In this study, an alcohol soluble novel naphthalene diimide (NDI)−thiophene-based cathode interface layer (CIL), PNDIT10N, is reported. PNDIT10N was synthesized in a facile three-step method, processed from environmentally friendly benzyl alcohol (BnOH) and employed in inverted polymer solar cells (PSCs). The three polymer donors TQ1, PTNT, and PTB7-Th were paired with the fullerene acceptor PC71BM for bulk heterojunction (BHJ) layers to evaluate the CIL. The modification of the indium tin oxide (ITO) electrode with a ∼3 nm thin layer of PNDIT10N yielded a significant reduction of 0.8 eV in the work function, reducing it from 4.6 to 3.8 eV, effectively transforming ITO to a functioning cathode. PSCs with a TQ1/PC71BM BHJ layer and incorporating a PNDIT10N interlayer were found to have a high Jsc value of 10.5 mA cm−2, Vocvalue of 909 mV, and an FF value of 68%, resulting in the highest PCE of 6.7% for TQ1 donor in the inverted device structure. Of note, the interface layer showed a good stability in ambient atmosphere for a 10 d indoor aging period, both in darkness and exposed to direct sunlight. Additionally, flexible PSCs incorporating slot-die coated PNDIT10N, processed from a BnOH− acetone solution, and BHJ layer in air achieved a PCE of 1.6%.Read less <