Polymer Electron Acceptors Based on Iso‐Naphthalene Diimide Unit with High LUMO Levels

The lowest unoccupied molecular orbital (LUMO) energy level ( E LUMO ) is a key parameter for polymer electron acceptors of all‐polymer solar cells (all‐PSCs). The state‐of‐the‐art polymer electron acceptors based on 1,4,5,8‐naphthalenediimide (NDI) unit suffer from low‐lying E LUMO , which leads to low open‐circuit voltage ( V OC ) of all‐PSCs. In this manuscript, the authors use 1,2,5,6‐naphthalenediimide (iso‐NDI) unit to develop a series of polymer electron acceptors with high‐lying E LUMO . Their thermal, optical, electrochemical, and charge transporting properties are systematically investigated and their applications in all‐PSCs are explored. The polymer based on iso‐NDI unit and bithiophene unit exhibits the E LUMO of −3.50 eV, which is higher than that of the corresponding polymer based on NDI unit ( E LUMO = −3.86 eV) by 0.36 eV. As a result, the all‐PSC devices exhibit V OC enhancement by 0.30 V. All‐PSC devices of these polymer electron acceptors with poly(3‐hexylthiophene) as the electron donor show the V OC of 0.90 V and power conversion efficiency of 0.27%. These results indicate that iso‐NDI unit is a promising building block to design polymer electron acceptors for all‐PSCs with high V OC .