Formulation engineering, featuring with different ingredients combined, is a useful strategy for making efficient electronic acceptors because it can achieve effects that cannot be obtained from its components when these are used singly. Herein, we show a contour plot with different formulations of isomeric phenyl-C-71-butyric acid methyl ester (PC71BM) as electron acceptors in poly(3-hexylthiophene) (P3HT):PC71BM polymer solar cells (PSCs). Upon altering fullerene formulation the PCEs change in the range of 0.87-4.01%. The optimized formulation consisting of- 80% a-PC71BM,- 10% 135-PC71BM, and - 10% 132-PC71BM exhibits the best PCE of 4.01%, which mainly benefits from the optimum morphology of P3HT-fullerene heterojunction film. The present work demonstrates a successful strategy by controlling morphology of PC71BM isomers to improve the PCE of P3HT-based PSCs, and consolidates the photovoltaic performance of polymer solar cells heavily dependent on the formulation of fullerene-based electron acceptors.