Sudhakar Shet

Enhanced Photoelectrochemical Responses of ZnO Films Through Ga and N Codoping

We report on the crystallinity and photoelectrochemical (PEC) response of ZnO thin films codoped by Ga and N. The ZnO:(Ga,N) thin films were deposited by cosputtering at room temperature and followed by postannealing at 500°C in air for 2h. We found that ZnO:(Ga,N) thin films exhibited significantly enhanced crystallinity compared to ZnO doped solely with N at the same growth conditions. Furthermore, ZnO:(Ga,N) thin films exhibited enhanced N incorporation over ZnO doped solely with N at high temperatures. As a result, ZnO:(Ga,N) thin films achieved dramatically improved PEC response, compared to ZnO thin films doped solely with N at any conditions. Our results suggest a general way to improve PEC response for wide-band-gap oxides.

ZnO nanocoral structures for photoelectrochemical cells

We report on synthesis of a uniform and large area of a new form of ZnO nanocorals. These nanostructures can provide suitable electrical pathways for efficient carrier collection as well as large surface areas for the photoelectrochemical (PEC) cells. PEC devices made from these ZnO nanocoral structures demonstrate significantly enhanced photoresponse as compared to ZnO compact and nanorod films. Our results suggest that the nanocoral structures could be an excellent choice for nanomaterial-based applications such as dye-sensitized solar cells, electrochromic windows, and batteries.

Carrier concentration tuning of bandgap-reduced p-type ZnO films by codoping of Cu and Ga for improving photoelectrochemical response

In this study, the synthesis of p-type ZnO films with similar bandgaps but varying carrier concentrations through codoping of Cu and Ga is reported. The ZnO:(Cu,Ga) films are synthesized by rf magnetron sputtering in O2 gas ambient at room temperature, followed by postdeposition annealing at 500°C in air for 2h. The bandgap reduction and p-type conductivity are caused by the incorporation of Cu. The tuning of carrier concentration is realized by varying the Ga concentration. The carrier concentration tuning does not significantly change the bandgap and crystallinity. However, it can optimize the carrier concentration to significantly enhance the photoelectrochemical response for bandgap-reduced p-type ZnO thin films.

Electrochemical deposition of copper oxide nanowires for photoelectrochemical applications

We report a two-step electrochemical process to grow copper oxide nanowires without the use of templates and surfactants. The first step is to electrochemically corrode amorphous Cu–W oxides pre-sputtered on fluorine-doped tin oxide (FTO) coated glass substrates under positive potentials. The second step is to use the corroded samples to grow copper oxide nanowires electrochemically under negative potentials. We find that copper oxide nanowires grown by this method exhibit excellent contact with the FTO-coated substrates leading to good charge transfer. The oxidation states, i.e., CuO or Cu2O, and morphology of nanowires can be controlled by the applied potentials. Cu2O nanowires grown using this method reveal moderate photoelectrochemical response and stability under illumination and under cathodic bias.

Synthesis and characterization of band gap-reduced ZnO:N and ZnO:(Al,N) films for photoelectrochemical water splitting

ZnO thin films with significantly reduced band gaps were synthesized by dopingN and codoping Al and N at 100 C. All the films were synthesized by radiofrequencymagnetron sputtering on F-doped tin-oxide-coated glass. We found that codoped ZnO:(Al,N) thin films exhibited significantly enhanced crystallinity compared with ZnO dopedsolely with N, ZnO:N, at the same growth conditions. Furthermore, annealed ZnO:(Al,N)thin films exhibited enhanced N incorporation over ZnO:N films. As a result, ZnO:(Al,N)films exhibited better photocurrents than ZnO:N films grown with pure N doping,suggesting that charge-compensated donor–acceptor codoping could be a potentialmethod for band gap reduction of wide-band gap oxide materials to improve theirphotoelectrochemical performance.I. INTRODUCTIONTransition-metal oxides are potential candidates forphotoelectrochemical (PEC) H

Understanding light-induced degradation of c-Si solar cells

We discuss results of our investigations toward understanding bulk and surface components of light-induced degradation (LID) in low-Fe c-Si solar cells. The bulk effects, arising from boron-oxygen defects, are determined by comparing degradation of cell parameters and their thermal recovery, with that of the minority-carrier lifetime (τ) in sister wafers. We found that the recovery of t in wafers takes a much longer annealing time compared to that of the cell. We also show that cells having SiN:H coating experience a surface degradation (ascribed to surface recombination). The surface LID is seen as an increase in the q/2kT component of the dark saturation current (J02). The surface LID does not recover fully upon annealing and is attributed to degradation of the SiN:H-Si interface. This behavior is also exhibited by mc-Si cells that have very low oxygen content and do not show any bulk degradation.

CoAl2O4–Fe2O3 p-n nanocomposite electrodes for photoelectrochemical cells

CoAl2O4–Fe2O3 p-n nanocomposite electrodes were deposited on Ag-coated stainless-steel substrates and annealed at 800 °C. Their photoelectrochemical (PEC) properties were investigated and compared with that of p-type CoAl2O4 films. We found that the nanocomposite electrodes exhibit much improved PEC photoresponse as compared to the reference p-type CoAl2O4 electrodes. We speculate that the enhancement is due to the formation of a three-dimensional junction between p-type CoAl2O4 and n-type Fe2O3 nanoparticles, which improves electron-hole separation, thus reducing charge recombination upon light illumination.

Characterizing damage on Si wafer surfaces cut by slurry and diamond wire sawing

We have measured and compared surface roughness and the degree of damage for wafers cut by three different sawing techniques - slurry, Ni-based diamond wire, and resin-based diamond wire sawing. The local damage was determined by angle polishing followed by defect etching, TEM, SEM/EBSD imaging and Raman imaging. It showed that each of the cutting processes produces a thin layer of amorphous Si at the surface and dislocation loops that can go about 1 μm deep below the surface. A new approach was used to quantify the average damage over a large area. We determined the effective surface recombination (SRV) as a function of depth. Because the effective SRV is a function of the carrier loss close to the surface, it is well-suited to define damage distribution at and below the surface. Wafers with surface damage were step etched in (HF:HNO3:CH3COOH::1:1:5), and the effective lifetime was measured with a Sinton system after each etching step, with iodine-ethanol passivation. The SRV plots as a function of depth, representing depth distribution of the damage, were compared for large groups of wafers cut by each technique. Our results show that for optimized cutting, all three cutting methods produce damage depth of about 5μm (each surface). However, the degree of damage is higher for slurry cut wafers.

Amorphous copper tungsten oxide with tunable band gaps

We report on the synthesis of amorphous copper tungsten oxide thin films with tunable band gaps. The thin films are synthesized by the magnetron cosputtering method. We find that due to the amorphous nature, the Cu-to-W ratio in the films can be varied without the limit of the solubility (or phase separation) under appropriate conditions. As a result, the band gap and conductivity type of the films can be tuned by controlling the film composition. Unfortunately, the amorphous copper tungsten oxides are not stable in aqueous solution and are not suitable for the application of photoelectrochemical splitting of water. Nonetheless, it provides an alternative approach to search for transition metal oxides with tunable band gaps.

Room-temperature silicon band-edge photoluminescence enhanced by spin-coated sol–gel films

Photoluminescence is observed at room temperature from phonon-assisted band-to-band emission in Si (1.067 eV peak) using unpatterned bulk p-type silicon wafer samples that were spin-coated with Er-doped (6 at. %) silica-gel films (0.13 m) and vacuum annealed (strongest emission for ~700 C). Comparative study of annealing behavior indicates two-orders of magnitude efficiency enhancement. Emission from Er +3 ions in the silica film is used to gauge relative emission strengths. Mechanisms for inducing