Anion-Doped Thickness-Insensitive Electron Transport Layer for Efficient Organic Solar Cells

Area of Science:

• Organic electronics
• Materials science
• Photovoltaics

Background:

• Interfacial materials are crucial for charge extraction, transport, and collection in organic solar cells (OSCs).
• Printable, large-area OSC modules require highly efficient and thickness-insensitive interfacial materials.
• Conjugated polyelectrolytes offer potential as electron-transport layers (ETLs) but require optimization.

Purpose of the Study:

• To develop efficient and thickness-insensitive ETLs for OSCs using doped conjugated polyelectrolytes.
• To investigate the effect of different alkali metal salt dopants with varying counter anions on ETL performance.
• To understand the mechanism behind performance enhancement in doped ETLs.

Main Methods:

• Synthesis of a medium bandgap benzothiadiazole-based conjugated polyelectrolyte, PFNBT-Br, soluble in water/alcohol.
• Doping PFNBT-Br with sodium hypophosphite (NaH2PO2) and sodium oxalate (Na2C2O4) to create ETLs.
• Fabrication and characterization of OSC devices with doped ETLs, evaluating performance under varying film thicknesses.

Main Results:

• Doping PFNBT-Br with sodium salts significantly improved OSC device performance.
• The ETL doped with sodium oxalate (Ox2-) exhibited superior performance compared to that doped with sodium hypophosphite (H2PO2-), particularly for thicker films (50 nm).
• Oxalate doping led to reduced series resistance, lower dark current, enhanced charge transport and extraction, and suppressed recombination.

Conclusions:

• N-doping of conjugated polyelectrolytes is a promising strategy for achieving thickness-insensitive interfacial layers in OSCs.
• Careful selection of dopant salts, specifically the counter anion, is critical for optimizing device performance.
• The findings pave the way for developing robust and efficient ETLs for printable, large-area organic electronic devices.