Solution-processable Li₂CO₃ And MoSe₂ Quantum Dots For Tailoring Electroluminescent Device Performance

Organic electroluminescent devices（OLEDs）and quantum dot electroluminescent devices（QLEDs）have attracted huge interest in new solid state lighting and flat panel display technologies due to their self light emission,high brightness,high responsiveness,ultra-thin and flexible properties.ultraviolet（UV）OLEDs with short-wave emission are widely used in excitation lighting sources,high-density information storage and biochemical sensing.However,the high potential barrier between the energy levels of indium tin oxide（ITO）which is used as the UV OLED electrode and UV-luminescent materials,the injection of carriers is seriously restricted.In this paper,novel carrier-injected materials of Li₂CO₃and MoSe₂quantum dots/carbon dots（QDs/CDs）have been developed based on solution-processable technology.High performance UV OLEDs are fabricated by modifying the ITO electrode interface to reduce the carrier injection barrier.Meanwhile,QLEDs are fabricated using MoSe₂-QDs/CDs as the luminescent material.Details of the researches are as follows.1)Solution-treated lithium carbonate（Li₂CO₃）-formic acid solution and Li₂CO₃-boric acid solution were used as electron injection layers for the modification of ITO electrodes.Analysis of atomic force microscopy,X-ray/UV photoelectron spectroscopy,current-voltage curves and impedance spectroscopy shows that the Li₂CO₃films have excellent surface topography and electronic properties.Using PBD broad band-gap molecules as luminescent layer,highly efficient inverted near-UV OLEDs was assembled.The devices assembled with an optimum concentration of 3 mg/ml Li₂CO₃-formic acid（7mg/ml Li₂CO₃-boronic acid）as the electron injection layer had a maximum irradiance of5.24 mW/cm²（2.28 mW/cm²）and an external quantum efficiency（EQE）of 2.47%（2.17%）.The devices exhibit near-UV luminescence with an electroluminescence（EL）peak of 404-406 nm and a full width at half maximum（FWHM）of 52-56 nm.2)Colloidal solutions of MoSe₂-QDs/CDs were synthesised by liquid phase exfoliation and their excellent optical properties were confirmed by transmission electron microscopy,liquid fluorescence tests and fluorescence lifetime tests.Raman spectroscopy,atomic force microscopy,scanning electron microscopy and X-ray/UV photoelectron spectroscopy measurements confirmed the excellent film morphology and exceptional electronic properties of MoSe₂-QDs/CDs.MoSe₂-QDs/CDs were applied to the electron injection layer of inverted near-UV OLEDs,the hole injection layer of conventional structured UV OLEDs and the light emitting layer of QLEDs,respectively.The inverted UV OLED was prepared using MoSe₂-QDs/CDs as the electron injection layer（hole injection layer）and PBD as the light emitting layer with an EL peak of 406 nm（392 nm）and a FWHM of 54 nm（53 nm）.The maximum EQE of the device was 0.65%（1.57%）and the irradiance was 2.32 mW/cm²@15 V（3.61 mW/cm²@14 V）.In addition,impedance spectroscopy showed that the UVO-treated MoSe₂-QDs/CDs have a strong hole injection capacity.The performance of the device with MoSe₂-QDs/CDs as the light-emitting layer（EQE of 0.13%）is slightly lower than that of the device with CDs as the light-emitting layer（EQE of 0.18%）.However,devices using MoSe₂-QDs/CDs as the light-emitting layer have a longer light-emitting lifetime in terms of device stability.