Crystalline organic materials are promising for organic light-emitting devices (OLEDs) due to their high carrier mobility and stability. However, challenges remain in controlling crystalline layers, integrating functional layers, and managing carrier/exciton processes for efficient exciton utilization. Recently, crystalline host matrix (CHM) with embedded nanoaggregates (NA) has emerged as a solution, combining high mobility and high exciton utilization efficiency. The CHM-NA architecture controls luminescence via exciton management and simplifies fabrication. While this approach has yielded high-performance blue C-OLEDs, extending it to white OLEDs (WOLEDs) requires new material systems and optimization strategies. Thermally activated delayed fluorescence (TADF) materials, which can convert triplet excitons to singlet excitons, offer the potential for 100% exciton utilization and high efficiency in WOLEDs, potentially replacing traditional fluorescent materials. This research proposes a novel CHM-NA structure for WOLEDs combining a fluorescent CHM with blue TADF nanoaggregates (TADFNA) and orange phosphorescent dopants (Phos.-D) to leverage the advantages of both high carrier mobility and high exciton utilization.

The introduction extensively reviews existing literature on crystalline OLEDs, highlighting the challenges in achieving high efficiency and the advantages of using crystalline materials. It discusses the use of polycrystalline thin films to combine facile deposition with the benefits of crystalline structure. The use of CHM-NA architectures for exciton management and improved performance is also reviewed. The literature on TADF materials and their application in high-efficiency WOLEDs is also discussed, emphasizing their potential to improve efficiency and stability compared to traditional fluorescent emitters.

The researchers fabricated a C-WOLED using a vacuum deposition process at 10⁻⁴ Pa. An ITO anode coated with PEDOT:PSS served as the hole injection layer. The main crystalline thin films were grown using the weak-epitaxy-growth (WEG) method. A BP1T layer induced oriented growth of subsequent layers. A 2FPPICz layer acted as both the hole transport layer and CHM. DMAC-DPS formed the TADF nanoaggregates. The CHM-NA structure was repeated four times. Orange phosphorescent material Ir(tptpy)₂acac (5 wt%) was co-evaporated with the 2FPPICZ in the third layer. BmPyPb served as the electron transport layer, followed by LiF and Al cathode. The exciton transition process involves initial exciton formation in DMAC-DPS nanoaggregates and subsequent energy transfer to the orange phosphorescent material. The morphology of the CHM-TADFNA layers was characterized using atomic force microscopy (AFM) and scanning electron microscopy (SEM). X-ray diffraction (XRD) was used to analyze the crystallinity of the films. The device performance was characterized by measuring current-voltage-luminance (I-V-L) characteristics, EQE, CIE coordinates, and electroluminescence (EL) spectra.

The fabricated CHM-TADFNA-D WOLED exhibited a low turn-on voltage of 2.7 V, a maximum power efficiency of 43.3 lm W⁻¹, a maximum current efficiency of 38.6 cd A⁻¹, and a maximum EQE of 12.8%. These results represent the highest performance among WOLEDs based on organic crystalline materials and are comparable to high-performance amorphous WOLEDs. The device demonstrated a rapid increase in luminance and current density at low driving voltages. Morphological analysis confirmed the successful formation of DMAC-DPS nanoaggregates on the crystalline 2FPPICz substrate without compromising film quality. The high carrier mobility of the crystalline thin films contributed to the enhanced photon output. Analysis of exciton distribution and energy transfer processes supported the design and efficiency of the device. A reference blue CHM-TADFNA device (Device B) was also characterized, exhibiting improved performance compared to an amorphous counterpart, demonstrating the benefits of the CHM-NA structure. Detailed analysis of Device B and Device W revealed stable spectral properties across a wide luminance range.

The high performance of the CHM-TADFNA-D WOLED is attributed to the synergistic effects of the crystalline host matrix, TADF nanoaggregates, and phosphorescent dopants. The CHM provides high carrier mobility leading to low driving voltages and fast exciton formation. The TADF nanoaggregates and phosphorescent dopants enhance exciton utilization efficiency. The well-designed energy transfer process prevents triplet exciton leakage and maximizes light output. The observed blue shift and narrowing in the EL spectrum of the CHM-TADFNA device indicate potential for fabrication of deep-blue devices. The findings demonstrate the successful integration of TADF materials into a crystalline architecture for efficient WOLEDs. The results are significant as they demonstrate the potential of crystalline materials for high-performance WOLEDs, offering advantages in efficiency and stability.

This work successfully demonstrated high-efficiency crystalline white organic light-emitting diodes based on a novel CHM-TADFNA-D architecture. The device achieved a record-high EQE of 12.8% for C-WOLEDs. The superior performance is attributed to the combined benefits of high carrier mobility, efficient exciton utilization, and a well-engineered energy transfer process. This research opens new avenues for developing next-generation WOLEDs with improved efficiency and stability.

While the study demonstrated high performance, further investigation could explore the long-term stability of the device under continuous operation. The influence of different dopant concentrations and the optimization of the energy transfer process could be further studied to potentially improve device efficiency. The scalability of the WEG method for large-area device fabrication also requires further investigation.