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

## TL;DR

This paper explores how modifying side-chains in non-fullerene acceptors can control exciton dynamics in organic solar cells.

## Contribution

A predictive side-chain engineering approach is introduced to modulate energy levels without affecting light absorption.

## Key findings

- Flexible alkyl groups stabilize electronic structures via inductive effects.
- Rigid aromatic groups cause conjugative perturbations and distinct energy shifts.
- PDIEH shows prolonged charge-separated lifetimes, while PDIIN shows faster recombination.

## Abstract

The last 2 years have witnessed rapid progress in organic
solar
cells (OSCs), and precise modulation of frontier orbital energies
without compromising light absorption is crucial for optimizing organic
semiconductors. This study presents a predictive side-chain engineering
approach for non-fullerene acceptors (NFAs) using quantum-chemical
calculations to identify substituents that selectively shift the HOMO
and LUMO levels while preserving the optical bandgap. Guided by structural
and excited-state simulations, two non-fullerene acceptors, PDIEH
and PDIIN, were designed, incorporating flexible 2-ethylhexyl and
rigid indanyl groups, respectively. Electronic structure modeling
revealed that flexible alkyl groups stabilize the electronic structure
via inductive effects, whereas rigid aromatic groups introduce partial
conjugative perturbations, resulting in distinct energy level shifts.
Despite similar optical gaps (∼2.27 eV), these electronic modulations
critically impact exciton dynamics as revealed by ultrafast transient
absorption spectroscopy (UTAS). PDIEH exhibits deeper LUMO levels
and prolonged charge-separated lifetimes, while PDIIN shows faster
recombination. This study establishes a chemically intuitive framework
where side-chain engineering enables predictive control of electronic
properties and charge behavior in organic photovoltaics.