Probing Side-Chain Engineering for Modulating Exciton Dynamics in Non‑fullerene Acceptors

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Organic solar cells (OSCs) have advanced rapidly, requiring precise control over electronic properties of organic semiconductors.
• Modulating frontier orbital energies (HOMO/LUMO) without affecting light absorption is key for optimizing OSC performance.

Purpose of the Study:

• To develop a predictive side-chain engineering strategy for non-fullerene acceptors (NFAs).
• To identify substituents that selectively tune HOMO/LUMO levels while maintaining the optical bandgap.
• To understand how side-chain structure influences electronic properties and charge dynamics in NFAs.

Main Methods:

• Utilized quantum-chemical calculations for substituent identification and electronic structure modeling.
• Employed structural and excited-state simulations to guide the design of NFAs.
• Applied ultrafast transient absorption spectroscopy (UTAS) to investigate exciton dynamics.

Main Results:

• Designed two NFAs, PDIEH (flexible 2-ethylhexyl) and PDIIN (rigid indanyl), with similar optical gaps (~2.27 eV).
• Flexible alkyl side chains induced inductive effects, stabilizing electronic structure and deepening LUMO levels.
• Rigid aromatic side chains caused partial conjugative perturbations, leading to distinct energy level shifts and altered charge behavior.
• PDIEH showed prolonged charge-separated lifetimes, while PDIIN exhibited faster recombination.

Conclusions:

• Established a chemically intuitive framework for predictive side-chain engineering in organic photovoltaics.
• Demonstrated that side-chain structure critically influences electronic properties and exciton dynamics.
• Side-chain engineering offers a powerful tool for controlling charge behavior and optimizing NFA performance in OSCs.