High-efficiency crystalline white organic light-emitting diodes

Crystalline organic materials are attractive for OLEDs due to high carrier mobility and structural stability from long-range ordered molecular arrangements. Since the first electroluminescence in organic crystals, C-OLEDs have faced challenges in precisely controlling crystalline layers, integrating functional layers, and efficiently managing carriers and excitons. Polycrystalline thin-film C-OLEDs prepared via weak-epitaxy-growth (WEG) can combine facile vacuum deposition with crystalline advantages such as high mobility and aligned dipoles. A crystalline host matrix (CHM) with embedded nanoaggregates (NA) leverages both high mobility hosts and high exciton utilization emitters, enabling exciton management and practical engineering for high performance. While not all materials are compatible with WEG, high-efficiency guests can form nanoaggregates on smooth CHM surfaces, maintaining CHM continuity while enabling efficient exciton use and simplifying fabrication compared with co-doping. To realize WOLEDs, thermally activated delayed fluorescence (TADF) materials are promising because RISC enables conversion of triplet to singlet excitons, theoretically allowing 100% exciton utilization. Blue TADF emitters can replace conventional fluorescent blue in hybrid WOLEDs to achieve high efficiency and stability. This work proposes a CHM-NA WOLED that combines a fluorescent CHM with blue TADF nanoaggregates and orange phosphorescent dopants. The CHM’s high mobility yields low driving voltages, while nanoaggregates and dopants enable full exciton utilization. A hybrid C-WOLED achieves a turn-on voltage of 2.7 V (at 1 cd m−2), maximum power efficiency of 43.3 lm W−1, current efficiency of 38.6 cd A−1, and EQE of 12.8%, representing the highest performance among crystalline-material WOLEDs. Low operating voltage and high photon output are attributed to engineered crystalline structures and efficient exciton management.

Devices were fabricated by vacuum deposition (∼1×10−4 Pa) on ITO anodes. The ITO was coated with a 40 nm PEDOT:PSS layer serving as a hole injection layer and to provide a flat surface for subsequent crystalline growth. Crystalline thin films were grown by weak-epitaxy-growth (WEG), enabling precise thickness control and large-area, continuous, single-crystal-like polycrystalline films. A 7 nm BP1T crystalline thin film was first grown to induce oriented growth of subsequent layers. The fluorescent deep-blue 2FPPICz served as both a hole transport layer (7 nm) and crystalline host matrix (5 nm). The blue TADF material DMAC-DPS was deposited as 1 nm layers to form uniform embedded nanoaggregates atop 2FPPICz. The CHM–TADF nanoaggregate bilayer was repeated four times to construct a multilayer emission region. An orange phosphorescent emitter, Ir(tptpy)2acac, was co-evaporated at 5 wt% into the third 5 nm 2FPPICz CHM sublayer. On top of the crystalline emission stack, amorphous electron-transport/injection and cathode layers were deposited: BmPyPb (10 nm), BmPyPb (40 nm), LiF (1 nm), and Al (150 nm). Reference blue CHM–TADFNA device (Device B) structure: ITO/PEDOT:PSS (40 nm)/BP1T (7 nm)/2FPPICz (7 nm)/EML 1 (24 nm)/BmPyPb (40 nm)/LiF/Al. Film morphology and crystallinity were characterized by AFM/SEM (showing stripe-like CHM crystal bars with oblate ellipsoid nanoaggregates on top) and out-of-plane XRD, which showed that embedding DMAC-DPS nanoaggregates did not introduce additional diffraction peaks and preserved BP1T and 2FPPICz crystalline features. Reported charge mobilities for WEG-grown 2FPPICz are ∼0.10 cm2 V−1 (holes) and ∼0.015 cm2 V−1 (electrons) by time-of-flight. Device operation and EL spectra were measured versus voltage/current density; photon output rate N was evaluated from N = EQE·J/e to compare CHM–TADFNA to amorphous devices. WOLED optimization (Device W) tuned the nanoaggregate/host stacking and phosphor placement to harvest triplet leakage and stabilize spectra.

- A hybrid crystalline WOLED (Device W) using a crystalline host matrix with embedded blue TADF nanoaggregates and an orange phosphorescent dopant achieved: turn-on voltage ~2.7 V (at 1 cd m−2); luminance 1000 cd m−2 at 3.2 V; ΔV (V1000 − Von) = 0.5 V; maximum forward-viewing EQE 12.8%; maximum current efficiency 38.6 cd A−1; maximum power efficiency 43.3 lm W−1. At 1000 cd m−2, EQE 11.2%, CE 33.6 cd A−1, PE 33.0 lm W−1. CIE remained stable from (0.42, 0.46) to (0.42, 0.45) across luminance 327 to 5812 cd m−2. - For the blue CHM–TADFNA device (Device B): Von ≈ 2.7 V; emission peak 471 nm, FWHM ~63 nm; EQEmax 7.22%; CIE (0.15, 0.20) at 1000 cd m−2; V1000 ≈ 4.3 V; ΔV = 1.6 V. Compared with reported amorphous DMAC-DPS devices, CHM–TADFNA had a lower Von, faster increases in luminance and current density at low voltages, and enhanced photon output rate N due to higher carrier mobility. - Photon output analysis (N = EQE·J/e) showed CHM–TADFNA devices have enhanced photoemission relative to amorphous counterparts owing to rapid exciton formation from higher mobility crystalline films. - EL spectra of CHM–TADFNA blue devices exhibited blue-shift and narrowing versus amorphous devices, indicating potential for deep-blue emission. - Identified triplet leakage pathway: the relatively low triplet energy of the 2FPPICz CHM (T1 = 2.49 eV) can draw triplet excitons from TADF nanoaggregates, reducing blue EQE; incorporating an orange phosphor with lower T1 (Ir(tptpy)2acac) effectively harvests this leaked energy, boosting WOLED efficiency and stabilizing spectra. - Structural analyses (AFM/SEM/XRD) confirmed that DMAC-DPS nanoaggregates form on CHM without degrading crystallinity; crystalline peaks of BP1T and 2FPPICz remain unchanged.

The study addresses the challenge of achieving efficient, low-voltage crystalline WOLEDs by combining a high-mobility crystalline host matrix with high-exciton-utilization emitters. In the CHM–TADFNA structure, excitons preferentially form within the DMAC-DPS nanoaggregates; the exposed nanoaggregate surfaces promote direct electron injection from the ETL, while holes are transported effectively through the crystalline 2FPPICz host. Although the lower triplet energy of the CHM can siphon triplet excitons from the TADF aggregates (diminishing blue device EQE), integrating an orange phosphorescent dopant (Ir(tptpy)2acac) harvests these triplets via energy transfer due to its lower T1, thereby recapturing otherwise lost excitons and enhancing overall WOLED efficiency. The high mobility of the crystalline CHM enables rapid exciton formation, fast ramping of luminance and current density at low voltages, and reduced series resistance and Joule heat losses, culminating in high photon output. Morphological and structural characterizations verify that embedding nanoaggregates does not disrupt the crystalline order of the host, allowing retention of high mobility while introducing efficient emissive centers. The resulting device exhibits record performance for crystalline WOLEDs and stable chromaticity over a wide luminance range, satisfying practical application requirements.

This work demonstrates a CHM–TADF nanoaggregate–dopant (CHM‑TADFNA‑D) architecture that delivers record performance for crystalline WOLEDs. By leveraging WEG-grown crystalline 2FPPICz and BP1T to provide high carrier mobility and incorporating DMAC-DPS nanoaggregates with an Ir(tptpy)2acac phosphor to fully utilize singlet and triplet excitons, the devices achieve low turn-on voltage, high EQE, high current and power efficiencies, and stable white emission across operating luminance. The approach simplifies fabrication relative to conventional co-doping, preserves crystalline film integrity, and enhances photon output through efficient exciton management. Future work can focus on expanding compatible material systems for WEG growth, further optimizing exciton energy alignment to minimize losses, improving operational stability and lifetime, and tailoring emission for full-color and display-specific requirements.

- The blue CHM–TADFNA device exhibits lower EQE than some reported doped/undoped DMAC-DPS amorphous devices, attributed to triplet leakage to the CHM due to its relatively low T1. - Not all organic materials are compatible with WEG-grown crystalline thin films, potentially limiting generalizability across emitter/host systems. - Operational stability and lifetime metrics are not reported in the provided text, leaving long-term performance unassessed.