Optimizing Active Layer Morphology of Organic Solar Cells by Constructing Random Copolymers with Simple Third Units.

Area of Science:

• Materials Science
• Polymer Chemistry
• Organic Electronics

Background:

• The performance of organic solar cells (OSCs) is highly dependent on the molecular structure and morphology of the donor polymer.
• Optimizing polymer backbone coplanarity and crystallinity is crucial for efficient charge transport and device efficiency.

Purpose of the Study:

• To investigate the impact of incorporating difluoro-substituted thiophene (2FT) and dicyano-substituted thiophene (2CNT) units into the PM6 polymer backbone via random copolymerization.
• To correlate structural modifications with changes in molecular crystallinity, morphology, charge transport properties, and ultimately, the power conversion efficiency (PCE) of organic solar cells.

Main Methods:

• Random copolymerization of the polymer PM6 with 2FT and 2CNT units.
• Characterization of the resulting copolymers' molecular structure, coplanarity, and crystallinity.
• Fabrication and performance testing of organic solar cells using these modified polymers.
• Analysis of charge carrier mobilities and morphology in the active layers.

Main Results:

• Incorporation of 2FT units enhanced polymer coplanity and molecular crystallinity, leading to improved charge mobilities and a PCE of 12.65% in PM6-10%2FT:IT4F based OSCs.
• Introduction of 2CNT units reduced coplanity and crystallinity, resulting in lower PCEs (<12%) for the 2CN-series random copolymers.
• A ternary device (PM6:L8-BO:PM6-20%2CN) achieved a superior PCE of 19.0% due to enhanced open-circuit voltage.

Conclusions:

• Enhancing the coplanarity of donor polymer backbones through the introduction of specific structural units (like 2FT) can significantly improve molecular crystallinity.
• Optimized polymer crystallinity and backbone coplanity lead to favorable morphology and enhanced charge transport in organic solar cells.
• Strategic molecular design, including the use of ternary blends, offers a promising pathway for achieving high-performance organic solar cells.