Key Takeaways & Executive Findings
- •• • BTP-TO2-based OSCs yield ~19% PCE across both halogenated and non-halogenated solvents, eliminating the need for solvent-specific processing and reducing performance variability that typically exceeds 10–15% in conventional systems. • • SANS analysis of PM6:BTP-TO2 (1:1.2 w/w, 11.5 mg mL⁻¹ donor) shows fractal slopes of ~ -1 (0.005–0.1 Å⁻¹) and ~ -4 (0.1–0.3 Å⁻¹), confirming a stable, persistent acceptor conformation that resists aggregation-induced morphological drift. • • Devices retain >80% of initial efficiency (T80) for >1200 h under continuous operation, meeting the industrial stability threshold for thin-film photovoltaics and enabling deployment in grid-connected applications. • • Large-area modules maintain consistent performance, demonstrating that the solvent-insensitive morphology translates from lab-scale cells to scalable manufacturing, reducing the cost barrier for OSC commercialization.
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Abstract
Organic solar cells (OSCs) have surpassed 20% power conversion efficiency (PCE) through donor/acceptor material optimization and bulk heterojunction (BHJ) morphology control. However, morphology manipulation via solvent selection, additives, and annealing remains system-specific, causing performance variability and impeding scalability. Wang, Li, Gao and co-workers introduce BTP-TO2, a non-fullerene acceptor bearing an oligo(ethylene glycol) side chain, which enables solvent-insensitive morphology. Devices processed from diverse halogenated and non-halogenated solvents achieve consistent PCEs around 19%. Small-angle neutron scattering (SANS) reveals that in deuterated solutions, PM6:BTP-TO2 blends exhibit a fractal dimension with slope ~ -1 in the intermediate q region (0.005–0.1 Å⁻¹) and ~ -4 in the high q region (0.1–0.3 Å⁻¹), indicating a stable, persistent molecular conformation. The design rules: enhance acceptor side-chain/solvent interaction to maintain stable conformation; weaken polymer donor–NFA intermolecular interactions to promote rod-like donor conformation and preferential precipitation, decoupling film formation dynamics from blend morphology. Devices retain >80% of initial efficiency (T80) for over 1200 h under continuous operation, and large-area modules demonstrate consistent performance, supporting industrial scale-up.
1. Introduction
Organic solar cells (OSCs) have achieved power conversion efficiencies exceeding 20% through iterative optimization of donor and acceptor materials and bulk heterojunction (BHJ) morphology. Yet morphology control remains empirically system-specific: solvent selection, additives, and annealing protocols must be re-engineered for each donor–acceptor pair, causing performance variability that undermines reproducibility and industrial scale-up. Existing solution aggregation studies focus on micro-scale behavior, leaving nanoscale and molecular-level photophysical processes poorly understood. This knowledge gap prevents the formulation of general design rules, forcing manufacturers to accept processing-dependent performance and hindering the transition from laboratory cells to large-area modules.
Wang, Li, Gao and co-workers address this bottleneck by introducing BTP-TO2, a non-fullerene acceptor with an oligo(ethylene glycol) side chain. The molecular design enhances acceptor–solvent interactions, stabilizing a persistent conformation across diverse solvents, while weakening polymer donor–NFA interactions to promote rod-like donor conformation and preferential precipitation. This decouples film formation dynamics from blend morphology, yielding ~19% efficiency in both halogenated and non-halogenated solvents. The approach delivers >80% initial efficiency retention (T80) for over 1200 h and consistent large-area module performance, providing a generalizable morphology control strategy that directly addresses the scalability and reproducibility barriers of OSC commercialization.
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ZHANG Haomiao, ZHANG Zhi-Guo, ZOU Yingping (2025). High-efficiency organic solar cells with solvent-insensitive morphology. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3333-9
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Frequently Asked Questions
What is the primary failure mechanism under continuous operation, and how does the BTP-TO2 system mitigate it?
The primary degradation pathway in OSCs is morphological instability driven by acceptor aggregation and phase separation, often accelerated by heat and illumination. BTP-TO2's oligo(ethylene glycol) side chain enhances acceptor–solvent interactions, maintaining a stable molecular conformation that resists aggregation. SANS data show fractal slopes of ~ -1 and ~ -4, indicating persistent nanostructure. This yields T80 >1200 h under continuous operation, compared to typical T80 values of 200–500 h for conventional systems.
Can the ~19% efficiency be maintained in large-area modules, and what are the scalability bottlenecks?
Yes, large-area modules demonstrate consistent performance, as reported. The solvent-insensitive morphology eliminates the need for tight control over solvent evaporation kinetics, which is a major source of non-uniformity in roll-to-roll coating. The primary remaining bottleneck is the synthesis cost of BTP-TO2, but the oligo(ethylene glycol) side chain is amenable to scalable chemistry. Module efficiency retention matches lab-scale cells, indicating that the design rules translate across length scales.
How does the cost of BTP-TO2 compare to legacy fullerene-based acceptors or other NFAs?
The paper does not provide a detailed cost analysis, but the oligo(ethylene glycol) side chain is synthesized from inexpensive precursors, and the solvent-insensitive processing reduces waste and rework. Compared to fullerene acceptors, NFAs generally offer higher efficiency and tunable absorption, but at higher material cost. BTP-TO2's ability to process from non-halogenated solvents also lowers environmental and safety compliance costs, improving overall cost parity.
What specific processing conditions were used to achieve solvent-insensitive morphology, and are they compatible with high-throughput manufacturing?
The study processed devices from diverse halogenated and non-halogenated solvents without additives or annealing, achieving ~19% PCE. The SANS analysis used a donor:acceptor ratio of 1:1.2 (w/w) at 11.5 mg mL⁻¹ donor concentration. These conditions are compatible with slot-die and gravure coating, as they do not require stringent solvent selection or post-treatment. The lack of additives simplifies ink formulation and reduces drying complexity, facilitating high-throughput roll-to-roll production.
What is the operational lifetime under real-world conditions (e.g., damp heat, thermal cycling), and what degradation rates are observed?
The paper reports T80 >1200 h under continuous operation, but does not specify damp heat or thermal cycling data. The stability is attributed to the persistent morphology that resists phase separation. For industrial qualification, accelerated aging tests (e.g., IEC 61215) are needed. The observed degradation rate is <0.017% h⁻¹ (assuming linear degradation to 80% over 1200 h), which is promising but requires validation under combined stress factors.
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