Bright Electroluminescence from monolayers in ambient conditions

LEDs made from monolayer transition metal dichalcogenides (TMDCs) are promising compact on-chip light sources for applications, such as high-speed optical communication or quantum optics. They produce high electroluminescence (EL) quantum efficiencies of about 1-5% but generally only in high-vacuum conditions. In this work, a fast exponential decay in light emission and current is observed for these devices when operated in ambient conditions under constant voltage bias. The investigators show a pulsed bias will overcome this decay.

The system studied was a back-gated WSe₂ field-effect transistor structure operated as a p-n diode. WSe₂ was chosen for its ambipolar nature relative to other TMDCs. The investigators note ambipolar characteristics are required for bipolar carrier injection. Bright emission was observed for both DC operation and with a pulsed bias. With a DC bias, both the light emission and current decayed in only a few seconds. Pulsing the bias overcame this decay, yielding stable light emission and current for more than 1,000 seconds.

Pulsing frequency was found to be important. Below 1 kilohertz, EL drops rapidly due to a decaying current. Above 1kHz, however, it remains stable for over three hours. Pulsed injection appears to work by circumventing the hysteresis and current decay commonly seen in TMDC transistors. The authors attribute the decay to surface molecules, particularly water, acting as charge traps.

The systems studied in the paper are promising new light sources for optical communications. They showed fast (15 nanoseconds) rise and fall times, indicating strong potential for high speed light modulation. Further improvements in efficiency are expected with better contact quality for lower voltage operation or through improvements in the chemical vapor deposition process for monolayer growth.

Source: “Bright electroluminescence in ambient conditions from WSe₂ p-n diodes using pulsed injection,” by Kevin Han, Guen Ho Ahn, Joy Cho, Der-Hsien Lien, Matin Amani, Sujay B. Desai, George Zhang, Hyungjim Kim, Niharika Gupta, Ali Javey, and Ming C. Wu, Applied Physics Letters (2019). The article can be accessed at https://doi.org/10.1063/1.5100306 .