Key Takeaways & Executive Findings
- •• • PM6:BTP-TO12 binary blend achieves a PCE of 18.2% under green-solvent (toluene) processing, demonstrating that balanced solubility and molecular packing order are critical for high efficiency. • • Ternary OSCs incorporating BTP-TO12 as a guest material reach a PCE of 19.5%, representing state-of-the-art performance for halogen-free solvent-processed devices and highlighting the potential for industrial scalability. • • Systematic variation of inner and outer side-chain lengths reveals that longer alkyl chains do not always improve performance; excessive solubility reduces molecular packing order, leading to lower fill factor and efficiency. • • BTP-TO12 exhibits low energy loss and well-controlled aggregation in toluene, which are key factors in achieving high open-circuit voltage and fill factor, as evidenced by the 19.5% PCE in ternary devices.
Abstract
The rapid development of halogen-free solvent-processed organic solar cells (OSCs) has been enabled by side-chain modification on small molecular acceptors, yet the structure-property relationship between inner/outer chain lengths and device performance remains unclear. This study systematically investigates five non-fullerene acceptors (NFAs) with varied side-chain positions and architectures, clarifying the effects of inner versus outer modifications on energy level distribution, film morphology, and carrier dynamics. Notably, longer alkyl chains are not always superior; excessive solubility reduces molecular packing order. The optimized PM6:BTP-TO12 blend achieves a power conversion efficiency (PCE) of 18.2%. Furthermore, ternary OSCs incorporating BTP-TO12 as a guest material reach a remarkable PCE of 19.5%, enhancing the performance of L8-BO-based devices processed with green solvents. This improvement is attributed to the low energy loss and well-controlled aggregation behavior of BTP-TO12 in environmentally friendly toluene. These findings establish a design guideline for side-chain engineering in green-solvent-processed OSCs, achieving state-of-the-art performance and advancing scalable, eco-compatible photovoltaic technologies.
1. Introduction
Organic solar cells (OSCs) have emerged as a promising renewable energy technology due to their lightweight nature, mechanical flexibility, low manufacturing costs, and compatibility with printing techniques. Over the past few decades, significant advancements in material design, interface engineering, and active layer morphology control have driven the power conversion efficiencies (PCEs) of single-junction OSCs based on non-fullerene acceptors (NFAs) beyond 20%. However, most high-performance OSCs still rely on halogenated solvents such as chloroform and chlorobenzene, which pose serious health and environmental hazards. The limited compatibility of current acceptor materials with green, halogen-free solvents hinders large-scale manufacturing, as spin-coated active layers in halogen-free systems often suffer from poor film quality, resulting in reduced PCEs and significant performance discrepancies between lab-scale devices and large-area modules.
Transitioning from lab-scale devices to industrial-scale modules using green solvents is crucial for practical applications. Among halogen-free solvents, toluene with its moderate boiling point (~110°C) offers a favorable balance between drying kinetics and film formation, helping to reduce over-aggregation, improve morphology, and minimize toxicity. However, the solubility of photoactive materials in such solvents remains a key limitation, often leading to inadequate donor–acceptor interfaces, poor charge transport, and increased recombination. Side-chain engineering plays a pivotal role in enhancing the solubility and morphological control of acceptor materials in green solvent systems. While side-chain modifications, typically at external (shoulder) or internal (bottom) positions on the molecular core, have been widely explored, systematic comparisons of their effects on device performance are lacking. This study addresses this gap by systematically varying inner and outer side-chain lengths and architectures in five NFAs, providing a comprehensive understanding of the structure-property relationship and demonstrating a design strategy for achieving high-efficiency green-solvent-processed OSCs.
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LIU Wei, LIANG Wenting, ZOU Bosen, ZHAO Chaoyue, WANG Feifei, ZENG Ping, AN Juan, KONG Zhen, NIU Jiaxin, CHEN Shangshang, YAN He, WANG Yufei, ZHANG Guangye, ZHANG Jianquan, HU Huawei (2026). Strategic Inner/Outer Side-Chain Tuning for High-Efficiency Green-Solvent-Processed Organic Solar Cells. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3828-y
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Frequently Asked Questions
What is the optimal side-chain length for achieving high efficiency in green-solvent-processed OSCs, and why do longer chains not always improve performance?
The study demonstrates that side-chain length must be balanced with solubility and molecular packing. While longer alkyl chains can enhance solubility in green solvents, excessive solubility reduces molecular packing order, leading to lower charge transport and fill factor. The optimal acceptor BTP-TO12, with a specific side-chain architecture, achieves a PCE of 18.2% in binary blends and 19.5% in ternary devices, indicating that a moderate chain length that maintains sufficient solubility without compromising crystallinity is critical.
How does the incorporation of BTP-TO12 as a guest material in ternary OSCs enhance device performance compared to binary systems?
BTP-TO12 as a guest material in ternary OSCs with L8-BO-based blends improves performance by reducing energy loss and controlling aggregation behavior in toluene. This leads to better film morphology and charge carrier dynamics, resulting in a PCE of 19.5%, which is higher than the binary PM6:BTP-TO12 PCE of 18.2%. The ternary approach leverages the complementary properties of the two acceptors to optimize light absorption and energy levels.
What are the key morphological and carrier dynamics factors that contribute to the high PCEs achieved with BTP-TO12?
BTP-TO12 exhibits well-controlled aggregation in toluene, leading to favorable film morphology with appropriate domain sizes and purity. This reduces charge recombination and enhances charge transport, as evidenced by improved fill factor and short-circuit current density. Additionally, low energy loss contributes to a high open-circuit voltage, collectively yielding PCEs above 18%.
How do the results of this study compare to other state-of-the-art green-solvent-processed OSCs in terms of PCE and scalability?
The ternary OSCs incorporating BTP-TO12 achieve a PCE of 19.5%, which is among the highest reported for halogen-free solvent-processed devices. This performance, combined with the use of toluene, a less toxic solvent, demonstrates the potential for scalable manufacturing. The study also provides a systematic design guideline for side-chain engineering, which can be applied to other acceptor systems to further improve efficiency and processability.
What are the implications of this work for the industrial production of OSCs using environmentally friendly solvents?
This work addresses a critical bottleneck in OSC industrialization: the reliance on hazardous halogenated solvents. By demonstrating high efficiencies (19.5%) with toluene, a greener solvent, and providing a clear structure-property relationship for side-chain design, the study paves the way for developing materials that are both high-performing and compatible with roll-to-roll manufacturing. This could accelerate the commercialization of OSCs as a sustainable energy technology.
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