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.

We have investigated the thermoelectric properties of amorphous InGaZnO (a-IGZO) thin films optimized by adjusting the carrier concentration. The a-IGZO films were produced under various oxygen flow ratios. The Seebeck coefficient and the electrical conductivity were measured from 100 to 400 K. We found that the power factor (PF) at 300 K had a maximum value of 82 × 10−6 W/mK2, where the carrier density was 7.7 × 1019 cm−3. Moreover, the obtained data was analyzed by fitting the percolation model. Theoretical analysis revealed that the Fermi level was located approximately above the potential barrier when the PF became maximal. The thermoelectric properties were controlled by the relationship between the position of Fermi level and the height of potential energy barriers.

We report the fabrication of highly reliable amorphous InGaZnO (a-IGZO) passivated by a polysilsesquioxane-based passivation layer using a simple solution process. Results show that a copolymer of methylsilsesquioxane and phenylsilsesquioxane is an effective passivation layer. a-IGZO thin film transistors (TFT) passivated by this copolymer showed a small threshold voltage (V th ) shift of 0.1 V during positive bias stress, very small V th shift of less than 0.1 V during negative bias stress and a minimal V th shift (∼ −2.4 V) during negative bias illumination stress. These results demonstrate the potential of easy to fabricate polysilsesquioxanebased passivation layers as effective passivation materials.Amorphous In-Ga-Zn-O (a-IGZO) has become a popular active channel material for thin-film transistors (TFT) because of properties such as low fabrication temperature, high mobility, low threshold voltage (V th ), small subthreshold swing (S) and large area uniformity. 1 Because of these impressive properties, a-IGZO is expected to replace conventional amorphous Si as channel material. However, instability during bias stress is a problem especially for unpassivated bottom gate type TFT. Ambient environmental effects, moisture, desorption of oxygen and water, and post-fabrication processes can harm the exposed back channel leading to the degradation of reliability. 2 Lee et. al. previously discussed how moisture can affect the negative bias thermal instability of a-IGZO TFT due to either the creation of water related defects that act as electron traps or the action of water as a shallow donor. 3 Park et. al. also examined the effect of adsorbed water on the back channel of an unpassivated TFT and claimed that the role of water molecules as an electron donor or in forming acceptor-like traps is thickness-dependent. 4 In addition, they showed that adsorbed oxygen on the back channel acts as an electron acceptor which leads to a positive V th shift ( V th ). 4 Several groups have already employed passivation layers such as SiO x , 5 TiO x , 6 SiN x , 7 and Al 2 O 3 , 8 among others, to address the problem. These passivation layers have been shown to be effective in reducing instability during standard positive and negative bias stress by inhibiting the degradation effects discussed earlier. Nevertheless, more work is needed to address the more pressing issue of instability after application of negative bias illumination stress (NBIS). Moreover, these passivation layers are usually coated using a more complicated vacuum process. In this research, we report the effect of using a polysilsesquioxane-based passivation layer fabricated using a simple solution process on a-IGZO TFT. The passivated TFT showed good stability during positive bias stress (PBS) and a very good stability during application of negative bias stress (NBS) and negative bias illumination stress (NBIS). ExperimentalTFT fabrication and passivation.-N-doped Si substrates (resistivity < 0.002 · cm) with a thermally oxidized layer of 100 nm SiO 2 was chosen a...

Amorphous indium gallium zinc oxide (a-IGZO) thin-film transistors (TFTs) having several metals, namely Ag, Ti, and Mo, as the source and drain electrodes were characterized. TFTs with Ti and Mo electrodes showed drain current–gate voltage characteristics without fluctuation. However, TFTs with Ag electrodes indicated a low noisy on-state current at a large channel length under a low drain–source voltage condition. The source and drain resistances [R s/d (Ω)] of the TFTs with each of the three metals were calculated from the I DS–V GS characteristics. The R s/d values of the Ag, Ti, and Mo samples reached 4 × 104, 2 × 104, and 1 × 104 Ω, respectively. This implies that a spatial potential barrier exists at the a-IGZO/Ag interface and that the resistance of the potential barrier changes with the application of gate voltage.