Germanium-integrated exciplex host for high-performance narrowband OLEDs with mitigated efficiency roll-off

Germanium-integrated exciplex host for high-performance narrowband OLEDs with mitigated efficiency roll-off | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Article Germanium-integrated exciplex host for high-performance narrowband OLEDs with mitigated efficiency roll-off Chuluo Yang, Xu Zhang, Zhanxiang Chen, Manli Huang, Jiahui Liu, and 1 more This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-6308443/v1 This work is licensed under a CC BY 4.0 License Status: Under Review Version 1 posted You are reading this latest preprint version Abstract Multi-resonance thermally activated delayed fluorescence (MR-TADF) materials open a heavy-metal-free avenue toward highly efficient, narrowband organic light-emitting diodes (OLEDs). However, their slow reverse intersystem crossing (RISC) kinetics lead to severe efficiency roll-off at practical driving currents, and existing strategies to improve RISC often require emitter-specific chemical modifications. Here we demonstrate a general strategy by integrating heavy-atom germanium into hole-transporting/electron-transporting hosts to construct an exciplex system that accelerates spin-flip dynamics. This design enhances RISC rates in diverse MR-TADF emitters via Förster resonance energy transfer without altering their electronic structure. The resulting blue and green OLEDs achieve external quantum efficiencies exceeding 40% with markedly suppressed efficiency roll-off relative to silicon-based counterparts. Notably, the green device exhibits a half-lifetime of 241 h at an initial luminance of 1,000 cd m -2 , surpassing previously reported OLEDs employing non-metallic heavy atoms. This general host-guest strategy advances MR-TADF systems toward practical high-performance displays. Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Organic LEDs Physical sciences/Materials science/Materials for optics/Lasers, LEDs and light sources/Organic LEDs Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Organic light-emitting diodes (OLEDs) combining high efficiency and narrowband emission are critical for meeting the growing demand for high-quality displays 1-3 . Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters, which enable efficient exciton utilization and narrowband emission, have emerged as promising alternatives to phosphorescent emitters reliant on heavy metals 4,5 . Pioneering work by Hatakeyama et al. demonstrated that MR-TADF emitters minimize structural reorganization and reduce the singlet-triplet energy gap through short-range charge-transfer transitions, achieving high device efficiency and narrow emission in OLEDs 6,7 . However, most high-performance MR-TADF materials still suffer from limited reverse intersystem crossing (RISC) rates, leading to severe efficiency roll-off at elevated driving currents 8,9 . Addressing efficiency roll-off in OLEDs requires accelerating exciton consumption, particularly of dark triplet excitons, to suppress detrimental exciton-exciton and exciton-polaron interactions 10-15 . In MR-TADF systems, introducing heavy atoms into the molecular framework 16-18 or periphery 19-21 can enhance spin-orbit coupling, thereby accelerating the RISC process 22 . This approach has enabled RISC rates exceeding 10 6 s -1 , with reported green-emitting devices achieving a state-of-the-art external quantum efficiency (EQE) of 36.8% alongside reduced roll-off 16 . Despite these significant advances, heavy atom-modified emitters remain limited in their adaptability, typically being case-by-case. We conceive a general strategy by integrating the heavy atom into the host molecular structure, which may apply to a broad range of emitters. Recent research has also focused on exciplex host systems formed by combining hole-transport (HT) and electron-transport (ET) materials 23-25 . These hosts improve charge balance in devices and, when paired with TADF sensitizers and terminal fluorescent emitters, enable high-efficiency OLEDs with suppressed efficiency roll-off. However, such systems typically require simultaneous co-evaporation of four independent components, complicating fabrication processes and demanding stringent control over deposition equipment. In this context we are driven to develop exciplex hosts with accelerated RISC, which not only reduce the delayed fluorescence lifetime of MR-TADF emitters without requiring TADF sensitizers but also deliver exceptional electroluminescent (EL) performance. Both blue and green OLEDs employing these hosts demonstrate high external quantum efficiencies over 40% alongside significantly reduced efficiency roll-off, showcasing their potential for simplified, high-performance device architectures 26,27 . Results Molecular design and photophysical properties The host-guest system developed in this work is illustarted in Fig. 1a. Our design leverages a host with heavy atom effect to enable rapid triplet exciton upconversion and subsequent energy transfer to the MR-TADF guest (see Figs. 1b,c for the chemical structures). Building on previously reported hosts with bulky triphenylsilyl units 28 , specifically SiCzCz (9-(3-(triphenylsilyl)phenyl)-9 H -3,9'-bicarbazole) and SiTrzCz2 (9,9'-(6-(3-(triphenylsilyl)phenyl)-1,3,5-triazine-2,4-diyl)bis(9 H -carbazole)), we replaced silicon (atomic number Z = 14) with germanium ( Z = 32) to boost spin-orbit coupling in the resulting exciplex. This substitution also minimally affected emission color, triplet energy (T 1 ), and geometric configuration of the HT/ET host pair (as detailed below). With the established design strategy in place, we synthesized 9-(3-(triphenylgermyl)phenyl)-9 H -3,9'-bicarbazole (GeCzCz) via the Buchwald-Hartwig coupling reaction and 9,9'-(6-(3-(triphenylgermyl)phenyl)-1,3,5-triazine-2,4-diyl)bis(9 H -carbazole) (GeTrzCz2) via Suzuki-Miyaura cross-coupling reaction (Supplementary Note 1 and Supplementary Figs. 1-8). Single-crystal X-ray diffraction analysis (Supplementary Figs. 9 and 10) revealed that while the C–Ge bond length (~1.95 Å) is slightly longer than the C–Si bond (~1.88 Å), the crystal structures of GeCzCz and its silicon analogue (SiCzCz) exhibited similar packing motifs, with no significant intra- or intermolecular π-π interactions. Instead, both structures displayed multiple intermolecular C–H···π interactions between phenyl rings of the triphenylgermyl (or triphenylsilyl) unit and hydrogens on adjacent phenyl rings, as well as between carbazole rings and hydrogens on phenyl rings of the triphenylgermyl (or triphenylsilyl) group. Similarly, the GeTrzCz2 crystal structure closely resembled that of SiTrzCz2, featuring intermolecular π-π interactions between triazine and carbazole rings, as well as between adjacent carbazole rings. These structural similarities between germanium- and silicon-based analogues enable direct investigation of heavy-atom effects on excited states, while minimizing confounding factors such as variations in triplet-state energy or charge transport properties. The steady-state photophysical properties of GeCzCz, GeTrzCz2, and their 1:1 molar blend (GeCzCz:GeTrzCz2) in solid films were further analyzed. As shown in Fig. 2a, both GeCzCz and GeTrzCz2 exhibited absorption bands below 350 nm, with emission maxima at 386 and 446 nm, respectively. The red-shifted and featureless photoluminescence (PL) spectrum of GeCzCz:GeTrzCz2 compared to those of the neat films (GeCzCz and GeTrzCz2) indicates the formation of an exciplex, with a maximum emission wavelength at 470 nm, consistent with previously reported SiCzCz:SiTrzCz2 system. Phosphorescence spectra at 77 K (Supplementary Fig. 11) revealed T 1 energies of 3.12 eV for GeCzCz and 2.91 eV for GeTrzCz2, both higher than those of the SiCzCz and SiTrzCz2 reference, supporting sufficient T 1 energy levels to suppress back energy transfer from dopant to host. As expected, the delayed fluorescence lifetime of GeCzCz:GeTrzCz2 was significantly longer than those of the GeCzCz and GeTrzCz2 neat films, confirming efficient RISC in the exciplex (Fig. 2b). Notably, replacing silicon with germanium reduced the delayed fluorescence lifetime from 1.9 μs (SiCzCz:SiTrzCz2) to 1.6 μs (GeCzCz:GeTrzCz2) and the prompt fluorescence lifetime from 186.8 ns to 173.0 ns (Supplementary Fig. 12 and Supplementary Table 1), demonstrating that the heavy atom effect of germanium accelerates both prompt and delayed fluorescence processes. Key kinetic constants, including singlet radiative decay rate ( k r,S ) and RISC rate ( k RISC ), were determined (Supplementary Note 2 and Supplementary Table 1) 29-31 . The germanium-based exciplex exhibited a 1.9-fold enhanced k r,S of 2.5 × 10 6 s -1 and a 1.8-fold higher PL quantum yield (54% vs. 30%) than the silicon-based reference (SiCzCz:SiTrzCz2). Importantly, the k RISC of GeCzCz:GeTrzCz2 also increased 1.1-fold to 7.6 × 10 5 s -1 compared to the silicon counterpart (SiCzCz:SiTrzCz2), highlighting the role of germanium in promoting the RISC process of exciplex. Additionally, density functional theory (DFT) calculations 31 revealed that the highest occupied molecular orbital (HOMO) of GeCzCz:GeTrzCz2 is localized on the bicarbazole units of GeCzCz, while the lowest unoccupied molecular orbital (LUMO) resides on the triazine moiety of GeTrzCz2 (Supplementary Fig. 13). This indicates that the spatial separation between HOMO and LUMO remains unaffected by the substitution of the heavy-atom germanium. Analysis of spin-orbit coupling matrix elements (SOCMEs) in the optimized T 1 geometry (Supplementary Fig. 14) showed a 2.98-fold increase from 0.265 cm -1 (SiCzCz:SiTrzCz2) to 0.791 cm -1 (GeCzCz:GeTrzCz2), consistent with the expected enhancement in spin-orbit coupling due to the higher atomic number. These results confirm accelerated spin-flip processes in the germanium-based exciplex, aligning with the time-resolved photoluminescence data. The photophysical properties of GeCzCz:GeTrzCz2 as the host were further analyzed and compared to the reference SiCzCz:SiTrzCz2 host. We selected two blue MR-TADF emitters 1,32 ( t -DABNA, single-boron; υ -DABNA: double-boron) and a green emitter 33 ( ω -DABNA, triple-boron), doping all at 1 wt% to minimize interchromophore quenching. Absorption spectra of the emitters overlapped with both hosts (Supplementary Fig. 15), ensuring efficient FRET. Förster radii ( R FRET ) for t -DABNA, υ -DABNA, and ω -DABNA were calculated as 4.8 nm, 7.8 nm, and 8.6 nm, respectively, based on spectral overlap between extinction spectrum of the emitter and PL spectra of the host. Time-resolved emission spectra (Fig. 3) revealed that the germanium-based exciplex host reduced delayed fluorescence lifetimes of t -DABNA, υ -DABNA, and ω -DABNA by 1.18-fold, 1.67-fold, and 2.14-fold, respectively, compared to the silicon-based exciplex host. Greater lifetime reductions for υ -DABNA and ω -DABNA correlated with their larger R FRET , which ensured more complete FRET. Concurrently, the k RISC for t -DABNA, υ-DABNA and ω -DABNA increased by 1.69-fold, 2.02-fold and 2.08-fold in the germanium-based exciplex host, with υ -DABNA achieving k RISC = 9.1 × 10 5 s -1 (Supplementary Table 2). These improvements, driven by enhanced SOCME of germanium-based exciplex host, demonstrate the host’s efficacy in accelerating RISC dynamics. OLED devices Motivated by the shortened delayed fluorescence lifetimes and enhanced k RISC of MR-TADF emitters in the germanium-based exciplex host, we evaluated their EL performance in optimized OLEDs. Target devices were fabricated with the structure: indium tin oxide (ITO)/1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN, 5 nm)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, 30 nm)/tris(4-carbazolyl-9-ylphenyl)amine (TCTA, 15 nm)/GeCzCz (15 nm)/GeCzCz:GeTrzCz2:dopant (25 nm, 0.50:0.49:0.01 w/w/w)/ GeTrzCz2 (20 nm)/1-(4-(10-([1,1'-biphenyl]-4-yl)anthracen-9-yl)phenyl)-2-ethyl-1 H -benzo[ d ]imidazole (ANT-BIZ, 30 nm)/8-hydroxyquinolinato lithium (Liq, 2 nm)/alumina (Al, 100 nm), where the emissive layer incorporated t -DABNA, υ -DABNA, or ω -DABNA as dopant. Control devices (A-C) were fabricated by substituting GeCzCz and GeTrzCz2 with their silicon-based counterparts, SiCzCz and SiTrzCz2, respectively. The chemical structures of the organic materials, energy-level diagram and EL performance are depitcted in Fig. 4 and Supplemenatry Fig. 16, with detailed parameters provided in Table 1. Figure 4b illustrates the EL spectra of the three target OLEDs employing the GeCzCz:GeTrzCz2 host, labeled as t -DABNA-based, υ -DABNA-based, ω -DABNA-based devices. The t -DABNA-based device exhibited deep-blue emission peaking at 460 nm, while the υ -DABNA-based device displayed pure blue emission at 469 nm. Despite its bluer emission, the t -DABNA device achieved CIE coordinates of (0.154, 0.141), less optimal than the υ -DABNA device’s (0.125, 0.113), due to incomplete Förster energy transfer from the host to the t -DABNA dopant. Notably, the ω -DABNA-based device emitted green light with CIE coordinates (0.154, 0.741), closely approaching the Rec. 2020 green standard (0.170, 0.797). Table 1 | Summary performance of the devices Device CE (cd . A -1 ) PE (lm W -1 ) EQE (%) Max (average) 1,000 cd m -2 Max (average) 1,000 cd m -2 Max (average) 1,000 cd m -2 5,000 cd m -2 t -DABNA 38.1 (35.6±1.3) 14.4 42.7 (39.9±1.4) 10.6 30.1 (29.3±0.5) 12.0 7.6 Control-A 38.7 (37.1±1.9) 10.2 43.5 (41.6±2.2) 6.0 26.9 (26.5±0.5) 7.5 4.5 υ -DABNA 43.6 (37.4±1.6) 30.4 48.9 (38.8±3.3) 27.7 40.1 (39.2±0.9) 32.8 24.4 Control-B 35.8 (32.3±2.9) 23.1 40.2 (35.2±4.6) 18.5 34.1 (33.1±1.1) 22.4 14.6 ω -DABNA 145.5 (142.1±2.3) 118.6 163.2 (159.4±2.6) 117.8 40.4 (39.3±0.7) 33.4 23.9 Control-C 130.7 (124.5±5.0) 87.3 146.6 (139.7±5.6) 79.4 35.3 (34.3±0.8) 23.7 10.5 The average device parameters in parentheses are based on the measurement of twenty independent devices. All fabricated devices exhibited nearly identical turn-on voltages of 2.8 V and high maximum luminance over 30,000 cd m -2 (Supplementary Fig. 17), indicating efficient exciton formation in the exciplex host followed by FRET to the guest (Langevin recombination). Target devices using the GeCzCz:GeTrzCz2 as host showed higher efficiencies and reduced efficiency roll-off compared to SiCzCz:SiTrzCz2-based ones (Control A-C). The t -DABNA-based device achieved a maximum EQE of 30.1% (Fig. 4d), surpassing Control-A (26.9%). However, incomplete energy transfer from the GeCzCz:GeTrzCz2 host to the t -DABNA guest (Fig. 4b) limited its EQE to 12.0% at 1,000 cd m -2 . In contrast, devices with complete host-to-guest energy transfer, specifically υ -DABNA and ω -DABNA-based devices, achieved peak EQEs of 40.1% and 40.4%, respectively. Their EQE reproducibility, shown in a histogram (Fig. 4c), averaged 39.2±0.9% ( υ -DABNA) and 39.3±0.7% ( ω -DABNA). At 1,000 cd m -2 , these devices retained EQE values of 32.8% ( υ -DABNA) and 33.4% ( ω -DABNA), and still remained 24.4% and 23.9% at 5,000 cd m -2 . Both significantly outperformed Control-B (22.4%/14.6%) and Control-C (23.7%/10.5%) at equivalent brightness levels. The suppressed roll-off in germanium-based devices stems from reduced bimolecular annihilation processes (e.g., triplet-triplet and triplet-polaron annihilation), enabled by accelerated spin-flip kinetics from enhanced SOCME via germanium’s heavy-atom effect. To the best of our knowledge, the υ -DABNA and ω -DABNA-based devices set record EQE values for OLEDs with CIEy ≤ 0.2 (blue) and CIEy ≥ 0.71 (green, NTSC standard), surpassing platinum-based phosphorescent and TADF-sensitized systems (Figs. 4h,i and Supplementary Table 3). The ω -DABNA-based device also exhibited enhanced operational stability, achieving a half-lifetime (LT 50 ) of 241 h at an initial luminance of 1,000 cd . m -2 (Fig. 4g). This surpasses the stability of previously reported green OLEDs utilizing non-metallic heavy atoms (Supplementary Table 4). These results validate the generality of heavy-atom-modified exciplex hosts for achieving efficient and stable OLEDs without complex guest synthesis. Conclusion By integrating HT and ET hosts containing germanium into an exciplex host aligned with the low-energy S 1 state of MR-TADF emitters, the intrinsically slow k RISC s of MR-TADF emitters were significantly accelerated. This principle was validated using blue ( υ -DABNA) and green ( ω -DABNA) MR-TADF emitters, which exhibited 2.02- and 2.08-fold enhancements in k RISC , reaching up to 9.1 × 10 5 s − 1 under photoexcitation. Resulting blue and green OLEDs achieved maximum EQEs exceeding 40%, with significantly reduced roll-off and improved operational stability. Our strategy not only advances MR-TADF-based OLED performance but also provides a universal approach to minimize efficiency roll-off without complex emitter synthesis, extendable to applications beyond optoelectronics. Declarations Competing interests SZU has filed patent applications on materials and devices. C.Y., X.Z., Z.C., J.L., M.H., J.M. are the authors of the invention. CN patent application no. 202411789408 (pending). Author contributions C.Y. supervised the projects and designed the hosts. X.Z. synthesized and characterized the hosts. Z.C., M.H. and J.L. fabricated the OLEDs and characterized the performance of the devices. X.Z. and Z.C. performed photophysical measurements. Z.C. performed theoretical calculations. Z.C., M.H., J.M. and C.Y. participated in discussions. Z.C. and M.H. contributed to the paper writing. C.Y. reviewed and edited the paper. All authors engaged in result analysis and provided feedback on the manuscript. Acknowledgements This work was funded by the National Natural Science Foundation of China (grant no. 52130308 to C.Y.; grant no. 52403237 to Z.C.; grant no. 52373192 to J.M.), the Shenzhen Science and Technology Program (grant nos. ZDSYS20210623091813040 to C.Y.; grant no. RCBS20231211090518026 to Z.C.) and the Foundation for Basic and Applied Research of Guangdong Province (grant no. 2022A1515110445 to Z.C.). Data availability The data that support the findings of this study are available from the corresponding author upon reasonable request. References Kondo Y , et al. Narrowband deep-blue organic light-emitting diode featuring an organoboron-based emitter. Nat. Photon. 13 , 678-682 (2019). Hua T , et al. Deep-blue organic light-emitting diodes for ultrahigh-definition displays. Nat. Photon. 18 , 1161-1169 (2024). Fan X-C , et al. Ultrapure green organic light-emitting diodes based on highly distorted fused π-conjugated molecular design. Nat. Photon. 17 , 280-285 (2023). 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High-resolution mass spectrometry was performed on a Thermo Scientific LTQ Orbitrap XL equipped with an electrospray ionization source. NMR spectra were measured on Bruker Advance 400 or 500 MHz spectrometers using tetramethylsilane as an internal standard and CDCl 3 as the solvent. Organic films for optical characterization were deposited onto clean quartz substrates by thermal evaporation under high vacuum. UV-vis absorption spectra were recorded on a Shimadzu UV-2600 spectrophotometer. Steady-state fluorescence and phosphorescence spectra were collected using a Hitachi F-7100 fluorescence spectrophotometer. Time-resolved fluorescence lifetimes were measured with a PicoQuant FluoTime 300 system employing a 375 nm picosecond pulsed UV laser. Absolute PL quantum yields were determined using a Hamamatsu C13534 UV-NIR spectrometer with an integrating sphere purged with dry argon; samples were excited at 320 nm. Theoretical calculations All DFT calculations were carried out using the Gaussian 16 software package 34 . Geometry optimizations were performed using the B3LYP functional with the DFT-D3(BJ) empirical dispersion correction 35 . The 6-31G(d) basis set was employed for hydrogen, carbon, nitrogen, and silicon atoms 36 , while the Stuttgart-Dresden effective core potential was applied for germanium 37 . Time-dependent DFT calculations for excited states were conducted at the CAM-B3LYP functional level with the same basis sets 38 . Spin-orbit coupling calculations were performed using the PySOC package 39 . Device fabrication and characterization Prior to device fabrication, ITO glass substrates were thoroughly cleaned. The substrates were then transferred to a high-vacuum deposition chamber (pressure ≤ 5.0×10 -5 Pa) for sequential layer deposition. Organic layers were thermally evaporated at 1.0 Å · s -1 , followed by 2 nm of Liq and 100 nm aluminum cathodes. All organic materials were purified via vacuum sublimation. The electroluminescence characteristics were measured using a Keithley 2400 DC source meter and a Hamamatsu C9920-12 EQE measurement system. Operational stability of encapsulated devices was evaluated under constant current density (initial luminance: 1,000 cd . m -2 ) using an FS-MP64 luminance meter (Suzhou FSTAR). References 34. Frisch, M. J. et al. Gaussian 16 revision B.01 (Gaussian Inc., 2016). 35. Grimme, S., Antony, J., Ehrlich, S. & Krieg, H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J. Chem. Phys. 132 , 154104 (2010). 36. Petersson GA, Bennett A, Tensfeldt TG, Al‐Laham MA, Shirley WA, Mantzaris J. A complete basis set model chemistry. I. The total energies of closed‐shell atoms and hydrides of the first‐row elements. J. Chem. Phys. 89 , 2193-2218 (1988). 37. Martin JML, Sundermann A. Correlation consistent valence basis sets for use with the Stuttgart-Dresden-Bonn relativistic effective core potentials: The atoms Ga-Kr and In-Xe. J. Chem. Phys. 114 , 3408-3420 (2001). 38. Chen Z , et al. Pivotal role of transition density in circularly polarized luminescence. Chem. Sci. 14 , 6022-6031 (2023). 39. Gao X, Bai S, Fazzi D, Niehaus T, Barbatti M, Thiel W. Evaluation of Spin-Orbit Couplings with Linear-Response Time-Dependent Density Functional Methods. J. Chem. Theory Comput. 13 , 515-524 (2017). Additional Declarations Yes there is potential Competing Interest. SZU has filed patent applications on materials and devices. C.Y., X.Z., Z.C., J.L., M.H., J.M. are the authors of the invention. CN patent application no. 202411789408 (pending). 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-6308443","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Article","associatedPublications":[],"authors":[{"id":438337911,"identity":"e0b24d64-9efe-423f-bee2-924c69d42531","order_by":0,"name":"Chuluo Yang","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAAyklEQVRIiWNgGAWjYDACdjBpw2AAoniI0sIMJtNI13KYBC38zTyGnwt+nU/cLpHA+OBtG4O8OSEtEod5jKVn9t1O3DkjgdlwbhuD4c4GAloMmHkMpHl7buduuJHAJs3bxpBgcICwFuPfvD3nQFrYfxOrxUya58cBsC3MRGmROMxWZs3bkFy/4czDZsk55yQMNxDSwt/evPk2zx87Y4PjyQc/vCmzkSdoCwMDhwEDYxuIwdgAspWgeiBgf8DA8IcYhaNgFIyCUTBiAQAhQT2Qx8MbEQAAAABJRU5ErkJggg==","orcid":"https://orcid.org/0000-0001-9337-3460","institution":"Shenzhen University","correspondingAuthor":true,"prefix":"","firstName":"Chuluo","middleName":"","lastName":"Yang","suffix":""},{"id":438337912,"identity":"c1c1ef83-52cf-4079-9521-a3f8c7572d78","order_by":1,"name":"Xu Zhang","email":"","orcid":"","institution":"Shenzhen University","correspondingAuthor":false,"prefix":"","firstName":"Xu","middleName":"","lastName":"Zhang","suffix":""},{"id":438337913,"identity":"2cdb0468-c05a-48e9-9aa8-3a28d8c9dd29","order_by":2,"name":"Zhanxiang Chen","email":"","orcid":"https://orcid.org/0000-0002-8264-0460","institution":"Shenzhen University","correspondingAuthor":false,"prefix":"","firstName":"Zhanxiang","middleName":"","lastName":"Chen","suffix":""},{"id":438337914,"identity":"496b23df-3598-445b-8da7-9fbbbced71d6","order_by":3,"name":"Manli Huang","email":"","orcid":"","institution":"Shenzhen University","correspondingAuthor":false,"prefix":"","firstName":"Manli","middleName":"","lastName":"Huang","suffix":""},{"id":438337915,"identity":"6331c599-57f0-4399-938d-40fc89d9c1d9","order_by":4,"name":"Jiahui Liu","email":"","orcid":"","institution":"Shenzhen University","correspondingAuthor":false,"prefix":"","firstName":"Jiahui","middleName":"","lastName":"Liu","suffix":""},{"id":438337916,"identity":"0c42b0ff-015a-425e-8935-ff462688bc8c","order_by":5,"name":"Jingsheng Miao","email":"","orcid":"","institution":"Shenzhen University","correspondingAuthor":false,"prefix":"","firstName":"Jingsheng","middleName":"","lastName":"Miao","suffix":""}],"badges":[],"createdAt":"2025-03-26 04:20:26","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-6308443/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-6308443/v1","draftVersion":[],"editorialEvents":[],"editorialNote":"","failedWorkflow":false,"files":[{"id":80781304,"identity":"1a748ecc-1e3a-44db-93ed-50461d0cf03a","added_by":"auto","created_at":"2025-04-17 04:39:34","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":146894,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eHost-guest system design\u003c/strong\u003e. \u003cstrong\u003ea\u003c/strong\u003e, Schematic illustration of reverse intersystem crossing (RISC) in the host and Förster resonance energy transfer (FRET) between host and MR-TADF emitters in EL devices. \u003cstrong\u003eb\u003c/strong\u003e, Chemical structures of the target germanium-based hosts (GeCzCz, GeTrzCz2) and reference silicon-based hosts (SiCzCz, SiTrzCz2). \u003cstrong\u003ec\u003c/strong\u003e, Chemical structures of the MR-TADF emitters used in this study.\u003c/p\u003e","description":"","filename":"floatimage1.png","url":"https://assets-eu.researchsquare.com/files/rs-6308443/v1/129c2a3f34ca7a07f44ada84.png"},{"id":80780542,"identity":"51f0385a-a658-4e3f-8fa4-f777e0fc2813","added_by":"auto","created_at":"2025-04-17 04:31:34","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":154108,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003ePL properties of germanium-based hosts.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e, UV-visible absorption spectra (open symbols) and PL spectra (filled symbols) of GeCzCz, GeTrzCz2, and GeCzCz:GeTrzCz2 in solid films. \u003cstrong\u003eb\u003c/strong\u003e, Transient PL decay curves of neat GeCzCz, GeTrzCz2, and the GeCzCz:GeTrzCz2 exciplex films.\u003c/p\u003e","description":"","filename":"floatimage2.png","url":"https://assets-eu.researchsquare.com/files/rs-6308443/v1/f4d222f23d594ddec492a326.png"},{"id":80780544,"identity":"c0180eca-5ea0-4052-afc5-64a1eac1e26e","added_by":"auto","created_at":"2025-04-17 04:31:34","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":387943,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eTransient emission properties of MR-TADF guest-doped films.\u003c/strong\u003e \u003cstrong\u003ea\u003c/strong\u003e, Time-dependent emission contours of SiCzCz:SiTrzCz2:t-DABNA and GeCzCz:GeTrzCz2:t-DABNA films, measured in the range of 400-650 nm. \u003cstrong\u003eb\u003c/strong\u003e, Time-dependent emission contours of SiCzCz:SiTrzCz2:\u003cem\u003eυ\u003c/em\u003e-DABNA and GeCzCz:GeTrzCz2:\u003cem\u003eυ\u003c/em\u003e-DABNA films, measured in the range of 420-580 nm. \u003cstrong\u003ec\u003c/strong\u003e, Time-dependent emission contours of SiCzCz:SiTrzCz2:\u003cem\u003eω\u003c/em\u003e-DABNA and GeCzCz:GeTrzCz2:\u003cem\u003eω\u003c/em\u003e-DABNA films, measured in the range of 420-630 nm.\u003c/p\u003e","description":"","filename":"floatimage3.png","url":"https://assets-eu.researchsquare.com/files/rs-6308443/v1/e665ccb37f20b9836ba8bd22.png"},{"id":80781306,"identity":"07912eab-8109-4d5e-9c84-536573f5c80a","added_by":"auto","created_at":"2025-04-17 04:39:34","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":353722,"visible":true,"origin":"","legend":"\u003cp\u003e\u003cstrong\u003eOptimized EL properties of OLEDs. a\u003c/strong\u003e, Diagram of the germanium-based device structure. \u003cstrong\u003eb\u003c/strong\u003e, EL specrtra recorded at 1,000 cd\u003csup\u003e.\u003c/sup\u003em\u003csup\u003e-2\u003c/sup\u003e. c, Statistical histogram of EQEs for different devices. \u003cstrong\u003ed-f\u003c/strong\u003e, EQE versus luminance characteristics of target devices (\u003cem\u003et\u003c/em\u003e-DABNA (\u003cstrong\u003ed\u003c/strong\u003e), \u003cem\u003eυ\u003c/em\u003e-DABNA\u003cstrong\u003e \u003c/strong\u003e(\u003cstrong\u003ee\u003c/strong\u003e),\u003cem\u003e ω\u003c/em\u003e-DABNA (\u003cstrong\u003ef\u003c/strong\u003e)) and control devices (control-A (\u003cstrong\u003ed\u003c/strong\u003e), control-B (\u003cstrong\u003ee\u003c/strong\u003e), control-C (\u003cstrong\u003ef\u003c/strong\u003e)). \u003cstrong\u003eg\u003c/strong\u003e, Half-lifetime (LT\u003csub\u003e50\u003c/sub\u003e) measurement of \u003cem\u003eω\u003c/em\u003e-DABNA-based device udner constant current density. \u003cstrong\u003eh\u003c/strong\u003e,\u003cstrong\u003ei\u003c/strong\u003e, Summary of CIEy coordinates versus maximum EQE (\u003cstrong\u003eh\u003c/strong\u003e) values and EQE at 1,000 cd\u003csup\u003e.\u003c/sup\u003em\u003csup\u003e-2\u003c/sup\u003e (\u003cstrong\u003ei\u003c/strong\u003e) for state-of-the-art blue (CIEy ≤ 0.2) and green (CIEy ≥ 0.71) OLEDs.\u003c/p\u003e","description":"","filename":"floatimage4.png","url":"https://assets-eu.researchsquare.com/files/rs-6308443/v1/af5332305bf2ff73b34654af.png"},{"id":80781622,"identity":"3fd49fd2-5d37-434c-984d-866b5b969ff9","added_by":"auto","created_at":"2025-04-17 04:47:34","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":1720693,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-6308443/v1/d524de6e-e9d3-4b78-a64b-b1d336198f8b.pdf"},{"id":80780552,"identity":"731b51f7-b423-4b48-9311-04af42b350ee","added_by":"auto","created_at":"2025-04-17 04:31:34","extension":"docx","order_by":1,"title":"","display":"","copyAsset":false,"role":"supplement","size":4530072,"visible":true,"origin":"","legend":"Supplementary Information","description":"","filename":"SupplementaryInformation.docx","url":"https://assets-eu.researchsquare.com/files/rs-6308443/v1/277e864e0514573e09c5eba1.docx"}],"financialInterests":"\u003cb\u003eYes\u003c/b\u003e there is potential Competing Interest.\nSZU has filed patent applications on materials and devices. C.Y., X.Z., Z.C., J.L., M.H., J.M. are the authors of the invention. CN patent application no. 202411789408 (pending).","formattedTitle":"Germanium-integrated exciplex host for high-performance narrowband OLEDs with mitigated efficiency roll-off","fulltext":[{"header":"Introduction","content":"\u003cp\u003eOrganic light-emitting diodes (OLEDs) combining high efficiency and narrowband emission are critical for meeting the growing demand for high-quality displays\u003csup\u003e1-3\u003c/sup\u003e. Multi-resonance thermally activated delayed fluorescence (MR-TADF) emitters, which enable efficient exciton utilization and narrowband emission, have emerged as promising alternatives to phosphorescent emitters reliant on heavy metals\u003csup\u003e4,5\u003c/sup\u003e. Pioneering work by Hatakeyama et al. demonstrated that MR-TADF emitters minimize structural reorganization and reduce the singlet-triplet energy gap through short-range charge-transfer transitions, achieving high device efficiency and narrow emission in OLEDs\u003csup\u003e6,7\u003c/sup\u003e. However, most high-performance MR-TADF materials still suffer from limited reverse intersystem crossing (RISC) rates, leading to severe efficiency roll-off at elevated driving currents\u003csup\u003e8,9\u003c/sup\u003e.\u003c/p\u003e\n\u003cp\u003eAddressing efficiency roll-off in OLEDs requires accelerating exciton consumption, particularly of dark triplet excitons, to suppress detrimental exciton-exciton and exciton-polaron interactions\u003csup\u003e10-15\u003c/sup\u003e. In MR-TADF systems, introducing heavy atoms into the molecular framework\u003csup\u003e16-18\u003c/sup\u003e or periphery\u003csup\u003e19-21\u003c/sup\u003e can enhance spin-orbit coupling, thereby accelerating the RISC process\u003csup\u003e22\u003c/sup\u003e. This approach has enabled RISC rates exceeding 10\u003csup\u003e6\u003c/sup\u003e s\u003csup\u003e-1\u003c/sup\u003e, with reported green-emitting devices achieving a state-of-the-art external quantum efficiency (EQE) of 36.8% alongside reduced roll-off\u003csup\u003e16\u003c/sup\u003e. Despite these significant advances, heavy atom-modified emitters remain limited in their adaptability, typically being case-by-case. We conceive a general strategy by integrating the heavy atom into the host molecular structure, which may apply to a broad range of emitters. Recent research has also focused on exciplex host systems formed by combining hole-transport (HT) and electron-transport (ET) materials\u003csup\u003e23-25\u003c/sup\u003e. These hosts improve charge balance in devices and, when paired with TADF sensitizers and terminal fluorescent emitters, enable high-efficiency OLEDs with suppressed efficiency roll-off. However, such systems typically require simultaneous co-evaporation of four independent components, complicating fabrication processes and demanding stringent control over deposition equipment. In this context we are driven to develop exciplex hosts with accelerated RISC, which not only reduce the delayed fluorescence lifetime of MR-TADF emitters without requiring TADF sensitizers but also deliver exceptional electroluminescent (EL) performance. Both blue and green OLEDs employing these hosts demonstrate high external quantum efficiencies over 40% alongside significantly reduced efficiency roll-off, showcasing their potential for simplified, high-performance device architectures\u003csup\u003e26,27\u003c/sup\u003e.\u003c/p\u003e"},{"header":"Results","content":"\u003cp\u003e\u003cstrong\u003eMolecular design and photophysical properties\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe host-guest system developed in this work is illustarted in Fig. 1a. Our design leverages a host with heavy atom effect to enable rapid triplet exciton upconversion and subsequent energy transfer to the MR-TADF guest (see Figs. 1b,c for the chemical structures). Building on previously reported hosts with bulky triphenylsilyl units\u003ca href=\"#_ENREF_1\" title=\"Sun, 2022 #25\"\u003e\u003csup\u003e28\u003c/sup\u003e\u003c/a\u003e, specifically SiCzCz (9-(3-(triphenylsilyl)phenyl)-9\u003cem\u003eH\u003c/em\u003e-3,9\u0026apos;-bicarbazole) and SiTrzCz2 (9,9\u0026apos;-(6-(3-(triphenylsilyl)phenyl)-1,3,5-triazine-2,4-diyl)bis(9\u003cem\u003eH\u003c/em\u003e-carbazole)), we replaced silicon (atomic number \u003cem\u003eZ\u003c/em\u003e = 14) with germanium (\u003cem\u003eZ\u003c/em\u003e = 32) to boost spin-orbit coupling in the resulting exciplex. This substitution also minimally affected emission color, triplet energy (T\u003csub\u003e1\u003c/sub\u003e), and geometric configuration of the HT/ET host pair (as detailed below).\u003c/p\u003e\n\u003cp\u003eWith the established design strategy in place, we synthesized 9-(3-(triphenylgermyl)phenyl)-9\u003cem\u003eH\u003c/em\u003e-3,9\u0026apos;-bicarbazole (GeCzCz) \u003cem\u003evia\u003c/em\u003e the Buchwald-Hartwig coupling reaction and 9,9\u0026apos;-(6-(3-(triphenylgermyl)phenyl)-1,3,5-triazine-2,4-diyl)bis(9\u003cem\u003eH\u003c/em\u003e-carbazole) (GeTrzCz2) \u003cem\u003evia\u003c/em\u003e Suzuki-Miyaura cross-coupling reaction (Supplementary Note 1 and Supplementary Figs. 1-8). Single-crystal X-ray diffraction analysis (Supplementary Figs. 9 and 10) revealed that while the C\u0026ndash;Ge bond length (~1.95 \u0026Aring;) is slightly longer than the C\u0026ndash;Si bond (~1.88 \u0026Aring;), the crystal structures of GeCzCz and its silicon analogue (SiCzCz) exhibited similar packing motifs, with no significant intra- or intermolecular \u0026pi;-\u0026pi; interactions. Instead, both structures displayed multiple intermolecular C\u0026ndash;H\u0026middot;\u0026middot;\u0026middot;\u0026pi; interactions between phenyl rings of the triphenylgermyl (or triphenylsilyl) unit and hydrogens on adjacent phenyl rings, as well as between carbazole rings and hydrogens on phenyl rings of the triphenylgermyl (or triphenylsilyl) group. Similarly, the GeTrzCz2 crystal structure closely resembled that of SiTrzCz2, featuring intermolecular \u0026pi;-\u0026pi; interactions between triazine and carbazole rings, as well as between adjacent carbazole rings. These structural similarities between germanium- and silicon-based analogues enable direct investigation of heavy-atom effects on excited states, while minimizing confounding factors such as variations in triplet-state energy or charge transport properties.\u003c/p\u003e\n\u003cp\u003eThe steady-state photophysical properties of GeCzCz, GeTrzCz2, and their 1:1 molar blend (GeCzCz:GeTrzCz2) in solid films were further analyzed. As shown in Fig. 2a, both GeCzCz and GeTrzCz2 exhibited absorption bands below 350 nm, with emission maxima at 386 and 446 nm, respectively. The red-shifted and featureless photoluminescence (PL) spectrum of GeCzCz:GeTrzCz2 compared to those of the neat films (GeCzCz and GeTrzCz2) indicates the formation of an exciplex, with a maximum emission wavelength at 470 nm, consistent with previously reported SiCzCz:SiTrzCz2 system. Phosphorescence spectra at 77 K (Supplementary Fig. 11) revealed T\u003csub\u003e1\u003c/sub\u003e energies of 3.12 eV for GeCzCz and 2.91 eV for GeTrzCz2, both higher than those of the SiCzCz and SiTrzCz2 reference, supporting sufficient T\u003csub\u003e1\u003c/sub\u003e energy levels to suppress back energy transfer from dopant to host. As expected, the delayed fluorescence lifetime of GeCzCz:GeTrzCz2 was significantly longer than those of the GeCzCz and GeTrzCz2 neat films, confirming efficient RISC in the exciplex (Fig. 2b). Notably, replacing silicon with germanium reduced the delayed fluorescence lifetime from 1.9 \u0026mu;s (SiCzCz:SiTrzCz2) to 1.6 \u0026mu;s (GeCzCz:GeTrzCz2) and the prompt fluorescence lifetime from 186.8 ns to 173.0 ns (Supplementary Fig. 12 and Supplementary Table 1), demonstrating that the heavy atom effect of germanium accelerates both prompt and delayed fluorescence processes. Key kinetic constants, including singlet radiative decay rate (\u003cem\u003ek\u003c/em\u003e\u003csub\u003er,S\u003c/sub\u003e) and RISC rate (\u003cem\u003ek\u003c/em\u003e\u003csub\u003eRISC\u003c/sub\u003e), were determined (Supplementary Note 2 and Supplementary Table 1)\u003csup\u003e29-31\u003c/sup\u003e. The germanium-based exciplex exhibited a 1.9-fold enhanced \u003cem\u003ek\u003c/em\u003e\u003csub\u003er,S\u003c/sub\u003e of 2.5 \u0026times; 10\u003csup\u003e6\u003c/sup\u003e s\u003csup\u003e-1\u003c/sup\u003e and a 1.8-fold higher PL quantum yield (54% vs. 30%) than the silicon-based reference (SiCzCz:SiTrzCz2). Importantly, the \u003cem\u003ek\u003c/em\u003e\u003csub\u003eRISC\u003c/sub\u003e of GeCzCz:GeTrzCz2 also increased 1.1-fold to 7.6 \u0026times; 10\u003csup\u003e5\u003c/sup\u003e s\u003csup\u003e-1\u003c/sup\u003e compared to the silicon counterpart (SiCzCz:SiTrzCz2), highlighting the role of germanium in promoting the RISC process of exciplex.\u003c/p\u003e\n\u003cp\u003eAdditionally, density functional theory (DFT) calculations\u003ca href=\"#_ENREF_1\" title=\"Chen, 2023 #39\"\u003e\u003csup\u003e31\u003c/sup\u003e\u003c/a\u003e revealed that the highest occupied molecular orbital (HOMO) of GeCzCz:GeTrzCz2 is localized on the bicarbazole units of GeCzCz, while the lowest unoccupied molecular orbital (LUMO) resides on the triazine moiety of GeTrzCz2 (Supplementary Fig. 13). This indicates that the spatial separation between HOMO and LUMO remains unaffected by the substitution of the heavy-atom germanium. Analysis of spin-orbit coupling matrix elements (SOCMEs) in the optimized T\u003csub\u003e1\u003c/sub\u003e geometry (Supplementary Fig. 14) showed a 2.98-fold increase from 0.265 cm\u003csup\u003e-1\u003c/sup\u003e (SiCzCz:SiTrzCz2) to 0.791 cm\u003csup\u003e-1\u003c/sup\u003e (GeCzCz:GeTrzCz2), consistent with the expected enhancement in spin-orbit coupling due to the higher atomic number. These results confirm accelerated spin-flip processes in the germanium-based exciplex, aligning with the time-resolved photoluminescence data.\u003c/p\u003e\n\u003cp\u003eThe photophysical properties of GeCzCz:GeTrzCz2 as the host were further analyzed and compared to the reference SiCzCz:SiTrzCz2 host. We selected two blue MR-TADF emitters\u003csup\u003e1,32\u003c/sup\u003e (\u003cem\u003et\u003c/em\u003e-DABNA, single-boron; \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA: double-boron) and a green emitter\u003ca href=\"#_ENREF_1\" title=\"Uemura, 2023 #48\"\u003e\u003csup\u003e33\u003c/sup\u003e\u003c/a\u003e (\u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA, triple-boron), doping all at 1 wt% to minimize interchromophore quenching. Absorption spectra of the emitters overlapped with both hosts (Supplementary Fig. 15), ensuring efficient FRET. F\u0026ouml;rster radii (\u003cem\u003eR\u003c/em\u003e\u003csub\u003eFRET\u003c/sub\u003e) for \u003cem\u003et\u003c/em\u003e-DABNA, \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA, and \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA were calculated as 4.8 nm, 7.8 nm, and 8.6 nm, respectively, based on spectral overlap between extinction spectrum of the emitter and PL spectra of the host. Time-resolved emission spectra (Fig. 3) revealed that the germanium-based exciplex host reduced delayed fluorescence lifetimes of \u003cem\u003et\u003c/em\u003e-DABNA, \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA, and \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA by 1.18-fold, 1.67-fold, and 2.14-fold, respectively, compared to the silicon-based exciplex host. Greater lifetime reductions for \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA and \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA correlated with their larger \u003cem\u003eR\u003c/em\u003e\u003csub\u003eFRET\u003c/sub\u003e, which ensured more complete FRET. Concurrently, the \u003cem\u003ek\u003c/em\u003e\u003csub\u003eRISC\u003c/sub\u003e for \u003cem\u003et\u003c/em\u003e-DABNA, \u003cem\u003e\u0026upsilon;-DABNA\u003c/em\u003e and \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA increased by 1.69-fold, 2.02-fold and 2.08-fold in the germanium-based exciplex host, with \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA achieving \u003cem\u003ek\u003c/em\u003e\u003csub\u003eRISC\u003c/sub\u003e = 9.1 \u0026times; 10\u003csup\u003e5\u003c/sup\u003e s\u003csup\u003e-1\u003c/sup\u003e (Supplementary Table 2). These improvements, driven by enhanced SOCME of germanium-based exciplex host, demonstrate the host\u0026rsquo;s efficacy in accelerating RISC dynamics.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eOLED devices\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eMotivated by the shortened delayed fluorescence lifetimes and enhanced \u003cem\u003ek\u003c/em\u003e\u003csub\u003eRISC\u003c/sub\u003e of MR-TADF emitters in the germanium-based exciplex host, we evaluated their EL performance in optimized OLEDs. Target devices were fabricated with the structure: indium tin oxide (ITO)/1,4,5,8,9,11-hexaazatriphenylenehexacarbonitrile (HAT-CN, 5 nm)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC, 30 nm)/tris(4-carbazolyl-9-ylphenyl)amine (TCTA, 15 nm)/GeCzCz (15 nm)/GeCzCz:GeTrzCz2:dopant (25 nm, 0.50:0.49:0.01 w/w/w)/ GeTrzCz2 (20 nm)/1-(4-(10-([1,1\u0026apos;-biphenyl]-4-yl)anthracen-9-yl)phenyl)-2-ethyl-1\u003cem\u003eH\u003c/em\u003e-benzo[\u003cem\u003ed\u003c/em\u003e]imidazole (ANT-BIZ, 30 nm)/8-hydroxyquinolinato lithium (Liq, 2 nm)/alumina (Al, 100 nm), where the emissive layer incorporated \u003cem\u003et\u003c/em\u003e-DABNA, \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA, or \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA as dopant. Control devices (A-C) were fabricated by substituting GeCzCz and GeTrzCz2 with their silicon-based counterparts, SiCzCz and SiTrzCz2, respectively. The chemical structures of the organic materials, energy-level diagram and EL performance are depitcted in Fig. 4 and Supplemenatry Fig. 16, with detailed parameters provided in Table 1.\u003c/p\u003e\n\u003cp\u003eFigure 4b illustrates the EL spectra of the three target OLEDs employing the GeCzCz:GeTrzCz2 host, labeled as \u003cem\u003et\u003c/em\u003e-DABNA-based, \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA-based, \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA-based devices. The \u003cem\u003et\u003c/em\u003e-DABNA-based device exhibited deep-blue emission peaking at 460 nm, while the \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA-based device displayed pure blue emission at 469 nm. Despite its bluer emission, the \u003cem\u003et\u003c/em\u003e-DABNA device achieved CIE coordinates of (0.154, 0.141), less optimal than the \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA device\u0026rsquo;s (0.125, 0.113), due to incomplete F\u0026ouml;rster energy transfer from the host to the \u003cem\u003et\u003c/em\u003e-DABNA dopant. Notably, the \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA-based device emitted green light with CIE coordinates (0.154, 0.741), closely approaching the Rec. 2020 green standard (0.170, 0.797).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTable 1 |\u0026nbsp;\u003c/strong\u003e\u003cstrong\u003eSummary performance of the devices\u003c/strong\u003e\u003c/p\u003e\n\u003ctable border=\"0\" cellspacing=\"0\" cellpadding=\"0\" width=\"100%\"\u003e\n \u003ctbody\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 10px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eDevice\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eCE (cd\u003csup\u003e.\u003c/sup\u003eA\u003csup\u003e-1\u003c/sup\u003e)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\u0026nbsp;\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e\u003cstrong\u003ePE (lm W\u003csup\u003e-1\u003c/sup\u003e)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\u0026nbsp;\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eEQE (%)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\u0026nbsp;\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\u0026nbsp;\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 10px;\"\u003e\u0026nbsp;\u003cp\u003e\u003cstrong\u003e\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eMax (average)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1,000 cd m\u003csup\u003e-2\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eMax (average)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1,000 cd m\u003csup\u003e-2\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e\u003cstrong\u003eMax (average)\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e1,000 cd m\u003csup\u003e-2\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e\u003cstrong\u003e5,000 cd m\u003csup\u003e-2\u003c/sup\u003e\u003c/strong\u003e\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 10px;\"\u003e\n \u003cp\u003e\u003cem\u003et\u003c/em\u003e-DABNA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e38.1 (35.6\u0026plusmn;1.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e14.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e42.7 (39.9\u0026plusmn;1.4)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e10.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e30.1 (29.3\u0026plusmn;0.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e12.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e7.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 10px;\"\u003e\n \u003cp\u003eControl-A\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e38.7 (37.1\u0026plusmn;1.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e10.2\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e43.5 (41.6\u0026plusmn;2.2)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e6.0\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e26.9 (26.5\u0026plusmn;0.5)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e7.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e4.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 10px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e43.6 (37.4\u0026plusmn;1.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e30.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e48.9 (38.8\u0026plusmn;3.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e27.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e40.1 (39.2\u0026plusmn;0.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e32.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e24.4\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 10px;\"\u003e\n \u003cp\u003eControl-B\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e35.8 (32.3\u0026plusmn;2.9)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e23.1\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e40.2 (35.2\u0026plusmn;4.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e18.5\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e34.1 (33.1\u0026plusmn;1.1)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e22.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e14.6\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 10px;\"\u003e\n \u003cp\u003e\u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e145.5 (142.1\u0026plusmn;2.3)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e118.6\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e163.2 (159.4\u0026plusmn;2.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e117.8\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e40.4 (39.3\u0026plusmn;0.7)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e33.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e23.9\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003ctr\u003e\n \u003ctd style=\"width: 10px;\"\u003e\n \u003cp\u003eControl-C\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e130.7 (124.5\u0026plusmn;5.0)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e87.3\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 15px;\"\u003e\n \u003cp\u003e146.6 (139.7\u0026plusmn;5.6)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e79.4\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 13px;\"\u003e\n \u003cp\u003e35.3 (34.3\u0026plusmn;0.8)\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e23.7\u003c/p\u003e\n \u003c/td\u003e\n \u003ctd style=\"width: 11px;\"\u003e\n \u003cp\u003e10.5\u003c/p\u003e\n \u003c/td\u003e\n \u003c/tr\u003e\n \u003c/tbody\u003e\n\u003c/table\u003e\n\u003cp\u003eThe average device parameters in parentheses are based on the measurement of twenty independent devices.\u003c/p\u003e\n\u003cp\u003eAll fabricated devices exhibited nearly identical turn-on voltages of 2.8 V and high maximum luminance over 30,000 cd m\u003csup\u003e-2\u003c/sup\u003e (Supplementary Fig. 17), indicating efficient exciton formation in the exciplex host followed by FRET to the guest (Langevin recombination). Target devices using the GeCzCz:GeTrzCz2 as host showed higher efficiencies and reduced efficiency roll-off compared to SiCzCz:SiTrzCz2-based ones (Control A-C). The \u003cem\u003et\u003c/em\u003e-DABNA-based device achieved a maximum EQE of 30.1% (Fig. 4d), surpassing Control-A (26.9%). However, incomplete energy transfer from the GeCzCz:GeTrzCz2 host to the \u003cem\u003et\u003c/em\u003e-DABNA guest (Fig. 4b) limited its EQE to 12.0% at 1,000 cd m\u003csup\u003e-2\u003c/sup\u003e. In contrast, devices with complete host-to-guest energy transfer, specifically \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA and \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA-based devices, achieved peak EQEs of 40.1% and 40.4%, respectively. Their EQE reproducibility, shown in a histogram (Fig. 4c), averaged 39.2\u0026plusmn;0.9% (\u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA) and 39.3\u0026plusmn;0.7% (\u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA). At 1,000 cd m\u003csup\u003e-2\u003c/sup\u003e, these devices retained EQE values of 32.8% (\u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA) and 33.4% (\u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA), and still remained 24.4% and 23.9% at 5,000 cd m\u003csup\u003e-2\u003c/sup\u003e. Both significantly outperformed Control-B (22.4%/14.6%) and Control-C (23.7%/10.5%) at equivalent brightness levels. The suppressed roll-off in germanium-based devices stems from reduced bimolecular annihilation processes (e.g., triplet-triplet and triplet-polaron annihilation), enabled by accelerated spin-flip kinetics from enhanced SOCME \u003cem\u003evia\u003c/em\u003e germanium\u0026rsquo;s heavy-atom effect. To the best of our knowledge, the \u003cem\u003e\u0026upsilon;\u003c/em\u003e-DABNA and \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA-based devices set record EQE values for OLEDs with CIEy \u0026le; 0.2 (blue) and CIEy \u0026ge; 0.71 (green, NTSC standard), surpassing platinum-based phosphorescent and TADF-sensitized systems (Figs. 4h,i and Supplementary Table 3). The \u003cem\u003e\u0026omega;\u003c/em\u003e-DABNA-based device also exhibited enhanced operational stability, achieving a half-lifetime (LT\u003csub\u003e50\u003c/sub\u003e) of 241 h at an initial luminance of 1,000 cd\u003csup\u003e.\u003c/sup\u003em\u003csup\u003e-2\u003c/sup\u003e (Fig. 4g). This surpasses the stability of previously reported green OLEDs utilizing non-metallic heavy atoms (Supplementary Table 4). These results validate the generality of heavy-atom-modified exciplex hosts for achieving efficient and stable OLEDs without complex guest synthesis.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eBy integrating HT and ET hosts containing germanium into an exciplex host aligned with the low-energy S\u003csub\u003e1\u003c/sub\u003e state of MR-TADF emitters, the intrinsically slow \u003cem\u003ek\u003c/em\u003e\u003csub\u003eRISC\u003c/sub\u003es of MR-TADF emitters were significantly accelerated. This principle was validated using blue (\u003cem\u003eυ\u003c/em\u003e-DABNA) and green (\u003cem\u003eω\u003c/em\u003e-DABNA) MR-TADF emitters, which exhibited 2.02- and 2.08-fold enhancements in \u003cem\u003ek\u003c/em\u003e\u003csub\u003eRISC\u003c/sub\u003e, reaching up to 9.1 \u0026times; 10\u003csup\u003e5\u003c/sup\u003e s\u003csup\u003e\u0026minus;\u0026thinsp;1\u003c/sup\u003e under photoexcitation. Resulting blue and green OLEDs achieved maximum EQEs exceeding 40%, with significantly reduced roll-off and improved operational stability. Our strategy not only advances MR-TADF-based OLED performance but also provides a universal approach to minimize efficiency roll-off without complex emitter synthesis, extendable to applications beyond optoelectronics.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e \u003ch2\u003eCompeting interests\u003c/h2\u003e \u003cp\u003eSZU has filed patent applications on materials and devices. C.Y., X.Z., Z.C., J.L., M.H., J.M. are the authors of the invention. CN patent application no. 202411789408 (pending).\u003c/p\u003e \u003c/p\u003e\u003ch2\u003eAuthor contributions\u003c/h2\u003e \u003cp\u003eC.Y. supervised the projects and designed the hosts. X.Z. synthesized and characterized the hosts. Z.C., M.H. and J.L. fabricated the OLEDs and characterized the performance of the devices. X.Z. and Z.C. performed photophysical measurements. Z.C. performed theoretical calculations. Z.C., M.H., J.M. and C.Y. participated in discussions. Z.C. and M.H. contributed to the paper writing. C.Y. reviewed and edited the paper. All authors engaged in result analysis and provided feedback on the manuscript.\u003c/p\u003e\u003ch2\u003eAcknowledgements\u003c/h2\u003e \u003cp\u003eThis work was funded by the National Natural Science Foundation of China (grant no. 52130308 to C.Y.; grant no. 52403237 to Z.C.; grant no. 52373192 to J.M.), the Shenzhen Science and Technology Program (grant nos. ZDSYS20210623091813040 to C.Y.; grant no. RCBS20231211090518026 to Z.C.) and the Foundation for Basic and Applied Research of Guangdong Province (grant no. 2022A1515110445 to Z.C.).\u003c/p\u003e\u003ch2\u003eData availability\u003c/h2\u003e \u003cp\u003eThe data that support the findings of this study are available from the corresponding author upon reasonable request.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eKondo Y\u003cem\u003e, et al.\u003c/em\u003e Narrowband deep-blue organic light-emitting diode featuring an organoboron-based emitter. \u003cem\u003eNat. Photon.\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 678-682 (2019).\u003c/li\u003e\n\u003cli\u003eHua T\u003cem\u003e, et al.\u003c/em\u003e Deep-blue organic light-emitting diodes for ultrahigh-definition displays. \u003cem\u003eNat. 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C\u003c/em\u003e \u003cstrong\u003e7\u003c/strong\u003e, 3082-3089 (2019).\u003c/li\u003e\n\u003cli\u003eUemura S\u003cem\u003e, et al.\u003c/em\u003e Sequential Multiple Borylation Toward an Ultrapure Green Thermally Activated Delayed Fluorescence Material. \u003cem\u003eJ. Am. Chem. Soc.\u003c/em\u003e \u003cstrong\u003e145\u003c/strong\u003e, 1505-1511 (2023).\u003c/li\u003e\n\u003c/ol\u003e"},{"header":"Methods","content":"\u003cp\u003e\u003cstrong\u003eGeneral information\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll commercially available reagents were used as received. GeTrzCz2 and GeCzCz were synthesized as detailed in the Supplementary Information. High-resolution mass spectrometry was performed on a Thermo Scientific LTQ Orbitrap XL equipped with an electrospray ionization source. NMR spectra were measured on Bruker Advance 400 or 500 MHz spectrometers using tetramethylsilane as an internal standard and CDCl\u003csub\u003e3\u003c/sub\u003e as the solvent. Organic films for optical characterization were deposited onto clean quartz substrates by thermal evaporation under high vacuum. UV-vis absorption spectra were recorded on a Shimadzu UV-2600 spectrophotometer. Steady-state fluorescence and phosphorescence spectra were collected using a Hitachi F-7100 fluorescence spectrophotometer. Time-resolved fluorescence lifetimes were measured with a PicoQuant FluoTime 300 system employing a 375 nm picosecond pulsed UV laser. Absolute PL quantum yields were determined using a Hamamatsu C13534 UV-NIR spectrometer with an integrating sphere purged with dry argon; samples were excited at 320 nm. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eTheoretical calculations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll DFT calculations were carried out using the Gaussian 16 software package\u003csup\u003e34\u003c/sup\u003e. Geometry optimizations were performed using the B3LYP functional with the DFT-D3(BJ) empirical dispersion correction\u003csup\u003e35\u003c/sup\u003e. The 6-31G(d) basis set was employed for hydrogen, carbon, nitrogen, and silicon atoms\u003csup\u003e36\u003c/sup\u003e, while the Stuttgart-Dresden effective core potential was applied for germanium\u003csup\u003e37\u003c/sup\u003e. Time-dependent DFT calculations for excited states were conducted at the CAM-B3LYP functional level with the same basis sets\u003csup\u003e38\u003c/sup\u003e. Spin-orbit coupling calculations were performed using the PySOC package\u003csup\u003e39\u003c/sup\u003e. \u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eDevice fabrication and characterization\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003ePrior to device fabrication, ITO glass substrates were thoroughly cleaned. The substrates were then transferred to a high-vacuum deposition chamber (pressure \u0026le; 5.0\u0026times;10\u003csup\u003e-5\u003c/sup\u003e Pa) for sequential layer deposition. Organic layers were thermally evaporated at 1.0 \u0026Aring;\u003csup\u003e\u0026middot;\u003c/sup\u003es\u003csup\u003e-1\u003c/sup\u003e, followed by 2 nm of Liq and 100 nm aluminum cathodes. All organic materials were purified via vacuum sublimation. The electroluminescence characteristics were measured using a Keithley 2400 DC source meter and a Hamamatsu C9920-12 EQE measurement system. Operational stability of encapsulated devices was evaluated under constant current density (initial luminance: 1,000 cd\u003csup\u003e.\u003c/sup\u003em\u003csup\u003e-2\u003c/sup\u003e) using an FS-MP64 luminance meter (Suzhou FSTAR).\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eReferences\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003e34. Frisch, M. J. et al. 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Phys.\u003c/em\u003e \u003cstrong\u003e114\u003c/strong\u003e, 3408-3420 (2001).\u003c/p\u003e\n\u003cp\u003e38. Chen Z\u003cem\u003e, et al.\u003c/em\u003e Pivotal role of transition density in circularly polarized luminescence. \u003cem\u003eChem. Sci.\u003c/em\u003e \u003cstrong\u003e14\u003c/strong\u003e, 6022-6031 (2023).\u003c/p\u003e\n\u003cp\u003e39. Gao X, Bai S, Fazzi D, Niehaus T, Barbatti M, Thiel W. Evaluation of Spin-Orbit Couplings with Linear-Response Time-Dependent Density Functional Methods. \u003cem\u003eJ. Chem. Theory Comput.\u003c/em\u003e \u003cstrong\u003e13\u003c/strong\u003e, 515-524 (2017).\u003c/p\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":true,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":false,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"nature-portfolio","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"","sideBox":"","snPcode":"","submissionUrl":"","title":"Nature Portfolio","twitterHandle":"","acdcEnabled":false,"dfaEnabled":false,"editorialSystem":"ejp","reportingPortfolio":"","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-6308443/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-6308443/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eMulti-resonance thermally activated delayed fluorescence (MR-TADF) materials open a heavy-metal-free avenue toward highly efficient, narrowband organic light-emitting diodes (OLEDs). However, their slow reverse intersystem crossing (RISC) kinetics lead to severe efficiency roll-off at practical driving currents, and existing strategies to improve RISC often require emitter-specific chemical modifications. Here we demonstrate a general strategy by integrating heavy-atom germanium into hole-transporting/electron-transporting hosts to construct an exciplex system that accelerates spin-flip dynamics. This design enhances RISC rates in diverse MR-TADF emitters \u003cem\u003evia\u003c/em\u003e Förster resonance energy transfer without altering their electronic structure. The resulting blue and green OLEDs achieve external quantum efficiencies exceeding 40% with markedly suppressed efficiency roll-off relative to silicon-based counterparts. Notably, the green device exhibits a half-lifetime of 241 h at an initial luminance of 1,000 cd m\u003csup\u003e-2\u003c/sup\u003e, surpassing previously reported OLEDs employing non-metallic heavy atoms. This general host-guest strategy advances MR-TADF systems toward practical high-performance displays.\u003c/p\u003e","manuscriptTitle":"Germanium-integrated exciplex host for high-performance narrowband OLEDs with mitigated efficiency roll-off","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2025-04-17 04:31:29","doi":"10.21203/rs.3.rs-6308443/v1","editorialEvents":[],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"nature-communications","isNatureJournal":true,"hasQc":false,"allowDirectSubmit":false,"externalIdentity":"NCOMMS","sideBox":"Learn more about [Nature Communications](http://www.nature.com/ncomms/)","snPcode":"","submissionUrl":"https://mts-ncomms.nature.com/","title":"Nature Communications","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"ejp","reportingPortfolio":"Nature Communications","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"520c6f65-55b5-4f5e-b0af-a6392d0376c6","owner":[],"postedDate":"April 17th, 2025","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"under-review","subjectAreas":[{"id":46676170,"name":"Physical sciences/Optics and photonics/Lasers, LEDs and light sources/Organic LEDs"},{"id":46676171,"name":"Physical sciences/Materials science/Materials for optics/Lasers, LEDs and light sources/Organic LEDs"}],"tags":[],"updatedAt":"2025-04-17T04:31:29+00:00","versionOfRecord":[],"versionCreatedAt":"2025-04-17 04:31:29","video":"","vorDoi":"","vorDoiUrl":"","workflowStages":[]},"version":"v1","identity":"rs-6308443","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-6308443","identity":"rs-6308443","version":["v1"]},"buildId":"8U1c8b4HqxoKbykW_rLl7","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}