Asymmetrical A-DA′D-A–type electron transport materials with enhanced electron mobility and water-resistant interface for perovskite solar cells

Organic small-molecule electron transport materials (ETMs) exhibit fantastic potential in achieving high power conversion efficiency (PCE) of perovskite solar cells (PSCs). In this work, the novel asymmetric naphthalene diimide (NDI) derivatives were designed by fused-ring engineering and end-group engineering based on the symmetric NDI-based E molecule. These asymmetric NDI derivatives are designed by tuning thiophene units (A1, A2, and A3), introducing heteroatoms into the donor (B1, B2), and introducing asymmetric end groups (C1, C2, and C3). Quantum chemical calculations show that the energy levels of ETMs match well with MAPbI₃. In addition, a strong linear correlation (R² > 0.96) is observed between the LUMO energies, adiabatic electron affinities, and reorganization energies. Notably, the electron mobility of the asymmetric molecule B1 is enhanced by 16 times (0.851 cm²V⁻¹s⁻¹) compared to the symmetric E molecule (0.053 cm²V⁻¹s⁻¹). The calculation shows that the designed asymmetric molecules exhibit robust interaction with perovskite, and the Bader charge indicates enhanced electron injection from the perovskite to the ETM. Furthermore, molecular dynamics simulations verified that the asymmetric structure (A2 and C3) can effectively prevent water from invading the perovskite surface. This asymmetric molecular design strategy provides insights for designing novel ETM for high performance PSCs.