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

Abstract

The lowest unoccupied molecular orbital (LUMO) energy level (ELUMO) 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 ELUMO, which leads to low open‐circuit voltage (VOC) 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 ELUMO. 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 ELUMO of −3.50 eV, which is higher than that of the corresponding polymer based on NDI unit (ELUMO = −3.86 eV) by 0.36 eV. As a result, the all‐PSC devices exhibit VOC enhancement by 0.30 V. All‐PSC devices of these polymer electron acceptors with poly(3‐hexylthiophene) as the electron donor show the VOC 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 VOC. image