Wooseok Yang

Band-gap-graded Cu2ZnSn(S1-x,Sex)4 Solar Cells Fabricated by an Ethanol-based, Particulate Precursor Ink Route

Solution processing of earth-abundant Cu2ZnSn(S1-x,Sex)4 (CZTSSe) absorber materials is an attractive research area in the economical and large-scale deployment of photovoltaics. Here, a band-gap-graded CZTSSe thin-film solar cell with 7.1% efficiency was developed using non-toxic solvent-based ink without the involvement of complex particle synthesis, highly toxic solvents, or organic additives. Despite the high series resistance due to the presence of a thick Mo(S,Se)x layer and Zn(S,Se) aggregates, a high short-circuit current density (JSC) was generated. In addition, there was no significant difference in open circuit voltages (VOC) between CZTS (0.517 V) and CZTSSe (0.505–0.479 V) cells, despite a significant band gap change from 1.51 eV to 1.24 eV. The high JSC and less loss of VOC are attributed to the effect of band gap grading induced by Se grading in the CZTSSe absorber layer. Our environmentally benign ink approach will enable the realization of low-cost, large-area, high-efficiency thin-film solar cells.

Solution-deposited Zr-doped AlOx gate dielectrics enabling high-performance flexible transparent thin film transistors

Although high dielectric constant (k) oxide thin film has been considered as a key element for high performance and low-voltage driven thin-film transistors (TFTs), there are no solution processable high-k oxide dielectrics that satisfy the stringent requirements of low-temperature processability, mechanical flexibility, and good TFT performance. Here, we demonstrate that the incorporation of a zirconium component that has strong bonding to oxygen enables a significant reduction in the processing temperature for soluble alumina dielectrics to as low as 250 °C. Based on these Zr-AlOx films, high performance, low operational voltage, flexible TFTs are achieved. Flexible TFTs operate well under a gate bias of 5 V, exhibiting a high saturation mobility of 51 cm2 V−1 s−1, an on/off current ratio of 104, and a low threshold voltage of 1.2 V with good mechanical flexibility. This is the first study demonstrating the mechanical flexibility of all-oxide soluble high-k dielectric–semiconductor-based TFTs with an emphasis on the influence of annealing temperature on the solution-deposited high-k oxide dielectric characteristics.

A solution-processed yttrium oxide gate insulator for high-performance all-solution-processed fully transparent thin film transistors

We studied high-k soluble yttrium oxide dielectrics for high performance oxide thin film transistors (TFTs). The electrical characteristics of yttrium oxide show leakage current density as less than 10−6 A cm−2 at 2 MV cm−1 regardless of annealing temperature and a high dielectric constant of nearly 16. For the first time, all solution-processed fully transparent ZnO-TFTs based on spin-coated YOx gate dielectric layers with a small interfacial trap density and high capacitance were demonstrated, exhibiting a field-effect mobility of 135 cm2 V−1 s−1, a threshold voltage of 1.73 V, and an on–off current ratio of 5.7 × 107 as well as low-voltage operation. In addition to microstructural and electrical analyses for solution-processed YOx dielectrics, we investigated the influences of dielectric–semiconductor interfacial quality on device parameters. Our results suggest that solution-processable fully transparent oxide TFTs have the potential for low-temperature and high-performance application in transparent, flexible devices.

Russell body gastritis associated with Helicobacter pylori infection: a case report

An unusual and rare gastric mucosal lesion histologically consisting of a localised accumulation of Russell bodies and Russell body-containing plasma cells, the so-called Mott cells, has been recognised only recently and termed as “Russell body gastritis”. This lesion, despite its densely monomorphous appearance is easily confirmed to be non-neoplastic by its polyclonal immunoreactive pattern to immunoglobulin light chains. However, the aetiology of Russell body gastritis is controversial and hence the optimal treatment for this disease has not been established. Two cases of Russell body gastritis associated with Helicobacter pylori infection are reported, and the possible role of H pylori infection in the pathogenesis is discussed.

Metal salt-derived In–Ga–Zn–O semiconductors incorporating formamide as a novel co-solvent for producing solution-processed, electrohydrodynamic-jet printed, high performance oxide transistors

We report the previously unrecognized co-solvent, formamide (FA), which can comprehensively improve both the device performance and bias stability of metal salt-derived, solution-processed In–Ga–Zn–O (IGZO) TFTs. By incorporating FA in IGZO precursor solutions, the chemical structures are tailored adequately for reducing the content of hydroxide and encouraging the oxygen vacancy formation, which has not been fulfilled in conventional chemical/physical approaches. Owing to such distinct chemical structural evolution, the field-effect mobility is enhanced dramatically by a factor of 4.3 (from 2.4 to 10.4 cm2 V−1 s−1), and the threshold voltage shift during a positive-bias stress test is suppressed effectively by a factor of 2.3 (from 9.3 to 4.1 V) for unpassivated devices. The addition of formamide to IGZO precursor solutions also facilitates electrohydrodynamic-jet (e-jet) printability, with which the directly printed device with a channel width of ∼30 μm is demonstrated successfully. In addition, a high performance, solution-processed IGZO transistor with a mobility of 50 cm2 V−1 s−1 is suggested through coupling a FA-added IGZO oxide semiconductor with a solution-processed zirconium aluminum oxide ((Zr,Al)2Ox) gate dielectric.

High performance and high stability low temperature aqueous solution-derived Li–Zr co-doped ZnO thin film transistors

Highly stable and high performance solution-processed amorphous oxide semiconductor thin film transistors (TFTs) were produced using a Li and Zr co-doped ZnO-based aqueous solution. Li and Zr co-doping at the appropriate amounts enhanced the oxide film quality in terms of enhanced oxygen bonding and reduced defect sites. The 0.5 mol% Li and 1.0 mol% Zr co-doped ZnO TFTs annealed at 320 °C exhibited noticeably lower threshold voltage shifts of 3.54 V under positive bias stress and −2.07 V under negative bias temperature stress than the non-doped ZnO TFTs. The transistors revealed a good device mobility performance of 5.39 cm2 V−1 s−1 and an on/off current ratio of 108 when annealed at 320 °C, compared to a mobility performance of 2.86 cm2 V−1 s−1 and an on/off current ratio of ∼107 when annealed at 270 °C. Our results suggest that Li and Zr co-doping can be a useful technique to produce more reliable and low temperature solution-processed oxide semiconductor TFTs.

Enhanced Performance of Solution-Processed Amorphous LiYInZnO Thin-Film Transistors

Solution-processed, amorphous lithium-doped YInZnO (L-YIZO) thin-film transistors (TFTs) are investigated. An appropriate amount of Li doping significantly enhances the field-effect mobility in TFT performance (~15 times greater than that of nondoped YIZO) without controlled annealing under water vapor or O(3)/O(2) environments. The addition of Li into solution-processed YIZO semiconductors leads to improved film quality, which results from enriched metal oxygen bonding and reduced defect sites, such as oxygen vacancies and hydroxyl groups. Li doping of an amorphous ionic oxide semiconductor (AIOS) could serve as an effective strategy for low-temperature and high-performance solution-processed AIOS TFTs.

La2O3 interface modification of mesoporous TiO2 nanostructures enabling highly efficient perovskite solar cells

Perovskite solar cell performance is critically dependent on the characteristics of the electron transporting layer (ETL) and its interface. This work demonstrates the importance of interface modification in which the charge transport kinetics is improved through band gap engineering of the interfaces in perovskite solar cells with La2O3. The surface of a mesoporous TiO2 ETL (mp-TiO2) on CH3NH3PbI3-based solar cells is modified using a simple dip-coating method involving a lanthanum salt solution. The layer thickness of La2O3 was optimized, and its effects on the conduction band position, interfacial charge recombination, and photovoltaic performance were investigated. Modifying the mp-TiO2 ETL with La2O3 resulted in a nearly 46% enhancement of the power conversion efficiency because of the energetically favourable band gap engineering, which allowed the fast transfer of photo-generated electrons from the perovskite absorber layer to the ETL. Our simple interfacial control approach uses a new efficient interface-modifying material, demonstrating that interface engineering could be a key strategy for improving the performance of perovskite solar cells.

Self-oriented Sb2Se3 nanoneedle photocathodes for water splitting obtained by a simple spin-coating method

Synthesis of one-dimensional nanostructured chalcogenide compounds using nontoxic and abundant constituents provides an important pathway to the development of commercially feasible photoelectrochemical water splitting. In this study, grass-like Sb2Se3 nanoneedle arrays are successfully fabricated on a substrate via a facile spin-coating method without any complicated processes such as templating, seed formation, or use of a vapor phase. Preferential [001] growth of the initial single-crystalline Sb2Se3 occurs during the first spin-coating, but interfacial defects are generated upon subsequent spin-coating iterations, resulting in annual-ring-like growth of Sb2Se3 nanoneedles. After sequential surface modification with TiO2 and Pt, the resistance to charge transfer from the photoelectrode to the electrolyte decreases significantly, yielding a remarkable record-high photocurrent of 2 mA cm−2 at 0 VRHE (4.5 mA cm−2 at −0.2 VRHE).

Photoelectrochemical Properties of Vertically Aligned CuInS2 Nanorod Arrays Prepared via Template-Assisted Growth and Transfer.

Although copper-based chalcopyrite materials such as CuInS2 have been considered promising photocathodes for solar water splitting, the fabrication route for a nanostructure with vertical orientation has not yet been developed. Here, a fabrication route for vertically aligned CuInS2 nanorod arrays from an aqueous solution using anodic aluminum oxide template-assisted growth and transfer is presented. The nanorods exhibit a phase-pure CuInS2 chalcopyrite structure and cathodic photocurrent response without co-catalyst loading. Small particles of CdS and ZnS were conformally decorated onto CuInS2 nanorods using a successive ion layer adsorption and reaction method. With surface modification of CdS/ZnS, the photoelectrochemical properties of CuInS2 nanorod arrays are enhanced via flat-band potential shift, as determined by analyses of onset potential and Mott-Schottky plots.