Toward Quantifying the Relation between Exciton Binding Energies and Molecular Packing

Area of Science:

• Materials Science
• Physical Chemistry
• Organic Electronics

Background:

• Exciton binding energy (Eb) reduction is crucial for organic solar cell efficiency.
• Understanding the relationship between molecular packing and Eb is vital but not well-established.

Purpose of the Study:

• To investigate the impact of molecular packing on exciton binding energy in A-D-A type nonfullerene acceptors.
• To elucidate the correlation between alkyl side chain length, molecular packing, and Eb.

Main Methods:

• Employed self-consistent quantum mechanics/embedded charge calculations.
• Examined Eb in crystals of nonfullerene acceptors with varying alkyl chain lengths.

Main Results:

• Molecular packing variations, influenced by alkyl chains, affect charge carrier polarization and Eb.
• Eb linearly increases with the ratio of void fraction to packing coefficient.
• An optimal alkyl chain length yielded a significantly low Eb of tens of meV.

Conclusions:

• Molecular packing is a critical factor in tuning exciton binding energy in organic semiconductors.
• Controlling molecular packing through side chain engineering offers a pathway to enhance organic solar cell performance.