Deep-red To Near-infrared CdTe/CdSe Core/Shell Quantum Dots:Synthesis And Application In Light-emitting Diodes

Quantum dots(QDs)have been widely used in the photoluminescence,biosensors,printable thin-film transistors(TFTs),solar cells and light-emitting diodes,due to the unique characteristics,including high photoluminescence(PL)quantum yield(QY),high monochromaticity,tunable emission wavelength,high stability,low cost and large-scale production.In recent years,light emitting diodes based on the quantum dots(QLEDs)have been received extensive attention by researchers.In particular,the performance of visible QLEDs has been very excellent,because of synthesizing the high-quality QDs.For example,for red QLEDs,the maximum external quantum efficiency(EQE)has been up to 20.5%,and the maximum luminance has reached 50000 cd/m~2.For green QLEDs,the maximum EQE and the maximum luminance have reached 23.68%and 210000 cd/m~2.Also,the deep-blue emitting QLEDs show a maximum luminance of 7600cd/m~2 and peak EQE of 12.2%.However,compared to the excellent visible QLEDs,deep-red and near-infrared emitting QLEDs developed relatively slowly,the main reason is as follows.Type I structure QDs with high PL QY still cannot be tuned to deep-red and near-infrared regions by simply controlling the size of QDs.Although type II structure QDs can present deep-red and near-infrared emission through simply controlling the size of QDs,both low PL QY and poor stability of type II QDs yet limit their application in deep-red and near-infrared QLEDs.Therefore,it is essential to prepare high-quality deep-red and near-infrared QDs,and then improve the performance of deep-red and near-infrared QLEDs.In view of the above questions,we optimized the experimental scheme to synthetize high quality deep-red and near-infrared QDs,and fabricate high-performance deep-red and near-infrared-emitting QLEDs based on these materials.The main research results of this dissertation is as follows:(1)High-efficiency deep-red quantum-dot light-emitting diodes with type-II CdSe/CdTe core/shell QDs as emissive layers.CdTe core QDs were synthetized by a hot injection method,then CdSe shells were grown onto the CdTe nanocrystals by a slightly modified successive ion layer adsorption and reaction(SILAR)procedure to produce a high quality CdTe/CdSe core/shell QDs with good monodisperse and high PL QY(63-68%).These QDs show good color tunability in the deep-red optical window from 640 to 680nm,good photochemical and thermal quenching stability.After optimizing the experimental condition,high efficiency deep-red emitting QLEDs were demonstrated by employing the CdTe/CdSe core/shell QDs as emitters.The deep-red QLEDs show a maximum luminance up to 17116 cd/m~2 and peak external quantum efficiency(EQE)of 6.19%,EQE is kept above 5%in a certain range of luminance(from 10 to 1000 cd/m~2)which is much higher than that of rare-earth complex and conjugated polymer-based phosphorescent organic light-emitting diodes PH-OLEDs(the maximum EQE is 2.7%and 1.1%,respectively.).These results signify a remarkable progress,not only in the synthesis of high-quality QDs but also in QLEDs that offer a practicle thought for the realization of next generation QD-based deep-red display and lighting.(2)High efficiency near-infrared QLEDs based on chloride treated Cd Te/CdSe QDs.A high quality near-infrared CdTe/Cd Se core/shell quantum dots were synthesized using the same method as(1).The QDs have high PL QY of 40-60%and good color tunability in the near-infared optical window from 740 to 850nm.Then,the long chain organic ligands covered in the surface of QDs were replaced by chloride ion to improve the carrier mobility between QDs and hole transport layer.Taking an example of near-infrared emitting QLEDs based on CdTe/CdSe QDs with the PL peak at 784nm,after chloride ions treated,the maximum radiance and peak EQE of device reached up to 66 mW/cm~2 and 7.2%,respectively,which increases by 4.5%and 26.3%in radiance and EQE,compared to that of QLEDs without chloride ions treated(53 mW/cm~2 and 5.7%).This improvement is attributed to adopting QDs with chloride ions treated that suppress the non-radiative recombination and improve the carrier mobility between QDs and hole transport layer.These results indicate that it is beneficial to use the near-infrared emitting QLEDs in night-vision-readable displays.