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Wang, Miaosheng

Publications and source records attributed to Wang, Miaosheng.

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Dual Emission Bands of a 2D Perovskite Single Crystal with Charge Transfer State Characteristics

Several hybrid halide 2D-perovskite species emit light with an emergent and controversial broadband emission Stokes-shifted down from the narrow band emission. This work uncovers the sub- and above-bandgap emission and absorption characteristics of PEA 2 PbI 4 prepared with gap states introduced during single crystal growth. Here, gap states led to coexistent intrinsic and heterostructured electronic frameworks that are selectively accessible with ultraviolet (UV) and infrared (IR) light, respectively, resulting in the phenomenon of photoluminescence (PL) switching from narrowband green to broadband red. Electron-energy dependent cathodoluminescence shows a relative increase in the broadband red PL intensity as the electron penetration depth increases from 30 nm to 2 μm, confirming the heterostructured framework is formed in the bulk of the crystal. Excitation–emission power slope of 2.5 and up-conversion pump transient absorption (TA) spectra suggest that the IR up-conversion excitation with red photoluminescence, peaked at 655 nm, is a multiphoton process occurring in the heterostructured framework through a nonlinear optical response. The energetic pathways toward the dual emission bands are revealed by pump–probe transient absorption spectroscopy, showing energetically broad gap states with high sensitivity to an IR pump are upconverted and subsequently quickly relax from high to low energy levels within 4 ps. Furthermore, the up-conversion red PL demonstrates a linear polarization with magnetic field effects, thus affirming that the band-like heterostructured framework is crystallographically aligned with characteristics of spatially extended charge-transfer states.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Photoinduced iodide repulsion and halides-demixing in layered perovskites

Mixing halides in metal halide perovskites (MHPs) is an effective approach for adjusting the MHPs bandgap for applications in tandem solar cells. However, mixed-halide (MH-) MHPs undergo light-induced phase segregation (LIPS) under continuous illumination. Therefore, understanding the mechanism of LIPS is necessary for developing stable MH-MHPs. Here, we investigated LIPS in layered (L) MHPs and discovered a critical role of spacer cations in LIPS. Through probing chemical changes of LIPS, we unveil light-induced iodide repulsion and the formation of Br-rich-phase in illuminated regions during LIPS. This discovery also gives insight into the LIPS process in three-dimensional (3D) MHPs. By further investigating LIPS in 3D MHPs, we reveal that LIPS induces not only the formation of Br-rich and I-rich domains but also an overall change of halide distribution along the film thickness direction. Furthermore, LIPS is more significant in bulk due to a larger population of photogenerated charge carriers. Overall, this study reveals the chemical mechanism of LIPS in MHPs and its potential effect on device performance, offering insight into understanding the LIPS mechanism and improving the stability of MHPs.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Photophysics and Spin-Physics Studies on Persistent Upconversion Luminescence from Nonlinearly Polarizable Ferroelectric-Like Lattice Prepared by Orderly Packing Donor–Acceptor Structures under Multiphoton Excitation

The ultralong-lived upconversion luminescence with the lifetime of 0.48 s in a broad spectral range (530–650 nm) is observed in CD49 (9-(3-(5-bromopyridin-3-yl)prop-2-yn-1-yl)-9H-carbazole) crystal designed with donor–acceptor (carbazole–pyridine) structures under infrared excitation, simultaneously accompanied with second harmonic generation (SHG). This phenomenon indicates orderly packing donor–acceptor structures form a nonlinearly polarizable ferroelectric-like lattice with ultralong-lived light-emitting states, leading to much prolonged nonlinear optical behaviors. The persistent upconversion luminescence together with SHG is largely reduced when lowering crystallinity. This implies that nonlinearly polarizable ferroelectric-like lattice provides the necessary condition to generate persistent upconversion luminescence. Evidently, persistent upconversion luminescence becomes completely lacking when only using ultralong-lived light-emitting states without nonlinearly polarizable ferroelectric-like lattice, exampled by 4-(dimethylamino)benzonitrile dispersed in polyvinyl alcohol matrix. Magneto-photoluminescence shows that persistent upconversion luminescence is essentially a super-delayed fluorescence from crystalline intermolecular charge-transfer excitons formed in the nonlinearly polarizable ferroelectric-like lattice. Magnetodielectrics indicate crystalline intermolecular charge-transfer excitons are coupled with nonlinearly polarizable ferroelectric-like lattice, leading to prolonged nonlinear optical behaviors shown as persistent upconversion luminescence through super delayed fluorescence. In conclusion, therefore, crystalline intermolecular charge-transfer excitons formed in nonlinearly polarizable ferroelectric-like lattice provide an interesting platform to generate prolonged nonlinear optical behaviors toward developing persistent upconversion luminescence under multiphoton excitation.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Spin-orbital coupling and slow phonon effects enabled persistent photoluminescence in organic crystal under isomer doping

When periodically packing the intramolecular donor-acceptor structures to form ferroelectric-like lattice identified by second harmonic generation, our CD49 molecular crystal shows long-wavelength persistent photoluminescence peaked at 542 nm with the lifetime of 0.43 s, in addition to the short-wavelength prompt photoluminescence peaked at 363 nm with the lifetime of 0.45 ns. Interestingly, the long-wavelength persistent photoluminescence demonstrates magnetic field effects, showing as crystalline intermolecular charge-transfer excitons with singlet spin characteristics formed within ferroelectric-like lattice based on internal minority/majority carrier-balancing mechanism activated by isomer doping effects towards increasing electron-hole pairing probability. Our photoinduced Raman spectroscopy reveals the unusual slow relaxation of photoexcited lattice vibrations, indicating slow phonon effects occurring in ferroelectric-like lattice. Here, we show that crystalline intermolecular charge-transfer excitons are interacted with ferroelectric-like lattice, leading to exciton-lattice coupling within periodically packed intramolecular donor-acceptor structures to evolve ultralong-lived crystalline light-emitting states through slow phonon effects in ferroelectric light-emitting organic crystal.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Optically Induced Static Magnetization in Metal Halide Perovskite for Spin‐Related Optoelectronics

Abstract Understanding the feasibility to couple semiconducting and magnetic properties in metal halide perovskites through interface design opens new opportunities for creating the next generation spin‐related optoelectronics. In this work, a fundamentally new phenomenon of optically induced magnetization achieved by coupling photoexcited orbital magnetic dipoles with magnetic spins at perovskite/ferromagnetic interface is discovered. The depth‐sensitive polarized neutron reflectometry combined with in situ photoexcitation setup, constitutes key evidence of this novel effect. It is demonstrated that a circularly polarized photoexcitation induces a stable magnetization signal within the depth up to 7.5 nm into the surface of high‐quality perovskite (MAPbBr 3 ) film underneath a ferromagnetic cobalt layer at room temperature. In contrast, a linearly polarized light does not induce any detectable magnetization in the MAPbBr 3 . The observation reveals that photoexcited orbital magnetic dipoles at the surface of perovskite are coupled with the spins of the ferromagnetic atoms at the interface, leading to an optically induced magnetization within the perovskite’s surface. The finding demonstrates that perovskite semiconductor can be bridged with magnetism through optically controllable method at room temperature in this heterojunction design. This provides the new concept of utilizing spin and orbital degrees of freedom in new‐generation spin‐related optoelectronic devices.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Identifying Photoinduced Dipolar Polarization and Orbit–Orbit Interaction between Excitons in Organic–Inorganic Hybrid Perovskites

Abstract Photoinduced polarization and orbit–orbit interaction are important issues in hybrid perovskites toward developing optoelectronic functionalities. This paper identifies that photoinduced polarization occurs in hybrid perovskites with mixed‐cation methylammonium (MA)/formamidinium (FA) (MA x FA (1− x ) PbI 3 ) by measuring bulk polarization at 1 MHz in a magnetic field. Interestingly, when the internal dipole moment is increased upon increasing the MA:FA ratio, the photoinduced dipolar polarization can be substantially enhanced, clarifying the controversial issue of whether photoexcitation can induce a dielectric polarization within dipolar polarization regime in hybrid perovskites. Furthermore, upon increasing photoinduced dipolar polarization, it is found that the intrinsic orbit–orbit interaction between excitons can be increased, revealed by monitoring photocurrent change (Δ J sc ) upon switching the photoexcitation between linear and circular polarizations. This presents that organic cations are directly involved in the orbit–orbit interaction within band structures. Clearly, the studies provide an insightful understanding of the dipole moment effects on photoinduced dipolar polarization and orbit–orbit interaction between excitons in hybrid perovskites toward controlling the optoelectronic properties.

Dou, Yixuan↗
DOE OSTI2020

Establishing charge-transfer excitons in 2D perovskite heterostructures

Charge-transfer excitons (CTEs) immensely enrich property-tuning capabilities of semiconducting materials. However, such concept has been remaining as unexplored topic within halide perovskite structures. Here, we report that CTEs can be effectively formed in heterostructured 2D perovskites prepared by mixing PEA 2 PbI 4 :PEA 2 SnI 4 , functioning as host and guest components. Remarkably, a broad emission can be demonstrated with quick formation of 3 ps but prolonged lifetime of ~0.5 μs. This broad PL presents the hypothesis of CTEs, verified by the exclusion of lattice distortion and doping effects through demonstrating double-layered PEA 2 PbI 4 /PEA 2 SnI 4 heterostructure when shearing-away PEA 2 SnI 4 film onto the surface of PEA 2 PbI 4 film by using hand-finger pressing method. The below-bandgap photocurrent indicates that CTEs are vital states formed at PEA 2 PbI 4 :PEA 2 SnI 4 interfaces in 2D perovskite heterostructures. Electroluminescence shows that CTEs can be directly formed with electrically injected carriers in perovskite LEDs. Clearly, the CTEs presents a new mechanism to advance the multifunctionalities in 2D perovskites.

36 MATERIALS SCIENCE↗
DOE OSTI2020

Tuning spin-orbit coupling towards enhancing photocurrent in hybrid organic-inorganic perovskites by using mixed organic cations

The presence of the heavy elements leads to strong spin-orbit coupling (SOC) in hybrid organic-inorganic perovskites (HOIPs), which plays an important role in the photovoltaic performance of HOIPs by changing the populations between bright and dark states. The organic cation is a critical composition and affects SOC via the interaction with the inorganic cage. In this work, we use magneto-photocurrent measurement to explore the SOC effect upon using mixed organic cations (methylammonium (MA) and formamidinium (FA)) with different dipole moments in Pb based perovskites. Magneto-photocurrent measurements showed that the internal magnetic parameter B0 is decreased from 402.41 mT to 180.18 mT and 104.58 mT with decreasing the MA/FA ratio. This provides evidence that changing the internal dipole moment by using mixed organic cations can essentially change the SOC in HOIPs. Simultaneously, the photocurrent is increased from 21.46 mA/cm 2 to 22.60 mA/cm 2 and 23.08 mA/cm 2 when the MA/FA ratio is changed from 1:0 to 0.9:0.1 and 0.7:0.3. Above all, our results indicate that the change in SOC by different organic cations can also be a factor affecting the performance of perovskite solar cells.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Two-Photon Up-Conversion Photoluminescence Realized through Spatially Extended Gap States in Quasi-2D Perovskite Films

A new approach to generate a two-photon up-conversion photoluminescence (PL) by directly exciting the gap states with continuous-wave (CW) infrared photoexcitation in solution-processing quasi-2D perovskite films [(PEA) 2 (MA) 4 Pb 5 Br 16 with n = 5] is reported. Specifically, a visible PL peaked at 520 nm is observed with the quadratic power dependence by exciting the gap states with CW 980 nm laser excitation, indicating a two-photon up-conversion PL occurring in quasi-2D perovskite films. Decreasing the gap states by reducing the n value leads to a dramatic decrease in the two-photon up-conversion PL signal. This confirms that the gap states are indeed responsible for generating the two-photon up-conversion PL in quasi-2D perovskites. Furthermore, mechanic-al scratching indicates that the different-n-value nanoplates are essentially uniformly formed in the quasi-2D perovskite films toward generating multi-photon up-conversion light emission. More importantly, the two-photon up-conversion PL is found to be sensitive to an external magnetic field, indicating that the gap states are essentially formed as spatially extended states ready for multi-photon excitation. Finally, polarization-dependent up-conversion PL studies reveal that the gap states experience the orbit–orbit interaction through Coulomb polarization to form spatially extended states toward developing multi-photon up-conversion light emission in quasi-2D perovskites.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Light-Ferroic Interaction in Hybrid Organic–Inorganic Perovskites

Given the remarkable performance of hybrid organic–inorganic perovskites (HOIPs) in solar cells, light emitters, and photodetectors, the quest to advance the fundamental understanding of the photophysical properties in this class of materials remains highly relevant. Recently, the discovery of ferroic twin domains in HOIPs has renewed the debate of the ferroic effects on optoelectric processes. This work explores the interaction between light and ferroic twin domains in CH 3 NH 3 PbI 3 . Due to strain and chemical inhomogeneities, photogenerated electrons and holes show a preferential motion in the ferroelastic twin domains. Density functional theory (DFT) shows that electrons and holes result in lattice expansion in CH 3 NH 3 PbI 3 differently. Hence, light generates strain in the ferroelastic domains due to preferential photocarrier motion, leading to a screening of strain variation. X-ray diffraction studies verify the DFT simulations and reveal that the photoinduced strain is light intensity dependent, and the photoexcitation is a prerequisite of inducing strain by light. This work extends the fundamental understanding of light-ferroic interaction and offers guidance for developing functional devices.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Poly(ethylene oxide)-assisted energy funneling for efficient perovskite light emission

Poly(ethylene oxide) (PEO) has been demonstrated as an effective polymer additive for fabrication of organic–inorganic hybrid perovskite light emitting devices due to its properties enabling pinhole-free film morphology, improved charge injection, and defect passivation. In this paper, we report a new aspect of PEO in assisting the energy funneling in formamidinium lead bromide (FAPbBr 3 ) thin films, leading to a very high photoluminescence quantum yield of ~62.1%. By mixing PEO with FAPbBr 3 at a desired ratio, self-assembled 3D polycrystals with sizes from ~20 to ~1500 nm were formed. The polycrystals contain inhomogeneous domains with a range of energetic excited states. Through fluorescence lifetime image microscopy, the in situ fluorescence emissions and the lifetimes of the domains were measured simultaneously, indicating energy funneling from the wide-bandgap domains (with small-size grains) into the narrow-bandgap domains (with large-size grains). PL spectroscopy on the nano-second scale was used to study the dynamics of charge transfer, which shows a reduced short-wavelength emission and narrowed full width at half maximum with increasing time. Our results also indicate that the optimal PEO doping ratio of 0.35 :1 can facilitate the most efficient charge carrier cascade for energy funneling. Furthermore, this study establishes the role of PEO-assisted energy funneling avoiding the defect-quenching effect in perovskites, for potential applications in perovskite LEDs and lasers.

29 ENERGY PLANNING, POLICY, AND ECONOMY↗
DOE OSTI2019

Investigating underlying mechanism in spectral narrowing phenomenon induced by microcavity in organic light emitting diodes

This paper reports our experimental studies on the underlying mechanism responsible for electroluminescence spectral narrowing phenomenon in the cavity-based organic lightemitting diodes. It is found that the microcavity generates an emerging phenomenon: a magneto-photoluminescence signal in Poly(9,9-dioctylfluorene-alt-benzothiadiazole) polymer under photoexcitation, which is completely absent when microcavity is not used. This provides an evidence that microcavity leads to the formation of spatially extended states, functioning as the intermediate states prior to the formation of Frenkel excitons in organic materials. This is confirmed by the magneto-electroluminescence solely observed from the cavity-based light-emitting diodes under electrical injection. Furthermore, the narrowed electroluminescence output shows a linear polarization, concurrently occurred with magnetoelectroluminescence. This indicates that the spatially extended sates become aligned towards forming coherent light-emitting excitons within the microcavity through optical resonance. Clearly, the spatially extended states present the necessary condition to realize electroluminescence spectral narrowing phenomenon towards lasing actions in cavity-based organic light-emitting diodes.

36 MATERIALS SCIENCE↗