Stable pure-green organic light-emitting diodes toward Rec.2020 standard

Achieving a wide colour gamut that meets the Rec.2020 standard while maintaining long operational stability is a central challenge for OLED displays. Multiple-resonance (MR) thermally activated delayed fluorescence (TADF) emitters can deliver narrowband spectra with excellent colour purity due to localized B/N multiple resonance effects and suppressed excited-state structural vibrations. However, despite progress in MR-TADF emitter design yielding CIE coordinates close to Rec.2020, operational durability remains insufficient for commercialization. Phosphor-sensitized fluorescence approaches can stabilize MR emitters but rely on rare metals (Ir, Pt). Pure-organic TADF assistants offer a metal-free alternative but have lagged in improving stability due to suboptimal assistant selection and undesirable exciton dynamics (quenching/annihilation, migration) under electrical excitation. Conventional p-/n-type hosts (e.g., mCBP, SF3-TRZ) can raise driving voltages and complicate charge balance due to polarity, with device behavior highly dependent on assistant doping levels that risk aggregation quenching at high concentrations. This study aims to control charge carrier and exciton behaviors—specifically the recombination zone (RZ) position and exciton management—to simultaneously achieve high color purity and superior stability in pure-green OLEDs approaching Rec.2020.

Prior work established MR-TADF emitters as promising for narrowband emission and high color purity through B/N multiple-resonance frameworks (Hatakeyama et al., 2016; subsequent advances in MR-TADF green/blue emitters). Enhancing operational lifetimes has been pursued through robust emitter design (large oscillator strength, fast RISC) and via sensitization using stable phosphors, which improves lifetime but uses scarce metals, limiting sustainability. TADF assistants have been explored to stabilize fluorescence devices (all-fluorescent architectures), yet stability gains have been limited, often due to poor charge/exciton management in the EML and imbalanced transport arising from host polarity and energy level mismatches. Traditional hosts like mCBP and SF3-TRZ can contribute to higher voltages and charge imbalance; device performance and stability may hinge on assistant concentration, with aggregation-induced quenching at higher loadings. These findings motivate exploring bipolar host matrices and rational energy level alignment to balance carriers, control the recombination zone, and suppress exciton quenching/annihilation without resorting to metal-based sensitizers.

- Materials and design: Selected a pure-green MR-type terminal emitter h-BNCO (~530 nm, FWHM 38 nm) for narrowband emission. Used a bipolar donor–acceptor host PIC-TRZ2 (HOMO −5.6 eV, LUMO −2.8 eV) to facilitate balanced carrier injection/transport and low voltage. Employed TADF assistants: primary 4CzIPN (HOMO −5.9 eV, LUMO −3.3 eV) to stabilize triplets; a comparison assistant 5Cz-TRZ (HOMO −5.9 eV, LUMO −3.0 eV). - Photophysical characterization: Measured UV–vis absorption and PL of components and blends; verified FRET by spectral overlap. Recorded PL of neat and doped films (1 wt% h-BNCO: PIC-TRZ2; 1 wt% h-BNCO: 8 wt% 4CzIPN: PIC-TRZ2). Performed transient PL decay to extract prompt lifetimes and delayed components; estimated FRET and RISC rates. - Device fabrication: OLED stack (ITO/HAT-CN/Tris-PCz/mCBP/EML/SF3-TRZ/Liq:SF3-TRZ/Liq/Al) with EMLs: D1 (1 wt% h-BNCO:mCBP), D2 (1 wt% h-BNCO:PIC-TRZ2), D3 (1 wt% h-BNCO:8 wt% 4CzIPN:PIC-TRZ2). A fourth device D4 added a functional spacer (5 nm of 1 wt% h-BNCO:PIC-TRZ2) between the 4CzIPN-doped EML and SF3-TRZ to relocate the recombination zone away from the EML/ETL interface. Devices were encapsulated and characterized. - Electrical/EL characterization: Measured EL spectra, EQE vs luminance, and operational stability (LT95 at 1000 cd m−2). Assessed recombination zone using 0.3 nm Ir(fliq)2(acac) exciton-sensing layers inserted at different EML depths. - Carrier transport analysis: Built hole-only and electron-only devices (HOD/EOD) for D2 and D3 compositions to determine relative mobilities and identify electron trapping by 4CzIPN. - Transient electroluminescence (EL): Recorded turn-on transients and turn-off decays to probe carrier accumulation and exciton dynamics; extracted triplet lifetimes and assessed RISC acceleration. - Controls and comparisons: Evaluated a device with 8 wt% 4CzIPN:PIC-TRZ2 (D5) to isolate trapping origin; assessed a device with 5Cz-TRZ assistant (D6) to study energy-level alignment effects. Varied spacer thickness and composition to optimize RZ position and performance.

- Efficient energy transfer and narrowband emission: Blend films (1 wt% h-BNCO:PIC-TRZ2 and with 8 wt% 4CzIPN) show narrow PL/EL (FWHM ~38–40 nm) with complete energy transfer to h-BNCO. - Photophysics and rates: PIC-TRZ2 exhibits long donor lifetime (τr ≈ 99 ns) leading to slower FRET rate (~4.9 × 10^5 s−1), yet efficient transfer due to low kr. 4CzIPN has shorter τr (17.0 ns) and accelerates transfer. RISC rates in blends increase versus mCBP host (to ~5.3–7.4 × 10^5 s−1). - Device performance (at 1000 cd m−2): • D1 (mCBP host): EQE 10.7% (rolloff 57.4%), LT95 = 46 h, CIE (0.23, 0.71). • D2 (PIC-TRZ2 host): EQE 18.1% (rolloff 27.3%), LT95 = 124 h, CIE (0.26, 0.70). • D3 (PIC-TRZ2 + 8 wt% 4CzIPN): EQE 15.7% (rolloff 3.7%), LT95 = 322 h, CIE (0.26, 0.68). • D4 (D3 + 5 nm 1 wt% h-BNCO:PIC-TRZ2 spacer): EQE 16.6% (rolloff 7.3%), LT95 = 437 h, CIE (0.27, 0.69). - Recombination zone (RZ) control: • D1 RZ near EML/SF3-TRZ interface; SF3-TRZ electromer peak (~625 nm) grows during operation, indicating exciton diffusion and degradation. • D2 RZ centered within EML due to bipolar PIC-TRZ2, yielding improved rolloff and stability. • D3 RZ shifts toward EML/SF3-TRZ interface because 4CzIPN’s deep LUMO (−3.3 eV) traps electrons; hole transport remains via PIC-TRZ2 (HOMO −5.6 eV). HOD/EOD confirm reduced electron mobility in D3 vs D2; hole mobility unchanged. • Adding a PIC-TRZ2-based spacer in D4 redistributes RZ away from the fragile EML/ETL interface, suppressing exciton diffusion to SF3-TRZ and improving lifetime to LT95 = 437 h. - Transient EL dynamics: • D3 shows initial EL spike then dip upon turn-on, attributed to electron trapping by 4CzIPN; behavior absent under photoexcitation. • Turn-off triplet lifetimes: D1 ≈ 22.9 µs (delayed dominates, ~0.97 fraction), D2 ≈ 21.3 µs, D3 ≈ 3.42 µs (accelerated RISC dominated by 4CzIPN), D4 ≈ 4.96 µs. - Assistant selection by energy levels: Replacing 4CzIPN with 5Cz-TRZ (shallower LUMO −3.0 eV) eliminates pronounced trapping/accumulation and shifts RZ toward the HTL side, underscoring the role of rational energy-level alignment. - Trade-offs with spacer thickness/composition: Thicker spacers (10–20 nm) increase peak EQE but worsen rolloff and LT95 at 1000 cd m−2; replacing the spacer composition degrades EQE and broadens FWHM. - Overall: Achieved high color purity (CIE y up to 0.69) and improved operational stability in purely organic OLEDs without rare-metal phosphors, approaching Rec.2020 requirements.

The study demonstrates that controlling carrier dynamics through energy-level engineering directly governs the recombination zone location, which in turn determines efficiency rolloff and operational stability. A deep-LUMO TADF assistant (4CzIPN) embedded in a bipolar host (PIC-TRZ2) traps electrons, shifting the RZ toward the EML/ETL interface where exciton–polaron and exciton–exciton annihilation are more probable, reducing EQE despite increased stability from faster RISC. By introducing a barrier-free PIC-TRZ2-based spacer, the RZ is moved away from the fragile interface and exciton diffusion into the ETL is suppressed. This redistribution mitigates interfacial degradation pathways, yielding both high color purity (narrow FWHM, CIE y ~0.69) and substantially extended lifetime (LT95 up to 437 h at 1000 cd m−2) while maintaining low driving voltage. The comparative results using a shallower-LUMO assistant (5Cz-TRZ) further confirm that rational energy-level alignment of assistant and host is critical for balancing charge transport, managing excitons, and optimizing stability toward Rec.2020-compliant pure-green OLEDs.

By integrating a narrowband MR emitter (h-BNCO), a bipolar host (PIC-TRZ2), and a carefully selected TADF assistant (4CzIPN), the work establishes a strategy to manage charge and exciton dynamics via energy-level alignment and recombination zone control. Transient EL analyses reveal that electron trapping by deep-LUMO assistants can displace the recombination zone to detrimental interfaces; employing a non-barrier PIC-TRZ2-based spacer relocates the zone and suppresses interfacial quenching. The optimized architecture achieves narrowband pure-green emission with high color purity (CIE y ~0.69), low rolloff, and markedly improved stability (LT95 = 437 h at 1000 cd m−2) without rare-metal phosphors. These insights into degradation mechanisms and RZ management provide a pathway toward commercial-level stability in pure-organic OLEDs meeting Rec.2020 color standards. Future work may further tune assistant/host energy levels and spacer designs to optimize both efficiency and durability across colors.

- Electron trapping by deep-LUMO 4CzIPN, while enhancing stability via faster RISC, shifts the recombination zone to the EML/ETL interface and reduces EQE; device performance reflects a trade-off between stability and efficiency. - The 4CzIPN-doped devices (D3/D4) show lower maximum EQE than PIC-TRZ2-only (D2), partly due to reduced EML PLQY and involvement of the spacer in exciton recombination. - Spacer optimization presents trade-offs: increasing spacer thickness can raise peak EQE but worsens efficiency rolloff and LT95 at practical luminance. - Long-term operation reveals some exciton diffusion toward the ETL (SF3-TRZ electromer signatures), indicating remaining interfacial degradation pathways that are mitigated but not eliminated. - Results are demonstrated for specific materials stacks; generality across other emitters/transport layers requires further validation.