Preparation,Optical Properties And Application Of Environmental-friendly Alloy Quantum Dots

Quantum dots(QDs)have been recognized as a new-generation of photoelectric materials owing to their advantages such as size-and shape-controlled optoelectronic properties,low-cost solution processibility and so on.During the past several decades,scientific synthesis methodologies for various types of QDs have been developed rapidly,and many researchers have found extensive applications in light-emitting diodes(LEDs),solar cells,bio-imaging etc.Because of its excellent photoelectric display effect,strong feasibility and low preparation cost,quantum dot light-emitting diodes(QLEDs)are favored by many researchers and highlighted in the field of display and lighting with practical applications.At the same time,its excellent performance has been widely used in display and lighting two major applications with far-reaching technical significance.Furthermore,QD materials are also outstanding for transport modification in all-inorganic perovskite solar cells(PSCs).However,most part of the predominant work focused on the Cd-based II-VI QDs,Pb-based IV-VI and perovskite ones,in which the heavy metals such as cadmium and lead are harmful for human health as well as the environment and detrimental to the commercial development of corresponding QDs materials.Therefore,it is of great significance to carry out research work on non-toxic QDs and relevant applications.This thesis synthesizes environmental-friendly QDs and investigates their applications in LEDs and solar cells,fully releasing the advantages of QDs,while considering the burden of traditional QDs materials on the environment and maintaining the lowest impact on the environment.Non-Cd kinds of multivariate QDs were prepared,such as AgInS₂/ZnS QDs and Ag-In-Ga-S(AIGS)QDs,whose advantages were embodied in electroluminescent diodes and perovskite solar cells.On the one hand,the structure of core/shell AgInS₂/ZnS QDs were optimized to improve the quantum yields of QDs.thus,to enhance the luminous efficiency of QLEDs.On the other hand,AIGS QDs with larger bandgap,which were derived from AgInS₂ QDs,acting were as hole modification layer were benificial for the enhancement of power conversion efficiency of all-inorganic PSCs.Specific innovations are described below:(1)AgInS₂(AIS)QDs are synthesized by a hot injection method.By adjusting the ratio of indium/silver(In/Ag=1,2,3,4,5),the AIS QDs exhibit a blue shift from 868 nm to 603 nm with the indium composition increases.Therein,the AIS QDs with the ratio of In/Ag=4 show a highest photoluminescent(PL)quantum yields(QYs)up to 57%.AIS QDs are coated with ZnS shell to passivate the surface defects,and the PL QYs of obtained core/shell AIS/ZnS QDs is increased to 72%.By using these AIS/ZnS QDs as light emitters,light emitting diodes are assembled for the first time with a stacked multi-layer structure ITO/PEDOT:PSS/Poly-TPD/QDs/ZnO:Mg/Al.The resulted electroluminescent(EL)device exhibits a maximum external quantum efficiency(EQE)of 1.25%and an open circuit voltage of 4.6 V corresponding to a maximum brightness of1120 cd/m².Although the performances of the as fabricated AIS/ZnS-based device lag much behind than those of the Cd-based ones,they are expected to be enhanced with much more studies on the synthesis of the QDs and the optimization of device structure.(2)AIGS QDs derived from AgInS₂ones were used as hole modification materials to enhance the power conversion efficiency of all-inorganic PSCs.The severe interface charge recombination caused by the large energy difference between perovskite material and carbon electrode significantly limits the further performance improvement of the PSCs.We apply innovatively multilayer of quaternary AIGS QDs with cascade-like valence bands as hole-transport materials to assemble all-inorganic PSCs,and the resultant all-inorganic PSCs exhibit a power conversion efficiency(PCE)of 8.46%,which is enhanced by 20.9%in comparison with 7%for the pristine device.The high performance of the PSCs indicates that sequential layers of AIGS QDs with cascade-like energy levels can facilitate the charge separation,reduce the barrier the holes crossing and suppress the charge recombination.Stack of QDs with cascade-like energy levels provide solution-processed PSCs with a new method to enhance device performance.(3)Energy levels of AIGS QDs were adjusted by altering amine ligands of different carbon chains.All AIGS QDs with different ligands were used as hole-transport materials to assemble all-inorganic PSCs.All the devices exhibited higher PCE than the control one,and the cell with the modification of QDs capped with dodecylamine show the champion PCE of 8.61%,which increased by 20.4%compared with the original blank device.These results indicated that ligands had influences on the hole extraction in all-inorganic PSCs.