Cross-linkable naphthalene diimide acceptors for applications in photovoltaic devices

Cross-linkable naphthalene diimide (NDI) electron-transporting materials (ETMs) bearing vinylbenzyl groups were designed, synthesized and evaluated for possible photovoltaic applications. A one-pot condensation followed by O-alkylation reaction was employed to synthesize new compounds. The new NDIs are sufficiently thermally and electrochemically stable owing to the presence of vinyl groups; all compounds are able to undergo thermal cross-linking by heating at 199°C and 213°C. Differential scanning calorimetry (DSC) and solubility tests revealed a two-step cross-linking mechanism: a low-temperature linear polymerization followed by high-temperature three-dimensional network formation. Cross-linking causes minimal changes in electron mobility in neat films, but in donor–acceptor blends it increases electron mobility by a factor of approximately 2–3, maintains stable hole transport, suppresses deep-trap formation and stabilizes quantum efficiency of photogeneration over time. These results establish vinylbenzyl-functionalized NDIs as promising, solution-processable n-type semiconductors for solvent-resistant, thermally robust electron-transport layers for perovskite or organic photovoltaics.