Bio

DR. SIMIYU JUSTUS

Brief

I am a Senior Lecturer in Physics at the University of Nairobi and a member of Condensed Matter/Solar Energy Research Group (in the Department of Physics, University of Nairobi) with 13 years teaching and research experience in areas of basic physics, engineering physics and electronics, solar energy, process control and photovoltaics. I am a certified Solar PV Trainer and I am the coordinator and lead trainer for Solar Photovoltaics with the University of Nairobi and an active member of the college committee for exhibitions in local and international fairs.

Publications


2016

Domtau, DL, Simiyu J, Ayieta EO, Muthoka B, Mwabora JM.  2016.  Optical and Electrical Properties Dependence on Thickness of Screen-Printed TiO2 Thin Films.. Journal of Materials Physics and Chemistry.. 4(1):1. Abstract

Effect of film thickness on the optical and electrical properties of TiO2 thin films was studied. Thin films of different thicknesses were deposited by screen printing method on fluorine doped tin oxide coated on glass substrate. The film thickness was determined by surface profile measurement. The thicknesses were 3.2, 8.2, 13.5 and 18.9 µm. Transmittance, reflectance and absorbance spectra were studied using UV-VIS-NIR spectrophotometer in the photon wavelength range of 200-2500 nm for transmittance and reflectance and 200-1200 nm for absorbance. Band gap and refractive index of the films were determined using these spectra. It was found that reflectance, absorbance, band gap and refractive index increase with film thickness while transmittance decreases with increase in thickness. I-V characteristics of the films were also measured by a 4- point probe. Electrical resistivity (

Domtau, DL, Simiyu J, Ayieta EO, Muthoka B, Nyakiti LO, Mwabora JM.  2016.  Effects of Film Thickness and Electrolyte Concentration on the Photovoltaic Performance of TiO2 Thin Films. Surface Reviews and Letters. 24 Abstract

Effects of film thickness and electrolyte concentration on the photovoltaic performance of TiO2 based dye-sensitized solar cell were studied. Nanocrystalline anatase TiO2 thin films with varying thicknesses (3.2-18.9 µm) have been deposited on FTO/glass substrates by screen printing method as work electrodes for dye-sensitized solar cells (DSSC). The prepared samples were characterized by UV-vis spectroscopy, atomic force microscopy/scanning tunneling microscopy (AFM/STM) and x-ray diffraction. The optimal thickness of the TiO2 photoanode is 13.5 µm. 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 18.9 µm (7.5 mA/cm2 and 0.687 V) are smaller than those of DSSC with a TiO2 film thickness of 13.5 µm (8.2 mA/cm2 and 0.711 V). 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 3.5 % was obtained in a DSSC with the TiO2 film thickness of 13.5 µm and redox concentrarion of 0.03 mol dm-3 under AM 1.5G illumination at 100 mW/cm2

Simiyu, J, Domtau DL, Ayieta EO, Asiimwe GM, Mwabora JM.  2016.  Influence of Pore Size on the Optical and Electrical Properties of Screen Printed TiO2 Thin Films. Advances in Materials Science and Engineering. 2016 Abstract

Influence of pore size on the optical and electrical properties of TiO2 thin films were studied. TiO2 thin films with different weight percentages (wt %) of carbon black were deposited by screen 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 300-1500 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. Currents and voltages (I-V) characteristics of the films were measured by a 4 point-probe. Resistivity (

Wafula, H, Juma A, Sakwa T, Robinson Musembi, Simiyu J.  2016.  A Surface Photovoltage Study of Surface Defects on Co-Doped TiO2 Thin Films Deposited by Spray Pyrolysis, Coatings. Abstract

Surface photovoltage (SPV) spectroscopy is a powerful tool for studying electronic defects on semiconductor surfaces, at interfaces, and in bulk for a wide range of materials. Undoped and Cobalt-doped TiO2 (CTO) thin films were deposited on Crystalline Silicon (c-Si) and Flourine doped Tin oxide (SnO2:F) substrates by chemical spray pyrolysis at a substrate temperature of 400◦C. The concentration of the Co dopant in the films was determined by Rutherford backscattering spectrometry and ranged between 0 and 4.51 at %. The amplitude of the SPV signals increased proportionately with the amount of Co in the films, which was a result of the enhancement of the slow processes of charge separation and recombination. Photogenerated holes were trapped at the surface, slowing down the time response and relaxation of the samples. The surface states were effectively passivated by a thin In2S3 over-layer sprayed on top of the TiO2 and CTO films.

Domtau, DL, Simiyu J, Muthoka B, Mwabora J.  2016.  Optical and Electrical Properties Dependence on Thickness of Screen-Printed TiO2 Thin Films. . Journal of Materials Physics and Chemistry. . 4(1):1-3.

2013

Wafula, HB, Simiyu J, Waita S, Aduda BO, Mwabora JM.  2013.  EFFECT OF NITRATION ON PRESSED TIO2 PHOTOELECTRODES FOR DYE-SENSITIZED SOLAR CELLS. Website

2012

Otakwa, R.V.M, Simiyu, J., Waita, S.M., Mwabora JM.  2012.  Dark J-V Characterization of a Dye-Sensitized Module. International Journal of Professional Practice (IJPP), . 3(1&2):165-168.
Otakwa, R.V.M, Simiyu, J., Waita SM, Mwabora JM.  2012.  Application of Dye-Sensitized Solar Cell Technology in the Tropics: Effects of Air Mass on Device Performance. International Journal of Renewable Energy Research (IJRER). 2(2):369-375.
Otakwa, R.V.M, Simiyu, J., Waita, S.M., Mwabora JM.  2012.  Application of Dye-Sensitized Solar Cell Technology in the Tropics: Effects of Radiation Intensity and Temperature on DSSC Performance. International Journal of Advanced Renewable Energy Research (IJARER). 1,2(4):17-25.

2011

2010

J. Nissfolk, K. Fredin, Simiyu J, Haeggmann L, Hagfeldt A, Boschloo G.  2010.  Interpretation of Small-modulation photocurrent transients in Dye-sensitized Solar Cells- A Film Thickness Study. Journal of Electroanalytical Chemistry . 646(1):91-99.

2009

Simiyu, J., Aduda BO, Mwabora JM.  2009.  Conduction Band Edge of (Ti,Sn)O2 Solid Mixtures Tuning for Photoelectrochemical Applications, 2 July. International Conference on Electroceramics. 1171(Special issue):S04-05., Arusha, Tanzania

2008

Simiyu, J., B.O A, Mwabora JM, Lindqvist S-E, Hagfeldt A, Boschloo G.  2008.  Titania Nanotubes Prepared by Synthesis Method for Dye Sensitized Electrochemical Solar Cells. African Physical Review Special Issue (Materials). 83(2):161.

2007

2004

"Simiyu, J., ", "Mwabora, M. J. ", "Aduda, B. O. ".  2004.  Anthocyanin Sensitized Nanoporous TiO2 PEC Solar Cells Prepared by Sol Gel Process. Progr. Colloid Polym. Science. 125(1):34-37.

2002

Simiyu, J., J. MM, B.O A.  2002.  Stability of Anthocyanin Sensitized TiO2 Photoelectrochemical (PEC) Solar Cells. Africa Journal of Science and Technology. 6(2):56-61.

2001

UoN Websites Search