[ Article ]

JOURNAL OF SENSOR SCIENCE AND TECHNOLOGY - Vol. 31, No. 4, pp.238-243

ISSN: 1225-5475 (Print) 2093-7563 (Online)

Print publication date 31 Jul 2022

Received 15 Jul 2022 Revised 27 Jul 2022 Accepted 30 Jul 2022

DOI: https://doi.org/10.46670/JSST.2022.31.4.238

Enhancing Performance of 1-aminopyrene Light-Emitting Diodes via Hybridization with ZnO Quantum Dots

Jong Hyun Choi¹^{, 2} ; Hong Hee Kim¹ ; Won Kook Choi¹^{, 3}^{, +}

1Center for Opto-electronic Materials and Devices, Korea Institute of Science and Technology, Seoul 02792, Korea
2School of Electrical Engineering, Korea University, 145, Anam-ro, Seongbuk-gu, Seoul, 02841, Korea
3KIST School, Department of Nanomaterials and Nano Science, University of Science and Technology, 217, Gajeong-ro, Yuseong-gu, Daejeon 34113, Korea

Correspondence to: ⁺ wkchoi@kist.re.kr

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License(https://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

In this study, a pyrene-core single molecule with amino (-NH2) functional group material was hybridized using ZnO quantum dots (QDs). The suppressed performance of the 1-aminopyrene (1-PyNH₂) single molecule as an emissive layer (EML) in light-emitting diodes (LEDs) was exploited by adopting the ZnO@1-PyNH₂ core–shell structure. Unlike pristine 1-PyNH₂ molecules, the ZnO@1-PyNH₂ hybrid QDs formed energy proximity levels that enabled charge transfer. This result can be interpreted as an improvement in surface roughness. The uniform and homogeneous EML alleviates dark-spot degradation. Moreover, LEDs with the ITO/PEDOT:PSS/TFB/EML/TPBi/LiF/Al configuration were fabricated to evaluate the performance of two emissive materials, where pristine-1-PyNH₂ molecules and ZnO@1-PyNH₂ QDs were used as the EML materials to verify the improvement in electrical characteristics. The ZnO@1-PyNH₂ LEDs exhibited blue luminescence at 443 nm (FWHM = 49 nm), with a turn-on voltage of 4 V, maximum luminance of 1500 cd/m², maximum luminous efficiency of 0.66 cd/A, and power efficiency of 0.41 lm/W.

Keywords:

Hybrid light-emitting diodes, 1-aminopyrene, ZnO, Quantum dots

1. INTRODUCTION

Since first reported by Tang and Van Slyke[1] in 1987, stable organic light-emitting diodes (OLEDs) have garnered increasing academic and industrial attention as a new generation display technology; currently, these OLEDs have been succeeded by liquid crystal flat panel displays. To realize full-color-based flat panel displays, red, green, and blue emitters are essential. Fluorescent emitting materials were first developed for single emitting organic materials. Thereafter, phosphorescent (Ph) emitting materials, including heavy metals such as iridium (Ir), were developed to overcome the low internal quantum efficiency (IQE) of fluorescent materials, which is 25%. Owing to the existence of heavy metals in these Ph materials, an IQE of approximately 75 % was achieved. However, blue Ph materials still suffer from challenging bottle-necks when used as commercial materials owing to their stability, due to the wide triplet bandgap and relatively short lifetime, as compared with those of red and green Ph materials. [2], [3] Furthermore, the so-called metal-free thermally activated delayed fluorescence (TADF) material developed by Adachi et al. [4] has emerged as a next-generation emitting material to overcome the abovementioned challenges. A specialized steric-hindered molecular structure enables an IQE of 100% during emission. [5] TADF emitters are molecules with a singlet (S₁)-triplet (T₁) energy gap of less than 25.6 meV, which corresponds to the amount of energy that can be activated at room temperature. Consequently, triplet excitation states can transform into singlet excitation states, which is termed as reverse intersystem crossing (RISC).

The effective design and synthesis of fluorescent molecules, including TADF materials, is based on the core-side concept. [6] This core significantly affects the emission wavelength and luminous efficiency. [7] In this approach, the side functional group is replaced with existing hydrogen atoms or other functional groups to alter the nature of the core molecules. This side moiety group helps control the solubility, arrangement, structure, and polarity of the emitter to prevent excimer formation. [7]

Organic materials are vulnerable to air and water; therefore, designing and synthesizing these specialized TADF molecules entail complex processes. In addition, OLEDs suffer from several drawbacks in terms of degradation. The general degradation in OLEDs has been categorized using three key factors by Hany Aziz et al. [8]: dark-spot degradation caused by surface morphology, catastrophic failure due to electrical shorts, and intrinsic degradation created during operation.

Here, we introduced ZnO quantum dots (QDs) into polycyclic aromatic hydrocarbons (f-PAHs) to induce the creation of ZnO@f-PAHs hybrid QDs. By combining organic and inorganic materials, we expect to overcome these degradations and realize stable blue emitters. Accordingly, a blue-chromophore pyrene moiety with a high-photoluminescence quantum yield (PLQY) was selected as the core group molecule, whereas an amine (-NH₂) moiety was selected as the side group to enable hybridization with the hydroxyl group (-OH) of the ZnO QDs. Blue-light emitter with a pyrene core and optimized side functional group of a diphenylamine moiety [7] and cyclic phosphazenes core with aminopyrene [9] were selected to enhance an electron-donating effect and improve the EL efficiency. However, hybrid QDs consisting of inorganic core and organic ligand of 1-aminopyrene is never used for blue emitting material.

Regarding the dark-spot degradation described above, the non-uniform surface morphology of EML in electroluminescence (EL) devices leads to localized current flows through the accumulation of excessive carriers within a specific area; this, in turn, results in the Joule heating phenomenon. Consequently, a “bubble” or “domelike” structure is created on the cathodes of the OLED device. [10-15] The delaminated areas, caused by the non-homogeneous film surface, lead to the loss of contact between the cathode and organic layer. [8] Therefore, the surface roughness after spin coating the EML layer is a key parameter related to enhancing device performance. Accordingly, this novel treatment suppresses the dark-spot degradation in pristine 1-PyNH₂ EML LEDs by enhancing their surface morphology.

Furthermore, an EL device with 1-PyNH₂ and ZnO@1-PyNH₂, used as the emission material, was fabricated to verify the performance of the organic–inorganic hybrid material.

2. EXPERIMENTAL

2.1 Synthesis of ZnO@1-PyNH₂ hybrid QDs as emitting material

Zinc acetate (Zn(CH₂COOH)₂), 99.99%), tetramethylammonium hydroxide pentahydrate (TMAH, ≥97%), dimethyl sulfoxide (DMSO, ≥99.7%), ethanol (≥99.5%, anhydrous), chlorobenzene (99.8%, anhydrous), 1-aminopyrene ((1-PyNH₂); C₁₆H₁₁N, ≥97%), and N, N-Dimethyl-formamide – anhydrous (DMSO, 99.8%) were purchased from Sigma-Aldrich.

2.1.1 Synthesis of ZnO QDs

The ZnO QDs were synthesized via the co-precipitation method. A precursor solution was prepared with 3 μmol Zn acetate in 30 mL DMSO and 10 mL 2-propanol. Thereafter, 5 μmol TMAH was prepared in a 10 mL solution as a reducing agent. These two solutions were dissolved at room temperature for 30 min. The agent solution was titrated using a syringe pump at 0.66 mL min^-1 for 1 h to produce the ZnO QDs at 70^oC. Subsequently, the ZnO QDs were extracted by washing away the excess acetone. Moreover, centrifugation was performed at 12,000 rpm for 10 min to collect the resultant ZnO QDs, which were then re-dispersed in 15 mL ethanol for use.

2.1.2 Synthesis of ZnO@1-PyNH₂ hybrid QDs

500 mg 1-Aminopyrene was uniformly dissolved in 40 mL N, N-Dimethylformamide (DMF). The ZnO QDs were dissolved in 300 mL of this DMF. These two solutions were mixed, and the ZnO QDs solution was continuously stirred to form a stable precursor. Consequently, it was heated to 120^oC and maintained at this temperature for 5 h for the formation of the ZnO@1-PyNH₂ hybrid QDs. Furthermore, toluene and acetone were used to wash the synthesized solution several times to obtain pure hybrid QDs. Finally, they were dispersed in ethanol.

2.2 Fabrication of EL devices

Two EL devices were fabricated to evaluate their performance with respect to the ITO (Indium Tin Oxide)/PEDOT:PSS/TFB/EML/TPBi/LiF/Al configuration, where EML denotes the emissive material layer. ITO was used as a transparent anode electrode, and PEDOT:PSS and TFB were used as the hole injection layer (HIL) and hole transport layer (HTL), respectively. 1-PyNH₂ molecules and ZnO@1-PyNH₂ hybrid QDs were adopted as EMLs for comparison. TPBi and LiF/Al were used as the electron transport layer (ETL) and electron injection layer (EIL) and the metal cathode, respectively.

A PEDOT:PSS layer was spin-coated on the patterned ITO glass with a pinwheel shape at 5000 rpm for 40 s; it was subsequently annealed at 130 °C for 30 min to evaporate the residual solvent. Thereafter, 10 mg/mL Poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4'-(N-(4-sec-butylphenyl) diphenylamine)] (TFB) solution in chlorobenzene was spin-coated on PEDOT:PSS at 4000 rpm for 40 s and then annealed at 130 ^oC for 30 min under a N₂ environment. 1-PyNH₂ molecule and ZnO@1-PyNH₂ hybrid QDs in DMF solutions used as EMLs were spin-coated at 5000 rpm for 40s and then annealed at 130 ^oC to evaporate DMF. Following this, 2, 2′, 2"-(1,3,5-Benzinetriyl)-tris(1-phenyl-1-H-benzimidazole) (TPBi) was thermally evaporated at a working pressure of 10^-6 Torr up to 40 nm. Finally, LiF and Al were thermally evaporated under the same high vacuum level (< 10^-6 Torr) to 1 nm and 100 nm, respectively, within the same chamber, without breaking the vacuum.

3. RESULTS AND DISCUSSION

3.1 Characteristics of synthesized materials

Fig. 1 presents the high-resolution transmission electron microscopy (HR-TEM) images of the ZnO QDs and ZnO@1-PyNH₂ hybrid QDs, along with a magnified image of the ZnO@1-PNH₂ QDs. Their average particle sizes were approximately 8.45 nm and 12.42 nm, calculated using Gatan. The black dotted lines enclosing the inner ZnO QDs indicate that 1-PyNH₂ molecules are wrapped around the core ZnO QDs. From a morphological perspective, it is evident that the synthesized ZnO@1-PyNH₂ hybrid QDs form a core–shell structure.

Fig. 1.

HR-TEM images of (a) ZnO QDs and (b) ZnO@1-PyNH2 hybrid QDs, and (c) magnified image of ZnO@1-PyNH2 from the red box.

Fig. 2 demonstrates the UV-Vis absorption spectra of (a) ZnO, (b) 1-PyNH₂, and (c) ZnO@1-PyNH₂. The absorption edge of each material corresponds to a wavelength of approximately 377 nm for (a) ZnO QDs, 420 nm for (b) 1-PyNH₂ molecule, and 391, 420 nm for (c) ZnO@1-PyNH₂ hybrid QDs. It can be concluded that the two absorption edges of 391 nm and 420 nm from (c) are closely related to 377 nm and 420 nm from (a) and (b).

Fig. 2.

UV-Vis absorption spectra of ZnO QDs, pristine 1-PyNH2 molecule, and ZnO@1-PyNH2 hybrid QDs. (Inset presents the magnified portion, indicated by the black ellipse, related to ZnO@1-PyNH2 hybrid QDs.)

The surface morphology closely related to the dark-spot degradation of EL devices was carefully analyzed using atomic force microscopy. After depositing each layer in the order of (a) TFB, (b) ZnO, (c) 1-PyNH₂, and (d) ZnO@1-PyNH₂, as described in Fig. 3, the corresponding measured root-mean-square values (R_q) of surface roughness were (e) 2 nm, (f) 1.15 nm, (g) 7.73 nm, and (h) 1.55 nm, respectively. Based on these results, the R_q value of (g) 1-PyNH₂ is nearly 5 times that of (h) ZnO@1-PyNH₂. Accordingly, it is highly expected that the ZnO@1-PyNH₂ hybrid QDs comprise a more suitable layer than the pristine 1-PyNH₂ molecule in terms of alleviating the dark-spot degradation of EL devices.

Fig. 3.

Schematic of (a) TFB, (b) ZnO, (c) 1-PyNH2, and (d) ZnO@1-PyNH2 structures. (e)–(h) Corresponding AFM images.

Fig. 4 shows the electronic energy diagram of the ZnO@1-PyNH₂ hybrid QDs, as constructed using photoluminescence (PL), photoluminescence excitation (PLE), UV-Vis spectroscopy, and ultraviolet photoemission spectroscopy (UPS). Accordingly, the values of the work function (ϕ) and valence band maximum can be estimated. As shown in Fig. 4, the conduction band (CB) of ZnO QDs is located -3.45 eV below the vacuum energy level (E_vac=0 eV), and the anti-bonding level (π^*) of C=N is closely located at -3.47 eV. Due to the proximity of the energy levels, it is expected that the charge transfer from the CB of the ZnO QDs to that of C=N (π^*) (II) and the subsequent radiative transition (III) to C=C (π) will generate and enhance blue emission.

Fig. 4.

Anderson electronic energy level diagram of ZnO@1-PyNH2 hybrid QDs.

3.2. Device performance

EL devices adopting 1-PyNH₂ and ZnO@1-PyNH₂ as the EMLs were fabricated to evaluate the performance of hybrid synthesized QDs. Fig. 5 shows the current density (J) (mA/cm²), luminance (cd/m²), current efficiency (cd/A), power efficiency (lm/W), and EL spectra of the 1-PyNH₂ EML LEDs with respect to the applied voltages (V_a’s). At V_a = 10 V, the 1-PyNH₂ EML LEDs exhibit poor performance, characterized by J = 83.509 mA/cm², L = 7.44 cd/m², 0.089 cd/A, and 0.028 lm/W. Furthermore, as shown in Fig. 5 (c), the peak center of EL is located at approximately 436 nm; however, as shown in the inset of Fig. 5, non-uniform EL was observed. This was attributed to the rough surface morphology of the 1-PyNH₂ EML.

Fig. 5.

Electrical characteristics of 1-PyNH2 EML LEDs. (a) J-V-L characteristic curve, (b) current efficiency, and (c) EL spectra (inset presents a photograph of EL).

Fig. 6 represents the J-V-L characteristic curves of the ZnO@1-PyNH₂ EML device. The current density of the ZnO@1-PyNH₂ EML device (J = 800 mA/cm²) is nearly one order of magnitude higher than that of 1-PyNH₂ at approximately 10 V. This indicates highly efficient electron–hole recombination in the hybrid QDs, as compared with that in the 1-PyNH₂ EL device. Similarly, the hybrid QD EML device exhibits a dramatically improved luminance of 1500 cd/m², as compared with that of the pristine-1-PyNH₂ device (13 cd/m²). Moreover, the current efficiency (CE) and power efficiency (PE) indicate maximum values of 0.185 cd/A and 0.073 lm/W, respectively, at 8 V for 1-PyNH₂. By contrast, maximum values of 0.66 cd/A and 0.41 lm/W were noted at 5 V for ZnO@1-PyNH₂. These results imply that the ZnO@1-PyNH₂ EML device is more energy-efficient than the 1-PyNH₂ device, because the former reaches its maximum efficiency at a lower V_a value. Moreover, as shown in Fig. 6 (d), the EL peak is located at 443 nm (FWHM = 49 nm); this corresponds to a deep blue color and a narrow FWHM, which is essential for high color purity when utilizing high-quality display panels as the blue source. This enhanced EL can be reasonably explained in terms of the surface morphology and electronic structure of the hybrid QDs. As indicated in Fig. 3 (g)–(h), the lower average surface roughness value of ZnO@1-PyNH₂ EML (R_a = 1.55 nm), relative to that of the sole 1-PyNH₂ EML (R_a = 7.73 nm), will increase the electrical contact area with the TPBi ETL. This, in turn, improves both the electron injection efficiency and EL uniformity. Moreover, considering the electronic energy level hierarchies at the interface between ZnO and 1-PyNH₂, as shown in Fig. 4, it can be easily assumed that electrons are transferred from the CB at -3.4 eV of ZnO to the anti-bonding (π^*) state (-3.4 eV) of the C=N non-bond and then radiatively transitioned to the bonding state (π) of the C=N non-bond.

Fig. 6.

Electrical characteristic curves of ZnO@1-PyNH2 hybrid QDs EML LEDs: (a) J-V-L, (b) current and power efficiencies, (c) EL spectra (inset presents a photograph of EL), and (d) EL spectrum and full width at half maximum (FWHM) at Va = 10 V.

4. CONCLUSIONS

In this study, we fabricated inorganic/organic ZnO@1-PyNH₂ core–shell structure hybrid QDs as EMLs and used them to improve the performance of organic 1-Pyrene EML LEDs. Consequently, dark-spot degradation in the 1-Pyrene EML LEDs was considerably suppressed, owing to the smoother surface of the ZnO@1-PyNH₂ hybrid QDs, as compared with that achieved using pristine 1-PyNH₂. Moreover, the electron transfer from ZnO to 1-PyNH₂, owing to the energy level hierarchy, is a crucial mechanism for enhancing the luminescence of ZnO@1-PyNH₂ hybrid QDs. Thus, the ITO/PEDOT:PSS/TFB/ ZnO@1-PyNH₂/TPBi/ LiF/Al LED achieved a bright blue emission, with a maximum luminance of 1500 cd/m² centered at 436 nm and an FWHM of 49 nm, turn-on voltage of 4 V, maximum CE of 0.66 cd/A, and PE of 0.41 lm/W.

Acknowledgments

This work was partially supported by the Materials, Components & Equipments Research Program, funded by the Gyeonggi Province (AICT11T2) and the KIST Institutional Program. This work was also partially supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF), funded by the Ministry of Science and ICT (Grant No. 2021R1A66A3A01087644).

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