Metal Halide Perovskites And Perovskite Related Hybrids For Blue And White Light Emitting Diodes

Metal halide perovskites have received significant attention in the past decade as a promising class of material for energy harvesting, light emission and detection as well as various other solid-state devices. The easy solution processability, high defect tolerance, abundance of raw materials and exceptional optoelectronic properties have warranted intense investigation into their fundamental material physics and their device integration. Particularly, metal halide perovskite light-emitting diodes (LEDs) are thought to hold promise for next generation displays due to their narrow and tunable emission, high radiative efficiency and easy processability. For instance, red and green perovskite LEDs have now achieved the high external quantum efficiency required for commercial applications, on par with more mature lighting technologies such as organic LEDs and quantum dot LEDs. However, there are still several roadblocks that need to be overcome before perovskite LEDs can be considered a commercially viable technology. These challenges include, device stability, material toxicity, mass-production and development of efficient blue LEDs. The development of efficient blue LEDs is a major milestone in any display technology as it allows the production of multi-color images by combining the three primary colors red, green and blue (RGB). Similarly, the development of efficient white LEDs with excellent white light quality displaying high color rendering indices is also as important for solid-state lighting applications. In this dissertation, we explore two major themes related to the use of perovskites and perovskite related metal halide hybrids in light-emitting applications. The first and the broader work involves the study of various strategies to enable the realization of efficient blue perovskite LEDs. Three of the main challenges in obtaining efficient blue perovskite LEDs have been identified as band gap and emission tuning, poor radiative efficiency of perovskite blue emitters and charge carrier imbalance that results in suboptimal device performance. These issues are addressed in the first theme of this dissertation by introducing effective band gap and emission tuning strategies, improving radiative efficiency of blue perovskite emitters through trap passivation and engineering the energy band edges of perovskite thin films to obtain favorable band alignment and enhanced charge balance. In this theme, two methods are presented in chapter 2 and 3 to achieve band gap and emission control while ensuring spectral stability. These include the synthesis of perovskite hollow nanocrystals and phase control of perovskite multiple quantum well thin films. Perovskite hollow nanocrystals are shown to enable band gap and emission tuning through the formation of a hollow 3D crystal structure and quantum confinement. Reduced grain boundaries and passivation of surface trap sites in these nanocrystalline thin films are also shown to result in high radiative efficiencies. Although the formation of perovskite quantum wells has been shown to be an effective strategy to control the band gap and emission, the crystallization of multiple quantum well phases during thin film formation and fast energy funneling across phases has limited the application of perovskite quantum wells in color tunable and blue LEDs. In chapter 3, we show the addition of diammonium salts enables phase control in perovskite multiple quantum well thin films, resulting in color tunable emission. The presence of diammonium salts was also found to increase radiative efficiency enabling the fabrication of pure blue perovskite LEDs with good efficiency. Finally, energy band edge control of quasi-2D perovskites is also presented as a viable solution to charge injection barriers that appear due to unfavorable band alignment in blue perovskite LEDs. By modifying the dipole moment of organic spacer cations in quasi-2D perovskites through the addition of electron donating or withdrawing substituent groups, rational band edge control is achieved which enables improved blue LED performance due to enhanced charge balance. In the second theme of this dissertation, the use of low-dimensional metal halide hybrids in broadband white LEDs for high color quality applications is shown. The broadband emission spectra of low-dimensional metal halide hybrids are shown to be ideal for white light applications that require full spectrum coverage and high color rendering. The dissertation concludes by presenting a few exciting routes that could be explored to further improve the performance of blue and white LEDs based on perovskites and perovskite related materials. The work presented in this dissertation contributes to the field of perovskite LEDs by exploring the structure-processing-property-performance phase space and providing alternative routes to obtain spectrally stable and efficient blue perovskite LEDs as well as excellent light quality white LEDs, which could help transition perovskite LEDs to commercial viability.