Key Takeaways & Executive Findings
- •• • The cross-linked blue emitters v-4CzBn and v-5CzBn achieve a singlet–triplet energy gap (ΔEST) ≤ 0.10 eV and a reverse intersystem crossing rate (kRISC) > 10^6 s−1, enabling efficient triplet harvesting in nondoped devices. • • Nondoped solution-processed OLEDs using S-4CzBn as the emitting layer deliver a maximum external quantum efficiency (EQE) of 12.01% and a maximum luminance of 11,141.15 cd m−2, demonstrating high electroluminescence performance. • • The devices exhibit an operational lifetime (T50) of 1375.66 hours at an initial luminance of 100 cd m−2, the longest reported for solution-processed OLEDs with cross-linked emitting layers, indicating superior stability. • • The thermal cross-linking reaction forms a three-dimensional covalent network that suppresses aggregation and crystallization, enhancing morphological stability and blocking moisture/oxygen ingress, which is critical for prolonged device operation.
Abstract
The solution process holds great promise for organic light-emitting diode (OLED) fabrication owing to its minimal material loss, simple processing and low equipment investment. However, solution-processed blue OLEDs still face the challenges of low electroluminescence efficiency and poor working stability. In this study, two new cross-linkable blue light-emitting molecules, v-4CzBn and v-5CzBn, were synthesized. They featured a multicarbazole-substituted benzonitrile donor–acceptor structure as the emitting core, with two vinyl phenyl units on the carbazole rings serving as cross-linking groups. A singlet–triplet energy gap of ΔEST ≤ 0.10 eV and a high reverse intersystem crossing rate (kRISC > 10^6 s−1) were achieved because the three-dimensionally confined covalent network structure formed through a thermal cross-linking reaction limited intramolecular motions and vibrational relaxations of luminescent units. Moreover, this structure suppressed irreversible morphological changes and structural deterioration of light-emitting units due to aggregation or crystallization, improving the light-emitting performance of the device. Nondoped solution-processed OLEDs with the structure of ITO/PEDOT:PSS/TFB/S-4CzBn/TPBi/LiF/Al exhibited blue emission with a peak at 488 nm, achieving a maximum external quantum efficiency of 12.01%, a maximum luminance of 11,141.15 cd m−2, and a T50 lifetime of 1375.66 h@100 cd m−2. This result represents the longest operational lifetime reported to date for solution-process devices with cross-linked emitting layers.
1. Introduction
Solution-processed OLEDs promise low-cost, large-area manufacturing, yet their commercial viability is hindered by inferior efficiency and operational stability compared to vacuum-deposited counterparts. The emitting layer (EML) in solution-processed devices often suffers from uncontrolled molecular packing and morphological degradation under electrical stress, leading to efficiency roll-off and shortened lifetimes. Cross-linking of luminescent molecules offers a route to stabilize the EML by forming a robust three-dimensional network, but prior cross-linkable emitters have shown limited efficiency and stability, particularly in the blue spectral region.
This work introduces two novel cross-linkable blue TADF emitters, v-4CzBn and v-5CzBn, which incorporate vinylphenyl cross-linking groups on a multicarbazole-benzonitrile donor–acceptor core. Thermal cross-linking creates a confined network that restricts intramolecular motion, reducing nonradiative decay and enhancing reverse intersystem crossing. The resulting nondoped devices achieve a record T50 lifetime of 1375.66 h at 100 cd m−2, directly addressing the stability bottleneck while maintaining high efficiency (EQE 12.01%). This breakthrough demonstrates the viability of cross-linked EMLs for practical solution-processed blue OLEDs.
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Qiming Li, Jiaxu Bai, Yinzhao Zhen, Wenkai Guan, Tianhao Wang, Hongli Liu, Shirong Wang, Xianggao Li (2026). Novel Cross-Linkable Blue Light Emitting Material and Its High Stability OLEDs by Solution Process. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4203-7
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Frequently Asked Questions
What is the underlying mechanism by which thermal cross-linking improves the operational stability of the blue OLEDs?
Thermal cross-linking forms a three-dimensional covalent network that restricts intramolecular motions and vibrational relaxations of the luminescent units, as evidenced by the small ΔEST (≤0.10 eV) and high kRISC (>10^6 s−1). This network suppresses aggregation and crystallization, preventing irreversible morphological changes and structural deterioration during operation. Additionally, the cross-linked layer acts as a barrier against moisture and oxygen penetration, mitigating degradation pathways and extending device lifetime to a T50 of 1375.66 h at 100 cd m−2.
How do the efficiencies of these cross-linked blue OLEDs compare with state-of-the-art solution-processed devices, and what are the trade-offs?
The nondoped devices achieve a maximum EQE of 12.01% and a maximum luminance of 11,141.15 cd m−2, which are among the highest reported for solution-processed blue OLEDs with cross-linked EMLs. The trade-off is that the cross-linking process requires thermal annealing, which may limit substrate choices and increase processing complexity. However, the extended operational lifetime (T50 > 1375 h) compensates for these processing constraints, making the approach attractive for applications demanding long-term stability.
What is the role of the donor–acceptor structure and the vinylphenyl cross-linking groups in achieving the observed photophysical properties?
The multicarbazole-substituted benzonitrile donor–acceptor structure provides a small singlet–triplet energy gap (ΔEST ≤ 0.10 eV), facilitating efficient reverse intersystem crossing (kRISC > 10^6 s−1) for TADF. The vinylphenyl groups enable thermal cross-linking, which creates a rigid network that suppresses nonradiative decay pathways and stabilizes the emitting layer. This combination yields high photoluminescence quantum yield and enhanced device stability.
What are the potential scalability and manufacturing challenges for integrating these cross-linkable materials into commercial OLED production?
The cross-linking reaction requires precise thermal annealing conditions (temperature and duration) to achieve optimal network formation without degrading the material. This adds a processing step compared to non-cross-linked systems. Additionally, the solubility and film-forming properties of the materials must be compatible with large-area coating techniques such as spin-coating or inkjet printing. The reported device performance, however, demonstrates that these challenges are surmountable, and the materials show promise for roll-to-roll manufacturing if annealing can be integrated inline.
How does the operational lifetime of 1375.66 h at 100 cd m−2 translate to practical application lifetimes, and what are the dominant degradation mechanisms?
At a typical display brightness of 100 cd m−2, a T50 of 1375.66 h corresponds to a median lifetime of about 57 days of continuous operation. For practical applications, this would need to be extended, but it is already the longest reported for solution-processed cross-linked EMLs. The dominant degradation mechanisms in OLEDs include chemical degradation of the emitter, interfacial degradation, and morphological instability. The cross-linked network addresses morphological degradation and blocks moisture/oxygen, but chemical degradation under electrical stress may still occur. Further improvements could focus on molecular design to enhance intrinsic photochemical stability.
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