Halide perovskites are emerging materials for next-generation display applications, thanks to their narrow emission linewidth and band gap tunability, capable of covering the entire range of visible light. Despite their short period of research, perovskite light emitting diodes (PeLEDs) have shown rapid progress in device external quantum efficiency (EQE) in the nearinfrared (NIR), red, and green emission wavelengths, and the record EQE has exceeded over 20 %. However there has been limited progress with blue emission compared to the red and green counterparts. In this review, the current status and challenges of blue PeLEDs are introduced, and strategies to produce spectrally stable blue PeLEDs are discussed. The strategies include 1) a mixed halide system in the form of 3-dimensional (3D) perovskites, 2) colloidal perovskite nanocrystals and 3) low dimensional perovskites, known as quasi-2D perovskites. In the mixed halide system, previous reports based on the compositional engineering of 3D perovskites to reduce spectral instability (i.e., halide segregation) will be discussed. Since spectral instability issue originate from the mixed halide composition in perovskites, the two other strategies are based on enlarging the band gap with a single halide composition. Finally, the prospects for each strategy are discussed, for further improvement in spectrally stable blue PeLEDs.

Abstract
1. Introduction
2. Current Status and Challenges in Blue PeLEDs
3. Strategies for Spectrally Stable Blue PeLEDs
    3.1. Compositional engineering in mixed halide system
    3.2. Colloidal nanocrystals in single halide system
    3.3. Dimensional engineering in single halide system
4. Features and Prospects of Pure Blue PerovskiteLEDs
5. Conclusion
Acknowledgement
References

• Byungha Shin(Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology) Corresponding author
• Jinu Park(Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology)
• Joonyun Kim(Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology)