A High‐Efficiency Wavelength‐Tunable Monolayer LED with Hybrid Continuous‐Pulsed Injection

High‐efficiency and wavelength‐tunable light‐emitting diode (LED) devices will play an important role in future advanced optoelectronic systems. Traditional semiconductor LED devices typically have a fixed emission wavelength that is determined by the energy of the emission states. Here, a novel high‐efficiency and wavelength‐tunable monolayer WS2 LED device, which operates in the hybrid mode of continuous‐pulsed injection, is developed. This hybrid injection enables highly enhanced emission efficiency (>20 times) and effective size of emission area (>5 times) at room temperature. The emission wavelength of the WS2 monolayer LED device can be tuned over more than 40 nm by driving AC voltages, from exciton emission to trion emission, and further to defect emission. The quantum efficiency of defect electroluminescence (EL) emission is measured to be more than 24.5 times larger than that from free exciton and trion EL emission. The separate carrier injection in the LED also demonstrates advantages in allowing defect species to be visualized and distinguished in real space. Those defects are assigned to be negatively charged defects. The results open a new route to develop high‐performance and wavelength‐tunable LED devices for future advanced optoelectronic applications.


Introduction
Layered transition metal dichalcogenide (TMD) monolayers have attracted consid erable interest for their unique electronic and optical properties. [1][2][3][4][5][6] These mono layers exhibit a directbandgap nature with a high quantum yield of light emission and are of particular interest for novel optoelec tronic device applications, such as a light emitting diode (LED). [7,8] Monolayer LED was demonstrated by forming a horizontal pn junction using electrostatic gates. [8] LEDs based on vertically stacked hetero structures with hexagonal boron nitride tunnel barriers have also been reported. [7,9] These two types of LED devices are based on direct current (DC) injection configu ration and require complicated device structures. As the counterpart of the DC driving LED, the alternating current (AC) driving LED, also known as pulsed LED, has attracted more attention recently. [10,11] Compared with DC LED structure, the AC LED only uses the twoterminal capacitor structure via a single metal-semiconductor contact, which significantly simplifies the fabrication. The electron charging and discharging effect in this capacitor structure will lead to pulsed light emission. In a DC LED device, electrons and holes are injected simultaneously, [7] while in an AC LED, electrons or holes are injected separately under different AC voltage polarities. [10] This separate injec tion feature opens a promising way to fabricated pulsed LED devices [12] and allows us to probe electroluminescence (EL) emission pathway and the carrier dynamic down to each injec tion cycle. [13] However, the recombination of electrons and holes can only occur on the contact edge between electrodes and 2D monolayer material, thus EL emission effective area is small. This drawback requires a very dense electrode pattern for large size LED devices, resulting in a bottleneck for the emerging ACdriving LED technology. In this work, we developed a novel hybrid continuouspulsed injection method in an ACdriving LED device, which enables electron-hole recombination to occur in areas far away from the metal-semiconductor contact edge. This can highly increase the effective EL emission area, enabling a more facile fabrication of electrode patterns and thus contributing to the adoption of pulsed LED technology.
On the other hand, wavelengthtunable LED devices are important and can enable many advanced optoelectronic High-efficiency and wavelength-tunable light-emitting diode (LED) devices will play an important role in future advanced optoelectronic systems. Traditional semiconductor LED devices typically have a fixed emission wavelength that is determined by the energy of the emission states. Here, a novel highefficiency and wavelength-tunable monolayer WS 2 LED device, which operates in the hybrid mode of continuous-pulsed injection, is developed. This hybrid injection enables highly enhanced emission efficiency (>20 times) and effective size of emission area (>5 times) at room temperature. The emission wavelength of the WS 2 monolayer LED device can be tuned over more than 40 nm by driving AC voltages, from exciton emission to trion emission, and further to defect emission. The quantum efficiency of defect electroluminescence (EL) emission is measured to be more than 24.5 times larger than that from free exciton and trion EL emission. The separate carrier injection in the LED also demonstrates advantages in allowing defect species to be visualized and distinguished in real space. Those defects are assigned to be negatively charged defects. The results open a new route to develop high-performance and wavelength-tunable LED devices for future advanced optoelectronic applications. www.advmat.de www.advancedsciencenews.com applications. [8,14,15] However, most semiconductor LED devices demonstrated so far have a fixed emission wavelength, which is determined by the energy of the emission states, such as bandgap edge, [16,17] excitons, [7,18,19] or defects. [20] Here, we dem onstrated a wavelengthtunable ACdriving WS 2 monolayer LED device. Its EL wavelength can be switched from exciton to trion emission, and finally to the defect emissions, by controlling the frequency of the driving signal. Thanks to the unique feature of separate injection of electrons and holes in the ACdriving LED platform, it allows us to visualize and distinguish defect species and the evolution of defect states. The quantum efficiency of the defect EL emission was measured to be ≈24.5 times larger than free exciton and trion EL emissions. Our results provide crucial insights into optimizing the performance of ACdriving LED devices for future practical applications and open a new route to study the defect states and defect engineering for the tunable LED device.

High-Efficiency Monolayer LEDs with Hybrid Continuous-Pulsed Injection
For LED device fabrication (Figure 1a inset), a mechanically exfoliated monolayer WS 2 flake was drily transferred onto a SiO 2 /Si substrate (275 nm thermal oxide on n + doped silicon), in contact with a gold electrode that was peeltransferred onto monolayer sample. [21,22] Part of the 1L WS 2 was under the gold electrode and the other part on the SiO 2 substrate. The meas ured EL emission spectrum, under AC voltage driving, was comparable with the photoluminescence (PL) spectrum from monolayer WS 2 (Figure 1a). A large effective area of EL light emission (Figure 1b) was observed from this monolayer WS 2 LED device (Figure 1b inset). Figure 1c shows the emission profile along the solid white line in Figure 1b. We observed EL emissions from locations that are 25 µm away from the edge of metal contact, which is around five times larger than the max imum length (≈5 µm) reported before. [10] The driving voltage and frequencydependent EL emissions were also measured (Figures S1 and S2, Supporting Information).
To understand this ultralong EL emission length, time resolved electroluminescence (TREL) measurements were performed (Figure 1d). Two EL decay curves (Figure 1d) from the WS 2 LED device emerged at both voltage rising and falling edges, showing dramatically different shapes and indicating the different carrier recombination mechanisms in these two decay processes. The hybrid continuouspulsed EL emission observed in 1L WS 2 LED device ( Figure 1d) is in sharp con trast to the symmetry pulsed EL decay curves observed from 1L WSe 2 LED devices from both a previous report [10] and our control experiments ( Figure S3, Supporting Information). The entire EL decay curve from 1L WS 2 was divided into four phases labeled as I-IV (Figure 1d). The corresponding schematic band diagrams are illustrated in Figure 1e. In phase I, when the driving voltage was quickly switched from −6.6 to +6.6 V, the energy band was bent upward, leading a large number of holes injected into WS 2 samples within a short period of time. The injected holes recombined with remaining electrons from previous cycle that did not have time to escape, resulting in a fast EL emission (lowintensity shoulder in Figure 1d). Those remaining electrons (labeled by purple color in Figure 1eI) were of different origin from those electrons due to the initial naturally ntype doping (labeled with red color). In phase II, the driving voltage was stabilized at +6.6 V, the holes were continuously injected into 1L WS 2 . Due to the naturally ntype doping in 1L WS 2 , the injected holes could find excess electrons to form excitons and emitted light as illustrated in Figure 1eII. The popula tion of injected holes was far less than those of free electrons from initial ndoping. The free electrons could move toward the Au metal edge and recombined with the holes continu ously injected into 1L WS 2 , leading to a steady increase of the EL intensity with time. In phase III, the injected holes would diffuse inward accompanying with radiative recom bination until they reached their longest diffusion length. Therefore, the emission area was extending inward from Au contact edge toward the far end, and finally it generated an ultralong emission length as depicted in Figure 1eIII. In the meantime, the EL intensity reached its maximum and then slowly decays with time. During this process, the Au contact continuously injected holes into the system to compensate holes consumption. This caused an almost flat decay curve and an ultralong decay lifetime. Here, the EL decay curve also explained the long EL emission time. Thus, this EL decay life time can be approximated to EL emission time. Actually, the EL emission time can be further extended if the AC frequency is decreased ( Figure S4, Supporting Information). The longest EL emission lifetime in our WS 2 AC LED device was found to be ≈1980 ns when AC frequency was 200 kHz. This ultralong EL lifetime was attributed to the highquality WS 2 sample and contaminationfree device fabrication method (Note S1, Supporting Information). In phase IV, when the applied AC voltage suddenly change its polarity (from +6.6 to −6.6 V), the negative bias voltage quickly bent the energy band downward, leading to the injection of a large number of electrons into 1L WS 2 . The injected electrons recombined with remaining holes from previous cycle that did not have time to escape (labeled by cyan color in Figure 1eIV), resulting in a short and intensive emission pulse ( Figure 1d and Figure 1eIV). After the depletion of free holes, no more light emission was detected even when a negative voltage bias was still applied. In phase IV, the light emission area was very close to Au con tact edge because electrons and holes only meet in the region close to the metal edge.
Here, we defined phases I and IV as pulsed injection/emis sion processes, while phases II and III as continuous injec tion/emission processes. The integrated EL intensities for both continuous mode (phases II and III) and pulsed mode (phases I and IV) were extracted as a function of AC frequency ( Figure S5a, Supporting Information). The total contribution of EL intensity from pulsed emissions did not change when the frequency increased. However, the EL intensity from con tinuous emissions dramatically increased with frequency increased. The intensity ratio between continuous emissions and pulsed emissions was much larger than 1, and it reached a maximum value of ≈24 under a driving frequency of 1 MHz ( Figure S5b, Supporting Information). These results suggest www.advmat.de www.advancedsciencenews.com that the continuous injection mode plays a significant role in improving the overall efficiency of 1L WS 2 AC LED devices.
The ntype 1L WS 2 LED device operates in a hybrid contin uouspulsed mode (with four phases I-IV), which is in sharp contrast to the purely pulsed mode (with only phases I and IV) observed in 1L WSe 2 LED device with neutral initial doping. [10] 1L WSe 2 sample shows neutral initial doping, [23][24][25] while the monolayer WS 2 sample shows naturally ntype doping because of its natural sulfur vacancy [26,27] and substitutional halo gens. [28,29] The ntype doping level of 1L WS 2 used in LED device can be estimated to be 7.5 × 10 11 cm −2 by using gatedependent PL spectra ( Figure S6 and Note S2, Supporting Information).
Adv. Mater. 2021, 33, 2101375 Figure 1. Monolayer WS 2 LED devices with large area and ultralong EL emission length at room temperature and the principle of the continuous-pulsed emission mode. a) EL and PL spectra measured for monolayer WS 2 devices (V ac = 7.5 V at 500 kHz). Inset: the schematic of AC LED devices. b) EL mapping at 619 nm of the recoded WS 2 AC LED device with a large area and the longest EL emission length (V ac = 7.5 V at 500 kHz). The white dashed line indicates the contacted edge of the gold electrode. Inset: the optical image of this WS 2 device. c) EL emission profile along the solid white line shown in (b). d) Time-resolved electroluminescence and their corresponding alternating driving voltage trace for WS 2 AC LED device. The driving voltages are ± 6.6 V and the driving frequency is 500 kHz. Two different decay behaviors indicate the unsymmetrical carrier recombination process at two voltage transients. e) Schematic band diagrams corresponding to four emission phases labeled as I, II, III, and IV in (d).

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To further confirm that the operation of the hybrid continuous pulsed mode in 1L WS 2 LED device is attributed to the initial doping, we used a ptype chemical doping technique to tune the initial doping of 1L WS 2 sample. We found that the contin uous mode can be significantly modulated using this method ( Figure S7 and Note S4, Supporting Information).

Temperature-Dependent EL Emission
The initial doping normally comes from thermally activated free carriers, and they should be very sensitive to tempera ture. To further explore and understand the dynamics of this hybrid continuouspulsed EL emission, temperaturedependent EL mappings and TREL measurements were performed. In Figure 2a-d, we show the EL emission mapping of the monolayer WS 2 under four different temperatures, with fixed driving voltage (V ac = 7.5 V) and frequency (500 kHz). The atomic force microscopy (AFM) image confirms that the WS 2 flake is mon olayer, as shown in Figure S8 (Supporting Information). A clear trend of EL quenching and emission area shrinking toward the Au electrode was observed when the temperature decreased. For comparison, the crosssection profiles of EL emission along the solid white line under 290 and 80 K were extracted as shown in Figure 2e,f, respectively. The EL emission length, which is defined as the distance value of full width at half maximum of EL emission profile, decreased from 5.822 µm at room temper ature to 0.016 µm at 80 K, leading to an ultrahigh shrinking factor of 364 times in the latter scenario. The temperature dependent EL emission length was further extracted and shown in Figure 2g. The corresponding EL emission mapping images are presented in Figure S9   www.advmat.de www.advancedsciencenews.com emission length saturated when temperature was higher than 230 K, which might result from the limited size of this specific 1L WS 2 sample (Figure 2a inset). When the temperature was below 230 K, the EL emission length dramatically decreased as temperature decreased (Figure 2g). In this AC driving LED structure, the injected carriers firstly tunneled through the air barrier under Au electrode into the monolayer WS 2 sample. Then, those accumulated carriers laterally diffused inward the monolayer WS 2. Therefore, the emission length is highly related with carrier diffusion ability as well as mobility. Con sidering this emission principle, the hole diffusion length can be considered as proportional to emission length. In 1L WS 2 , the carrier mobility can be separated into two regimes, which are insulating regime at low gate voltages and metallic regime at high voltages. [26] In the temperaturedependent EL measure ment, the driving voltage was fixed at 7.5 V, the LED system was under insulating regime. In this regime, the carrier mobility behavior follows the frameworks of thermally activated and var iable range hopping models. [30] Therefore, the carrier mobility decreases as the temperature decreases. The reduced carrier mobility of holes makes them harder to diffuse inward the monolayer WS 2 , and finally those holes are radiatively recom bined with electrons that are close to the Au metal edge. The emission area was shrinking toward the Au contact and gener ated a short emission length.
The EL emission intensity from the same single point (indi cated in Figure 2a) as a function of temperature is presented in Figure 2h. The EL intensity monolithically decreased with tem perature. We attribute this to the reduction of intrinsic carrier concentrations which are thermally activated. The measured temperaturedependent EL intensity data was well fitted by a thermalactivation model (Note S5, Supporting Information), as shown by the red fitting line in Figure 2h. From the fitting, the ionization energy can be extracted to be 140.4 meV, which reasonably matches with the calculated activation energy of our Cldoped WS 2 (≈180 meV). [31] To understand the temperaturedependent carrier injec tion dynamics in time domain, temperaturedependent TREL measurements were also performed and presented in Figure 2i. The corresponding temperaturedependent PL and EL spectrum are presented in Figure S10 (Supporting Infor mation). At the 290 K, a hybrid continuouspulsed injection/ emission mode with four phases was observed, as previ ously discussed. However, the pulsed injection/emission in phases I and IV vanished when the temperature was lower than 200 K. This suggests that a completely pulsed AC LED device [10] might not operate at low temperature, owing to the possibility of remaining carriers escape quickly at low temper ature when switching the polarity of voltage bias (Figure 1eI  and 1eIV). On the other hand, the continuous mode (phases II and III) can still operate at low temperature (Figure 2i). The EL emission intensity and lifetime from phases II and III also significantly reduce as temperature decrease from 290 to 80 K. This behavior is consistent with thermally activated free car rier model presented above. When temperature decreased, the population of thermally activated electrons and holes will both reduce. The TREL oscillation may come from the cur rent oscillation from our instruments ( Figure S11 and Note S6, Supporting Information).

Wavelength-Tunable EL Emission Modulated by AC Driving Frequency
At 80 K, the EL emission peak wavelength can be tuned from 599 nm (exciton emission) to 609.6 nm (trion emission), fur ther to 628 nm (defect state 1, D1) and 638 nm (defect state 2, D2) (Figure 3a and Figure S12, Supporting Information), when the AC driving frequency changes from 100 kHz to 1 MHz. Those two defect emission peaks from EL spectra match with the defect peaks from PL spectra ( Figure S13, Supporting Infor mation). Under low driving frequencies (<300 kHz), the exciton and trion emissions dominate the EL spectra (Figure 3a (top)). When the driving frequency is higher than 300 kHz, the emis sion from defect state 1 (D1) start to become dominant; when the driving frequency is higher than 800 kHz, the emission from defect state 2 (D2) start to become dominant in the EL spectra (Figure 3a,b and Figure S12, Supporting Information). It is worth to note that this wavelength tunability was observed at the temperature below 110 K. We attribute this to thermal activated defect localized exciton dissociation. The thermal acti vation energy for defect bound excitons in monolayer WS 2 is only ≈36 meV, while the exciton binding energy is ≈300 meV. [27] Thus, defect localized excitons are typically observed under low temperature.
The wavelength tunability by AC driving frequency can be explained by the driving frequency dependent electrical doping to the 1L WS 2 sample. In each cycle of AC LED operation, there are more electrons than holes injected into 1L WS 2 , leading to an effective ntype doping. A higher driving frequency will effectively lead to a higher ntype doping to 1L WS 2 samples. This is evidenced by following three folds. First, the Fermi level of Au metal was pinned close to the conduction band of 1L WS 2 , which is suggested by the slightly unipolar transport curves measured from a 1L WS 2 fieldeffect transistor (FET) device ( Figure S14, Supporting Information). The closetocon duction Fermi pinning will be more beneficial to the injection of electrons than holes. Second, the measured integrated PL from phases II and III per cycle increased when the driving fre quency increases ( Figure S3b, Supporting Information), which suggested that the 1L WS 2 gets more ndoped as driving fre quency increases. Third, at 80 K, the PL intensity ratio between trion and exciton peaks increased when driving frequency is switched from 100 to 200 kHz.
The observed defect states (D1 and D2) might be the charged defect states sitting above the valence band maximum, which have been demonstrated by both experiments and theoretical calculations in 1L WS 2 . [29] Under low driving frequency, 1L WS 2 sample has a low ntype doping and the quasi Fermi level of holes is below the defect levels ( Figure 3c). The defect levels are fully occupied by holes and will be involved in the EL process. When the AC driving frequency increases, the quasiFermi level of holes moves upward, causing the defect levels to be unoccu pied. The injected holes can be quickly captured by those defect levels, resulting in the transition of the EL emissions from exciton/trion to the lowerenergy defect states (Figure 3a,b).
The ratio between the integrated EL intensities from those two defects and that from exciton is also plotted as a function of driving frequency (Figure 3b inset). The ratio value for D1 was around 10 for frequencies greater than 600 kHz, while www.advmat.de www.advancedsciencenews.com the ratio value for D2 (with larger trapping potential) reached a maximum of 24.5 at 900 kHz. Those values suggest highly enhanced internal quantum efficiencies from the electrically pumped defect states in 1L WS 2 than that from free excitons (Note S7, Supporting Information), which is hardly seen in optically pumped WS 2 monolayer samples. [32] In an optical pumping process, electrons and holes are optically excited under certain optical selection rules, and they can relax into conduction band minima and valence band maxima to forming free excitons quickly. [32] Those free excitons can recombine radiatively before they are captured by the defectinduced traps. [32] In sharp contrast to the optical excitation, the electrical pumping will inject carriers to conduction or valence bands without strict selection rules (Note S8, Supporting Informa tion). Those randomly injected electrons or holes will take a much longer time to form excitons (Figure 2i). Those defect states can capture electrons and holes, facilitating the forma tion of excitons, leading to higher quantum yield from those defect states. This phenomenon has also been observed in WS 2 and WSe 2 based single defect emitters. [20,33] To understand the origin of the large enhancement of quantum yield from the defect states, we used TREL to char acterize the timeresolved emission dynamics of the defects, exciton and trion species as depicted in Figure 3d. From the TREL curves, it took 170.2 and 156.8 ns for exciton and trion emissions to reach the maximum value, respectively; in Adv. Mater. 2021, 33, 2101375   Figure 3. AC-driving-frequency-induced wavelength-tunable EL emission. a) Frequency-dependent EL spectra from the monolayer WS 2 AC LED at 80 K (V ac = 7.5 V). b) Frequency normalized integrated intensities of four emission species versus AC driving frequency. Inset: defects and exciton emission ratio as function of AC driving frequency. c) Schematic band explanation of frequency-induced exciton/trion to defect 1 (D1) and defect 2 (D2) transition. E fp represents the quasi-Fermi level of holes. d) Time-resolved electroluminescence of exciton, trion, D1, and D2 species under driving frequency at 500 kHz.
www.advmat.de www.advancedsciencenews.com contrast, the required time to reach the two defect states, D1 and D2, are 14.3 and 15.9 ns, respectively, a much shorter time than for free excitons and trions. This confirms the above expla nation that defect states can facilitate the formation of excitons in 1L WS 2 ACdriven LED device. Likewise, the radiative EL emission lifetime for D1 and D2 were measured to be 46.6 and 43.3 ns, respectively, i.e., much shorter than those from free excitons (181.2 ns) and trion (192.4 ns). This further confirms the explanation that those defect states can result in enhanced EL quantum yield.

Visualization of Exciton/Trion to Defect Emission Transition and Defect Level Evolution
Visualizing the spatial distribution of EL emissions from indi vidual defects would allow us to better understand their transi tion dynamics, and thus to design highperformance lighting devices. Here, we conducted the lowtemperature EL mapping measurements and directly visualized the transition dynamics of three individual defects in 1L WS 2 AC driving LED devices.  www.advmat.de www.advancedsciencenews.com the AC driving WS 2 LED device with different driving fre quency. Three bright emission spots were observed (labeled as S1, S2, and S3) under driving frequency of 200 kHz. When the driving frequency increased, the middle spot S2 emerged and became the dominant bright spot. By converting the spa tial axis, which is perpendicular to the edge of Au electrode (dashed line shown in Figure 4a-c), to wavelength dimension, the EL spectra collected from three narrow rectangular areas (a width of 95 µm) covering those three spots, respectively, were extracted as shown in Figure 4d-f. The extraction method is provided in the Experimental Section and Figure S15 (Sup porting Information). Under driving frequency of 200 kHz, the EL spectra for S1 and S2 contains a significant contribu tion from defect states, while the contribution from exciton and trion dominated the EL spectra of S3. Under the driving frequencies of 600 kHz and 1 MHz, the defect emissions domi nated the EL spectra for all three areas. Interestingly, the central defect emission wavelength was found at 628 nm (D1) for S1 and 638 nm (D2) for S2 and S3. Note that the intensity from S2 was more than ten times higher than that from S3, despite coming from the same defect state D2. This suggests that the local doping level is highly uniform and might be significantly affected by the appearance of the defects.
Here, we have successfully distinguished different defect types and visualized the evolution of different individual defects in real space by using this AC driving EL emission mapping. The origin of two defect emissions (D1 and D2) was attributed to the localization of excitons in the negatively charged defects type I, [29] which has been observed by scanning tunneling microscopy/spectroscopy (STM/STS) and COtip noncon tact atomic force microscopy (ncAFM). On basis of STS, the negatively charged defects type I exhibit electronic signatures showing several states above the valence band maximum, [29] which is consistent with our experimental observation.
In our experiment, the beauty of our AC injection is that it can separately inject holes or electrons. In this way, it provides a better chance for us to monitor injection dynamics. The EL defect emission in our sample only appears in hole injection section as shown in TREL measurement (Figure 3c), which is consistent with STS dI/dV spectra results. [29] This suggests that the two defect states in our experiment are negatively charged. They prefer trap holes rather than electrons. On the basis of the STS dI/dV spectra, we attribute the emission peaks of 628 and 638 nm to the negatively charged type I defects with different energy depths. Those negatively charged defects can be triggered by increasing AC driving frequency as shown in Figure 4g.

Conclusion
We have demonstrated a novel hybrid continuouspulsed injec tion method in an AC driving LED device, which allows elec tron-hole recombination to occur in the areas far away from the metal-semiconductor contact edge. This led to a large increase of the effective emission areas and emission efficiency in an AC driving LED device. The continuouspulsed injection concept demonstrated in this work can be extended to other 2D materials under this AC driving LED structure for achieving a larger effec tive emission area and reducing fabrication costs. For example, this hybrid injection method can be used in monolayer MoSe 2 devices to study the electrically pumped biexciton and higher order excitons dynamic behaviors. Meanwhile, the large emis sion area lightemitting diode based on monolayer MoTe 2 can be achieved by using this method to bridge the optical emission range between the visible and nearinfrared regimes in the lay ered material field. Moreover, the wavelengthtunable AC driving WS 2 monolayer LED device was demonstrated. Its EL emission wavelength can be switched from exciton to trion emission, and finally to the defect emissions, by controlling the frequency of the driving signal. The quantum efficiency of defect EL emis sion was measured to be ≈24.5 times larger than free exciton and trion EL emission. The ability to realize highintensity defect emission opens opportunities in the field of defect engineering and defect based single photon emission. In addition, thanks to the unique feature of separate injection of electrons and holes in the AC driving LED platform, we could visualize and distinguish defect species and the evolution of defect states in real space. Those defect levels were assigned to negatively charged defects. Our results not only illustrate important insights for further understanding the AC LED working principle and providing a general guideline for rationally designing high performance AC driving LED, but they also open a new route to study the defect states and defect engineering for the tunable LED device.

Experimental Section
Device Fabrication: The monolayer WS 2 and WSe 2 samples were mechanically exfoliated onto the SiO 2 /Si substrate where had been already patterned 100 nm thickness gold electrodes by conventional photolithography. Another 100 nm thickness gold electrode was peeled off from SiO 2 /Si substrate through a homemade micromanipulator and then drop it between samples and electrodes to bridge them together. The WS 2 crystal was purchased from HQ graphene with chemical vapor transport (CVT) growth method containing chlorine (Cl). The WSe 2 crystal was purchased from 2D semiconductor with flux zone growth method containing no halide elements.
Optical Characterization: PL and EL measurements were conducted using an in-house developed photoluminescence setup equipped with an Andors iDu416 charge-coupled detector and a Kymera 328i spectrometer. A 532 nm diode-pumped solid state (DPSS) laser was used as the excitation source. The EL signal was generated by applying an amplified square wave generated from the function generator. All EL-related measurements were conducted with a microscope-compatible chamber (Linkam Chamber) with electrical feedthrough. For temperaturedependent measurements, a low-temperature controller was linked to the chamber and while using liquid nitrogen as the coolant. Timeresolved PL and EL (TRPL and TREL) measurements were conducted in a setup that incorporates with an avalanche photodetector and a timecorrelated single-photon counting (TCSPC). For TREL measurement, the reference signal from function generator was synchronized to TCSPC as a start time tagger signal. The histogram measurement was performed by TCSPC, where the resolution of TCSPC could go down to 8 ps. It is noted that all EL spectra, TREL and EL mapping spectra were generated with 0.7-1 V peak-to-peak voltage with frequency ranging from 100 kHz to 1.5 MHz. All spectra were corrected with the instrument response.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author. www.advmat.de www.advancedsciencenews.com