R. Thangavel

Bandgap tuning in highly c-axis oriented Zn1−xMgxO thin films

We propose Mg doping in zinc oxide (ZnO) films for realizing wider optical bandgap in highly c-axis oriented Zn1−xMgxO (0 ≤ x ≤ 0.3) thin films. A remarkable enhancement of 25% in the bandgap by 30% Mg doping was achieved. The bandgap was tuned between 3.25 eV (ZnO) and 4.06 eV (Zn0.7Mg0.3O), which was further confirmed by density functional theory based wien2k simulation employing a combined generalized gradient approximation with scissor corrections. The change of stress and crystallite size in these films were found to be the causes for the observed blueshift in the bandgap.

Solution-processed Cu2XSnS4 (X = Fe, Co, Ni) photo-electrochemical and thin film solar cells on vertically grown ZnO nanorod arrays

This article presents the photo-electrochemical and photovoltaic performances of solution processed Cu2XSnS4 (X = Fe, Co, Ni) thin films on ZnS coated vertically aligned ZnO nanorod (NR) arrays for the first time. The optical, structural and electrical properties of all the synthesized structures were investigated using various characterization techniques. The electrocatalytic activities of the superstrate structures were characterized by taking cyclic voltammetry (CV) measurements and performing electrochemical impedance spectroscopy (EIS) analyses. Furthermore, the photovoltaic performances of the devices with architecture ITO/ZnO-NRs/ZnS/Cu2XSnS4/Au were investigated under AM 1.5 illumination condition. The thin film solar cells with Cu2FeSnS4 (CFTS), Cu2CoSnS4 (CCTS) and Cu2NiSnS4 (CNTS) photoactive layers exhibited power conversion efficiency (PCE) values of 2.73%, 3.23% and 2.71% respectively. All the devices exhibit good electrochemical as well as photovoltaic performance under illuminations and have shown potential to fabricate nontoxic solution processed high efficient devices.

Structural, optical and photo-electrochemical properties of hydrothermally grown ZnO nanorods arrays covered with α-Fe2O3 nanoparticles

A low cost hydrothermal method and subsequent wet-chemical process has been used for the preparation of a ZnO nanorod (NR) array film grown on tin doped indium oxide (ITO) coated glass substrates, post decorated by α-Fe2O3 nanoparticles (NPs). The effect of the amount of α-Fe2O3 NPs on the properties of ZnO NRs such as electron scattering, surface roughness, photo-charge transport and interface recombination has been investigated. Furthermore, the optimized α-Fe2O3 NPs@ZnO exhibited enhanced light absorption, charge separation and charge transport properties, as characterized by UV-vis absorption spectra and the photo-electrochemical characterization by Linear Sweep Voltammetry (LSV) and Electrochemical Impedance Spectroscopy (EIS). An optimum amount of α-Fe2O3 NPs on the ZnO NRs enhanced the photocurrent density by 163% and the photo-conversion efficiency by more than 2.5 times compared to those of the bare ZnO NRs. These results indicated that the prepared α-Fe2O3 NPs@ZnO NRs could serve as an efficient photo-electrode material for photo-electrochemical cells.

Chemical synthesis, characterization and theoretical investigations of stannite phase CuZn2AlS4 nanocrystals

Stannite phase CuZn2AlS4 (CZAS) nanocrystals have been grown for the first time using hot injection and hydrothermal processes. As-synthesized nanocrystals in the shape of nanowires (from a hot injection process) and nanoplates (from a hydrothermal process) show band gaps of 1.64 eV and 1.69 eV, respectively, which is attractive for use as solar cell absorber materials. From first principles study, we find that CZAS exhibits direct band characteristics with a high absorption coefficient of >104 cm−1. Based on energy considerations the stannite structure is found to be more stable than the kesterite, wurtzite, orthorhombic and zinc blende type structures. The total and partial density of states (DOSs) indicate that hybridization between Zn-s and S-p like bands occurs near the Fermi level.

Facile synthesis, and the optical and electrical properties of nanocrystalline ZnFe2O4 thin films

Nanostructured zinc ferrite (ZnFe2O4) thin films with uniform grain size distribution have been synthesized using a citrate–nitrate sol–gel and spin coating method. The formation and impact of intrinsic oxygen vacancies in all of the grown samples have been explained with various structural, optical and electrical properties. Low temperature four probe resistivity measurements imply that the conduction mechanism involved impurity and valance band contributions. The high donor density and oxygen vacancies have improved the photocurrent density of the optimized sample from 36.85 to 65.87 mA cm2. The optimized sample showed a reduced band gap of 2.316 eV and electrochemical impedance spectroscopy under dark and visible light illumination conditions revealed significantly improved charge transfer properties of the samples. These samples have improved optical absorption and electrical conduction with enhanced photocurrent density and have promise for solar photoelectrochemical applications.

Photo-electrochemical properties and electronic band structure of kesterite copper chalcogenide Cu2–II–Sn–S4 (II = Fe, Co, Ni) thin films

Herein, we report a spin coating route for the deposition of multifunctional quaternary copper chalcogenide Cu2–II–Sn–S4 (II = Fe, Co and Ni) thin films on ITO glass substrates. The surface morphology of the thin films is a uniform porous like structure with an average film thickness of 1.5 μm. The formation of single phase Cu2FeSnS4 (CFTS), Cu2CoSnS4 (CCTS) and Cu2NiSnS4 (CNTS) thin films was confirmed by Raman spectroscopy, which reveals two principal Raman peaks of the A1 phonon mode. Due to the presence of the d6, d7 and d8 electronic configurations within Fe2+, Co2+ and Ni2+ ions, respectively, all the materials show a spin magnetic moment per ion of 3.33 μB, 2.45 μB and 1.31 μB for CFTS, CCTS and CNTS, respectively, as described by the electronic band structure calculation (WIEN2k code). All the thin films have estimated band gap values of 1.87 eV, 1.57 eV and 1.74 eV, respectively, with enhanced photocurrent properties and electrocatalytic activity. Significantly improved charge transfer properties of the thin films were observed from electrochemical impedance spectroscopy analysis under dark and visible light illumination conditions. These thin films are promising for use in photoelectrochemical and solar cell applications.

Erratum to: Structural and optical properties of Na doped ZnO nanocrystalline thin films synthesized using sol–gel spin coating technique

Sodium (Na) doped Zinc oxide (ZnO) thin films have been deposited on a glass substrate by the sol–gel spin coating method. Effect of doping with various percentages of Na at a particular annealing temperature of 500 °C is studied. The samples were characterized by X-ray diffraction (XRD), micro-photoluminescence, Raman and Polarized Raman spectroscopy. The X-ray diffraction and micro-Raman spectroscopy confirmed the presence of Na substitution in zinc oxide and the wurtzite structure of the lattice is retained. An enhancement of resonant Raman scattering processes as well as longitudinal optical phonon overtones up to the fifth order were observed in the micro Raman spectra. The similar values of depolarization ratios obtained from Polarized Raman studies recommend no change in the symmetry. Photoluminescence showed a strong emission peak in the near UV at 3.2 eV and negligible visible emission.

Hydrothermal growth of undoped and boron doped ZnO nanorods as a photoelectrode for solar water splitting applications

Undoped and boron-doped ZnO nanorods (NRs) were grown on ITO glass substrates by using hydrothermal techniques. The as grown nanorods were investigated by using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), UV–visible spectroscopy, and photoelectrochemical study. XRD spectra reveal the confirmations regarding the hexagonal wurtzite structure along with preferential orientation (002). The observation of (002) peak shows a red shift. The average size distribution of NRs in doped and undoped sample ranges from 169 to 191 nm. The absorption spectra clearly revealed the band gap tunability feature of the samples with a change in doping percentage. Photoluminescence spectra clearly indicate the presence of oxygen defects. Photocurrent density as high as ∼0.622 and 2.6 mA/cm2 were obtained for undoped and 6% B-doped ZnO NRs arrays respectively, at +0.44 V vs. Ag/AgCl electrode under visible light AM 1.5 G (100 mW/cm2) in 0.1 M electrolyte solutions of NaOH. More enhancement in photoconversion efficiency (PCE) from 0.491 to 2.054% was observed for undoped ZnO NRs and optimum 6% B-doped ZnO in 0.1 M NaOH electrolyte solution.Graphical abstract

Gd doping effect on structural, electrical and magnetic properties of ZnO thin films synthesized by sol-gel spin coating technique

Nanocrystalline Gd-doped ZnO thin films were deposited on sapphire (0001) substrates using sol-gel spin coating technique. The structural and optical properties of deposited thin films were characterized by X-ray diffraction (XRD) and micro Raman spectroscopy. Structural and optical studies show that the doped Gd ions occupy Zn sites retaining the wurtzite symmetry. Photoluminescence (PL) studies reveal the presence of oxygen vacancies in Gd doped ZnO thin films. The resistivity of Gd doped ZnO thin film decreases with increase in Gd doping upto 4%. Gd-doped ZnO films demonstrate weak magnetic ordering at room temperature.

A comparative study of structural and optical properties of Cu2ZnSnS4 nanocrystals for photovotaic applications

The Cu 2 ZnSnS 4 (CZTS) nanocrystals were successfully synthesized by hot injection method. The synthesized nanocrystals were characterized by the X-ray diffraction (XRD) and UV–visible spectrometer. X-ray diffraction studies reveal polycrystalline films exhibiting kesterite structures with preferential orientation along (112) direction. From UV studies revealed that the CZTS nanocrystals band gap was tuned between 1.6 eV (for 1 hour, stirring) and 1.4 eV (for 2 hours stirrer) and its compared, structural and optical properties of CZTS by density functional theory based on Wien2K (FP-LAPW) simulation employing a combined Tran-Blaha modified Becke-Johnson (TB-MBJ) potential with scissor corrections. From this calculation, it was found that the compound is in direct nature. The estimated lattice constant and bandgap value is in good agreement with earlier reported results. The optical band-gap energy of the CZTS nanocrystals is 1.4 eV, which is quite close to the optimum value for solar cell.