Bio

BIOGRAPHY

Professor Julius Mwakondo Mwabora earned a B.Sc. (Physics major) and M.Sc. in Physics from the University of Nairobi (UoN) in 1990 and 1993, respectively. He received his Ph.D. in Physics from the University of Dar es Salaam in 1999. He joined the Department of Physics at the UoN in 1992 as a Graduate Assistant, was promoted Tutorial Fellow in 1994, to Lecturer in 2000, to Senior Lecturer in 2003, to Associate Professor in 2009, and to Full Professor in 2014. He was the Coordinator, Self Study Centre, UoN from 2006 to 2013.

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Publications


2017

Zipporah, M, Rohit P, Robinson M, Ralph S.  2017.  First-principle investigation of structural, electronic and magnetic properties of Co2VIn and CoVIn Heusler compounds. AIP Advances. 7 Abstract

Investigation of the structural, electronic and magnetic properties of full-Heusler Co2VIn as well as half-Heusler CoVIn Cobalt based Heusler compounds using density functional theory (DFT) leads to the general conclusion that Co2VIn and CoVIn are half-metallic materials with a gap at the Fermi level in the minority states and majority states respectively. A Hubbard-like Coulomb correlation term U has been included in the DFT (DFT+U) for the computation of the electronic and magnetic properties of the compounds. The structural properties have been calculated for the paramagnetic and ferromagnetic phases, and both Co2VIn and CoVIn are found to be stable in the ferromagnetic phase. The calculated magnetic moments are 2 μB2 μB and 0.9 μB0.9 μB per formula unit for Co2VIn and CoVIn respectively.

2016

Muiva, CM, Sathiaraj TS, King JG.  2016.  Optical properties of amorphous Se90-XIn10SbX thin film alloys. Journal of Alloys and Compounds. 689:432–438. Abstract

Ternary thin film alloys of Se90-XIn10SbX (x = 1, 4, 10, 15 and 20) were synthesised by flash evaporation of the pre-melt quenched bulk samples under a vacuum of 10−5 Torr. Optical absorption analysis pointed to indirect allowed transitions as the mechanism of excitation across the energy gap. The optical band gap (Eg) was evaluated on the basis of Wemple-Didomenico single oscillator model and Tauc's extrapolation method in the spectral region where the absorption coefficient, α ≥ 104 cm−1. The refractive index (n), complex dielectric constant (ε), band tailing parameter (B), plasma frequency (ωp), single oscillator parameters (Eo and Ed) and lattice dielectric constant (εL) were deduced for each alloy. The compositional dependence of optical and dielectric parameters was explained on the basis of chemical bond approach. The observed shift in the trends of Eg, Ed, εL and ωp values at the composition where Sb = 4 at% was correlated to the usual chemical threshold at this composition.

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

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.

Muiva, CM, Mwabora JM.  2016.  Optical and Dielectric properties of Amorphous ternary Se90-xIn10Sbx thin films,. Journal of Alloys and Compounds. 689:432-438. Abstract

Ternary thin film alloys of Se90-XIn10SbX (x = 1, 4, 10, 15 and 20) were synthesized by flash evaporation of the pre-melt quenched bulk samples under a vacuum of 10-5 Torr. Optical absorption analysis pointed to indirect allowed transitions as the mechanism of excitation across the energy gap. The optical band gap (Eg) was evaluated on the basis of Wemple-Didomenico single oscillator model and Tauc's extrapolation method in the spectral region where the absorption coefficient, α ≥ 104 cm-1. The refractive index (n), complex dielectric constant (ε), band tailing parameter (B), plasma frequency (ωp), single oscillator parameters (Eo and Ed) and lattice dielectric constant (εL) were deduced for each alloy. The compositional dependence of optical and dielectric parameters was explained on the basis of chemical bond approach. The observed shift in the trends of Eg, Ed, εL and up values at the composition where Sb = 4 at% was correlated to the usual chemical threshold at this composition.

2015

Mwabora, JM, Muiva CM, Sathiaraj TS, Maabong K.  2015.  Observation of Meyer-Neldel rule in Se100−XM(M=In, Sb)X chalcogenide glasses. Ceramics International. 41(2):2348–2352. Abstract

Non-isothermal Differential Scanning Calorimetric (DSC) studies were done on Se100−XM(M=In, Sb)X chalcogenide glasses prepared by the melt quenching technique. The composition-dependent activation energy for glass to crystal transformation (ΔEc) and the pre-exponential factor (Ko) of the Arrhenius expression were obtained on the basis of the Augis and Bennett׳s method. A linear dependence between ΔEc and lnK0 was observed in both systems implying the existence of compensation effects of the Meyer-Neldel type. These compensation effects confirmed the applicability of Meyer-Neldel Rule (MNR) for the non-isothermal crystallization in the present systems.

Muiva, CM, Sathiaraj S, Mwabora JM, Maabong K.  2015.  Observation of Meyer-Nedel Rule in Se100-X(M = In, Sb)X and Se100-XIn10SbX chalcogenide glasses. Ceramics International . 41(2 Part A):2348-2352.

2014

Mwathe, PM, Robinson Musembi, Munji M, Odari B, Munguti L, Ntilakigwa AA, Mwabora JM, Njoroge W, Aduda B, Muthoka B.  2014.  Surface Passivation effect on CO2 sensitivity of Spray Pyrolysis deposited Pd-F:SnO2 thin films. Advances in Materials. 3(5):38-44.paper_patrick_musembi_2014__advances_in_materials_10.10.2014.pdf
Mulama, AA, Oduor AO, Muiva CM, Muthoka B.  2014.  Investigation of the Effect of film Thickness on the Optical Properties of Amorphous Se 85-xTe15Sbx. , African Journal of Physical Sciences . 1(1):38-42.
Mulama, AA, Oduor AO, Muiva C, Mwabora JM.  2014.  Optical properties and Raman studies of amorphous Se-Bi thin films. The African Review of Physics . 9:33-38.Website

2013

Kianji, CK, kaniaru NW, Mutai PK, Mwabora JM.  2013.  Importance of Law and Policy on Successful Utilization of Nuclear Technology for Electricity Generation. Proceedings of the Mechanical Engineering Annual Conference on Sustainable Research and Innovation. :82-90., , Jomo Kenyatta University of Agriculture and Technology, Nairobi, 24-26th April 2013
Matu, LN, Mwabora JM.  2013.  The Role of Communication in a Nuclear Power Programme. Proceedings of the Mechanical Engineering Annual Conference on Sustainable Research and Innovation. :82-90., , Jomo Kenyatta University of Agriculture and Technology, Nairobi, 24-26th April 2013
Mulwa, W, Makau NW, Amolo GO, Lutta S, Okoth MDO, Mwabora JM, Musembi RJ, Maghanga CM, Gateru R.  2013.  Structural and Electronic Properties of TiO2, Nb:TiO2 and Cr:TiO2: A first principles study. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :9., United Kenya Club Abstract

Substitutional Nb donor and Cr acceptor states in Anatase and Rutile TiO2 have been studied using generalized gradient approximation (PBE-GGA) employing pseudopotentials and plane wave basis sets in bulk. The calculations reveal that, on doping the Rutile structure with Cr and Nb atoms, new states were found to occur within the band gap, principally between 8.67 eV and 10.56 eV. These states are due to Nb_4d and Cr_3d orbitals. For the anatasse structure, states due to the dopants occurred between 6.663 eV and 8.939 eV. It was also observed that during the 2% doping with Cr and Nb, there were fewer new states in the band gap compared to many new states realized during the 4% doping and this happened in both Rutile and Anatase phases of TiO2. This shows that a higher doping concentration of 4% results in more energy states and hence more carriers, thus making TiO2 a better conductor than either 2% doping or pure TiO2. This study found that doping TiO2 (Anatase and Rutile) with either Cr or Nb at 2% and 4%, resulted in the removal of the energy band gap, implying improved conductivity compared to pure TiO2 [1] which exhibits insulating properties.

Mulama, AA, Mwabora JM, Oduor AO, Muiva C.  2013.  Optical Properties of Flash Evaporated Thin Films: Effect of Film Thickness. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :10., United Kenya Club Abstract

Thin films of 〖 Se〗_(100-x) 〖Bi〗_x (x=0.0,1.5,3.0,4.5 and 6.0 at.%) deposited by flash evaporation technique, have been investigated in the wavelength range of 500nm-1000nm. It is found that the effect of increasing bismuth content on the as deposited films led to increase in the absorption coefficient, reflectance, refractive index, extinction coefficient, real and imaginary parts of dielectric constant while transmittance and optical band gap energy decreased. On the other hand, reflectance, absorption coefficient, extinction coefficient, refractive index, band gap energy, real and imaginary parts of dielectric constant increased with increase in film thickness but transmittance decreased.

Mengwa, VK, N. W. Makau, Amolo GO, Lutta S, Okoth MDO, Mwabora JM, Musembi RJ, Maghanga CM, Gateru R.  2013.  A Density Functional Theory Study of Electronic Structure of Rutile (110) Surfaces with Catechol Adsorbate. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :13., United Kenya Club, Nairobi, 28-29th November 2013 Abstract

Titanium dioxide (TiO2) is used as semiconductor in the dye sensitized solar cell (DSSC), amongst many other applications. Thus coupled with a suitable sensitizer such as catechol, the study of surface electronic structure of TiO2 will improve light harvesting and electron transfer processes in DSSC. The distribution of states in clean and catechol terminated four and five layer TiO2 (110) rutile surfaces were investigated. All calculations in this work were done by quantum espresso code which uses plane waves and pseudopotentials. The slabs were modelled by four and five layers with vacuum width of 20 Å. The results showed that the (110) stoichiometric TiO2 four layer surface had band gap of 2.1 eV, a value less than band gap value of 2.2 eV of similar catechol bound TiO2 surface. There was an increase in the band gap value of 0.32 eV for the catechol bound TiO2 (110) rutile five layer surface compared to that of clean stoichiometric TiO2 (110) surface. The HOMO in four and five layered TiO2 (110) surfaces was found to lie above the valence band edge. The LUMO in both surfaces was located in the conduction band, and hence the band gap of the molecule was in the range of 4.0 eV. These findings have showed that the energy level alignment of catechol coupled to TiO2 is a suitable model to study electron transfer processes that occur in dye sensitized solar cell.

Magero, D, Makau NW, Amolo GO, Lutta S, Okoth MDO, Mwabora JM, Musembi RJ, Maghanga CM, Gateru R.  2013.  Hydrogen as an alternative fuel: An ab-initio study of Lithium Hydride and Magnesium Hydride. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :16., United Kenya Club Abstract

Limited energy resources and growing pollution associated with conventional energy production have stimulated the search for cleaner, cheaper and more efficient energy technologies. Hydrogen as a fuel is seen as one of the promising energy technologies alternative to fossil fuel. Metal hydrides have been suggested as potential candidates for the bulk storage of hydrogen. In this study, ab-initio calculations of metal hydrides that are promising candidates for hydrogen storage applications, that is, magnesium hydride (MgH2) and lithium hydride (LiH) was carried out using the Quantum Espresso computer code. The calculated quantities were the equilibrium structural parameters namely, the electronic properties as well as the thermodynamic properties. The calculated lattice parameters for MgH2 were a = 4.54 Å and c = 3.019 Å. Both values of a and c are in good agreement with experimental values of a = 4.501 Å and c = 3.01 Å. The calculated lattice parameter for LiH was a = b = c = 3.93 Å. The lattice parameter of LiH shows a correlation of approximately -3.79% with the experimental value of 4.083 Å. Thermodynamic properties of LiH were investigated by performing density functional theory within the quasi harmonic approximation. The temperature dependence of the heat capacity at constant volume CV, the Helmholtz free energy ∆F, the internal energy ∆E and the entropy ∆S was obtained. The thermodynamic properties and formation enthalpies are in good agreement with the experimental data.

Eneku, JP, Tom O, Mwabora JM.  2013.  Fabrication and Characterization of Aluminium and Gallium Mono and Co-doped Zinc Oxide Thin Films by Radio Frequency Sputtering for Photovoltaic Applications. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :26., United Kenya Club Abstract

This study intends to realize a novel thin film material for photovoltaic applications. TiO2 that has a large band gap of 3.2eV is sensitized to visible light via the use of dyes in the Gratzel cell. The dye monolayer when excited by light photons, electron-hole pairs are generated, electrons are injected into the conduction band of TiO2, while the holes are transported to the counter electrode by diffusion. The use of dye and wet electrolyte material has associated instability problems which threatens the suitability of this type of solar cell for commercialization purposes.

The objective of this proposed study is to come up with a semiconductor material of a smaller band-gap which can be used to fabricate a solar cell. This is to be achieved by doping the metal oxide (TiO2) with germanium utilizing the property of the semiconductor nanodot band gap variation with the size. The reduction of the band gap is expected to broaden the wavelength range of the incident light that can be absorbed by the material. This involves the use of large band gap materials (TiO2,) in the form of a thin film that acts as the matrix within which atoms of Ge are added by doping. This enables the tailoring of the band gap of the matrix semiconductor (TiO2,) to absorb incident radiation of a wide range of wavelengths. Film deposition will be done using the sputtering method. Substrate temperatures will be varied for deposition in order to vary the phase. Annealing of the deposited films will be done at different temperatures. The films will then be investigated using various techniques to establish their structural, optical, electrical and opto-electrical properties. The results of the investigation will help to optimize the material performance for fabrication of the solar cells of high efficiency and low cost.

Mengwa, VK, Makau NW, Amolo GO, Lutta S, Okoth MDO, Musembi RJ, Maghanga CM, Mwabora JM.  2013.  A Density Functional Theory Study of Electronic Structure of Rutile (110) Surfaces with Catechol Adsorbate. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :13., United Kenya Club, Nairobi, 28-29th November 2013 Abstract

Titanium dioxide (TiO2) is used as semiconductor in the dye sensitized solar cell (DSSC), amongst many other applications. Thus coupled with a suitable sensitizer such as catechol, the study of surface electronic structure of TiO2 will improve light harvesting and electron transfer processes in DSSC. The distribution of states in clean and catechol terminated four and five layer TiO2 (110) rutile surfaces were investigated. All calculations in this work were done by quantum espresso code which uses plane waves and pseudopotentials. The slabs were modelled by four and five layers with vacuum width of 20 Å. The results showed that the (110) stoichiometric TiO2 four layer surface had band gap of 2.1 eV, a value less than band gap value of 2.2 eV of similar catechol bound TiO2 surface. There was an increase in the band gap value of 0.32 eV for the catechol bound TiO2 (110) rutile five layer surface compared to that of clean stoichiometric TiO2 (110) surface. The HOMO in four and five layered TiO2 (110) surfaces was found to lie above the valence band edge. The LUMO in both surfaces was located in the conduction band, and hence the band gap of the molecule was in the range of 4.0 eV. These findings have showed that the energy level alignment of catechol coupled to TiO2 is a suitable model to study electron transfer processes that occur in dye sensitized solar cell.

  2013.  Hydrogen as an alternative fuel: An ab-initio study of Lithium Hydride and Magnesium Hydride. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :16., United Kenya Club Abstract

Limited energy resources and growing pollution associated with conventional energy production have stimulated the search for cleaner, cheaper and more efficient energy technologies. Hydrogen as a fuel is seen as one of the promising energy technologies alternative to fossil fuel. Metal hydrides have been suggested as potential candidates for the bulk storage of hydrogen. In this study, ab-initio calculations of metal hydrides that are promising candidates for hydrogen storage applications, that is, magnesium hydride (MgH2) and lithium hydride (LiH) was carried out using the Quantum Espresso computer code. The calculated quantities were the equilibrium structural parameters namely, the electronic properties as well as the thermodynamic properties. The calculated lattice parameters for MgH2 were a = 4.54 Å and c = 3.019 Å. Both values of a and c are in good agreement with experimental values of a = 4.501 Å and c = 3.01 Å. The calculated lattice parameter for LiH was a = b = c = 3.93 Å. The lattice parameter of LiH shows a correlation of approximately -3.79% with the experimental value of 4.083 Å. Thermodynamic properties of LiH were investigated by performing density functional theory within the quasi harmonic approximation. The temperature dependence of the heat capacity at constant volume CV, the Helmholtz free energy ∆F, the internal energy ∆E and the entropy ∆S was obtained. The thermodynamic properties and formation enthalpies are in good agreement with the experimental data.

  2013.  Optical and Electrical Properties of Magnesium Doped Zinc Oxide for Photovoltaic Applications. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :27., United Kenya Club Abstract

This research is an experimental design which came up with the optical and electrical properties of magnesium doped zinc oxide for photovoltaic applications. The specific objectives are to determine the Optical properties, and electrical properties of Magnesium doped ZnO of Magnesium doped ZnO. These objectives are to be achieved by sputtering three types of targets on to glass slides. The targets that will be used are two targets of magnesium doped ZnO with compositions of ZnO: Mg being 95:5 wt% and 90:10 wt%, both with a purity of 99.99% and a third target of un-doped ZnO with a purity of 99.99% . The optical properties will be determined using a double beam spectrophotometer in the UV/VIS/NIR regions. Thus the absorption, transmission and reflection properties will be focused on. The electrical properties will be determined using four point probe and the film resistance, sheet resistance and film conductivity will be looked at.

Mwonga, PV, Makau NW, Amolo GO, Lutta S, Okoth MDO, Musembi RJ, Maghanga CM, R. Gateru, Mwabora J.  2013.  Ab-initio Studies of Point Defects in : A Density Functional Approach. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :29., United Kenya Club, Nairobi, 28-29th November 2013 Abstract

Titanium dioxide has been intensively studied as a wide band gap transition metal oxide due to its n-type semi-conducting property which makes it to have many applications in industry. Some of the observed conductivity arises from its intrinsic point defects. The structural properties and electronic band structures of TiO2 (rutile and anatase) phases, have been investigated using ab-initio methods. The structural properties were obtained using generalized gradient approximation (GGA) employing pseudopotentials and plane wave basis sets. For the two phases of TiO2, the calculated equilibrium lattice constants, bulk moduli and bond lengths were found to be in good agreement with other recent theoretical calculations and also with experimental data. After introduction of various defects to the perfect super cell, the Ti-O bond lengths were altered greatly. The apical bond lengths changed from a constant 1.959 Å to a range of values (1.718 - 1.861) Å, and the equatorial bond lengths changed from a constant 2.006 Å to a range of values (2.072 - 2.231 ) Å for rutile TiO2. The apical bond lengths changed from a constant 1.956 Å to a range of values (1.782 - 1.830) Å, and the equatorial bond lengths changed from a constant 2.050 Å to a range of values (2.112 - 2.214) Å, for anatase TiO2. Also altered were Ti-O-Ti angles, from the two constants (99.93, 131.04)° to a range of values (88.86 - 95.69 and 132.01 - 143.49)° for rutile TiO2. For anatase TiO2, Ti- O-Ti angles changed from the two constants (103.81, 152.39)° to a range of values (93.59 - 149.91 and 156.74 - 176.05)°. Electronic properties were investigated too. Perfect rutile and anatase super cells gave band gaps of 2.24 eV and 2.44 eV, respectively, underground-state conditions. Valence bandwidths (VB) and conduction bandwidths (CB) were also obtained for both phases. VB of 5.6 eV and CB of 1.654 eV were observed for rutile TiO2, while VB of 4.76 eV and CB of 2.35 eV were observed for anatase TiO2; all in good agreement with experimental values. This study also investigated the defect formation enthalpies of Frenkel and Schottky defects in both rutile and anatase phases of TiO2. This study also considered point defect stability in rutile and anatase phases of TiO2. The formation energies for oxygen and titanium atoms, defects were found to be in agreement with the experimental values, especially the case of rutile oxygen atom vacancy. Both Frenkel and Schottky defects were found to induce new energy states in titanium dioxide. Normally band gaps are reduced in defective TiO2 crystals, and in this study, reduced energy band gaps were reported for all the defective super cells. In rutile, the metal oxide gaps were found to almost vanish due to the presence of oxygen atom vacancy, oxygen atom Frenkel and titanium Frenkel defects. These gave direct energy band gaps: 0.35 eV, 0.207 eV and 0.327 eV, respectively. Defects in anatase phase showed a similar trend, with the least energy band gap being reported for the case titanium interstitial (0.041 eV, which is indirect). With such infinitesimal gaps, these otherwise insulating oxides can with ease become conducting metal oxides, by either increasing the temperatures or pressure since these calculations were done at 0 K and 0 pressure. It can thus be said that intrinsic point defects in titanium dioxide do contribute to the improvement of the electrical conductivity of this oxide.

Alfred, A, Musembi RJ, Waita SM, Mwabora JM.  2013.  Effects of Oxygen Partial Pressure and Substrate Temperature on Optical Properties of Sputter Deposited Thin Films. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :31., United Kenya Club Abstract

The effects of oxygen partial pressure and substrate temperature on optical properties of CuCrO_2 thin films deposited on float glass substrate by reactive DC magnetron sputtering system using CuCr alloy targets have been studied. The sputtering was performed in Argon (Ar) and Oxygen (O_2) atmosphere and the substrate temperature varied up to 263 °C. The optical constants: refractive index, n, extinction coefficient, k, dielectric constant, ε, and absorption coefficient, α, at different oxygen partial pressure and substrate temperatures were determined from measured transmittance and reflectance data fitted in SCOUT software for wavelength range 200-2247 nm. The optical studies gave energy band gap of about 2.47 eV at 0.153 μbar Po2 and 3.7 eV at 263 °C substrate temperature. The values obtained for Urbach energy were 0.27-0.31 eV for samples prepared at Po2 between 0.153-0.187 μbar and 0.81-1.45 eV for those prepared at substrate temperature of 263 °C and as-grown film, respectively.

Muramba, V, Mageto M, Gaitho F, Odari V, Musembi, Simiyu J.  2013.  Preparation and Characterization of Transparent and Conducting Doped Tin Oxide. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :33., United Kenya Club
Otakwa, RVM, Simiyu J, Mwabora JM.  2013.  The Complementary of Dye-Sensitized and Amorphous Silicon Photovoltaics in Field Application in the Tropics. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :38., United Kenya Club, Nairobi, 28-29th November 2013 Abstract

The complementarity of the Dye-Sensitized and Amorphous Silicon (a-Si) Photovoltaic (PV) modules has been investigated under different outdoor air mass (AM), irradiance intensity and temperature conditions. The performance of the Dye-Sensitized module (DSM) was investigated in Nairobi, Kenya and its performance compared with that of a-Si modules investigated in Lagos, Nigeria. The DSM’s good response to short wavelength radiation caused it to perform better at increased AM values than what has been reported of a-Si PV modules. On the other hand, studies on a-Si showed that its performance favors low AM conditions. The DSM was also found to generally perform better than what is reported of a-Si under irradiance and temperature dependence, but a-Si PV modules’ performance was reported to be remarkable at increased irradiance conditions. These results show that the Dye-Sensitized and the a-Si technologies complement each other’s performance when subjected to the outdoor field AM, irradiance and temperature conditions. These findings are useful in PV sizing, especially in the Building Integrated Photovoltaics (BIPV) in the tropics.

Njoroge, W, Wangati CK, Karanja PK, Musembi RJ, Simiyu J, Mwabora JM.  2013.  Deposition and Characterization of Thin Film Deposited by DC-RF Co-sputtering for Photovoltaic Application. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :40., United Kenya Club, Nairobi, 28-29th November 2013 Abstract

Solar cell has the potential of being the main drive to economic prosperity as it is one of the most promising sources of cheap, environmentally friendly and renewable energy. Crystalline silicon based technology currently dominate the solar energy market. However, it is generally expensive and the cell efficiency has reached 24.7% hence approaching theoretical expected maximum of 30%. In order to reduce cost of production, focus is shifting towards thin film based I-III-VI family chalcopyrite compounds where cheaper CuInxGa1-xSe2 absorber semiconductor is reported to have attained the highest efficiency of 20.3 %. This study intends to fabricate and characterize a compound of copper, aluminum, boron and selenium (CuAlxB1-x Se2 ) thin film. The compound is based on I-III-IV family of chalcopyrite which has generated a lot of interest as an absorber material for solar cells due to their high absorption coefficients. The research procedure will involve deposition of CuAlxB1-xSe2 thin film by DC and RF magnetron sputtering of CuAlB alloy and selenium targets respectively. The deposition is done using Edwards Auto 360 RF and DC magnetron vacuum system. Characterization of the resulting thin film based on structural and optoelectronic properties is done using X-Ray diffraction (XRD), X-Ray photoelectron spectroscopy (XPS), Scanning Electron microscopy (SEM), UV-Visible-IR Spectrometer, and the Hall Effect. The outcome of this research will provide fundamental practical science and engineering knowledge base on structural and optoelectronic properties of CuAlxB1-x Se2 compound as solar absorber material among other optoelectronic applications. In general the study will contribute towards achieving greater efficiency in production of “green” energy.

Ajuoga, P, Ogacho A, Aduda B, Mwabora JM.  2013.  Niobium Doped Effects of Doping Concentration on the Optical Properties of. 1st Young Scientist MSSEESA Conference on Materials Science and Solar Cell Technology. :47., United Kenya Club Abstract

The optical band gaps and crystal structure were investigated on niobium doped TiO2 (for atomic niobium concentrations ranging from 0.02 –0.06 at. % in the composite) prepared by high temperature diffusion method. The Nb:TiO2 films displayed an enhanced visible light absorption with a red shift of 18.2 nm of the optical absorption edge from 394 nm for pure TiO2 film to 412.2 nm for 0.04 at. % niobium concentration representing a band gap lowering of 0.181eV due to the donor–type behavior of niobium. As the niobium concentration increased, the enhancement in light absorption at the investigated concentration range goes through a maximum at 0.04 at. % of Nb5+ with minimum band gap of 3.017eV. Despite higher rutilization, at the doping temperature of 850oC used, crystal sizes (39–43 nm) obtained from X-ray diffraction spectra depicted a significant increase in surface areas which is attributed to retardation of anatase - rutile phase transformation caused by Nb:TiO2 matrix.

Mwabora, J, Robinson Musembi, Aduda B, Marin R, Fostiropoulos K, Lux-Steiner M.  2013.  Light Soaking Induced Increase in Conversion Efficiency in Solar Cells Based on In(OH)xSy/Pb(OH)xSy. Material Sciences and Applications. Abstract

Light soaking characterization on complete SnO2:F/TiO2/ln(OH)xSy/PEDOT:PSS/Au, Pb(OH)xS)pEDOT:PSS/Au, eta solar cell structure
as well as on devices which do not include one or both TiO2 and/or PEDOT:PSS layers has been conducted. Additionally,
studies of SnO2:F/In(OH)xSy/PEDOT:PSS/Au solar cell have been performed. The power conversion
efficiency and the short circuit current density have been found to increase with light soaking duration by a factor of
about 1.6 - 2.7 and 2.1 - 3, respectively. The increase in these two parameters has been attributed to the filling up of trap
states and/or charge-discharge of deep levels found in In(OH)xSy. These effects take place at almost fill factor and open
circuit voltage being unaffected by the light soaking effects.

Otakwa, RVM, Simiyu J, Mwabora JM.  2013.  Dye-Sensitized and Amorphous Silicon Photovoltaic (PV Devices' Outdoor Performance: A Comparative Study. International Journal of Emerging Technological in advanced Engineering . Abstract

The performance of a dye-sensitized solar module (DSSM) has been investigated under different air mass (AM), irradiance intensity and temperature conditions in Nairobi, Kenya. The good response of the DSSM to short wavelength radiation made it perform well at increased AM values as compared to what is reported of Amorphous Silicon (a-Si) photovoltaic (PV) devices. The DSSM performed better compared to what is reported of a-Si PV devices under irradiance and temperature dependence. The results are useful in PV sizing, especially in the area of Building Integrated Photovoltaics (BIPV) in Kenya and the tropics.

Robinson Musembi, Aduda B, Mwabora J, Rusu M, Fostiropoulos K, Lux-Steiner M.  2013.  Effect of Recombination on Series Resistance in eta Solar Cell Modified with In(OH)xSy Buffer Layer. International Journal of Energy Engineering. 3(3):183-189. AbstractWebsite

Transport mechanism studies in TiO2/In(OH)xSy/Pb(OH)xSy/PEDOT:PSS eta solar cell have been carried out. The characterizations have been performed both in the dark and under varying illumination intensity for temperature range 200 K – 320 K. Calculations from ideality factor have shown that the recombination process of the eta solar cell in the dark to be tunneling enhanced, while under illumination it is thermally activated and takes place through exponentially distributed energy recombination levels. The temperature has been found to influence series resistance of the solar cell. Series resistance has been found to be high at low temperature and low at higher temperature, thus we can conclude that the recombination is thermally activated.

Keywords: Eta Solar Cell, Recombination, Series Resistance, Buffer Layer

Cite this paper: Robinson Musembi, Bernard Aduda, Julius Mwabora, Marin Rusu, Konstantinos Fostiropoulos, Martha Lux-Steiner, Effect of Recombination on Series Resistance in eta Solar Cell Modified with In(OH)xSy Buffer Layer, International Journal of Energy Engineering, Vol. 3 No. 3, 2013, pp. 183-189. doi: 10.5923/j.ijee.20130303.09.

2012

Aseey, AA, Mwabora JM.  2012.  Poverty as a factor in determining Education achievement in Kenya: A case Study of Nyanza and Coast Provinces in Kenya. Oral Presentation, Regional Symposium on Poverty and Human Right. , Lake Naivasha County Club, 28-30 September, 2011
Muiva, C, Santhiaraj S, Mwabora JM.  2012.  Chemical Bond approach to Crystallisation Kinetics and Thermal Stability in some Se90-xIn10Sbx Chalcogenide Glassy Alloys. Proceedings 2nd African Materials Science and Engineering Network (AMSEN) workshop. :33-34., Silver Springs Hotel, Nairobi, 21-23rd March 2012
Muiva, C, Santhiaraj S, Mwabora JM, Maabong K.  2012.  Optical Properties of flash evaporated amorphous Se100-xInx thin films for CuInSe2 solar cell applications. Proceedings of the IASTED conference on Power and Energy Systems. :86-90., Gaborone, Botswana
Otakwa, RVM, Simiyu J, Waita SM, Mwabora JM.  2012.  Application of Dye-Sensitized Solar Cell Technology in the Tropics: Effect of Radiation Intensity and Temperature on DSSC Performance. International Journal of Advanced Renewable Energy Research . 1(2):109-116. AbstractWebsite

Effects of radiation intensity and temperature on the performance of a dye-sensitized solar module (DSSM) have been investigated in a tropical area in Nairobi, Kenya. Outdoor measurements were performed on cloudless days at normal incidence of the incoming solar beam radiation to the module. A series of current-voltage (I-V) characterizations were carried out at different solar radiation intensities and module temperatures. The module performance parameters: Short circuit current density, (Jsc), Open circuit voltage, (Voc), Fill factor, (FF) and solar-to-electricity conversion efficiency, (η) were extracted from the I-V curves. Better efficiencies were observed at lower than higher radiation intensities. There was also an overall improved performance at elevated temperatures. The results may be useful during fabrication of dye-sensitized solar cells meant for use in the tropics.

Muiva, CM, Santhiaraj S, Mwabora JM.  2012.  Chemical Bond Approach to Optical Properties of some Flash Evaporated Se100-xSbx Chaloconide Alloys. European Physical Journal Applied Physics. 59:10301p1-10301p7.
Otakwa, RM, Simiyu J, Waita SM, Mwabora JM.  2012.  Application of Dye-Sensitized Solar Cell Technology in the Tropics: Effect of Air Mass on Device Performance. International Journal of Renewable Energy Research. 3(3):369-375. AbstractWebsite

Abstract - The performance of a Dye-Sensitized Solar Module (DSSM) of active area 175.12 cm2 has been investigated at a tropical climate area located 1.28˚S, 35.81˚E. Outdoor current density-voltage (J-V) characterizations were carried out at different AM values. The DSSM’s performance parameters; short circuit current density (Jsc), open circuit voltage (Voc), fill factor (FF) and solar-to-electricity conversion efficiency (η) were extracted from the J-V characteristics. The DSSM’s Voc reduced linearly by 2.05% from 8.31 V to 8.14 V as AM increased from 1 to 1.09. Jsc reduced linearly by 26.06% from 1.04 x 10-3 Acm-2 to 7.69 x 10-4 Acm-2 as AM increased from 1 to AM 1.09. FF increased linearly by 19.05% from 0.51 to 0.63 as AM increased from 1 at 1.09. η increased by 32.77% from 1.77% to 1.19% as AM increased from 1 to 1.09. The DSSM performed better during afternoon than morning hours. The results may be useful in tuning Dye-Sensitized Solar Cells (DSSCs) meant for use in the tropics. The design of Net Zero Energy (NZE) buildings in the tropics can also benefit from these findings.

Otakwa, RVM, Simiyu J, Waita SM, Mwabora JM.  2012.  Dark Current Voltage (I-V) Characteristics of a Commercialized Dye-Sensitized Solar Module Ideal for use in the Tropics. International Journal of Professional Practice. 3(1&2):165-168.

2011

Muiva, CM, Santhaiaraj ST, Mwabora JM.  2011.  Thermal and compositional defects in chemical spray pyrolysed indium selenide (In2Se3) thin films: Effects on film properties . Journal of Optoelectronics and Advanced Material. 13(9):1240-1245. Abstract

Polycrystalline In2Se3 semiconducting thin films were synthesized by chemical spray pyrolysis and their properties investigated. Strong dependence of structural and opto-electronic properties on film composition was observed. Absorption coefficient (α) at normal incidence was determined and an optimised direct optical band gap (Eg) of 1.92 eV was obtained at a substrate temperature (Tsub) of 673 K. Whereas low Tsub favoured incorporation of impurities in the films, elevated Tsub
had the effects of introducing textural and structural defects with modifications in the film properties. The crystallinity of the films increased with switch from chalcogen rich to chalcogen deficient films.

Keywords: In2Se3, Spray pyrolysis, Chalcopyrite buffers, Opto-electronic property

Cosmas M. Muiva, Santhiaraj S, Mwabora JM.  2011.  Thermal and Compositional defects in Chemical Spray Pyrolysed Indium Selenide (In2Se3) Thin Films: Effects on film properties. Journal of Non Crystalline Solids . 357:3726-3733.

2010

and C. Muiva, SMSJM.  2010.  Optical, electrical and structural properties of Se-In-Bi chalcogenide glasses: Phase change materials for data storage. Proceedings1stAfrican Materials Science and Engineering Network (AMSEN) workshop. :15-15., Windhoek, Namibia, 27-29th January, 2010

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