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.