Charge carrier balance in scalable TMDC-based light emitting devices

2-dimensional transition metal dichalcogenides (TMDCs) such as WS₂ are ultrathin materials with huge oscillator strength, strong in-plane bonds, and in the case of monolayers a direct bandgap. These outstanding properties have stimulated the development of various concepts for light-emitting devices, often based on micrometer-sized flakes. Recently, a scalable device architecture has been suggested, where wafer-scale TMDCs grown by metal-organic chemical vapor deposition (MOCVD) have been embedded between electron and hole-supporting layers to form a vertical p-i-n architecture.

However, the luminance of TMDC-based LEDs in CW operation at room temperature is still limited to 50 cd/m² for microscale LEDs based on mechanically exfoliated WS₂ and to about 1 cd/m² for scalable, 6 mm² large devices, respectively. One key requirement for optimizing the luminance is a balanced electron and hole injection. In this work, we address the electron-hole balance in scalable WS₂-based LEDs by systematically varying the carrier transport layers. Implementing the Mg-doped ZnO electron transport layer (ETL) led to a systematic increase of 2D-LEDs efficiency, luminance up to ~3 cd/m², and a reduced current density. Additionally, a change in the electron-blocking layer thickness allows for the control of the operating onset voltage of the LEDs. Our experimental study is supported by device simulations, emphasizing the impact of a balanced electron and hole injection for efficient LEDs based on scalable 2D TMDCs.