No abstract

We study the optoelectronic properties of a type-II heterojunction (HJ) comprising a monolayer of the transition metal dichalcogenide (TMDC), WS, and a thin film of the organic semiconductor, 3,4,9,10-perylene tetracarboxylic dianhydride (PTCDA). Both theoretical and experimental investigations of the HJ indicate that Frenkel states in the organic layer and two-dimensional Wannier-Mott states in the TMDC dissociate to form hybrid charge transfer excitons at the interface that subsequently dissociate into free charges that are collected at opposing electrodes. A photodiode employing the HJ achieves a peak external quantum efficiency of 1.8 ± 0.2% at a wavelength of 430 ± 10 nm, corresponding to an internal quantum efficiency (IQE) as high as 11 ± 1% in these ultrathin devices. The photoluminescence spectra of PTCDA and PTCDA/WS thin films show that excitons in the WS have a quenching rate that is approximately seven times higher than in PTCDA. This difference leads to strong wavelength dependence in IQE.

Half-light half-matter quasiparticles termed exciton-polaritons arise through the strong coupling of excitons and cavity photons. They have been used to demonstrate a wide array of fundamental phenomena and potential applications ranging from Bose-Einstein like condensation 1-3 to analog Hamiltonian simulators 4,5 and chip-scale interferometers 6 . Recently the two dimensional transition metal dichalcogenides (TMDs) owing to their large exciton binding energies, oscillator strength and valley degree of freedom have emerged as a very attractive platform to realize exciton-polaritons at elevated temperatures 7 . Achieving electrical injection of polaritons is attractive both as a precursor to realizing electrically driven polariton lasers 8,9 as well as for high speed light-emitting diodes (LED) for communication systems 10 . Here we demonstrate an electrically driven polariton LED operating at room temperature using monolayer tungsten disulphide (WS2) as the emissive material. To realize this device, the monolayer WS2 is sandwiched between thin hexagonal boron nitride (hBN) tunnel barriers with graphene layers acting as the electrodes 11,12 . The entire tunnel LED structure is embedded inside a one-dimensional distributed Bragg reflector (DBR) based microcavity structure. The extracted external quantum efficiency is ~0.1% and is comparable to recent demonstrations of bulk organic 13 and carbon nanotube based polariton electroluminescence (EL) devices 14 . The possibility to realize electrically driven polariton LEDs in atomically thin semiconductors at room temperature presents a promising step towards achieving an inversionless electrically driven laser in these systems as well as for ultrafast microcavity LEDs using van der Waals materials.