The donor polymers (PM7, PM7 D1 and PM7 D2) used in this study were synthesized according to Jones et al. with similar number average molecular weights between 38 kg/mol and 41 kg/mol with dispersity below 2.5 as shown in Table . The molecular acceptor, ITIC-4F, with > 99% purity was purchased from Brilliant Matters, Inc. The neat polymer solutions were prepared by dissolving each polymer in chlorobenzene (anhydrous, 99.8%, Sigma-Aldrich, Inc) to obtain a solution of 20 mg/mL concentration and stirred at 40 o C for over 12 hours and cooled down to room temperature for 2-3 hours before each experiment. The blend solutions for device fabrication were prepared by mixing each polymer with the small molecule acceptor at 1:1 weight ratio and dissolving the mixture in chlorobenzene to obtain a solution with a total concentration of 40 mg/mL. These blend solutions were also stirred at 40 °C for over 12 hours and cooled to room temperature for 2-3 hours before device fabrication.