Hexafluoroquinoxaline Based Polymer for Nonfullerene Solar Cells Reaching 9.4% Efficiency

Area of Science:

• Organic electronics
• Materials science
• Photovoltaics

Background:

• Donor-acceptor copolymers are crucial for organic photovoltaics (OPVs).
• Tuning molecular energy levels is key to improving OPV performance.

Purpose of the Study:

• To synthesize a new electron acceptor unit, hexafluoroquinoxaline (HFQx), based on quinoxaline.
• To develop a novel donor-acceptor (D-A) copolymer (HFQx-T) incorporating the HFQx unit.
• To evaluate the performance of the HFQx-T copolymer in organic photovoltaic devices.

Main Methods:

• Synthesis of hexafluoroquinoxaline (HFQx) and the HFQx-T copolymer.
• Fabrication of blended films using HFQx-T:ITIC.
• Two-step annealing process (thermal annealing and solvent vapor annealing).
• Characterization of photovoltaic properties, including open-circuit voltage (V_oc), short-circuit current density (J_sc), and fill factor (FF).

Main Results:

• The introduction of six fluorine atoms onto quinoxaline yielded the HFQx unit.
• The HFQx-T copolymer, featuring a benzodithiophene derivative donor and HFQx acceptor, was successfully synthesized.
• Fluorine atoms significantly tuned the HOMO energy level, increasing V_oc.
• Enhanced absorption and altered film morphology led to improved J_sc and FF.
• The HFQx-T:ITIC blend achieved a power conversion efficiency (PCE) of 9.4% with V_oc = 0.92 V, J_sc = 15.60 mA/cm², and FF = 65% after annealing treatments.

Conclusions:

• The novel HFQx electron acceptor and HFQx-T copolymer show promise for OPVs.
• Fluorination is an effective strategy for enhancing OPV performance.
• The developed materials offer a potential pathway for high-efficiency organic solar cells.