Copper oxide nanostructured thin films processed by SILAR for optoelectronic applications.

The lack of high-functioning p-type semiconductor oxide material is one of the critical challenges that face the widespread performance of transparent and flexible electronics. Cu x O nanostructured thin films are potentially appealing materials for such applications because of their innate p-type semi-conductivity, transparency, non-toxicity, abundant availability, and low-cost fabrication. This review summarizes current research on Cu x O nanostructured thin films deposited by the SILAR technique. After a brief introduction to the advantages of Cu x O semiconductor material, diverse approaches for depositing and growing such thin films are discussed. SILAR is one of the simplest deposition techniques in terms of better flexibility of the substrate choice, the capability of large-area fabrication, budget-friendly, deposition of stable and adherent film, low processing temperature for the film fabrication as well as reproducibility. In addition, various fabrication parameters such as types of copper salts, pH of precursors, number of cycles during immersion, annealing of as-deposited films, doping by diverse dopants, and growth temperature affect the rate of fabrication with the structural, electrical, and optical properties of Cu x O nanostructured thin films, which led the technique unique to study extensively. This review will include the recent progress that has recently been made in different aspects of Cu x O processed by the SILAR. It will describe the theory, mechanism, and factors affecting SILAR-deposited Cu x O. Finally, conclusions and perspectives concerning the use of Cu x O materials in optoelectronic devices will be visualized.

Optoelectronic properties and ultrafast carrier dynamics of copper iodide thin films.

As a promising high mobility p-type wide bandgap semiconductor, copper iodide has received increasing attention in recent years. However, the defect physics/evolution are still controversial, and particularly the ultrafast carrier and exciton dynamics in copper iodide has rarely been investigated. Here, we study these fundamental properties for copper iodide thin films by a synergistic approach employing a combination of analytical techniques. Steady-state photoluminescence spectra reveal that the emission at ~420 nm arises from the recombination of electrons with neutral copper vacancies. The photogenerated carrier density dependent ultrafast physical processes are elucidated with using the femtosecond transient absorption spectroscopy. Both the effects of hot-phonon bottleneck and the Auger heating significantly slow down the cooling rate of hot-carriers in the case of high excitation density. The effect of defects on the carrier recombination and the two-photon induced ultrafast carrier dynamics are also investigated. These findings are crucial to the optoelectronic applications of copper iodide.

Facile synthesis of Cu2O nanorods in the presence of NaCl by successive ionic layer adsorption and reaction method and its characterizations.

Cuprous oxide (Cu2O) nanorods have been deposited on soda-lime glass substrates by the modified successive ionic layer adsorption and reaction technique by varying the concentration of NaCl electrolyte into the precursor complex solution. The structural, electrical and optical properties of synthesized Cu2O nanorod films have been studied by a variety of characterization tools. Structural analyses by X-ray diffraction confirmed the polycrystalline Cu2O phase with (111) preferential growth. Raman scattering spectroscopic measurements conducted at room temperature also showed characteristic peaks of the pure Cu2O phase. The surface resistivity of the Cu2O nanorod films decreased from 15 142 to 685 Ω.cm with the addition of NaCl from 0 to 4 mmol and then exhibited an opposite trend with further addition of NaCl. The optical bandgap of the synthesized Cu2O nanorod films was observed as 1.88-2.36 eV, while the temperature-dependent activation energies of the Cu2O films were measured as about 0.14-0.21 eV. Scanning electron microscope morphologies demonstrated Cu2O nanorods as well as closely packed spherical grains with the alteration of NaCl concentration. The Cu2O phase of nanorods was found stable up to 230°C corroborating the optical bandgap results of the same. The film fabricated in presence of 4 mmol of NaCl showed the lowest resistivity and activation energy as well as comparatively uniform nanorod morphology. Our studies demonstrate that the nominal presence of NaCl electrolytes in the precursor solutions has a significant impact on the physical properties of Cu2O nanorod films which could be beneficial in optoelectronic research.

Effect of heterojunction structures on photoelectrochemical properties of ZnTe-based photocathodes for water reduction.

Photoelectrochemical (PEC) properties of ZnTe-based photocathodes with various structures were investigated to clarify the effective structure for the water reduction reaction. Samples with n-ZnO/ZnTe/p-ZnTe and n-ZnS/ZnTe/p-ZnTe heterostructures showed superior PCE properties to the samples without a heterojunction. In particular, the n-ZnS/ZnTe/p-ZnTe sample exhibited a large photocurrent even at a low applied potential in an electrolyte containing Eu3+ ion as an electron scavenger. Appreciable H2 evolution with a constant rate (approximately 87 µmol cm-2 h-1) was also observed over the sample loaded with Pt deposits under visible-light irradiation (>420 nm): faradaic efficiency of almost 100% was obtained, indicating no unfavorable side reaction occurred in the sample.

Influence of the Substrate, Process Conditions, and Postannealing Temperature on the Properties of ZnO Thin Films Grown by the Successive Ionic Layer Adsorption and Reaction Method.

Here, we report the effect of the substrate, sonication process, and postannealing on the structural, morphological, and optical properties of ZnO thin films grown in the presence of isopropyl alcohol (IPA) at temperature 30-65 °C by the successive ionic layer adsorption and reaction (SILAR) method on both soda lime glass (SLG) and Cu foil. The X-ray diffraction (XRD) patterns confirmed the preferential growth thin films along (002) and (101) planes of the wurtzite ZnO structure when deposited on SLG and Cu foil substrates, respectively. Both XRD and Raman spectra confirmed the ZnO and Cu-oxide phases of the deposited films. The scanning electron microscopy image of the deposited films shows compact and uniformly distributed grains for samples grown without sonication while using IPA at temperatures 50 and 65 °C. The postannealing treatment improves the crystallinity of the films, further evident by XRD and transmission and reflection results. The estimated optical band gaps are in the range of 3.37-3.48 eV for the as-grown samples. Our experimental results revealed that high-quality ZnO thin films could be grown without sonication using an IPA dispersant at 50 °C, which is much lower than the reported results using the SILAR method. This study suggests that in the presence of IPA, the SLG substrate results in better c-axis-oriented ZnO thin films than that of deionized water, ethylene glycol, and propylene glycol at the optimum temperature of 50 °C. Air annealing of the samples grown on Cu foils induced the formation of Cu x O/ZnO junctions, which is evident from the characteristic I-V curve including the structural and optical data.

Three-dimensional band structure and surface electron accumulation of rs-CdxZn1-xO studied by angle-resolved photoemission spectroscopy.

Three-dimensional band structure of rock-salt (rs) CdxZn1-xO (x = 1.0, 0.83, and 0.60) have been determined by angle-resolved photoemission spectroscopy (ARPES) using synchrotron radiation. Valence-band features shift to higher binding energy with Zn content, while the conduction band position does not depend strongly on Zn content. An increase of the indirect band gap with Zn-doping is larger than that of the direct band gap, reflecting a weaker hybridization between Zn 3d and O 2p than that between Cd 4d and O 2p. Two-dimensional electronic states due to the quantization along surface normal direction are formed in the surface accumulation layer and show non-parabolic dispersions. Binding energy of the quantized two-dimensional state is well reproduced using an accumulation potential with the observed surface band bending and the characteristic width of about 30 Å.

Synthesis of Copper-Antimony-Sulfide Nanocrystals for Solution-Processed Solar Cells.

The p-type nanocrystals (NCs) of copper-based chalcogenides, such as CuInSe2 and Cu2ZnSnS4, have attracted increasing attention in photovoltaic applications due to their potential to produce cheap solution-processed solar cells. Herein, we report the synthesis of copper-antimony-sulfide (CAS) NCs with different crystal phases including CuSbS2, Cu3SbS4, and Cu12Sb4S13. In addition, their morphology, crystal phase, and optical properties were characterized using transmission electron microscopy, X-ray diffractometry, UV-vis-near-IR spectroscopy, and photoemission yield spectroscopy. The morphology, crystal phase, and electronic structure were significantly dependent on the chemical composition in the CAS system. Devices were fabricated using particulate films consisting of CAS NCs prepared by spin coating without a high-temperature treatment. The CAS NC-based devices exhibited a diode-like current-voltage characteristic when coupled with an n-type CdS layer. In particular, the CuSbS2 NC devices exhibited photovoltaic responses under simulated sunlight, demonstrating its applicability for use in solution-processed solar cells.