Carbon Dots as a sustainable new platform for organic light emitting diode

: In the last 10 years, carbon dots (CDs) synthesized from renewable organic resources have been gathered a considerable amount of attention in different fields for their peculiar photoluminescent properties. Moreover, the synthesis of CDs fully responds to the principles of the circular chemistry and the concept of safe-by-design. This review will focus on the different strategies for the incorporation of CDs in organic light-emitting devices (OLEDs) and on the study of the impact of CDs properties on the OLEDs performance. The main current research outcomes and highlights are summarized to guide users towards the fully exploitation of use these materials in optoelectronic applications.


Introduction
The growing demand for electronic devices for an ever-increasing number of applications means that green and sustainable electronics are no longer just a dream, but a pressing need [1].
In this context the electronics and optoelectronics based on organic semiconductor showed, in the last few years, significant growth in many areas dominated by conventional electronics [2]. The foremost advantage of organic materials is that they are cheap, lightweight, easy to be processed and flexible [3]. However, the most effective synthetic methodologies [4] currently employed for producing organic semiconductors are affected by safety and environmental issues which may seriously prevent their large-scale production. In this regard, the application of principles of green chemistry for the development of waste-minimized and cleaner synthetic approaches to semiconductor synthesis is essential for propelling the field of organic electronics thus moving towards increasingly sustainable electronic devices [5].
In fact, the use of organic materials to build electronic devices [6] holds the promise that future electronic manufacturing methods will rely on safer and more abundant raw materials [7]. The vision is for resource-efficient synthetic methodologies, whereby both devices themselves and manufacturing of those devices use less and safe materials.
The first pillar on which the new sustainable industrial revolution is based is inevitably the development of new materials that are possibly safe and sustainable by design [8].
Among the emerging classes of materials able to meet these needs, carbon dots (CDs), member of nanocarbon family that comprise, differently from the best known carbon nanotube [9], discrete quasi-spherical nanoparticles with sizes below 10 nm, are being attracting considerable interest. Since their first discovery in 2006, CDs [10,11] have gained ever-increasing attention due to their fascinating properties like unique optical behaviour, tunable emission, multiple functional groups, excellent biocompatibility, chemical and photo-stability, low toxicity and low-cost. More importantly, their properties can be tailored by controlling the size, shape and heteroatom doping of CDs and by modifying their surfaces and edges, due to the remarkable quantum confinement effect (QCE), surface effect and edge effect [12], (Fig.1). They are considered as promising green alternatives to fluorescent dyes [13][14][15] and generally to toxic metallic colloidal semiconductor nanocrystals, and are advocated for diversified applications such as sensing, bioimaging, fingerprint detection, gene delivery , solar cells, printing inks and optoelectronic applications in general [12,[16][17][18][19] (Fig.1 and 2). Preparation methods of CDs reported so far can broadly be grouped into top-down and bottomup approaches. The top-down strategy [20][21][22][23][24][25][26][27] involves the breakdown of bulk carbon sources like graphite, carbon nanotubes, and nanodiamonds into fluorescent CDs by employing techniques like arc discharge, laser ablation, chemical oxidation in strong acid and electrochemical synthesis. Conversely, in the bottom-up approach [28-32, 33, 34] the CDs are formed from molecular precursors by applying solvothermal/hydrothermal methods, ultrasound/microwave treatments, or simple thermal combustion.
Although a large variety of techniques and starting materials were employed for the production of CDs, the demand for the sustainable synthetic routes, that adhere to the principles of green chemistry, is prominent. It has been demonstrated that CDs can be prepared in principle from any starting materials that contain carbon. This, in particular, guarantees that most by-products of the food supply chain can be reused to produce CDs. Agriculture products contain a myriad of natural molecules that can make up a diverse source of surface functional groups in CDs formation.
A relevant feature for CDs sustainability has to do with the synthetic methodologies for their fabrication. CDs can be produced hydrothermally, that is, by heating the starting materials in water under atmosphere or elevated pressure [28][29][30][31]34,35]. Consequently, the cost of production is low, and the operation is easy, relatively safe, and free of organic solvents. Furthermore, shorter processing time and lower energy consumption for CD manufacture can be obtained when microwave is used as the heating source [36,37].
Toxicity studies of the CDs were performed with both plants and animals (mice) revealing good biocompatibility [38,39,40] and opened the way not only to their bio-applications, but also to a biodegradable electronics [19,40]. With this perspective, in the present minireview we will discuss CDs obtained with a bottom up approach as it is the one that best adheres to the principles of green chemistry. They can in fact be obtained from renewable sources or waste [37], the synthesis methodologies are simple and inexpensive, and they do not require the use of metal catalysts or chlorinated solvents. We will focalized on CDs as a sustainable new platform for OLEDs [42,43] without deepening the methods of synthesis on which relevant works have already been written [28][29][30][31][32][33][34][35][36][37]44]. In Fact, among the various interesting applications of organic electronics, the OLEDs are certainly those that have already carved out a slice of the market [42] and for this reason the development of sustainable active materials and green technologies can already help the economy in a GREEN turn.
In the next Section 2, we will make a brief introduction on the type and optical properties of these luminescent carbon materials. Section 3 will cover their different application in electroluminescence devices as active layer or as charge regulating layer. In the last section, we will give perspectives for CDs including potential applications and possible development in particular as promising candidate for a safe-by-design material. We hope that this review will provide critical insights to inspire new exciting discoveries on CDs from both fundamental and practical standpoints so that the realization of their potential in the optoelectronic area can be facilitated.
Since their fortuitous discovery in 2004 by Xu et al. [10] and subsequently by Sun et al. in 2006 [11], CDs attracted a great deal of attention. Generally, CDs are 0-dimension nanocarbons with a typical size of less than 10 nm, although ∼60 nm size CDs have also been reported. CDs are quasispherical nanoparticles consisting of amorphous and crystalline parts, mainly composed of carbon. The height of CDs ranges from 0.5 to 5 nm depending on the preparation methods, with a crystal lattice with an interlayer difference of 0.34 nm, which corresponds to the (002) interlayer spacing of graphite [12][13][14]. CDs are often confused, or associated, with graphene quantum dots (GQDs) [15] which are always part of the family of carbon based nanomaterials, but have different characteristics and origins. In fact, GQDs are nanofragments of graphene, they have one or few layers of graphene and usually a larger horizontal dimension than in height with thickness of less than 2 nm exhibiting a typical fringe spacing of 0.24 nm associated with the (100) in-plane lattice spacing of graphene and they are generally produced from graphene or graphene oxide via top-down approaches [14].
Over the last 15 years, CDs have been synthesized with different approaches (i.e. top down and bottom up) however, only recently sustainable precursors and methodology have been deeply investigated for their production [45,46]. These approaches look for sustainable materials which are low-cost, scalable, industrially and economically attractive, and based on renewable and highly abundant resources. This means that CDs synthesis can meet the requirements of the circular chemistry. Interestingly, after preparation from solution, CDs can be functionalized with complex surface groups, especially oxygen based functional groups, such as carboxyl and hydroxyl, which impart excellent water solubility and suitable chemically reactive groups for surface passivation and derivatization with various organic materials. These surface groups make great contributions to the optical properties of CDs. For instance, upon surface passivation, the fluorescence properties of CDs can be enhanced. [47,48] Surface functionalization modifies the physical properties of CDs as their solubility in aqueous and non-aqueous solvents. In addition, the large conjugated structure endows CDs with some important characteristics, like good photostability, high surface area, and robust surface grafting [49]. Their electronic structures can be tuned by their size, shape, surface functional groups and heteroatom doping, as theoretical investigated and experimental confirmed by several groups [50][51][52][53][54]. The tunability of optoelectronic properties by modifying synthetic parameters and precursor strictly resembled the conjugated polymers features [55].
As mentioned above the CDs emission properties are their most amazing characteristics [41]. and significant advances have been made in the last years reaching photoluminescence quantum yield (PLQY) up to 80% in CDs produced from citric acid as renewable precursor, or bright and stable PLQY of 26% converting toxic cigarette butts [28,54].
Mostly, CD are blue emitter, but emission from ultraviolet to near-infrared was reported [55,56] as well as white light emission [59]. In general their PL spectra are symmetrical and broad with large Stokes shifts (mainly due to the CDs large size distribution) and the PL spectra usually have an excitation-dependent behaviour, with the emission peak varying with the excitation wavelength [22,58].
The emission mechanism of CDs is a longstanding debate and several hypotheses have been proposed [41] (see Fig.3) such as i) size-dependent emission, ii) surface state-derived luminescence, and iii) embedded molecular luminophore [61]. Regardless of the type of mechanism, it has been shown that the CDs emission could be regulated by controlling their size (mainly referring to sp2 carbon domains), heteroatom doping, and surface functionalization [60,61,62]. The first hypothesis proposed is based on the size of CDs [61]. . Yuan and co-workers synthetized multicolour-emitting CDs with different dimensions from citric acid (CA) and diaminonaphthalene (DAN) by controlling the process parameter. CDs showed average sizes of about 1.95, 2.41, 3.78, 4.90 and 6.68 nm (Fig.4a), with a corresponding blue (430 nm), green (513 nm), yellow (535 nm), orange (565 nm), and red (565 nm) emission, respectively (Fig.4b). In according with results, they deduced that by increasing the size of CDs and consequently the conjugated π-domain, the bandgap decreases ( fig.4c) [65]. A second hypothesis is related to the surface states of CDs. Ding et al. [66] synthetized tunable photoluminescent CDs by one pot hydrothermally synthesis (Fig.5a). Noteworthy, these CDs had comparable particle size and carbon core, but variable degree of oxidation of the surface state. Therefore, a gradual reduction in their band gaps and a red shift in their emission peaks from 440 to 625 nm (Fig.5b,c) was observed by increasing incorporation of oxygen species into their surface structures ( fig.5d) [66]. Also Miao et al. [67] hypothesized a similar mechanism. They modulate the CDs emission from 430 to 630 nm by controlling the degree of graphitization and the number of surface -COOH groups by changing the molar ratios of CA to urea at different temperatures (Fig.6). The increasing number of -COOH groups on the surface, increase the electronic delocalization and the emission wavelength is consequently red-shifted [64]. Another relevant hypothesis for CDs emission is molecular luminophore-derived emission or molecular state emission. Small molecules or oligomeric luminophores could be produced during CD synthesis and these luminophores could be attached to the surface of CD backbones allowing CDs bright emission properties [68]. Song. et al. [69] studied the chemical structure and PL mechanism of CDs form CA and ethylenediamine (EDA). They proved the presence of a type of bright blue fluorophore and that the CDs emission was a result of small molecules, polymer clusters, and carbon cores. Indeed, the fluorophore may be attached to the carbon core, that may strongly affect the PL properties of the CDs

OLED-based Carbon dots
CDs with amazing properties such as optical characteristics and carbon's intrinsic merits of high stability, low-cost, and environment-friendliness, find a natural and practical application as components in OLED technology.
In the last decade the interest in OLED based on CDs (hereafter CD-OLEDs) has been growth and increasing number of research groups have started to investigate in this field.
We provide a recent advisor about CD-OLEDs, illustrating the dual employment of CDs as direct emitter, both as neat layer or as guest in host:guest systems, and as a charge regulating interlayer (Tab.1 and Fig.7). Particular attention will be devoted to the strategies used to prevent aggregationinduced quenching in the solid-state and to tune the emission colour. Notably, but out from the purpose of the present review, many groups used CDs also as a remote emitter, endowing blue commercial LEDs with a colour converting filter based on CDs embedded in poly(methylmethacrylate) (PMMA) or other matrices. The blue LEDs emission was tuned from blue to red by altering the film thickness of the filter or the doping concentration of CDs [68,69,70].

CDs as emitter
In 2011    Ding and colleagues [66] proposed short-chain passivated CDs in fact short-chain cap shortens the distances of carriers from CDs, which probably facilitates the injection of carriers into CDs . Their CDs were synthesized via a one-step hydrothermal approach using phthalic acid and ethylenediamine, the resulting CDs aqueous solution featured a PLQY of 29.3% and good filmforming ability. CD-OLEDs were fabricated by a solution processing method and the devices exhibited a stable blue EL peak at 410 nm at 6-9 V. Despite the improvement of the carrier transfer ability of CDs achieved by the short-chain modification, the lower PLQY compared with that of longchain passivated CDs and the not optimal device architecture (see energy barrier at the anode interface in Figure 11a), were the most probable explanation for the observed lower performance of devices ( Fig.11b-d). Zhang at al. in 2013 [75] observed for the first time, a multi-colored (bright blue, cyan, magenta and white) EL from CDs of the same size (3.3 nm). Such a switchable EL behavior has not previously observed in single nanomaterial emitting layer OLEDs. This all solution processed device consisted of a CDs emissive layer sandwiched between an organic HTL and an organic or inorganic ETL (electron transporting layer) with typical architecture ITO/PEDOT:PSS/PolyTPD/CDs/TPBI ( were poly-TPD was poly-(N,N0-bis(4-butylphenyl)-N,N0-bis(phenyl) benzidine and TPBI 1,3,5-tris(Nphenylbenzimidazol-2-yl) benzene)) or ZnO/LiF/Al (Fig.12a). The devices structure was adjusted to control the J and therefore the EL spectra. By increasing LiF thickness from 1 at 5 nm, or replacing LiF and TPBi with ZnO, the emitted color changed, with the applied voltage, from blue, cyan, magenta, and white from the same carbon dots (Fig.12b,d). The LMAX obtained was 24 cd/m 2 for the blue-emitting OLED at low current injection. The LMAX observed in devices incorporating ZnO nanoparticles was higher (90 cd/m 2 ) thanks to the higher electron mobility of ZnO with respect to organic ETLs.
To solve the problem of quenching and aggregation of CDs since 2017 a host guest approach [76,77] was proposed with CDs as guest component and, usually, the PVK as a host matrix.
Yuan et al. [65] compared the performance of a neat CDs film and CDs dispersed in PVK. They reported the synthesis of bright multicolor fluorescent CDs (called MCBF-CDs) from blue to red with PLQY up to 75% for blue fluorescence through a facile solvothermal method. Their CDs were Ndoped, highly surface passivated with a high degree of crystallinity. CDs were firstly employed directly as an active layer for monochromatic OLEDs with conventional simple architecture ITO/PEDOT:PSS/CDs/TPBI/Ca/Al (Fig.13a) (Fig.13c,d). The EL spectra showed voltage-independent behavior as well as no temporal degradation, that are of great significance for display technology. In the B-OLEDs, LMAX reached136 cd/m 2 with ηc of 0.084 cd/A (Fig.13b). In the devices incorporating a blend of green CD into a PVK polymer matrix (5wt.% ratio) with ITO/PEDOT:PSS/PVK:MCBF-CDs/TPBI/Ca/Al architecture (Fig.13d) Yuan et al. [65].reached a LMAX and ηc as high as 2050 cd/m 2 and 1.1 cd/A with a low turn on voltage (VON = 3.9 V) (Fig.13e). In the broad EL spectrum two peaks were identified, centered at 410 and 517 nm and assigned to PVK and green MCBF-CD emission, (Fig.13f). The CIE coordinate of (0.30, 0.33), very close to ideal white, resulted from the energy transfer from PVK to CD and form the direct charge injection to CD.
In 2018 Xu et al. [78] synthesized oleophilic CDs, with a PLQY of 41%, by a one-pot microwave carbonization method (Fig.14a) to study the impact of CD aggregation as a limiting factor for the brightness of the CD-LEDs.
They fabricated CD-OLEDs, incorporating a neat CD active layer, with the simple architecture ITO/PEDOT:PSS/CD/TPBI/LiF/Al. The devices featured modest performance (Fig.14), with yellow emission peaked at 554 nm, LMAX of 5.7 cd/m 2 at 10 V and CIE coordinates od (0.36, 0.42), most probably because of aggregation of the CDs. The impact of CDs aggregation on the brightness of CD-OLEDs was studied also by blending the CDs in PVK. Yellow and white EL were observed by tuning the doping concentration of the active layer. The yellow EL, mainly derived from direct carrier trapping, reached LMAX of 339.5 cd/m 2 with excellent color stability. The white CD-OLEDs exhibited a CRI value of 83 with CIE coordinates of (0.29, 0.33) and high LMAX of 544.2 cd/m 2 . The white EL resulted from the contemporary emission from PVK and CDs, and the good brightness was attributed to a suitable balance between holes and electrons in the emitting layer. (Fig.15) [78]. In the last 3 years the research on LEDs based on CDs increase and focalized, almost exclusively, on CDs dispersed in PVK as emitting layer. The next two articles showed that amination could be exploited to improve the performance of devices.
Yuan et al. [79] presented deep-blue light-emitting materials and devices based on CDs that outperform deep-blue emitting LEDs based on Cd 2+ /Pb 2+ materials. CDs were synthesized by solvothermal treatment using CA and DAN as precursors. To enables efficient high-color purity, an additional surface amination step using ammonia liquor and hydrazine hydrate under high temperature was performed. This second step, decreasing the number of defects of CDs and thus suppresses non-radiative pathways, increases the quantum yield up to 70%.
CD-OLEDs were manufactured with the architecture ITO/PEDOT:PSS/TFB/PVK:CDs/TPBI/LiF/Al (Fig.16a). The corresponding energy level diagram (Fig.16b) show a mitigation of energy barrier for both electrons and holes injections thanks to the suitable selection of the different layers. The EL spectra remained at 440 nm across the range of voltage explored, and are in good agreement with the corresponding PL emission peaks indicating that an efficient energy transfer from PVK to CDs takes place. L and EQE as a function of V and J, respectively, are shown in Fig.16d,e. LMAX, ηc and EQE reached remarkable values of 5240 cd/m 2 , 2.6 cd/A and 4% respectively.
After operating continuously for 3 hours at 1000 cd/m 2 , the OLED retained 50% of initial luminance (Fig.16f) without evident changes in EL spectrum. At the same time Jia and co-workers [80] demonstrated the effectiveness of the electron-donating group passivation strategy to impart in CD finely tuned properties for their application as emitters in CD-OLEDs. Specifically, they developed three efficient red-emissive CDs, by p-phenylenediamine and N,N-dimethyl-, N,N-diethyl-,and N,N-dipropyl obtaining CDs-NMe2, -NEt2, and -NPr2, respectively (Fig.17a), with the aim to fabricate warm-light WOLEDs.
Thanks to theoretical investigations, they revealed that the CD emission originated from the rigid π-conjugated skeleton structure, while -NR2 passivation played a key role in inducing charge transfer excited state in the π-conjugated structure to afford high PLQY (up to 86%).
Moreover, the polar -NR2 groups are responsible for the good solubility of CDs in organic solvent and then processability by low-cost spin-coating technique. The CDs were dispersed in PVK.
Finally, WOLEDs-1,2,3 displayed voltages-stable EL spectra with a LMAX in the range of 5248-5909 cd/m 2 (Fig.17f) and a ηc of 3.65-3.85 cd/A (Fig.17g). Yuan et al. [81] recently addressed the issue of broad emission, usually observed in CD, which fundamentally limit their application in displays.
They synthetized multi-colored narrow bandwidth emission from triangular CDs (T-CDs). They demonstrated that the molecular purity and high crystallinity of the triangular CDs are indispensable to obtain the high color-purity. The triangular structure and the narrow bandwidth emission of triangular allowed to reduce dramatically electron-phonon coupling.
Conventional simple structure ITO/PEDOT:PSS/active layer/TPBI/Ca/Al (Fig.18b) was used for the fabrication of OLEDs from blue to red with the T-CDs blended in PVK as active emission layer. HOMO/LUMO energy levels of T-CDs were located within those of PVK then electrons and holes can be efficiently transferred from PVK to T-CDs emitter. The multi-colored OLEDs based on the T-CDs demonstrated high color-purity (FWHM of 30-39 nm) peaked at 476, 510, 540 and 602 nm for B-, G-, Y-, and R-LEDs respectively (Fig.16f-i), a ηc of 1.22-5.11 cd/A (Fig.18c) and LMAX of 1882-4762 cd/m 2 , rivalling the well-developed inorganic QDs-based LEDs. Finally, the LEDs exhibited outstanding stability (Fig.18e) which is of great significance foe display technology. Singh and colleagues [82] fabricated the first flexible OLED (Fig.19a,b) based on CDs employing self-assembled 2D array of CDs embedded in PVK emission layer. The flexible device switched on at 4.3 V and exhibited a blue/cyan emission peaked at about 500 nm (comparable to PL emission, Fig.19c) with LMAX of 350 cd/m 2 and ηc of 0.22 cd/A (Fig.19d,e), whereas the corresponding OLED device based on the rigid glass substrate featured a LMAX of 700 cd/m 2 and ηc of 0.27 cd/A. Fig.19f shows the CD 2D island in the emitter layer. The host-guest energy transfer as main mechanism for CDs emission was exploited by Xu et al. [83] for the fabrication of solution processed blue CD-LEDs with ultrahigh brightness. Oleophylic CDs with PLQY of 41% were obtained using an anhydrous citric acid as carbon precursor and hexadecylamine as passivation agent by one-step microwave carbonization method. CDs with different doping concentrations were blended with PVK and incorporated into a simple and PEDOTfree device architecture ITO/PVK:CDs/TPBI/LiF/Al. The EL spectra of devices were dominated by the 474 nm CD emission peak with a contribution from PVK at around 410 nm. When the doping concentration was higher than 25 wt.%, a progressive weakening of PVK contribution in favour of an enhanced CD emission was observed. For 30 wt.% doping ratio, the LMAX reached 569.8 cd/m 2 at a driving voltage of 12.5 V, and CIE coordinated of (0.22, 0.27) located in the blue light region. Also in this case, the long chain passivating ligands on the oleophylic CDs surface were responsible for the unideal transporting performance that limited the device efficiencies, but it should also be considered that the devices does not contain the commonly employed PEDOT:PSS layer.

CDs as charge regulating Interlayer
The possibility to tune CD's energy levels and intrinsic charge carrier transport, allows and alternative use of CD besides the incorporation as emitting layer. The following paragraph demonstrated that CDs, employed as interlayer (like interfacial layer also called charge regulating layer) in device's architectures based on different class of emitters, may improve the overall performance of the LEDs. The CD layer was incorporated both as ETL and HTL in either direct and inverted LEDs architecture (Fig.7b,c).
In 2017 Park et al. [84] explore the possibility to enhance performance of nanocrystal LEDs (NC-LEDS) based on quantum dots active layer. NC-LEDs typically take advantage of the incorporation of organic or inorganic interfacial layers as charge regulators to ensure charge balancing and high performance [85]. They investigated the roles played by CDs N-doped interlayer inserted by spincoating between the PVK as HTL and the QD emitting layer (core/shell CdSe/CdS) [86,87]. In fact, QD-LEDs basically consisted of a multilayer architecture ITO/PEDOT:PSS/PVK/with or without N-CD/QD/ZnO/Al (Type A and B, Fig.21a). They showed that CDs HTL decrease the highest hole barrier height thus leading to a more balanced carrier injection and improved hole conduction capability. Moreover, they demonstrate that CDs interlayer acted as a resonant energy donor layer to the QD layer.
Both type A and B devices exhibited good electrical rectification, but the leakage current of type A QD-LEDs was significantly suppressed by inserting the N-CD layer (see lower J for type A device with respect to type B ones in the ohmic range below 0.5 V in Fig.21c). Noteworthy, the LMAX observed in LED incorporating CDs outperformed type B LEDs, showing 3500 and 20 cd/m 2 , and the ηc were 0.63 and 0.044 cd/A, respectively (Fig.21d,e). EL of both types of LEDs was dominated by the emission peak at 622 nm under applied voltage > 4 V (Fig.21f) with CIE coordinates of (0.66, 0.33) corresponding to the highly pure red emission (Fig.21g). In a very recently publication Paulo-Mirasol et al. [88] described the use CDs as HTL, but in an inverted LED architecture. They synthetized four N-doped CDs by citric acid as precursor of carbon skeleton and p-phenylenediamine, EDA, urea and HDA as precursor of capping ligand and show that the nature of the capping ligand influences directly the optoelectronic properties of CDs (Tab.2). The architecture of the device consisted of ZnO nanoparticles layer directly deposited on ITO, of an emissive conjugated polymer, the well-known and commercially available poly (9,9-dioctylfluorenealt-benzothiazole or F8BT, and of the CDs (Fig.22a) as HTL. The devices were completed by Au anode.
The LEDs displayed moderate L values, but always clearly superior to the control CD-free device (Fig.22b). In the case of EDA capping, the VON was very low, which indicates the existence of an excellent charge injection capability. On the other hand, devices prepared with urea and HAD CDs showed the highest luminance.  Alam et al. [89] reported the use of CDs synthesized with one step hydrothermal process using banana leaf precursor, as ETL in direct architecture LEDs with polyfluorene derivative as emitting layer (Fig.23a). The introduction of CD as ETL reduces the barrier for electron injection which in turn lowered the VON. The EL spectra showed dominant peaks typical of PFO, but slightly red-shifted compared to literature. Zang et al. [90] demonstrated that stable, abundant, and easily-tosynthesize CDs are suitable as solution processable thin transparent films, serving as the cathode surface modifier in inverted LED architecture. This CD ETL can minimize the charge injection/extraction energy barrier, improve the interface contact property, and smooth the electron transport pathways in various optoelectronic devices such as perovskite solar cells and QD-LEDs. A series of CDs with varying content of either amine or carboxyl groups at their surface were synthesized using varying ratios of common precursors CA and EDA. Thanks to film consisting in a blend of CDs in ZnO, the WF of commonly used ITO substrates was modified over a broad range to become suitable as electron injection electrode in inverted device architecture (Fig.24a,b). Specifically, CD modifiers with abundant amine groups reduced the ITO's WF from 4.64 to 3.42 eV, while those with abundant carboxyl groups increased it to 4.99 eV. Regarding the LED technology, they manufactured inverted architecture CsPbI3 QD-LEDs with EQE increased, thanks to the incorporation of the CD layer, from 4.8% to 10.3% and the LMAX increases from 951 to 1605 cd/m 2 (Fig.24c,e) and that of CdSe/ZnS QD-LEDs increases from 8.1% to 21.9% and ηc increases from 34.7 cd/A at 93.8 cd/A (Fig.24d,f) .
The proposed approach may hold true for CDs with other surface functional groups, which guides us toward more ideal interface materials, and offering chances to lower the production costs of various solution-processed optoelectronic devices with an improved performance. (d) Luminance versus current density of CdSe/ZnS QD LEDs modified with different concentrations of CDs as given on the frame (in mg/mL); the EL spectrum of the LED employing the CDs (concentration 0.03 mg/mL) modifier is given as an inset. (f) EQE and current efficiency versus current density of CdSe/ZnS QD LEDs with or without CDs modifier introduced at different concentrations (in mg/mL) provided on the frame. Reproduced with permission from ref. [90] Copyright 2019 WILEY-VCH Verlag GmbH & Co.

Summary and Outlook
CDs are materials with a great potential from multiple points of view. In fact, despite being relatively young materials, whose discovery is placed between 2004 and 2006, their development has already led to particularly encouraging results in many disciplines including biosensing, photonics and optoelectronics.
The interest in this class of materials is both academic and applicative. In fact, numerous efforts have been made to understand and modulate their chemical and physical properties and nevertheless, although different mechanisms have been highlighted, much remains to be done, especially in reproducibility and in the deep understanding of their size, shape, composition/doping properties dependence.
With regard to their application in OLEDs, as here reported, they have been used both as active layers and as charge regulation layers.
As active layers have been used dispersed in the matrix to avoid aggregation quenching, but also as neat film, leading to increasingly encouraging results mostly when their surface was passivated by hexadecylamine (HDA) agent [71,82].
Recently, CDs have been tested as charge regulating layer (both for holes and for electrons), in either direct or inverted device's architectures, highlighting their great versatility linked to the huge number of possible modifications which, as we have pointed out, are related to composition, shape, size, and surface groups. Importantly, CDs used as ETL in OLED with direct architecture were synthetized with one step hydrothermal process using banana leaf as raw material.
In our opinion, the importance of these materials consists in the possibility of declining them to the concept of safe-by-design which is substantially at the basis of the circular economy (Fig. 1A).
To promote sustainability in the electronics industry, a paradigm shift needs to occur in economic practices from linear to circular. According to the Ellen McArthur Foundation, new electronics must be designed for sustainability from the get-go [91].
Technical performance is not the only driving force for the design but also the use of sustainable precursor (from waste or renewable materials), biodegradability, toxicity, synthetic methodology, production of toxic waste are only some of key parameters to be taken into consideration.
In this perspective, although CDs are currently still very far from the performance of other active materials commonly used in LED and OLED such as metallic quantum dots or organic semiconductors, in a circular perspective they are already able to meet some fundamental requirements.
For this reason, we hope that this review can offer a contribution and be a source of inspiration for the transition towards a "circular organic electronics".