Researchers from Shenzhen Technology University and collaborative institutions, led by Professor Guangye Zhang, have achieved a major milestone in organic solar cell (OSC) technology. Their latest work, published in Nano-Micro Letters, introduces a novel additive engineering strategy that enables 20.0% power conversion efficiency (PCE) in binary OSCs processed entirely with non-halogenated solvents. This advancement not only sets a new benchmark for toluene-processed OSCs but also offers a scalable and eco-friendly approach aligned with industrialization needs.

Why This Innovation Matters

• Certified 20% Efficiency with Toluene: A remarkable PCE of 20.0% (certified at 19.7%) is achieved in PM6/BTP-eC9 OSCs, surpassing many halogen-solvent-processed devices.
• Green and Scalable Fabrication: The entire process uses toluene—a non-halogen, high-boiling solvent—avoiding toxic solvents like chloroform, which are unsuitable for large-scale manufacturing.
• Universal Additive Strategy: The strategy is shown to be effective across multiple systems, including PM6/PYF-T-o, PM6/L8-BO, and PM6/PJ1-γ, offering wide applicability.

Key Innovation: Additive-Guided Secondary Nucleation Control

To address the longstanding challenge of morphology control in sequentially processed OSCs (SqP), the team introduced two isomeric additives, ODBC and PDBC, into either the donor or acceptor layers. These additives regulate the swelling and aggregation behaviors of the active materials through non-covalent interactions, allowing for precise modulation of film morphology.

• Dipole-Driven Control: ODBC, with a higher dipole moment, promotes earlier nucleation and tighter π–π stacking, leading to enhanced crystallinity and exciton dissociation.
• Additive Placement Matters: Adding the additive into the acceptor layer rather than the donor layer produces significantly better morphology and charge transport properties.
• Drying Kinetics Optimization: The high boiling points of ODBC/PDBC extend the drying window, promoting homogeneous phase separation and vertical phase segregation.

Unprecedented Device Performance

• PM6/PYF-T-o Devices: Achieved a champion PCE of 38%, surpassing chloroform-casted blend devices (16.81%) and marking the highest efficiency for this binary system.
• PM6/BTP-eC9 Devices: With ODBC and 2PACZ as a hole transport layer, a certified PCE of 19.7% is reached—the highest ever for non-halogenated solvent-processed binary OSCs.
• Improved Charge Dynamics:

• • High short-circuit current (J_SC) up to 2 mAcm^-2.
  • Balanced and enhanced carrier mobilities (μ_e≈ μ_h ≈ 7.7 × 10⁻⁴ cm² V^-1 s^-1).
  • Reduced bimolecular recombination and improved charge extraction, validated through photo-CELIV and transient photovoltage measurements.

Mechanistic Insights and Morphology Control

• In Situ Absorption and AFM: Extended drying times and smaller RMS roughness confirm finer microstructures.
• GIWAXS Analysis: Improved π–π stacking (d-spacing ~3.67 Å, CCL ~11.14 Å) supports better charge transport.
• Vertical Composition Profiling: FDDLAS and FLAS analyses reveal more uniform donor/acceptor distribution, promoting efficient exciton dissociation and charge collection.
• Ultrafast Spectroscopy: Transient absorption experiments indicate enhanced hole transfer and stronger interfacial charge generation in ODBC-treated devices.

Robust Stability and Broad Applicability

• Stability: Devices with ODBC show superior light and thermal stability, retaining over 88% of PCE after 500 hours of continuous illumination.
• Universality: The additive strategy was extended to various binary OSC systems, consistently improving performance:

• • PM6/L8-BO: Increased from 16.95% to 16%.
  • PM6/PJ1-γ: Increased from 14.36% to 41%.

Future Outlook

This study establishes a practical, scalable pathway toward high-efficiency, halogen-free OSCs by leveraging additive-induced secondary nucleation control in the SqP framework. The use of isomeric additives like ODBC offers a powerful handle for morphology tuning, enabling enhanced crystallinity, charge transport, and long-term stability. With compatibility across multiple systems and processing conditions, this strategy holds promise for industrial-scale, environmentally friendly production of organic photovoltaics.

Stay tuned for more from Professor Guangye Zhang’s team as they continue to push the frontiers of green, high-performance organic solar technologies！