High Brightness And Multicolor Electroluminescent Devices Based On Carbon Quantum Dot/Poly(N-vinylcarbazole) Doped Light Emitting Layer

Carbon quantum dots(CQDs)are zero-dimensional carbon nanoparticles with particle size less than 10 nm.As a new class of quantum dots(QDs),CQDs not only inherit the excellent luminescent properties of traditional QDs,but also have the advantages of wide availability of raw materials,low toxicity,environmentalfriendlinessandlowcost.Therefore,QD-based electroluminescent light emitting diodes using CQDs as emitting layer are expected to open up a new research field and show good commercial prospects in solid-state lighting and full-color displays.However,CQDs in solid state suffer obvious fluorescence quenching and aggregation,which inevitably reduces the performance of the device.Therefore,it is essential to overcome the solid-state luminescent quenching of electroluminescent CQDs.In this paper,the structure optimization of CQD emitting layer and device was conducted to suppress the aggregation of CQDs,thus improving the dispersion of CQDs,suppressing the luminescence quenching of the device,and improving the carrier transporting performance of CQDs,so as to improve the electroluminescent properties of the devices.The main research content and results are as follows:(1)Electroluminescent properties of long-chain passivated solid-state luminescent CQDs:Aiming at overcoming the luminescent quenching of electroluminescent CQDs,the long-chain passivated CQDs(L-CQDs)with solid-state luminescent properties were prepared by one-step microwave carbonization using citric acid as carbon source,hexadecylamine as surface passivator and octadecene as solvent.As-prepared L-CQDs are quasi-spherical,with average particle size of 2.5 nm,and have high degree of crystallinity.The LUMO level of L-CQDs is–3.39 eV,the HOMO level of L-CQDs is–6.43 eV.L-CQDs exhibit excitation-dependent PL emission and the QY of 41%.Moreover,they exhibit obvious solid-state luminescence as a result of the abundant long-chain alkanes on the surface.The CQD-based electroluminescent light emitting diodes(CQD-LEDs)were prepared by using L-CQDs as emitting layer,which give the brightness output of 5.7 cd/m~2 at the voltage of 10.0 V.The results show that CQD-LEDs have obvious luminescent quenching owing to the serious aggregation of L-CQDs in CQD-based emitting layer.Moreover,the CQDs have poor transporting performance owing to their long chain passivating ligands.Therefore,the CQD-LEDs with low brightness are resulted.(2)Bright multicolor doped CQD-LEDs by using long-chain passivated CQDs as the dopant.Aiming at overcoming the serious aggregation of CQDs in CQD-based emitting layer,the host-guest doping method was adopted to improve the dispersion of CQDs,and to suppress the luminescent quenching of the CQD-LEDs.In host-guest system,CQDs act as the guest,conductive polymer poly(N-vinylcarbazole)(PVK)as the host.White,yellow and blue CQD-LEDs were obtained from the same oleophilic L-CQDs by tuning the structure of the devices.The white CQD-LEDs exhibit a high color rendering index(CRI)of 83 with a luminance of 455.2 cd/m~2,the yellow CQD-LEDs show the maximum brightness of 339.5 cd/m~2 and excellent color stability,the blue CQD-LEDs show the maximum brightness of 569.8 cd/m~2.The results show that the dispersion of CQDs is significantly improved by doping L-CQDs into PVK to form a uniform emitting layer.Hence,the luminescence quenching of CQD-LEDs is resisted and the brightness of CQD-LEDs is improved by using host-guest doping method.(3)Doped CQD-LEDs by using short-chain passivated CQDs(S-CQDs)as the dopant:aiming at improving the poor transporting performance of long-chain passivated CQDs(L-CQDs),a new class of oleophilic CQDs with high sp~2 conjugation carbon in core and short-chain surface capping ligands agent on surface were prepared to improve the transporting performance of the CQD-doped emitting layer.The oleophilic S-CQDs were prepared by a two-step method using citric acid and urea as carbon sources and octylamine as surface ligands.As-prepared S-CQDs exhibit excitation-dependent PL emission,with the QY of 34.8%,and have the average particle size of 5.95 nm and the high degree of crystallinity and sp~2 conjugation.Their LUMO level is–4.08 eV and HOMO level is–6.41 eV.The results show that the S-CQDs have unideal electroluminescent performance,although they have better transporting performance,which is probably related to the luminescent quenching of CQDs owing to their abundant surface defect,because the surface defects release energy in a non-radiative process,resulting in non-radiative Auger recombination.