Highly Ordered Polymorphism of Small Molecule Acceptor Delivering Efficient and Stable Binary Organic Solar Cells

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Organic solar cells (OSCs) achieve high efficiencies (>20%) with narrow bandgap small molecule acceptors (SMAs).
• Precise control over active layer morphology (crystallinity, phase distribution) is crucial for optimizing OSC performance.
• Controlling SMA morphology is challenging due to aggregation, with current methods yielding disordered polymorphs and lacking scalability.

Purpose of the Study:

• To develop a novel, scalable method for inducing highly ordered polymorphs in SMAs.
• To investigate the impact of SMA polymorphism on active layer morphology and photovoltaic performance.
• To enhance organic solar cell efficiency through controlled molecular ordering.

Main Methods:

• Utilizing 4-bromochlorobenzene as a volatile solid additive during mild annealing (60 °C).
• Inducing the formation of a highly ordered polymorph of the BTP-eC9 small molecule acceptor.
• Characterizing the optical properties and crystal structure of the induced polymorph.
• Fabricating and testing organic solar cells based on the PM6:BTP-eC9 blend.

Main Results:

• First demonstration of a highly ordered BTP-eC9 polymorph with ideal crystal-like optical properties (enhanced anisotropy, refractive index, extinction coefficients).
• Formation of a well-organized PM6 donor arrangement and an optimal bicontinuous network morphology.
• Achieved power conversion efficiency of 19.53% for PM6:BTP-eC9 OSCs, increasing to 20.32% with an antireflection layer.

Conclusions:

• A scalable and effective strategy using volatile solid additives for controlling SMA polymorphism and enhancing OSC performance was established.
• The study highlights the critical role of polymorphism in optimizing photovoltaic performance in organic solar cells.
• This approach offers a pathway to further improve OSC efficiencies by precise control over molecular ordering.