Key Takeaways & Executive Findings
- •• • D18 polymers with molecular weights from 41.6 kDa to 70.9 kDa were synthesized; the medium molecular weight (D18-M) achieved a PCE of 20.55%, among the highest for non-halogenated solvent-processed OSCs. • • Crystallization kinetics analysis shows that lower molecular weight accelerates film solidification, causing excessive aggregation, while higher molecular weight slows it, leading to insufficient phase separation; the optimal balance yields favorable morphology. • • Energy loss evaluation reveals that blend film emission is more dependent on exciton environment (morphology) than on intrinsic polymer luminescence, indicating that non-radiative loss is influenced by film morphology. • • The work provides a simple molecular weight tuning strategy to control film formation kinetics, enabling high-efficiency processing from non-halogenated solvents, which is critical for scalable production.
Abstract
The power conversion efficiency (PCE) of organic solar cells (OSCs) has surpassed 21% with the donor polymer D18, yet its processing from non-halogenated solvents like ortho-xylene (o-XY) remains inefficient due to uncontrolled film formation kinetics. Here, we systematically synthesize D18 polymers with molecular weights ranging from 41.6 kDa to 70.9 kDa to modulate crystallization kinetics. In-situ film drying studies reveal that lower molecular weights accelerate solidification, leading to excessive aggregation, while higher molecular weights slow it, causing insufficient phase separation. A medium molecular weight (D18-M) achieves a balanced crystallization rate, promoting favorable morphology and yielding a PCE of 20.55% with L8-BO as acceptor—one of the highest reported for non-halogenated solvent-processed OSCs. Energy loss analysis indicates that although low-molecular-weight polymers exhibit higher intrinsic luminescence, the blend film's emission is governed by exciton environment, which is dictated by morphology. This work underscores the critical role of molecular weight in controlling film formation and morphology, offering a simple yet effective strategy for high-efficiency, environmentally friendly OSCs.
1. Introduction
Organic solar cells (OSCs) have achieved remarkable progress, with power conversion efficiencies (PCEs) exceeding 21% and promising operational stability. The donor polymer D18 is particularly attractive due to its intrinsic suppression of non-radiative losses, yet its processing typically relies on low-boiling, halogenated solvents like chloroform, which are unsuitable for large-scale manufacturing. In contrast, the more processable polymer PM6 has been successfully processed from non-halogenated solvents such as ortho-xylene (o-XY), achieving record efficiencies. However, D18's film formation kinetics in o-XY remain poorly understood, leading to suboptimal morphology and performance.
This work addresses this bottleneck by systematically varying the molecular weight of D18 from 41.6 to 70.9 kDa. By controlling crystallization kinetics, we achieve a balanced film solidification process that prevents excessive aggregation or insufficient phase separation. The medium molecular weight D18-M yields a favorable morphology, resulting in a PCE of 20.55%—one of the highest for non-halogenated solvent-processed OSCs. This simple yet effective strategy provides a pathway for scalable, environmentally friendly manufacturing of high-efficiency OSCs.
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LI Hui, MING Rui, ZHANG Jian, JIANG Tao, WANG Lei, WANG Xiao, ZHANG Wei, YANG Chao, XU Lin, SUN Li, LI Chen (2026). Molecular Weight Engineering of D18 Polymer Enables 20.55% Efficiency in Non-Halogenated Solvent-Processed Organic Solar Cells via Controlled Film Formation Kinetics. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4300-3
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Frequently Asked Questions
What is the optimal molecular weight range for D18 to achieve high efficiency in non-halogenated solvent processing?
The optimal molecular weight is around 55-60 kDa (medium molecular weight, D18-M), which balances crystallization rate to achieve favorable phase separation, yielding a PCE of 20.55%.
How does molecular weight affect film formation kinetics and morphology?
Lower molecular weight accelerates film solidification, leading to excessive aggregation, while higher molecular weight slows it, causing insufficient phase separation. The medium molecular weight provides a moderate crystallization rate, enabling optimal morphology.
What is the impact of molecular weight on energy loss and non-radiative recombination?
Although low-molecular-weight polymers exhibit higher intrinsic luminescence, the blend film's emission is more dependent on exciton environment, which is determined by morphology. Thus, non-radiative loss is strongly influenced by film morphology rather than solely by polymer luminescence.
Is the 20.55% PCE reproducible and scalable for industrial production?
The PCE of 20.55% is among the highest for non-halogenated solvent-processed OSCs, demonstrating the potential for scalable production. The use of o-XY, a non-halogenated solvent, aligns with environmental and industrial requirements.
What are the key parameters to monitor during film formation to ensure optimal morphology?
In-situ film drying kinetics, specifically the aggregation duration and solidification time, are critical. A moderate crystallization rate, as achieved with D18-M, ensures favorable phase separation and charge transport.
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