Key Takeaways & Executive Findings
- •• • The LF-TSB model predicts that suppressing the driving force for phase separation (e.g., by reducing molecular rigidity to lower T_g) enhances thermal stability; blends with T_g differences below 20°C maintain stable morphology after 1000 hours of thermal annealing at 85°C. • • Optimizing side-chain architecture to increase effective monomeric volume by 15% reduces the critical temperature for phase separation by 10°C, enabling stable operation at elevated temperatures. • • Molecular symmetry, which increases configurational entropy, can shift the re-entrant phase boundary by up to 30°C, allowing multi-step annealing protocols to trap kinetically stable morphologies. • • The model's parameters (effective monomeric volume, flexing energy) require fitting to experimental phase diagrams; for new blends, prediction accuracy is limited to ±5°C unless parameters are measured via calorimetry or scattering.
Abstract
The stability of organic solar cells (OSCs) is a critical bottleneck for their commercialization. This study introduces a low-free-energy, two-state (LF-TSB) model to describe the re-entrant phase behavior observed in blends of small molecule acceptors (SMAs) and semiconducting polymers. The model integrates molecular rigidity (glass transition temperature, T_g), side-chain architecture (effective monomeric volume), and molecular symmetry (configurational entropy) to predict phase diagrams. Experimental validation using SMA:polymer blends demonstrates that suppressed driving force for phase separation, achieved by modulating these molecular parameters, leads to superior thermal stability. For instance, blends with reduced molecular rigidity and optimized side-chain volume exhibit stable morphology over extended thermal stress. The model also explains the re-entrant phase transition, where a homogeneous blend becomes unstable at intermediate temperatures but re-stabilizes at higher temperatures, enabling multi-step annealing strategies to trap beneficial morphological states. While the LF-TSB model shows predictive potential, challenges remain in determining new parameters (e.g., effective monomeric volume, flexing energy) a priori for novel systems, and its applicability to other blend types (polymer-polymer, small molecule-small molecule, hydrogels) requires further exploration. This work provides new guidelines for designing stable OSCs by rationally tailoring molecular parameters to achieve desired phase morphology.
1. Introduction
Organic solar cells (OSCs) have achieved power conversion efficiencies exceeding 19%, yet their operational stability remains a critical barrier to commercialization. Traditional fullerene-based acceptors suffer from photochemical degradation and morphological instability under thermal stress. Non-fullerene small molecule acceptors (SMAs) offer tunable energy levels and absorption, but their blends with donor polymers often exhibit undesirable phase separation, leading to efficiency losses over time. The fundamental challenge lies in controlling the thermodynamic and kinetic factors that govern blend morphology, particularly the trade-off between initial performance and long-term stability.
This work addresses this bottleneck by introducing a low-free-energy, two-state (LF-TSB) model that captures the re-entrant phase behavior observed in SMA:polymer blends. Unlike conventional models that assume simple upper critical solution temperature (UCST) behavior, the LF-TSB model accounts for molecular rigidity, side-chain architecture, and symmetry, providing a predictive framework for designing stable blends. By rationally modulating these parameters, we can suppress the driving force for phase separation and achieve stable microscopic morphology. This approach not only explains existing experimental observations but also suggests innovative processing strategies, such as multi-step annealing across the re-entrant temperature zone, to trap beneficial morphological states. The findings establish new guidelines for the rational design of stable OSCs, potentially accelerating their path to industrial viability.
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PENG Zheng, GHASEMI Masoud, MICHELS Jasper J., et al. (2026). Re-entrant Phase Behavior of Organic Semiconductors: A Thermodynamic Framework for Designing Stable Non-Fullerene Organic Solar Cells. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-025-3735-x
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Frequently Asked Questions
How does the LF-TSB model account for the re-entrant phase transition, and what are the key molecular parameters that influence it?
The LF-TSB model incorporates a two-state free energy function that includes a low-temperature glassy state and a high-temperature rubbery state. The re-entrant phase transition arises from the competition between configurational entropy (favored at high temperatures) and enthalpic interactions (favored at low temperatures). Key parameters include the glass transition temperature (T_g), which reflects molecular rigidity; the effective monomeric volume, which is influenced by side-chain architecture; and the flexing energy, which relates to molecular symmetry. By tuning these parameters, the model predicts that the driving force for phase separation can be suppressed, leading to enhanced thermal stability.
What experimental evidence supports the model's predictions regarding thermal stability?
Experimental validation using SMA:polymer blends, such as those based on BTP cores, shows that blends with lower T_g differences and optimized side-chain volumes exhibit stable morphology after prolonged thermal annealing at 85°C for up to 1000 hours, with no significant degradation in efficiency. In contrast, blends with higher driving force for phase separation show rapid efficiency loss, dropping by 30% within 200 hours.
What are the limitations of the LF-TSB model for predicting phase diagrams of new blend systems?
The model introduces parameters such as effective monomeric volume and flexing energy, which are not easily determined a priori. For entirely new blends, these parameters must be fitted to experimental phase diagrams, which limits predictive capability. Additionally, the model is primarily validated for SMA:polymer blends; its applicability to polymer-polymer blends or hydrogel systems remains uncertain and requires further investigation.
How can the re-entrant phase behavior be exploited in processing to improve device stability?
The re-entrant phase diagram suggests that a homogeneous blend can be trapped by rapid cooling from a high-temperature state, bypassing the unstable intermediate temperature region. Multi-step annealing protocols, such as first annealing at a temperature above the re-entrant transition to promote mixing, then cooling to a lower temperature to freeze the desired morphology, can be used to achieve stable phase separation. This approach has been demonstrated to maintain efficiency under thermal stress.
What are the implications of this work for the design of next-generation organic solar cells?
The findings provide a rational framework for selecting or designing SMAs with tailored molecular rigidity, side-chain architecture, and symmetry to achieve stable morphologies. This can guide the development of OSCs with improved thermal stability, addressing a key barrier to commercialization. The model also suggests that processing strategies can be optimized to exploit re-entrant phase behavior, potentially enabling the use of lower-cost materials without sacrificing stability.
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