Scalable Bilayer MoS2‐Based Vertically Inverted p–i–n Light‐Emitting Diodes

Herein, a vertically inverted p–i–n architecture of light‐emitting diodes (LEDs) is designed for manufacturing feasibility and demonstrated scalable bilayer MoS2‐based LEDs. A 4 inch scale bilayer MoS2 is prepared by a two‐step growth method allocating the pre‐deposition of a few‐nm thick metal film and post‐sulfurization. To apply bilayer MoS2 for an active layer in LEDs, the film is transferred over ZnO nanoparticle layers, an electron transfer layer, and then the rest of the LED components are constructed by thermal deposition. This vertically inverted LED architecture allows individual organic or inorganic components to incorporate without degradation during the wet‐transfer process and transfer electron or hole carriers across separate layers, resulting in efficient radiative recombination in the MoS2 emitting layer. MoS2‐based LEDs exhibit electroluminescence of ≈5.41 cd m−2 throughout four active areas of 6.25 mm2 at a driving voltage of 7 V. Therefore, this achievement can overcome the drawbacks of existing transition metal dichalcogenides (TMDs)‐based optoelectrical applications and extend its potential in various fields, such as flexible, ultrathin, or transparent displays.


Introduction
3][4] DOI: 10.1002/admi.202300319   Accordingly, extensive research on TMDs has been vigorously studied in various fields, including electronic devices, smart sensors, flexible circuits, and optoelectronic applications.[9] The 2D nature of TMDs, in addition, facilitates the significantly enhanced Coulomb interaction attributed to quantum confinement.This allows exciton which is a neutral quasiparticle of electron and hole attached by Coulomb force to exhibit considerable binding energy, [10][11][12][13][14] resulting in the feasible observation of exciton in TMDs at room temperatures.
From a geometry perspective, TMD-based LEDs could be mainly allocated into a p-n lateral junction and vertical stacking architectures.The p-n junction formation in TMDs can be carried out by many approaches, such as tailoring the energy structures in a plane or forming p-and n-TMD heterostructures.J. S. Ross et al. formed lateral p-n junctions in monolayer WSe 2 using a boron nitride thin film with underlying splitting gates to demonstrate electrically tunable LEDs. [15]Y. J. Zhang et al. applied ambipolar WSe 2 transistors for tunable p-i-n LEDs emitting circularly polarized electroluminescence (EL). [16]These approaches possibly achieved detectable EL with acceptable optoelectronic performances.However, their emitting efficiencies are relatively low, attributed to 1D line junctions.To improve LED efficiency, meanwhile, R. Cheng et al. produced a wide active layer of p-type WSe 2 and n-type MoS 2 vertical junctions and observed a significant EL signal with high efficiency. [17]However, EL was promoted in a region near electrodes out of the active layers.This indicates that electron and hole carriers are hardly injected through the lateral contacts with electrodes, addressing that it is inevitable for scalable LED demonstration of the current injection layers wrapping up the active area.
In this regard, many studies on vertical stacking TMD-based LEDs have been proposed to achieve high efficiency and largearea emission.One of the representative strategies is constructing van der Waals heterojunctions of metallic-insulatingsemiconducting (MIS) TMDs.F. Withers et al. assembled single and multi-quantum wells of graphene/hBN/MoS 2 (or WS 2 ) as an essential chunk to realize an extrinsic quantum efficiency of ≈10% and uniform EL from tens of μm 2 -scale active area. [18]S. Wang et al. also demonstrated MIS van der Waals heterojunction-based WS 2 LEDs by efficient hole carrier injection attributed to Fowler-Nordhein tunneling. [19]However, these studies mainly fabricated devices by mechanical exfoliation or manual transfer of individual layers.This is unfavorable for mass-production and scalable LED markets, hindering further optoelectronic applications as a bottleneck.
Herein, we designed an inverted p-i-n architecture to be compatible with scalable bilayer MoS 2 grown by a two-step growth method [20] and transferred by a concise wet transfer [21]

Results and Discussion
Figure 1a depicts an inverted p-i-n architecture of scalable bilayer MoS 2 LEDs.Indium thin oxide (ITO) and zinc oxide nanoparticles (ZnO NPs) are used as a cathode electrode and an electron transport layer (ETL) by thermal deposition and spin coating, respectively.Scalable bilayer MoS 2 film is transferred above the ZnO NPs layer and functions as the active layer (or emitting layer (EL)) with atomic thickness.Then, 4,4′-Bis(Ncarbazolyl)-1,1′-biphenyl (CBP), MoO 3 , and Al are employed as a hole transport layer, a hole injection layer, and an anode electrode.ITO electrodes are generally utilized as an anode to inject and transfer hole carriers due to their high work function. [22]owever, in this study, scalable bilayer MoS 2 is transferred with a poly(methyl methacrylate) (PMMA) supporting layer, so the PMMA should be removed through acetone rinse to obtain a clean interface between MoS 2 and adjacent carrier injection layers.Thus, instead of organic carrier injection layers that easily dissolve in acetone, ZnO NP-based ETL is formed above ITO.
ITO electrodes are utilized for the cathodes for manufacturing feasibility.This unique inverted p-i-n architecture allows scalable LED demonstration based on not only MoS 2 , but also other grown TMDs.
Figure 1b illustrates the energy level diagram of scalable bilayer MoS 2 LEDs.For a better visibility, band bending or possible interaction between each component wasn't marked in this energy band diagram.The work function (WF) of electrodes is 4.7 eV for ITO [22] and 4.3 eV for Al, [23] respectively.In ZnO NPs, the valence band maximum and conduction band minimum are 7.6 and 4.0 eV, respectively.In bilayer MoS 2 , the conduction band minimum and valence band maximum are 4.5 and 6.4 eV, along with an energy bandgap of 1.9 eV.In CBP, HOMO and LUMO are 6.0 and 2.9 eV. [24]Besides, the WF of MoO 3 is ≈5.5 eV. [25]Electron carriers readily inject from the ZnO NPs layer, and hole carriers efficiently migrate from the CBP layer, preferentially promoting the radiative recombination of the electron and hole carriers accumulated in MoS 2 active layers.The detailed carrier injection and transport mechanism in the inverted p-i-n LED will be addressed later.
For the LED array formation, bilayer MoS 2 should be prepared in a large area with high uniformity and quality.Figure 2a illustrates the two-step growth method of MoS 2 film. [20]A 1 nm thick Mo film is deposited on a clean Si/SiO 2 substrate by an RF sputter with a power of 150 W for 220 s under an Ar flow of 100 sccm and sulfurized at 950 °C for 15 min by a gas-phase precursor, H 2 S. For further understanding of as-synthesized MoS 2 , TEM and Raman analyses are performed.As shown in Figure 2a inset, two (or partially three) atomic layers coalesce into the films.A plane-viewed TEM image of MoS 2 shows a coalescence of three MoS 2 grains in nm scale, colored with red, green, and yellow corresponding to each FFT pattern (Figure 2b).In the green region, we cropped the part and extracted the inverted FFT pattern to confirm Mo and S alignment along the armchair direction. [26]igure 2d presents line profiles of Mo and S atomic alignments along the white line in Figure 2c.This indicates highly ordered and well-aligned Mo─S atomic layers inside a particular grain.
Figure 2e,f shows the Raman spectrum measured under 532 nm laser irradiation and an actual image of 4 inch MoS 2 film.Two peaks emerge at 406.01 and 384.80 cm −1 , corresponding to the A 1g and E 1 2g modes of bilayer MoS 2 with a difference of 21.21 cm −1 . [27]Moreover, the 4 inch MoS 2 film is divided into 100 pixels and extracted its Raman modes to evaluate its uniformity.2g modes are highly consistent and uniform for all 100 pixels.This suggests that the entire films are significantly uniform, and the number of layers is precisely controlled on the atomic scales.
X-ray photoelectron spectroscopy (XPS) and PL analysis were carried out to characterize scalable MoS 2 film.Figure 3a,b shows the Mo 3d and the S 2p spectra obtained from XPS analysis.In Mo 3d spectrum, intense doublets appear at 229.5 and 232.6 eV, corresponding to Mo 4+ 3d 5/2 and 3d 3/2 of MoS 2 . [28]A shallow peak at 226.6 eV corresponds to S 2s of MoS 2 .Besides, doublets in the S 2p core level emerge at 162.3 and 163.5 eV, indicating the S 2− 2p 3/2 and 2p 1/2 of MoS 2 , respectively.It is in great accordance with the Raman results.Figure 3c shows a PL peak position map of scalable MoS 2 film with a scale bar of 500 μm.The entire film shows almost analogous PL peaks at ≈1.87 eV.Some minor PL peak variations are attributed to the monolayer or multilayer MoS 2 occasionally formed on regions.The mixed layers in one film commonly occur on diverse growth methods. [29]As shown in Figure 3d, bilayer MoS 2 presents two significant PL peaks associated with A and B excitons of MoS 2 . [30,31]These split excitons are attributed to the spin-orbit splitting of the valence band at the K-point of the Brillouin zone.As-grown bilayer MoS 2 exhibits a strong PL peak at 1.87 eV (660 nm) corresponding to A exciton, resulting in red emission of PL and EL. Figure 3e presents a histogram of the PL peak positions extracted from the 10 000 PL spectra in Figure 3c.Most PL peaks (≈99.22%) are localized from 1.84 to 1.88 eV.Thus, the scalable MoS 2 film is suitable to be EL of flexible LEDs.It is worth understanding why bilayer MoS 2 is used as the EL instead of monolayer.The PL signals of bilayer MoS 2 could be weaker than those of monolayer due to the energy band modulation from a direct bandgap in monolayer to an indirect bandgap in bi-or few-layers.However, the LED fabrication process involves HF-assisted wet transfer, which might degrade the optoelectrical characteristics of the transferred MoS 2 .If the grown samples are bi-or-trilayer MoS 2 , there is a good chance that their quality will be maintained even if undesirable damage occurs during the transfer.Herein, we successfully demonstrated the LEDs based on large-area MoS 2 synthesized by twostep growth for the first time.
Meanwhile, bilayer MoS 2 should be transferred to the ZnO NPs film on ITO/glass substrate.Here, we demonstrate a hydrofluoric acid (HF) solution-assisted wet transfer procedure.Figure S1a (Supporting Information) illustrates the schematic diagrams of the transfer procedure of bilayer MoS 2 .At first, poly(methyl methacrylate) (PMMA) solution is spin-coated on the bilayer MoS 2 and annealed at 180 °C for 90 s.We break all edges of PMMA-coated MoS 2 film and immerse PMMA/MoS 2 samples in 10% HF solution, consequently separating PMMA/MoS 2 films from the substrate.The free-standing PMMA/MoS 2 films are transferred to the target substrate.Figure S1b (Supporting Information) shows an actual image of the transferred MoS 2 with a scale bar of 5 mm.The transferred MoS 2 exhibits dominant doublets in the Raman spectrum and an intense PL peak in the PL spectrum under 532 nm laser illumination, as shown in Figure S1c,d (Supporting Information).It implies that MoS 2 films safely retain with no critical harm despite the exposure to HF solution during the transfer process.
Next, we demonstrated MoS 2 -based LEDs and examined their emitting properties, including current density, luminance, and electroluminescence (EL).Most LEDs with organic or quantum dot-based emitting layers undergo ohmic, trap-limited, and space-charged limited current under sweeping applied biases. [32]s shown in Figure 4a, the MoS 2 -based LED exhibits a diode-like behavior of current density with respect to voltage, indicating that it also experiences three conduction regimes (ohmic, trap-limited space charge, and trap-filled limited conduction) [33] as the driving voltage increases.Under reverse bias, EL devices barely show any current, so we measured the forward bias regime for EL characterization.Figure 4a   ≈0.17 cd m −2 at 4.0 V (Figure 4b), emitting red light originating from efficient photon recombination at the MoS 2 emitting layer.Here, we determined the turn-on voltage as a forward bias where the luminance is readily promoted and exceeds 0.01 cd m −2 since the current barely across the MoS 2 -based LEDs in a lowbias regime.MoS 2 -based LEDs exhibit a relatively high turn-on voltage, an average of 3.93 ± 0.24 V (see Figure S2, Supporting Information) In general, the turn-on voltage and corresponding current onset highly depend on the bandgap of emitting layers.In contrast, recent studies on TMDs-based light emitting devices [19,[34][35][36] have reported relatively higher turn-on voltages up to 5.5 V, compared with their bandgaps, i.e., 1.9 eV for monolayer MoS 2 and 2.3 eV for monolayer WS 2 .This could be attributed to the tunneling resistance that carriers inevitably experience while applying bias along out-of-plane in TMDs.Nevertheless, the detailed mechanism hasn't been fully understood yet.Then, a peak luminance is up to 5.41 cd m −2 at 7.8 V, exhibiting relatively superior performance than the existing studies on the grown TMDbased ELs, such as WS 2 -based p-i-n LEDs. [35]This indicates that the inverted p-i-n architecture functions appropriately, allowing carriers to transfer across adjacent layers under voltage biasing.
Figure 4c,d presents the spectral variation of MoS 2 -based LEDs as a function of applied biases.Under increasing voltages from 4 to 7 V, the MoS 2 -based LED shows a dominant EL peak at 634 nm in agreement with the PL peak of bilayer MoS 2 . [31]An EL peak associated with ZnO NPs at 396 nm emerges following the MoS 2 emission. [37]Since both MoS 2 and ZnO contribute to the overall EL, we would like to extract the emission originating solely from MoS 2 based on their relative spectra weights.The luminance contributed exclusively by MoS 2 in the LED shown in Figure 4  Meanwhile, the EL of MoS 2 shows the blueshift in the A exciton by ≈80 meV compared to its PL (Figure 3c).It could be attributed to several reasons, including in-plane compression from a stacking LED structure, [33] charge transfer between adjacent layers, [38] HF-induced MoS 2 thinning, [39] and external biasinduced excitons. [40]Among these factors, excitons populated in MoS 2 in the presence of external biases predominantly contribute to the observed blueshift in EL.Luminance of MoS 2 could stem from neutral A and B excitons and negatively charged trions, which bind two electrons and one hole and possess a lower bond energy than neutral excitons. [41]When an external bias is applied to the electrodes of an LED, electrons, and holes migrate through the layered structures and are injected into the MoS 2 EL.This facilitates promoting the formation of more excitons via injected electron-hole column interactions than the PL of MoS 2 while simultaneously inhibiting the accumulation of trions inherently present in grown MoS 2 .The dominant exciton population derived from injected carriers in MoS 2 enables the blueshift of EL compared to the inherent PL of MoS 2 , where excitons and trions coexist.
Consistent with bias-dependent EL spectra (Figure 4c), MoS 2 emits red light at the turn-on voltage and slightly shifts its emitting color to magenta at 7 V (Figure 4d).The current efficiency is shown in Figure S3 (Supporting Information).The current efficiency of 0.0017545 cd A −1 was obtained at a current density of 69 mA cm −2 .Moreover, Figure S4 (Supporting Information) presents a cycle test where an LED repeats turning on and off with different biases and a static test where the LED keeps its emission bright at a fixed voltage of 6 V over time (see Video S1, Supporting Information) Thus far, the optoelectrical performance of MoS 2 -based LEDs with the inverted p-i-n architecture was elucidated in terms of EL and related features.Next, we would like to investigate a driving force of efficient carrier transport in the inverted p-i-n MoS 2based LEDs.As depicted in Figure 1b, ITO and Al electrodes are utilized as the cathode and the anode in MoS 2 -based LEDs, respectively.This is contrary to their typical roles in regular p-i-n LEDs, where they function as an anode and a cathode due to their work functions.The reversed electrode roles offer promising fabrication availability and scalability for 2D TMDs-based LEDs, particularly involving the wet transfer of TMDs.However, this would inhibit carriers from migrating through layers and following exciton formation at the EL since carriers at electrode interfaces experience considerable energy barriers.Intriguingly, MoS 2 -based LEDs exhibit considerably superior carrier injection and transport, leading to efficient exciton formation and recombination.Some driving forces in the inverted p-i-n LEDs would play a role in overcoming energy barriers for electrons and holes and boosting exciton generation.
To better understand the carrier transport mechanism, we split the device structures into two parts by carrier that predominantly flow through layers.Figure 5a,b illustrates band alignment for electron and hole transport.Electrons are primarily injected from the ITO cathode, pass through the ZnO NPs, and finally reach the MoS 2 EL.The most crucial parameter in electron injection efficiency at the cathode interface is the LUMO offset binding energy (E L ), which is the energy difference between the LUMO of a material and the work function of a cathode. [42]The small E L facilitates electron migration from the cathode to the ETL.The E L at the ITO/ZnO interface is relatively suitable for electron transport when a sufficient bias is applied.
In contrast, the LUMO offset between MoO 3 and Al is considerably large, so it could be challenging that hole carriers are injected and transferred from the Al anode to the valence band of the MoO 3 EIL.However, the luminance and EL properties of MoS 2 -based LEDs indicate that holes can flow across layers under an external bias, although the considerable energy barrier at the Al interface would restrain hole injection.This implies something else plays out to provide effective routes for holes, and we focused on the MoO 3 HIL.[45] To benchmark those studies, we investigated the characteristics and energy band structures of MoO 3 when introduced in layered devices.MoO 3 is a well-known high-work function material that exhibits n-type behavior. [46]Its high work function enables dipole rearrangement at the interface when forming a heterojunction, leading to energy level realignment along the junction. [45]Additionally, MoO 3 inherently contains oxygen vacancies after deposition, which create gap states near its Fermi level.These gap states provide extra routes for hole carriers in an anode. [47]Based on these properties, we could hypothesize the mechanism behind efficient hole injection in the inverted p-i-n LED structure.When the MoO 3 is introduced as HIL between the CBP HTL and the Al anode, it rearranges interfacial dipoles along the junctions and creates an internal electric field across the CBP/MoO 3 /Al region.This allows electrons located at the HOMO of CBP to migrate into the conduction band of MoO 3 .When an external bias is applied, the transferred electrons at MoO 3 and the resultant holes at the HOMO of CBP could flow toward the ITO cathode and form excitons at the MoS 2 EL by binding with injected electrons from the ZnO ETL.Simultaneously, under the external bias, holes could be directly injected through gap states of MoO 3 .These two effects synergistically contribute to enhanced hole injection and transport in the inverted p-i-n LEDs.Therefore, the inverted p-i-n architecture of the MoS 2 -based LEDs offers potential for scalable TMDs-based LEDs with promising manufacture availability and a novel design strategy for hole injection-enhanced LEDs.

Conclusion
In conclusion, MoS 2 -base LEDs with the inverted p-i-n architecture were newly designed and demonstrated using scalable bilayer MoS 2 grown via the sulfurization of a few-nm thick Mo film.We successfully produced 4 inch scale bilayer MoS 2 films and transferred them onto the ZnO nps-coated ITO substrates.After depositing the rest of the LED components, device performances of MoS 2 -based LEDs were evaluated in terms of current density and luminescence.They exhibited relatively high luminescence of up to 5.41 cd m −2 throughout large active areas of 6.25 mm 2 , indicating efficient carrier transports and prompt radiative recombination under applied bias.Note that the MoO 3 EIL could boost hole injection and transport due to the interfacial dipole rearrangement attributed to its high work function and gap states derived from inherent oxygen vacancies, allowing the inverted p-i-n MoS 2 -based LEDs to present superior optoelectrical properties compared to existing studies on TMD-based LEDs.Therefore, this achievement could overcome the drawbacks of existing TMDs-based optoelectrical applications and extend their potential in various fields, such as flexible, ultrathin, or transparent displays.

Experimental Section
Synthesis, Characterization, and Transfer Methods of 4 inch Bilayer MoS 2 Films: Si/SiO 2 substrates were immersed in acetone and isopropyl alcohol and sonicated for 20 min.Then, 1 nm thick Mo film was deposited on the cleaned Si/SiO 2 substrates by the RF sputter with the power of 150 W for 220 s under an Ar flow of 100 sccm.Pre-deposited Mo film was annealed and sulfurized at 950 °C for 15 min under the gas condition of Ar:H 2 :H 2 S = 5: 50: 1, rapidly cooled down by opening the furnace.
For the characterization of bilayer MoS 2 , Raman, PL, and PL mapping analyses were carried out by a Raman spectrometer with a 532 nm layer.XPS results were calibrated at a C 1s core level of 284.8 eV.In addition, plane-viewed and cross-viewed TEM analyses were conducted by transferring MoS 2 to the Cu-TEM grids and lifting out, respectively.
For the MoS 2 transfer to the desirable substrates, PMMA was spincoated on the bilayer MoS 2 and soft-baked at 180 °C for 90 s.To allow MoS 2 film to facilitate the lamination from the Si/SiO 2 substrates, corners of the PMMA-coated MoS 2 were cut by a diamond cutter, and the rest was floated on a 10% HF solution.Then, MoS 2 was entirely detached from the substrate and floated alone since the HF solution could penetrate the MoS 2 and SiO 2 interface and etch the SiO 2 .Substrate-free MoS 2 was scooped up with slide glass and rinsed with diluted water.Finally, MoS 2 was transferred on the desired substrates, i.e., ZnO np-coated ITO substrates.
Fabrication of Bilayer MoS 2 -Based LEDs: The MoS 2 -based inverted LEDs were fabricated on indium-tin-oxide (ITO) coated glass substrates.The substrates were sequentially cleaned with isopropyl alcohol and then rinsed with deionized water.After the patterned ITO substrates were treated in ultraviolet-ozone for 15 min, ZnO NPs were deposited on ITO substrates by spin-casting at a spin rate of 1000 rpm for 60 s.For the fabrication of MoS 2 EL, MoS2 was transferred to the ZnO NPs as described before.The organic materials and metals were deposited by thermal evaporation without breaking the vacuum.4,4′-bis (carbazol-9-yl) biphenyl (CBP) for hole transport layer, MoO 3 , and Al were thermally evaporated with a deposition rate of ≈1 Å s −1 for CBP, ≈0.5 Å s −1 for MoO 3 , and, ≈3 Å s −1 for Al electrode.
Characterization of Bilayer MoS 2 -Based LEDs: The current densityvoltage-luminance (J-V-L) characteristics of the devices were measured by using a spectroradiometer (CS2000, Konica Minolta) with a Keithley 2400 source meter under ambient conditions.From these J-V-L measurements, the changes in the luminance and current efficiency of the devices as a function of the applied voltage were studied systematically.

Figure 1 .
Figure 1.Scalable bilayer MoS 2 LEDs.a) The inverted p-i-n device architecture and b) energy level diagram.
to demonstrate large-area red LEDs.A 4 inch scale bilayer MoS 2 was reproducibly grown and exhibited highly uniform Raman and photoluminescence signals throughout the entire surfaces, allowing the LEDs to emit consistent, bright EL in a large area.In experiments, 2 × 2 bilayer MoS 2 -based LEDs could achieve EL at 634 nm with an average luminance of ≈5.41 cd m −2 at a bias of 7 V, and individual active areas are 6.25 mm 2 .To the best of our knowledge, cm 2 -scale bilayer MoS 2 -based LED arrays are demonstrated for the first time.Therefore, this study may give insight and open new possibilities for TMDs-based optoelectronic displays.

Figure 2 .
Figure 2. Scalable bilayer MoS 2 film on a 4 inch diameter SiO 2 /Si substrate.a) Schematic illustration of the fabrication process (inset = cross-sectional TEM image of MoS 2 ).b) Plane TEM image of MoS 2 (inset = a corresponding FFT pattern).c) Cropped TEM image and its inverted FFT image.d) Atomic profile of white line in (c).e) Raman spectrum of MoS 2 .f) Real image of 4 inch scale MoS 2 .Raman peak maps of g) A 1g , h) E 1 2g , and i) peak differences between A 1g and E 1 2g , respectively.

Figure 2g -
Figure 2g-i presents the Raman peak maps of A 1g , E 12g , and their peak differences.The individual A 1g and E 1 2g modes are highly consistent and uniform for all 100 pixels.This suggests that the entire films are significantly uniform, and the number of layers is precisely controlled on the atomic scales.X-ray photoelectron spectroscopy (XPS) and PL analysis were carried out to characterize scalable MoS 2 film.Figure3a,bshows the Mo 3d and the S 2p spectra obtained from XPS analysis.In Mo 3d spectrum, intense doublets appear at 229.5 and 232.6 eV, corresponding to Mo 4+ 3d 5/2 and 3d 3/2 of MoS 2 .[28]A shallow peak at 226.6 eV corresponds to S 2s of MoS 2 .Besides, doublets in

Figure 3 .
Figure 3. Characterization of scalable bilayer MoS 2 .XPS spectra.a) Mo 3d and b) S 2p.c) PL peak position map (scale bar = 500 μm), d) PL spectrum, and e) PL peak distribution under  ex of 532 nm.
inset presents a photograph of the MoS 2based LED turned on with an emission area of 6.25 mm 2 at a given bias of 7 V.The luminance of the MoS 2 -based LED reaches

Figure 4 .
Figure 4. Characterization of scalable bilayer MoS 2 LED.a) I−V characteristics (inset: a photograph of MoS 2 LED turned on at 7 V).b) Voltage-dependent luminescence curve (the dashed lines depict the turn-on voltage at 4 V).c) Voltage-dependent EL spectra.d) CIE chromaticity diagram with a trajectory of EL emission colors under varying voltage.

was 3 .
83 cd m −2 .The average luminance of the MoS 2 -exclusive emission in demonstrated MoS 2 -based LEDs is 4.31 ± 0.68 cd m −2 , with the best result of 6.02 cd m −2 .

Figure 5 .
Figure 5. Band alignment of an inverted p-i-n LED based on MoS 2 for a) electron and b) hole injection and transport.Insets illustrate rearranged dipoles at the MoO 3 /CBP and Al/MoO 3 interfaces under biasing.