Electroluminescence Quantum Efficiency Research Articles

Effects of polystyrene on morphology and electrical performance of solution-processed thermally activated delayed fluorescent organic light-emitting diodes

A polymer was added to the emissive-layer (EML) solution of thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs) to enhance their electrical and optical performances. A blue 4CzFCN TADF emitter and m-CBP host were used with 15 wt% dopant, and 1 wt% polystyrene (PS) was added to this EML solution. The PS enhanced the surface morphology, electroluminescence (EL), and external quantum efficiency (EQE). Energy transfer between the host and the dopant material was verified by analyzing the photoluminescence and time-resolved photoluminescence. After PS treatment, the surface roughness was reduced, according to the root-mean-square value obtained using atomic force microscopy. Finally, we fabricated solution-processed TADF-based OLEDs with an EQE of 4.7% at 7.7 V and a maximum luminance of 3700 cd/m2 at 10 V.

Organic Planar Heterojunction Solar Cells and Photodetectors Tailored to the Exciton Diffusion Length Scale of a Non‐Fullerene Acceptor

AbstractWhile non‐fullerene acceptors (NFAs) have recently been demonstrated to exhibit long‐range exciton diffusion, most organic photovoltaic and photodetector studies still focus on blended polymer: NFA systems. Herein, a 40 nm exciton diffusion length for IT4F excitons is determined, and it is demonstrated that sharp interface, planar heterojunction (PHJ) IT4F/PM6 devices with the IT4F layer thickness matched to this diffusion length yield optimized photovoltaic and photodetector performance. The PHJ devices yield an enhanced device open‐circuit voltage relative to bulk heterojunction (BHJ) devices, associated with suppressed bimolecular recombination losses. The PHJ architecture also results in a ≈100‐fold increase in electroluminescence (EL) quantum efficiency relative to the BHJ device, correlated with a shift from charge transfer state EL for the BHJ to IT4F exciton dominated EL for the PHJ, attributed to significant hole injection from PM6 into IT4F. Of particular note, the PHJ architecture is shown to suppress dark leakage current, resulting in 83 times higher photodetector detectivity at −2 V bias than the equivalent BHJ device.

Narrowband Emissive Thermally Activated Delayed Fluorescence Materials

AbstractOrganic thermally activated delayed fluorescence (TADF) materials have attracted significant research interest in the field of organic electronics because of their inherent advantage of 100% exciton utilization capability in organic light‐emitting diodes (OLEDs) without the use of noble metals. However, despite their high internal electroluminescence quantum efficiencies approaching unity, broad emission spectra with sizable full width at half maxima (FWHM; 60–100 nm) present a critical issue that must be solved for their application in ultrahigh‐definition OLED displays. Recently, a new paradigm of TADF materials featuring the multiple resonance (MR) effect based on heteroatom‐doped polycyclic aromatic frameworks, referred to as MR‐TADF materials, has emerged and garnered considerable research interest owing to their remarkable features of efficient narrowband emissions with extremely small FWHMs (≤30 nm). Currently, MR‐TADF materials occupy a prominent position in the cutting‐edge research on organic light‐emitting materials from both chemical and physical perspectives. This review article focuses on recent progress in narrowband emissive MR‐TADF systems from the perspective of molecular design, photophysical properties, and electroluminescence performance in OLEDs. The current status and future prospects of this advanced material technology are discussed comprehensively.

Multiple resonance type thermally activated delayed fluorescence by dibenzo [1,4] azaborine derivatives.

We studied the photophysical and electroluminescent (EL) characteristics of a series of azaborine derivatives having a pair of boron and nitrogen aimed at the multi-resonance (MR) effect. The computational study with the STEOM-DLPNO-CCSD method clarified that the combination of a BN ring-fusion and a terminal carbazole enhanced the MR effect and spin-orbit coupling matrix element (SOCME), simultaneously. Also, we clarified that the second triplet excited state (T2) plays an important role in efficient MR-based thermally activated delayed fluorescence (TADF). Furthermore, we obtained a blue–violet OLED with an external EL quantum efficiency (EQE) of 9.1%, implying the presence of a pronounced nonradiative decay path from the lowest triplet excited state (T1).

Organic Photovoltaic Cells Based on Nonhalogenated Polymer Donors and Nonhalogenated A-DA'D-A-Type Nonfullerene Acceptors with High VOC and Low Nonradiative Voltage Loss.

Compared with other all-inorganic/organic-inorganic hybrid solar cells, the large voltage loss (Vloss) of organic photovoltaic (OPV) cells, especially the nonradiative voltage loss (ΔVnonrad), limited the further improvement of performance. Although A-DA'D-A-type Y-series nonfullerene acceptors (NFAs) largely improve the power conversion efficiencies (PCEs) to 18%, the open-circuit voltage (VOC) of this kind of material was still restricted to below 1.0 V. Herein, we designed and synthesized a narrow bandgap (Eg = 1.41 eV) acceptor BTA77 with an A-DA'D-A-type backbone containing a nonhalogenated terminal group to achieve high electroluminescence efficiency and high VOC. Combined with the nonhalogenated polymer PBDB-T with a conjugated thiophene side chain, BTA77 realized a VOC of 0.944 V, a Vloss of 0.552 V, and a PCE of 13.75%, which is one of the highest PCEs based on nonhalogenated A-DA'D-A-type acceptors with VOC > 0.9 V. After further blending with the nonhalogenated donor polymer PBT1-C with a conjugated phenyl side chain, the VOC increases to 1.021 V with a super low ΔVnonrad of 0.14 V owing to the greatly improved electroluminescence external quantum efficiency (EQEEL) of 4.42 × 10-3. Our results indicate that there is still a large room to decrease the ΔVnonrad and increase VOC by synergistic molecular engineering of p-type polymers and n-type small molecules.

High-Performance Organic Laser Semiconductor Enabling Efficient Light-Emitting Transistors and Low-Threshold Microcavity Lasers.

An organic light-emitting transistor (OLET) is a candidate device architecture for developing electrically pumped organic solid-state lasers, but it remains a critical challenge because of the lack of organic semiconductors that simultaneously possess a high solid-state emission efficiency (Φs), a high and balanced ambipolar mobility (μh,e), and a large stimulated emission cross-section. Here, we designed a molecule of 4,4'-bis(2-dibenzothiophenyl-vinyl)-biphenyl (DBTVB) and prepared its ultrathin single-crystal microplates with herringbone packing arrangements, which achieve balanced mobilities of μh = 3.55 ± 0.5 and μe = 2.37 ± 0.5 cm2 V-1 s-1, a high Φs of 85 ± 3%, and striking low-threshold laser characteristics. Theoretical and experimental investigations reveal that a strong electronic coupling and a small reorganization energy ensure efficient charge transport; meanwhile, the exciton-vibration effect and negligible π-π orbital overlap give rise to highly emissive H-aggregates and facilitate laser emission. Furthermore, OLET-based DBTVB crystals offer an internal quantum efficiency approaching 100% and a record-high electroluminescence external quantum efficiency of 4.03%.

Ultrafast Triplet–Singlet Exciton Interconversion in Narrowband Blue Organoboron Emitters Doped with Heavy Chalcogens

AbstractNarrowband emissive organoboron emitters featuring the multi‐resonance (MR) effect have now become a critical material component for constructing high‐performance organic light‐emitting diodes (OLEDs) with pure emission colors. These MR organoboron emitters are capable of exhibiting high‐efficiency narrowband thermally activated delayed fluorescence (TADF) by allowing triplet‐to‐singlet reverse intersystem crossing (RISC). However, RISC involving spin‐flip exciton upconversion is generally the rate‐limiting step in the overall TADF; hence, a deeper understanding and precise control of the RISC dynamics are ongoing crucial challenges. Here, we introduce the first MR organoboron emitter (CzBSe) doped with a selenium atom, demonstrating a record‐high RISC rate exceeding 108 s−1, which is even higher than its fluorescence radiation rate. Furthermore, the spin‐flip upconversion process inCzBSecan be accelerated by factors of ≈20000 and ≈800, compared to those of its oxygen‐ and sulfur‐doped homologs (CzBOandCzBS), respectively. UnlikeCzBOandCzBS, the photophysical rate‐limiting step inCzBSeis no longer RISC, but the fluorescence radiation process; this behavior is completely different from the conventional time‐delaying TADF limited by the slow RISC. Benefitting from its ultrafast exciton spin conversion ability, OLEDs incorporatingCzBSeachieved a maximum external electroluminescence quantum efficiency as high as 23.9 %, accompanied by MR‐induced blue narrowband emission and significantly alleviated efficiency roll‐off features.

Ultrafast Triplet-Singlet Exciton Interconversion in Narrowband Blue Organoboron Emitters Doped with Heavy Chalcogens.

Narrowband emissive organoboron emitters featuring the multi-resonance (MR) effect have now become a critical material component for constructing high-performance organic light-emitting diodes (OLEDs) with pure emission colors. These MR organoboron emitters are capable of exhibiting high-efficiency narrowband thermally activated delayed fluorescence (TADF) by allowing triplet-to-singlet reverse intersystem crossing (RISC). However, RISC involving spin-flip exciton upconversion is generally the rate-limiting step in the overall TADF; hence, a deeper understanding and precise control of the RISC dynamics are ongoing crucial challenges. Here, we introduce the first MR organoboron emitter (CzBSe) doped with a selenium atom, demonstrating a record-high RISC rate exceeding 108 s-1 , which is even higher than its fluorescence radiation rate. Furthermore, the spin-flip upconversion process in CzBSe can be accelerated by factors of ≈20000 and ≈800, compared to those of its oxygen- and sulfur-doped homologs (CzBO and CzBS), respectively. Unlike CzBO and CzBS, the photophysical rate-limiting step in CzBSe is no longer RISC, but the fluorescence radiation process; this behavior is completely different from the conventional time-delaying TADF limited by the slow RISC. Benefitting from its ultrafast exciton spin conversion ability, OLEDs incorporating CzBSe achieved a maximum external electroluminescence quantum efficiency as high as 23.9 %, accompanied by MR-induced blue narrowband emission and significantly alleviated efficiency roll-off features.

Low-cost and high-performance poly(thienylene vinylene) derivative donor for efficient versatile organic photovoltaic cells

The utilization of donor materials with complex structures obviously increases the costs of organic photovoltaic (OPV) cells. Therefore, low-cost and high-performance are two issues that must be considered when designing polymer donors for the preparations of large-area OPV cells. Here, a poly(thienylene vinylene) derivative, named PTVT-BT was reported. PTVT-BT has a very simple completely non-fused molecular structure. PTVT-BT shows planar molecular structure and obvious solution aggregation effect. Besides, PTVT-BT demonstrates a high hole mobility up to the magnitude of 10-2 cm-2 V-1 s-1. The external quantum efficiency of electroluminescence of PTVT-BT is 7 × 10-3. As a result, the OPV cell based on PTVT-BT:eC9 demonstrates a power conversion efficiency (PCE) of 16.31%, which is the highest value among the poly(thienylene vinylene)- and polythiophene-based OPV cells. The tandem OPV cell with PTVT-BT as the donor of the sub-cell yields an outstanding PCE of 18.49%. Besides, the indoor OPV cell based on PTVT-BT:BTA3 exhibits a PCE of 27.30% under a light-emitting diode (LED) illumination of 1000 lux (2700 K). This study indicates that PTV-derivative is a kind of promising material for the future OPV industrialization.

13‐5: Quantum‐Dot‐in‐Perovskite Near‐Infrared Light‐Emitting Diodes

Quantum dots represent a unique class of solution‐processed semiconductors with unprecedented optoelectronic properties such as charge transport capability and light absorption/emission. Regarding the latter, the electroluminescence of these nanocrystals can spun a wide wavelength range from the visible to the near infrared through size engineering. However, they exhibit low photoluminescence quantum yield and inferior charge transport properties which hinders their application to light‐emitting diodes. On the other hand, halide perovskite semiconductors exhibit high charge carrier mobility and photoluminescence quantum efficiency approaching unity. However, their emission maxima cannot spun the whole near infrared range due to bandgap induced limitations. Here, we report unique combinations of these classes of semiconductors that allowed the fabrication of efficient near infrared electroluminescent hybrid quantum dot‐in‐perovskite device. In the first approach, core‐shell carbon polymer quantum dots were embedded into the perovskite emissive layer to induce passivation of defect sites and modify the nanomorphology hence resulting to significant efficiency enhancement. In the second approach, silica‐encapsulated silver sulphide quantum dots served as the emissive nanocrystals that were dispersed in a caesium‐containing triple cation perovskite matrix. The latter acted as a highly conductive medium that also offers effective passivation to the emissive nanocrystals. As a result, high external quantum efficiency for a long near infrared wavelength peak electroluminescence was achieved.

Blue Thermally Activated Delayed Fluorescence with Sub‐Microsecond Short Exciton Lifetimes: Acceleration of Triplet–Singlet Spin Interconversion via Quadrupolar Charge‐Transfer States

AbstractExciton lifetime is a critical factor in determining the performance of optoelectronic functional systems and devices. Thermally activated delayed fluorescence (TADF) emitters that can concurrently achieve a high fluorescence quantum yield and short exciton lifetime are desirable for application in organic light‐emitting diodes (OLEDs) with suppressed efficiency roll‐off. Herein, phenoxaborin and xanthone‐cored TADF emitters with quadrupolar electronic structures are reported to exhibit sub‐microsecond TADF lifetimes as short as 650 and 970 ns, respectively, while preserving high fluorescence quantum yields. By extending the El‐Sayed rule to the quadrupolar π‐systems, the contribution of doubly degenerate charge‐transfer excited states induced by dual donor units can enhance the spin–orbit coupling between them, leading to a spin‐flip acceleration between the excited triplet and singlet states. This electronic feature is advantageous for mitigating exciton annihilation processes in the emission layer, thereby reducing the efficiency roll‐offs in OLEDs. Consequently, a high external electroluminescence quantum efficiency over 20% can be retained, even under operating the device at a high luminance of 1000 cd m−2.

Asymmetric electron acceptor enables highly luminescent organic solar cells with certified efficiency over 18%

Enhancing the luminescence property without sacrificing the charge collection is one key to high-performance organic solar cells (OSCs), while limited by the severe non-radiative charge recombination. Here, we demonstrate efficient OSCs with high luminescence via the design and synthesis of an asymmetric non-fullerene acceptor, BO-5Cl. Blending BO-5Cl with the PM6 donor leads to a record-high electroluminescence external quantum efficiency of 0.1%, which results in a low non-radiative voltage loss of 0.178 eV and a power conversion efficiency (PCE) over 15%. Importantly, incorporating BO-5Cl as the third component into a widely-studied donor:acceptor (D:A) blend, PM6:BO-4Cl, allows device displaying a high certified PCE of 18.2%. Our joint experimental and theoretical studies unveil that more diverse D:A interfacial conformations formed by asymmetric acceptor induce optimized blend interfacial energetics, which contributes to the improved device performance via balancing charge generation and recombination.

Overcoming Efficiency Limitation of Cluster Light-Emitting Diodes with Asymmetrically Functionalized Biphosphine Cu4I4 Cubes.

Electroluminescent (EL) nanoclusters holding promise for new-generation cluster light-emitting devices (CLEDs) rapidly emerge. However, slow radiation and serious quenching of cluster emitters largely limit the device performance. Herein, we report two monofunctionalized biphosphine chelated Cu4I4 clusters [DMACDBFDP]2Cu4I4 and [DPACDBFDP]2Cu4I4. The asymmetric modification and electron-donating effect of acridine groups lead to the iodine-to-ligand charge transfer predominant excited states of the clusters, which feature thermally activated delayed fluorescence with markedly improved singlet radiative rate constants and reduced triplet nonradiative rate constants. As consequence, compared to the nonfunctionalized parent cluster, [DPACDBFDP]2Cu4I4 achieves 16-fold increased photoluminescence (81%) and 20-fold increased EL (19.5%) quantum efficiencies. Such new-record efficiencies make CLEDs achieve the state-of-the-art performance of all kinds of EL technologies.

Ambipolar Self-Host Functionalization Accelerates Blue Multi-Resonance Thermally Activated Delayed Fluorescence with Internal Quantum Efficiency of 100.

Emerging multi-resonance (MR) thermally activated delayed fluorescence (TADF) emitters can combine 100% exciton harvesting and high color purity for their organic light-emitting diodes (OLED). However, the highly planar configurations of MR molecules lead to intermolecular-interaction-induced quenching. A feasible way is integrating host segments into MR molecules, namely a "self-host" strategy, but without involving additional charge transfer and/or vibrational components to excited states. Herein, an ambipolar self-host featured MR emitter, tCBNDADPO, is demonstrated, whose ambipolar host segment (DADPO) significantly and comprehensively improves the TADF properties, especially greatly accelerated singlet radiative rate constant of 2.11 × 108 s-1 and exponentially reduced nonradiative rate constants. Consequently, at the same time as preserving narrowband blue emission with an FWHM of ≈28nm at a high doping concentration of 30%, tCBNDADPO reveals state-of-the-art photoluminescence and electroluminescence quantum efficiencies of 99% and 30%, respectively. The corresponding 100% internal quantum efficiency of tCBNDADPO supported by an ultrasimple trilayer and heavily doped device demonstrates the feasibility of the ambipolar self-host strategy for constructing practically applicable MR materials.

Domain modulation and energetic disorder in ternary bulk-heterojunction organic solar cells

The compatibility of the third component mixing into binary blends primarily decides the device performance in ternary organic solar cells (t-OSCs). We added a wide-bandgap PCDTBT polymer as a third component into PTB7-Th: PC 71 BM binary blend, which improves photoconversion efficiency (PCE) with efficient energy transfer, enhanced light absorption, and better morphology of the ternary blends. The enhanced PCE is attributed to the absorption improvement and non-radiative Förster resonance energy transfer (FRET) between PTB7-Th/PCDTBT polymers and the formation of a more fine bi-continuous interpenetrating network with a large donor/acceptor interfacial area with improved phase separation in the ternary system. Simultaneously, the addition of PCDTBT into PTB7-Th: PC 71 BM binary blend promotes the energetic disorder at the donor/acceptor interface in t-OSCs. This energetic disorder is studied in terms of Urbach energy and electroluminescence quantum efficiency. It was observed that the addition of PCDTBT into PTB7-Th: PC 71 BM affects the Urbach energy of PTB7-Th: PC 71 BM, which changes in bulk from 42 to 40.50 meV, and 42.70–50 meV for donor/acceptor interface. Interestingly, the energetic disorder at the donor/acceptor interface increases while the disorder in bulk decreases after the addition of PCDTBT polymer. Our results indicate that incorporating a wide-bandgap polymer as a third component is an appropriate way to design high-performance devices. • The addition of PCDTBT polymer into the matrix of PTB7-Th promotes the domain modulation with high J sc and FF , resulting in high PCE in ternary OSCs. • More bi-continuous interpenetrating donor/acceptor networks with improved phase separation in the ternary blend of PTB7-Th: PCDTBT: PC 71 BM. • Urbach energy and EL QE provide a better insight into the charge carrier's bulk and interfacial properties in ternary OSCs.

Fused π‐Extended Multiple‐Resonance Induced Thermally Activated Delayed Fluorescence Materials for High‐Efficiency and Narrowband OLEDs with Low Efficiency Roll‐Off

AbstractThe simultaneous achievement of multiple‐resonance thermally activated delayed fluorescence (MR‐TADF) materials with strong narrowband emission and efficient reverse intersystem crossing (RISC) process can further promote the advancement of organic light‐emitting diodes (OLEDs). Herein, a new strategy is proposed to achieve two π‐extended MR‐TADF emitters (NBO and NBNP) peaking at 487 and 500 nm via fusing conjugated high‐triplet‐energy units (carbazole, dibenzofuran) into boron‐nitrogen (B/N) framework, aiming to increase charge transfer delocalization of the B/N skeleton and minimize singlet‐triplet energy gap (∆EST). This strategy endows the two emitters with full width at half maximum of 27 and 29 nm, and high photoluminescence efficiencies above 90% in doped films, respectively. Additionally, considerable rate constants of RISC are obtained due to the small ∆EST (0.12 and 0.09 eV) and large spin‐orbital coupling values. Consequently, the OLEDs based on NBO and NBNP show the maximum external electroluminescence quantum efficiency of up to 26.1% and 28.0%, respectively, accompanied by low‐efficiency roll‐off. These results provide a feasible design strategy to construct efficient MR‐TADF materials for OLEDs with suppressed efficiency roll‐off.

Heterostructure from PbS Quantum Dot and Carbon Nanotube Inks for High‐Efficiency Near‐Infrared Light‐Emitting Field‐Effect Transistors

AbstractLight‐emitting field‐effect transistors (LEFETs) are emerging optoelectronic devices able to display simultaneously electrical switching as transistors and electroluminescence emission as light emitting diodes. Lead chalcogenide colloidal quantum dots (CQDs) allow achieving light emission in a very broad spectral range, covering the near‐infrared (NIR) and the short‐wavelength infrared (SWIR) regions, which cannot be reached with other solution‐processable materials. Therefore, the use of lead chalcogenide CQDs as active layer in LEFETs opens the possibility for very narrow and switchable light sources in the NIR and SWIR range. The recently reported, first fully solid‐state lead chalcogenide (PbS) CQD based LEFET shows an electroluminescence (EL) quantum efficiency of 1.3 × 10−5 at room temperature and of about 1% below 100 K. To overcome the limits of a previous report, an active material comprising two sequentially deposited layers is designed, the first of PbS CQDs displaying n‐type transport and the second of polymer‐wrapped semiconducting carbon nanotubes displaying p‐type dominated transport. With this double layer system, LEFETs displaying a well‐balanced ambipolar transport, charge carrier mobility of about 0.2 cm2 V−1 s−1 for both electrons and holes, and EL external quantum efficiency reaching 1.2 × 10−4 at room temperature are obtained.

Ternary strategy: An analogue as third component reduces the energy loss and improves the efficiency of polymer solar cells

Ternary strategy is a convenient and effective method to boost the performance of polymer solar cells (PSCs). Utilizing a ternary strategy to trade-off between the energy loss and the efficiency of devices however requires further exploration. Here, through the hydroxyl (–OH) and acetoxy (-OCOMe) substitution at β-position of the IC terminal group, we developed two new synthetic acceptors, BTIC-OH-β and BTIC-OCOMe-β, which were designed to confine the morphology aggregation. Introduction of an analogue as the third component provides a simple but efficient way to further balance the short current density (Jsc) and open-circuit voltage (Voc), leading to a champion efficiency based on PBDB-T:PBDB-TF:BTIC-OCOMe-β, effectively as high as 12.45%. The results were examined mainly in terms of the morphology characterization, electroluminescence external quantum efficiency (EQEEL), steady-state photoluminescence (PL) and transient technology. It suggested fine-tuning of the morphology by ratio modulation, reduction of the energy loss, construction of a promising pathway for charge transfer in the ternary system and enhancing the carrier extraction. In this way, a ternary strategy with an analogue donor could provide more routes to higher-quality solar cells.

Novel Oligomer Enables Green Solvent Processed 17.5% Ternary Organic Solar Cells: Synergistic Energy Loss Reduction and Morphology Fine-Tuning.

The large non-radiative recombination is the main factor that limits state-of-the-art organic solar cells (OSCs). In this work, two novel structurally similar oligomers (named 5BDTBDD and 5BDDBDT) with D-A-D-A-D and A-D-A-D-A configuration are synthesized for high-performance ternary OSCs with low energy loss. As third components, these PM6 analogue oligomers effectively suppress the non-radiative recombination in OSCs. Although the highest occupied molecular orbital (HOMO) levels of 5BDTBDD and 5BDDBDT are higher than that of PM6, the oligomers enabled ultra-high electroluminescence quantum efficiency (EQEEL ) of 0.05% and improved VOC , indicating suppressing non-radiative recombination overweighs the common belief of deeper HOMO requirement in third component selection. Moreover, the different compatibility of 5BDTBDD and 5BDDBDT with PM6 and BTP-BO4Cl fine-tunes the active layer morphology with synergistic effects. The ternary devices based on PM6:5BDTBDD:BTPBO4Cl and PM6:5BDDBDT:BTP-BO4Cl achieve a significantly improved PCEs of 17.54% and 17.32%, representing the state-of-the art OSCs processed by green solvent of o-xylene. The strategy using novel oligomer as third component also has very wide composition tolerance in ternary OSCs. This is the first work that demonstrates novel structurally compatible D-A type oligomers are effective third components, and provides new understanding of synergetic energy loss mechanisms towards high performance OSCs.

Achieving Ultimate Narrowband and Ultrapure Blue Organic Light-Emitting Diodes Based on Polycyclo-Heteraborin Multi-Resonance Delayed-Fluorescence Emitters.

To achieve an ultimate wide color gamut for ultrahigh-definition displays, there is great demand for the development of organic light-emitting diodes (OLEDs) enabling monochromatic, ultrapure blue electroluminescence (EL). Herein, high-efficiency and ultrapure blue OLEDs based on polycyclo-heteraborin multi-resonance thermally activated delayed fluorescence (MR-TADF) materials, BOBO-Z, BOBS-Z, and BSBS-Z, are reported. The key to the design of the present luminophores is the exquisite combination and interplay of multiple boron, nitrogen, oxygen, and sulfur heteroatoms embedded in a fused polycyclic π-system. Comprehensive photophysical and computational investigations of this family of MR-TADF materials reveal that the systematic implementation of chalcogen (oxygen and sulfur) atoms can finely modulate the emission color while maintaining a narrow bandwidth, as well as the spin-flipping rates between the excited singlet and triplet states. Consequently, OLEDs based on BOBO-Z, BOBS-Z, and BSBS-Z demonstrate narrowband and ultrapure blue EL emission, with peaks at 445-463nm and full width at half maxima of 18-23nm, leading to Commission Internationale de l'Éclairage-y coordinates in the range of 0.04-0.08. Particularly, for OLEDs incorporating sulfur-doped BOBS-Z and BSBS-Z, notably high maximum external EL quantum efficiencies of 26.9% and 26.8%, respectively, and small efficiency roll-offs are achieved concurrently.