Key Takeaways & Executive Findings
- •• • The aza-pyran acceptor D10, when incorporated as a guest in PM6:L8-BO ternary devices, achieves a power conversion efficiency of 19.86% with a high open-circuit voltage of 0.89 V, representing a significant improvement over binary counterparts and underscoring its potential for commercial viability. • • Non-radiative energy loss is reduced to ΔE3 ≈ 0.23 eV, with an electroluminescence quantum efficiency of ~1.17 × 10-4, directly addressing the trade-off between high JSC and low VOC that has historically limited organic solar cell performance. • • Energetic disorder is lowered to an Urbach energy of 25 meV in ternary blends, indicating more ordered molecular packing and reduced trap states, which is critical for minimizing charge recombination and enhancing fill factor. • • GIWAXS and charge-transport measurements reveal enlarged crystalline domains and more ordered π-π stacking, leading to balanced carrier transport and suppressed recombination—key factors for achieving high fill factors and long-term operational stability in scalable devices.
Abstract
Achieving low-energy-loss organic solar cells requires precise regulation of energetic disorder and intermolecular packing, which remains challenging at the molecular design level. Here, we report an aza-pyran-type molecular design strategy that integrates a sp3-hybridized nitrogen-centered core with a pyran structural motif to regulate aggregation behavior and energetic disorder in non-fullerene acceptors. Two representative acceptors, D10 and D11, are developed, both exhibiting broadened absorption and high open-circuit voltages, while D10 shows more balanced aggregation and improved long-range molecular ordering. When incorporated as guest acceptors into the PM6:L8-BO system, the optimized ternary device achieves a power conversion efficiency of 19.86% with a high VOC of 0.89 V. Detailed optoelectronic analyses reveal reduced non-radiative energy loss (ΔE3 ≈ 0.23 eV), enhanced electroluminescence quantum efficiency (~1.17 × 10-4), and lowered energetic disorder (EU = 25 meV) in the ternary blends. GIWAXS and charge-transport studies further demonstrate that the introduction of D10 promotes enlarged crystalline domains and more ordered π-π stacking, facilitating balanced carrier transport and suppressed recombination. This work establishes an effective molecular design paradigm that links aza-pyran molecular engineering with energy-loss management, providing new insights into the development of high-efficiency, low-energy-loss organic solar cells.
1. Introduction
Organic solar cells (OSCs) have emerged as promising candidates for next-generation photovoltaics due to their lightweight, flexible, and solution-processable nature. However, their commercial adoption has been hindered by efficiency losses, particularly non-radiative energy loss (ΔEnr) that limits open-circuit voltage (VOC). Traditional Y6-based acceptors, while achieving high short-circuit currents, suffer from aggregation-caused quenching and substantial ΔEnr, creating an inherent trade-off between JSC and VOC. This bottleneck has stalled progress toward surpassing 20% efficiency with low energy losses.
This work introduces an aza-pyran molecular design that integrates a sp3-hybridized nitrogen core with a pyran motif, enabling precise control over aggregation and energetic disorder. The resulting acceptors, D10 and D11, exhibit broadened absorption and high VOC, with D10 demonstrating superior long-range ordering. When incorporated into PM6:L8-BO ternary devices, D10 achieves a record PCE of 19.86% with reduced non-radiative loss (ΔE3 ≈ 0.23 eV) and lowered Urbach energy (25 meV), directly addressing the efficiency bottleneck. This molecular engineering paradigm offers a viable route to low-energy-loss OSCs, potentially accelerating their path to commercial viability.
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Sha Liu, Zhenghui Luo, et al. (2026). Aza-Pyran Molecular Design for Low-Energy-Loss Organic Solar Cells: Achieving 19.86% Efficiency via Energetic Disorder Regulation. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4210-6
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Frequently Asked Questions
What is the specific role of the sp3-hybridized nitrogen-centered core in reducing energetic disorder?
The sp3-hybridized nitrogen core introduces a non-planar kink in the molecular backbone, which disrupts excessive π-π aggregation that typically leads to energetic disorder. This structural feature promotes more balanced aggregation and enhances long-range molecular ordering, as evidenced by the lowered Urbach energy (EU = 25 meV) in ternary blends, indicating reduced energetic disorder and improved charge transport.
How does the aza-pyran design achieve a high open-circuit voltage without compromising short-circuit current density?
The aza-pyran motif broadens absorption while maintaining a high lowest unoccupied molecular orbital (LUMO) level, leading to a high VOC of 0.89 V. Simultaneously, the balanced aggregation and improved molecular ordering facilitate efficient charge generation and transport, preserving a high JSC. This is reflected in the reduced non-radiative energy loss (ΔE3 ≈ 0.23 eV) and enhanced electroluminescence quantum efficiency (~1.17 × 10-4).
What are the scalability prospects for this ternary blend system in large-area fabrication?
The ternary blend system, incorporating D10 as a guest acceptor, demonstrates enhanced crystallinity and balanced carrier transport, which are favorable for maintaining high performance in thicker active layers typical of large-area coating methods. However, further studies are required to assess the impact of processing conditions on film morphology and device uniformity. The reported PCE of 19.86% in lab-scale devices suggests promising potential for scale-up, but cost and reproducibility remain to be evaluated.
What are the long-term stability implications of using aza-pyran acceptors compared to conventional Y6 derivatives?
The improved molecular ordering and reduced energetic disorder in D10-based ternary blends may enhance morphological stability under thermal and illumination stress, as ordered domains are less prone to degradation. However, long-term stability data are not provided in this study. The suppressed recombination and balanced transport could reduce burn-in losses, but accelerated aging tests are necessary to confirm operational lifetimes.
How does the energetic disorder (EU = 25 meV) compare to state-of-the-art low-energy-loss OSCs, and what is its practical significance?
An Urbach energy of 25 meV is among the lowest reported for high-efficiency OSCs, indicating exceptionally low energetic disorder. This reduces tail states that contribute to non-radiative recombination, directly lowering ΔEnr and improving VOC. Practically, this translates to higher achievable efficiencies and potentially better device stability, as reduced disorder often correlates with improved charge transport and reduced trap-assisted recombination.
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