Browsing by Author "Zhang, Hao"

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    Direct visualization of ultrafast lattice ordering via resonant electron-phonon coupling in 2D hybrid perovskites
    (2021-08-25) Zhang, Hao; Mohite, Aditya D.
    Understanding and tracing the dynamic interactions between charge carriers and crystal lattice has remained a challenge in photo-excited semiconductors. Specifically, a direct visualization of carrier-lattice interactions (electron-phonon coupling) provides crucial insights in such as hot-carrier lifetime and recombination rates, which are key factors for device fabrications. In this study, we’ve monitored the transient structural response of two-dimensional perovskites (2DPKs) by tracking the evolution of the wavevector-resolved electron diffraction patterns, through femtosecond ultrafast electron diffraction (UED). Analysis of the Bragg peak intensities reveals a unique positive response, suggesting an ultrafast lattice ordering resulting from a resonant transfer from hot-carriers to perovskite lattice. Simulated structural response and calculation of vibrational modes may reveal the presence of non-polar optical phonons, which induces in-plane octahedral tilts and a reduction of distortion toward symmetrized phase, consistent with the predicted electron-phonon interactions through optical deformation potential via non-polar modes. Finally, we show the strength and the dynamics of the reported electron-phonon coupling process can be further tailored via alternating the organic layers of 2D perovskites, resulting in difference in mechanical rigidity and intrinsic distortions. This study on the structural dynamics of 2D perovskites may provide fundamental information on tailoring specific electron-phonon coupling channels, paving a pathway for the design of efficient perovskite-based opto-electronic devices.
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    High-phase purity two-dimensional perovskites with 17.3% efficiency enabled by interface engineering of hole transport layer
    (Elsevier, 2021) Sidhik, Siraj; Wang, Yafei; Li, Wenbin; Zhang, Hao; Zhong, Xinjue; Agrawal, Ayush; Hadar, Ido; Spanopoulos, Ioannis; Mishra, Anamika; Traoré, Boubacar; Samani, Mohammad H. K.; Katan, Claudine; Marciel, Amanda B.; Blancon, Jean-Christophe; Even, Jacky; Kahn, Antoine; Kanatzidis, Mercouri G.; Mohite, Aditya D.
    State-of-the-art p-i-n-based 3D perovskite solar cells (PSCs) use nickel oxide (NiOX) as an efficient hole transport layer (HTL), achieving efficiencies >22%. However, translating this to phase-pure 2D perovskites has been unsuccessful. Here, we report 2D phase-pure Ruddlesden-Popper BA2MA3Pb4I13 perovskites with 17.3% efficiency enabled by doping the NiOX with Li. Our results show that progressively increasing the doping concentration transforms the photoresistor behavior to a typical diode curve, with an increase in the average efficiency from 2.53% to 16.03% with a high open-circuit voltage of 1.22 V. Analysis reveals that Li doping of NiOX significantly improves the morphology, crystallinity, and orientation of 2D perovskite films and also affords a superior band alignment, facilitating efficient charge extraction. Finally, we demonstrate that 2D PSCs with Li-doped NiOX exhibit excellent photostability, with T99 = 400 h at 1 sun and T90 of 100 h at 5 suns measured at relative humidity of 60% ± 5% without the need for external thermal management.
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    Unraveling exciton photo-physics in low-dimensional perovskites towards classical and quantum light emissions
    (2024-04-11) Zhang, Hao; Mohite, Aditya
    Organic-inorganic (hybrid) halide perovskite, an arising class of low-cost semiconductor materials, has gained great research interest due to the intriguing photo-physical properties as well as great potentials in photovoltaics and light emitting applications. Such chemically-altered semiconductor platforms enable physical tunability of charge carriers in different dimensionalities, ranging from 3D bulk materials to 0D quantum dots, resulting in novel physical behaviors and light emission properties. Despite great research attention and wide applications in opto-electronics, the fundamental physical properties of the electron-hole pair quasiparticles termed excitons, such as exciton-phonon interactions in 2D perovskites, and the fine structures in perovskite nanocrystals, are still underexplored. In this thesis, we will focus on the exciton properties in two specific type of perovskites systems: two-dimensional perovskites and zero-dimensional perovskite quantum dots. Using a series of spectroscopic and structural characterizations, we will investigate how excitons in 2D perovskites interact with the lattice vibrations and structural dynamics, and the intrinsic exciton behaviors in buried quantum dots, such as fine structure splitting and quantum light emissions at the single-dot level. In the first part, we study the exciton-phonon coupling and carrier dynamics using the ultrafast spectroscopy and stead-state cryogenic spectroscopies, which provides a correlated prospective of the light-induced structural dynamics and origin of exciton-phono couplings in multi-layered 2D perovskites. We suggest that the creation of a dense electron–hole plasma triggers the relaxation of lattice distortion at shorter timescales by modulating the crystal cohesive energy. We also demonstrate close to 3D like exciton-LO phonon coupling, as well as unique light-matter interactions such as exciton-polaritons and lasing properties in 2D perovskites. In the second part, we will demonstrate a novel material platform of perovskite-based quantum emitters, by embedding FAPbI3 based perovskite quantum dots (QDs) into the wide-bandgap 3D perovskite FAPbBr3 using one-step solution processed technique. Spectroscopic characterizations reveal the quantum nature of light emission from the buried QDs, as well as rich exciton fine structures such as triplets and singlets with assistance of magneto-spectroscopy. Such buried QDs exhibit a clear photon-antibunching signature, with second-order correlation function g2(0) to be ~0.15 at T = 6K. Photoluminescence suggests ultra narrow emissions lines with 130 μeV FWHM. High-resolution transmission electron microscope (HR-TEM) confirms the presence of nanometer-sized domains, which indicates the formation of quantum dots during the rapid crystallization of the precursor solvent. The embedded emitters exhibit a mono-exponential radiative decay (τ = 300 ps), with additional multi-exciton states from bi-excitons and trions, as well as temperature-dependent linewidth broadening and phonon sidebands, expected for colloidal FA-based nanocrystals. In addition, direct spectroscopic signatures of the exciton fine states - such as triplets splitting and singlet states brightening - are clearly resolved under magnetic field, revealing the spectral origin and rich photo-physics from the embedded QDs. Furthermore, we have firstly demonstrated the capability of electrical-driven single-photon emission in perovskite by sandwiching the system between electron-transport and hole-transport layers. Our results may pave the pathway of on-chip integration of low-cost single-photon sources for quantum optical systems.