# Hydrogen‐Bonding Networks Enabled by Trace Water for Morphological Design in Ternary Organic Solar Cells

## TL;DR

Trace amounts of water help control the structure of organic solar cells, improving their performance by guiding material arrangement.

## Contribution

Water is redefined as a functional additive that enables morphology control through hydrogen-bonding networks in ternary organic solar cells.

## Key findings

- Trace water forms hydrogen-bonding networks that stabilize alloy-like morphologies in organic solar cells.
- A co-solvent strategy with a water-to-chloroform ratio of 0.06:1 enables practical morphology control.
- The hydrogen-bonding mechanism works across various solvent environments including CB, DMSO, and THF.

## Abstract

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.

Trace water transforms from an impurity to a design handle for ternary organic solar cells. During solvent evaporation, minimal amounts organize transient hydrogen‐bonding networks that steer component migration and set the morphology. The mechanism enables reproducible morphology locking and identifies a practical co‐solvent threshold near H2O:CF = 0.06:1 with relevance to scalable solution processing.