J. M. Mwabora

Effect of Ga Incorporation and Film Thickness on the Optical Properties of as-Deposited Amorphous GaxSel-x Thin Films

Flash evaporated amorphous GaxSe1-x (x = 0, 1, 2, 3, and 4 atomic %) on glass substrates have been investigated within a 500nm-1500nm spectral range. Film thicknesses explored were ; 200±10nm, 265±10nm, 330±10nm, and 400±10nm. The effect of film thickness and gallium content on the as-deposited thin films has been established. As the gallium content increases, both the optical transmittance and band gap energy decrease. Increase in film thickness led to a decrease in optical transmittance and an increase in the band gap energy. The other optical parameters like the absorption coefficient, extinction coefficient, refractive index, real part and imaginary part of dielectric constant increase with increase in gallium content and film thickness.

EFFECTS OF TiO2 FILM THICKNESS AND ELECTROLYTE CONCENTRATION ON PHOTOVOLTAIC PERFORMANCE OF DYE-SENSITIZED SOLAR CELL

Effects of film thickness and electrolyte concentration on the photovoltaic performance of TiO2-based dye-sensitized solar cell (DSSC) were studied. Nanocrystalline anatase TiO2 thin films with varying thicknesses (3.2–18.9μm) have been deposited on FTO/glass substrates by screen printing method as work electrodes for DSSC. The prepared samples were characterized by UV-Vis spectroscopy, Atomic Force Microscopy/Scanning Tunneling Microscopy (AFM/STM) and X-ray diffraction (XRD). The optimal thickness of the TiO2 photoanode is 13.5μm. Short-circuit photocurrent density (Jsc) increases with film thickness due to enlargement of surface area whereas open-circuit voltage decreases with increase in thickness due to increase in electron diffusion length to the electrode. However, the Jsc and Voc of DSSC with a film thickness of 18.9μm (7.5mA/cm2 and 0.687V) are smaller than those of DSSC with a TiO2 film thickness of 13.5μm (9.9mA/cm2 and 0.734V). This is because the increased thickness of TiO2 thin film resulted in the decrease in the transmittance of TiO2 thin films hence reducing the incident light intensity on the N719 dye. Photovoltaic performance also depends greatly on the redox couple concentration in iodide∖triiodide. Jsc decreases as the redox concentration increases as a result of increased viscosity of the solution which lowers ion mobility. Similarly, Voc decreases as the electrolyte concentration increases due to enhanced back electron transfer reaction. An optimum power conversion efficiency of 4.3% was obtained in a DSSC with the TiO2 film thickness of 13.5μm and redox concentration of 0.03mol dm−3 under AM 1.5G illumination at 100mW/cm2.

Metallic Cu and In films deposited by d.c. magnetron sputtering

Cu and In films have been deposited by direct current (d.c.) magnetron sputtering on glass substrates under different deposition conditions. Deposition temperature, partial pressure in the vacuum chamber, and thickness have been found to be the most critical parameters for achieving low resistivity films. Resistivity of In films was found to be more sensitive to deposition parameters than for Cu films. For the deposition parameters studied in this work, changes in the resistivity of Cu films are of the order of 101, compared to In film resistivity changes of order of 101 to 1011.

Enhanced performance of Sb2S3 mesoscopic sensitized solar cells employing TiO2:Nb compact layer

This paper reports on the enhancement of charge transport and recombination by niobium doped compact layers of TiO2 in a solar cell with Sb2S3 absorber layer by characterizing both thin films of TiO2:Nb and working solar cell devices with the layer stack FTO/cp-TiO2:Nb/mp-TiO2/Sb2S3/P3HT/MoOx/Ag. The electron transport layers of TiO2 doped with 0.14 and 0.27 at.% Nb were prepared by spin coating and have no structural change as determined from the analysis of GIXRD spectra. SEM images show thin pin hole free layers of the cp-TiO2:Nb on FTO crystals that are agglomerates of particles. Analysis of the current–voltage curves of the solar cells with Sb2S3 as the absorber material showed increased short-circuit current, fill factor and power conversion efficiency from 1.3 to 1.7%. The enhancement of the device performance is attributed to substitution of Ti ions with Nb ions in the TiO2 resulting in a change in the band alignment of the solar cells with Nb content. This results in increase in charge recombination resistance in the Sb2S3 layer as determined from the analysis of the impedance spectroscopy measurements.

Influence of Pore Size on the Optical and Electrical Properties of Screen Printed Thin Films

Influence of pore size on the optical and electrical properties of TiO2 thin films was studied. TiO2 thin films with different weight percentages (wt%) of carbon black were deposited by screen printing method on fluorine doped tin oxide (FTO) coated on glass substrate. Carbon black decomposed on annealing and artificial pores were created in the films. All the films were 3.2 µm thick as measured by a surface profiler. UV-VIS-NIR spectrophotometer was used to study transmittance and reflectance spectra of the films in the photon wavelength of 300–900 nm while absorbance was studied in the range of 350–900 nm. Band gaps and refractive index of the films were studied using the spectra. Reflectance, absorbance, and refractive index were found to increase with concentrations of carbon black. There was no significant variation in band gaps of films with change in carbon black concentrations. Transmittance reduced as the concentration of carbon black in TiO2 increased (i.e., increase in pore size). Currents and voltages ( ) characteristics of the films were measured by a 4-point probe. Resistivity ( ) and conductivity ( ) of the films were computed from the values. It was observed that resistivity increased with carbon black concentrations while conductivity decreased as the pore size of the films increased.

Optical Properties of Flash Evaporated Amorphous Se 100-X In X Thin Films for CuInSe 2 Solar Cell Applications

Amorphous Se100-XIn X thin films were prepared by flash evaporation of the pre-melt quenched bulk samples. The samples were characterised and the suitability of each composition assessed towards applications as a buffer layers in CuInSe2 photovoltaic cells. XRD studies confirmed that all the samples were amorphous. There was a descending dependence of complex dielectric constant and refractive index on wavelength which levelled in the infra-red (IR) region. A red shift in the optical energy gap Eg opt from 1.76 eV for In = 5 at% to 1.46 eV for In = 20 at% was observed. The Eg opt values were in the range that is compatible with solar energy conversion.

Anthocyanin-sensitized nanoporous TiO2 photoelectrochemical solar cells prepared by a sol–gel process

Photoelectrochemical solar cells comprising a colloidal TiO2 photoelectrode and sensitized with anthocyanin pigments, delphinidin purple and cyanidin 3,5-diglucose extracted from Hibiscus sabdariffa and Ribes nigra plants, respectively, have been fabricated. A sunlight-to-electricity conversion efficiency of 3.16% under simulated solar light was obtained with the cell sensitized with the delphinidin purple dye. Open-circuit photovoltages of 0.2–0.3 V and short-circuit photocurrents of 15–30 mA/cm2> were obtained, which points to efficient charge-carrier injection at the semiconductor/electrolyte interface. The cells also showed a high activation energy of between 0.3–0.5 eV.