Columnar-Structured Low-Concentration Donor Molecules in Bulk Heterojunction Organic Solar Cells

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Bulk heterojunction (BHJ) organic solar cells (OSCs) rely on the nanostructure of donor and acceptor materials for efficient charge generation and transport.
• The precise arrangement of donor molecules significantly impacts the performance of OSCs, particularly in vacuum-deposited systems.

Purpose of the Study:

• To investigate the impact of donor molecule arrangement on the performance of 1,1-bis-(4-bis(4-methyl-phenyl)-amino-phenyl)-cyclohexane (TAPC):C70 BHJ organic solar cells.
• To understand the critical donor concentration threshold for forming efficient charge transport pathways.

Main Methods:

• Fabrication of TAPC:C70 based organic solar cells using vacuum deposition.
• Kelvin probe force microscopy (KPFM) to measure contact potential difference and analyze morphology.
• Analysis of trap-assisted charge injection to probe charge transport limitations.

Main Results:

• Even low donor (TAPC) concentrations (around 10%) induce columnar structures, facilitating efficient hole transport.
• Donor concentrations below 10% lead to disconnected hole pathways, hindering charge transport.
• Balanced electron and hole mobility was observed due to preserved hole mobility at low donor concentrations.

Conclusions:

• Columnar donor structures are crucial for efficient hole transport and high performance in TAPC:C70 OSCs.
• Optimizing donor concentration is key to maximizing internal quantum efficiency and power conversion efficiency.
• A power conversion efficiency of 6.24% was achieved in inverted TAPC:C70 (1:9) OSCs, demonstrating the effectiveness of controlled morphology.