Silicon Quantum Dots With Octyl And Phenylpropyl Ligands For Light-Emitting Diodes

In recent years,quantum dot(QD)materials have become the subject of intense research interest in a number of areas including photovoltaics,optoelectronics,and biological sensing.Among semiconductor quantum dots of various constituents,silicon quantum dots(Si QDs)deserve special attention due to the intrinsic advantages of natural abundance,relative low cost,ease of processing,low toxicity and biological compatibility.Compared with traditional bulk silicon materials,photoluminescence could be obtained from Si QDs under ambient temperature,which could be used to fill the gap of the utilization of silicon materials in semiconductor luminescence.By far,Si QDs have shown great promise in the fields of biological imaging,photovoltaics and optoelectronics.However,numerous challenges of Si QDs remain unresolved,of which the most severe are the low monochromaticity and the unsatisfactory photoluminescence quantum yield(PLQY),thus limiting the development of the light-emitting diodes(LEDs)based on Si QDs.Fortunately,according to recent research,the gradual refinement of the capping ligands on the surface of Si QDs could effectively improve the performance of the LEDs.In this paper,we compare the effect of two different ligands,octyl(alkyl)and phenylpropyl(aryl)ligands on the optical properties of Si QDs.We also perform hybrid surface modification with both kinds of ligands on the surface of Si QDs and study the influence on the optical properties.Based on the first part of work,we fabricate corresponding Si-QD LEDs.Through analysis of the Si-QD LEDs,we further investigate the effect of two kinds of ligands on the performance of Si-QD LEDs.The main contents include:1.We synthesize free-standing Si QDs via the non-thermal plasma system and carry out the surface modification with the conventional method of hydrosilylation.1-octene and allylbenzene are used as capping agents to obtain Octyl-Si QDs(alkyl ligands)and Phenylpropyl(PhPr)-Si QDs(aryl ligands).In addition,we carry out hybrid surface modification with different ratios of 1-octene and allylbenzene to obtain Si QDs capped by both octyl and phenylpropyl ligands.Compared with Octyl-Si QDs,it is found that PhPr-Si QDs more efficiently emit light due to the more oxidized and less defective surface.In addition,the optical properties of hybrid surface-modified Si QDs are influenced by both kinds of ligands.2.Based on the singly and hybrid surface-modified Si QDs,we fabricate Si-QD LEDs with the structure of ITO/PEDOT:PSS/poly-TPD/PVK/Si QDs/ZnO/Ag.Compared to Octyl-Si QDs LEDs,PhPr-Si QDs LEDs exhibit larger current density and higher optical power density.However,the charge injection is more seriously unbalanced in PhPr-Si QDs LEDs,and thus the external quantum efficiency(EQE)is much lower than that of Octyl-Si QDs LEDs.In addition,the highest optical power density of 0.64 mW/cm2 is obtained from PhPr-Si QD LEDs and the optimum EQE of 6.2%is obtained from Octyl-Si QD LEDs.It is also found that the overall performance of the Si-QD LEDs is preserved based on hybrid surface-modified Si QDs.