High Exciton Diffusion Coefficients in Fused Ring Electron Acceptor Films

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Organic photovoltaic (OPV) cells typically require nanostructured bulk heterojunctions due to limited exciton diffusion lengths.
• This nanostructure, however, can impede efficient charge collection and reduce overall device performance.

Purpose of the Study:

• To investigate exciton diffusion dynamics in a novel fused-ring electron acceptor blended with a donor material.
• To determine if enhanced exciton diffusion can overcome limitations of traditional OPV morphologies and improve performance.

Main Methods:

• Utilized temperature-dependent ultrafast exciton annihilation measurements to quantify exciton diffusion.
• Analyzed molecular and packing factors influencing exciton diffusion in the organic films.

Main Results:

• Resolved a quasi-activationless exciton diffusion coefficient of at least 2 × 10^-2 cm²/s, significantly higher than typical organic semiconductors.
• Identified molecular rigidity, planar structure, high chromophore density, and low disorder as key factors enhancing exciton diffusion via resonant energy transfer.
• Achieved OPV performance exceeding fullerene-based cells.

Conclusions:

• Enhanced three-dimensional exciton diffusion, driven by molecular design, substantially improves OPV performance by facilitating charge separation and transport.
• Optimized domain structure and suppressed recombination lead to higher fill factors.
• Further improvements in exciton diffusion could potentially negate the necessity for bulk heterojunction architectures in OPVs.