Stable Perovskite Quantum Dots Using a ZnX₂-Trioctylphosphine-Oxide—Application for High-Performance Full-Color Light-Emitting Diode

Perovskite is a very promising material that is being extensively studied at the bulk and nano-size scales because it has outstanding optical properties, including high quantum efficiency and narrow emission spectra. However, perovskite has stability issues related to heat, air, and light. To overcome these, highly stable perovskite quantum dots (PeQDs) are developed using excess Zn precursor and trioctylphosphine-oxide (TOPO). Some of smaller ions can enter the perovskite structure, such as rubidium or potassium, and can change their diffraction by changing the tolerance factor. In our ZnX₂cases, the XRD peak pattern does not change indicating that Zn ions do not affect the perovskite structure. Our PeQDs were synthesized using excess zinc halide and TOPO, which needs a higher reaction temperature (230 °C) compared with conventional methods (180 °C) because the reaction solution should be transparent for homogeneous reaction. In particular, it is clarified that Zn and TOPO are combined and these complexes are attached to the surface of the PeQDs through ³¹P NMR. They not only have high quantum efficiency and sharp full width at half maximum values (15–30 nm) but also have improved long-term stability at high temperature. Among the blue, green, red particularly green device has the longest lifetime and blue device has the shortest lifetime. It can be explained by that the fact that the current efficiency of the green device is more than three times higher than for the other colors, which means that the joules for heating the green device are lower and the morphology of the film is superior to other colors. In the case of the green device, the stabilities of the pristine PeQD device and TOPO-Zn treated PeQD devices were also compared. The pristine device showed a very short lifetime of 1.5 min at 1.0 mA cm⁻². As seen in the thermal stability test, the structure of PeQDs is very weak, and cannot maintain the electrical and thermal energy after fabrication. However, TOPO-Zn treated PeQDs can emit light for a longer time compared to the pristine PeQDs because of their structural stability. These results appear to have an impact on the device as the stability of materials is also improved. Additionally, XPS measurements are conducted for a detailed surface analysis of PeQDs, finding that the TOPO-Zn complex effectively decrease Pb-O bonding in the lattice. Perovskite full-color electroluminescence (EL) devices are fabricated using PeQDs and 9,9-bis[4-[(4-ethenylphenyl)methoxy]phenyl]-N2,N7-di-1-naphthalenyl-N2,N7-diphenyl-9H-fluorene-2,7-diamine (VB-FNPD) as a new cross-linkable hole transporting material. The VB-FNPD has a high-hole carrier mobility compared to the PVK as conventional hole-transporting layer. As a result of EL performance, they have high EQE (%) and current efficiency(Cd A⁻¹) of (7.12%, 9.93 Cd A⁻¹) for red, (6.06%, 32.5 Cd A⁻¹) for green, and(0.56%, 0.88 Cd A⁻¹) for blue-emitting devices, respectively.