We present alternative methods to oxidize a graphene layer through ultraviolet (UV)/ozone (O3)-treatment, resulting in chemically homogeneous graphene oxide (GO), and then to reduce GO through UV-irradiation. Both UV/O3-treatment and UV-irradiation were performed at room temperature in atmospheric pressure for only several minutes and did not involve any wet chemical treatments. The quantity of doped oxygen, determined using X-ray photoelectron spectroscopy, increased after oxidation and decreased after reduction. The quantity of doped oxygen reached its maximum, which was around 20% (approximately one oxygen atom in every five or six carbon atoms) after performing UV/O3-treatment for 6 to 10 min. Conducting UV/O3-treatment for around 6 or 10 min resulted in a chemically homogeneous GO surface with only oxygen epoxide groups on the graphene surface. Performing UV/O3-treatment beyond 15 min as well as multiple turns of UV/O3-treatment could lead to the formation of defects and carbonyl groups on graphene lattice. The oxygen quantity gradually decreased after conducting 6 min UV-irradiation several times, indicating that the resulting GO was successfully reduced. How the doped oxygen atoms distributed on graphene surface was directly investigated using scanning tunneling microscopy. Moreover, changes in electrical properties of three identical single-layer graphene field-effect transistors (G-FETs) after being oxidized through UV/O3-treatment were investigated. The electron mobility of G-FETs decreased after oxidation; however, it recovered after irradiating the oxidized G-FETs with UV-lights, indicating that GO was successfully reduced nonthermally through UV-irradiation. The reversibility in electron mobility was confirmed even after conducting redox processes twice. Furthermore, the reversibility of oxidation was also verified from the graphene lattice disorder point of view using Raman spectroscopy. We concluded that UV/O3-treatment produced chemically homogeneous GO that is nonthermally reversible through UV-irradiation, and changes in the electron mobility were nonthermally reversible also.

Demonstration and characterization of an ambipolar high mobility transistor in an undoped GaAs/AlGaAs quantum well Appl. Phys. Lett. 102, 082105 (2013) Investigation of the charge transport mechanism and subgap density of states in p-type Cu2O thin-film transistors Appl. Phys. Lett. 102, 082103 (2013) Negative gate-bias temperature stability of N-doped InGaZnO active-layer thin-film transistors Appl. Phys. Lett. 102, 083505 (2013) A pH sensor with a double-gate silicon nanowire field-effect transistor Appl. Phys. Lett. 102, 083701 (2013) Extrinsic and intrinsic photoresponse in monodisperse carbon nanotube thin film transistors Appl. Phys. Lett. 102, 083104 (2013) Additional information on Appl. Phys. Lett.

Engineering of photonics for antireflection and electronics for extraction of the hole using 2.5 nm of a thin Au layer have been performed for two- and four-terminal tandem solar cells using CHNHPbI perovskite (top cell) and p-type single crystal silicon (c-Si) (bottom cell) by mechanically stacking. Highly transparent connection multilayers of evaporated-Au and sputtered-ITO films were fabricated at the interface to be a point-contact tunneling junction between the rough perovskite and flat silicon solar cells. The mechanically stacked tandem solar cell with an optimized tunneling junction structure was ⟨perovskite for the top cell/Au (2.5 nm)/ITO (154 nm) stacked-on ITO (108 nm)/c-Si for the bottom cell⟩. It was confirmed the best efficiency of 13.7% and 14.4% as two- and four-terminal devices, respectively.

Organic−inorganic perovskite solar cells have attracted much attention as high performance and low-cost photovoltaic devices. Because it consists of p-type hole transport layer, perovskite layer, and n-type electron transport layer similar to a p−i−n structure, it works effectively even under low-illuminance conditions, such as indoor lighting. In this work, we focused on the characteristics of perovskite solar cells under lowilluminance conditions, and a detailed investigation was carried out. The open-circuit voltage yielded at around 70% of AM1.5 at 0.1 mW/cm 2 illuminance, which is similar to that under indoor lighting. From impedance spectroscopy, it was suggested that the planartype structure solar cell provided better resistance characteristics than that of the mesostructured cell for indoor applications. Comparing the characteristics of these types of solar cells, planar-type solar cells show higher voltage than mesostructured cells under lowilluminance conditions. These results have shown important implications for various applications of perovskite solar cells.

Efficient hole transport layer (HTL) is crucial for realizing efficient perovskite solar cells (PSCs). In this study, nickel‐oxide (NiOX) thin‐films are investigated as a potential HTL for PSCs. The NiOX films are prepared by electron‐beam physical vapor deposition at low temperatures. The crystalline properties and the work function are determined by X‐ray diffraction and photoelectric yield spectroscopy. The transmission and the complex refractive index of the films are determined by optical spectroscopy and ellipsometry. Furthermore, PSCs are fabricated and characterized. The short‐circuit current density (Jsc) of the PSC is limited by the optical loss due to the NiOx front contact. The optical losses of the front contact are quantified by optical simulations using finite‐difference time‐domain simulations, and a solar cell structure with improved light incoupling is designed. Furthermore, the electrical characteristics of the solar cell are simulated by finite element method simulations. As a result, it is found that the optical losses can be reduced by 70%, and the light incoupling can be improved so that the JSC can be increased by up to 12%, allowing for the realization of PSCs with an energy conversion efficiency of 22%. Findings from the numerical simulations are compared with experimentally realized results.