A Comparative Study on Hole Transfer Inversely Correlated with Driving Force in Two Non-Fullerene Organic Solar Cells

Area of Science:

• Materials Science
• Organic Electronics
• Photovoltaics

Background:

• Organic solar cells (OSCs) offer a promising alternative to traditional silicon-based photovoltaics due to their flexibility and low manufacturing costs.
• Efficient charge transfer and transport are critical for maximizing power conversion efficiency in OSCs.
• Understanding the relationship between molecular energy levels and device performance is essential for designing next-generation organic electronic materials.

Purpose of the Study:

• To investigate the impact of the energy level difference between donor and acceptor materials on hole transfer rates in OSCs.
• To compare the performance of OSCs utilizing PBDBT and PBDBT-SF as donors with an IT-4F acceptor.
• To elucidate the factors contributing to enhanced short-circuit current (J_SC) in OSC devices.

Main Methods:

• Fabrication and characterization of two types of OSCs using IT-4F as the acceptor and PBDBT or PBDBT-SF as donors.
• Measurement of photovoltaic parameters, including short-circuit current (J_SC).
• Analysis of the energy level alignment (Δ_HOMO) between donor and acceptor materials and its correlation with charge transfer dynamics.

Main Results:

• A smaller energy level difference (Δ_HOMO = 0.31 eV) between PBDBT-SF and IT-4F resulted in a higher J_SC compared to a larger Δ_HOMO (0.45 eV) in PBDBT:IT-4F OSCs.
• The rate of hole transfer from IT-4F to the donor materials was found to be inversely proportional to Δ_HOMO.
• The superior performance of PBDBT-SF:IT-4F devices was attributed to faster hole transfer, reduced recombination, and efficient charge extraction, supported by favorable film morphology and balanced charge carrier mobilities.

Conclusions:

• Optimizing the energy level alignment (Δ_HOMO) is crucial for enhancing hole transfer efficiency in OSCs.
• Achieving high-performance OSCs requires a holistic approach, considering not only energy levels but also charge transport, film morphology, and material compatibility.
• The findings provide valuable insights for the rational design of novel donor materials for high-efficiency organic solar cells.