Quasi 2D perovskite materials for optoelectronics device application

Abstract

Lead halide perovskites have emerged as excellent semiconducting materials. They have excellent properties such as high defect tolerance and being solution processable direct bandgap semiconductors with tunable bandgap. These properties make them promising candidates for optoelectronic applications. Recently reported performance of perovskite light emitting diodes (peLEDs) have reached that of organic LEDs in red and green emission. However, the development of blue emitting peLEDs is mainly impeded by low efficiency and poor color stability. Blue emitting perovskites are usually prepared via halide mixing approach where emission wavelength can be tuned by the halide composition. However, mixed halide perovskites tend to segregate into inhomogeneous thin film which results in redshifted emission. On the other hand, blue emitting perovskite can be achieved with quasi 2D (q2D) perovskite structure. For example, Ruddlesden-Popper perovskite structure consists of self-assembled quantum well structure where lead halide perovskite is sliced by a layer of spacer cations. The bandgap of q2D perovskite is determined by the number of perovskite layers (n). Since q2D perovskites have large binding energy and good stability, blue emitting q2D perovskites are highly desirable for light emitting application. This thesis is devoted to blue emitting perovskite with Ruddlesden-Popper structure. To achieve pure blue emission, q2D perovskite with n=2 phase is targeted. Two common spacer cations, n-butylammonium (BA) and phenethylammonium (PEA), are employed. In the first part, the effects of spacer cation in q2D perovskite are investigated. Both single crystal samples of BA2MAPb2Br7 and PEA2MAPb2Br7 were prepared via slow temperature cooling method. Under UV laser exposure, the emission of BA sample was shifted to green while that of PEA sample remained the same. During thin film preparation, phase pure BA2MAPb2Br7 sample was obtained while PEA2MAPb2Br7 sample always contained impurity phases. Stability tests in both thin film and single crystal samples suggest that BA based q2D perovskite has inferior stability to UV exposure while PEA based sample can endure it. In order to achieve to achieve stable blue emitting q2D perovskite, spacer mixing approach is suggested. From XRD result, the q2D perovskites with mixed spacer cations ((BA0.5PEA0.5)2MAPb2Br7) has distinct structure from its parent materials. The mixed structure was found to be compatible with a range of BA-PEA ratios. Also, the approach was shown to be generalized to other spacer combinations. The mixed BA-PEA perovskite has demonstrated properties of high phase purity from BA spacer and improved stability from PEA spacer. Furthermore, both BA and mixed BA-PEA samples were used as emissive materials in LEDs. For BA based device, the emission was shifted to sky blue due to structural rearrangement during device fabrication. On the other hand, PL and EL spectra of mixed BA-PEA perovskite have negligible difference, implying improved stability of the mixed perovskite. Also, the device showed excellent spectral stability over time and voltage bias. Results indicate that q2D perovskite with mixed spacer cations exhibits advantages over that with single spacer cation. Also, it suggests a new route to tailor q2D perovskite with different spacer cations.