The Asymmetric Strategy of Small‐Molecule Materials for Organic Solar Cells

Abstract

The conjugated small‐molecule materials of organic solar cells have always played a crucial role in light‐harvesting, charge transport, morphology optimization, and the attainment of efficient devices. The advancement of novel materials and the understanding of underlying molecular design rules serve as the driving force for furthering efficient and stable photovoltaic devices. Among a variety of design principles, the symmetry‐breaking strategy, which is well developed in 3,9‐bis(2‐methylene‐(3‐(1,1‐dicyanomethylene)‐indanone))‐5,5,11,11‐tetrakis(4‐hexylphenyl)‐dithieno[2,3‐d:2′,3′‐d’]‐s‐indaceno[1,2‐b:5,6‐b’]dithiophene (ITIC)‐series acceptors, recently demonstrates great potential in small‐molecule acceptors and donors for realizing high power conversion efficiency. In this review, in order to give a deep insight on asymmetric strategy, the small‐molecule acceptors and donors are systematically summarized with asymmetric structure to elucidate structure‐performance relationship, molecular packing behaviors, and morphology evolution. Not only the delicate balance between open circuit voltage and short‐circuit current density, but also the reductions in charge recombination and non‐radiative recombination are considered to play the key points in improving the photovoltaic performance when the asymmetric molecule is used as host or guest materials. Finally, concise challenges and outlooks are provided for the future development and application of asymmetric molecules and symmetry‐breaking strategies.

Asymmetrical Diketopyrrolopyrrole Derivatives with Improved Solubility and Balanced Charge Transport Properties

“…The decrease in molecular weight has an obvious effect on reducing the melting point of the molecules (hence increasing the solubility); the lower symmetry has a positive effect on solubility as well, as previously reported [ 49 ]. Indeed, the desymmetrization strategy already proved to afford improved solubility in a class of organic semiconductor derivatives applied in organic photovoltaics [ 50 ]. This molecular design made it possible to obtain soluble derivatives by using the short 2-ethylhexyl tail (that in the case of the previously reported dyes was sufficient to achieve a good solubility only for furan derivatives), helping the overall synthetic procedure and reducing the percentage of the inactive, insulating part in the final molecular structure.…”

Nonfullerene Acceptors with Aromatic Heterocycle Substitutions for Efficient Organic Solar Cells

“…Additionally, various strategies for molecular modification have emerged. Among these, side chains ensure sufficient solubility to facilitate solution processing, while alterations to the central backbone and end group have proven effective in optimizing the optoelectronic properties of the molecules. − The compound 1,1-dicyanomethylene-3-indanone (IC) is recognized for its potent electron-withdrawing capability as an end group, rendering it a favorable option for SMAs. Functionalizing the end group is widely recognized as an effective means of adjusting solubility, optical band gap, energy level, and intermolecular stacking of SMAs, ultimately leading to high-performance photovoltaic devices. − End group functionalization typically focuses on two aspects: π-conjugated extension and substituent modification. − Extending the π-conjugation enhances the electronic delocalization, thereby improving the charge transportation.…”