The neat polymer solutions (20 mg/mL concentration in chlorobenzene) were first investigated using UV-Vis spectroscopy as shown in Fig. . The UV-Vis spectra of PM7 solution contains 0-0 and 0-1 vibronic peaks at 609 nm and 572 nm respectively indicating that the PM7 polymer backbone is highly ordered. On the other hand, PM7 D1 and D2 solutions both show blue-shifted absorption spectra without vibronic peaks suggesting increased backbone disorder with a shorter effective conjugation length. Furthermore, the similarity of the solution and film UV-Vis spectra of PM7 (Fig. and) suggests PM7 may be strongly aggregated in solution. On the other hand, the film UV-Vis spectra of PM7 D1 and D2 becomes more red-shifted, and the 0-0 and 0-1 vibronic peaks start to emerge which indicates that the polymer is becoming more planarized due to further aggregation from solution to the film state. However, UV-Vis spectroscopy measurements alone are not conclusive to compare the solution aggregation state of these three polymers.