Luminescence in sulfides : a rich history and a bright future

Sulfide-based luminescent materials have attracted a lot of attention for a wide range of photo-, cathodo- and electroluminescent applications. Upon doping with Ce3+ and Eu2+, the luminescence can be varied over the entire visible region by appropriately choosing the composition of the sulfide host. Main application areas are flat panel displays based on thin film electroluminescence, field emission displays and ZnS-based powder electroluminescence for backlights. For these applications, special attention is given to BaAl2S4:Eu, ZnS:Mn and ZnS:Cu. Recently, sulfide materials have regained interest due to their ability (in contrast to oxide materials) to provide a broad band, Eu2+-based red emission for use as a color conversion material in white-light emitting diodes (LEDs). The potential application of rare-earth doped binary alkaline-earth sulfides, like CaS and SrS, thiogallates, thioaluminates and thiosilicates as conversion phosphors is discussed. Finally, this review concludes with the size-dependent luminescence in intrinsic colloidal quantum dots like PbS and CdS, and with the luminescence in doped nanoparticles

Non-toxic near-infrared (NIR) LEDs

Summary: Harnessing cost-efficient printable semiconductor materials as near-infrared (NIR) emitters in light-emitting diodes (LEDs) is extremely attractive for sensing and diagnostics, telecommunications, and the biomedical sciences. However, the most efficient NIR LEDs suitable for printable electronics rely on emissive materials containing precious transition metal ions (such as platinum), which have triggered concerns about their poor biocompatibility and sustainability. Here, we review and highlight the latest progress in NIR LEDs based on non-toxic and low-cost functional materials suitable for solution-processing deposition. Different approaches to achieve NIR emission from organic and hybrid materials are discussed, with particular focus on fluorescent and exciplex-forming host-guest systems, thermally-activated delayed fluorescent molecules, aggregation-induced emission fluorophores, as well as lead-free perovskites. Alternative strategies leveraging photonic microcavity effects and surface plasmon resonances to enhance the emission of such materials in the NIR are also presented. Finally, an outlook for critical challenges and opportunities of non-toxic NIR LEDs is provided

Thin film electroluminescent displays produced using sol-gel methods

An inverted double insulating thin film electroluminescent (TFEL) display has been fabricated using all sol-gel methods. This involved the production of three film types, an insulating material, a conducting film and a luminescent film. The layers have been evaluated individually and the combination effects are also looked at. The optimised film choice for the display is then given. This investigation focused on manganese doped zinc sulphide (ZnS:Mn), which has a strong orange emission due to the Mn2+ 4 Ti(4G) -» 6Ai(6S) transitions. It is produced from sol-gel deposited zinc oxide films. The oxide films are converted to zinc sulphide by annealing in a hydrogen sulphide-containing atmosphere. The conversion process was investigated and it was found that it takes place in a two-step process that is controlled by diffusion. The parameters of the conversion were optimised to produce the doped zinc sulphide having the changes in structure and composition as a function of sulphidation temperature and annealing time. It has been found that, after an initial “dead time”, conversion takes place in a two-step manner where, for an initial period of approximately 60 mins. little diffusion takes place followed by faster diffusion with a diffusion coefficient of 7.8xl0~18 mV1, which is independent of sulphur concentration. It is also found that the sulphide forms the hexagonal, wurtzite phase with a strong (002) orientation. The emission due to the Mn2+ 4Ti(4G) 6Ai(6S) transitions has been investigated using photoluminescence (PL), cathodoluminescence (CL), and electroluminescence (EL) and the correlation between the luminescence produced by the various methods has been studied. A comparison of the spectra using PL, CL and EL has shown how these excitation methods can be used to quantify the electroluminescence produced by the zinc sulphide. The luminescent properties of ZnS:Mn films have been investigated for their application as an emission layer in the thin film electroluminescent display. The luminescence of the device depends on the structure of the device and various structures were fabricated and the luminous output investigated. It was found in this case that the optimum structure is a five layer inverted structure. The luminescent properties also depend on the insulating films used in the device and their properties. In this case two insulating materials were investigated. The insulating films used in the device were both tantalum pentoxide (Ta20s) and silicon dioxide (SiC>2 ). Devices using both materials have been produced. The properties and the interaction between the emission layer and the insulating layer were investigated. The effect of the crystal structure of the Ta20s on the luminescent properties of the device was also investigated. The luminescent characteristics of the fabricated devices have been measured and a comparison is made with the characteristics of sol-gel TFEL devices using SiC>2 insulators. The TFEL devices with the Ta2 0 5 insulators have shown higher stability and more reliability in theiroperation Therefore Ta2Os is the preferred choice as an insulating material The conducting material chosen for this device is aluminium doped zinc oxide (ZnO Al) as it is a transparent conductor that is compatible with the insulating materials and enhances the performance of the devic

Growth optimisation and laser processing of thin film phosphors for electroluminescent displays

This thesis presents results of a study of ZnS:Mn thin film phosphors used in Thin Film ELectroluminescent (TFEL) and Laterally Emitting TFEL (LETFEL) devices, examining techniques for phosphor growth optimisation and post deposition processing in order to strengthen development of novel TFEL devices. To achieve this, thin films of phosphor were deposited using RF magnetron sputtering to investigate the use of co-sputtering in order to optimise dopant concentration. 800 nm films of ZnS:Mn were simultaneously co-sputtered from ZnS and ZnS:Mn (1 wt.%) solid targets. The thin films were deposited at different manganese concentrations by varying the relative RF power applied to each target. The films were deposited directly onto 100 mm diameter (100) n-type silicon substrates, or onto a layer of 300 nm of Y2O3 to fabricate electroluminescent test devices. Luminescence from the phosphor films was characterised via photoluminescent excitation using a 337 nm pulsed N2 laser, with the photoluminescence (PL) optimum obtained at 0.38 ZnS:Mn power ratio. Electroluminescence (EL) from TFEL devices were excited by applying a sinusoidal waveform voltage at a frequency of 1 kHz with maximum luminance obtained at 0.36 ZnS:Mn power ratio

Organic Light-Emitting Diodes based on New Promising Materials

The present work focuses on the investigation of two types of new materials, phosphorescent and near-infrared, for the fabrication of solution-processible Organic Light-Emitting Diodes (OLEDs). After the introduction of the theoretical background in the first part, the second part concentrates on phosphorescent OLEDs based on copper transition metal complexes. The photophysical properties of the copper complexes, the phosphorescent host and the interlayers were studied before the fabrication of phosphorescent OLEDs. Despite the various colours exhibited by the metal complexes all devices emit white light. The possible formation of an exciplex at the guest/host interface was thus investigated. Finally the influence of the solvent on the morphologies of the films and the performances of the devices were studied. The third part focuses on near-infrared OLEDs obtained by using two different strategies. First by using a near-infrared copolymer emitting at 880 nm and incorporating it in green and red hosts and second by the creation of what is believed to be an exciplex at the interface between a hole injection layer and twisted organic molecules that emit at 515 and 540 nm. In both cases pure infra-red light above 800 nm was achieved