Hexaaqua Metal Complexes for Low-Temperature Formation of Fully Metal Oxide Thin-Film Transistors

We investigated aqueous metal complex-based oxide semiconductor films formed with various ligands, such as chloride, acetate, fluoride, and nitrate. Nitrate ligand-based indium­(III) precursor was easily decomposed at low temperature due to the replacement of all nitrate ions with water during solvation to form the hexaaqua indium­(III) cation ([In­(H₂O)₆]³⁺). Hexaaqua indium­(III) cation was a key complex to realize high-quality oxide films at low temperature. Additionally, Al₂O₃-based high-<i>k</i> dielectric was also employed by using a nitrate precursor, and the hexaaqua aluminum­(III) cation ([Al­(H₂O)₆]³⁺) was confirmed. This complex-based Al₂O₃ film showed high breakdown voltage and stable capacitance under high frequency operation compared to organic solvent-based Al₂O₃ films. We successfully demonstrated aqueous-based In₂O₃ TFTs with Al₂O₃ high-<i>k</i> gate dielectrics formed at 250 °C with a wide gate voltage operation and high saturation mobility and on/off ratio of 36.31 ± 2.29 cm² V^–1 s^–1 and over 10⁷, respectively

Direct Light Pattern Integration of Low-Temperature Solution-Processed All-Oxide Flexible Electronics

The rise of solution-processed electronics, together with their processing methods and materials, provides unique opportunities to achieve low-cost and low-temperature roll-to-roll printing of non-Si-based devices. Here, we demonstrate a wafer-scale direct light-patterned, fully transparent, all-solution-processed, and layer-by-layer-integrated electronic device. The deep ultraviolet irradiation of specially designed metal oxide gel films can generate fine-patterned shapes of ∼3 μm, which easily manifest their intrinsic properties at low-temperature annealing. This direct light patterning can be easily applied to the 4 in. wafer scale and diverse pattern shapes and provides feasibility for integrated circuit applications through the penetration of the deep ultraviolet range on the quartz mask. With this approach, we successfully fabricate all-oxide-based high-performance transparent thin-film transistors on flexible polymer substrates

Morphology Evolution of High Efficiency Perovskite Solar Cells via Vapor Induced Intermediate Phases

Morphology is critical component to achieve high device performance hybrid perovskite solar cells. Here, we develop a vapor induced intermediate phase (VIP) strategy to manipulate the morphology of perovskite films. By exposing the perovskite precursor films to different saturated solvent vapor atmospheres, e.g., dimethylformamide and dimethylsufoxide, dramatic film morphological evolution occurs, associated with the formation of different intermediate phases. We observe that the crystallization kinetics is significantly altered due to the formation of these intermediate phases, yielding highly crystalline perovskite films with less defect states and high carrier lifetimes. The perovskite solar cells with the reconstructed films exhibits the highest power conversion efficiency (PCE) up to 19.2% under 1 sun AM 1.5G irradiance, which is among the highest planar heterojunction perovskite solar cells. Also, the perovskite solar cells with VIP processing shows less hysteresis behavior and a stabilized power output over 18%. Our work opens up a new direction for morphology control through intermediate phase formation, and paves the way toward further enhancing the device performances of perovskite solar cells

The Interplay between Trap Density and Hysteresis in Planar Heterojunction Perovskite Solar Cells

Anomalous current–voltage (<i>J</i>–<i>V</i>) hysteresis in perovskite (PSK) solar cell is open to dispute, where hysteresis is argued to be due to electrode polarization, dipolar polarization, and/or native defects. However, a correlation between those factors and <i>J</i>–<i>V</i> hysteresis is hard to be directly evaluated because they usually coexist and are significantly varied depending on morphology and crystallinity of the PSK layer, selective contacts, and device architecture. In this study, without changing morphology and crystallinity of PSK layer in a planar heterojunction structure employing FA_0.9Cs_0.1PbI₃, a correlation between <i>J</i>–<i>V</i> hysteresis and trap density is directly evaluated by means of thermally induced PbI₂ regulating trap density. Increase in thermal annealing time at a given temperature of 150 °C induces growth of PbI₂ on the PSK grain surface, which results in significant reduction of nonradiative recombination. Hysteresis index is reduced from 0.384 to 0.146 as the annealing time is increased from 5 to 100 min due to decrease in the amplitude of trap-mediated recombination. Reduction of hysteresis by minimizing trap density via controlling thermal annealing time leads to the stabilized PCE of 18.84% from the normal planar structured FA_0.9Cs_0.1PbI₃ PSK solar cell

Boosting Responsivity of Organic–Metal Oxynitride Hybrid Heterointerface Phototransistor

Amorphous metal oxides are attractive materials for various sensor applications, because of high electrical performance and easy processing. However, low absorption coefficient, slow photoresponse, and persistent photoconductivity of amorphous metal oxide films from the origin of deep-level defects are obstacles to their use as photonic applications. Here, we demonstrate ultrahigh photoresponsivity of organic–inorganic hybrid phototransistors featuring bulk heterojunction polymers and low-bandgap zinc oxynitride. Spontaneous formation of ultrathin zinc oxide on the surface of zinc oxynitride films could make an effective band-alignment for electron transfer from the dissociation of excitons in the bulk heterojunction, while holes were blocked by the deep highest occupied molecular orbital level of zinc oxide. These hybrid structure-based phototransistors are ultrasensitive to broad-bandwidth photons in ultraviolet to near-infrared regions. The detectivity and a linear dynamic range exceeded 10¹² Jones and 122.3 dB, respectively

Printable Ultrathin Metal Oxide Semiconductor-Based Conformal Biosensors

Conformal bioelectronics enable wearable, noninvasive, and health-monitoring platforms. We demonstrate a simple and straightforward method for producing thin, sensitive In₂O₃-based conformal biosensors based on field-effect transistors using facile solution-based processing. One-step coating <i>via</i> aqueous In₂O₃ solution resulted in ultrathin (3.5 nm), high-density, uniform films over large areas. Conformal In₂O₃-based biosensors on ultrathin polyimide films displayed good device performance, low mechanical stress, and highly conformal contact determined using polydimethylsiloxane artificial skin having complex curvilinear surfaces or an artificial eye. Immobilized In₂O₃ field-effect transistors with self-assembled monolayers of NH₂-terminated silanes functioned as pH sensors. Functionalization with glucose oxidase enabled d-glucose detection at physiologically relevant levels. The conformal ultrathin field-effect transistor biosensors developed here offer new opportunities for future wearable human technologies

Ultrathin Organic Solar Cells with Graphene Doped by Ferroelectric Polarization

Graphene has been employed as transparent electrodes in organic solar cells (OSCs) because of its good physical and optical properties. However, the electrical conductivity of graphene films synthesized by chemical vapor deposition (CVD) is still inferior to that of conventional indium tin oxide (ITO) electrodes of comparable transparency, resulting in a lower performance of OSCs. Here, we report an effective method to improve the performance and long-term stability of graphene-based OSCs using electrostatically doped graphene films via a ferroelectric polymer. The sheet resistance of electrostatically doped few layer graphene films was reduced to ∼70 Ω/sq at 87% optical transmittance. Such graphene-based OSCs exhibit an efficiency of 2.07% with a superior stability when compared to chemically doped graphene-based OSCs. Furthermore, OSCs constructed on ultrathin ferroelectric film as a substrate of only a few micrometers show extremely good mechanical flexibility and durability and can be rolled up into a cylinder with 7 mm diameter

Mechanical and Environmental Stability of Polymer Thin-Film-Coated Graphene

A uniform polymer thin layer of controllable thickness was bar-coated onto a chemical vapor deposition (CVD) grown monolayer graphene surface. The effects of this coating layer on the optical, electric, and tribological properties were then investigated. The thin polymer coating layer did not reduce the optical transmittance of the graphene films. The variation in the sheet resistance of the graphene films after the coating depended on the interaction between polymer and graphene. The top coating layer can maintain the high conductivity of chemical doped graphene films under long-term ambient conditions compared with uncovered doped samples. Friction tests demonstrated that the polymer coating layer can enhance both the friction force and the coefficient of friction of the graphene films and protect the graphene against damage in the repeated sliding processes

Large-Area, Ultrathin Metal-Oxide Semiconductor Nanoribbon Arrays Fabricated by Chemical Lift-Off Lithography

Nanoribbon- and nanowire-based field-effect transistors (FETs) have attracted significant attention due to their high surface-to-volume ratios, which make them effective as chemical and biological sensors. However, the conventional nanofabrication of these devices is challenging and costly, posing a major barrier to widespread use. We report a high-throughput approach for producing arrays of ultrathin (∼3 nm) In₂O₃ nanoribbon FETs at the wafer scale. Uniform films of semiconducting In₂O₃ were prepared on Si/SiO₂ surfaces via a sol–gel process prior to depositing Au/Ti metal layers. Commercially available high-definition digital versatile discs were employed as low-cost, large-area templates to prepare polymeric stamps for chemical lift-off lithography, which selectively removed molecules from self-assembled monolayers functionalizing the outermost Au surfaces. Nanoscale chemical patterns, consisting of one-dimensional lines (200 nm wide and 400 nm pitch) extending over centimeter length scales, were etched into the metal layers using the remaining monolayer regions as resists. Subsequent etch processes transferred the patterns into the underlying In₂O₃ films before the removal of the protective organic and metal coatings, revealing large-area nanoribbon arrays. We employed nanoribbons in semiconducting FET channels, achieving current on-to-off ratios over 10⁷ and carrier mobilities up to 13.7 cm² V^–1 s^–1. Nanofabricated structures, such as In₂O₃ nanoribbons and others, will be useful in nanoelectronics and biosensors. The technique demonstrated here will enable these applications and expand low-cost, large-area patterning strategies to enable a variety of materials and design geometries in nanoelectronics