Simple A-D₂-A Nonfullerene Acceptors for Efficient Binary Bulk Heterojunction Organic Solar Cells.

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Nonfullerene electron acceptors (NFAs) are crucial for organic photovoltaics (OPVs).
• Developing efficient and simpler NFAs is key to improving OPV performance and commercial viability.
• Nonfused ring systems offer potential advantages in molecular design and synthesis.

Purpose of the Study:

• To synthesize and characterize two new nonfused ring NFAs with an A-D2-A motif: (dicarbazolyl)bis(2-(3-oxo-2,3-dihydro-1H-inden-1-ylidene)malononitrile) (MDCzM-4H) and its fluorinated analog (MDCzM-4F).
• To investigate the optical and electronic properties of these novel NFAs.
• To evaluate their performance in bulk heterojunction solar cells when blended with the PTB7-Th copolymer.

Main Methods:

• Synthesis and characterization of new NFA materials.
• Thin-film absorption and fluorescence spectroscopy.
• Density Functional Theory (DFT) and Time-Dependent DFT (TDDFT) calculations.
• Electrochemical measurements for HOMO/LUMO energy level determination.
• Fabrication and characterization of organic photovoltaic devices (ITO/PEDOT:PSS/active layer/PFN-Br/Ag).

Main Results:

• The synthesized NFAs (MDCzM-4H and MDCzM-4F) exhibit strong absorption in the visible to near-IR region (540-700 nm) due to intramolecular charge transfer.
• Both NFAs fluoresce in the near-IR (peak ~750 nm) with short lifetimes (75-410 ps).
• Determined HOMO (~-5.7 eV) and LUMO (~-4.0 eV) energy levels are suitable for photovoltaic applications.
• Binary blends of MDCzM-4H/PTB7-Th and MDCzM-4F/PTB7-Th achieved power conversion efficiencies (PCE) of 10.17% and 14.09%, respectively.
• The fluorinated MDCzM-4F NFA demonstrated superior performance with a PCE of 14.09% (JSC = 20.92 mA cm-2, VOC = 0.965 V, FF = 0.698).

Conclusions:

• The successful synthesis and high performance of nonfused ring NFAs demonstrate their potential in organic photovoltaics.
• These simpler NFA structures provide a viable alternative to more complex fused-ring systems.
• The results highlight a promising direction for developing next-generation organic solar cells with improved efficiency and simpler manufacturing.