Scalable Large-Area p–i–n Light-Emitting Diodes Based on WS₂ Monolayers Grown via MOCVD

Abstract

Transition metal dichalcogenides (TMDCs) represent a novel and sustainable material basis for ultrathin optoelectronic devices. Although various approaches toward light-emitting devices, e.g., based on exfoliated or chemical vapor deposited (CVD) TMDC monolayers, have been reported, they all suffer from limited scalability and reproducibility required for industrial fabrication. Here, we demonstrate a light-emitting device in a scalable approach by embedding metal−organic (MO-)­CVD WS2 monolayers into a vertical p–i–n device architecture using organic and inorganic injection layers. Red electroluminescence is emitted from an active area of 6 mm2 starting already at a driving voltage of about 2.5 V.

Flexible Large‐Area Light‐Emitting Devices Based on WS2 Monolayers

“…The luminescent layer is a three‐atom‐thick WS 2 monolayer with a room temperature photoluminescence (PL) emission around 2.04 eV. The fully coalesced TMDC monolayer has been grown by MOCVD on a 2” sapphire (0001) wafer and was transferred on top of the poly‐TPD layer using a polystyrene auxiliary layer in a semi‐dry approach [ 20 ] with modifications as described in the Supporting Information (Device Preparation). A layer of ZnO nanoparticles (NP) is spin‐coated on top of the active material and serves as electron injection/transport layer as well as hole blocking layer.…”

“…The slight shift in peak wavelength—compared to the PL signal of the as‐grown WS 2 monolayer on sapphire (see Figure S3, Supporting Information)—might have different reasons. In previous works it has been shown that the PL peak shifts after the transfer of TMDC layers from the sapphire substrate into the device environment due to the change of the adjacent material and thus the dielectric environment [ 20,22 ] (see also Figure S3, Supporting Information). In addition, the emission energy can be influenced by built‐in fields of the p–n junction, by the external electric fields when applying a bias voltage and—in particular—by emerging Joule heating effects with increasing current density.…”

“…At an applied voltage of 3 V, the LED emits with a luminance of about 5 × 10 −4 cd m −2 which continuously increases reaching a value of nearly 1 cd m −2 at a voltage of 9 V. In the red spectral region, this value corresponds to a radiance of about 1 µW sr −1 cm −2 . Taking into account that the internal quantum efficiency of these WS 2 monolayers was previously estimated to be about 10 −2 % [ 20 ] and could be improved by further optimizing the growth process, our device architecture has the potential to achieve brightness values exceeding the technically relevant number of 1000 cd m −2 . These EL and luminance measurements were performed on unstrained devices under ambient conditions, i.e., without any encapsulation.…”

Laser‐ and Ion‐Induced Defect Engineering in WS2 Monolayers

“…[ 1,2 ] This makes them attractive for optoelectronic device applications. [ 3–5 ] TMDCs also show excellent spin–orbit coupling (control over polarization‐dependent band excitation), mobility, and strong exciton binding energies. [ 6–10 ] monolayers show higher luminescence quantum yield (≈6%) when compared with other TMDC monolayers (<1%).…”

Centimeter‐Scale and Visible Wavelength Monolayer Light‐Emitting Devices

“…Although the WSe 2 devices were bright, their emission is in the near‐infrared regime (750 nm peak emission). Recently, millimeter‐scale WS 2 devices operated in the visible wavelength regime were demonstrated using a vertical architecture with quantum dots and polymers as electron/hole injection layers, but a more efficient device is still demanded . In this regard, we report a centimeter‐scale, bright, visible light‐emitting device based on WS 2 monolayers synthesized via CVD using a simple capacitor structure.…”