Efficient and stable hybrid perovskite-organic light-emitting diodes with external quantum efficiency exceeding 40 per cent

The study addresses the need for display LEDs that meet the Rec. 2020 color gamut, highlighting metal-halide perovskites as uniquely capable due to their narrowband emissions (FWHM ≤ 20 nm) and tunable bandgaps (410–850 nm). Despite recent progress in perovskite LEDs (PeLEDs) achieving EQEs approaching 30%, their practical use is hindered by intrinsic electric-field-driven instability. The authors propose integrating perovskite emitters with mature OLED technologies in a tandem configuration to leverage high color purity, existing OLED-compatible vacuum processing, and to improve operational lifetime through reduced current at a given luminance. The research aims to realize efficient, stable, and high color-purity hybrid perovskite–organic LEDs by optimizing an interconnecting layer (ICL) for superior charge generation, transport, and reduced Joule heating.

- Device architecture: A tandem hybrid LED consisting of a bottom PeLED and a top OLED connected by an interconnecting layer (ICL). The PeLED structure: ITO/TFB:PVK/perovskite/TPBi/Bphen:Cs2CO3/Al. The OLED structure: ITO/HAT-CN/MoO3/CBP/CBP:Ir(ppy)2(acac)/TPBi/LiF/Al. - Interconnecting layer (ICL): Designed as HAT-CN/MoO3/CBP to serve as a high-transparency, efficient charge generation and separation interface. HAT-CN acts as the electron-generation/separation material, CBP as the hole-generation/separation material, and an ultrathin MoO3 (~1 nm) is inserted between HAT-CN and CBP as a charge enhancement layer to promote p-type interface doping, strengthen the built-in electric field, facilitate carrier separation/transport, and reduce resistive losses. - Energy level rationale: MoO3 has a deeper LUMO than HAT-CN and lies below the CBP HOMO, promoting electron transfer from CBP to HAT-CN and enhancing hole density in CBP; thin Al and ultrathin MoO3 aid electron separation and hole tunneling, respectively, enabling efficient carrier injection into both subunits. - Simulation and modeling: Drift–diffusion simulations solving continuity and Poisson equations, with field-dependent Miller–Abrahams hopping for interfacial transport across Bphen:Cs2CO3/ICL/CBP, were used to compare a conventional CGL (HAT-CN/CBP) versus modified CGL (m-CGL, HAT-CN/MoO3/CBP). Outputs included spatial distributions of charge density, electric field, and recombination rate. - Electrical/optical characterization of ICLs: Fabricated ICL test diodes ITO/HAT-CN/MoO3/CBP/Al with MoO3 thicknesses from 0 to 3 nm. Measured J–V in forward/reverse bias to assess charge generation/quenching symmetry and transport, and C–V at 1 kHz using LiF double-insulating layers to suppress electrode injection and probe the onset of interfacial charge generation. - Morphology and conductivity: AFM to assess surface roughness of HAT-CN with/without ultrathin MoO3; c-AFM to probe local conductivity; XPS to verify MoO3 diffusion into HAT-CN and associated conductivity enhancement. - Device performance: Compared EL spectra, J–V, L–V, EQE–J among single PeLED, single OLED, and hybrid LED. Varied MoO3 thickness in the ICL to optimize trade-off between electrical enhancement and optical transmittance. Assessed device color metrics (FWHM, CIE) and large-area operation (30 mm × 30 mm). - Thermal and stability testing: Monitored surface temperature versus luminance for hybrid devices with CGL vs m-CGL to quantify Joule heating. Lifetime tests (in glovebox, no encapsulation) for hybrid devices based on quasi-2D/3D PeLEDs coupled with OLEDs; estimated T50 at 100 cd m−2 using an acceleration model (L^n T50 = constant, n = 1.61).

- Color purity and spectra: Hybrid LED exhibits a peak at 516 nm with a narrow FWHM of 31 nm, much narrower than OLED alone (67 nm), approaching high color purity; CIE shifted from (0.28, 0.64) with CGL to (0.25, 0.67) with m-CGL. - Efficiency and luminance: Peak EQE of 43.42% and current efficiency of 156 cd A−1; maximum luminance of 176,166 cd m−2. Hybrid EQE approaches the sum of sub-units (PeLED 21.07% + OLED 21.33%). V_on reduced from 8.5 V (CGL) to 5 V (m-CGL). - Charge generation/separation: Simulations show interfacial charge densities increase 1.9× from 9.53 × 10^11 cm−3 (CGL) to 1.83 × 10^12 cm−3 (m-CGL). Radiative recombination rates in both emitters increase by ~1.8× with MoO3 insertion. Electric field within ICL region is notably enhanced with MoO3. - ICL electrical properties: J–V of ICL test devices becomes more symmetric and current increases with MoO3 thickness (0–3 nm), indicating strengthened charge generation and transport. C–V shows onset voltage for charge generation reduced to ~7 V for 3 nm MoO3, versus >15 V without MoO3. - Morphology/conductivity: AFM shows similar roughness with/without MoO3; c-AFM indicates increased local current from ~2 nA to ~8 nA after MoO3 deposition; XPS evidences partial diffusion of MoO3 into HAT-CN, improving conductivity. - Thermal management: At 10,000 cd m−2, surface temperature reduced from 48.7 °C (CGL) to 38.3 °C (m-CGL), indicating reduced Joule heating. Comparable temperatures (~25 °C) at 1,000 cd m−2. - Thickness optimization: Optimal MoO3 thickness ~1 nm for maximum EL efficiency; further thickening reduces transmittance (hurting outcoupling) and increases current density, leading to efficiency roll-off and thermal degradation. - Stability: Hybrid LEDs with m-CGL show T50 = 13.2 h at L0 = 15,000 cd m−2; CGL devices show T50 = 4.3 h at L0 = 5,000 cd m−2. Using acceleration (n = 1.61), estimated T50 at 100 cd m−2 is 42,080 h for m-CGL devices (~18× improvement over CGL). Slight EL spectral changes observed after lifetime or under different biases. - Scalability: Demonstrated bright green emission at 7.5 V over a large active area (30 mm × 30 mm).

The tandem hybrid architecture successfully integrates the narrow-linewidth, high-color-purity emission of perovskites with the maturity and stability advantages of OLEDs. By closely matching the PL peaks of the PeLED and OLED, the hybrid device minimizes reabsorption losses and allows the perovskite’s sharp emission to dominate, yielding a narrowed overall EL linewidth. The key enabler is the engineered ICL (HAT-CN/MoO3/CBP): the ultrathin MoO3 layer deepens the interfacial energy landscape, enhances p-type interface doping in CBP, strengthens the built-in field, and facilitates balanced and efficient charge generation/separation. This results in higher interfacial charge densities, greater recombination rates in both sub-units, reduced turn-on voltage, and suppressed Joule heating. The reduced thermal load is crucial for operational stability, consistent with the improved lifetimes observed and the known advantages of tandem devices operating at lower current density for a given luminance. Optimization identifies 1 nm MoO3 as the sweet spot balancing electrical benefits with optical transmittance to maximize outcoupling and efficiency. Overall, the results directly address the instability and efficiency challenges of PeLEDs by leveraging an optimized tandem configuration and interfacial engineering.

This work demonstrates a high-performance green hybrid perovskite–organic tandem LED featuring an efficient interconnecting layer of HAT-CN/MoO3/CBP. The devices achieve a narrow emission linewidth (~31 nm), peak EQE exceeding 43%, maximum luminance over 176,000 cd m−2, reduced operating voltage, lower thermal load, and markedly improved operational lifetime (estimated T50 ~42,080 h at 100 cd m−2). The approach is compatible with different perovskite systems and large-area fabrication, underscoring strong potential for advanced display applications. Future work is implied in further optimizing ICL thickness/transmittance trade-offs, extending to other emission colors, and advancing encapsulation and outcoupling strategies to enhance stability and efficiency.

- Full author affiliation details for all contributors are not provided in the excerpt. Device stability tests were performed in a glove box without encapsulation; long-term behavior in ambient conditions remains unreported. - The lifetime at 100 cd m−2 is extrapolated using an acceleration model (n = 1.61); actual long-term stability under practical operation may vary. - Increasing MoO3 thickness beyond the optimum degrades optical transmittance and induces higher current density, leading to efficiency roll-off and heating effects. - Slight EL spectral shifts under different biases and after stress indicate residual spectral stability concerns. - Reported thermal measurements are surface temperatures; internal junction temperatures are not directly measured.