Methylated naphthalene additives with various melting and boiling points enable a win–win scenario of optimizing both cost and efficiency of polymer solar cells†

Using additives to fine-tune morphology is one of the effective strategies to enhance the performance of polymer solar cells (PSCs). Currently, high-performance additives mostly are halogenated materials or high-boiling-point solvents, which inevitably affect the environment and device stability. Herein, three structurally similar naphthalene derivatives, 1-methylnaphthalene (1-MN), 2-methylnaphthalene (2-MN), and 2,7-dimethylnaphthalene (2,7-MN), were employed as solvent/solid additives in view of their various melting and boiling points. These non-halogenated additives are readily available and low-cost. All three additives have good volatility and can be completely removed from the active layer, which is beneficial to enhance device stability. Moreover, the volatile solid additive 2-MN showed the best ability to control the active layer morphology, ultimately achieving an improved efficiency of 18.70% for PM6:L8-BO-based PSCs compared to the other two additives and the control device without an additive. The increased performance was attributed to the increased charge mobilities, dissociation/collection efficiencies, charge extraction rates and carrier lifetimes, and reduced charge recombination. Furthermore, the results demonstrate that 2-MN has good universality in diverse device systems and can enhance thermal stability and photostability of PSCs. Our results suggest that methyl-substituted naphthalene derivatives would be low-cost, environmentally friendly and high-performance additives to tune morphology and enhance the efficiency and stability of PSCs.