Performance optimization of solution-processed TADF-OLEDs using core-identical small, medium, and high molecular weight hosts

Abstract

The physical interactions between the host and emitter in solution-processed organic light-emitting diodes (OLEDs) significantly influence device performance. In this study, we designed and synthesized a novel solution-processable blue multi-resonance (MR) emitter, 4FlDABNA, which features narrow-band blue emission and excellent solubility with a sterically protected emissive core to mitigate aggregation-induced quenching. Using this bulky emitter, we investigated the effect of host materials with varying molecular weights (MWs) on OLED performance while maintaining a fixed conjugated core unit, CzCzPh. Three hosts were synthesized: CzCzPh-mAd (low-MW), Cy-2(Ph-mCzCz) (medium-MW), and P(Ph-mCzCz) (high-MW). All hosts shared the same CzCzPh core and exhibited similar energy levels and optical properties, as well as high triplet energy levels, making them compatible with blue emitters. OLEDs based on the low-MW CzCzPh-mAd and medium-MW Cy-2(Ph-mCzCz) outperformed those based on the high-MW P(Ph-mCzCz). Notably, the medium-MW host Cy-2(Ph-mCzCz) maintained stable device performance even under high-temperature drying conditions. These findings suggest that medium-MW Cy-2(Ph-mCzCz) provides an optimal balance between molecular stability, thin-film morphology, and device efficiency, making it a promising host for solution-processed MR-OLEDs.

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1. Introduction

Organic light-emitting diodes (OLEDs) have emerged as a key device in display applications due to their high color purity, excellent color reproduction, lightweight design, and mechanical flexibility.^1–3 In recent years, OLED technology has advanced significantly with the development of phosphorescent materials and thermally activated delayed fluorescence (TADF) materials.^4–6 Conventional phosphorescent emitters rely on heavy metal complexes to induce spin–orbit coupling, whereas TADF emitters consist of organic compounds, enabling efficient reverse intersystem crossing (RISC).⁷ Consequently, TADF-based OLEDs can achieve an internal quantum efficiency (IQE) approaching 100%, comparable to that of phosphorescent emitters.^8,9

However, donor–acceptor (D–A)-structured TADF emitters often suffer from broad emission spectra, low color purity, and reduced photoluminescence quantum yield (PLQY) due to molecular geometric relaxation and vibrational (or rotational) effects.^10–12 To overcome these limitations, multi-resonance (MR) emitters have been actively developed following the concept introduced by Hatakeyama. In MR emitters, N and B atoms are strategically confined at para-positions within a π-resonance structure. This design separates the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO), suppresses vibrational relaxation, and promotes short-range charge transfer.¹³ As a result, MR emitters offer narrow emission with a full width at half maximum (FWHM) of ≤30 nm, a low Stokes shift, and high PLQY, demonstrating great potential for the development of high-performance, high-color-purity, narrow-bandwidth TADF emitters.^14–16

Unlike vacuum deposition processes, solution-processed OLED fabrication requires special attention of the solubility of both the host and the emitter, as it directly influences thin-film formation. In solution-processed OLEDs, aggregation of host molecules—which constitute the majority of the emissive layer (EML)—as well as phase separation between the host and the emitter, can disrupt film uniformity and hinder efficient energy transfer. These issues ultimately lead to a significant decline in device performance.^17,18 Furthermore, high-boiling-point solvents like toluene (110 °C) and chlorobenzene (132 °C) are predominantly used in solution processes. Their residues must be thoroughly removed during the high-temperature drying process, as remaining solvents may lead to physical/chemical defects within the thin films.¹⁹ Thus, optimizing the molecular structure of hosts and emitters is essential to improve film uniformity, prevent phase separation, and ensure the thermal stability of the EML.^20,21 In particular, MR emitters must be designed to minimize aggregation-caused quenching (ACQ) by limiting intermolecular interactions between emissive cores, while maintaining compatibility with host matrices.^22,23

Despite significant advancements in solution-processed OLEDs, the impact of host molecular weight (MW) on the internal morphology of the EML during the solvent-drying process remains insufficiently studied. To address this, it is essential to synthesize low-, medium-, and high-MW hosts with identical resonance units and similar Frontier molecular orbitals and excited-state energy levels, as well as to analyze their impact on device performance. Such studies are crucial for identifying optimal host-emitter combinations in solution processing. However, systematic studies on this topic are scarce.

In this study, medium-MW Cy-2(Ph-mCzCz) and high-MW P(Ph-mCzCz) hosts, based on the CzCzPh core, were synthesized using σ-linkers such as cyclohexane and a vinyl polymer backbones. These results were then compared with those of the low-MW CzCzPh-mAd host. CzCzPh-mAd exhibits excellent charge mobility and solubility but suffers from limited thermal stability and poor film-forming ability, while the P(Ph-mCzCz) host provides high thermal stability but low charge mobility and solubility. In contrast, the Cy-2(Ph-mCzCz) host combines the advantages of both materials, achieving superior charge mobility, solubility, thermal stability, and film-forming characteristics (Fig. 1).

Fig. 1 Comparative illustration of small-, medium-, and high-MW hosts for OLEDs.

Due to the use of non-conjugated linkers, all three hosts exhibited similar Frontier molecular orbitals and excited-state energy levels, demonstrating high triplet energy levels (T₁ > 3.0 eV) suitable for blue emitters. Using a t-DABNA-based blue MR emitter (4FlDABNA) with a sterically shielded structure, we conducted a systematic study on the influence of the molecular structure of the host on the performance of OLED devices. In the host, CzCzPh-mAd and Cy-2(Ph-mCzCz) showed superior energy transfer efficiency and higher PLQY compared to P(Ph-mCzCz). Although CzCzPh-mAd and Cy-2(Ph-mCzCz) exhibited similar hole mobilities, the high-MW P(Ph-mCzCz) showed significantly lower charge mobility owing to the irregular arrangement of the CzCzPh units within the insulating polymer backbone. When incorporated into OLED devices using 4FlDABNA, the CzCzPh-mAd- and Cy-2(Ph-mCzCz)-based devices achieved high external quantum efficiency (EQE) values of 14.8% and 15.4%, respectively, outperforming the P(Ph-mCzCz)-based device. Furthermore, under high-temperature drying conditions (130 °C), the EML based on the medium-MW Cy-2(Ph-mCzCz) host retained excellent film uniformity and stable device performance, whereas the low-MW CzCzPh-mAd host exhibited instability. These results demonstrate that medium-MW hosts effectively combine the strengths of both small- and high-MW systems while minimizing their weaknesses, positioning them as highly promising materials for achieving both molecular stability and optimal performance in solution-processed MR-TADF OLEDs.

2. Experimental

2.1. Materials

All reagents used for the synthesis were purchased from Acros Organics and Sigma-Aldrich Co. Reagent-grade solvents, including toluene, were purified via standard distillation methods prior to use. Compounds 1, 5, and 10, required for synthesizing the three host materials, were prepared according to previously reported methods.^24,25

2.2. Synthesis

2.2.1 Synthesis of 4,4′-(cyclohexane-1,1-diyl)bis(2-bromoaniline) (2). Compound 1 (2.6 g, 10 mmol) was dissolved in 100 mL of dimethylformamide (DMF) in a round-bottom flask under a nitrogen atmosphere. A DMF (10 mL) solution of N-bromosuccinimide (NBS, 3.56 g, 20 mmol) was slowly added to the flask. The mixture was stirred at 25 °C for 9 h. After the reaction was complete, the product was precipitated by adding water and subsequently collected via filtration. To remove residual water, the crude product was dissolved in dichloromethane (DCM) and dried using magnesium sulfate. Purification was performed by silica gel column chromatography with DCM as the eluent. The final product was washed with hexane, followed by filtration, yielding compound 2 as a white powder (3.4 g, 80%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 7.28 (d, J = 2.1 Hz, 2 H), 6.93 (dd, J = 8.4, 2.1 Hz, 2 H), 6.61–6.67 (m, 2 H), 3.93 (br. s., 4 H), 2.05–2.14 (m, 4 H), 1.48–1.54 (m, 6 H). ¹³C NMR (125 MHz, CDCl₃) δ (ppm): 141.43, 140.08, 130.74, 127.23, 115.68, 109.36, 44.64, 37.12, 26.21, 22.73.

2.2.2 Synthesis of 3,3′-(cyclohexane-1,1-diyl)bis(bromobenzene) (3). Compound 2 (2.1 g, 5.0 mmol) was dissolved in 60 mL of ethanol and heated to 50 °C. Sulfuric acid (7.6 mL, 0.2 mol) was then gradually added, and the mixture was further heated to 70 °C with continuous stirring. The reaction was conducted under a nitrogen atmosphere, and an aqueous solution of sodium nitrite (NaNO₂, 1.0 g, 15 mmol) was slowly added over 45 min. The mixture was continuously stirred at 100 °C for 24 h. Once the reaction was complete, the mixture was cooled to room temperature and then chilled in an ice bath. The precipitate was collected by filtration and purified via silica gel column chromatography using hexane as the eluent. The product was washed with methanol and filtered to obtain compound 3 as a white powder (1.7 g, 86%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 7.45 (t, J = 1.83 Hz, 2 H), 7.31 (d, J = 7.48, 1.68 Hz, 2 H), 7.13–7.21 (m, 4 H), 2.22–2.29 (t, 4 H), 1.55–1.61 (m, 4 H), 1.48–1.54 (m, 2 H). ¹³C NMR (125 MHz, CDCl₃) δ (ppm): 150.33, 130.12, 129.94, 128.88, 125.95, 122.71, 46.28, 36.92, 26.09, 22.69.

2.2.3 Synthesis of Cy-2(Ph-mCzCz) (4). Under a nitrogen atmosphere, compound 3 (1.0 g, 2.5 mmol), 2,9′-bi-9H-carbazole (1.7 g, 5.0 mmol), and sodium tert-butoxide (NaO^tBu, 0.58 g, 6.0 mmol) were dissolved in 50 mL of toluene. Palladium(II) acetate (5.6 mg, 25 μmol) and tri-t-butylphosphonium tetrafluoroborate (29 mg, 0.10 mmol) were added to the solution, which was stirred at 120 °C for 12 h. After completion of the reaction, the mixture was cooled to room temperature and filtered through a Celite pad. The crude product was purified via silica gel column chromatography using a mixture of DCM and hexane (1 : 3, v/v) as the eluent. The purified product was filtered using methanol to obtain compound 4 as a white powder (1.4 g, 63%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.27 (s, 2 H), 8.18 (d, J = 7.78 Hz, 4 H), 8.08 (d, J = 7.63 Hz, 2 H), 7.62–7.67 (m, 4 H), 7.57 (d, J = 7.93 Hz, 2 H), 7.50 (d, J = 7.78 Hz, 2 H), 7.41–7.47 (m, 4 H), 7.34–7.40 (m, 12 H), 7.26–7.30 (m, 6 H), 2.49 (s, 4 H), 1.76 (s, 4 H), 1.62–1.68 (m, 2 H). ¹³C NMR (125 MHz, CDCl₃) δ (ppm): 150.67, 141.81, 141.38, 139.79, 137.50, 130.05, 129.84, 126.66, 126.22, 126.12, 125.81, 125.47, 124.29, 124.23, 123.08, 122.88, 120.58, 120.33, 120.23, 119.57, 119.47, 110.68, 109.95, 109.76, 46.65, 36.95, 26.23, 22.96. MALDI-TOF (M): m/z: 896.34 [M]⁺ (calcd: 896.39).

2.2.4 Synthesis of 9-(3-bromophenyl)-9H-3,9′-bicarbazole (6). Compound 5 (2.5 g, 8.8 mmol) and K₃PO₄ (3.8 g, 18 mmol) were dissolved in toluene (50 mL) in a nitrogen-purged flask. Subsequently, 1-bromo-3-iodobenzene (1.7 mL, 14 mmol) was added, and the solution was further purged with nitrogen. Under a nitrogen atmosphere, trans-1,2-diaminocyclohexane (0.43 mL, 3.6 mmol) and CuI (0.34 g, 1.8 mmol) were introduced, and the mixture was stirred at 100 °C overnight. After cooling to room temperature, the reaction mixture was filtered through a Celite pad. The crude product was purified via silica gel column chromatography using a mixture of DCM and hexane (1 : 4, v/v) as the eluent. The purified product was precipitated in hexane and filtered to obtain compound 5 as a white powder (2.6 g, 59%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.30 (s, 1 H) 8.20 (d, J = 7.78 Hz, 2 H), 8.12 (d, J = 7.78 Hz, 1 H), 7.85 (t, J = 1.91 Hz, 1 H), 7.68 (d, J = 8.05, 1.39 Hz, 1 H), 7.63 (ddd, J = 7.93, 1.91, 1.14 Hz, 1 H), 7.53–7.61 (m, 3 H), 7.46–7.53 (m, 2 H), 7.39–7.46 (m, 4 H) 7.30–7.37 (m, 3 H). ¹³C NMR (125 MHz, CDCl₃) δ (ppm): 141.75, 141.19, 139.60, 138.73, 131.27, 130.88, 130.22, 130.14, 126.82, 125.84, 125.72, 125.63, 124.52, 123.37, 123.08, 123.02, 120.74, 120.63, 120.27, 119.62, 119.51, 110.69, 109.93, 109.72.

2.2.5 Synthesis of 3-(9H-[3,9′-bicarbazol]-9-yl)benzaldehyde (7). Compound 6 (4.0 g, 8.2 mmol) was dissolved in distilled tetrahydrofuran (THF, 60 mL) in a 250 mL two-neck round flask under a nitrogen atmosphere. The solution was stirred and cooled to −78 °C, and n-BuLi (5 mL, 2.5 M in hexane, 13 mmol) was added dropwise. The reaction mixture was maintained at −78 °C for 40 min, followed by adding DMF (1.2 g, 16.4 mmol). The reaction mixture was stirred overnight at 25 °C. After the reaction was complete, the mixture was hydrolyzed using an aqueous HCl solution. The crude product was extracted and purified by silica gel column chromatography using a mixture of DCM and hexane (1 : 2, v/v) as the eluent. The product was further refined by filtering through hexane, and compound 7 was obtained as a pale-yellow powder (1.3 g, 37%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 10.20 (s, 1 H), 8.29–8.37 (d, 1 H), 8.20–8.27 (d, 3H), 8.16 (d, J = 7.8 Hz, 1 H), 8.07 (d, J = 7.6, 1.3 Hz, 1 H), 7.96–8.01 (d, 1 H), 7.89 (t, J = 7.7 Hz, 1 H), 7.56–7.66 (t, 2 H), 7.46–7.56 (m, 2 H), 7.41–7.46 (m, 4 H), 7.36–7.41 (t, 1 H), 7.34 ppm (t, J = 7.8, 6.3, 1.8 Hz, 2 H). ¹³C NMR (125 MHz, CDCl₃) δ (ppm): 191.16, 141.78, 141.22, 139.62, 138.57, 138.28, 132.86, 130.92, 130.39, 128.98, 127.59, 126.93, 125.89, 125.73, 124.69, 123.18, 123.13, 120.90, 120.76, 120.30, 119.67, 119.64, 110.58, 109.82, 109.74.

2.2.6 Synthesis of 9-(3-vinylphenyl)-9H-3,9′-bicarbazole (8). Compound 7 (1.2 g, 2.75 mmol) and methyltriphenylphosphonium iodide (1.2 g, 3.0 mmol) were dissolved in distilled THF (20 mL) under a nitrogen atmosphere. A THF solution of potassium t-butoxide (KO^tBu, 0.34 g, 3.0 mmol) was slowly added to this solution. The reaction was performed in a foil-wrapped flask to prevent light exposure. After the reaction was complete, the mixture was filtered to remove salt byproducts. The crude product was then purified via silica gel column chromatography using a mixture of DCM and hexane (1 : 2, v/v) as the eluent. Compound 8 was isolated as a white powder (790 mg, 66%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.30 (d, J = 1.53 Hz, 1 H), 8.20 (d, J = 7.78 Hz, 2 H), 8.13 (d, J = 7.78 Hz, 1 H), 7.70 (t, J = 1.75 Hz, 1 H), 7.54–7.66 (m, 5 H), 7.47–7.51 (m, 2 H), 7.40–7.44 (m, 4 H), 7.29–7.36 (m, 3 H), 6.85 (dd, J = 17.55, 10.99 Hz, 1 H), 5.89 (d, J = 17.55 Hz, 1 H), 5.41 (d, J = 10.99 Hz, 1 H). ¹³C NMR (125 MHz, CDCl₃) δ (ppm): 141.87, 141.58, 140.01, 139.75, 137.74, 135.92, 130.20, 129.88, 126.68, 126.43, 125.85, 125.71, 125.53, 124.80, 124.34, 123.10, 122.92, 120.58, 120.38, 120.27, 119.59, 119.50, 115.49, 110.85, 110.15, 109.80.

2.2.7 Synthesis of P(Ph-mCzCz) (9). P(Ph-mCzCz) was synthesized via free radical polymerization using purified 2,2′-azobis(2-methylpropionitrile) (AIBN) as the initiator. A mixture of 9-(3-vinylphenyl)-9H-3,9′-bicarbazole (500 mg, 1.2 mmol) and AIBN (7.6 mg, 0.046 mmol) was dissolved in chlorobenzene (3.0 mL) and stirred under a nitrogen atmosphere at 80 °C for 48 h. After completion of the polymerization, the reaction mixture was precipitated in methanol to recover the crude product. The crude polymer was purified via sequential Soxhlet extraction using acetone, hexane, and DCM. The final product was precipitated from methanol to obtain a white powder (380 mg, 76%). M_n = 17.1 kDa, PDI = 2.60. Elemental composition: CHN: calcd: C 87.90, H 6.07, N 6.03; Found: C 86.96, H 5.39, N 6.32.

2.2.8 Synthesis of 2-bromo-5-(t-butyl)-N¹,N¹,N³,N³-tetraphenylbenzene-1,3-diamine (11). Toluene (50 mL) was added to a reaction mixture containing compound 10 (4.6 g, 10.0 mmol), diphenylamine (3.7 g, 22.0 mmol), Pd(OAc)₂ (0.1 g, 0.44 mmol), Xantphos (0.5 g, 0.88 mmol), and NaO^tBu (2.0 g, 20.8 mmol) while stirring to ensure complete dissolution. The reaction mixture was further stirred at 110 °C under a nitrogen atmosphere overnight. Once the reaction was complete, the mixture was cooled to room temperature and filtered through a Celite pad. The solvent was then evaporated under reduced pressure, and the crude product was purified by silica gel column chromatography using a mixture of DCM and hexane (1 : 6, v/v) as the eluent. The product was recrystallized in methanol to obtain a white powder (3.3 g, 60.0%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 7.19–7.24 (m, 8 H), 7.16 (s, 2 H), 6.98 (dd, J = 8.6, 1.0 Hz, 8 H), 6.92–6.96 (m, 4 H), 1.19 (s, 9 H). ¹³C NMR (125 MHz, CDCl₃) δ (ppm): 153.20, 147.09, 146.85, 128.95, 126.44, 123.75, 122.77, 121.82, 121.58, 34.71, 31.13, 31.05.

2.2.9 Synthesis of (2-bromo-5-(t-butyl)-N¹,N¹,N³,N³-tetrakis(4-(9-(4-(t-butyl)phenyl)-9H-fluoren-9-yl)phenyl)benzene-1,3-diamine) (12). In a reaction vessel, compound 11 (1.4 g, 2.5 mmol) and 9-(4-(t-butyl)phenyl)-9H-fluoren-9-ol (3.1 g, 10.0 mmol) were dissolved in DCM (20 mL). Then, methanesulfonic acid (0.7 mL, 10.0 mmol) was added dropwise under a nitrogen atmosphere at room temperature. The reaction mixture was stirred for 3 h. After completion, an aqueous solution of sodium bicarbonate was gradually added to neutralize the mixture. The organic layer was extracted with deionized water and DCM, dried over Na₂SO₄, and filtered. After removing the solvent under reduced pressure, the crude product was purified by silica gel column chromatography using DCM as the eluent. Compound 12 was obtained as a white powder via recrystallization in methanol (1.4 g, 67.0%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 7.72 (d, J = 7.5 Hz, 8 H), 7.36 (d, J = 7.5 Hz, 8 H), 7.28–7.33 (m, 8 H), 7.15–7.22 (m, 16 H), 7.05–7.11 (m, 8 H), 7.03 (s, 2 H), 6.94–7.00 (m, 8 H), 6.68–6.76 (m, 8 H), 1.21–1.26 (m, 36 H), 1.09 (s, 9 H). ¹³C NMR (125 MHz, CDCl₃) δ (ppm): 152.94, 151.69, 149.06, 146.46, 144.96, 142.76, 140.02, 138.78, 128.64, 127.67, 127.50, 127.19, 127.11, 126.24, 124.98, 123.57, 120.73, 119.97, 64.54, 34.63, 34.29, 31.31, 30.99, 29.69. MALDI-TOF (M): m/z: 1651.78 [M − Br]⁺ (calcd: 1651.87).

2.2.10 Synthesis of 4FlDABNA (13). A 2.5 M solution of n-BuLi (0.18 mL, 0.43 mmol) was slowly introduced into a solution of compound 12 (0.6 g, 0.36 mmol) in t-butylbenzene (20 mL) while stirring at −40 °C under a nitrogen atmosphere. The mixture was gradually brought to room temperature and stirred for 2 h. Subsequently, boron tribromide (BBr₃, 90 mg, 0.36 mmol) was added at –40 °C, and the reaction was maintained at 25 °C for an additional hour. After cooling to 0 °C, N,N-diisopropylethylamine (0.12 mL, 0.72 mmol) was added. The mixture was then heated to 130 °C and stirred for 6 h. Upon completion, the mixture was cooled to room temperature, and methanol (20 mL) was added to quench any remaining BBr₃. The reaction mixture was then separated, and the organic layer was extracted using water and DCM. The combined organic extracts were dried over Na₂SO₄, concentrated under vacuum, and purified by silica gel column chromatography using a mixture of DCM and hexane (1 : 3, v/v) as the eluent, yielding a yellow solid (0.32 g, 55.0%). ¹H NMR (500 MHz, CDCl₃) δ (ppm): 8.78 (s, 2 H), 7.78 (d, J = 7.5 Hz, 4 H), 7.67 (d, J = 7.5 Hz, 4 H), 7.43 (d, J = 8.5 Hz, 4 H), 7.46 (d, J = 7.6 Hz, 4 H), 7.35–7.39 (m, 4 H), 7.27–7.31 (m, 4 H), 7.22–7.26 (m, 8 H), 7.13–7.21 (m, 18 H), 7.01 (m, J = 7.9, 1.8 Hz, 8 H), 6.68 (d, J = 9.2 Hz, 2 H), 5.92 (s, 2 H), 1.27 (s, 18 H), 1.18–1.24 (s, 18 H), 0.88–0.92 (s, 9 H). ¹³C NMR (125 MHz, CDCl₃) δ (ppm): 154.96, 152.34, 151.19, 149.50, 148.60, 146.63, 146.37, 146.31, 143.06, 142.18, 140.66, 140.15, 139.84, 135.62, 134.15, 131.06, 130.54, 130.17, 127.99, 127.76, 127.68, 127.53, 127.41, 126.91, 126.42, 126.15, 125.40, 125.22, 124.09, 120.22, 119.77, 116.59, 102.32, 64.99, 64.88, 35.23, 34.35, 34.29, 31.58, 31.32, 31.31, 30.78, 22.65, 14.13. MALDI-TOF (M): m/z: 1661.77 [M + H]⁺ (calcd: 1660.87).

3. Results and discussion

3.1 Design strategy, synthesis, and characterization

Unlike host materials for vacuum deposition, solution-processable hosts must exhibit excellent solubility in organic solvents and high compatibility with bulky emitters in the solid film state. In this study, we aimed to diversify the physical properties of host materials without altering the core structure by employing a strategy to modulate molecular weight (MW) without extending conjugation. To achieve this, we linked the core units via sigma (σ) bonds in both Cy-2(Ph-mCzCz) and P(Ph-mCzCz) structures, thereby preserving the intrinsic electronic properties of the core while tuning the molecular weight.

As shown in Fig. 1, we designed p-type hosts with small-, medium-, and high-molecular-weight structures based on 9-phenyl-9H-3,9′-bicarbazole (CzCzPh) to evaluate their applicability as hosts for solution-processed OLEDs. The small-molecule host CzCzPh-mAd was recently reported by our research group,²⁶ where the poor thin-film formation ability of CzCzPh in solution processing was improved by incorporating an adamantane group at the meta-position of the phenyl ring. Despite structural modifications, the intrinsic molecular properties of the CzCzPh core were retained due to its σ-bonded linkage.

Cy-2(Ph-mCzCz) was synthesized through a Buchwald–Hartwig amination reaction, whereas the P(Ph-mCzCz) monomer was prepared via a Wittig reaction and subjected to free radical polymerization using AIBN as the initiator (Scheme 1). The small- and medium-MW hosts exhibited excellent solubility in chlorobenzene and toluene at 25 °C. In contrast, P(Ph-mCzCz) is readily soluble in chlorobenzene but requires agitation to achieve complete dissolution in toluene. To select a suitable blue emitter for these hosts, we synthesized 4FlDABNA, a solution-processable MR-TADF emitter with a sterically shielded structure, as illustrated in Scheme 2.

Scheme 1 Synthetic procedure for Cy-2(Ph-mCzCz) and P(Ph-mCzCz). (i) NBS, DMF, room temperature (rt), 9 h; (ii) H₂SO₄, NaNO₂, ethanol, 70 °C, 24 h; (iii) Pd(OAc)₂, P(t-butyl)₃ HBF₄, NaO^tBu, toluene, reflux, 12 h; (iv) 1-bromo-3-iodobenzene, CuI, trans-1,2-diaminocyclohexane, K₃PO₄, reflux, overnight; (v) n-BuLi, DMF, THF, −78 °C; (vi) methyltriphenylphosphonium iodide, KO^tBu, THF, rt, overnight; (vii) AIBN, chlorobenzene, 80 °C, 48 h.

Scheme 2 Synthetic procedure for 4FlDABNA. (i) Diphenylamine, Pd(OAc)₂, Xantphos, NaO^tBu, toluene, reflux, overnight; (ii) CH₃SO₃H, DCM, rt, 3 h; (iii) n-BuLi, BBr₃, N,N-diisopropylethylamine, t-butylbenzene.

Following the same design strategy as the previously reported 4FlCzBN, 4FlDABNA incorporates a t-butyl group into the core structure, shifting the emission wavelength to blue and enabling it to function as a sky-blue MR-TADF emitter.²⁷ Compared to t-DABNA bearing t-butyl groups, the synthesized 4FlDABNA emitter showed good solubility in DCM, toluene, and chlorobenzene.

3.2 Theoretical calculations

Density functional theory (DFT) calculations were performed at the B3LYP/6-31G(d) level to investigate the optimized structures and Frontier molecular orbitals (FMOs) of the three hosts (Fig. 2 and Fig. S22, ESI†). For the polymeric host, P(Ph-mCzCz), a simplified dimer structure of CzCzPh was used in the calculations. Additionally, the electronic transition characteristics were analyzed by calculating the natural transition orbitals (NTOs), with the results presented in Fig. S23 (ESI†). The HOMO/LUMO energy levels for CzCzPh-mAd, Cy-2(Ph-mCzCz), and P(Ph-mCzCz) were calculated as −5.21 eV/−0.94 eV, −5.01 eV/−1.01 eV, and −5.10 eV/−0.86 eV, respectively. Additionally, their T₁ values were determined as 3.15, 3.13, and 3.12 eV, respectively, indicating comparable excited-state energy levels. The HOMO distribution was delocalized across the carbazole units for all the hosts, whereas the LUMO distribution was localized in the central carbazole unit. In Cy-2(Ph-mCzCz) and P(Ph-mCzCz), the CzCzPh cores operate independently because of their connection via nonconjugated (σ-bonded) linkers. As shown in Fig. S22 (ESI†), the HOMO and HOMO−1, as well as the LUMO and LUMO+1, of Cy-2(Ph-mCzCz) and P(Ph-mCzCz) were nearly degenerate (Table S1, ESI†). These results indicate that structural modifications involving σ-bonds have a negligible impact on the electronic properties of the CzCzPh core. Theoretical calculations indicate that the optical and photophysical properties of the three hosts are nearly identical despite the differences in MW and structure. This validates the design strategy for modulating MW while preserving the inherent properties of the CzCzPh structure.

Fig. 2 Optimized geometries, HOMO, LUMO, S₁, and T₁ of (a) CzCzPh-mAd, (b) Cy-2(Ph-mCzCz), and (c) P(Ph-mCzCz) obtained by DFT calculations at the B3LYP/6-31G(d) level.

As shown in Fig. S24 (ESI†), the HOMO and LUMO energy levels of 4FlDABNA, featuring a phenyl-fluorene peripheral moiety, were −4.61 eV and −1.04 eV, respectively, closely matching those of t-DABNA (−4.58 eV and −0.96 eV, respectively). Similarly, the electronic transition energies and natural transition orbitals (NTOs) of 4FlDABNA and t-DABNA were nearly identical, as shown in Fig. S25 (ESI†). The fluorene group in the side structure of 4FlDABNA is oriented orthogonally to the central plane of the DABNA core, which is expected to inhibit dense packing of the emitters. Notably, 4FlDABNA exhibited a slightly reduced singlet–triplet energy splitting (ΔE_ST) of 0.42 eV compared to 0.50 eV for t-DABNA. This reduction is attributed to the distortion of two phenyl rings near the boron atom caused by the phenyl-fluorene peripheral group.²⁸ Consequently, 4FlDABNA is expected to facilitate the RISC process due to its smaller ΔE_ST. Moreover, all three hosts, with T₁ values exceeding 3.0 eV, are well-suited for the blue 4FlDABNA emitter in MR-TADF OLEDs.

3.3. Thermal properties of hosts and emitter

Thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) were used to assess the thermal stability and phase transition behavior of the synthesized hosts. According to the TGA results, the decomposition temperature (T_d) increased with increasing MW of the hosts (T_d = 385 °C for CzCzPh-mAd, T_d = 503 °C for Cy-2(Ph-mCzCz), T_d = 425 °C for P(Ph-mCzCz)) (Fig. S26, ESI†). Cy-2(Ph-mCzCz) and P(Ph-mCzCz) exhibited significantly higher T_d values than CzCzPh-mAd, demonstrating superior thermal stability. Their high T_d values indicated their potential for stable and durable OLED operation under prolonged high-temperature conditions.

The DSC results (Fig. S26b, ESI†) show the glass transition temperatures (T_g) of the hosts. CzCzPh-mAd exhibited a T_g of 133 °C, indicating relatively poor thermal deformation resistance. The T_g of Cy-2(Ph-mCzCz) was increased to 186 °C, reflecting its enhanced rigidity and thermal resistance due to the medium-MW. As expected, the polymeric P(Ph-mCzCz) host exhibited the highest T_g (244 °C) because its high MW increased intermolecular interactions and restricted chain mobility. The excellent thermal stability of P(Ph-mCzCz) suggests a lower risk of phase separation and aggregation within the active layer during thermal treatment. On the other hand, the thermal stability of the 4FlDABNA emitter was also evaluated (Fig. S27, ESI†). It exhibited a high T_d of 537 °C, and its complex steric structure restricted molecular mobility and reduced amorphous regions, resulting in no detectable T_g up to 350 °C.

3.4 Photophysical and electrochemical properties

The photophysical properties of the three hosts, CzCzPh-mAd, Cy-2(Ph-mCzCz), and P(Ph-mCzCz), along with the MR emitters, t-DABNA and 4FlDABNA, were comprehensively analyzed. Fig. 3 shows their UV-vis absorption, room-temperature photoluminescence (RTPL), and low-temperature photoluminescence (LTPL) spectra in both toluene solution and thin-film states, and Table 1 summarizes the key optical properties.

Fig. 3 UV-vis absorption (Abs), fluorescence (FL, recorded at 298 K), and phosphorescence (LTPL, recorded at 77 K) spectra of (a) CzCzPh-mAd, (b) Cy-2(Ph-mCzCz), (c) P(Ph-mCzCz), (d) t-DABNA, and (e) 4FlDABNA in both solution (toluene) and film states. Each film was prepared using toluene as the solvent.

Table 1 Photophysical and electrochemical properties of CzCzPh-based hosts and DABNA-based emitters

Compound	Absorption^a (nm)		PL^a (nm)		E S/ET^b (eV)	ΔEST (eV)	E g (eV)	Energy level (eV)
	Solution	Film	Solution	Film				HOMO^d	LUMO^e
a Measured in toluene solution and the thin-film state. b S1 and T1 energies were obtained from the onset of the fluorescence and phosphorescence spectra in toluene at 298 and 77 K, respectively. c 1240/λcut-off. d HOMO (eV) = −e (4.8 V + Eox − EFerrocene). e LUMO (eV) = HOMO (eV) + Eg (eV).
CzCzPh-mAd	343	344	373	377	3.46/3.07	0.39	3.33	−5.59	−2.26
Cy-2(Ph-mCzCz)	343	345	374	386	3.46/3.04	0.42	3.35	−5.61	−2.26
P(Ph-mCzCz)	343	344	383	387	3.46/3.01	0.46	3.37	−5.61	−2.24
t-DABNA	445	448	461	463, 531	2.78/2.58	0.20	2.66	−5.37	−2.71
4FIDABNA	448	449	468	471	2.77/2.63	0.14	2.62	−5.29	−2.67

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The three hosts exhibited similar absorption spectra in both solution and thin-film states, with primary absorption peaks around 295 nm and additional peaks near 343–345 nm, corresponding to the π–π* and n–π* transitions of the carbazole unit. In a dilute toluene solution, CzCzPh-mAd, Cy-2(Ph-mCzCz), and P(Ph-mCzCz) exhibited PL peaks at 373 nm, 374 nm, and 383 nm, respectively. In the pure thin-film state, the emission bands showed a redshift of 4–12 nm, which can be ascribed to weak intermolecular interactions among the CzCzPh cores in the solid state.

For 4FlDABNA, an absorption peak corresponding to the S₀ → S₁ transition of the DABNA unit was observed at 448 nm. In the solution, 4FlDABNA displayed a PL peak at 468 nm, slightly redshifted compared to t-DABNA (λ_em = 461 nm) (Fig. 3d and e). In the pure thin film, t-DABNA showed emission peaks at 463 and 531 nm, corresponding to monomer emission and excimer formation, respectively. In contrast, 4FlDABNA exhibited a single, narrow emission peak at 471 nm, indicating that its molecular design effectively suppresses aggregation of the emitter cores. The similarity between its emission profiles in solution and solid-state films suggests that the incorporation of bulky side chains successfully minimizes aggregation in the solid state.²⁷ This steric hindrance inhibits π–π stacking interactions, thereby reducing non-radiative decay and emission quenching, which ultimately enhances both the emission efficiency and color purity of OLED devices.

From the LTPL spectra measured at 77 K, the T₁ energy levels of CzCzPh-mAd, Cy-2(Ph-mCzCz), and P(Ph-mCzCz) were determined to be 3.07, 3.04, and 3.01 eV, respectively. These values agree with the theoretical calculations, confirming that the T₁ levels of the three hosts are sufficiently high for blue-light emission applications. Similarly, the S₁ and T₁ energy levels of 4FlDABNA were determined to be 2.74 and 2.61 eV, respectively, with a ΔE_ST value of 0.13 eV, significantly lower than that of t-DABNA (ΔE_ST = 0.22 eV), consistent with theoretical predictions. The low ΔE_ST in 4FlDABNA facilitates the RISC, confirming its TADF characteristics.

Cyclic voltammetry (CV) was conducted to investigate the HOMO and LUMO energy levels of the three hosts (Fig. S28, ESI†). The oxidation potentials (E_ox) of CzCzPh-mAd, Cy-2(Ph-mCzCz), and P(Ph-mCzCz) in the thin-film state were measured as +1.14, +1.16, and +1.16 eV, respectively. Based on these values, their HOMO/LUMO levels were calculated as −5.59 eV/−2.26 eV, −5.61 eV/−2.26 eV, and −5.61 eV/−2.24 eV, respectively. Similarly, the HOMO/LUMO levels of t-DABNA and 4FlDABNA were determined to be −5.37 eV/−2.71 eV and −5.29 eV/−2.67 eV, respectively. These results suggest favorable energy alignment for efficient charge carrier transport from the host to the emitter in the EMLs of the OLED device.

3.5 Energy transfer efficiency in host:emitter blend systems

TRPL measurements were conducted on three host systems doped with 4FlDABNA emitter at a concentration of 4 wt%. The TRPL decay curves are shown in Fig. 4, and the measured Förster resonance energy transfer (FRET) lifetimes (τ_FRET) and rate constants (k_FRET) are summarized in Table S2 (ESI†). For the CzCzPh-mAd:4FlDABNA and Cy-2(Ph-mCzCz):4FlDABNA films, the τ_FRET values were found to be 3.88 ns and 4.03 ns, respectively, indicating that FRET occurred relatively fast between the host and emitter. Their k_FRET values were calculated as 2.58 × 10⁸ s⁻¹ and 2.48 × 10⁸ s⁻¹, respectively. The rapid energy transfer is primarily attributed to the strong spectral overlap between the emission of the host and the absorption of the emitter, along with the well-ordered molecular structure, which reduces the spatial separation between the host and emitter, thereby enhancing the energy transfer efficiency. In contrast, for the polymeric host system P(Ph-mCzCz):4FlDABNA, the τ_FRET and k_FRET values were measured as 5.38 ns and 1.86 × 10⁸ s⁻¹, respectively. The increase in τ_FRET and decrease in k_FRET indicate a less efficient FRET process compared with the small- and medium-MW host systems. The emission spectra of the three hosts and the absorption spectrum of 4FlDABNA exhibit similar overlaps (Fig. S29a, ESI†), suggesting that the observed differences in the FRET dynamics are primarily attributed to molecular alignment and host-emitter interactions. The presence of spatial disorder and amorphous morphology in the polymeric host system can impede the FRET process. Additionally, the spatial alignment and molecular packing between the polymeric host and emitter are not optimized.²⁹

Fig. 4 Time-resolved photoluminescence (TRPL) signals measured at 390 nm (λ_ex = 340 nm) for both neat films and doped films of (a) CzCzPh-mAd, (b) Cy-2(Ph-mCzCz), and (c) P(Ph-mCzCz).

In summary, the small-molecular-weight CzCzPh-mAd and medium-molecular-weight Cy-2(Ph-mCzCz) systems exhibit more efficient energy transfer to the emitter compared to the polymeric P(Ph-mCzCz) system. The lower energy transfer efficiency observed in P(Ph-mCzCz)-based films increases the probability of non-radiative decay, potentially leading to degraded device performance. These results emphasize the critical role of host selection in determining energy transfer efficiency, which is a key design factor for optimizing OLED performance. As shown in Fig. S29b (ESI†), the PL decay behavior of each blended film was measured at an emission wavelength of 470 nm, corresponding to 4FlDABNA, under excitation at 340 nm. As expected, the thin films of the three hosts containing 4FlDABNA exhibit similar prompt and delayed emission behaviors consistent with the characteristic TADF behavior of the emitter.

3.6 Thermal stability analysis of EML morphology

The thermal stability of thin films comprising the EML significantly affects the device's lifetime and efficiency. In this study, the thermal stability of thin films doped with 4FlDABNA, based on three different hosts, was evaluated by measuring surface roughness (R_q) and surface profiles before and after drying at 130 °C for varying durations (Fig. 5). For the small-MW host CzCzPh-mAd, the doped film displayed a sharp increase in R_q (from 0.322 to 0.589 nm) with prolonged drying time. Atomic force microscopy (AFM) images revealed that extended drying led to pronounced surface irregularities. In contrast, the doped films based on Cy-2(Ph-mCzCz) and P(Ph-mCzCz) exhibited only minor change in R_q over time.

Fig. 5 Atomic force microscopy (AFM) images (5 μm × 5 μm) and surface profiles of doped films of CzCzPh-mAd (a)–(d), Cy-2(Ph-mCzCz) (e)–(h), and P(Ph-mCzCz) (i)–(l) obtained at different drying times: (a), (e) and (i) 0 min, (b), (f) and (j) 10 min, (c), (g) and (k) 20 min, and (d), (h) and (l) 40 min. Each host was doped with 4 wt% 4FlDABNA and processed using toluene as the solvent.

The lower thermal stability of the CzCzPh-mAd films is attributed to the low-MW and low T_g, which facilitate molecular mobility and rearrangement under thermal energy, ultimately leading to morphological degradation.³⁰ In contrast, the doped films of Cy-2(Ph-mCzCz) and P(Ph-mCzCz) demonstrated enhanced thermal stability owing to their larger molecular structures and increased molecular entanglement, which collectively contribute to structural integrity under high-temperature conditions.

In the case of the polymeric host, the P(Ph-mCzCz) blend film exhibited a significantly higher R_q value than the CzCzPh-mAd and Cy-2(Ph-mCzCz) hosts, even at room temperature. This increased roughness is likely attributed to the aggregation of polymer chains during the film formation process, resulting from the relatively low solubility of the polymer in toluene.

3.7 Electrical properties and charge balance analysis of EMLs

To elucidate the mechanisms underlying the performance of OLED devices, the charge balance within the EML was investigated by fabricating hole-only devices (HODs) and electron-only devices (EODs). The current density–voltage (J–V) characteristics for CzCzPh-mAd, Cy-2(Ph-mCzCz), and P(Ph-mCzCz) doped with 4 wt% 4FlDABNA are presented in Fig. S30 (ESI†).

Both CzCzPh-mAd and Cy-2(Ph-mCzCz) showed relatively higher current densities than P(Ph-mCzCz). For small- and medium-MW hosts, the hole and electron current densities were well-matched under the same applied voltage, suggesting effective charge injection and transport within the EML (Fig. S30, ESI†). Proper charge balance is critical for minimizing exciton quenching and achieving efficient device operation. However, for P(Ph-mCzCz), a significantly lower current density was observed at the same voltage for both the hole-only and electron-only configurations.

The hole mobility, measured using the space-charge-limited current (SCLC) method, revealed that P(Ph-mCzCz) exhibited hole mobility approximately 1000 times lower than that of CzCzPh-mAd and Cy-2(Ph-mCzCz) (Fig. S31, ESI†). This suggests that charge injection and transport in the polymer host system are less efficient than those in the small- and medium-MW host systems. The high MW of the polymer host, its insulating vinyl backbone, and the disordered arrangement of core units within the amorphous film can limit charge mobility in the emissive layer (EML) and hinder overall charge transport. Therefore, small- and medium-MW hosts, which provide superior charge balance and transport efficiency, are more advantageous for OLED applications from a device performance perspective.

3.8 Performance evaluation of OLED devices

In this study, OLED devices were fabricated via a solution processing method using 4FlDABNA as the emitter. In the device, poly(3,4-ethylenedioxythiophene):poly(styrenesulfonic acid) (PEDOT:PSS) was used as the hole-injection layer, and 1,3-bis(3,5-dipyrid-3-ylphenyl)benzene (BmPyPB) acts as the electron-transport layer. The OLED devices were constructed with the following configuration: glass substrate/ITO anode/PEDOT:PSS (30 nm)/host: 4 wt% 4FlDABNA (30 nm)/BmPyPB (50 nm)/LiF (1 nm)/Al (100 nm). Fig. 6a shows the energy-level diagram of the OLED devices, and Table 2 summarizes their performance parameters.

Fig. 6 (a) Device configurations and energy diagram. (b) Current density–voltage–luminance (J–V–L) characteristics. (c) External quantum efficiency (EQE), power efficiency (PE), and current efficiency (CE) curves for the devices. (d) Electroluminescence (EL) spectra of CzCzPh-based host films doped with 4 wt% 4FlDABNA. Inset: Emission image of the actual device.

Table 2 Device performance of solution-processed TADF-OLEDs utilizing CzCzPh-based hosts doped with 4 wt% 4FlDABNA

Host	Dopant	V on (V)	η c (cd A^−1)	η p (l m W^−1)	L (cd m^−2)	η ext (%)	λ EL (nm)	FWHM (nm)	CIE(x, y)	PLQY (%)
a Turn-on voltage at 1 cd m^−2. b Maximum current efficiency. c Maximum power efficiency. d Maximum luminance. e Maximum external quantum efficiency.
CzCzPh-mAd	4FlDABNA	3.6	16.6	13.0	1692	14.8	472	28.7	(0.12, 0.14)	79.5
Cy-2(Ph-mCzCz)		3.51	17.7	13.9	1998	15.4	472	30.1	(0.12, 0.16)	71.2
P(Ph-mCzCz)		4.17	3.52	2.76	437	2.37	472	30.5	(0.14, 0.15)	47.4

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Evaluating the performance of OLED devices based on CzCzPh-mAd, Cy-2(Ph-mCzCz), and P(Ph-mCzCz) provides key insights into the influence of the MW and structure of the hosts on device performance. The current density–voltage–luminance (J–V–L) characteristics revealed the turn-on voltages (V_on) of devices based on CzCzPh-mAd, Cy-2(Ph-mCzCz), and P(Ph-mCzCz) were 3.6 V, 3.6 V, and 4.2 V, respectively (Fig. 6b). The polymeric host P(Ph-mCzCz) exhibited a higher V_on, which is attributed to its lower hole mobility. Fig. 6c illustrates the luminance-dependent power efficiency (PE), current efficiency (CE), and external quantum efficiency (EQE) of the devices.

The device based on CzCzPh-mAd achieved a CE of 16.6 cd A⁻¹, a PE of 13.0 l m W⁻¹, and a maximum EQE of 14.8%. Meanwhile, the device based on Cy-2(Ph-mCzCz) recorded a CE of 17.7 cd A⁻¹, a PE of 13.9 l m W⁻¹, and a maximum EQE of 15.4%. The excellent performance of both host-based devices reflects their superior charge transport properties and efficient exciton recombination, which are expected to enable optimal device operation even at high luminance levels. Furthermore, the high PLQY and efficient energy transfer mechanisms contributed to the overall enhancement of performance.

Conversely, the device based on P(Ph-mCzCz) showed the lowest performance among the three hosts, with a CE of 3.52 cd A⁻¹, a PE of 2.76 l m W⁻¹, and a maximum EQE of 2.37%. The significantly deteriorated device performance is likely attributed to relatively poor surface morphology and inefficient charge transport between the host and emitter, as well as the relatively small k_FRET, leading to suboptimal energy transfer. The electroluminescence (EL) spectra in Fig. 6d indicate a consistent emission peak centered at 472 nm for all devices. These results demonstrate that Cy-2(Ph-mCzCz) is the most effective host for optimizing the efficiency and luminance when combined with the 4FlDABNA emitter. To further verify the observed trends in host performance, we fabricated additional OLED devices using another D–A type TADF emitter, 2,3,4,5,6-pentakis(3,6-di-tert-butyl-9H-carbazol-9-yl)benzonitrile (5TCzBN). The performance of these devices was evaluated and compared. Similar trends to those previously observed were confirmed in this experiment as well (Fig. S32 and Table S4, ESI†).

Another crucial factor in solution-processed OLED fabrication is the morphological stability of the EML film during the drying process, which is necessary to remove the residual solvent after EML deposition. As observed in the AFM study, when the drying time at high temperatures was extended, the R_q of small-MW CzCzPh-mAd-based EML increased significantly. Next, we investigated the effect of the EML drying time on the EQE of the OLEDs.

As shown in Fig. 7, extending the drying time to 40 min at 130 °C in small-MW host-based EMLs resulted in a considerable decrease in EQE compared to the other two host systems (Table S3, ESI†). In contrast, for the polymeric host system, the efficiency remained almost unchanged, regardless of the drying time. However, its inherently poor charge-transport capability makes it difficult to achieve high device efficiency. Ultimately, it was confirmed that the medium-MW Cy-2(Ph-mCzCz) host provided a balance between device efficiency and drying stability, making it a promising solution-processable host material for high-performance OLEDs.

Fig. 7 L-EQE curves of OLED based on (a) CzCzPh-mAd, (b) Cy-2(Ph-mCzCz), and (c) P(Ph-mCzCz). (d) EQE variation as a function of drying time at 130 °C, where the pristine EML (t = 0 min) was pre-dried at 100 °C for 10 min.

4. Conclusions

In this study, CzCzPh-based host materials and DABNA derivatives were developed via a solution process to achieve high-performance sky-blue MR-TADF OLEDs. All three synthesized CzCzPh-based hosts exhibited high T₁ levels exceeding 3.0 eV and similar HOMO/LUMO energy levels.

From an optoelectronic perspective, CzCzPh-mAd and Cy-2(Ph-mCzCz) demonstrated higher hole mobilities and superior energy-transfer efficiency compared to P(Ph-mCzCz). In contrast, in terms of thermodynamic stability, Cy-2(Ph-mCzCz) and P(Ph-mCzCz) showed better thin-film stability than CzCzPh-mAd after drying. The solution-processed OLED based on 4FlDABNA and Cy-2(Ph-mCzCz) achieved a maximum EQE of 15.4%, with blue emission centered at 472 nm, a narrow FWHM of 30 nm, and excellent thermal stability. Such performance surpasses most narrow-band blue devices fabricated through a solution process, with a CIEy value below 0.2. (Table S5, ESI†) This study represents a significant milestone in the development of solution-processable narrow-band blue OLEDs based on sterically shielded bulky MR emitters and medium-MW hosts, paving the way for future research in this area.

Author contributions

Shinyoung Kim and Chae Yeong Park designed and synthesized the p-type hosts and evaluated the device performance. Nagaraju Peethani synthesized the blue emitter. Ha Yeon Kim and Sungnam Park conducted spectroscopic studies and time-resolved photoluminescence measurements. Haeun Kwak, Subin Kwon, and Yeseo Lee performed thermal analyses and contributed to discussions on device performance. Chang Seop Hong measured the absolute photoluminescence quantum yield (PLQY). Min Ju Cho and Dong Hoon Choi supervised the research and contributed to the preparation of the manuscript.

Data availability

The data (instrumentation, synthetic procedure, structural characterization data, theoretical calculations, spectroscopic data, thermal analysis data, device fabrication method, and device performance data) that support this article is available in the article itself and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge financial support from the National Research Foundation of Korea (NRF-2019R1A6A1A11044070 and 2022R1A2B5B02001454, RS-2023-00238064) and LG Display Co. Limited (Q2228311, 2023). The Korea Basic Science Institute is also acknowledged for providing MALDI data.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d5tc01813g

‡ S. Kim, C. Y. Park, and N. Peethani equally contributed to this work.

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