Inkjet printing of heavy-metal-free quantum dots-based devices: a review

Inkjet printing (IJP) has become a versatile, cost-effective technology for fabricating organic and hybrid electronic devices. Heavy-metal-based quantum dots (HM QDs) play a significant role in these inkjet-printed devices due to their excellent optoelectrical properties. Despite their utility, the intrinsic toxicity of HM QDs limits their applications in commercial products. To address this limitation, developing alternative HM-free quantum dots (HMF QDs) that have equivalent optoelectronic properties to HM QD is a promising approach to reduce toxicity and environmental impact. This article comprehensively reviews HMF QD-based devices fabricated using IJP methods. The discussion includes the basics of IJP technology, the formulation of printable HMF QD inks, and solutions to the coffee ring effect. Additionally, this review briefly explores the performance of typical state-of-the-art HMF QDs and cutting-edge characterization techniques for QD inks and printed QD films. The performance of printed devices based on HMF QDs is discussed and compared with those fabricated by other techniques. In the conclusion, the persisting challenges are identified, and perspectives on potential avenues for further progress in this rapidly developing research field are provided.

One fascinating characteristic of QDs is their confinement of electrons and holes in all three dimensions, known as quantum confinement (QC), which occurs strongly when the QD size is smaller than the excitonic Bohr radius.This confinement differs from their bulk semiconductor counterparts.Original content from this work may be used under the terms of the Creative Commons Attribution 4.0 licence.Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.
The emission color varies with the size of the QDs due to shifts in both the conduction band (CB) and valence band (VB) (see figure 2) [19].Moreover, QDs possess high photoluminescence quantum yields (PLQY), narrow emission width (quantified by the full width at half maximum (FWHM)), and nanosecond photoluminescent (PL) lifetime (from a few ns to hundreds of ns) [20].
The ink formulation to transport the functional material to the surface and eliminating the coffee ring effect (CRE) are key to obtaining high quality QD films by IJP methods.QD inks cannot be printed unless the rheological property is adjusted for the printer, which means appropriate solvents need to be selected.Controlling the three-phase contact line and the drying rate at the periphery and apex of the droplet pose challenges to achieving a flat pattern after ink deposition, known as the CRE [21].It can be alleviated by the Marangoni effect (ME) [22], UV curing with the addition of the photoinitiator (PI) [23], electrowetting (EW) [24], and engineering the substrate surface [25,26].
Motivated by the rapid development of the IJP technique and HMF QDs, this review explores the mechanism, printing factors, challenges, and solutions of the IJP technique.Several Figure 1.HMF QDs and inkjet-printed HMF QDs-based optoelectrical devices.InP QD adapted with permission from [200], copyright 2021 ACS.ZnO QD adapted with permission from [142], Copyright 2015 ACS.CQD adapted with permission from [201], copyright 2016 ACS.Cs 3 Sb 2 Br 9 QD adapted with permission from [202], copyright 2017 ACS.GQD reprinted with permission from [16], copyright 2020 Elsevier.CuInS 2 QD adapted from [85].ZnSe QD adapted with permission from [203], copyright 2015 RSC.Si QD adapted with permission from [204], copyright 2012 ACS.QLED reprinted with permission from [11], copyright 2021 ACS.Humidity sensor adapted with permission from [12], copyright 2012 RSC.UV-shield coating adapted with permission from [13], copyright 2020 RSC.Synaptic transistor adapted with permission from [14], copyright 2022 ACS.Photodetector reprinted with permission from [15], copyright 2017 ACS.Anticounterfeit reprinted with permission from [199], copyright 2019 ACS.Color conversion layer reprinted with permission from [17], CC BY 4.0.Solar cell reprinted with permission from [18], CC BY 3.0.characteristic HMF QDs and characterization techniques of QD inks and printed QD films are briefly discussed in the next section.Subsequently, some inkjet-printed optoelectrical devices based on HMF QDs are summarized.Last, we propose the remaining obstacles and offer insights into potential paths to advance in this burgeoning realm of research.

Basics of IJP
The IJP mechanism operates by expelling micro-sized inks from μm-sized nozzles onto a designated substrate under computer control [27].Upon droplet evaporation through heating, distinct patterns are formed.IJP has two standard ejection modes: continuous and drop-on-demand (DOD).The DOD mode, favored for its lower cost and higher ink utilization ratio, prevails over the continuous mode.In the DOD mode, ink droplets are managed using either a piezoelectric transducer or a thermal resistor (see figure 3(a)).IJP offers several advantages over alternative thin film deposition methods, such as spin coating (SC) [28], lithography [29], and vacuum evaporation [30].These benefits include a straightforward process, high automation, cost-effectiveness, and the absence of masks, thereby showcasing its significant potential for industry.However, the volume of ink droplets limits the lateral printing resolution (>10 μm), and IJP instruments cannot currently handle highly viscous inks (>30 mPa s).Although IJP does not claim the top spot among printing methods such as screen printing [31], offset printing [32], and gravure printing [33], it strikes a favorable balance between high resolution and printing speed.

Ink formulation and key parameters
Optimizing ink formulation is crucial for ensuring good printability before printing.Key factors affecting ink formulation include viscosity, surface tension (ST), suspended particle size, solute concentration, substrate wettability, and solvent properties [34].The particle size in the ink should ideally be smaller than 1/100th of the nozzle size.Low viscosity and ST often result in the generation of satellite droplets, while high viscosity and ST can lead to nozzle clogging or lack of control over droplet formation.In general, printability can be assessed by the Ohnesorge number (the Znumber) [35],  [36].In some cases, longer ligands are replaced by shorter ones for carrier transportation, influencing solvent selection.Secondly, solvents should not damage the inner printer cartridge, making strong acids and alkalise unsuitable.Thirdly, these solvents should have low volatility and a high boiling point (ideally >100 °C) to avoid risks of nozzle blockage.Fourthly, solvent viscosity and ST should fall within appropriate ranges to achieve a favorable Z value and avoid the generation of satellites [37].Additionally, compatibility with the substrate or the pre-deposited layer on the substrate is essential (see figure 3(d)) [38,39].Table 1 shows the rheological properties of some solvent systems for printing.Sometimes, a small amount of polymer is added to adjust the ink's viscosity and ST [40,41].Moreover, the substrate's surface energy should surpass that of the ink's ST, resulting in a low contact angle (=90°), which facilitates the spreading of ink droplets after depositing on the substrate.
To ensure the formation of single droplets without satellites, various printing factors, such as jetting waveform, firing frequency, and voltage, need examination.Drop ejection, triggered by the electrical signal controlled by the jetting waveform with multiple segments, can be optimized by adjusting the amplitude and duration of each segment.For less viscose ink, a low firing frequency and voltage are set, resulting in a lower drop velocity and volume.Drop spacing (DS), the center distance between neighboring droplets, affects printed pattern behavior.Increasing DS from 50 to 100 μm changes the contact line from a smooth line to isolated drops (see figure 3(e)) [42].Furthermore, substrate temperature is a crucial factor affecting the uniformity and morphology of the resultant pattern [43].

The CRE problem and the solutions
While IJP offers numerous advantages, it encounters an inevitable challenge known as the CRE.Once a droplet is deposited on the substrate, the three-phase contact line becomes fixed, leading to higher evaporation rate at the edge  compared to the apex.This results in solvent and solute transport to the edge, forming an outermost ring of material [21].
To mitigate the CRE, the ME is introduced to balance the capillary effect.This involves transporting solvents from regions of lower ST to those with higher ST.There are two types of Marangoni flows.The first is the concentration-driven Marangoni flow, achieved by introducing an additional solvent [44][45][46] or surfactant [41,47], such as a binary solvent system.As the low-BP solvent evaporates first at the contact line, a difference in ST between the edge and the center generates an inward Marangoni flow, balancing the outward capillary flow and ultimately achieving a uniform pattern (see figure 4(a)) [48].The strength of the Marangoni flow, u, is proportional to the ST gradient along the liquidvapor surface ( g ∆ ) [49] which dependent on the volume ratio of the two solvents.
A strong Marangoni flow causes a bump at the center of the droplet, and a balance is reached when the ratio of solvents is appropriately adjusted.In addition to the binarysolvent system, a ternary-solvent strategy was proposed for more precise manipulation of the Marangoni flow [50,51].
Another type of Marangoni flow is thermally driven by a nonuniform temperature distribution, exhibiting circulating motion in the droplet.The initial flow direction depends on the thermal conductivity ratio (K R = K s /K l ) between the substrate (K s ) and the liquid (K l ) [52].If K R is greater than 2, indicating an efficient conductor substrate, heat transfers from the contact line to the droplet center because it is warmest at the contact line.Conversely, when K R is less than 1.45, the flow reverses due to the highest evaporation rate at the contact line, and the droplet temperature cannot be maintained without sufficient energy.For 1.45 < K R < 2, the Marangoni flow direction depends on the critical contact angle ( c q ) (see figure 4(b)), with the critical thermal conductivity ratio given by, Heating the substrate introduces a thermal gradient, impacting the flow direction, and therefore the substrate temperature can adjust the strength of the ME [43,53,54].
In addition to introducing the ME, researchers have proposed alternative methods to prevent the occurrence of CRE.Surface engineering of the substrate surface, involving modification in roughness and topography with structures like micro-pillar arrays [25] and porous structures with small pore sizes [26], confines solutes in these structures, making it difficult for them to migrate due to internal flows and resulting in a flat pattern (see figure 4(c)).Surface engineering, although a direct approach, is time-consuming and involves processes like lithography and etching.Lee et al addressed the issue by adding a PI into the ink to crosslink QDs.The QD film polymerized upon UV exposure once the droplets were deposited on the substrate (see figure 4(d)) [23].Some low-boiling-point solvents can be used, and the addition of the PI helps increase the viscosity of the ink.However, proper matching of ligands and PI is crucial; otherwise, polymerization may not occur under UV curing.EW employs alternating current with a frequency between a few Hz and a few tens of kHz to prevent contact line pinning due to timedependent electrostatic forces and internal flows (see figure 4(e)) [24].This method eliminates the need for additives or heat in the system, and the ink does not directly contact the electrode.However, the liquid must be conductive, limiting its application with certain QD inks.

HMF QDs
The optical properties of QDs play a crucial role in the performance of QD-based devices.In this section, we provide a brief summary of the properties, synthesis methods, advantages, and disadvantages, as well as surface modification of several typical HMF QDs commonly used in optoelectronics (see table 2).InP QDs [55], copper indium sulfide (CuInS 2 ) QDs [56], LFP QDs [7], carbon-based QDs [57], Si QDs [10], and ZnO QDs [58] have been comprehensively reviewed by other researchers.QDs are prone to degradation due to various molecular interactions with oxygen and water, such as photoactivation [59], photooxidation [60], and photo-corrosion [61], potentially impacting their optical performance positively or negatively.This physical and chemical adsorption of oxygen and water can significantly influence overall behavior of QDs.Researchers have proposed different strategies to mitigate degradation caused by oxygen, moisture, and illumination, including ion passivation, ligand engineering, and surface encapsulation.

InP QDs
InP QDs feature a large Bohr radius (∼10 nm), high color purity, and share synthesis protocols with Cd-based QDs.Various InP QDs synthesis methods have been developed, including hot injection [62], heat-up [63], seeded growth [64], cation exchange [65], microwave-assisted synthesis [66], and a microfluidic method [67].Due to the susceptibility of In and P to oxidation and photodegradation, ion passivation and shell coatings are introduced to suppress surface state generation and enhance chemical stability.Agents such as fluorides (e.g.HF, NH 4 F) and Lewis acids are used to etch away P dangling bonds and displace In 3+ ions [68,69], mitigating the quenching effect by suppressing electron traps (see figure 5(a)).Encapsulating the core with shell materials, such as ZnS [70], ZnSe [71], ZnSe 1−x S x [72,73], GaP [74], CdSe [75], and ZnO [76], proves more effective in confining electrons and holes, thereby enhancing luminescence and stability.

CuInS 2 (CIS) QDs
CIS QDs exhibit significant absorption efficiency (∼10 5 cm −1 ), a long PL lifetime (>200 ns), and a high tolerance for stoichiometric ratio deviations, resulting in the formation of high-density defect states without altering the crystal shape [77].However, the FWHM is extremely broad (>90 nm), attributed to both size polydispersity and variations in QD donor-acceptor pairs [78].The mainstream mechanism of radiative recombination involves the bonding of a delocalized electron from a donor state with a hole localized in an acceptor level, leading to donor-acceptor pair (DAP) recombination (see figure 5(b)) [77,79].An alternative theory suggests that the Cu + -related trap state serves as the primary luminescent center, while the presence of Cu 2+ counterparts leads to the degradation of excitons [80].Synthesis methods for CIS closely resemble those for InP QDs [81,82].Efficient tuning of the bandgap, FWHM reduction, and improvement in PLQY and stability are achieved by doping metallic cations [83,84].ZnS is extensively used as the shell material, forming a type-I structure due to its small lattice mismatch with CIS core (2%) [85,86].

Perovskite is generally expressed as ABX
), typically forming a cubic phase.PQDs show exceptional tolerance for defects, strong optical absorption, low exciton binding energy, a long carrier diffusion length, and versatile processibility.To reduce the toxicity of PQDs, Pb has been replaced by Group 14 elements (Sn [87] and Ge [88]), Group 15 elements (Sb [89], Bi [90]), and monovalent and trivalent elements (double halide perovskite A 2 BB'X 3 ; B = B + , B' = B 3+ ) [91] to prepare LFP QDs. Figure 5(c) illustrates the structures of single and double halide perovskites [92].LFP QDs can be synthesized through hot injection [87], recrystallization [93], and ligand-assisted reprecipitation [94].The ionic nature of perovskites leads to significant degradation under external environmental conditions, especially for Sn II -based perovskites.Strategies such as doping [95], encapsulation [96], and modification of surface ligands, A cations, and crystal shapes have been employed to enhance PL and stability [97].

Si QDs
Bulk Si transforms into to a direct bandgap material with a significant increment in PL intensity when the particle size is <5 nm [98].The advantages of Si QDs include earth abundance, good biocompatibility, and a long PL lifetime.Si QDs have two types of PL-fast band (F-band) and slow band (Sband) emission, correlated with the QDs size and decay time [99].Synthesis methods for Si QDs include physical approaches such as laser ablation [100] and plasma synthesis [101], as well as chemical methods like electrochemical etching [102], reduction of silicon halides [103], decomposition of Si precursors [104], and oxidation of Zintl salts [105].Particle size, surface groups, and doping significantly impact the PL properties of Si QDs [106][107][108][109].The emission peak and PLQY of Si QDs can be altered by changing the chain length and electronegativity of ligands without varying the Si QDs size (see figure 5(d)) [110][111][112].Unfortunately, shell coating is not effective for Si QDs due to undesirable lattice matching with wide band gap semiconductors.Therefore, ion doping remains the only effective method to tune the carrier concentration and mobility of Si QDs [113,114].

Carbon-based QDs
CQDs were accidentally discovered in single-wall carbon nanotubes [115], featuring a carbongenic core and an sp 3 -hybridized matrix of oxygen/nitrogen-containing surface functional groups [116,117].CQDs possess high electron mobility, a long PL lifetime, water-solubility, strong absorption from UV to IR, and tunable emission without surface passivation.However, the emission wavelength of CQDs is broad (>80 nm) due to the strong coupling of electrons and holes and a wide distribution of particle sizes.Synthesis methods for CQDs are classified into two sections: the topdown method (ultrasonication [118], laser ablation [119], electrochemical oxidation [120], plasma treatment [121]) and the bottom-up method (solvothermal [122], microwavemediated [123], electrochemical carbonization [124]).Doping with heteroatoms enhances the PL by altering surface configuration and electron distribution (see figure 5(e)) [125,126].
GQDs, a subset of CQDs, possess an infinite Bohr exciton radius, an edge effect [127], and a non-zero bandgap [128], distinguishing them from 2D graphene (see figure 5(f)).The edge effect and amphiphilic nature grant outstanding dispersity in various buffers.GQDs are typically synthesized from graphene or graphene oxides using methods similar to CQDs [129].In comparison to CQDs, GQDs show stronger crystallinity and fewer defects owing to a higher content of crystalline sp 2 carbon, resulting in a higher PLQY.Despite this difference, they have many similar optical properties related to absorption, fluorescence, and up-conversion PL.The PL mechanism of GQDs involves the QDs regulated by the number of aromatic rings and surface-related states associated with charge transfer occurring between the sp 2 carbon network and surface states.Factors such as preparation methods, size and shape, doping heteroatoms, and passivation influence the PLQY, emission, stability, and PL lifetime of GQDs [130][131][132].

ZnO QDs
ZnO QDs exhibit a very small exciton Bohr radius of ∼0.9 nm [133], with nanoparticles (NPs) smaller than 3.6 nm regarded as ZnO QDs due to their strong QC effect [134].The PL of ZnO QDs originates from near-band-edge emission in the UV region and the recombination of acceptor/donerbound excitons in the visible region, differing from bulk ZnO due to numerous surface-related defects (zinc interstitials, oxygen interstitials, oxygen vacancies, zinc vacancies, and their complexes) [135][136][137].ZnO QDs can be synthesized through sol-gel [138], ultrasonic [139], chemical vapor deposition (CVD) [139], and radio frequency (RF)-based methods [140].ZnO QDs synthesized via the sol-gel method are prone to quenching in water, leading to the introduction of ligands to improve the PL and stability of ZnO QDs in water [141,142].Doping ZnO QDs is more complex than with other QDs, as it limits nanocrystal growth and further reduces size.Moreover, most doping occurs at the surface rather than in the core.Despite these challenges, successful doping of rare earth metals and transition metals in ZnO QDs has been achieved [143,144] (see figure 5(g)).

QD inks
The rheological properties of QD inks must be tested to ensure printability before actual printing.The flow through the printhead nozzle is significantly influenced by the ink's viscosity, specifically shear viscosity, which is a crucial parameter.Due to the combination of high flow rates and small nozzle diameters, the ink operates within a regime characterized by high shear rates (10 3 −10 4 s −1 ) [145].Viscosity also plays a crucial role in the ink-filling process within the printhead ink chamber.While some inks exhibit 'Newtonian' behavior, where viscosity remains constant with applied shear rate, the introduction of additives such as polymers or other materials can alter this property, leading to 'non-Newtonian' behavior, where viscosity is no longer consistent with the shear rate (see figure 6(a)) [146].Notably, shear rates are rarely provided in reports.ST influences droplet formation, the occurrence of satellites, and interactions with the substrate.ST measurements are conducted using force or optical tensiometers.The optical tensiometry, also known as the pendant drop method, involves capturing the silhouette of an axisymmetric droplet and iteratively applying the Young-Laplace equation to find the shape factor and calculate ST [147].Viscosity and ST of QD inks slightly decrease with increasing temperature.CA serves as an indicator of a liquid's capacity to wet a solid surface.The configuration of a sessile droplet on a surface depends on both the ST of the fluid and the characteristics of the surface (see figure 6(b)) [148].The surface is considered wettable and hydrophilic if CA <90°, which is essential for printing.It is necessary to check the stability of QD inks over time as QDs can aggregate and degrade.Unfortunately, many reports lack information on this aspect.Optical-based techniques like UVvis spectroscopy and fluorescence spectroscopy are employed to monitor turbidity, fluorescent peak, and PLQY of QD inks (see figure 6(c)) [50].
The process of evaporation for a liquid droplet on a surface is intricate, involving particle-particle, particle-substrate, particle-flow, and particle-interface interactions.Observing the evolution and lifetime of QD droplets facilitates understanding of the CRE.Evaporation modes are classified into constant radius mode [149], constant angle mode [150], stick-slide mode [151], and stick jump mode [152].The contact radius and CA can be calculated using equations summarized by Wilson et al [153].Theoretical studies on nanofluid droplet drying typically focus on two key aspects: the dynamics of evaporation in and around a sessile droplet, and the transportation or deposition patterns of NPs.Methodologies include approaches based on the Navier-Stokes equations [21,154], diffusion-limited aggregation [155], dynamic density function theory [156], and the kinetic Monte Carlo methods [157].Evaporation conditions of droplets (relative humidity [158], substrate temperature (see figure 6(d)) [43], binary system (see figure 6(e)) [44], droplet array (see figure 6(f)) [159] and physical properties of solutes (NPs size (see figure 6(g)) [160], shape [161], and concentration [162]) impact the CRE.Despite achievements, most studies were carried out on marco-sized droplets with a single solvent and large particles (size >100 nm).Future efforts should focus on experimental and modeling studies of the drying of single picolitre-sized QD droplets with binary or ternary systems and QD droplet arrays with various configurations.

QD films
There are several techniques to characterize the CRE and assess the thickness, order, and uniformity of thin QD films after printing.Fluorescence microscopy (FM) provides a straightforward way to observe the CRE at the microscale (see figure 7(a)).A uniform pattern without coffee rings emits consistent light intensity, avoiding dark spots at the center.High-resolution 3D profilometry (see figure 7(a)) and atomic force microscopy (AFM, see figure 7(b)) offer insights into topography and morphology.Cross-section transmission electron microscopy (TEM) allows each layer, in a multilayered structure composed of different materials, to be directly distinguished (see figure 7(c)).Ellipsometry is an optional method to determine thickness, optical properties, and roughness by fitting the amplitude and phase of light with different wavelengths.The accuracy of fitting results depends on the accuracy of created model [163].The PLQY of QD films is crucial for devices like QLED because QDs are closepacked in the device rather than in the form of colloidal QDs.However, QD films typically show much lower PLQY than their colloidal counterparts due to non-radiative Foster resonance energy transfer (FRET, see table 3) [164].The stability test of QD films is rarely reported.Small angle (<10°) and wide-angle x-ray scattering (SAXS/WAXS) are employed to study the ordering type and orientation of the assembled structure on a millimeter scale, revealing the growth kinetics of QDs superlattices (see figure 7(d)) [165].Structural changes resulting from chemical reactions, such as ligand exchange, can be monitored in real-time during the drying process.X-ray diffraction assesses the crystallographic structure and chemical composition of QD films, while x-ray photoelectron spectroscopy determines the binding states of the elements and elemental composition, analyzing the products generated from the interaction between QDs and oxygen and water.Moreover, novel techniques may be employed in the future, such as the integration of FM and AFM and fast detection of QD film's uniformity on the macroscale.

QLEDs
QLEDs are considered to be the next generation of self-emissive displays, offering a longer lifespan, faster response time (ns), lower power consumption, and higher viewing angle compared to LCD and OLED devices [166].The structure of a QLED device is similar to the OLED, consisting of a substrate, anode, hole injection layer (HIL), hole transport layer (HTL), emissive layer (EL), electron transport layer (ETL), electron injection layer (EIL), and cathode (see figure 8(a)).
Common techniques for assembling QLED devices, such as SC, photolithography, and thermal evaporation, are conducted in low-oxygen and low-moisture conditions to avoid QD degradation.In contrast, IJP offers advantages such as reduced material usage, high-resolution pattern design without a mask, and no need for post-structuring like wiping or laser scribing.Currently, most environmentally friendly, inkjet-printed QLED devices use InP QDs, a field that is still in its infancy (see table 4).Efforts have been made to improve the performance of inkjet-printed InP QLED, such as the PL stability, current leakage, and injection balance between holes and electrons.Firstly, suppressing FRET between closedpacked QDs by engineering the thickness of the ZnS shell [167].The PLQY of the InP/ZnS/ZnS QD film reached 58%, which was twice the value of the InP/ZnS QD film (see figure 8(b)).The printed blue InP QLED showed a maximum luminance and EQE of 91 cd m −2 and 0.15% (see figure 8(c)).Secondly, the inter-QD spacing was reduced by PI-mediated cross-linkage between InP QDs [23].The cross-linked QD film with PI was more stable than without PI due to the homogenous QD surface, sustaining the original luminescence for 4000 s (see figure 8(d)).Additionally, the PI was mixed with ZnMgO NPs to reduce the current leakage from the ETL and Auger recombination (AR) in the high-J regime, achieving high luminescence of 2540 cd m −2 for the printed QLED (see figure 8(e)).Thirdly, the device structure design was optimized to enhance light extraction efficiency.For example, Bai et al achieved a red inkjet-printed InP QLED with the highest EQE of 8.1% by nanoimprinting periodic ZnO microlens arrays on the glass substrate (see figure 8(f)) [168].The arrays reduced the total reflection when the angle of incidence of emitted light at the interface between glass and air exceeds the critical angle.Efficiency improvements can be made by improved balancing of the injection of holes and electrons, typically by modifying the HTL.TFB is the most popular HTL due to its higher hole mobility than other HTLs; however, it is not compatible with the commonly used ink solvent-CHB and causes parasitic emission [169].Zhan et al tailored the HTL by sequentially SC a layer of TFB, chlorobenzene, and PVK, suppressing ink erosion and maintaining hole transport (see figures 8(g), (h)) [54].For commercialization, Park and coworkers inkjet-printed four soluble layers with subpixel size of 120 μm × 40 μm and successfully fabricated a large-area active-matrix QLED device with a resolution of 217 ppi (see figures 8(i), (j)) [11].
IJP is typically conducted in an ambient atmosphere rather than an inert atmosphere, and the weak covalent bond between In and P makes InP QDs less stable in ambient conditions compared to HM QDs [64,170].The relatively low viscosity of QD dispersion poses a challenge in printing, preventing effective QD packing and accelerating the trapping of O 2 /H 2 O during manufacturing.This leads to the formation of inhomogeneous and highly void films.Currently, the performance of inkjet-printed environmentally friendly QLEDs remains low due to issues such as FRET, less uniform QD film, degradation during air printing, and a significant drop in the PLQY of QD films [45,50].Efforts should be made in the following aspects to improve the performance and pave the way for commercialization.First, improving the stability of HMF QDs in air or creating an inert atmosphere for printing.Second, exploring other candidates with excellent PL and air stability to replace Cd/Pb/Hgbased QDs.Third, finding ways to suppress AR [171], FRET [172], and field-induced quenching [173].Last, exploring new carrier injection/transport materials for a more balanced injection of holes and electrons.
To date, most inkjet-printed PVs are organic PVs (OPVs).OPVs are potential candidates for achieving largescale fabrication because of the freedom of shaping, semitransparency, lightweight, flexibility, and high PCE [179].Modular shapes and sizes remain obstacles to the integration and application of OPVs.Roll-to-roll (R2R) technology has been developed to manufacture low-cost and efficient OPVs, but several deposition techniques and structuring are needed after that, increasing the investments of relevant equipment.IJP avoids these post-processes and fabricates products from prototypes to medium-scale with the advantages of DOD and freedom of design.The basic structure of PV device is similar to that of the QLED device, and ZnO QDs are also unitized as the ETL due to their excellent electron mobility [180].Jung et al reported the all-inkjet-printed, all-air-processed OPV cells (OPVCs) with the structure of PEDOT:PSS/PCDTBT: PCBM/ZnO/Ag for the first time (see figure 9(c)) [181].They formulated the PCDTBT:PC 70 BM ink in a ternary solvent rather than a single solvent to obtain a homogenous film (see figure 9(d)).The OPVs printed by using the ternary solvent mixed with chlorobenzene, mesitylene, and chloroform with the volume ratio of 5:4:1 exhibited the optimal performance (see figure 9(e)).To avoid the usage of halogenated solvents, Eggenhuisen et al employed veratrole and o-xylene as the solvent of the ink [18].The fully inkjetprinted inverted OPVC showed a PCE of 1.7%, which was 23% lower than the spin-coated counterpart due to the longtime printing at ambient atmosphere (see figures 9(f), (g)).In the same year, they reported a printed large-scale OPV module with an active area of 92 cm 2 and PCE of 0.98% (see figure 9(h)) [182].The printed photoactive layer just showed a minor performance loss when compared with the spincoated layer from chlorobenzene.
However, inkjet-printed HMF QDs-based PVs are scarce, with most printed PVs being OPVs, and the majority of HMF QDs-based PVs are fabricated via SC (see table 5).Future work should prioritize the following aspects.Firstly, develop and optimize new ink formulations by selecting an appropriate solvent system, a topic rarely addressed but welldocumented in the context of printed QLEDs.Secondly, address issues related to the formation and stability of LFP QD films, with particular attention to the quenching step postprinting.Thirdly, explore substitutes for unstable and expensive materials in future inkjet-printed PV modules.For example, ITO, a brittle and costly electrode, increases the overall cost of PV modules and hinders their application in flexible PVs.

PDs
PDs transform optical signals into electrical signals, which are crucial in light detection and optical communication.QDs are emerging as alternatives to traditional bulk materials like Si [183], GaN [184], and perovskite films [185], with the aim of reducing production costs.QDs-based PDs can be categorized into UV PD, visible PD, and infrared PD based on the bandgap of QDs.Recent advancements in HM QDs-based PDs have enhanced performance through ligand exchange, structure design, and new preparation methods (see table 6).While there are only a few examples of inkjet-printed ZnO QDs-based PDs, this is due to the ZnO QDs' stability in air.Ink formulation and substrate temperature influence film smoothness (see figure 10(a)) [186].IJP with QDs incorporation has shown improved PD performance.Cook et al printed the photoconductive PD using the ZnO precursor (ZnOPr) and a mixture of ZnOPr and ZnO QDs (ZnOPrQDs) inks, respectively (see figure 10(b)) [15].The ZnOPrQDsbased PD showed a higher R (383.6A W −1 ) and on/off ratio (2470) than the ZnOPr-based counterpart (14.7 A W −1 and 949), which was ascribed to the nanoporous structure with improved crystallinity and surface-to-volume ratio.However, the increased surface-to-volume ratio also increased the response time (see figure 10(c)).Despite comparable performance to counterparts assembled by CVD [187], RF sputtering [188], and SC [189], the ZnO QDs-based UV PDs still lag behind state-of-the-art analogs [190,191].
Improving device performance through the combination of other functional materials is an efficient strategy.Gong et al fabricated ZnO QD/graphene heterojunction PDs that combined strong QC and high charge mobility by printing ZnO QDs on the graphene FET (GFET) [191].The resultant PD exhibited a high photoresponsivity (R) of 9.9 × 10 8 A W −1 , a photoconductive gain of 3.6 × 10 9 , and UV detectivity of >10 15 Jones, which was attributed to a clean van der Waals interface between ZnO QDs and graphene by removing zinc acetate on ZnO QDs (see figure 10(d)).This interface facilitates efficient exciton dissociation and enables effective charge transfer across the ZnO QD/GFET heterojunction upon UV illumination.In addition, the textured ZnO QDs film by IJP is beneficial to form another homogenous WO 3 film on the top.The printed WO 3 film was more homogenous as it printed at a higher temperature and showed a higher R [192].Printing QD on flexible substrates enables the fabrication of soft and transparent PDs (see figure 10(e)) [193].The reliable performance under multiple bending cycles provides a potential application

Anticounterfeit tags
Anti-counterfeiting techniques that are difficult to duplicate yet easy to implement are therefore highly desired in medicine and food packaging.Fluorescent materials, including supramolecular NPs [194], polymer dots [195], and QDs [196] have been employed due to their concealability and ease of use.However, current fluorescent anti-counterfeiting methods often rely on a single emission, and the materials used can be expensive, toxic, or prone to photobleaching.
QDs offer size-dependent emission.Carbon-based QDs are an alternative material for preparing aqueous ink and printing.CQD inks exhibit consistent emission characteristics in their steady state, yet they possess unique and clearly distinguishable fluorescence lifetimes.This feature enables the exclusive use of fluorescence lifetime imaging for authenticating security tags [197].The fluorescence of CQD ink can be tuned by surface modification and controlling the aggregation [198].Chen's group prepared full-color-range GQD inks dispersed in glycerol-containing ethanol by mixing different ratios of RGB GQDs, providing more information [16].The inkjet-printed patterns can be encrypted by applying different irradiations, showing various colors under UV light but appearing colorless under natural light (see figures 11(a), (b)).The printed GQDs patterns did not blench under UV exposure within 3 h, indicating greater stability than organic fluorophores-based ink.Tan et al integrated lanthanide-doped NaYF 4 up-conversion NPs (UCNPs) and CQDs into mesoporous silica to synthesize dual-mode luminescent UCNPs@CDs@mSiO 2 nanohybrid inks that demonstrated UC and down-conversion luminescence under a 980 nm laser and 365 nm UV light respectively [199].Then they decrypted the information of a standard Code-93 barcode that was printed on a medicine box based on the width of fluorescent bars and spaces in the data region, i.e. 'HUT', 'UHV', and '141' from up to bottom (see figure 11(c)).Water-soluble ZnO QDs are a promising anticounterfeiting material as well.The stability of aqueous ZnO ink can be enhanced by adding additives like polyvinylpyrrolidone [146].
The fluorescent ink's role in anti-counterfeiting primarily manifests in two aspects.First, it reveals colored fluorescence when exposed to ultraviolet light excitation.Second, it involves fluorescence quenching and recovery under external conditions.Both approaches center on the characteristics of the fluorescent dyes, neglecting to address the inherent anticopy functionality of the anti-counterfeiting method itself.Addressing inherent functionality is crucial for practical applications in food and drug packaging.Improving the fluorescent properties and stability of HMF QD ink, such as modifying QDs surface and adding polymers, is essential.Stability considerations include maintaining sustained fluorescence intensity and resistance to water or other chemicals under ambient conditions.Additionally, printing more advanced tags/patterns is necessary to counter sophisticated counterfeiting attempts.

Summary and outlook
In conclusion, this review explores the innovative use of IJP technology for fabricating devices based on HMF QDs.IJP's scalability, low cost, and high resolution make large-scale manufacturing feasible, potentially leading to the commercialization and widespread adoption of HMF QDs-based devices in optoelectronics and biomedical applications.By addressing the environmental concerns and leveraging the advantages of IJP, it contributes to the ongoing quest for efficient, safe, and environmentally sustainable optoelectronic technologies.Optimizing ink formulations with suitable solvent systems and additives is crucial to achieve good printability and homogenous film.Studies of the experimental and theoretical evaporation of QD droplets shed light on the CRE and QDs assembly.Introducing ME and additives, engineering the substrate surface, and using EW are effective ways to achieve uniform patterns without coffee rings.The performance of HMF QDs-based devices depends on reliable HMF QDs with excellent optical properties.Engineering the composition and surface of HMF QDs through doping, ion passivation, shell coating, and ligand exchange can further enhance their optical properties.
While alternatives to HMF QDs and inkjet-printed HMF QDs-based devices have been explored, their performance still lags behind devices assembled by other methods like SC, limiting practical applications.Several persisting challenges have not been fully addressed.First of all, synthesizing HMF QDs is expensive and time-consuming, and most of them are dispersed in organic solvents, which are harmful to the environment and human health and restrict the solvent selection when considering the compatibility of the cartridge.
Secondly, the experimental and simulation of real-time evaporation of single picolitre-sized QD droplets in binary solvent systems under different substrate temperatures are not well studied, which is important to understand how the QDs are assembled during drying.Thirdly, techniques to examine the uniformity of printed large-scale patterns rapidly and accurately are lacking.Finally, the efficiency and lifetime of printed HMF QDs-based devices need improvement due to stability issues of HMF QDs in air atmosphere.
Therefore, future research should focus on the following aspects to match or surpass the capabilities of traditional HMQDs based counterparts.Firstly, seek eco-friendly and air-stable alternatives with outstanding performance and develop new recipes for low-cost large production by machine learning.Investigate aging behaviors and defectrelated luminance mechanisms of HMF QDs and develop new strategies for improving their stability and PL under various external conditions (e.g.UV light, heat, moisture, and oxygen).Secondly, developing water-based QD inks for greener printing.Printing in an inert atmosphere can be an optional method to prevent QDs degradation.Additionally, combine IJP with other technologies like R2R on curved or arbitrary surfaces.Thirdly, propose new techniques to characterize the in situ drying of QD droplets and the quality of printed macroscale QD films.Lastly, enhance the performance and lifetime of HMF QD-based devices by redesigning the device architecture and developing new functional materials.For example, explore flexible HMF QD-based devices for lightweight and wearable applications, and balance the injection of holes and electrons by developing new carrier transport/injection layers for displays and PVs.

Figure 3
Figure 3(b)  illustrates the correlation between printability and the associated numerical values, with the optimal Z value falling within the range of 1-14.This condition is sufficient but not strictly necessary.Regarding solvent selection, specific requirements must be met.Firstly, solvents should allow QDs to form a stable dispersion without compromising their PLQY (see figure3(c))[36].In some cases, longer ligands are replaced by shorter ones for carrier transportation, influencing solvent selection.Secondly, solvents should not damage the inner printer cartridge, making strong acids and alkalise unsuitable.Thirdly, these solvents should have low volatility and a high boiling point (ideally >100 °C) to avoid risks of nozzle blockage.Fourthly, solvent viscosity and ST should fall within appropriate ranges to achieve a favorable Z value and avoid the generation of satellites[37].Additionally, compatibility with the substrate or the pre-deposited layer on the substrate is essential (see figure3(d))[38,39].Table1shows the rheological properties of some solvent systems for printing.Sometimes, a small amount of polymer is added to adjust the ink's viscosity and ST[40,41].Moreover, the substrate's surface energy should surpass that of the ink's ST, resulting in a low contact angle (=90°), which facilitates the spreading of ink droplets after depositing on the substrate.To ensure the formation of single droplets without satellites, various printing factors, such as jetting waveform, firing frequency, and voltage, need examination.Drop ejection, triggered by the electrical signal controlled by the jetting waveform with multiple segments, can be optimized by adjusting the amplitude and duration of each segment.For less viscose ink, a low firing frequency and voltage are set, resulting in a lower drop velocity and volume.Drop spacing

Figure 2 .
Figure 2. Size-dependent emission and corresponding energy level diagram of QDs.The top photo is adapted with permission from [200].Copyright 2021 ACS.

Figure 4 .
Figure 4. (a) The evaporation process of a binary solvent system with 75 vol.%chlorobenzene and 25 vol.%dodecane, leading to the selfassembly of molecules.Reprinted with permission from [48].Copyright 2008 Wiley.(b) The thermal Marangoni flow direction is determined by K R .The temperature increases in the direction of the arrows outside the half droplets.Reprinted with permission from [52].Copyright 2007 APS.(c) The droplet evaporates on the silicon surface with micropillar arrays.Reprinted with permission from [25].Copyright 2012 RSC.(d) Cross-linking PI and surface ligands of QDs to prepare a densely packed QD film through UV curing.Adapted from [23].CC BY 3.0.(e) A schematic of the droplet drying process with and without EW.Adapted with permission from [24].Copyright 2011 RSC.

Figure 5 .
Figure 5. (a) PL spectra and PL photographs of InP core before and after HF treatment.Adapted with permission from [64].Copyright 2019 Springer Nature.(b) DAP recombination of CIS QDs.The PL quenches with increasing the concentration of DAP defect states.Reprinted with permission from [77].Copyright 2012 Wiley.(c) The schematic structures of single and double halide perovskites.Adapted with permission from [92].Copyright 2018 RSC.(d) Altering the PLQY of Si QDs by employing various capping ligands with varying linear aliphatic chain lengths, where N represents the number of carbon atoms in the capping ligand.Reprinted from [112].CC BY 3.0.(e) PL spectra of Cl-doped CQDs at varied excitation wavelengths.Reprinted with permission from [126].Copyright 2020 ACS.(f) UV-vis spectra of GQDs with the number of carbon atoms of 168 (1), 132 (2), and 170 (3).The inset shows the structures of the three GQDs.Adapted with permission from [128].Copyright 2010 ACS.(g) The photocatalytic degradation of phenol by ZnO QDs with and without doping Tb, Er, La, and Eu.Reprinted with permission from [144].Copyright 2019 Elsevier.

Figure 6 .
Figure 6.(a) The dynamic viscosity of ZnO QD aqueous ink with polyvinylpyrrolidone. Reprinted from [146].CC BY 4.0.(b) The CA of the QD ink on glass and the bank with no surface treatment, UV exposure of 20 min, and octadecyltrichlorosilane (OTS) treatment of 30 s. Reprinted with permission from [148].Copyright 2020 Elsevier.(c) The stability of perovskite QD inks using binary and ternary solvents.Adapted with permission from [50].Copyright 2022 Wiley.(d) PL images of the MAPbBr 3 /PVA hybrid ink printed at substrate temperatures of 30 °C-60 °C.Reprinted with permission from [43].Copyright 2021 Wiley.(e) The pattern shape versus the volume ratio of the more volatile solvent, quantified by the ratio of the radius of the flatten section of the drop (R flat ) to the radius of the drop (R).Reprinted with permission from [44].Copyright 2021 APS.(f) The experimental and theoretical volume evolutions of droplet arrays with different configurations.Reprinted with permission from [159].Copyright 2023 APS.(g) The top view and side view of dried nanofluids with 2-nm Au particles, 11-nm Al 2 O 3 particles, and 30-nm CuO particles.Reprinted with permission from [160].Copyright 2007 ACS.

Figure 9 .
Figure 9. (a) A schematic of printed Si QD patterns on multi-crystalline Si PVs.(b) Efficiency and open-circuit voltage of Si QD-coated PVCs as a function of DS.Figures 9(a), (b) reprinted with permission from [175].Copyright 2012 ACS.(c) The device structure of an allinkjet-printed OPVC.(d) Microscopy images of inkjet-printed PCDTBT:PC 70 BM films using inks with chlorobenzene and ternary solvents.(e) J-V curves of inkjet-printed OPVCs with different volume ratios of ternary solvents.Figures 9(c)-(e) reprinted with permission from [181].Copyright 2014 Wiley.(f) The assembling process of a fully inkjet-printed OPVC.(g) The PCE of OPVCs with multiple printed layers.Figures 9(f), (g) reprinted from [18].CC BY 3.0.(h) A photo (left) and schematic (right) of a printed OPV module with 48-unit cells.Reprinted with permission from [182].Copyright 2015 Elsevier.

Figure 10 .
Figure 10.(a) 3D AFM images of PbS QD films printed by different solvent types and substrate temperatures.Adapted with permission from [186].Copyright 2019 ACS.(b) A schematic of IJP ZnOPr and ZnOPrQDs inks.(c) The dynamic response of PDs printed from ZnOPr and ZnOPrQDs inks.Figures 10(b), (c) reprinted with permission from [15].Copyright 2017 ACS.(d) The photoresponsivity (solid line) and gain (dashed line) vary with the 340 nm UV light intensity at different V sd values of 1, 5, and 10 V, respectively.Reprinted with permission from [191].Copyright 2017 ACS.(e) A photo of flexible PDs fabricated by IJP.(f) The fatigue test of flexible printed PDs under bending.Figures 10(e), (f) reprinted with permission from [193].Copyright 2017 RSC.
in monitoring UV exposure on human skin (see figure 10(f)).However, the airtight polymer film may cause inflammation, and the lifetime of flexible PDs has not been well studied.While significant improvements in the performance of spin-coated photodetectors based on HM QDs have been achieved, eco-friendly QDs-based photodetectors by IJP are rarely reported.The suggestions are as follows: (1) improve electron mobility and stability of HMF QDs, especially LFP QDs, through surface engineering.(2) Develop printable HMF QD inks akin to those used in QLED, optimizing jetting conditions for homogenous QD film formation.(3) Enhance the performance of printed PDs by optimizing device structures and incorporating function materials into the ink.

Table 1 .
A summary of physical properties of QD ink formulations.

Table 2 .
The optical performance of state-of-the-art HMF QDs.

Table 3 .
The PLQY of state-of-the-art QD dispersions and films.

Table 6 .
A performance summary of HMF QDs-based PDs.