Environmentally benign solvent-processed photoactive materials are urgently required to replace the current toxic solvent processing, especially for nonfullerene organic solar cells (NFOSCs). In this regard, we report an efficient new π-conjugated polymer (P1) for high performance NFOSCs. The careful choice of side chain, number-average molecular weight (Mn) tuning (low, medium, and high Mn, denoted as P12, P19, and P32), and dedicated device engineering of NFOSCs resulted an impressive device performance. Interestingly, P19-based NFOSCs achieved a high power conversion efficiency (PCE) of over 10% with nonhalogenated solvents, which is much superior than the P12- and P32-based NFOSCs. The high performance of P1-M-Mn polymer is very well correlated with its high short-circuit current density and fill factor values attained via well-defined active layer morphology. Moreover, the energy losses of the optimized devices are lower, which is very well correlated with the high Voc of the NFOSCs. Additionally, the ambient stability of the NFOSCS processed from nonhalogenated solvents is relatively higher than halogenated solvents because of the better morphological stability of nonhalogenated processed films. Overall, our study provides deep insight on Mn impact on nonhalogenated processing and thereby efficiency enhancement in environmentally sustainable NFOSCs.