Boron-containing electron transport materials based on naphthalene diimide for organic solar cells: a theoretical study

Electron transport materials (ETMs) are crucial for extracting and transporting electrons from the active layer to the cathode in organic solar cells (OSCs). In this work, we designed a series of ETM candidates (E1-E6) based on the experimentally synthesized (N,N-dimethylamino)propyl naphthalene diimide (NDIN) molecule by changing the nitrogen (N) atoms with boron (B) atoms at different positions. The electronic properties, electron transfer mobility, and interfacial properties were investigated using density functional theory (DFT) and time-dependent DFT (TD-DFT). The computed results indicate that introducing boron atoms into side chains enhances electron mobility with more pronounced effects as the number of boron atoms increases, while inserting boron atoms into the core ring decreases the electron mobility. Importantly, the E4 molecule exhibits the most promising performance, where its electron mobility is about twice that of NDIN and the binding energy of E4 with the acceptor is approximately 40% higher than that of NDIN. We also constructed donor/acceptor/electron transport material (D/A/E) interfaces and found that the introduction of ETMs produces new charge transfer (CT) states in the low energy region. Compared to NDIN, the incorporation of E4 further expands the pathways for generating CT states, thereby enhancing exciton separation in the active layer and increasing the short-circuit current density (J_sc). This work not only provides valuable insights into future experimental research on ETMs but also offers guidance on the strategic incorporation of heteroatoms into ETMs for the rational design of high-efficiency OSC components.