Abstract
This thesis focuses on the structure-morphology-performance relationship in organic solar cells (OSCs), deeply exploring the complex connection between the active layer film morphology and device efficiency based on high-performing non-fullerene acceptor (NFA) Y6 and its derivatives. Even though the power conversion efficiency (PCE) of OSCs has crossed the 20% threshold, which is still below that of some inorganic and perovskite solar cells, controlling the nanoscale structure of the thin films remains a critical challenge in the field. This thesis addresses this research gap by employing advanced computational simulation techniques to elucidate the molecular-level mechanisms governing film formation and morphology optimization through additive engineering.The first research project utilizes all-atom molecular dynamics (AA-MD) simulations to investigate the impact of nonvolatile solvent additives with varying degrees of π-conjugation on the nanoscale packing of Y6 molecules. The results reveal that van der Waals (vdW) interactions between the end groups of Y6 and the solvent additive molecules are the dominant driving force. Notably, stronger vdW interactions are observed with increasing π-conjugation in the additive, facilitating the insertion of π-conjugated additives between neighboring Y6 molecules, creating more space that allows Y6 molecules to relax into more ordered packing arrangements. This enhanced interaction can also lead to shorter intermolecular distances between Y6 molecules, which is crucial for efficient charge transport.
The second project shifts focus to the influence of a volatile solvent additive, specifically the 2-chlorothiophene (2Cl-Th) solvent, on the active layer morphology and excitonic coupling. Through a combination of experimental measurements and theoretical simulations, the results demonstrate that the 2Cl-Th solvent fosters tighter intermolecular interactions with the NFA compared to the conventional chlorobenzene (CB) solvent. This stronger interaction effectively suppresses over-aggregation and retards crystallization of the acceptor, thereby reducing trap states. Critically, the resulting closer distance between donor and acceptor molecules in the 2Cl-Th treated blend significantly enhances the local excited state to charge-transfer (LE-CT) excitonic coupling (tLE−CT). This enhanced coupling not only promotes efficient exciton splitting but also mitigates non-radiative recombination losses, leading to impressive performance in 2Cl-Th-treated devices: PCEs of 19.8% for small-area devices with excellent operational stability (T80 lifetime of 586 hours), and 17.0% for large-area devices.
The final part of the thesis employs coarse-grained molecular dynamics (CGMD) simulations to explore the large-scale morphology evolution of polymer-NFA blends in ternary organic solar cells (TOSCs) during solution processing. This work develops a CG model capable of capturing key morphological features such as domain sizes and purity, specific interface areas, and the ratio of crystallites. The CGMD simulation results highlight that acceptor aggregation is the primary factor dictating phase separation and domain morphology in these systems. While the incorporation of a third acceptor material (Y6-F) does not significantly alter the domain characteristics of the donor polymer PM6, its impact on acceptor domain size is highly dependent on the blend composition. In PM6:Y6:Y6-F ternary blend films, donor crystallites constitute 28-37% of the blend, while acceptor crystallites maintain a uniform contribution of approximately 40% across different formulations. Notably, backmapping simulations reveal that the third acceptor component enhances packing efficiency within acceptor-rich domains, contributing to improved PCEs. Thus, by integrating advanced structural measurements with computational modeling, this approach offers crucial insights into the effects of ternary components on device performance, paving the way for the rational design and optimization of next-generation organic photovoltaics.
| Date of Award | 28 Nov 2025 |
|---|---|
| Original language | English |
| Awarding Institution |
|
| Supervisor | Alex JEN (Supervisor) & Xian-Kai Chen (External Co-Supervisor) |
Cite this
- Standard