The open-circuit voltage (Voc) of all-polymer solar cells (all-PSCs) is typically lower than 0.9 V even for the most efficient ones.Large energy loss is the main reason for limiting Voc and efficiency of all-PSCs.Herein,through materials design using electron deficient building blocks based on bithiophene imides,the lowest unoccupied molecular orbital (LUMO) energy levels of polymer acceptors can be effectively tuned,which resulted in a reduced energy loss induced by charge generation and re-combination loss due to the suppressed charge-transfer (CT) state absorption.Despite a negligible driving force,all-PSC based on the polymer donor and acceptor combination with well-aligned energy levels exhibited efficient charge transfer and achieved an external quantum efficiency over 70％ while maintaining a large Voc of 1.02 V,leading to a 9.21 ％ efficiency.Through various spectroscopy approaches,this work sheds light on the mechanism of energy loss in all-PSCs,which paves an avenue to achieving efficient all-PSCs with large Voc and drives the further development of all-PSCs.