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.
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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