Next, we select the conditions to vary the solution aggregate structure to the largest extent possible to understand its impact on solution coated thin film morphology and charge transport. For this purpose we choose three temperatures at which the aggregate structures in three classes of solvents are distinct and solvent vapor pressures are matched for subsequent solution coating: 115 °C for CN, 55 °C for DCB and 60 °C for decane. As shown in Figure and discussed above, at selected temperature, polymers dissolved in CN exhibit a featureless low-energy absorption band with a significant blue-shift compared to absorption at RT, indicating a disordered chain conformation . Polymers in decane at 60 °C, however, show a very pronounced vibronic progression in lower-energy UV-vis band with a high 0-0/0-1 peak intensity ratio (A0-0/A0-1) of 1.32. This indicates a strong J-coupling between the head-to-tail arranged transition dipole moments of the monomer units , which is due to highly planarized chain conformation possibly resultant from strong π-π interactions in such a poor solvent for the backbone. On the other hand, DCB gives a vibronic structure but with a smaller A0-0/A0-1 value of 1.09, suggesting a more torsional backbone conformation compared with the decane case. We further confirm our inference and characterize the aggregate structure in three cases using a combination of SAXS, electron diffraction and electron microscopy techniques. Our group has recently developed models to fit and interpret SAXS profiles of conjugated polymer solutions . Based on this prior work, we analyze our SAXS profiles by considering two populations of scatterers present in the solutions: single polymer chains that are individually dispersed in solution and solution aggregates of which the detailed structure is dependent on the selectivity of the solvent. A summary of SAXS fitting parameters is listed in Table . As small SFC model representing single polymer chains with a fitted persistence length of 6.7 nm in solution, a large SFC model representing fiber-like aggregates with a diameter ~30 nm and a power-law of 4 attributed to surface scattering of even larger fiber-like aggregates with diameter out of the measured q range. In contrast to the CN case, the aggregates in DCB exhibit a π-π stacking peak centered at 0.23 Å -1 with a FWHM of 0.10 Å -1 in the electron diffraction pattern (Figure , S7, and S9). This corresponds to a π-π stacking distance dπ of 4.37 Å. The SEM image shown in Figure further confirms that the aggregates are 1-D fiber-like. We thus refer the aggregates in DCB as semicrytalline fibers with both π-π and lamellar stacking internally. For the agglomerates-like aggregates in decane which are composed of randomly oriented fibers with a diameter around 100 nm (Figure and S8), the SAXS profile is comprised of two components: a small SFC model representative of polymer chains with a persistence length of 7.4 nm in solution and a large SFC model corresponding to those 100 nmwide fibers which is consistent with scanning electron microscope (SEM) images of sample freeze dried at 60 ºC (Figure ). The absence of the lamellar stacking peak in SAXS indicates no side-chain association. However, electron diffraction reveals a closer π-π stacking inside the aggregates than in the case of DCB (Figure and S7), consistent with our understanding that decane is a poorer solvent for the backbone than DCB. The π-π stacking distance dπ is 4.10 Å calculated from the electron diffraction peak centered at 0.244 Å -1 with a FWHM of 0.091 Å -1 (Figure ). Therefore, the aggregates in decane form large aggloromates comprised of thick fibers with internal backbone stacking but no side chain stacking. from SAXS as the aliphatic side chains tend to be well solubilized by decane according to "like dissolves like". The polymer aggregates possess a highly planar conformation, inferred from solution UV-vis absorption spectrum that shows a pronounced vibronic progression with a relatively high A0-0/A0-1 ratio. The strongly packed chains aggregate to form 100-nm-wide fibers which further agglomerate into micron-scale aggregates visible under optical microscope. As for the mutual solvent DCB, the polymers adopt a tortional conformation with a persistence of 6.7 nm. The polymers aggregate into fiber-like structures with a diameter around 30 nm from SAXS fitting (Figure and Table ) and supported by the SEM image (Figure ). SAXS and electron diffraction reveal the semicrystalline nature of the fiber aggregates as both π-π and lamellar stacking peak are observed (Figure and 1e), which is a result of DCB's mutual yet moderate solubilizing ability towards the backbone and side chains. The π-π and lamellar stacking distance are 4.37 Å and 2.3 nm, respectively. Our recent work has demonstrated that, when dissolved in a mutual solvent such as chloroform and chlorobenzene, PII-2T polymer chains/fiber aggregates adopts chiral helical structure shown in Figure and S10 . Unlike the amorphous ones in CN, the semicrystalline aggregates in DCB give a vibronic progression in the UV-vis low-energy absorption band which we attribute to aggregation-induced backbone planarization (Figure ). The vibronic progression in DCB, however, exhibits a lower A0-0/A0-1 ratio than that in decane possibly owing to a relatively tortional/helical chain conformation in the aggregates. This is also consistent with a larger π-π stacking distance (4.37 Å in DCB vs.