Ph.D. (Doctor of Philosophy)
Department of Chemistry and Biochemistry
This dissertation comprehensively studies the optoelectronic properties of organic-inorganic hybrid perovskites to fundamentally answer their foundations of outstanding performance on solar cells, photodetectors, nanowire lasers and other optoelectronic applications. Specifically, a novel type of charge carrier-lattice interaction was discovered in perovskite methylammonium lead iodide (CH3NH3PbI3), where photoluminescence lifetime of photoinduced carriers is strongly dependent on the rotational frequency of CH3NH3+, as modulated via substitution of hydrogens with deuterium atoms in the organic cation. In addition, two-dimensional Ruddlesden-Popper perovskite (CH3NH3)2Pb(SeCN)2I2 was first synthesized and characterized in the field, and its photoluminescence properties were systematically examined. The existence of intensive photoluminescence peak with small full width at half maximum (35.8 nm) along with clearly observed photoluminescence decay kinetics (4.6 ns) indicates a highly homogeneous band structure and distinctive semiconductor properties of this material. By contrast, thiocyanate-based perovskite counterpart (CH3NH3)2Pb(SCN)2I2 showed much broader photoluminescence peak (full width at half maximum: 171.1 nm) with no detectable time-resolved photoluminescence, thus signifying the negative impact of sulfur atom on the optoelectronic function of such material.
Moreover, it was found that the moisture stability of CH3NH3PbI3 is significantly enhanced after incorporating divalent anion Se2- into the material lattice. 10% w/w PbSe doped CH3NH3PbI3 films were chemically stable after 72 hrs of aging in 100% humidity at 40 °C, while pristine CH3NH3PbI3 film was completely degraded after only 30 min. of aging under the same condition. As PbSe doped CH3NH3PbI3 films maintained the perovskite structure, a top power conversion efficiency of 10.4% with 85% retention after 624 hours of aging in ambient air was achieved on an unencapsulated 10% w/w PbSe doped CH3NH3PbI3 solar cell, in contrast to 16% retained power conversion efficiency after 432 hrs of aging on CH3NH3PbI3 cell. Meanwhile, the incorporated Se2- also effectively suppressed iodine diffusion in solar cell, thereby leading to greatly improved chemical stability of the silver electrodes. To explore chemical pathways that can additionally enable greater thermal stability of perovskite materials, which is another crucial factor required for environmental stability of photovoltaics under operating conditions, isotropic Cs+ cation was doped into CH3NH3PbI3 structure for heat stress tests, where perovskite (CH3NH3)0.95Cs0.05PbI3 thin film showed increased thermal stability up to 200 ˚C, at which pristine CH3NH3PbI3 thin film was completely decomposed to PbI2. As a bonus, (CH3NH3)0.95Cs0.05PbI3 perovskite single crystal exhibited prolonged carrier lifetime in comparison with pristine CH3NH3PbI3 counterpart, which can be one of the reasons responsible for the superior photovoltaic performance witnessed on Cs+-doped perovskite solar cells.
To further evaluate the effects of alkali metal ion on the physical properties and optoelectronic performance of perovskite materials, ultraviolet (UV) photodetectors based on formamidinium lead chloride (HC(NH2)2PbCl3) perovskite nanorods were fabricated. It was found that HC(NH2)2PbCl3 nanorods grown with LiCl additive displayed much enhanced photocurrents under UV illumination. Most importantly, LiCl:HC(NH2)2PbCl3 photodetector exhibits unprecedented distinguishability towards 254-nm and 365-nm UV photons with temporally varying intensities, as in the form of bipolar photocurrents/photovoltages with periodically oscillating amplitudes. Such discovery in perovskite optoelectronics will realize ciphering, acquisition and decoding of optical information embedded in UV photons, and will contribute to the development of novel communication technology.
Gong, Jue, "Fundamental Studies of Chemical Stability and Carrier Process in Hybrid Perovskite Materials" (2018). Graduate Research Theses & Dissertations. 7071.
Northern Illinois University
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