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The synthesis of carbon dots (CDs) with tailored properties commonly requires time-consuming trial-and-error experimentation, in part because of a poorly understood and controlled chemical conversion of the precursor material. Here, we first report on the solid-state pyrolysis or solvothermal conversion of an ortho-aminophenol (oAP) precursor, comprising ortho-disposed amino and hydroxyl groups on a benzene ring. We find that both conversion reactions resulted in a two emission-colour product, which could be separated into distinct blue-emitting CDs (bCDs, λpeak = 420 nm) and yellow-emitting CDs (yCDs, λpeak = 565 nm) by repetitive column chromatography. Systematic characterization revealed that both CDs comprise a planar graphene-like interior, but that they are distinguished by that the bCDs comprise an intermixed significant amino-rich fluorophore while the yCDs instead comprise a pyridinic-rich fluorophore. This implies that the bCDs are formed via activation of the amino group of the oAP precursor, whereas the synthesis of the yCDs constituted a simultaneous activation of both the amino and hydroxyl groups. With this knowledge at hand, we managed to direct the chemical conversion of the oAP precursor to yield either solely bCDs or yCDs by adding a catalyst (either the Lewis acid AlCl3·6H2O or the Lewis base NaOH) that selectively and efficiently activated only one of the reaction pathways. This demonstration is important in that it shows that the synthesis of CDs with desired properties can be realized with efficient rational instead of trial-and-error means.

Deep-blue (DB) emitters that feature high photoluminescence quantum yield (PLQY) and narrow spectral bandwidth are desired for a variety of optoelectronic applications, particularly for lighting, illumination, and lasing. Currently favored DB emitters constitute quantum dots comprising cadmium or lead and organic compounds derived from petroleum, but they suffer from toxicity and sustainability issues. Here, we report the solvothermal synthesis of DB-emitting carbon dots (DB-CDs) using bioderivable phloroglucinol as the sole starting material, which exhibit a peak emission wavelength of 403 nm, narrow spectral full width at half-maximum of 35 nm, and high PLQY of 61% in ethanol. The DB-CDs with a planar structure are demonstrated to comprise distinct graphene segments in a polyether-cross-link network, with the former functioning as the fluorophore. The application merit of the DB-CDs is exemplified by their implementation as the gain medium in a random laser device, which exhibits a threshold optical power density of 40.5 kW cm-2. This study thus demonstrates a path toward efficient and sustainable deep-blue emitters, which can be exploited in practical applications.

The light-emitting electrochemical cell (LEC) forms a p-n junction doping structure by bipolar electrochemical doping during its initial operation. The light emission originates from the p-n junction region through the formation and radiative decay of excitons. The width of this emission zone (EZ) is important since it strongly affects the emission losses by exciton quenching, the outcoupling efficiency, and the drive voltage. The challenge is that it has proven very difficult to determine the width of the dynamic EZ in LECs. Here, this issue is addressed through the presentation of a method that fits simulated angle-resolved emission spectra to measured spectra, using the EZ width and position as the two free parameters. For improved accuracy, a linear polarizer is employed for the selective detection of s-polarized emission and a half-cylinder outcoupling structure for enhanced spectral output. The method is finally employed on a common conjugated-polymer LEC, and it is derived that its EZ width decreases during the initial operation, that the steady-state EZ width is equal to ≈20% of the active-material thickness at a current density of 10 mA cm−2, and that the steady-state EZ width appears to decrease with increasing current density.

Emissive carbon dots (CDs) that are synthesized from biomass can be highly sustainable, but the number of reported biomass-derived CDs that emit in the ultraviolet (UV) range is small. Moreover, current commercial UV-emitting materials rely heavily on the use of non-sustainable resources, such as rare metals, heavy metals, and petroleum chemicals. This yields that the development of efficient biomass-derived UV-CDs is desired. Here, we report on the hydrothermal conversion of a common green-tea extract (Polyphenon 60) into UV-CDs, which feature a photoluminescence (PL) peak wavelength of 384 nm, a full width at half maximum of 72 nm, and a photoluminescence quantum yield (PLQY) of 17% in water. By shifting to a lower-polarity solvent of 3-phenoxyanisole, the PLQY is strongly enhanced to 81%, and the PL peak blue-shifts to 370 nm, while the maximum solubility is lowered. These observations support the notion that the UV-CDs feature aggregation-induced emission and that they are endowed with hydrophilic surface groups. Moreover, the findings of excitation-wavelength-independent PL and a nanosecond-level short emission lifetime reveal that it is a single distinct fluorophore that produces the UV emission. We finally report preliminary results that the UV-CDs exhibit potential for inhibiting the proliferation of cancer cells.

Light-emitting electrochemical cells (LECs) are promising candidates for fully solution-processed lighting applications because they can comprise a single active-material layer and air-stable electrodes. While their performance is often claimed to be independent of the electrode material selection due to the in situ formation of electric double layers (EDLs), we demonstrate conceptually and experimentally that this understanding needs to be modified. Specifically, the exciton generation zone is observed to be affected by the electrode work function. We rationalize this finding by proposing that the ion concentration in the injection-facilitating EDLs depends on the offset between the electrode work function and the respective semiconductor orbital, which in turn influences the number of ions available for electrochemical doping and hence shifts the exciton generation zone. Further, we investigate the effects of the electrode selection on exciton losses to surface plasmon polaritons and discuss the impact of cavity effects on the exciton density. We conclude by showing that we can replicate the measured luminance transients by an optical model which considers these electrode-dependent effects. As such, our findings provide rational design criteria considering the electrode materials, the active-material thickness, and its composition in concert to achieve optimum LEC performance.

The p-i-n junction structure that develops dynamically under an applied bias via electrochemical doping (ECD) is of key importance for the performance of light-emitting electrochemical cells (LECs). While the complex electronic and ionic processes that govern its transient formation have been extensively studied by both experiments and drift-diffusion modeling, less attention has been given to the steady-state junction as a function of voltage. Here we study the formed p-i-n structure of a polymer LEC at the steady state by measuring and analyzing its most distinctive feature: the current density-voltage-luminance characteristics. Unexpectedly, we find that the effective conductance of the p-i-n structure exhibits a positive correlation with the applied bias, a behavior not predicted by existing LEC drift-diffusion models. We attribute this discrepancy to the assumption in these models of a constant density of mobile ions. Hence, we present a modified model in which the ECD level - represented by the number of ions with which the organic semiconductor is doped - scales with the applied voltage, implying a voltage-dependent doping efficiency. We validate this hypothesis using electron spin resonance spectroscopy and drift-diffusion modeling, while additionally establishing that only a small fraction of the available ions in our system, which increases from 1% to 3% with increasing bias, contributes to the ECD and is necessary for efficient LEC operation. These findings not only provide fundamental insights into the operational mechanism of LECs but also have direct implications for the broader organic mixed ionic and electronic conductor community.

Intrinsically stretchable emissive devices that are thin, lightweight and low-cost are highly desired for, e.g., wearable electronics, where they can enable facile communication and interaction with, and adaptability to, dynamic environments. The light-emitting electrochemical cell (LEC) is a candidate for the fulfillment of these challenging requirements, since its robust and air-stabile device architecture renders it a good fit for cost-efficient, ambient-air printing and coating fabrication of intrinsically stretchable thin-film device architectures. Here, we report on the design and pioneering fabrication of such an intrinsically stretchable LEC by non-interrupted spray-coating under ambient air and show that such an optimized, and potentially low-cost, thin-film LEC can deliver uniform light emission from a lightweight device architecture even at 30% lateral elongation.

The light-emitting electrochemical cell (LEC) is a good fit for scalable ambient-air coating and printing, since it can deliver efficient emission from a robust three-layer architecture comprising solely air-stable and soluble materials. However, a drawback is that hitherto employed emitters for printed LECs are either conjugated polymers that are efficiency and purification limited or small molecules that are difficult to solution process into uniform thin films. The recent advent of dendrimers that emit by thermally activated delayed fluorescence (TADF) promises to address all these issues, since they can efficiently utilize all (both singlet and triplet) excitons for the light emission, be of high purity because of their well-defined structure, and feature high solubility and good film-forming capacity by the virtue of being equipped with branched dendrons. Herein, an asymmetric second-generation TADF-dendrimer, tBuCz2m2pTRZ, is combined with an ionic-liquid electrolyte for the formulation of a tuned ink, which is used for a bar-coating fabrication of uniform LEC active-material films featuring a high photoluminescence quantum yield of 84%. This opportunity is ultimately utilized for the pioneering demonstration of a bar-coated TADF-dendrimer-LEC, which delivers uniform and bright green luminance of 350 cd m−2 at an external quantum efficiency of 1.2%.

Organic semiconductors that deliver emission with a wavelength exceeding 900 nm can enable a wide range of applications, but the unfortunate fact is that only a small number of such emitters have been synthesized and have demonstrated emissive function in devices. Here, this issue is addressed through the design and synthesis of two terpolymers that comprise a low energy-gap thiophene-thiadiazolobenzotriazole (TBTzTD) donor–acceptor unit as the minority guest incorporated in a majority donor–acceptor conjugated copolymer host, being either thiophene-quinoxaline (TQ) or thiophene-difluoroquinoxaline (TQ2F). These terpolymers are further endowed with high solubility in benign hydrophilic solvents through the grafting of branched oligo(ethylene glycol) side chains onto the quinoxaline unit. The application function of the terpolymer emitters is demonstrated through their implementation in light-emitting electrochemical cells (LECs). It is notable from a sustainability perspective that the emitter is metal free and that the single-layer LEC active material is cast from an environmentally benign water:ethanol solvent blend. From an application perspective, it is attractive that the terpolymer-LEC devices feature a very fast turn-on to significant radiance at low voltage and that they deliver emission with a peak wavelength of 935 nm with TQ-TBTzTD as the emitter and 950 nm with TQ2F-TBTzTD as the emitter.

The attainment of white emission from a light-emitting electrochemical cell (LEC) is important, since it enables illumination and facile color conversion from devices that can be cost-efficient and sustainable. However, a drawback with current white LECs is that they either employ non-sustainable metals as an emitter constituent or are intrinsically efficiency limited by that the emitter only converts singlet excitons to photons. Organic compounds that emit by thermally activated delayed fluorescence (TADF) can address these issues since they can harvest all excitons for light emission while being metal free. Here, we report on the first white LEC based on solely metal-free TADF emitters, as accomplished through careful tuning of the energy-transfer processes and the electrochemically formed doping structure in the single-layer active material. The designed TADF-LEC emits angle-invariant white light (color rendering index = 88) with an external quantum efficiency of 2.1 % at a luminance of 350 cd/m².