Pioneering electrochemical cells emitting white light

In an article recently published in the journal Advanced Optical Materials, Scientists have presented novel concepts for device fabrication and materials design that allowed them to create the first white light-emitting electrochemical cell (LEC) and contributed to the development of carbon dot-based LECs.

Background

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

In particular, carbon dots are suitable for emitters of thin-film lighting devices in this context due to their non-toxicity, tunable electro-/photoluminescent properties, and large-scale, eco-friendly, and relatively easy production. However, their effective incorporation into the active layer remains a major challenge due to visible phase separation and aggregation-induced emission quenching in thin films and poor compatibility with host materials in organic solvents during device fabrication for solvent-based deposition techniques and standard electron/hole transport layers.

Although recent efforts have successfully addressed the challenge through various approaches, including surface modification and incorporation of carbon dots in micelles and hydrophilic-solid matrix, the green solvent-dependent fabrication technique has not yet been developed and has not provided moderate device performance.

Proposed approach

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

Initially, a simple, rapid, scalable, and economical water-based microwave-assisted method was used to synthesize BN-CDs, which featured an amorphous carbon core doped with B and N. Commercially available and cheap precursors such as urea, boric acid, and citric acid were used.

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

Although BN-CDs showed excitation-independent and bright (42% photoluminescence quantum yield) narrow blue emission with a peak wavelength of 440 nm in dilute aqueous solution, they were not emissive in thin films due to aggregation-induced quenching.

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

In the preparation of the thin film via spin-coating, SPPO13 was dissolved in cyclohexanone at a concentration of 10 mg mL⁻¹ after heating for 2 h at 50 °C, while BN-CD was dispersed in ethanol 80% at a concentration of 8 mg mL⁻¹. The researchers chose cyclohexanone as the solvent due to its high green score, while ensuring the orthogonality of the layer with the underlying poly(3,4-ethylenedioxythiophene):polystyrene sulfonate (PEDOT:PSS) layer in the device architecture.

The composition of the last layer is SPPO13:THABF₄:BN-CDs:TMPE in the mass ratio of 1:0.2:0.6:0.2. The synthesized solutions were centrifuged on quartz slides for 30 s at 2000 rpm, which led to obtaining homogeneous films. AFM was used to monitor the morphology of the obtained BN-CDs films over an area of ​​100 µm², while a spectrofluorometer FS5 with an integrating sphere SC-30 was used to measure the photoluminescence quantum yield and spectral values.

The importance of this work

The results confirmed the presence of 0.14% B and 5.1% N in BN-CD. In contrast to carbon dots without heteroatoms, N doping increased the photoluminescence quantum yield, while B doping resulted in higher stability against chemical stress and photobleaching.

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

Both factors led to the peculiar electroluminescence behavior of the white emission LEC cells associated with a maximum luminance of 40 cd m⁻² and a significantly improved stability in the range of hours compared to the previous LEC cells based on monochromatic carbon dots, characterized by a stability of less than one minute.

These results indicate that the complex interaction between n–π BN-CD^* surface states and ionic electrolyte and control of the emission zone position were key to control/tune the device performance and chromaticity in the near future. Thus, better host matrix design combined with carbon dot surface functionalization will be crucial to further improve the performance of BN-CD based white LEC.

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

Journal Reference

Cavinato, L.M. and others. (2024) Blue carbon dots doped with boron and nitrogen for white-light-emitting electrochemical cells. Advanced Optical Materials2400618. DOI: 10.1002/adom.202400618, https://onlinelibrary.wiley.com/doi/full/10.1002/adom.202400618

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