Charge Carrier Balance in Scalable TMDC-Based Light Emitting Devices

2-dimensional transition metal dichalcogenides (TMDCs) such as WS₂are 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 [1], 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 [2-4].<br/> However, the luminance of TMDC-based LEDs in cw operation at room temperature is still limited to 50 cd/m²for microscale LEDs based on mechanically exfoliated WS₂ [5] and to about 1 cd/m² for scalable, 6 mm² large devices [2], respectively. One key requirement for optimizing the luminance is a balanced electron and hole injection into the active TMDC layer. In this work, we address the electron-hole balance in scalable WS₂-based LEDs by systematically varying the architecture of electron injection layers (EIL). We fabricated three different LED archetypes: type 1 includes a ZnO EIL, type 2 contains Mg-doped ZnO as an EIL, and type 3 is a reference device without an EIL. At 5V forward bias, we observed a reduction of the current density by a factor of 10 in type 2 devices as compared to type 1. Simultaneously, the EQE was found to increase by an order of magnitude in the type 2 device compared to type 1, and a luminance of up to 3 cd/m² was obtained for type 2 devices. We attribute this to an improved electron-hole balance in type 2 devices, caused by a reduced efficiency of electron injection. Our interpretation is supported by device simulations using NextNano, which emphasizes the impact of a balanced electron and hole injection for efficient LEDs based on scalable 2D TMDCs.<br/><br/>References:<br/>[1] Jiang Pu et al. Adv. Mater. 30, 1707627 (2018)<br/>[2] D. Andrzejewski et al., ACS Photonics. 6, 1832-1839 (2019)<br/>[3] D. Andrzejewski et al., Adv. Opt. Mat. 8, 2000694 (2020)<br/>[4] H. Myja et al., Nanotechnology 34, 285201 (2023)<br/>[5] D. Andrzejewski et al., Nanoscale, 11, 8372–8379 (2019)