Electroluminescence from zinc oxide nanoparticles/organic nanocomposites
Due to its wide band gap (3.3eV) and high exciton binding energy (60meV), zinc oxide (ZnO) has attractive properties for optoelectronics, such as superior UV emission characteristics, high stability and room-temperature luminescence. Recent research has demonstrated strong UV and visible photoluminescence, as well as optically pumped lasing in ZnO nanowires.1–4 However, only a few ZnO-based electroluminescence (EL) devices have been fabricated to date using ZnO nanowires, nanorods, or epitaxial films, due to inconvenient and expensive preparation methods.5–8 As a result, stable and low-cost ZnO EL devices are still in the development phase.
In our work, we use ZnO nanoparticles to fabricate ZnO EL devices by a spin-coating method and to study their EL properties.9 A light-emitting layer of ZnO nanoparticles is obtained using a phase-segregation technique. With phase-segregation, the ZnO nanoparticles and TPD:PMMA, an organic hole-transporting polymer, divide into two layers following the spin-coating process that enhances electron and hole recombination in the ZnO nanoparticles. Our method has the significant advantage of lowering the fabrication cost of ZnO-based devices.
The fabrication procedure is illustrated in Figure 1. First, the indium-tin oxide (ITO) glass substrate is sequentially cleaned using deionized water, acetone, and isopropyl alcohol. The ZnO nanoparticles, TPD, and PMMA are dissolved in either chloroform or an appropriate chloroform:toluene mixture. The solution is then spin-coated on the substrate with a sheet resistance of 7O/? (ohms per square) (Merck, Taiwan). The samples are then baked at 60° C for 2h to remove the solvent and enhance the adherence between the ITO substrate and the nanocomposite film. Afterwards, 2000Å of aluminum (Al) is deposited onto the ZnO composite layer by thermal evaporation under a vacuum of 9×10-6Torr.
Figure 2(a) shows the structure of our ZnO EL device. As a result of phase segregation, the ZnO nanoparticles distribute in a layer above the TPD:PMMA film. The phase segregation occurs because the solubility of ZnO nanoparticles in chloroform is different from that of TPD:PMMA, which can then segregate from them during spin coating. To examine phase segregation in the resulting composite thin films, we use confocal microscopy. Figure 2(b) shows the cross-sectional image of a typical thin film. It is clear that most ZnO nanoparticles are distributed on top of the film, indicative of successful phase segregation.
The energy band diagram of a ZnO nanoparticle/TPD:PMMA layer is sketched in Figure 3. TPD is the hole-transporting material. Holes are injected from the ITO contact into the highest occupied molecular orbital of the TPD matrix, and transported to the valence band of the ZnO nanoparticles. Similarly, electrons are injected from the Al cathode into their conduction band. Hence, the holes are combined with the electrons in the ZnO nanoparticles, forming excitons. The maximum emission in the spectrum of the EL device with ITO/TPD:PMMA:ZnO nanoparticles/Al corresponds to the band gap of the ZnO nanoparticles.9
Figure 4 displays a photograph of blue light emission from an EL device with 90nm ZnO nanoparticles at a forward bias of 7V, showing a characteristic 392nm blue color.9 The emitting area is 0.7×0.3cm. The full width at half-maximum of the emission peak of this blue light is as narrow as 35nm at ambient temperature. It is worth noting that the EL spectrum is free of oxygen defect-related emission.9
In summary, we have fabricated a low-cost ZnO EL device using spin coating to achieve phase segregation. We also showed that phase segregation enhances the probability of electron and hole recombination in the ZnO nanoparticles. This method should also be applicable to other inorganic-organic systems for the development of EL devices.
