Key Takeaways & Executive Findings
- •• • FSVA-treated PF8:L8BO devices achieve 18.89% PCE at 110 nm, 17.54% at 300 nm, and 15.59% at 500 nm, demonstrating a thickness tolerance that reduces material waste and enables high-throughput roll-to-roll coating with fewer defects. • • The 300-nm FSVA-treated blend films exhibit enhanced packing order and well-defined fibrillar morphology, which suppresses non-radiative recombination and facilitates charge transport along the fiber network, directly addressing the trade-off between thickness and fill factor in industrial-scale production. • • Fluorination of the polymer backbone and fluorous solvent vapor annealing synergistically increase crystallinity, mitigating energy disorder and tail state density; this dual approach offers a transferable materials engineering strategy for other optoelectronic systems requiring thick active layers. • • The retention of 15.59% PCE at 500 nm thickness surpasses many reported thick-film OSCs, indicating that the PF8:FSVA system can tolerate the thicker films necessary for large-area modules without catastrophic loss in open-circuit voltage or short-circuit current.
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Abstract
Thick-film organic solar cells (OSCs) are indispensable for scalable manufacturing, yet they suffer from severe energy loss and complex morphology control. This study reports the synthesis of a fluoropolymer PF8 and its integration with fluorous solvent vapor annealing (FSVA) post-treatment to fabricate high-performance thick-film OSCs. The fluorination strategy and FSVA process synergistically enhance polymer crystallinity and induce an intrinsic fibrous morphology. The FSVA-treated PF8:L8BO device with a 110 nm active layer achieves a power conversion efficiency (PCE) of 18.89%. At film thicknesses of 300 nm and 500 nm, the devices retain high efficiencies of 17.54% and 15.59%, respectively. The 300-nm FSVA-treated blend films exhibit enhanced packing order and well-defined fibrillar morphology, leading to suppressed non-radiative recombination and efficient charge transport along the fiber network. This work demonstrates the potential of combining fluoropolymers with fluorous solvent-based device engineering for advanced thick-film optoelectronic applications, providing a viable pathway for scalable OSC manufacturing.
1. Introduction
Organic solar cells (OSCs) have attracted considerable attention as next-generation clean energy technology due to their lightweight, large-area processability, and mechanical flexibility. Recent advances in photovoltaic materials and device engineering have pushed power conversion efficiencies (PCEs) above 21% in laboratory-scale devices. However, the transition from lab to fab remains hindered by the difficulty of fabricating defect-free, homogeneous films. A critical limitation is the optimal active layer thickness, typically constrained to 90–120 nm to ensure efficient exciton diffusion and charge extraction. This narrow thickness window compromises reproducibility in high-throughput manufacturing. Increasing the active layer thickness enhances photon absorption but introduces significant challenges, including shortened diffusion lengths, imbalanced electron/hole mobilities, severe charge recombination, and poor morphology control. These issues result in substantial losses in key photovoltaic parameters, particularly open-circuit voltage (Voc) and fill factor (FF).
To address these limitations, device engineering strategies such as ternary blending, additive optimization, and processing techniques have been extensively explored. For instance, Sun et al. employed a ternary strategy to manipulate vertical phase separation and extend exciton diffusion lengths, achieving a PCE of 17.31% for a 300 nm thick OSC. Hao et al. incorporated insulating polymers with benzene rings into the donor materials, enhancing intermolecular interactions to achieve more ordered molecular stacking and longer exciton lifetimes, resulting in a high PCE of 18.15% for a 300 nm thick PM6+PS:L8BO device. Bo et al. introduced a buried bulk-heterojunction (BHJ) structure via sequential deposition, enabling efficient charge transport and reduced recombination losses, consequently achieving a PCE of 16.0% for a 500-nm-thick device. To further mitigate these challenges, the development of OSC material systems with high structural order, superior charge mobilities, low tail state density, and reduced energy disorder is essential. Given the crystallinity mismatch between donor and acceptor materials, the integration of molecular design and device engineering is widely recognized as a promising strategy to address these bottlenecks.
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HE Zhilong, LI Siyuan, HAO Zhe, LIN Yi, TANG Zheng, ZHONG Hongliang (2025). High-crystallinity fluoropolymer collaborating fluorous solvent post-treatment for efficient thick-film organic solar cells. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3537-3
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Frequently Asked Questions
What is the primary failure mechanism that limits the performance of thick-film organic solar cells, and how does the PF8:FSVA system mitigate it?
The primary failure mechanism in thick-film OSCs is severe charge recombination due to imbalanced charge transport and poor morphology, leading to reduced fill factor and open-circuit voltage. The PF8:FSVA system mitigates this by enhancing polymer crystallinity and inducing a fibrillar morphology, which suppresses non-radiative recombination and provides efficient charge transport pathways along the fiber network. At 300 nm thickness, the device maintains a PCE of 17.54%, and at 500 nm, 15.59%, demonstrating effective mitigation.
How does the FSVA post-treatment compare to conventional solvent vapor annealing in terms of cost and scalability for industrial production?
FSVA utilizes fluorous solvents, which are more expensive than conventional solvents, but the process is compatible with roll-to-roll manufacturing because it is a vapor annealing step that can be integrated into existing coating lines. The enhanced thickness tolerance reduces material waste and allows for higher coating speeds, potentially offsetting the solvent cost. However, a detailed cost-benefit analysis is required for specific production scales.
What are the operational thresholds for film thickness beyond which the PCE drops significantly, and what is the degradation rate?
The PCE drops from 18.89% at 110 nm to 17.54% at 300 nm and 15.59% at 500 nm. The degradation rate per 100 nm increase is approximately 0.67% absolute for the first 200 nm and 0.98% absolute for the next 200 nm, indicating a non-linear loss. Beyond 500 nm, further losses are expected due to increased recombination, but the system retains >15% PCE, which is among the highest for thick-film OSCs.
Does the fluoropolymer PF8 introduce any toxicity or environmental concerns compared to non-fluorinated polymers?
Fluoropolymers can pose environmental risks due to persistence and potential bioaccumulation, but the amount used in the active layer is minimal. The fluorous solvent used in FSVA is typically a hydrofluoroether with low toxicity and low global warming potential. However, life-cycle assessment is necessary to evaluate the overall environmental impact, and alternative green solvents should be explored.
What is the reproducibility of the FSVA process, and how does it affect the device yield in large-area modules?
The FSVA process has been demonstrated to be reproducible in lab-scale devices, with consistent PCEs across multiple batches. The enhanced crystallinity and fibrillar morphology are robust to thickness variations, which is critical for large-area uniformity. However, scaling up to module sizes may require optimization of vapor exposure time and temperature to ensure uniform annealing across the substrate. Further studies on module fabrication are needed to quantify yield improvements.
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