2D and quasi-2D perovskites for optoelectronic applications

Abstract

Organic-inorganic halide perovskites (OIHPs) are emerging solution processable semiconductors for various optoelectronic applications. Given their exceptional properties including long carrier diffusion length, bandgap tunability, solution processability and optoelectronic properties, phenomenal advancement was triggered in efficiency of light harvesting and light-emitting devices (LEDs). While the external quantum efficiency (EQE) of perovskite LEDs surged over 20% for green, red and infrared color emission, the development for blue emitting perovskite LEDs is still lagging behind. Moreover, widely studied 3D perovskites suffer from intrinsic structural instability. In this thesis, these issues will be tackled through the utilization of lower dimensionality layered perovskite (2D and quasi-2D) semiconductors. Through composition engineering, 3D perovskites exhibit good bandgap tunability for visible color emitting LEDs. However, blue emitting mixed halide 3D perovskites show low PLQY, and phase segregation is favorable under continuous high operation bias resulting in spectral instability. With enhanced quantum confinement and dielectric confinement effect, 2D and quasi-2D perovskites with high photoluminescence quantum yield (PLQY) and high color purity can be achieved. Suitable bandgap can be tuned through control of the width of quantum wells with chemical composition engineering. In the first part of the thesis, comprehensive investigation on the effect of alkylammonium chain length in phenyl-alkyl spacer cations on the perovskite film phase distribution was conducted. The correlation between the crystal structure, aromatic cations arrangement and optical properties were elucidated. Blue emitting perovskite films with good PLQY were obtained. Consequently, efficient sky-blue perovskite LEDs were successfully fabricated and characterized. It is well known that 3D perovskites are instable under humid ambient environment. To remedy this issue, the incorporation of quasi-2D perovskites into 3D perovskite for light harvesting applications was investigated. The exceptional stability of quasi-2D perovskites can be attributed to the hydrophobic nature of spacer cation bilayers. Method to introduce monovalent and divalent spacer cations on top of 3D perovskite for Ruddlesden-Popper and Dion-Jacobson perovskite formation was developed, and their stability in ambient environment was discussed in detail. Perovskite solar cells with improved performance and prolonged stability under accelerated aging according to ISOS protocols were attained. On the other hand, deposition method of ambient processed perovskite films was developed in an attempt to achieve stable perovskite films and lower production cost. Comparison of quasi-2D Ruddlesden-Popper and Dion-Jacobson perovskite films deposited under ambient conditions was evaluated, and their optical properties were characterized. Lengthened lifetime of ambient processed perovskite LEDs was achieved.