Deepening the LUMO: Brominated Naphthalene Diimide Electron Transport Layers for Low-Hysteresis Perovskite Solar...

Precise energy level alignment at the interfaces between the charge transport layers, active layer, and electrodes plays a key role in maximizing photovoltaic performance and operational stability in perovskite solar cells (PSCs). Organic electron transport layers (ETLs) have received little attention compared to their inorganic counterparts but offer the unique advantage of facile functionalization for fine-tuning of electronic properties. Here, we report the design, synthesis, and characterization of two benzyl-phosphonic acid (BnPA)-functionalized naphthalene diimide (NDI) derivatives, NDI-(BnPA)₂ and Br₂-NDI-(BnPA)₂ as organic ETLs for PSCs in an n-i-p device configuration. Bromination of the NDI core at the 4,9-positions deepens the lowest unoccupied molecular orbital (LUMO) in Br₂-NDI-(BnPA)₂ by 0.29 eV, enabling improved energy alignment with the conduction band minimum of the perovskite. Devices incorporating Br₂-NDI-(BnPA)₂ demonstrated enhanced short-circuit current density (J _SC), reduced hysteresis, and a maximum power conversion efficiency of 13.67%, compared to 13.20% for the unsubstituted NDI-(BnPA)₂ analog. The deeper LUMO of Br₂-NDI-(BnPA)₂ is hypothesized to facilitate more efficient electron extraction, suppress interfacial charge accumulation, and reduce field-driven ion migration, collectively contributing to the observed reduction in hysteresis. These results highlight the effectiveness of combining molecular-level LUMO tuning with robust interfacial anchoring to advance the performance and durability of organic ETLs in PSCs.