Herein, we report an all-polymer solar cell with a PCE of over 5% fabricated with non-halogenated solvent. Our method of polymer side-chain engineering using polystyrene enhanced the solubility of polymers in toluene. The phase separation size of the polymer–polymer blend was controlled by tuning the additive concentration. Three different additives were employed and studied. To the best of our knowledge, this is the highest performing all-polymer solar cell fabricated with both non-halogenated solvent and non-halogenated additive, which highlights its potential toward environmentally friendly manufacturing of all-polymer organic solar cells.