface.

Figure 5.28. The molecular percentage of FBT-biTh and DIO on the top 25% region of the solution calculated during the solvent drying process. A Pictorial representative of the solution was also given, the thin layer marked in yellow are the CB molecular removed in each drying cycle. Grey: CB, red: DIO, blue: FBT-biTh. All alkyl chains on the FBT-biTh molecules are omitted for clarity.

To further understand this, we explored the orientation of the FBT-biTh molecules in the solutions We used two angles to determine how the molecule aligns with respect to the surface. First, we calculated the dihedral angle between the backbone central DTS unit and the xy-plane, defined by the surface of the solution. We also calculated the angle between the molecular long axes (defined as the vector of the backbone) and the z-axis, which is vertical to the surface of the solution. We calculated these two angles for each FBT-biTh and plotted the population of each angle at 20 ° intervals. As shown in Figure 5.29, with the increasing of DIO concentration, the population of two angles in the 80-100 range increases, while the first dihedral angle around 80-100 ° made the molecular backbone vertical to the surface, the second angle around 80-100 ° puts the molecule long axes along

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with the solution surface. The combination of these two factors reveals that the orientation of the solute is in an upright orientation with respect to the solution surface. A close-up view of the system reveals that the primary driver for this orientation is determined by the alkyl chains. While the two alkyl chains on the termini of the molecule keep the molecular backbone parallel to the surface, and the two alkyl chains on the center DTS unit lead the molecular backbone perpendicular to the interface. As shown in Figure 5.29 (D, E), the molecules in blue and black have the same orientations in terms of the center alkyl chains on DTS pointing to the interface, while the molecule red is the only exception that has the center alkyl chains pointing down.

As discussed previously, the interaction between two FBT-biTh molecules is maximized when the backbone has maximal overlap. In this situation, the linear molecule should have the most potential to enhance the interaction. A side view of the aggregation further reveals that most of the molecules preserved the linear conformation, as shown in blue and black in Figure 5.29. The only exception is the red molecule, which has the “banana shape”

conformation. This is due to the fact that while the center alkyl chains on this molecule is pointing away from the interface, the molecule has to maintain a banana shape in order to keep the terminal alkyl chain close to the interface. The mismatch of the conformation caused a disorder point in this structure and can potentially affect the charge transfer in the film.

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Figure 5.29. A: Angle distribution of the DTS unit to the xy plane of the FBT-biTh molecule. The xy plane is defined as the surface of the vac-solution system. B: Angle distribution of the FBT-biTh backbone to the z axes. The Z-axes was defined as the vector perpendicular to the vac-solution interface. C: Chemical structure of the FBT-biTh shows the inner DTS unit. D: A top view snapshot of one aggregate in the solution during drying process. The red and black molecules are highlighted to represent the two different orientations to the vac-solution interface. Grey: CB molecules. Red, blue, black: FBT-biTh.

All hydrogen atoms on the FBT-biTh molecules are omitted for clarity.

These aggregation studies did not result in large clusters of ordered molecules. This was due a function of i) the rotational freedom on each molecule make it too difficult to aggregate in an orderly fashion, as the molecules can change conformation in solution; and ii) aggregation is by nature a slow event, and the model system size and the time given in

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the simulation is not sufficient for large aggregates to form. Although these drying simulations can provide an initial view of the formation of local clusters, the nature of the molecule limited further study of the aggregation.