Bottom-up synthesized carbon nitride quantum dot-based light-emitting diodes

Abstract

Carbon nitride quantum dots (CNQDs) represent an emerging class of polymeric semiconductor nanomaterials that combines environmental friendliness with facile synthesis, solution-processability and tunable optoelectronic properties. Despite these advantages, their application in optoelectronic devices remains largely unexplored. Here we demonstrate high-performance light-emitting diodes (LEDs) employing thermally polymerized CNQDs as the emissive layer. The synthesized CNQDs exhibit uniform size distribution, long-term colloidal stability, and a remarkable photoluminescence quantum yield. Detailed charge transport analysis reveals matched electron–hole mobility of CNQDs, enabling efficient radiative recombination. The optimized CNQD-LED architecture achieves breakthrough performance metrics: a low turn-on voltage of 2.8 V, a maximum luminance of 885 cd m⁻², and a record external quantum efficiency of 2.14%. This study not only establishes CNQDs as viable alternatives to conventional heavy-metal QDs but also provides a general framework for developing sustainable optoelectronic materials, paving the way for environmentally benign display technologies.

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Introduction

Quantum dots (QDs) have garnered significant attention due to their unique size-tunable electronic and optical properties.^1–3 These distinctive characteristics have facilitated the widespread use of QDs in various applications, such as display technologies, biomedical imaging, solar cells, and sensors, highlighting their substantial potential.^4–7 Particularly, the tunable emission wavelengths, high color purity, and superior fluorescence efficiency of QDs set them apart from conventional fluorescent materials, positioning them as promising candidates for next-generation light-emitting diode (LED) technologies.^8–10 Traditional QD-LEDs typically utilize semiconductor materials like CdS, CdSe, PbS, and InP as the emissive layer, which have been extensively investigated in terms of optical and electrical properties and achieved good device performance.^11–14 However, these QDs often contain heavy metals, and their synthesis process requires stringent conditions (e.g. high temperatures and inert environments) and generates lots of organic waste.

Carbon nitride QDs (CNQDs), as a class of polymeric semiconductor nanomaterials, have emerged as promising alternatives to conventional metal-based QDs due to their high photoluminescence efficiency, non-toxicity, cost-effective synthesis, and tunable optoelectronic properties.^15–18 Despite their potential, the application of CNQDs in LEDs remains underexplored, with only a handful of studies demonstrating prototype devices. Current CNQD-LEDs suffer from notably poor electroluminescence performance, characterized by low luminance and external quantum efficiency (EQE). In 2018, He et al. reported the first LED featuring blue emitting CNQDs as the emissive layer.¹⁹ Although the device exhibited low luminance (∼20 cd m⁻² at 20 V), this study provided the initial proof of concept that CNQDs can be employed in LEDs. The following year, He et al. developed a one-pot methylamine intercalation-stripping method to prepare blue-emitting CNQDs and the optimized device exhibited a maximum luminance of 605 cd m⁻², approximately 30 times greater than that in the initial report.²⁰ Regrettably, the current efficiency was only 0.09 cd A⁻¹. More recently, Chen et al. demonstrated a single-component CNQD-based white electroluminescent device, which reached a maximum luminance of 4.91 cd m⁻² at 17 V, pointing to a new route for white LED development.²¹ Collectively, these studies establish the potential of CNQDs for electroluminescence applications, while simultaneously highlighting the urgent need to improve device performance.

Herein, for the first time, we report a high-performance LED based on bottom-up thermally polymerized CNQDs. The resulting CNQDs achieve a photoluminescence quantum yield (PLQY) of 43%, representing a leading value among comparable materials. The synthesized CNQDs demonstrate exceptional charge transport properties, with an electron mobility of 1.20 × 10⁻⁶ cm² V⁻¹ s⁻¹ and a hole mobility of 3.97 × 10⁻⁵ cm² V⁻¹ s⁻¹, indicating superior charge transport performance. We further fabricated solution-processed green-emitting CNQD-LED devices with CIE chromaticity coordinates of (0.29, 0.46). Compared to previously reported CNQD-LEDs, our device achieves a significant breakthrough: an EQE of 2.14% with a luminance of 855 cd m⁻² at a 5.8 V driving voltage. This groundbreaking advancement not only redefines the performance limits of CNQD-LEDs but also establishes a milestone new standard for their optoelectronic applications, while revealing their transformative potential as low-cost, eco-friendly advanced materials.

Results and discussion

The CNQDs were synthesized via a bottom-up thermal polymerization method using malic acid and urea as precursors. Transmission electron microscopy (TEM) reveals that the as-prepared CNQDs were uniformly distributed (Fig. 1a). Fig. S1 shows the size distribution of CNQDs, ranging from 0.5 to 5 nm with an average diameter of approximately 2.5 nm. The high-resolution TEM (HR-TEM) images clearly demonstrate that the CNQDs possess a spherical morphology with high crystallinity (Fig. 1b).²² Two kinds of distinct lattice spacings of 0.21 nm and 0.34 nm were detected (Fig. 1c). From the X-ray diffraction (XRD) pattern, the most prominent diffraction peak of CNQDs is located at 22.1°. The diffraction peak at 13.0° corresponds to the in-plane arrangement of the heptazine units of C₃N₄. The diffraction peaks at 22.1° and 13.0° correspond to the interplanar spacings of 0.21 nm and 0.34 nm, respectively.^16,23 Moreover, compared to the previously reported bulk C₃N₄, the relative intensity of the diffraction peak at 13.0° in CNQDs is significantly weaker, suggesting the disrupted in-plane repeating unit structure.^20,24 The CNQDs may contain various functional groups such as carboxyl, hydroxyl, and amino groups, endowing the product with excellent water solubility. The Fourier transform infrared (FT-IR) spectrum of CNQDs is shown in Fig. 1e. The strong vibrational peaks of CNQDs at 1333/1450 cm⁻¹ and 781 cm⁻¹ are attributed to the stretching vibrations of CN heterocycles and the vibrations of tri-s-triazine units, respectively.²⁵ The absorption peaks at 1622 cm⁻¹ and 1669 cm⁻¹ correspond to C=N and C=O vibrations, respectively, indicating the possible incorporation of oxygen atoms into CNQDs.¹⁶ The broad peak in the range of 3208–3446 cm⁻¹ originates from the stretching vibrations of N–H and O–H bonds.^26,27 In the ¹³C liquid nuclear magnetic resonance (NMR) spectrum of CNQDs, the chemical shifts at 157.2 ppm and 162.8 ppm correspond to the N–C=N and C–NH_x groups in the characteristic tri-s-triazine units, respectively (Fig. 1f).²⁸

Fig. 1 (a), (b) TEM, (c) HR-TEM, (d) XRD, (e) FT-IR spectrum, and (f) ¹³C liquid-state NMR spectrum of CNQDs. (g) C 1s core-level, (h) N 1s core-level, and (i) O 1s core-level XPS spectra of CNQDs.

X-ray photoelectron spectroscopy (XPS) was conducted to further investigate the elemental composition and chemical states of CNQDs. The CNQDs are composed of three elements: carbon (C), nitrogen (N), and oxygen (O), as shown in Fig. S2. The high-resolution C 1s spectrum exhibits four distinct peaks centered at 284.8, 286.3, 288.6, and 289.9 eV, assigned to C–C/C=C, C–O, N–C=N, and O–C=O bonds, respectively (Fig. 1g).²³ The presence of C–O and O–C=O bonds confirms the incorporation of oxygen-containing functional groups in the CNQDs. In the N 1s spectrum, three characteristic peaks are observed at binding energies of 398.8, 399.8, and 401.3 eV, corresponding to C=N–C, C–N–C, and N–(C)₃, respectively (Fig. 1h).²³ The O 1s spectrum can be deconvoluted into three components at 531.4, 532.5, and 533.8 eV, representing C=O, C–O, and O–H, respectively (Fig. 1i).²⁹ These XPS findings are in good agreement with the FT-IR spectroscopic results. Collectively, the structural analysis confirms that the CNQDs primarily consist of tri-s-triazine units as the fundamental framework, while their surface is modified with oxygen- and nitrogen-containing functional groups.

The ultraviolet-visible (UV-Vis) absorption and PL spectra reflect the optical properties of CNQDs. As shown in Fig. 2a, a characteristic absorption peak is observed at 435 nm, with an absorption edge around 590 nm. The PL spectrum exhibits asymmetric emission peaks, which are attributed to multiple electron transitions from the antibonding orbital π* to π states and lone pair states of the conjugated and bridged N atoms, the exciton self-trapping effect and various surface states.^30,31 During PL measurements, ground-state electrons absorb photons and are excited to higher energy levels. The excited electrons gradually relax to lower energy levels, while some electrons recombine with holes during the relaxation process, resulting in a broad and asymmetric PL spectrum. The PLQY reaches as high as 43%. Meanwhile, the colloidal solution of the CNQDs in isopropanol showed no changes or precipitation over six months storage, demonstrating excellent stability (Fig. S3). As illustrated in Fig. 2b, the emission peak shows a redshift with increasing excitation wavelength, which further confirms the multiple electron transitions in CNQDs. We further investigated the exciton recombination dynamics in CNQDs using time-resolved PL spectroscopy (Fig. 2c). The PL decay curve can be well fitted with a biexponential decay model, yielding two lifetimes: τ₁ = 0.8 ns and τ₂ = 5.5 ns. The longer lifetime component can be ascribed to a radiative recombination process, while the shorter one is associated with a non-radiative recombination process. Additionally, ultraviolet photoelectron spectroscopy (UPS) was employed to study the valence electronic structure of CNQDs, revealing that the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels are approximately −6.57 eV and −4.47 eV, respectively (Fig. S4).³²

Fig. 2 (a) UV-Vis absorption and PL spectra of CNQD powder. (b) Normalized PL spectra of CNQDs under different excitation light. (c) PL lifetime of CNQDs under a 375 nm picosecond laser excitation.

In order to further investigate the electrical properties of CNQDs and provide a theoretical basis for the design of LED device structures, the electron and hole mobilities were estimated using the space charge limited current (SCLC) method.^33–35 The carrier mobility was calculated using the following formula:

where J is the current density, ε₀ is the vacuum permittivity (8.85 × 10⁻¹⁴ F cm⁻¹), ε_r is the relative dielectric constant (generally takes 3 for organic semiconductors), μ is the carrier mobility, V is the applied bias, and d is the film thickness.²⁷ As shown in Fig. 3a and c, the electron mobility (μ_e) and hole mobility (μ_h) of CNQDs were estimated by constructing the electron-only device ITO/LiF/B3PYMPM/CNQDs/B3PYMPM/LiF/Al and hole-only device ITO/HAT-CN/TAPC/CNQDs/TAPC/HAT-CN/Al. The results indicate that the electron mobility of CNQDs is 1.20 × 10⁻⁶ cm² V⁻¹ s⁻¹, and the hole mobility is 3.97 × 10⁻⁵ cm² V⁻¹ s⁻¹ (Fig. 3b and d). In QD-LED devices, the electron mobility of the electron transport layer (ETL) material is generally higher than the hole mobility of the hole transport layer (HTL) materials, resulting in faster electron transport compared to hole transport. This imbalance in carrier mobilities can lead to a situation where electron injection significantly exceeds hole injection as carriers move from the electrodes to the emission material layer (EML), causing severe carrier imbalance.^36–39 The higher hole mobility and lower electron mobility of CNQDs may help promote more balanced carrier injection and ultimately enhance the device performance.

Fig. 3 (a) Energy level structure diagram of the electron-only device. (b) Electron mobility of CNQDs was calculated by the SCLC method. (c) Energy level structure diagram of the hole-only device. (d) Hole mobility of CNQDs was calculated by the SCLC method.

To investigate the application potential of CNQDs in LEDs, a series of electroluminescent devices were fabricated by spin-coating CNQD solution as the emitting layer. TPBi (device I), PO-T2T (device II), and B3PYMPM (device III) with various mobility levels were selected for ETL materials, and the devices were constructed as ITO/PEDOT:PSS/TFB/PVK/CNQDs/ETL/LiF/Al (Fig. 4a). In this device configuration, PEDOT:PSS serves as the hole injection layer (HIL) to reduce the hole injection barrier and enhance hole injection from the anode. TFB and PVK are employed as the HTL, facilitating more efficient hole transport to the EML. LiF and Al function as the electron injection layer and cathode, respectively. TPBi (device I), due to its excellent exciton binding ability, moderate electron mobility of 1 × 10⁻⁵ cm² V⁻¹ s⁻¹,⁴⁰ favorable energy level alignment, and excellent film-forming properties, is widely used as an ETL material for QDs-LEDs. As shown in Fig. 4c, the current density of device I increases uniformly with the applied bias voltage, reaching a maximum current density of 342 mA cm⁻², indicating good electron and hole injection characteristics. The device exhibits a low turn-on voltage of approximately 4.0 V and achieves a maximum luminance of 280 cd m⁻² at 6.8 V (Fig. 4d). However, its EQE is only 0.30% (Fig. 4e). This may be attributed to the well-aligned LUMO level of TPBi with the conduction band of conventional CdSe/InP QDs, but the significant energy level mismatch with the conduction band of CNQDs results in an electron injection barrier, which in turn affects the recombination efficiency.

Fig. 4 (a) and (b) Energy level diagram of CNQDs-LEDs. (c) Current density–voltage curves, (d) luminance–voltage curves, (e) EQE–current density curves of devices I, II, III, and IV. (f) Electroluminance spectra of device IV. Inset is the photograph of device IV under operation.

Carrier balance and energy level alignment are critical factors influencing the performance of QDs-LEDs. Therefore, selecting an appropriate HTL/ETL and optimizing energy level alignment to facilitate efficient electron and hole injection into the EML are two effective strategies for enhancing QD-LED performance. Initially, device II was fabricated using PO-T2T as the ETL, which possesses an ultra-high mobility of 2 × 10⁻³ cm² V⁻¹ s⁻¹ and a deeper energy level structure.⁴¹ PO-T2T not only significantly improves the electron mobility, optimizing electron injection, but also effectively compensates for the intermediate LUMO energy level between the CNQDs and LiF, significantly reducing the electron transport barrier. This results in a lower driving voltage and a substantial improvement in electron injection, with a maximum current density of 1210 mA cm⁻², three times that of device I (Fig. 4c), and achieving an extremely low turn-on voltage of 2.8 V. Additionally, the deep HOMO level of PO-T2T allows it to act as both an electron transport layer and a hole blocker, facilitating the injection of more electrons and restricting more holes in the EML, thereby greatly enhancing the electron–hole recombination efficiency. Device II achieve a maximum luminance of 556 cd m⁻² at 5.2 V (Fig. 4d), and the EQE increases to 0.83% (Fig. 4e). Additionally, considering that the electron mobility of PO-T2T is much higher than that of CNQDs, this may lead to carrier injection imbalance, significantly increasing non-radiative recombination and preventing the device from achieving an ideal EQE. Therefore, we chose B3PYMPM, which has an intermediate energy level similar to that of PO-T2T and a mobility (2 × 10⁻⁵ cm² V⁻¹ s⁻¹) between those of PO-T2T and TPBi, to fabricate device III.⁴² As shown in Fig. 4c, the current density of device III lies between those of device I and device II, with effective regulation of electron injection. The reasonable energy level alignment results in a turn-on voltage of 3.0 V. However, unfortunately, at 5.6 V, the maximum luminance of device III is only 168 cd m⁻², and the EQE is only 0.42%.

To further enhance device performance, we considered constructing a composite ETL to achieve a more rational stepwise energy level alignment and approach carrier balance, thus optimizing electron injection. Compared to B3PYMPM, TpPyPB has similar energy levels but a higher electron mobility of 8 × 10⁻³ cm² V⁻¹ s⁻¹,⁴³ which allows it to meet the energy level alignment requirements while enabling appropriate adjustment of electron mobility. Therefore, TpPyPB was combined with B3PYMPM to form a composite ETL.^31,44,45 By adjusting the thickness ratio of the composite ETL, we fabricated a series of devices (Fig. S5, Table S1) and ultimately obtained the optimal device structure: ITO/PEDOT:PSS/TFB/PVK/CNQDs/TpPyPB/B3PYMPM/LiF/Al (Fig. 4b). The introduction of TpPyPB significantly enhanced the device performance. The construction of a stepwise energy level structure reduced the electron injection barrier, resulting in an exceptionally low turn-on voltage of 2.8 V. The construction of a composite ETL effectively modulates the electron transport characteristics of the ETL, leading to carrier balance and significantly improving device efficiency. As a result, device IV achieves a maximum luminance of 855 cd m⁻² at 5.8 V, with a maximum current efficiency of 3.61 cd A⁻¹ (Fig. S6) and a record maximum EQE of 2.14% among CNQD-LEDs (Fig. 4e, Table S2). Compared to previous reports, all performance metrics of CNQD-LEDs are significantly improved. The electroluminance spectrum of device IV exhibits an emission peak at 517 nm with a FWHM of 116 nm (Fig. S7), and the corresponding CIE chromaticity coordinates are (0.29, 0.46) (Fig. S8). Moreover, the electroluminance intensity gradually increases with the applied voltage without any significant spectral shift (Fig. 4f), indicating that the increase in voltage does not alter the dominant emission mechanism, demonstrating good optical stability of the material.

Conclusion

In summary, high-quality CNQDs were successfully synthesized via a simple bottom-up thermal polymerization method. The synthesized CNQDs achieve a PLQY of up to 43% and exhibit matched charge transport properties. Based on these QDs, we further optimized the green-emitting QD-LED device structure: ITO/PEDOT:PSS/PVK/CNQDs/TpPyPB/B3PYMPM/LiF/Al. The device achieved an EQE of 2.14% and a luminance of 855 cd m⁻² under a low turn-on voltage, significantly outperforming previously reported CNQD-LEDs. These results not only demonstrate the vast potential of CNQDs in optoelectronic applications but also lay the foundation for their practical use in QD-LED devices. However, there remains a significant discrepancy between the actual and theoretical efficiency of CNQD-LEDs, indicating the need for further optimization of their optoelectronic properties and a deeper exploration of their electroluminescence mechanism.

Experimental section

Materials

Malic acid, urea and isopropanol were purchased from Shanghai Macklin Biochemical Technology Co., Ltd. Poly(3,4-ethylenedioxythiophene)-poly(styrene sulfonate) (PEDOT:PSS) CLEVIOS PVP Al 4083 was purchased from Heraeus; poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-(4,4′-(N-(4-sec-butylphenyl))diphenylamine)] (TFB) and poly(9-vinylcarbazole) (PVK) were purchased from American Dye Source; 1,3,5-tris(1-phenyl-1H-benzimidazol-2-yl)benzene (TPBi), 2,4,6-tris[3-(diphenylphosphinyl)phenyl]-1,3,5-triazine (PO-T2T), 4,6-bis(3,5-bis(pyridin-3-yl)phenyl)-methyl pyrimidine (B3PYMPM), and 1,3,5-tris(4-pyridyl-4-phenyl)benzene (TpPyPB) were purchased from Xi'an Yuri Solar Co., Ltd. All reagents were used without further purification.

Synthesis of CNQDs

A mixture of 0.134 g malic acid and 1.08 g urea in a molar ratio of 1 : 18 was put in a glass bottle containing 5 mL isopropanol. The mixture was then heated to 180 °C and maintained at this temperature for 1 hour in an oven. After cooling to room temperature, the sample was collected for subsequent use.

Preparation of electron-only and hole-only devices

The ITO glass with size of 2 cm × 2 cm were washed in an ultrasonic bath with deionized water, acetone, and isopropanol for 10 min each repetition and treated with oxygen plasma for 10 min. The substrate was transferred to a vacuum evaporation chamber, where HAT-CN (5 nm) and TAPC (35 nm) were sequentially deposited by thermal evaporation for hole-only devices (LiF (1 nm) and B3PYMPM (35 nm) were deposited instead for electron-only devices). Subsequently, the substrate was transferred to an N₂-filled glovebox, and the CNQD/isopropanol solution (40 mg mL⁻¹) was spin-coated at 4000 rpm for 60 s without annealing. Finally, the substrate was transferred back to the vacuum evaporation chamber, where TAPC (35 nm), HAT-CN (5 nm), and Al (80 nm) were sequentially deposited by thermal evaporation for hole-only devices (B3PYMPM (35 nm), LiF (1 nm), and Al (80 nm) were deposited instead for electron-only devices).

Preparation of CNQD-LEDs

For fabrication of the devices, the ITO glass with size of 2 cm × 2 cm was washed in an ultrasonic bath with deionized water, acetone, and isopropanol for 10 min each repetition and treated with oxygen plasma for 10 min. The PEDOT:PSS solution was spin-coated onto the ITO glass substrate at 4000 rpm for 60 s, followed by annealing at 150 °C for 20 minutes, the substrate was then transferred to an N₂-filled glove box. TFB and PVK were dissolved in chlorobenzene (CB) at concentrations of 1.0 and 3.0 mg mL⁻¹, respectively. The TFB/CB solution was spin-coated at 2000 rpm for 45 s and annealed at 120 °C for 20 min; the PVK/CB solution was spin-coated at 4000 rpm for 60 s and annealed at 120 °C for 20 min; the CNQD/isopropanol solution (40 mg mL⁻¹) was spin-coated at 4000 rpm for 60 s without annealing. Finally, the substrate was transferred to a vacuum evaporation chamber, where TPBi, B3PYMPM, PO-T2T, TpPyPB, LiF, and Al were deposited by thermal evaporation.

Characterization

The structures of the samples were characterized using TEM (JEOL, JEM-F200, Japan), XRD (PANalytical B.V., X'Pert-PRO MPD diffractometer, Holland), FT-IR (Thermo Fisher, Nicolet 6700 FT-IR spectrometer, USA), XPS (Thermo Fisher, ESCALAB XI + spectrometer, USA), and UPS (Thermo Fisher, ESCALAB XI + spectrometer, USA). The optical and photophysical properties of the samples were characterized by using UV-Vis absorption spectra (Agilent, Cary 5000 spectrophotometer, USA), PL spectra (Ocean Optics, QEPro spectrometer, USA), and time-resolved PL (Edinburgh, FLS1000, UK) with a system equipped with a 375 nm picosecond pulsed laser. The PLQY measurements were performed on a QEPro spectrometer (Ocean Optics, USA) equipped with an integrating sphere and a 385 nm LED as excitation light.

LED device performance test

All devices were tested in an N₂-filled glove box at room temperature. The Keithley 2450 sourcemeter provides a stable current and voltage output for the devices. Then QEPro spectrometer (Ocean Optics) connecting with an integrating sphere is used to collect the optical signal of the EL device.

Author contributions

J. X. conceived the idea and designed the experiments. X. W. and B. L. prepared the samples and LEDs and performed the characterization. Y. X., M. Z. and F. D. assisted with related tasks. L. W. and Q. C. helped in analyzing the experimental results. J. X., X. W. and B. L. co-wrote the manuscript. All authors read and commented on the manuscript.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Supplementary information (SI) is available, including full-fange XPS, photo of QDs solution, UPS data and performance of CNQDs-LEDs. See DOI: https://doi.org/10.1039/d5qi01479d.

Conflicts of interest

The authors declare no competing interests.

Acknowledgements

This work was supported by the Taishan Scholar Program, Qingdao Natural Science Foundation (25-1-1-168-zyyd-jch), and 111 Project of China (D20017).

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