Unveiling the Role of Donor–Acceptor Attachment Sites in Molecular Design for High-Performance Organic Solar Cells: A Theoretical Study

Organic solar cells (OSCs) based on π-conjugated small molecules are attracting growing interest as lightweight, flexible, and solution-processable photovoltaic technologies compatible with sustainable energy strategies. Within this class of materials, acceptor–donor–acceptor (A-D-A) chromophores have emerged as a particularly versatile platform, where subtle modifications of donor–acceptor connectivity can substantially alter energy levels, optical gaps, and charge-transport descriptors. Understanding how these intramolecular design choices control key electronic and photophysical properties is therefore essential for the rational development of next-generation OSC active layers.

In this contribution, we present a systematic theoretical investigation of a family of A-D-A small molecules constructed from an electron-rich pentathiophene (PTP) core and electron-deficient trimethylxanthine (TMX) terminal units. By varying only the attachment sites of the two TMX fragments around the PTP ring, four symmetry-distinct isomers are generated within a single chemical platform, allowing us to isolate the effect of donor–acceptor connection pattern without changing the underlying building blocks. Ground-state geometries and electronic structures are obtained through density functional theory (DFT), while excited-state properties are explored using time-dependent DFT (TD-DFT) in an implicit solvent environment. The workflow includes the calculation of frontier orbital energies, density-of-states profiles, chemical reactivity indices, and exciton binding energies, together with simulated absorption and emission spectra and an analysis of charge-transfer character.

Charge-transport descriptors are evaluated within the non-adiabatic Marcus hopping framework from intramolecular reorganization energies, Gibbs free energy differences, and effective electronic couplings extracted from frontier-orbital splittings. In addition, a semi-empirical photovoltaic model is employed to estimate open-circuit voltage, light-harvesting efficiency, fill factor, and normalized power-conversion descriptors based on the computed energetics and oscillator strengths.

The presentation will focus on how this integrated DFT/TD-DFT and Marcus theory framework links donor–acceptor attachment sites to changes in electronic structure, optical response, and charge-transport metrics across the four PTP/TMX isomers. Emphasis will be placed on extracting general structure–property relationships and qualitative design rules that can inform the future synthesis and device integration of A-D-A chromophores for organic solar cells, rather than on reproducing specific experimental devices or quantitative efficiencies.