Browsing by Author "Simiyu, Justus"
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Publication Characteristics of TiO2 Compact Layer prepared for DSSC application(Path of Science, 2018-11-04) Musila, Nicholas; Munji, Mathew; Simiyu, Justus; Masika, Eric; Nyenge, Raphael; Kineene, Miriam; Musila, Nicholas; Munji, Mathew; Simiyu, Justus; Masika, Eric; Nyenge, Raphael; Kineene, MiriamDye-sensitized solar cells (DSSCs) offer an economically reliable and suitable alternative in moderating the challenges presented by the existing convectional photovoltaic cells. However, the efficiency of dye-sensitized solar cells has remained relatively low. For this reason, this research was aimed at studying the characteristics of TiO2 compact layer that can be applied in DSSCs as a way of improving efficiency. To achieve this, TiO2 compact layer was deposited on a conductive glass substrate by using Holmarc’s Spray Pyrolysis system, using Ultrasonic Spray Head and spraying in the vertical geometry. X-ray Diffraction studies revealed that TiO2 compact layer was of anatase phase and had tetragonal crystalline structure. Raman spectroscopy showed that the most intense peak appeared at 142 cm-1 due to the external vibration of the anatase structure. Hall Effect studies revealed that TiO2 compact layer has a high density of charge carriers’ value of 1.25 × 1019 cm-3 hence it can be used in DSSC applications.Publication Compositional analysis and optical properties of Co doped TiO2 thin films fabricated by spray pyrolysis method for dielectric and p hotocatalytic applications(Science Direct, 2017-01-01) Wafula, Henry Barasa; Musembi, Robinson Juma; Juma, Albert Owino; Tonui, Patrick; Simiyu, Justus; Sakwa, Thomas; Prakash, Deo; Verma, K. D.Cobalt doped TiO2 thin films (CTF) deposited by spray pyrolysis has been studied. The compositional analysis has been done using RBS method, while optical spectroscopy has been done by measuring the transmittance and reflectance of the films. The CTF thin films were prepared by doping TiO2 at different concentration levels of Co which was varied between 0 and 4.51 at.%. The optical transmittance of the thin film has been found to be about 80% in the visible and near infra red regions. The calculated optical band gap has been observed to shift by about 0.22 eV, this shows a high potential for application as a dielectric and a Photocatalyst material.Publication Effect of TiO2 Compact Layer on DSSC Performance(Social Science Research Network, 2018-09-30) Musila, Nicholas; Munji, Mathew; Simiyu, Justus; Masika, Eric; Nyenge, RaphaelDye-sensitized solar cells offer an economically reliable and suitable alternative in moderating the challenges presented by the existing convectional photovoltaic cells. Whereas, for convectional solar cells the semiconductor adopts both the duty of light absorption and charge carrier transport, these two functions are separated in dye-sensitized solar cells. However, the efficiency of dye-sensitized solar cells has remained relatively low. For this reason, this research was aimed at how to increase the dye-sensitized solar cells performance. To achieve this, compact cover of TiO2 was deposited on a conductive glass substrate by using Holmarc’s Spray Pyrolysis system, using Ultrasonic Spray Head and spraying in vertical geometry, while TiO2 nanoparticles and nanotubes were deposited by screen printing technique on top of a transparent conducting FTO glass slide with or without the TiO2 compact layer. Transmission characteristics showed that introducing TiO2 compact layer on the conductive film lowers the transmission while reflectance properties were less than 15 % for all the prepared thin films. SEM micrographs showed that TiO2 nanotubes had a skein-like morphology with abundant number of nanotubes intertwined together to form a large surface area film. Solar cell performance properties revealed that introducing compact layer to dye-sensitized solar cells improved the performance by 145 % (from 1.31 % to 3.21 %) while TiCl4 treatment on compact layered dye-sensitized solar cells increased the efficiency by 28.79 % (from 0.66 % to 0.85 %).Publication Influence of Pore Size on the Optical and Electrical Properties of Screen Printed(Hindawi Publishing Corporation, 2016-09-27) Domtau, Dinfa Luka; Simiyu, Justus; Ayieta, Elijah Omollo; Asiimwe, Godwin Mwebeze; Mwabora, Julius MwakondoInfluence 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.Publication Optical Properties and Analysis of OJL Model’s Electronic inter-band Transition Parameters of TiO2 Films(Path of Science, 2018-07-31) Musila, Nicholas; Munji, Mathew; Simiyu, Justus; Masika, Eric; Nyenge, Raphael; Musila, Nicholas; Munji, Mathew; Simiyu, Justus; Masika, Eric; Nyenge, RaphaelTitanium dioxide is a wide band gap semiconductor responsible for the bright white appearance in most substances. This material has many unique properties due to its extra-ordinary chemical stability. TiO2 has a conduction band that closely matches the excited energy level of organic dyes hence it is used in fabrication of photo-anode electrode of dye sensitized solar cell. However, the optical properties and the density of states of TiO2 thin films determine the performance of dye sensitized solar cell fabricated from TiO2 photo-anode electrode. For this reason, the purpose of this study was to investigate the optical properties and the OJL electronic inter-band transition analysis of TiO2 nanoparticle thin films. Under the OJL model, the expressions of density of states were specified for the optical transition from the valence band to the conduction band. The TiO2 nanoparticles were prepared using sol-gel and hydrothermal methods and deposited on a conductive glass substrate by screen printing and spray pyrolysis techniques. SEM analysis revealed that TiO2 nanoparticles were spongy and had unevenly sphere-shaped profile while TiO2 nanotubes had a skein-like morphology with abundant number of nanotubes intertwined together. This study showed that TiO2 thin films have both direct and indirect band-gaps. The OJL Gap energy (E0) values were observed to be between 30273.2356 and 31072.0000 wavenumbers which translated to band-gap energies between 3.744 and 3.843 eV. From these findings showed that TiO2 films prepared could be used in the fabrication of high performing dye-sensitized solar cell.Publication Structural and optical properties of phosphorous and antimony doped ZnO thin films deposited by spray pyrolysis: a comparative study(International Advanced Research Journal in Science, Engineering and Technology, 2017) Nyarige, Justine S.; Sebastian, Waita; Simiyu, Justus; Mureramanzi, Silas; Aduda, BenardA study of structural and properties of pure (undoped) Zinc oxide (ZnO) and phosphorous (P) and Antimony(Sb) doped Zinc Oxide films has been carried out. The films were deposited by an automated spray pyrolysisequipment on both microscope glass at various elevated temperatures (270 oC - 420 oC) and on fluorine doped tinOxide (FTO) substrates at 420 oC. Structural characterization using Raman spectroscopy showed the presence of themain peak for ZnO at 437 cm-1for all the films. Antimony doped films showed other peaks associated with the dopingbut phosphorous doping did not show extra peaks. Optical characterization using a UV-VIS-NIR Shimadzu (ModelDUV 3700) double beam spectrophotometer provided both reflectance and transmittance data and Scout software wasused to compute the band gap. At a wavelength of 600 nm, the average transmittance of the pure ZnO films was ~62% while it was transmittance was ~85 % and ~80 % for Sb and P doped films respectively, an increase of ~23 % and~18 % respectively. For the undoped ZnO films, high deposition temperatures led to band gap narrowing from 3.25eVto 3.10eVwhile doping resulted in band gap widening from 3.10 eV to 3.30 eV (for P-doped) and 3.10 eV to 3.33 eV(for Sb-doped),an observation confirmed by the increased transmittance on doping. The band gap narrowing for ZnOfilms makes the film become a better materials for visible light absorption which is good for photovoltaic applications.The wide gap broadening on doping makes the film more transparent to solar radiation making it suitable fortransparent conducting oxide applications.Publication The effect of mesoporous TiO2 pore size on the performance of solid-state dye sensitized solar cells based on photoelectrochemically polymerized Poly(3,4-ethylenedioxythiophene) hole conductor(Science Direct, 2016-08-20) Zhang, Jinbao; Pazoki, Meysam; Simiyu, Justus; Johansson, Malin B.; Cheung, Ocean; Häggman, Leif; Johansson, Erik M. J.; Vlachopoulos, Nick; Hagfeldt, Anders; Boschloo, GerritPhotoelectrochemical polymerization of poly(3,4-ethylenedioxythiphene) (PEDOT) has recently been introduced and widely investigated for fabrication of the hole transporting material (HTM) in highly efficient solid state dye sensitized solar cells (sDSCs). In this work, the effects of the surface area and pore size of TiO2 film were for the first time investigated in the sDSCs employing the in-situ polymerizated PEDOT HTM. Three different varieties of mesoporous TiO2 particles with controllable surface area and pore size were synthesized through the basic route in order to study the corresponding sDSC photovoltaic performances. It was found that the pore size plays an important role in the kinetics of the photoelectrochemical polymerization (PEP) process and the formation of the PEDOT capping layer. Larger pore sizes provided a more favourable pathway for the precursor diffusion through the mesoporous pores during the PEP process, which contributed towards a more efficient PEP. However, the interfacial contact area between the formed polymer and the dyes on the surface of TiO2 particle would be lower in the case of larger pore sizes, which consequently caused a less efficient dye regeneration process. Electronic diffusion on the other hand was improved for larger particle sizes. Employing an organic dye LEG4 and the self-made TiO2 with an optimal pore size of 25nm and particle size of 24nm, the sDSCs showed a promising power conversion efficiency (PCE) of 5.2%, higher than 4.5% for the commercial TiO2 Dyesol DSL-30. By measuring the dye regeneration yield and the kinetics through photoinduced absorption, it was observed that the homemade TiO2 based device had more efficient dye regeneration compared to the Dyesol based device, which could result from the better interfacial contact between the PEDOT and the dye. This work provides important information on the effect of meso-pore size on sDSCs and points to the necessity of further photoanode optimization toward the enhancement of the PCE of polymeric hole conductor-based DSCs.