Abstract
Metal-halide perovskite solar cells combine high photovoltaic efficiency with low-temperature solution processing, but their performance and durability remain sensitive to precursor coordination, nucleation, drying, grain-boundary formation, and interfacial degradation. This critical narrative review evaluates plant-derived secondary metabolites as process-active additives rather than assuming that biological origin alone confers sustainability or passivation. Evidence is organized for alkaloids and xanthines, polyphenols and phenolic acids, and terpenoids and carotenoids, with explicit separation of experimentally demonstrated mechanisms, computational support, molecular analogy, and design hypotheses. Carbonyl-mediated coordination provides the clearest mechanistic basis for caffeine and related xanthines, whereas selected phenolic compounds have been associated, in separate studies, with coordination, crystallization control, and suppression of Sn²⁺ oxidation. Direct photovoltaic evidence now includes selected monoterpenoids, the sesquiterpene lactone artemisinin, and carotenoids, although the results remain molecule- and process-specific and do not justify class-wide generalization. The review maps molecular descriptors onto precursor chemistry, coating and crystallization variables, film structure, recombination, ion migration, device performance, and stability. Translation beyond laboratory spin coating will require specification-controlled additives, standardized stability protocols, process-compatible formulations, and iterative computational–experimental screening. This evidence-based framework identifies secondary-metabolite scaffolds as a promising but molecule-specific platform for perovskite process engineering, whose sustainability benefit remains to be established through life-cycle assessment.