Solution-Processed 8‑Hydroquinolatolithium as Effective Cathode Interlayer for High-Performance Polymer Solar Cells

Solution-processed 8-hydroxyquinolinatolithium (s-Liq) was successfully applied as an efficient cathode interlayer in bulk heterojunction polymer solar cells (PSCs), giving rise to enhancement in device performance. The ultraviolet photoelectron spectra results revealed that the presence of s-Liq could lower work function of Al cathode, allowing for the ohmic contacts with the fullerene acceptor for better electron extraction and also a larger work function difference between the two electrodes, which leads to an increase in open-circuit voltage (<i>V</i>_oc). Scanning Kelvin probe microscopy study on the surface potential of active layers suggested that an interfacial dipole was formed in the s-Liq interlayer between the active layer and the Al cathode, which enhanced the intrinsic built-in potential in the device for better charge transportation and extraction. Consequently, the <i>V</i>_oc, fill factor, and current density of the device can be improved by the introduction of s-Liq interlayer, leading to a power conversion efficiency (PCE) improvement. With PTB7 (or PTB7-Th) as the donor and PC₇₁BM as the acceptor, the s-Liq-based PSC devices exhibited a PCE of 8.37% (or 9.04%), much higher than those of devices with the evaporated Liq (7.62%) or commonly used PFN (8.14%) as the cathode interlayer. Moreover, the s-Liq-based devices showed good stability, maintaining 75% (in N₂) and 45% (in air) of the initial PCE after 7 days, respectively. These results suggest the great potential of s-Liq as cathode interlayer material for high-performance solar cells application

Robust Interfacial Exchange Bias and Metal–Insulator Transition Influenced by the LaNiO₃ Layer Thickness in La_0.7Sr_0.3MnO₃/LaNiO₃ Superlattices

Artificial heterostructures based on LaNiO₃ (LNO) have been widely investigated with the aim to realize the insulating antiferromagnetic state of LNO. In this work, we grew [(La_0.7Sr_0.3MnO₃)₅-(LaNiO₃)_<i>n</i>]₁₂ superlattices on (001)-oriented SrTiO₃ substrates by pulsed laser deposition and observed an unexpected exchange bias effect in field-cooled hysteresis loops. Through X-ray absorption spectroscopy and magnetic circular dichroism experiments, we found that the charge transfer at the interfacial Mn and Ni ions can induce a localized magnetic moment. A remarkable increase of exchange bias field and a transition from metal to insulator were simultaneously observed upon decreasing the thickness of the LNO layer, indicating the antiferromagnetic insulator state in 2 unit cells LNO ultrathin layers. The robust exchange bias of 745 Oe in the superlattice is caused by an interfacial localized magnetic moment and an antiferromagnetic state in the ultrathin LNO layer, pinning the ferromagnetic La_0.7Sr_0.3MnO₃ layers together. Our results demonstrate that artificial interface engineering is a useful method to realize novel magnetic and transport properties

Atomic-Scale Magnetism of Cr-Doped Bi₂Se₃ Thin Film Topological Insulators

Magnetic doping is the most common method for breaking time-reversal-symmetry surface states of topological insulators (TIs) to realize novel physical phenomena and to create beneficial technological applications. Here we present a study of the magnetic coupling of a prototype magnetic TI, that is, Cr-doped Bi₂Se₃, in its ultrathin limit which is expected to give rise to quantum anomalous Hall (QAH) effect. The high quality Bi_2–<i>x</i>Cr_<i>x</i>Se₃ epitaxial thin film was prepared using molecular beam epitaxy (MBE), characterized with scanning transimission electron microscopy (STEM), electrical magnetotransport, and X-ray magnetic circularly dichroism (XMCD) techniques, and the results were simulated using density functional theory (DFT) with spin–orbit coupling (SOC). We observed a sizable spin moment <i>m</i>_spin = (2.05 ± 0.20) μ_B/Cr and a small and negative orbital moment <i>m</i>_orb = (−0.05 ± 0.02) μ_B/Cr of the Bi_1.94Cr_0.06Se₃ thin film at 2.5 K. A remarkable fraction of the (Cr_Bi–Cr_I)³⁺ antiferromagnetic dimer in the Bi_2–<i>x</i>Cr_<i>x</i>Se₃ for 0.02 < <i>x</i> < 0.40 was obtained using <i>first-principles</i> simulations, which was neglected in previous studies. The spontaneous coexistence of ferro- and antiferromagnetic Cr defects in Bi_2–<i>x</i>Cr_<i>x</i>Se₃ explains our experimental observations and those based on conventional magnetometry which universally report magnetic moments significantly lower than 3 μ_B/Cr predicted by Hund’s rule

Enhancing the Spin–Orbit Coupling in Fe₃O₄ Epitaxial Thin Films by Interface Engineering

By analyzing the in-plane angular dependence of ferromagnetic resonance linewidth, we show that the Gilbert damping constant in ultrathin Fe₃O₄ epitaxial films on GaAs substrate can be enhanced by thickness reduction and oxygen vacancies in the interface. At the same time, the uniaxial magnetic anisotropy due to the interface effect becomes significant. Using the element-specific technique of X-ray magnetic circular dichroism, we find that the orbital-to-spin moment ratio increases with decreasing film thickness, in full agreement with the increase in the Gilbert damping obtained for these ultrathin single-crystal films. Combined with the first-principle calculations, the results suggest that the bonding with Fe and Ga or As ions and the ionic distortion near the interface, as well as the FeO defects and oxygen vacancies, may increase the spin–orbit coupling in ultrathin Fe₃O₄ epitaxial films and in turn provide an enhanced damping

Enhancing Magnetic Ordering in Cr-Doped Bi₂Se₃ Using High‑<i>T</i>_C Ferrimagnetic Insulator

We report a study of enhancing the magnetic ordering in a model magnetically doped topological insulator (TI), Bi_2–<i>x</i>Cr_<i>x</i>Se₃, via the proximity effect using a high-<i>T</i>_C ferrimagnetic insulator Y₃Fe₅O₁₂. The FMI provides the TI with a source of exchange interaction yet without removing the nontrivial surface state. By performing the elemental specific X-ray magnetic circular dichroism (XMCD) measurements, we have unequivocally observed an enhanced <i>T</i>_C of 50 K in this magnetically doped TI/FMI heterostructure. We have also found a larger (6.6 nm at 30 K) but faster decreasing (by 80% from 30 to 50 K) penetration depth compared to that of diluted ferromagnetic semiconductors (DMSs), which could indicate a novel mechanism for the interaction between FMIs and the nontrivial TIs surface