The recent dramatic rise in power conversion efficiencies (PCEs) of perovskite solar cells (PSCs) has triggered intense research worldwide. However, high PCE values have often been reached with poor stability at an illuminated area of typically less than 0.1 square centimeter. We used heavily doped inorganic charge extraction layers in planar PSCs to achieve very rapid carrier extraction, even with 10- to 20-nanometer-thick layers, avoiding pinholes and eliminating local structural defects over large areas. The robust inorganic nature of the layers allowed for the fabrication of PSCs with an aperture area >1 square centimeter that have a PCE >15%, as certified by an accredited photovoltaic calibration laboratory. Hysteresis in the current-voltage characteristics was eliminated; the PSCs were stable, with >90% of the initial PCE remaining after 1000 hours of light soaking.

http://science.sciencemag.org/content/sci/350/6263/944.full.pdf

Efficient and stable large-area perovskite solar cells with inorganic charge extraction layers

This review article first presents a summary of current understanding of the magnetic properties and intrinsic ferromagnetism of transition-metal (TM)-doped ZnO films, which are typical diluted magnetic oxides used in spintronics. The local structure and magnetic behavior of TM-doped ZnO are strongly sensitive to the preparation parameters. In the second part, we discuss in detail the effects of doping elements and concentrations, oxygen partial pressure, substrate and its orientation and temperature, deposition rate, post-annealing temperature and co-doping on the local structure and subsequent ferromagnetic ordering of TM-doped ZnO. It is unambiguously demonstrated that room-temperature ferromagnetism is strongly correlated with structural defects, and the carriers involved in carrier-mediated exchange are by-products of defects created in ZnO. The third part focuses on recent progress in TM-doped ZnO-based spintronics, such as magnetic tunnel junctions and spin field-effect transistors, which provide a route for spin injection from TM-doped ZnO to ZnO. Thus, TM-doped ZnO materials are useful spin sources for spintronics.

Ferromagnetism and possible application in spintronics of transition-metal-doped ZnO films

The development of transparent p-type oxide semiconductors with good performance may be a true enabler for a variety of applications where transparency, power efficiency, and greater circuit complexity are needed. Such applications include transparent electronics, displays, sensors, photovoltaics, memristors, and electrochromics. Hence, here, recent developments in materials and devices based on p-type oxide semiconductors are reviewed, including ternary Cu-bearing oxides, binary copper oxides, tin monoxide, spinel oxides, and nickel oxides. The crystal and electronic structures of these materials are discussed, along with approaches to enhance valence-band dispersion to reduce effective mass and increase mobility. Strategies to reduce interfacial defects, off-state current, and material instability are suggested. Furthermore, it is shown that promising progress has been made in the performance of various types of devices based on p-type oxides. Several innovative approaches exist to fabricate transparent complementary metal oxide semiconductor (CMOS) devices, including novel device fabrication schemes and utilization of surface chemistry effects, resulting in good inverter gains. However, despite recent developments, p-type oxides still lag in performance behind their n-type counterparts, which have entered volume production in the display market. Recent successes along with the hurdles that stand in the way of commercial success of p-type oxide semiconductors are presented.

/pdf/recent-developments-in-p-type-oxide-semiconductor-materials-4iad1efbja.pdf

Recent Developments in p-Type Oxide Semiconductor Materials and Devices.

Dilute magnetic oxides are transparent, wide-bandgap materials that behave ferromagnetically when doped with a few percent of a magnetic 3d cation. The magnetism, which appears well below the cation percolation threshold, cannot be understood in terms of the conventional theory of magnetism in insulators; nor can a carrier-mediated ferromagnetic exchange mechanism account for the magnitude of the Curie temperatures, which are well in excess of 400 K. The phenomenon is observed in thin films and nanocrystals, but not in well-crystallized bulk material. Experimental artefacts and segregation of secondary ferromagnetic phases can explain some observations, but the existence of a novel type of magnetism related to defects other than the magnetic dopants is a likely inference from the data.

/pdf/dilute-magnetic-oxides-1ckkbdqg3p.pdf

Dilute magnetic oxides

https://escholarship.org/content/qt2xf0c3gx/qt2xf0c3gx.pdf?t=pxd659

Recent progresses on physics and applications of vanadium dioxide

We report room temperature ferromagnetism in single crystal Ce1−xCoxO2−δ (x⩽0.05) films deposited on a LaAlO3(001) substrate. Films were grown by a pulsed laser deposition technique and were thoroughly characterized using x-ray diffraction, high-resolution transmission electron microscopy coupled with electron energy loss spectroscopy and scanning transmission electron microscopy-Z contrast, x-ray photoelectron spectroscopy, optical transmission spectroscopy, and magnetic measurements. These films are transparent in the visible regime and exhibit a very high Curie temperature ∼740–875K with a giant magnetic moment. Our results indicate that the ferromagnetic property is intrinsic to the CeO2 system and is not a result of any secondary magnetic phase or cluster formation.

/pdf/ferromagnetism-in-co-doped-ceo2-observation-of-a-giant-30cxmu5vd7.pdf

Ferromagnetism in Co doped CeO2: Observation of a giant magnetic moment with a high Curie temperature

Li doped NiO (LixNi1−xO) thin films were epitaxially grown along [111] orientation on c-sapphire by pulsed laser deposition. The structural, electrical, and optical properties of the films were investigated using x-ray diffraction, four probe technique, and UV-visible spectra, respectively. The epitaxial growth of [111] Li doped NiO on [0001] sapphire was determined by using high resolution x-ray Φ scan. Effects of the deposition condition and Li doping concentration variations on the electrical and optical properties of Li doped NiO films were also investigated. The analysis of the resistivity data show that doped Li ions occupy the substitutional sites in the films, enhancing the p-type conductivity. The minimum resistivity of 0.15 Ω cm was obtained for Li0.07Ni0.93O film. The activation energy of Li doped NiO films were estimated to be in the range of 0.11–0.14 eV. Based upon these values, a possible electrical transport mechanism is discussed. A p-n heterojunction has also been fabricated for the opti...

Effect of Li doping in NiO thin films on its transparent and conducting properties and its application in heteroepitaxial p-n junctions

We have made a comparative study of epitaxial growth of VO2 thin films on c-cut (0001) and r-cut (11¯02) sapphire substrates, and the semiconductor to metal transition (SMT) characteristics of these films have been correlated with their structural details. On c-sapphire, VO2 grows epitaxially in (002) orientation. These (002) oriented VO2 films have 60° twin boundaries due to three equivalent in-plane orientations. The epitaxial VO2 films on r-sapphire consisted of two orientations, namely (200) and (2¯11). The coexistence of these two orientations of VO2 has been explained on the basis of similarity of atomic arrangements in (200) and (2¯11) planes. The thermal hysteresis (ΔH), sharpness of the transition (ΔT), and the transition temperature for VO2 films on c-sapphire were found to be 4.8, 8.5, and 72.6 °C, respectively, which were higher than the corresponding values of 3.3, 5.4, and 60.3 °C for films on r-sapphire. The SMT temperature for VO2 films on c-sapphire was close to the bulk value of 68.0 °C....

Semiconductor-metal transition characteristics of VO2 thin films grown on c- and r-sapphire substrates

We have been able to achieve semiconductor-to-metal transition (SMT) temperature in VO2 thin films close to the values reported for bulk VO2 single crystals. This was achieved by complete relaxation of misfit strain, which leads to a negligible tension/compression along VO2 [001], upon introduction of NiO buffer layer on c-plane sapphire substrate. In this paper, we discuss the mechanism behind complete relaxation of misfit strain which occurs under the paradigm of domain-matching epitaxy, where integral multiples of planes match across the interface. NiO buffer layers were grown in situ, prior to the VO2 deposition, using pulsed-laser deposition technique. X-ray θ-2θ, φ, and pole figure scans were performed for structural characterization of the VO2/NiO/Al2O3 (0001) heterostructure. All the constituent layers of the heterostructure were found to be epitaxial with orientation relationship: (020)VO2∥(111)NiO∥(0001)Al2O3 and ⟨100⟩VO2∥⟨110⟩NiO∥⟨101¯0⟩Al2O3. Parameters related to SMT, such as hysteresis and t...

Near bulk semiconductor to metal transition in epitaxial VO2 thin films

We report controlled modifications in the semiconductor-to-metal transition characteristics of VO2 single-crystal thin films induced by swift heavy ion (SHI) irradiation with varying ion fluences. At very high energies of ions (200 MeV Au), the electronic stopping (~2009 eV/A) dominates over nuclear stopping (~16 eV/A). Under these extreme electronic excitation conditions caused by electronic stopping and the passage of SHIs through the entire thickness of the film, creation of certain unique type of defects and disordered regions occurs. X-ray diffraction, Raman spectroscopy, infrared transmission spectroscopy, x-ray photoelectron spectroscopy (XPS), and electrical measurements were performed to investigate the characteristics and role of these defects on structural, optical, and electrical properties of VO2 thin films. XPS and electrical resistivity measurements suggest that the ion irradiation induces localized defect states that appear to correlate well with the creation of disordered regions in the VO2 thin films. The high-energy heavy-ion irradiation changes the transition characteristics drastically from a first-order to a second-order transition (electronic—Mott type). The low-temperature conductance data for these ion-irradiated films fit well with the quasiamorphous model for resistivity of VO2, where ion irradiation is believed to create mid-bandgap defect states.

Alok Gupta

Papers

Ferromagnetism in Co doped CeO2: Observation of a giant magnetic moment with a high Curie temperature

Effect of Li doping in NiO thin films on its transparent and conducting properties and its application in heteroepitaxial p-n junctions

Semiconductor-metal transition characteristics of VO2 thin films grown on c- and r-sapphire substrates

Near bulk semiconductor to metal transition in epitaxial VO2 thin films

Electronic excitation induced controlled modifications of semiconductor-to-metal transition in epitaxial VO2 thin films