Gate Controlled Excitonic Emission in Quantum Dot Thin Films.

Formation of charged trions is detrimental to the luminescence quantum efficiency of colloidal quantum dot (QD) thin films as they predominantly undergo nonradiative recombination. In this regard, control of charged trion formation is of interest for both fundamental characterization of the quasi-particles and performance optimization. Using CdSe/CdS QDs as a prototypical material system, here we demonstrate a metal-oxide-semiconductor capacitor based on QD thin films for studying the background charge effect on the luminescence efficiency and lifetime. The concentration ratio of the charged and neutral quasiparticles in the QDs is reversibly controlled by applying a gate voltage, while simultaneous steady-state and time-resolved photoluminescence measurements are performed. Notably, the photoluminescence intensity is modulated by up to 2 orders of magnitude with a corresponding change in the effective lifetime. In addition, chip-scale modulation of brightness is demonstrated, where the photoluminescence is effectively turned on and off by the gate, highlighting potential applications in voltage-controlled electrochromics.

Efficiency Roll-Off Free Electroluminescence from Monolayer WSe2.

Exciton-exciton annihilation (EEA) is a nonradiative process commonly observed in excitonic materials at high exciton densities. Like Auger recombination, EEA degrades luminescence efficiency at high exciton densities and causes efficiency roll-off in light-emitting devices. Near-unity photoluminescence quantum yield has been demonstrated in transition metal dichalcogenides (TMDCs) at all exciton densities with optimal band structure modification mediated by strain. Although the recombination pathways in TMDCs are well understood, the practical application of light-emitting devices has been challenging. Here, we demonstrate a roll-off free electroluminescence (EL) device composed of TMDC monolayers tunable by strain. We show a 2 orders of magnitude EL enhancement from the WSe2 monolayer by applying a small strain of 0.5%. We attain an internal quantum efficiency of 8% at all injection rates. Finally, we demonstrate transient EL turn-on voltages as small as the band gap. Our approach will contribute to practical applications of roll-off free optoelectronic devices based on excitonic materials.