A series of naphthalene diimide-based polymers incorporated with branched oligo(ethylene glycol) (OEG) side chains are developed to enable green-processed, high-performance n-type OECTs. The OEG side chains afford processability in ethanol/water solvents. Importantly, taking advantage of the amphiphilicity of OEG-based polymers, ethanol/water solvents selectively solvate hydrophilic OEG side chains, while producing tight π−π stacks of hydrophobic backbones. This enables highly edge-on-oriented polymer packing, and thus excellent lateral charge transport. In particular, the fine-tuned OEG side chains of P(NDIMTEG-T) provide compact backbone packing, effective polaron generation, and superior electrochemical stability. The resultant OECT shows the best electrical/electrochemical performance, represented by a figure-of-merit (μC*) of 0.56 F V−1 cm−1 s−1. This study demonstrates the aqueous processing in OECTs and the design rule of n-type organic mixed ionic-electronic conductors.