Objective  Flexible self-luminescent devices are the frontier subject in the field of illumination and display. Compared to organic light-emitting diodes, inorganic semiconductors offer more stable physical and chemical properties. However, crystalline inorganic films do not have good flexibility and are easy to crack after bending. Low-dimensional nanowire array structures have become a new breakthrough in the research of flexible devices. Wide bandgap semiconductor zinc oxide (ZnO) is an ideal material for developing short wavelength luminescence and laser devices. ZnO is also one of the most researched nanomaterials currently. Recently, all inorganic cesium lead halide perovskite materials (CsPbX₃ (X=I, Br, Cl)) have sparked a new research boom due to their excellent optical properties. Cesium lead halide perovskite quantum dots have high luminescent quantum yields. By dispersing perovskite quantum dots as fluorescent layers in nanowire arrays, the high fluorescence efficiency of perovskite materials can be utilized to achieve visible band flexible devices.

  Methods  ZnO nanowire array was prepared by hydrothermal synthesis method. Zinc foil was used as a flexible substrate. Due to the polycrystalline nature of the zinc foil substrate and the random orientation of surface Zn grains, nano ZnO layer was deposited on the pre-cleaned substrate surface by sputtering method as the seed layer for nanowire growth (Fig.1). In order to construct MIS heterojunction devices, MgO layer was deposited on the surface of ZnO nanowire arrays by magnetron sputtering. Au electrode with a diameter of 1 mm was deposited on the surface of MgO layer using thermal evaporation method and mask technology. The MIS junction light emitting device based on ZnO/MgO nanowire array heterostructure was constructed (Fig.9). Then, CsPbBr₃&CsPbI₃ perovskite quantum dots were prepared by thermal injection method. Lastly, CsPbBr₃&CsPbI₃ quantum dots/PMMA saturated solution were spin coated and dispersed into the ZnO nanowire array to form a packaging layer. Using MIS heterojunction as a short wavelength excitation source and perovskite quantum dots as a fluorescent layer, a flexible fluorescent LED prototype device is constructed (Fig.13).

  Results and Discussions   As the concentration of precursor solution is 20 millimoles per liter, the size of the top and root of the nanowire is uniform, showing a highly oriented growth perpendicular to the substrate direction (Fig.3). ZnO nanowires exhibit a strong ultraviolet emission peak near 375 nm with a narrow width of ~23 nm, which can be attributed to exciton related transition recombination (Fig.4). In the visible wavelength range of 450-700 nm, the emission related to deep level defects commonly observed in ZnO was almost invisible. The photoluminescence spectra of ZnO nanowire arrays exhibit a high UV visible luminescence ratio, indicating that the prepared nanowires have high crystal and optical quality, further reflecting the high crystallinity of the nanowires. After depositing the MgO film layer, the UV emission peak position, line width, and deep level emission of the ZnO nanowire array remained almost unchanged. Compared to the uncoated nanowire array, the UV intrinsic emission of ZnO increased by ~10.3% after depositing MgO layer (Fig.8). Nanowire light-emitting devices exhibit pure ultraviolet emission under different bias voltages, and no deep level defect related emission in the visible region is observed. Electrically pumped UV random lasing was realized under very low forward bias voltage from this flexible light-emitting device (Fig.10). As to the flexible light-emitting devices of nanowire/quantum dot composite system, the proportion of green light is larger than red light and blue light (Fig.14). Therefore, the device exhibits warm white light.

  Conclusions  In this work, ZnO/MgO nanowire array heterostructures were prepared and synthesized on flexible zinc foil substrate, and MIS junction light-emitting prototype device was constructed. By changing the concentration of precursor in hydrothermal synthesis, controllable synthesis of ZnO nanowire arrays with appropriate surface density was achieved. The deposition of MgO not only formed a continuous coating layer, but also effectively passivated the non radiative recombination centers on the nanowire surface, further improving the UV intrinsic emission of ZnO nanowires. In this flexible heterojunction devices of nanowire arrays, ultraviolet electroluminescence and electrically pumped lasing from ZnO nanowires have been achieved. Subsequently, CsPbBr₃&CsPbI₃ perovskite quantum dots were synthesized using thermal injection method, and the quantum dots/PMMA saturated solution were spin coated and dispersed into ZnO nanowire arrays. A flexible fluorescent warm white LED prototype device was constructed using MIS heterojunction as a short wavelength excitation source and perovskite quantum dots as a fluorescence layer. This paper explores new development ideas for flexible luminescence and display.