Silver nanowire electrodes for transparent light emitting devices based on WS2 monolayers

Transition metal dichalcogenide (TMDC) monolayers with their direct band gap in the visible to near-infrared spectral range have emerged over the past years as highly promising semiconducting materials for optoelectronic applications. Progress in scalable fabrication methods for TMDCs like metal-organic chemical vapor deposition (MOCVD) and the ambition to exploit specific material properties, such as mechanical flexibility or high transparency, highlight the importance of suitable device concepts and processing techniques. In this work, we make use of the high transparency of TMDC monolayers to fabricate transparent light-emitting devices (LEDs). MOCVD-grown WS2 is embedded as the active material in a scalable vertical device architecture and combined with a silver nanowire (AgNW) network as a transparent top electrode. The AgNW network was deposited onto the device by a spin-coating process, providing contacts with a sheet resistance below 10 Ω sq−1 and a transmittance of nearly 80%. As an electron transport layer we employed a continuous 40 nm thick zinc oxide (ZnO) layer, which was grown by atmospheric pressure spatial atomic layer deposition (AP-SALD), a precise tool for scalable deposition of oxides with defined thickness. With this, LEDs with an average transmittance over 60% in the visible spectral range, emissive areas of several mm2 and a turn-on voltage of around 3 V are obtained.

For new applications like see-through displays, smart windows or head-up displays, transparent LEDs are of great interest and have already been reported based on organic (OLED), quantum-dot (QLED) and perovskite (PeLED) materials [29,30]. TMDCs, that provide high optical transparency [31] and, moreover, even mechanical flexibility [32] due to their sub-nm thickness [1,33], are another promising material group for the application in transparent or flexible devices. Such properties are rarely discussed in literature among the existing scalable device concepts and are employed only once for transparent [16] and flexible [19] architectures, respectively. For a 6 mm 2 transparent device Lien et al [16] used indium tin oxide (ITO) contacts on gated CVD WSe 2 to obtain pulsed electroluminescence (EL) in alternating current (AC) operation, while Andrzejewski et al [19] reported a vertical device architecture, where a strain-dependent bandgap shift [34,35] of ∼ 30 meV ε(%) −1 was shown by bending the device . Within this work, we demonstrate transparent light emitting devices based on TMDCs by using transparent electrodes on both sides of a vertical architecture. In contrast to the AC driven device presented in [16], the current flow is perpendicular to the substrate surface and the device is operated with a direct current (DC). Scalable technologies were applied consistently throughout the device fabrication process. Spin-coating of hole transport layers (HTLs) was followed by transfer of metal-organic chemical vapor deposition (MOCVD)-grown WS 2 as the active material. For direct deposition of a zinc oxide (ZnO) electron transport layer (ETL) a low-temperature atmospheric-pressure spatial atomic layer deposition (AP-SALD) process was developed. As a transparent top contact, silver nanowires (AgNWs) were spin-coated to build a conductive network.

Experimental methods
The fabrication of a transparent LED starts with a 1.5 × 1.5 cm 2 glass substrate with patterned ITO as the anode contact for the devices. Layers of poly (3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) and poly[N,N'-bis(4butylphenyl)-N,N'-bisphenylbenzidine] (poly-TPD) are spincoated for hole injection, hole transport and electron blocking, respectively. As the active material monolayer WS 2 grown by MOCVD on c-plane sapphire is transferred onto the organic HTLs with a surface-energy assisted process, adapted and modified from Gurarslan et al [18,36]. Transfer is performed with a polystyrene supporting layer spin-coated onto the WS 2 on sapphire, which is later removed with toluene from the device.
For the MOCVD of WS 2 monolayers, tungsten hexacarbonyl (W(CO) 6 ) and di-tertbutyl sulphide (DTBS) are used as precursors and the growth is conducted in a commercial AIXTRON multi-wafer reactor [28]. For detailed information on the growth parameters see supplementary material. Photoluminescence (PL) measurements of the asgrown sample show peak emission at 610 nm and a full width at half maximum (FWHM) value of about 17 nm (see supplementary figure S1(a)). Variation of excitation power density over six orders of magnitude does not change the peak position and results in only small variations of the FWHM. The integrated intensity rises proportionally to the excitation power density, which demonstrates a nearly constant photoluminescence quantum yield for the as-grown WS 2 monolayer (see supplementary figures S1(b) and S1(c)).
To realize a semi-transparent cathode, an ETL of polycrystalline ZnO is deposited onto the active material by AP-SALD. AP-SALD has emerged as a cost-effective, highthroughput and scalable method for integration of atomic layer deposition into nanofabrication processes [37,38]. With diethyl zinc as the precursor and deionized water as the oxidizing agent, 40 nm of ZnO are formed on the heated substrate surface by an oscillating movement of the sample under a stationary reactor head. The reactor is supplied with the precursors by bubbling inert nitrogen gas through the precursors. The custom built setup used for this work is similar to systems described in detail elsewhere [37,[39][40][41]. A schematic image is shown in supplementary figure S2. To prevent major damage of the TMDC monolayer during deposition of ZnO the growth temperature was limited to a maximum of 150°C and it was possible to reduce it down to 75°C within process optimization. This would in principle allow the use of flexible substrates like polyethylene terephthalate (PET) or polyethylene naphthalate (PEN), whose processabilities are mainly limited by their glasstransition temperatures of 78°C for PET and 120°C for PEN, respectively [42][43][44].
Silver nanowires (AgNWs) are spin-coated (process information can be found in supplementary material and supplementary figure S3) to form a AgNW network and serve as a transparent conductive film (TCF) cathode for the device. A schematic energy band diagram showing energetic values for isolated layers and the principal function of the devices can be found in supplementary figure S4. TCF AgNW networks were chosen, as they can compete with established ITO films regarding electrical and optical properties [45] and can be used in flexible applications [46].
As a reference, a AgNW network was prepared on a blank glass substrate following the same process flow as on the device. Figure 1(a) shows an image of a AgNW film on a glass substrate with some larger nanowire agglomerates visible as spots on the sample. In figure 1(b) the AgNW network is depicted in a scanning electron microscopy (SEM) image showing the crossing of individual nanowires to build the network. The average length of the nanowires was estimated from measurements of 40 nanowires by SEM to be 88 μm ± 39 μm with an average nanowire width of 49 nm ± 15 nm, determined from 64 SEM measurements as well (see supplementary figure S5). For the pristine AgNW electrodes on glass the sheet resistance was found to be below 10 Ω sq −1 , which is in the typical range of ITO thin films [47]. The spectral transmittance of the AgNW electrodes was measured using a UV-Vis spectrometer and corrected by the transmittance of a blank glass substrate (see figure 1(c)). The average transmittance in the visible spectral range (380−750 nm) was determined for two different samples to be 77% ± 2% and 74% ± 1%, respectively. The feature visible between 300 nm and 400 nm can be attributed to surface plasmon resonances as typically observed for AgNWs [48][49][50].

Results and discussion
Using the vertical architecture shown in figure 2(a), transparent 2D-LEDs were fabricated with spin-coated PEDOT: PSS and polyTPD HTLs, the MOCVD-grown WS 2 monolayer, a ZnO ETL by AP-SALD and AgNW electrodes deposited by spin-coating. A cross-sectional SEM image can be found in supplementary figure S6. From that, the layer thickness of ITO, the combined HTLs and ZnO can be derived as about 140 nm, 75 nm and 40 nm, respectively. Figure 2(b) shows a photograph of a finalized device with six LEDs which can be individually contacted by the Al contacts at the sample edges. The emitting area of ∼ 6 mm 2 (area marked in red in figure 2(b)) is defined by the overlap of the prepatterned ITO and the layers formed into bars by mechanical removal of the active material between the individual LEDs. The devices are sufficiently transparent to read text in see-through perspective, and an average transmittance of 63% ± 10% in the visible spectral range is determined from a transmittance spectrum depicted in figure 2(c). As a reference, the transmittance spectrum of an ITO-coated glass substrate used for device fabrication is presented as well. In the transmittance spectrum of the 2D-LED, the WS 2 A, B and C excitonic transitions can be identified at 620 nm, 524 nm and 440 nm, respectively. This is valid for all fabricated devices, as can be seen in supplementary figure S7. The presence of these features proves the integrity of the WS 2 monolayer in the device architecture after subsequent  processing steps. The drop of transmittance in the ultraviolet range is caused by the device substrate, as can be seen in the spectrum of the ITO/glass substrate in figure 2(c) as well. Additional absorption below ∼420 nm can be attributed to polyTPD and ZnO.
Applying a bias voltage of 7 V results in a current density of about 0.5 A cm −2 , and the 2D-LED emits red light with a peak wavelength of 650 nm and a FWHM of about 42 nm (see figure 3(a) for a representative EL spectrum of the device using ZnO deposited at 150°C). Comparing this with the PL peak position at about 610 nm, obtained from measurements of the as-grown WS 2 on sapphire (see supplementary figure  S1), a pronounced red shift of about 40 nm (corresponding to ∼130 meV) is present for the EL. For interpretation of this red shift, it has to be considered that the dielectric environment of the monolayer is changed in the device structure. Dielectric screening effects [51] or doping by charge transfer [52] are possible reasons for a red-shift. In addition, a change from dominant emission of neutral excitons on the sapphire substrate to trion emission in the device can explain red shifts up to ∼40 meV [53,54]. This effect could originate from a limited hole injection efficiency because of the potential barrier for holes between TPD and the valence band of WS 2 (see supplementary figure S4), leading to an unbalanced charge carrier concentration in the active region. Furthermore, at high current densities, device heating might lead to a bandgap narrowing of the WS 2 monolayer [18]. All these effects cancontribute to the observed stronger red-shift of the EL with respect to the PL measured on sapphire.
In figure 3(b) the current density of the transparent 2D-LED is plotted as a function of the applied voltage, showing current densities rising non-linearly from 0.1 to 0.6 A cm −2 between 2 V and 7 V, which is reasonable for 2D-LEDs [12,18,19]. For the different ZnO deposition temperatures only a slight variation in current density is observed with no systematic trend. We define the turn-on voltage of the device as the value at which a luminance of 10 −4 cd m −2 is measured. For the three devices turn-on voltages of 2.8 V for 75°C ZnO deposition temperature, 3.2 V for 110°C deposition temperature and 2.9 V for 150°C ZnO deposition temperature, are achieved. Figure 3(c) shows the voltagedependent luminance for all three devices, indicating no systematic effect with ZnO deposition temperature. All devices reach a luminance of 10 −2 cd m −2 ata voltage of 5 V and a maximum luminance was measured at 7 V and 1.1 A cm −2 to be 8.8·10 −2 cd m −2 for the device using 150°C AP-SALD ZnO (see supplementary figure S8). The key specifications for the best device are also summarized in supplementary table S1. The inset in figure 3(c) shows a photograph of the 75°C AP-SALD ZnO device in use at 7 V in front of printed letters. It clearly visualizes the red luminescence of WS 2, with the letters still being visible in the background. From the measured EL spectra, coordinates for the CIE 1931 xy chromaticity space of x = 0.682 and y = 0.285 can be derived. The bidirectional emission of the transparent devices was tested by EL collection from both sides of the device, resulting in comparable spectra (see supplementary figure S9). It can also be seen that the emission lacks homogeneity, which might be a consequence of localized current paths through the device due to the randomly distributed density of AgNW. The use of an additional current spreading layer or a more homogeneous distribution of AgNW by process adjustments could possibly help to achieve better homogeneity. As AgNW networks tend to degrade in atmospheric conditions on time scales of months [55], advanced encapsulation concepts [56][57][58][59] must be considered for stabilization. Storing the devices in nitrogen atmosphere helps to maintain their functionality as well, allowing the measurement of the EL spectra presented in supplementary figure S9 one year after device fabrication.
Beyond the presentation of a working concept for fully transparent 2D-LEDs, a critical evaluation of the device performance is needed to rate the potential for future applications. For this, 2D-LED concepts have to provide sufficient device brightness for their desired application. Absolute measures like the luminance for photometric or radiant intensity or radiance for radiometric units, respectively, have to be considered when evaluating the potential of a concept. Efficiency measures like the external quantum efficiency (EQE) are only applicable in combination with a sufficient luminance, as they might suggest a good device performance, while in fact they are limited to low injection densities and therefore do not provide sufficient output for practical applications. For the existing 2D-LED concepts, only the AC driven transient-EL device [16] and the vertical architecture [13,18,19], which we expand byour concept for transparent LEDs here, provide absolute valueson these key performance data. The average output power density of a 6 mm 2 transient-EL device using CVD-grown WSe 2 and operated at 400 kHz is given as 0.62 μW cm −2 [16] and vertical 2D-LEDs based on either exfoliated WS 2 flakes or MOCVD-grown WS 2 show maximum luminance of about 50 cd m −2 and 1 cd m −2 at 14 V and 7 V, respectively, corresponding to radiances of about 60 μW Sr −1 cm −2 and 1.1 μW Sr -1 cm −2 [13,18].
With the maximum luminance of about 8.8 × 10 −2 cd m −2 , corresponding to 0.12 μW Sr −1 cm −2 measured for the transparent 2D-LED here, and the reasonable luminance that have already been achieved [13,16,18,19], an application of TMDCs in lighting or display technology is still viable, while obviously further optimization is needed.
For MOCVD-grown TMDCs, the low internal quantum yield, measured here to be in the order of 10 −4 for the WS 2 used for device fabrication (see supplementary figure S1), is currently a limiting factor. It is expected that high defect densities are present in TMDCs grown via MOCVD [60][61][62], limiting the quantum yield due to non-radiative recombination [63]. Low temperature PL analysis revealed distinct defect related emission, supporting this hypothesis [18]. Further, injection barriers and a missing charge balance in the active region can strongly affect device performance. While the reasonable current densities and turn-on voltages obtained for the transparent 2D-LEDs suggest low potential barriers, an unbalanced charge carrier ratio can enhance non-radiative recombination and carrier spill-over [64] and severely affect device performance.

Conclusion
Transparent LEDs based on MOCVD-grown WS 2 were fabricated in a vertical, scalable device architecture using AgNW networks as top electrodes. The AgNWs were reproducibly deposited with a two-step spin-coating process to achieve sheet resistances below 10 Ω sq −1 and an absolute transmittance of nearly 80% over the whole visible range. With the use of 40 nm ZnO as electron transport layer deposited with AP-SALD at 150°C, a maximum luminance of 8.8 × 10 −2 cd m −2 was achieved at 7 V. Functional devices could be fabricated even with a reduced ZnO deposition temperature of 75°C . These results combine various industrially relevant technologies into one vertical device architecture, scalable to any desired size. Potential fields of applications include, e.g. transparent displays, smart windows and augmented reality applications. Therefore, this approach justifies future efforts towards LEDs based on 2D materials with relevant performance data, including specific properties like transparency, flexibility and ultra-low thickness.