Hydrogen-Bonding Networks Enabled by Trace Water for Morphological Design in Ternary Organic Solar Cells.

Morphology evolution is critical to the performance of functional materials, but strategies for its control remain largely empirical. Here, we identify a counterintuitive role of water (H2O) as a morphology-regulating agent in ternary organic solar cells (OSCs), traditionally considered an impurity. Molecular dynamics simulations reveal that the dual hydrogen-bonding capacity of H2O drives the formation of dynamic hydrogen-bonding networks (HBNs). Continuous HBNs facilitate the migration of the third component into donor-enriched domains through encapsulation, thereby stabilizing alloy-like morphologies. While this HBN-driven transition fails in single-donor solvent systems such as ethanol, it extends to both fullerene and non-fullerene blends in multi-donor or acceptor environments. To render the mechanism applicable in organic processing, we adopted a co-solvent strategy and identified a critical regime at a water-to-chloroform volume ratio of 0.06:1. At this threshold, trace H2O reproduces the alloy-like behavior in neat H2O without compromising solubility, providing practical utility for device processing. Analyses of H2O containing CB, DMSO, and THF co-solvents further confirm the general applicability of the HBN mechanism across distinct solvent environments. This work redefines H2O as a functional additive, establishes HBN engineering as a general framework for morphology control, and suggests broader implications for functional materials governed by weak interactions.