Browsing by Author "Tsai, Hsinhan"

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    Design principles for electronic charge transport in solution-processed vertically stacked 2D perovskite quantum wells
    (Springer Nature, 2018) Tsai, Hsinhan; Asadpour, Reza; Blancon, Jean-Christophe; Stoumpos, Constantinos C.; Even, Jacky; Ajayan, Pulickel M.; Kanatzidis, Mercouri G.; Alam, Muhammad Ashraful; Mohite, Aditya D.; Nie, Wanyi
    State-of-the-art quantum-well-based devices such as photovoltaics, photodetectors, and light-emission devices are enabled by understanding the nature and the exact mechanism of electronic charge transport. Ruddlesden-Popper phase halide perovskites are two-dimensional solution-processed quantum wells and have recently emerged as highly efficient semiconductors for solar cell approaching 14% in power conversion efficiency. However, further improvements will require an understanding of the charge transport mechanisms, which are currently unknown and further complicated by the presence of strongly bound excitons. Here, we unambiguously determine that dominant photocurrent collection is through electric field-assisted electron-hole pair separation and transport across the potential barriers. This is revealed by in-depth device characterization, coupled with comprehensive device modeling, which can self-consistently reproduce our experimental findings. These findings establish the fundamental guidelines for the molecular and device design for layered 2D perovskite-based photovoltaics and optoelectronic devices, and are relevant for other similar quantum-confined systems.
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    Highly efficient photoelectric effect in halide perovskites for regenerative electron sources
    (Springer Nature, 2021) Liu, Fangze; Sidhik, Siraj; Hoffbauer, Mark A.; Lewis, Sina; Neukirch, Amanda J.; Pavlenko, Vitaly; Tsai, Hsinhan; Nie, Wanyi; Even, Jacky; Tretiak, Sergei; Ajayan, Pulickel M.; Kanatzidis, Mercouri G.; Crochet, Jared J.; Moody, Nathan A.; Blancon, Jean-Christophe; Mohite, Aditya D.
    Electron sources are a critical component in a wide range of applications such as electron-beam accelerator facilities, photomultipliers, and image intensifiers for night vision. We report efficient, regenerative and low-cost electron sources based on solution-processed halide perovskites thin films when they are excited with light with energy equal to or above their bandgap. We measure a quantum efficiency up to 2.2% and a lifetime of more than 25 h. Importantly, even after degradation, the electron emission can be completely regenerated to its maximum efficiency by deposition of a monolayer of Cs. The electron emission from halide perovskites can be tuned over the visible and ultraviolet spectrum, and operates at vacuum levels with pressures at least two-orders higher than in state-of-the-art semiconductor electron sources.
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    Hybrid Perovskite Materials for Stable Optoelectronics Applications
    (2018-04-20) Tsai, Hsinhan; Ajayan, Pulickel M.; Lou, Jun; Mohite, Aditya D.
    Organic-inorganic hybrid perovskites materials have grab enormous attention in the material research community. This is because of their exceptional semiconducting properties such as direct band gap, free carrier generation, broad absorption range, long diffusion length and carrier lifetime, which enable highly efficient photovoltaic device over 22%, surpassing other classical semiconductors in few years. However, one fundamental bottleneck remains in this system that mitigates such material from wide use that are reproducibility, photo- and chemical instability. These are found to be closely related to the structure of the material, such as the hydroscopic nature of the organic cation, symmetry of the crystal structure and degree of thin film crystallinity. Therefore, the focus of this thesis is to understand the basic mechanism in structure that dominate the stability of perovskites materials properties and design robust hybrid perovskite structures through organic cation engineering. Through synthetic approach, we found that the bulky organic molecules can be inserted to the perovskite lattice, forming a layered structure with quantum and dielectric confinement, called Ruddlesden-Popper (RP) perovskites. Employing our previously developed hot-casting method, we were able to obtain near single crystalline thin film with preferred orientation. With the protection of the bulky molecules, the stability of the perovskite layers is much extended. Beyond the photovoltaics, the oriented, highly crystalline thin film can facilitate the current injection in light-emitting diodes (LEDs) which had high radiance with 1 % EQE in device performance. The devices also had low turn-on voltage which can decrease the energy consumption and benefit to lighting applications. This thesis demonstrates the detail solutions for fundamental problems and the research results can help to promote technologies and push the limits in hybrid perovskites optoelectronics society.
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    Supramolecular block copolymer photovoltaics through ureido-pyrimidinone hydrogen bonding interactions
    (Royal Society of Chemistry, 2016) Lin, Yen-Hao; Nie, Wanyi; Tsai, Hsinhan; Li, Xiaoyi; Gupta, Gautam; Mohite, Aditya D.; Verduzco, Rafael
    A challenge in the development of bulk heterojunction organic photovoltaics (BHJ OPVs) is achieving a desirable nanoscale morphology. This is particularly true for polymer blend OPVs in which large-scale phase separation occurs during processing. Here, we present a versatile approach to control the morphology in polymer blend OPVs through incorporation of self-associating 4 2-ureido-4[1H]-pyrimidinone (UPy) endgroups onto donor and acceptor conjugated polymers. These UPy functionalized polymers associate to form supramolecular block copolymers during solution blending and film casting. Atomic force microscopy measurements show that supramolecular associations can improve film uniformity. We find that the performance of supramolecular block copolymer OPVs improves from 0.45% to 0.77% relative to the non-associating conjugated polymer blends at the same 155 °C annealing conditions. Impedance measurements reveal that UPy endgroups both increase the resistance for charge recombination and for bulk charge transport. This work represents a versatile approach to reducing large-scale phase separation in polymer–polymer blends and directing the morphology through supramolecular interactions.