CVD-grown mWS2 has a high defect density comprising S vacancies formed during the growth process, limiting the device efficiency. Also, cracks and holes are generated during the dry transfer since a mWS2 is a polycrystal bound by weak van der Waals forces. The S vacancies led to emission from the defect levels in both the EOD and HOD, even when no charges were injected as shown in FIG. 16A. The physical defects led the EQE to vary by orders of magnitude even within the same growth run. The defects were non-radiative, appearing as the dark spots on the device emitting surface, as shown by the image in FIG. 13B.
The electroluminescence spectra showed emission from mWS2 but not from the organic host in FIG. 12C, demonstrating efficient F?rster transfer of the excitons generated at the EML/ETL interface, into mWS2. The spectrum shows a bathochromic shift depending on the drive current. In FIG. 16A, the photoluminescence of mWS2 in the EOD, excited with a 532 nm laser, is shown as a function of current density, with the deconvolution of the spectrum using two Lorentzians with exciton and trion emission peaks at wavelengths of λ1=617 nm and 628 nm, respectively. The trion peak intensity increases with the current density, as expected. The laser selectively excites A excitons of mWS2 (?2.0 eV), but not the higher energy (?2.4 eV) B excitons, allowing for the omission of their spectra in the peak fits. The ratio between the emission intensity of excitons and the increased emission intensity of trions due to the charge injection was calculated using the law of mass action, shown in Equation 5 below:
