Dissertation Submitted to the Department of Materials Science and Engineering and the Graduate School of Yonsei University in partial fulfillment of the requirements for the degree of Doctor of Philosophy 

By Do Hoon Kim --February 2022

ABSTRACT

 Development of micro light-emitting diode (LED) pixel array for ultra-high definition (UHD) displays is underway based on LED semiconductor chips, organic LEDs (OLEDs), and quantum dot LEDs (QLEDs). However, these devices have the drawbacks of high cost and complex processes as well as technical problems. Such as an increase in the cost due to the additional transfer process of semiconductor chips and an inaccuracy of mechanical positioning during repeated transfer process. Moreover, OLEDs have the advantage of being applicable to flexible and stretchable substrates, but require expensive organic materials and large-scale equipment. In case of QLEDs, these are not able to be used as a light source because of unstable electroluminescence (EL) property, thus they are used as color filters with a backplane. So, introduction of candidate of new luminescent materials is urgently needed. The Perovskite has an adjustable optical band gap, which can be tuned by changing halide anions in the entire visible region. In particular, a primary advantage of the perovskite is that it can be fabricated by simple solution process at low temperatures and this enables the perovskite to be useful for low-cost and large-area micro LED applications. Furthermore, the perovskite LEDs (PeLEDs) are expected to be suitable for nextgeneration displays because they have exhibited unprecedented improvements of luminescence efficiency in a short time compared to conventional LEDs. However, despite these advantages of perovskites, in the case of CsPbI3 crystals for realizing red emission, a high-temperature post-annealing process is essential for suppressing the formation of δ- phase (tilted octahedral) crystals and promoting the formation of a stable α-phase (cubic). In general, a high-temperature process results in better crystallinity with rapid crystal growth. However, perovskite crystals become large and exhibit many surface defects resulting in a rough surface, long diffusion length of excitons, and dissociation of excitons; these factors lead to non-radiative recombination and a high leakage current. Therefore, several strategies, such as the addition of hydrophilic polymer and ligands to the perovskite precursor, have been studied to prevent the surface defects in PeLEDs. In this dissertation, it was demonstrated that functional groups of poly(2-ethyl-2- oxazoline) (PEOXA) lead to coordination bonds with the metal cations of perovskite. PEOXA can decrease formation temperature of the perovskite nanocrystals and improve phase stability as well. PEOXA added to a CsPbBr0.6I2.4 precursor solution successfully suppressed the formation of δ-phase (tilted octahedral) crystals and promoted the formation of stable α-phase (cubic) CsPbBr0.6I2.4 nanocrystals.

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