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This study has proposed to use a well-defined oligomer F4TBT4 to replace its analogue polymer as electron acceptor toward tuning the phase separation behavior and enhancing the photovoltaic performance of all-polymer solar cells. It has been disclosed that the oligomer acceptor favors to construct pure and large-scale phase separation in the polymer:oligomer blend film in contrast to the polymer:polymer blend film. This gets benefit from the well-defined structure and short rigid conformation of the oligomer that endows it aggregation capability and avoids possible entanglement with the polymer donor chains. The charge recombination is to some extent suppressed and charge extraction is also improved. Finally, the P3HT:F4TBT4 solar cells not only output a high VOC above 1.2 V, but also achieve a power conversion efficiency of 4.12%, which is two times higher than the P3HT:PFTBT solar cells and is comparable to the P3HT:PCBM solar cells. The strategy of constructing optimum phase separation with oligomer to replace polymer opens up new prospect for the further improvement of the all-polymer solar cells. [ABSTRACT FROM AUTHOR]