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Pioneering White Light-Emitting Electrochemical Cells

In an article recently published in the journal Advanced Optical Materials, researchers demonstrated novel concepts concerning device fabrication and material design to realize the first white light-emitting electrochemical cell (LEC) and contribute to developing carbon dot-based LECs.

Pioneering White Light-Emitting Electrochemical Cells
Study: Pioneering White Light-Emitting Electrochemical Cells. Image Credit: DKai/Shutterstock.com

Background

LECs, electroluminescent single-layered devices, possess a simple architecture that depends on the mobile ions' presence in the active layer. These devices offer moderate lighting performance at low-cost production. Recently, LECs have evolved as disposable or reusable lighting devices due to a growing emphasis on fulfilling sustainability goals. The integration of biogenic and/or sustainable electrolytes and emitters is a leading example of this trend.

Specifically, carbon dots are suitable for thin-film lighting device emitters in this context owing to their non-toxicity, tunable electro-/photo-luminescent properties, and large-scale, green, and relatively easy production. However, effectively incorporating them in the active layer remains a major challenge due to prominent phase separation and aggregation-induced emission quenching within thin films, and poor compatibility with host materials in organic solvents upon device fabrication for solvent-based deposition techniques and standard electron/hole transport layers.

Although recent efforts have successfully addressed the challenge through different approaches, including surface modification approaches and incorporation of carbon dots in micelles and a hydrophilic-solid matrix, a fabrication technique that depends on green solvents is yet to be realized, attaining moderate device performances.

The Proposed Approach

In this study, researchers described the use of blue-emitting boron (B)- and nitrogen (N)-doped carbon dots (BN-CDs), rationalizing their photoluminescence behavior in solution and ion-based thin-films to synthesize white LECs. Two new device fabrication and material design concepts proposed to realize the white LECs were the key contributions of this work.

Initially, a simple, quick, scalable, and cost-effective water-based microwave-assisted method was used to synthesize BN-CDs, which featured an amorphous carbon core doped using B and N, using commercially available and cheap precursors, such as urea, boric acid, and citric acid.

The synthesized BN-CDs were characterized using atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy.

Although the BN-CDs displayed an excitation-independent and bright (42% photoluminescence quantum yield) narrow blue emission with 440 nm peak wavelength in diluted aqueous solution, they were not emissive in thin films owing to aggregation-induced quenching.

This issue was addressed using a hydrophilic host matrix based on a mixture of trimethylolpropane ethoxylate (TMPE) and tetrahexylammonium tetrafluoroborate (THABF4) as ion electrolyte and amorphous 2,7-bis(diphenylphosphoryl)-9,9′-spirobifluorene (SPPO13).

During the thin-film preparation through spin coating, the SPPO13 was dissolved in cyclohexanone at 10 mg mL⁻¹ upon heating for 2 h at 50 °C, while BN-CDs were dispersed in ethanol 80% at 8 mg mL⁻¹. Researchers selected cyclohexanone as solvent owing to its high green score while ensuring layer orthogonality with the underlying layer of poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) in the device architecture.

The final layer's composition was SPPO13:THABF4:BN-CDs:TMPE in a 1:0.2:0.6:0.2 mass ratio. The synthesized solutions were spin-coated on quartz slides for 30 s at 2000 rpm, leading to homogeneous films. AFM was utilized to monitor the morphology of the obtained BN-CDs films in a 100 µm² area, while an FS5 Spectrofluorometer with integrating sphere SC-30 was employed to measure the photoluminescence quantum yield and spectra values.

Importance of this Work

Results confirmed the presence of 0.14% B and 5.1% N in BN-CDs. Contrary to heteroatom-free carbon dots, the N-doping enhanced photoluminescence quantum yields, while the B-doping resulted in higher stabilities toward chemical stress and photobleaching.

The homogenous thin films displayed an excitation-dependent emission covering the entire visible range due to the interaction between the ions and the emitting n−π* surface states/interaction of the ion electrolyte with the peripheral functionalization of the BN-CDs and an efficient host to the BN-CD energy transfer.

Both factors led to a peculiar electroluminescence behavior in LECs with a white-emission associated with a maximum 40 cd m⁻² luminance and a substantially improved stability in the range of hours compared to the prior-art monochromatic carbon dot-based LECs with stabilities of less than one minute.

These findings indicated that the complex interaction between the BN-CDs' n–π* surface states and the ion electrolyte and the control of the emissive zone’s position were the key to control/tune device performance and chromaticity in the near future. Thus, a better host matrix design coupled with a surface functionalization of carbon dots will be crucial to further improve the BN-CD-based white LEC performance.

Overall, this work embraced the principles of green optoelectronics by employing abundant and cost-effective emitters, performing water-based synthesis, and using low-toxicity solvents for device fabrication.

Journal Reference

Cavinato, L. M. et al. (2024) Blue-Emitting Boron- and Nitrogen-Doped Carbon Dots for White Light-Emitting Electrochemical Cells. Advanced Optical Materials, 2400618. DOI: 10.1002/adom.202400618, https://onlinelibrary.wiley.com/doi/full/10.1002/adom.202400618

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Samudrapom Dam

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Samudrapom Dam

Samudrapom Dam is a freelance scientific and business writer based in Kolkata, India. He has been writing articles related to business and scientific topics for more than one and a half years. He has extensive experience in writing about advanced technologies, information technology, machinery, metals and metal products, clean technologies, finance and banking, automotive, household products, and the aerospace industry. He is passionate about the latest developments in advanced technologies, the ways these developments can be implemented in a real-world situation, and how these developments can positively impact common people.

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