On the Molecular Origin of Charge Separation at the Donor–Acceptor Interface

• Published in: Advanced energy materials 2018-04, Vol.8 (12), p.n/a
• Main Authors: Sini, Gjergji, Schubert, Marcel, Risko, Chad, Roland, Steffen, Lee, Olivia P., Chen, Zhihua, Richter, Thomas V., Dolfen, Daniel, Coropceanu, Veaceslav, Ludwigs, Sabine, Scherf, Ullrich, Facchetti, Antonio, Fréchet, Jean M. J., Neher, Dieter
• Format: Article
• Language: English
• Subjects: Chemical Sciences; Deformation mechanisms; Density functional theory; Diimide; donor–acceptor interfaces; Energy gradient; energy gradients; geometrical deformations; Heterojunctions; Molecular structure; nonfullerene acceptors; Optoelectronics; organic photovoltaics; photocurrent generation; Photovoltaic cells; polymer solar cells; Polymers; Polythiophene; Separation; Solar cells
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.201702232

Fullerene‐based acceptors have dominated organic solar cells for almost two decades. It is only within the last few years that alternative acceptors rival their dominance, introducing much more flexibility in the optoelectronic properties of these material blends. However, a fundamental physical understanding of the processes that drive charge separation at organic heterojunctions is still missing, but urgently needed to direct further material improvements. Here a combined experimental and theoretical approach is used to understand the intimate mechanisms by which molecular structure contributes to exciton dissociation, charge separation, and charge recombination at the donor–acceptor (D–A) interface. Model systems comprised of polythiophene‐based donor and rylene diimide‐based acceptor polymers are used and a detailed density functional theory (DFT) investigation is performed. The results point to the roles that geometric deformations and direct‐contact intermolecular polarization play in establishing a driving force (energy gradient) for the optoelectronic processes taking place at the interface. A substantial impact for this driving force is found to stem from polymer deformations at the interface, a finding that can clearly lead to new design approaches in the development of the next generation of conjugated polymers and small molecules. Understanding photocurrent generation at the molecular level is one of the greatest challenges in organic solar cell research. By performing an experimental and theoretical in‐depth study, the authors are able to correlate molecular structure with charge generation efficiency in prototypical polymer solar cells. These results provide clear structure–property relationships that will help to design a new generation of nonfullerene acceptors.