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Yan, Han

Publications and source records attributed to Yan, Han.

At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

Elucidate the Thermal Degradation Mechanism of Y6‐Based Organic Solar Cells by Establishing Structure‐Property Correlation

Abstract Organic solar cells (OSCs) achieved performance booming benefiting from the emerging of non‐fullerene acceptors, while inadequate device stability hampers their further application. At present, the prevalent belief attributes the inevitable thermal degradation of OSC device to morphological instability caused by excessive phase separation and crystallization in the active layer during device operation. However, it is inapplicable for state‐of‐art Y6‐based devices which strongly degrade before large‐scale morphology change. Herein, an alternative degradation mechanism is elucidated wherein molecular orientation change and demixing induced performance degradation in Y6‐based devices. Distinct from IT‐4F‐based counterpart, Y6‐based devices suffer severe thermal degradation dominated by open‐circuit voltage (V OC ) and fill factor (FF) losses. TheV OC loss is attributed to molecular orientation transition of polymer donors from edge‐on to face‐on, leading to a strong built‐in potential reduction and increase in non‐radiative loss due to energy level shifting. As forFFdecay, discontinuous acceptor phases result in electron mobility decrease by over orders of magnitude, originating from the increased molecular stacking and phase separation. This work reveals the thermal degradation mechanism for Y6‐based devices and correlates the photoelectric properties with morphology instability, which will offer guidance for improving the stability of high‐performance OSCs.

Chemistry↗
DOE OSTI2023

Eliminating the Imbalanced Mobility Bottlenecks via Reshaping Internal Potential Distribution in Organic Photovoltaics

The imbalanced carrier mobility remains a bottleneck for performance breakthrough in even those organic solar cells (OSCs) with recorded power conversion efficiencies (PCEs). Herein, a counter electrode doping strategy is proposed to reshape the internal potential distribution, which targets to extract the low mobility carriers at far end. Device simulations reveal that the key of this strategy is to partially dope the active layer with a certain depth, therefore it strengthens the electric field for low mobility carriers near counter electrode region while avoids zeroing the electric field near collection electrode region. Taking advantage of these, PCE enhancements are obtained from 15.4% to 16.2% and from 16.9% to 18.0%, respectively, via cathode p-doping and anode n-doping. Extending its application from opaque to semitransparent devices, the PCE of dilute cell rises from 10.5% to 12.1%, with a high light utilization efficiency (LUE) of 3.5%. The findings provide practical solutions to the core device physical problem in OSCs.

14 SOLAR ENERGY↗
DOE OSTI2023

Beyond single tetrahedron physics of the breathing pyrochlore compound Ba 3 ⁢Yb 2 ⁢Zn 5 ⁢O 11

Recently, a new class of quantum magnets, the so-called breathing pyrochlore spin systems, have attracted much attention due to their potential to host exotic emergent phenomena. Here, we present magnetometry, heat capacity, thermal conductivity, muon-spin relaxation, and polarized inelastic neutron scattering measurements performed on high-quality single crystal samples of the breathing pyrochlore compound Ba 3 ⁢Yb 2 ⁢Zn 5 ⁢O 11 . We interpret these results using a simplified toy model and provide insight into the low-energy physics of this system beyond the single tetrahedron physics proposed previously.

36 MATERIALS SCIENCE↗
DOE OSTI2023

N-dopants optimize the utilization of spontaneously formed photocharges in organic solar cells

Non-fullerene acceptors (NFAs) not only bring rapid efficiency progress to organic solar cells (OSCs) but have also generated scientific interest in re-evaluating the photocharge generation route via spontaneous or heterojunction-assisted charge separation. In this work, we propose the addition of n-dopants as a strategy to optimize the utilization of spontaneously formed photocharges in NFAs and construct ultralow donor-content dilute bulk heterojunction (BHJ) devices to validate the dopant's roles. N-dopants enhance the transport length of spontaneously formed photocharges over twofold and alleviate the bimolecular recombination with balanced carrier mobility during charge extraction. This improves the photocurrent of ultralow donor-content dilute BHJ devices by over 2.4 times. Moreover, these benefits are successfully conducted to the BHJ devices, and enhancement of conversion efficiencies is observed in both the 100 nm and 400 nm thick OSCs. Our results provide a feasible way to optimize the intrinsic photocharges in NFAs and renew the possibility of efficient single-component OSCs as inorganic photovoltaic techniques.

14 SOLAR ENERGY↗
DOE OSTI2022

Magnetic field effects in an octupolar quantum spin liquid candidate

Quantum spin liquid (QSL) is a disordered state of quantum-mechanically entangled spins commonly arising from frustrated magnetic dipolar interactions. However, QSL in some pyrochlore magnets can also come from frustrated magnetic octupolar interactions. Although the key signature for both dipolar and octupolar interaction-driven QSL is the presence of a spin excitation continuum (spinons) arising from the spin quantum number fractionalization, an external magnetic field-induced ferromagnetic order will transform the spinons into conventional spin waves in a dipolar QSL. By contrast, in an octupole QSL, the spin waves carry octupole moments that do not couple, in the leading order, to an external magnetic field or to neutron moments but will contribute to the field dependence of the heat capacity. Here, in this study, we use neutron scattering to show that the application of a large external magnetic field to Ce 2 Zr 2 O 7 , an octupolar QSL candidate, induces an Anderson-Higgs transition by condensing the spinons into a static ferromagnetic ordered state with octupolar spin waves invisible to neutrons but contributing to the heat capacity. Our theoretical calculations also provide a microscopic, qualitative understanding for the presence of octupole scattering at large wave vectors in Ce 2 Sn 2 O 7 pyrochlore, and its absence in Ce 2 Zr 2 O 7 . Therefore, our results identify Ce 2 Zr 2 O 7 as a strong candidate for an octupolar U(1) QSL, establishing that frustrated magnetic octupolar interactions are responsible for QSL properties in Ce-based pyrochlore magnets.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2022

Molecular Doping Increases the Semitransparent Photovoltaic Performance of Dilute Bulk Heterojunction Film with Discontinuous Polymer Donor Networks

The semitransparent and colorful properties of organic solar cells (OSCs) attract intensive academic interests due to their potential application in building integrated photovoltaics, wearable electronics, and so forth. The most straightforward and effective method to tune these optical properties is varying the componential ratio in the blend film. However, the increase in device transmittance inevitably sacrifices the photovoltaic performance because of severe carrier recombination that originates from discontinuous charge-transport networks in the blend film. In this work, a strategy is proposed via the molecular-doping strategy to overcome these shortcomings. It is discovered that p-doping is able to release the trapped holes in segregated polymer domains leading to short-circuit current enhancement, while n-doping is more effective to fill the bandgap states producing a higher fill factor. More importantly, either type of doping improves the photovoltaic performance in the semitransparent photovoltaic devices. These discoveries provide a new pathway to breaking the compromise between the photovoltaic performance and optical transmittance in semitransparent OSCs, and hold promise for their future commercialization.

14 SOLAR ENERGY↗
DOE OSTI2020

A versatile strategy for controlled assembly of plasmonic metal/semiconductor hemispherical nano-heterostructure arrays

Recent advances in manipulating plasmonic properties of metal/semiconductor heterostructures have opened up new avenues for basic research and applications. Herein, we present a versatile strategy for the assembly of arrays of plasmonic metal/semiconductor hemispherical nano-heterostructures (MSHNs) with control over spacing and size of the metal/semiconductor heterostructure array, which can facilitate a wide range of scientific studies and applications. The strategy combines nanosphere lithography for generating the metal core array with solution-based chemical methods for the semiconductor shell that are widely available and kinetically controllable. Periodic arrays of Au/Cu 2 O and Ag/Cu 2 O heterostructures are synthesized to demonstrate the approach and highlight the versatility and importance of the tunability of plasmonic properties. The morphology, structure, optical properties, and elemental compositions of the heterostructures were analyzed. Finally, this strategy can be important for understanding and manipulating fundamental nanoscale solid-state physical and chemical properties, as well as assembling heterostructures with desirable structure and functionality for applications.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2019

Nonvolatile Electric Control of the Anomalous Hall Effect in an Ultrathin Magnetic Metal

Abstract Low‐dimensional magnetism has been boosted by the recent discovery of the ferromagnetism in layered two‐dimensional (2D) semiconductors. Although the macroscopic magnetic moments of 2D ferromagnets are weak, the anomalous Hall effect (AHE) can serve as a versatile electric probe to their magnetic properties. Here, the nonvolatile electric‐field manipulation of the AHE in an ultrathin metallic ferromagnet with perpendicular magnetic anisotropy at room temperature is reported, which is achieved by the electrostatic modulation of the longitudinal resistivity via a ferroelectric substrate without varying magnetization. Therefore, this work demonstrates an electric‐field‐controlled room‐temperature memory device based on the zero‐magnetic‐field anomalous Hall resistance of an ultrathin ferromagnet. More importantly, the experimental results disentangle magnetization and anomalous Hall resistance. As a result, the study reveals a linear decrease of anomalous Hall conductivity with normal conductivity, which is distinct from previous scaling relations. Accordingly, this work manifests a universe avenue to harnessing the AHE in low‐dimensional magnetic materials.

Feng, Zexin↗
DOE OSTI2019

Vibronic Effects in the Ultrafast Interfacial Electron Transfer of Perylene-Sensitized TiO 2 Surfaces

We combine ultrafast transient absorption (TA) spectroscopy and nonadiabatic quantum dynamics simulations to describe the real-time unfold of vibronic effects on the photoabsorption of TiO 2 anatase sensitized with the (perylen-9-yl)carboxylate dye (Pe-COOH/TiO2). The excited state is mapped in time and frequency by ultrafast broadband spectroscopy while atomistic quantum dynamics is used to simulate the self-consistent vibronic effects. The TA map shows the lifetime of the electronic population generated in the S 1 state of the dye and the rise of the absorption D 0 –D 1 of the cation. The theoretical analysis reveals that the electron transfer from perylene into TiO2 is complete within 20 fs, in agreement with the 12 fs experimental measurement. Because of the structural relaxation produced by the photoinduced electron transfer, the optical gap decreases by 390 meV, in agreement with the D 0 –D 1 transition band. Furthermore, the reorganization energy estimated to be around 220 meV is mostly due to the energy shift of the HOMO level, since the electron transfer occurs in the wide-band limit with little dependence on reorganization energy modes. By assuming the Condon approximation and by making use of the mixed quantum/classical trajectories of the Pe-COOH/TiO 2 system, the absorption spectrum is calculated, and the broad features of the transient absorption spectrum are correlated to excited-state nuclear reorganization effects of the adsorbate dye. The reorganization energy modes are identified by the power spectrum of the velocity autocorrelation function, which shows the occurrence of nonequilibrium modes within the range 1000–1800 cm –1 as in-plane asymmetric C–C vibrations in the perylene dye. The vibrational modes with the strongest influence on the optical gap contribute to shifting the absorption spectrum up in energy by ~2000 cm –1 . Finally, the overall agreement between theory and experiment reveals the capabilities of both methods to study vibronic effects in molecular and extended systems.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2018

A novel role for Bcl2l13 in promoting beige adipocyte biogenesis

Highlights: • Bcl2l13 expression is induced in Browning of white adipose tissues in Mice. • Knockdown of Bcl2l13 in beige adipocytes represses adipogenesis and thermogenic programing. • Bcl2l13 induces thermogenic programing by regulating mitochondrial dynamic and biogenesis. Bcl2l13 is a member of the Bcl-2 family that has been found to play a central role in regulating apoptosis. Recently Bcl2l13 has been reported to induce mitophagy as a functional mammalian homolog of Atg32. However, the role of Bcl2l13 in adipose tissue has not been investigated yet. In the present study, we found that Bcl2l13 expression was increased in white adipose tissue browning process stimulated by cold exposure or β3-adrenergic agonist CL-316,243 in vivo as well as during brown adipocytes differentiation in vitro. Moreover, Bcl2l13 disruption dramatically inhibited the browning program of preadipocytes, evidenced by reduced Prdm16, Ucp1, Dio2 and Adrb3 expression. Our findings revealed that the inhibition effect of Bcl2l13 disruption on browning program may be independent of altering autophagy activity, but through regulating mitochondrial dynamic and biogenesis, supported by decreased mitochondrial fission/fussion genes, PGC-1α and mitochondrial respiratory chain complexes expression. Taken together, our study uncovered a novel function of Bcl2l13 in adipocytes differentiation and promoting browning program.

60 APPLIED LIFE SCIENCES↗
DOE OSTI2018

Lewis Acid Doping Induced Synergistic Effects on Electronic and Morphological Structure for Donor and Acceptor in Polymer Solar Cells

Abstract Due to the attraction of optimizing the electronic structure beyond chemical synthesis, molecular doping has recently aroused wide interest in the field of organic solar cells. However, the selection of limited dopants confines its successful application. Inspired by the Lewis base characteristics of the photovoltaic materials, the Lewis acid as novel dopant is introduced in organic solar cells. In both fullerene and nonfullerene based blends, Lewis acid doping leads to increased photovoltaic performance. Detailed experiments reveal that Lewis acid doping has a synergistic effect on modifying the polymer's electronic properties and the acceptor's nanostructure even at low doping concentration, and these are simultaneously responsible for the device improvements. Based on the mechanism studies, it is proposed that the Lewis acid‐doped polymers anions produce induced dipole on the acceptor, this increases the intermolecular interaction and facilitates the morphology optimization. It is believed that the synergistic effect by Lewis acid doping greatly expands the application of doped organic solar cells, in concert with other existing methods to yield higher efficiency values.

Yan, Han↗