Photoluminescence Properties Of Cd-Free Semiconductor Quantum Dots And Their Applications For White Light-Emitting Diodes

Colloidal quantum dots (QDs) are solution-processed nanoscale crystals ofsemiconducting materials. The QDs with unique size-dependent optical propertieshave been widely applied in light-emitting diodes (LEDs), photovoltaic cells, andbiological labels. After three decades’ development of synthesis technology for QDs,the photoluminescence (PL) quantum yield (QY) of QDs has reached90%.Moreover, the external quantum efficiency of QD-LEDs has increased to18%fromless than0.01%, which approaches to that of organic light emitting diodes (OLEDs).However, the intrinsic toxicity of elements such as cadmium potentially hinders theirultimate research transformation and commercialization. Zinc chalcogenide dopedwith transition metal ions and I-III-VI based semiconductor nanomaterials havemarkedly low toxicity and large ensemble Stokes shift, avoiding the self-absorptionprocess. These properties make them ideal for different optical applications. Thesecadmium-free QDs exhibit size-and composition-tunable emission from the visibleto NIR region and the PL QY up to70~85%, which make them relevant toapplications in solid state lighting and full color displays. Despite all, theperformances of LEDs based on cadmium-free QDs are much lower than thosebased on CdSe QDs. The energy transfer between QDs and charge transporting materials and the QD emission thermal stability are very important to optimize theperformances of QD-LEDs. Therefore, we study the energy transfer between chargetransporting materials and cadmium-free CuInS2-based QDs, the thermal stability ofcadmium-free Mn-doped QDs, and the applications of cadmium-free Cu-doped QDsin white LEDs. The original works are organized as follows:(1) The energy transfer processes from organic charge transporting materials(CTMs) to CuInS2-ZnS alloyed (ZCIS) QDs with different emission wavelengthwere studied by steady-state and time-resolved PL spectroscopy. The change in thePL excitation intensity of the ZCIS QDs and the PL decay time of the CTMs clearlydemonstrated an efficient energy transfer process in the ZCIS/CTM blend films. Itwas found that the efficiency of F rster resonance energy transfer significantlyincreases with increasing the particle size and decreasing the Zn content in the QDs,which is well consistent with the estimated F rster radii (R0) varying from3nm to5nm. In addition, the PL quenching of the QDs related to the charge separationprocess was also observed in some of the samples. The energy transfer and chargeseparation processes in the films were well explained based on the band alignmentbetween the ZCIS QDs and CTMs.(2) The thermal stability of luminescence is important for application of QDs inlight-emitting devices. The temperature-dependent PL intensities and decay times ofMn-doped ZnS, ZnSe, and ZnSeS alloyed core/shell QD films were studied in thetemperature range from80to500K by steady-state and time-resolved PLspectroscopy. It was found that the thermal stability of Mn-doped QD emissions wassignificantly dependent on the shell thickness and the host bandgap, which washigher than that of workhorse CdSe QDs. Nearly no PL quenching took place inMn:ZnS QDs with a thick ZnS shell, which kept a high PL QY of~50%even at500K. And the thermally stable PL was also observed in highly luminescent Mn:ZnSeand Mn:ZnSeS QDs with the quenching temperature over200oC. Further, thestability of Mn-doped QDs with different shell thickness at high temperature wasalso examined through heating-cooling cycling experiments. The PL quenching in the thick shell-coated Mn-doped QDs was almost totally recovered. The PLquenching mechanisms of the Mn2+ion emissions were discussed.(3) The efficient white LEDs based on Cu:ZnInS/ZnS core/shell QDs withsuper large Stokes shifts were fabricated. The composition-controllableCu:ZnInS/ZnS QDs with tunable emission from deep red to green wereprepared by a one-pot noninjection synthetic approach. The high performanceCu:ZnInS QD-white LEDs with colour rendering index up to96, luminousefficiency of70-78lm/W, and colour temperature of3800-5760K weresuccessfully fabricated by integration of red and green Cu-doped QDs.Negligible energy transfer between Cu-doped QDs was clearly found bymeasuring the PL lifetimes of the QDs, consistent with the small spectraloverlap between QD emission and absorption. The experimental resultsindicated low toxic Cu:ZnInS/ZnS QDs could be suitable for solid statelighting.