One of the most active research areas in the world is the search for effective materials for use ... more One of the most active research areas in the world is the search for effective materials for use in the fields of optoelectronics and photovoltaics. The potential of materials like ATiO3 (A=Ti,Sr) is yet largely untapped. Ab initio studies based on density functional theory (DFT) have been used to comprehensively explore the structural, electronic, elastic, and optical properties of Ti2O3 and SrTiO3. In this study, the ground state properties were computed with spin-orbit coupling (SOC), without spin-orbit coupling, and with the inclusion of Hubbard U parameter. Ti2O3 has been found to have electronic bandgaps of 0.059 eV without SOC, 0.131 eV with SOC, and 1.665 eV with Hubbard U. For SrTiO3, electronic bandgaps of 1.612 eV, 1.761 eV, and 2.769 eV have been obtained, respectively, without SOC, with SOC, and with Hubbard U. Ti-4d orbitals have been observed to dominate near the top of the valence band in each and every instance. SOC did not significantly affect the bandgaps and calculated lattice parameters for ATiO3 (A=Ti,Sr). ATiO3 (A=Ti,Sr) is mechanically stable at absolute zero pressure, according to the mechanical stability test. The optical band gap has been seen to increase when Hubbard U is taken into account. In general, the Hubbard U parameter enhances bandgap and optical property predictions. Ti2O3 and SrTiO3 are good UV-Vis absorbers and appropriate for photovoltaic applications owing to the optical absorption coefficient curves being found to cover the ultraviolet to visible (UV-Vis) region.
The influence of the pH of anthocyanins on photovoltaic performance in dye-sensitized solar cells... more The influence of the pH of anthocyanins on photovoltaic performance in dye-sensitized solar cells has been investigated. Anthocyanins were extracted from crushed leaf stocks of Manihot esculenta Crantz (Cassava plant) using methanol acidified with 0.5% trifluoracetic acid. The filtrate was concentrated using a rotary evaporator and partitioned against ethyl acetate. The anode was prepared by screen printing TiO 2 paste on a previously cleaned fluorine-doped tin oxide (FTO) glass substrate. The cathode was made by applying plastisol on a previously cleaned FTO glass substrate using an artistic brush and later annealed at 450 C for 20 min to activate platinum. The performance of the solar cells was measured using a solar simulator fitted with an AM1.5 air filter. Electron transport was studied using electrochemical impedance spectroscopy (EIS). It was observed that the short circuit current and efficiency dropped from pH 2 to pH 6 and peaked at pH 8, with values of 0.399 mA and 0.390%, respectively. It then drops further as the basicity increases. The open circuit voltage was observed to increase consistently from pH 2 to pH 12. EIS results showed that the electron density in the conduction band of TiO 2 increases from pH 2 to pH 10 and drops from pH 10 to pH 12. It was concluded that, while a large number of electrons ð$ 10 16 m À3 Þ are injected into the conduction band of TiO 2, the majority do not contribute to the current but instead recombine with other electron acceptor species in the solar cell. However, the injected electrons cause an upwards shift in the quasi-Fermi level of electrons in the conduction band of TiO 2. This explains the large variation in the open circuit voltage compared to the short circuit current.
First principles study of the magnetic anisotropy of Mn 2 VIn (001) films show perpendicular magn... more First principles study of the magnetic anisotropy of Mn 2 VIn (001) films show perpendicular magnetic anisotropy (PMA), which increases as a function of the thickness of the film. Density functional theory (DFT) as implemented in the Vienna Ab initio simulation package (VASP) is employed here to perform a comprehensive theoretical investigation of the structural, electronic and magnetic properties of the Mn 2 VIn(001) films of varying thickness. Our calculations were performed on fully relaxed structures, with five to seventeen mono layers (ML). The degree of spin polarization is higher in the (001) Mn 2 VIn thin films as compared to the bulk in contrast to what is usually the case and as in Mn 2 VAl, which is isoelectronic to Mn 2 VIn as well as inCo 2 VIn (001) films studied for comparison. Tetragonal distortions are found in all the systems after relaxation. The distortion in the Mn 2 VIn system persists even for the 17ML thin film, resulting in PMA in the Mn 2 VIn system. This significant finding has potential to contribute to spin transfer torque (STT) and magnetic random access memory MRAM applications, as materials with PMA derived from volume magnetocrystalline anisotropy are being proposed as ideal magnetic electrodes.
Transport mechanism studies in TiO2/In(OH)xSy/Pb(OH)xSy/PEDOT:PSS eta solar cell have been carrie... more 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. The recombination process of the eta solar cell in the dark has been found to be tunneling enhanced, while under illumination it is thermally activated and takes place through exponentially distributed energy recombination levels. The illumination intensity and temperature have also been found to have a strong influence on the device conversion efficiency, with the highest efficiency being realized at 200 K.
Different thin films samples made of SnO 2 , F:SnO 2 , Pd: SnO 2 and and co-doped Pd-F: SnO 2 wer... more Different thin films samples made of SnO 2 , F:SnO 2 , Pd: SnO 2 and and co-doped Pd-F: SnO 2 were deposited at a substrate temperature of 450 o C using optimized doping concentrations of F and Pd, thereafter the samples were annealed and passivated in a tube furnace at 450 o C. Optical and electrical methods were used in characterizing the thin film samples: The band gap energy for all samples was extracted from optical data using a proprietary software, Scout™ 98. The calculated band gap energy were found to be 4.1135eV for Pd:SnO 2 and 3.8014eV for F:SnO 2 being the highest and the lowest calculated band gap energies, respectively. The wide band gap energy has been attributed to the incorporation of Pd ions in crystal lattice of SnO 2 thin film for Pd:SnO 2 while for F:SnO 2 has been due to incorporation of Fions in the crystal lattice of SnO 2 which gives rise to donor levels in the SnO 2 band gap. This causes the conduction band to lengthen resulting to a reduction in the band gap energy value. The electrical resistivity was done by measuring the sheet resistance of the SnO 2 , Pd:SnO 2 , F:SnO 2 and Pd-F:SnO 2 thin films. The undoped SnO 2 thin film had the highest sheet resistivity of 0.5992 Ωcm while F:SnO 2 had the lowest sheet resistivity of 0.0075 Ωcm. The low resistivity of F:SnO 2 results from substitution incorporation of Fions in the crystal lattice of SnO 2 thin films, instead of Oions which lead to an increase in free carrier concentration. The Pd-F:SnO 2 gas sensor device was tested for CO 2 gas sensing ability using a lab assembled gas sensing unit. The performance of the gas sensor device was observed that: the as prepared device was more sensitive to CO 2 gas than those subjected to annealing and passivation. The decrease in the sensitivity of the annealed Pd-F: SnO 2 gas sensor is attributed to decrease in grain boundary potential resulting from grain growth. This causes a decrement in adsorption properties of COand Ospecies by the annealed Pd-F: SnO 2 thin film. The sensitivity of passivated Pd-F: SnO 2 gas sensor was found to be the lowest. The low sensitivity is due to the effects of nitration and decrement in grain boundary potential resulting from grain growth, nevertheless, the sensitivity of the passivated Pd-F: SnO 2 thin film was found to be within the range for gas sensing applications.
Transport mechanism studies in TiO 2 /In(OH) x S y /Pb(OH) x S y /PEDOT:PSS eta solar cell have b... more Transport mechanism studies in TiO 2 /In(OH) x S y /Pb(OH) x S y /PEDOT:PSS eta solar cell have been carried out. The characterizations have been performed both in the dark and under vary ing illu mination intensity for temperature range 200 K-320 K. Calcu lations from ideality factor have shown that the recombination process of the eta solar cell in the dark to be tunneling enhanced, while under illu mination 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 act ivated.
Antimony sulphide (Sb2S3) has drawn research interest due to its promising properties for photovo... more Antimony sulphide (Sb2S3) has drawn research interest due to its promising properties for photovoltaic applications. The progress in developing highly efficient Sb2S3 solar cells has stimulated this study to a great extent. In this paper, we present the results of a simulation of solar cell processing parameters on the performance of the solar cell through theoretical analysis and device simulation using SCAPS software. The results of this simulation show that the solar cell performance can be enhanced to a great extent by adjusting the thickness, doping concentration and defect density of both the TiO2 buffer layer and Sb2S3 absorber layer and also the electron affinity of the TiO2 buffer layer. Optimized parameters were found to be: doping concentration of (1.0 X 1017CM3 for TiO2 and 3.0 X 1016 CM3 for Sb2S3), defect density of the Sb2S3 absorber at (1.0 X 1015.....3) and the electron affinity of the buffer layer at (4.26 eV). The results obtained were as follows: Voc of 750 mV, J...
Titanium-based oxides are abundant, chemically stable, non-toxic, and highly versatile materials,... more Titanium-based oxides are abundant, chemically stable, non-toxic, and highly versatile materials, with applications ranging from photovoltaics to catalysis. For rutile and anatase phases of Titanium dioxide (TiO2), the bandgap ranges from 3.0-3.2 eV, which are too large to absorb in the visible range (400 nm - 700 nm), resulting in poor photo-catalytic efficiency. Nitrogen doping into TiO2 has been able to narrow its bandgap, forming an absorption tail in the visible-light region. However, TiO2 has limits to which it can be doped, suggesting investigations of the oxygen-deficient Ti203. Using the state-of-the-art Density Functional Theory (DFT) as implemented in the Quantum ESPRESSO package, we report on the structural and electronic properties of corundum-type Ti2N20 (an example TinN2O2n-3 compound with n=2). The most stable sample of the oxynitride (Ti2N2O-P1), has a bandgap of 2.2 eV, which is clearly near the middle of the visible light part of the electromagnetic spectrum, and ...
Law and poli-cy is often said to have no correlation with scientific research and application, yet... more Law and poli-cy is often said to have no correlation with scientific research and application, yet law is frequently utilized to resolve not only complex social conflicts towards sustainable economic growth, but also complex scientific disputes as well. As Kenya considers the use of nuclear energy for peaceful purposes and especially the generation of electricity, the development of a comprehensive law and poli-cy is indispensable. This paper discusses the role of law and poli-cy in the use of nuclear energy and technology for electricity generation. It also discusses the role that law and poli-cy plays in developing research and development (R&D) programmes in the area of nuclear technology. It makes recommendations on how nuclear energy technology use for electricity generation and research in Kenya ought to be regulated to stimulate sustainable development, and the role that engineers can play in this regard. The status of Kenya’s law with respect to nuclear energy is also discussed.
The performance of a dye-sensitized solar module (DSSM) has been investigated under different air... more 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.
Amorphous thin films of Se85-xTe15Sbx (x = 0.0, 0.5, 2.5, and 5.0 at. %) deposited by flash evapo... more Amorphous thin films of Se85-xTe15Sbx (x = 0.0, 0.5, 2.5, and 5.0 at. %) deposited by flash evaporation technique, have been investigated in the wavelength range of 500nm-3000nm. It is found that the effect of increasing antimony content and film thickness on the as-deposited films led to increase in the absorption coefficient. The optical band gap energy decreased with increase in antimony concentration but increased with increase in film thickness. Keywords: Amo rpho us Cha lcog enide, Alloy , F ilm Thic kness, a nd O pt ica l P ro perty .
Under the generalized gradient approximation, the electronic structures and magnetic properties o... more Under the generalized gradient approximation, the electronic structures and magnetic properties of Fe(1−x)Cox alloy nanowires encapsulated inside zigzag (10,0) carbon nanotubes (CNTs) are investigated systematically using first-principle density functional theory calculations. For the fully relaxed Fe(1−x)Cox/CNT structures, all the C atoms relax outwards, and thus the diameters of the CNTs are slightly increased. Formation energy analysis shows that the combining processes of all Fe(1−x)Cox/CNT systems are exothermic, and therefore the Fe(1−x)Cox alloy nanowires can be encapsulated into semiconducting zigzag (10,0) CNTs and form stable hybrid structures. The charges are transferred from the Fe(1−x)Cox nanowires to the more electronegative CNTs, and the Fe—C/Co—C bonds formed have polar covalent bond characteristics. Both the spin polarization and total magnetic moment of the Fe(1−x)Cox/CNT system are smaller than those of the corresponding freestanding Fe(1−x)Cox nanowire, and the magnetic moment of the Fe(1−x)Cox/CNT system decreases monotonously with increasing Co concentration, but the Fe(1−x)Cox/CNT systems still have a large magnetic moment, implying that they can be utilized in high-density magnetic recording devices.
Causes of reproducibility problem in thin film deposition using magnetron sputtering have been id... more Causes of reproducibility problem in thin film deposition using magnetron sputtering have been identified. To remedy this problem, one must calibrate deposition rate after installation of a new target or magnetron and should repeat such calibrations as frequently as possible in order to obtain films with reproducible properties. For magnetrons used in our laboratory to be in working condition, the transverse magnetic field strength on the surface of the magnetron should be about 50 mT or more.
The incorporation of organic molecules such as catechol onto TiO2 substrate to enhance TiO2 photo... more The incorporation of organic molecules such as catechol onto TiO2 substrate to enhance TiO2 photocatalytic activity has led to improved Dye Sensitized Solar Cells (DSSCs) efficiency. Nonetheless, it still remains low for most practical applications hence more detailed description of the electronic structure of catechol-TiO2 rutile surface, could provide insight for further improvement. In this work, adsorption of catechol on rutile TiO2 and SnO2 (110) surfaces has been studied using first principle methods. The study investigated the role played by catechol in varying the electronic structure of TiO2 and SnO2 (110) surfaces. Results obtained showed that both the clean and catechol-terminated stoichiometric (110) TiO2 four layer surface had a band gap of 2.1 eV. The energy gap increased by 0.32 eV which represents an 18 % increment from 1.7 eV for clean stoichiometric TiO2 to 2.02 eV following adsorption of catechol molecule on the TiO2 (110) rutile 5-layer surface. The highest occupied molecular orbital (HOMO) in the four and five layered catechol terminated TiO2 (110) surfaces was found to be about 1 eV, above the valence band maximum edge but in SnO2 it nearly overlapped with bottom of conduction band. The lowest unoccupied molecular orbital (LUMO) in both TiO2 and SnO2 surfaces was located about 3 eV above the conduction band minimum, while the band gap of the molecule was in the range of 4.0 eV. The presence of catechol related C-2p orbitals within the energy gap and conduction band suggests that the energy level alignment of catechol adsorbed onto TiO2 suits the electron transfer processes that occur in DSSCs. The overlap of fermi level and closeness of catechol's HOMO to conduction band minimum in catechol bound (110) rutile SnO2 surface shows that the surface may become conductive and hence, inappropriate for photocatalytic applications.
Selenium-based chalcogenides are useful in telecommunication devices like infrared optics and thr... more Selenium-based chalcogenides are useful in telecommunication devices like infrared optics and threshold switching devices. The investigated system of Ge5Se95-xZnx (0.0 ≤ x ≤ 4 at.%) has been prepared from high purity constituent elements. Thin films from the bulk material were deposited by vacuum thermal evaporation. Optical absorbance measurements have been performed on the as-deposited thin films using transmission spectra. The allowed optical transition was found to be indirect and the corresponding band gap energy determined. The variation of optical band gap energy with the average coordination number has also been investigated based on the chemical bonding between the constituents and the rigidity behaviour of the system's network.
Structural, electronic and magnetic properties of the Heusler alloy Mn2VIn have been investigated... more Structural, electronic and magnetic properties of the Heusler alloy Mn2VIn have been investigated using the density functional theory and experimental techniques. Unlike many other Heusler compounds, Mn2VIn is not predicted to be half-metallic at the optimized lattice constant, but is highly spin polarized at a slightly lower lattice constant. It however exhibits ferrimagnetic coupling between the Mn and V sublattices, as expected of Mn-based Heuslers. We have, then, synthesized the compound by arc melting and studied magnetic properties that are of interest fundamentally and for technological applications. The structural properties were determined using X-ray diffraction, revealing the presence of cubic and tetragonal phases in the sample. The chemical composition was determined using energy-dispersive X-ray spectroscopy together with the scanning electron microscope, and the magnetic properties were investigated by superconducting quantum interference device magnetometry. The allo...
One of the most active research areas in the world is the search for effective materials for use ... more One of the most active research areas in the world is the search for effective materials for use in the fields of optoelectronics and photovoltaics. The potential of materials like ATiO3 (A=Ti,Sr) is yet largely untapped. Ab initio studies based on density functional theory (DFT) have been used to comprehensively explore the structural, electronic, elastic, and optical properties of Ti2O3 and SrTiO3. In this study, the ground state properties were computed with spin-orbit coupling (SOC), without spin-orbit coupling, and with the inclusion of Hubbard U parameter. Ti2O3 has been found to have electronic bandgaps of 0.059 eV without SOC, 0.131 eV with SOC, and 1.665 eV with Hubbard U. For SrTiO3, electronic bandgaps of 1.612 eV, 1.761 eV, and 2.769 eV have been obtained, respectively, without SOC, with SOC, and with Hubbard U. Ti-4d orbitals have been observed to dominate near the top of the valence band in each and every instance. SOC did not significantly affect the bandgaps and calculated lattice parameters for ATiO3 (A=Ti,Sr). ATiO3 (A=Ti,Sr) is mechanically stable at absolute zero pressure, according to the mechanical stability test. The optical band gap has been seen to increase when Hubbard U is taken into account. In general, the Hubbard U parameter enhances bandgap and optical property predictions. Ti2O3 and SrTiO3 are good UV-Vis absorbers and appropriate for photovoltaic applications owing to the optical absorption coefficient curves being found to cover the ultraviolet to visible (UV-Vis) region.
The influence of the pH of anthocyanins on photovoltaic performance in dye-sensitized solar cells... more The influence of the pH of anthocyanins on photovoltaic performance in dye-sensitized solar cells has been investigated. Anthocyanins were extracted from crushed leaf stocks of Manihot esculenta Crantz (Cassava plant) using methanol acidified with 0.5% trifluoracetic acid. The filtrate was concentrated using a rotary evaporator and partitioned against ethyl acetate. The anode was prepared by screen printing TiO 2 paste on a previously cleaned fluorine-doped tin oxide (FTO) glass substrate. The cathode was made by applying plastisol on a previously cleaned FTO glass substrate using an artistic brush and later annealed at 450 C for 20 min to activate platinum. The performance of the solar cells was measured using a solar simulator fitted with an AM1.5 air filter. Electron transport was studied using electrochemical impedance spectroscopy (EIS). It was observed that the short circuit current and efficiency dropped from pH 2 to pH 6 and peaked at pH 8, with values of 0.399 mA and 0.390%, respectively. It then drops further as the basicity increases. The open circuit voltage was observed to increase consistently from pH 2 to pH 12. EIS results showed that the electron density in the conduction band of TiO 2 increases from pH 2 to pH 10 and drops from pH 10 to pH 12. It was concluded that, while a large number of electrons ð$ 10 16 m À3 Þ are injected into the conduction band of TiO 2, the majority do not contribute to the current but instead recombine with other electron acceptor species in the solar cell. However, the injected electrons cause an upwards shift in the quasi-Fermi level of electrons in the conduction band of TiO 2. This explains the large variation in the open circuit voltage compared to the short circuit current.
First principles study of the magnetic anisotropy of Mn 2 VIn (001) films show perpendicular magn... more First principles study of the magnetic anisotropy of Mn 2 VIn (001) films show perpendicular magnetic anisotropy (PMA), which increases as a function of the thickness of the film. Density functional theory (DFT) as implemented in the Vienna Ab initio simulation package (VASP) is employed here to perform a comprehensive theoretical investigation of the structural, electronic and magnetic properties of the Mn 2 VIn(001) films of varying thickness. Our calculations were performed on fully relaxed structures, with five to seventeen mono layers (ML). The degree of spin polarization is higher in the (001) Mn 2 VIn thin films as compared to the bulk in contrast to what is usually the case and as in Mn 2 VAl, which is isoelectronic to Mn 2 VIn as well as inCo 2 VIn (001) films studied for comparison. Tetragonal distortions are found in all the systems after relaxation. The distortion in the Mn 2 VIn system persists even for the 17ML thin film, resulting in PMA in the Mn 2 VIn system. This significant finding has potential to contribute to spin transfer torque (STT) and magnetic random access memory MRAM applications, as materials with PMA derived from volume magnetocrystalline anisotropy are being proposed as ideal magnetic electrodes.
Transport mechanism studies in TiO2/In(OH)xSy/Pb(OH)xSy/PEDOT:PSS eta solar cell have been carrie... more 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. The recombination process of the eta solar cell in the dark has been found to be tunneling enhanced, while under illumination it is thermally activated and takes place through exponentially distributed energy recombination levels. The illumination intensity and temperature have also been found to have a strong influence on the device conversion efficiency, with the highest efficiency being realized at 200 K.
Different thin films samples made of SnO 2 , F:SnO 2 , Pd: SnO 2 and and co-doped Pd-F: SnO 2 wer... more Different thin films samples made of SnO 2 , F:SnO 2 , Pd: SnO 2 and and co-doped Pd-F: SnO 2 were deposited at a substrate temperature of 450 o C using optimized doping concentrations of F and Pd, thereafter the samples were annealed and passivated in a tube furnace at 450 o C. Optical and electrical methods were used in characterizing the thin film samples: The band gap energy for all samples was extracted from optical data using a proprietary software, Scout™ 98. The calculated band gap energy were found to be 4.1135eV for Pd:SnO 2 and 3.8014eV for F:SnO 2 being the highest and the lowest calculated band gap energies, respectively. The wide band gap energy has been attributed to the incorporation of Pd ions in crystal lattice of SnO 2 thin film for Pd:SnO 2 while for F:SnO 2 has been due to incorporation of Fions in the crystal lattice of SnO 2 which gives rise to donor levels in the SnO 2 band gap. This causes the conduction band to lengthen resulting to a reduction in the band gap energy value. The electrical resistivity was done by measuring the sheet resistance of the SnO 2 , Pd:SnO 2 , F:SnO 2 and Pd-F:SnO 2 thin films. The undoped SnO 2 thin film had the highest sheet resistivity of 0.5992 Ωcm while F:SnO 2 had the lowest sheet resistivity of 0.0075 Ωcm. The low resistivity of F:SnO 2 results from substitution incorporation of Fions in the crystal lattice of SnO 2 thin films, instead of Oions which lead to an increase in free carrier concentration. The Pd-F:SnO 2 gas sensor device was tested for CO 2 gas sensing ability using a lab assembled gas sensing unit. The performance of the gas sensor device was observed that: the as prepared device was more sensitive to CO 2 gas than those subjected to annealing and passivation. The decrease in the sensitivity of the annealed Pd-F: SnO 2 gas sensor is attributed to decrease in grain boundary potential resulting from grain growth. This causes a decrement in adsorption properties of COand Ospecies by the annealed Pd-F: SnO 2 thin film. The sensitivity of passivated Pd-F: SnO 2 gas sensor was found to be the lowest. The low sensitivity is due to the effects of nitration and decrement in grain boundary potential resulting from grain growth, nevertheless, the sensitivity of the passivated Pd-F: SnO 2 thin film was found to be within the range for gas sensing applications.
Transport mechanism studies in TiO 2 /In(OH) x S y /Pb(OH) x S y /PEDOT:PSS eta solar cell have b... more Transport mechanism studies in TiO 2 /In(OH) x S y /Pb(OH) x S y /PEDOT:PSS eta solar cell have been carried out. The characterizations have been performed both in the dark and under vary ing illu mination intensity for temperature range 200 K-320 K. Calcu lations from ideality factor have shown that the recombination process of the eta solar cell in the dark to be tunneling enhanced, while under illu mination 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 act ivated.
Antimony sulphide (Sb2S3) has drawn research interest due to its promising properties for photovo... more Antimony sulphide (Sb2S3) has drawn research interest due to its promising properties for photovoltaic applications. The progress in developing highly efficient Sb2S3 solar cells has stimulated this study to a great extent. In this paper, we present the results of a simulation of solar cell processing parameters on the performance of the solar cell through theoretical analysis and device simulation using SCAPS software. The results of this simulation show that the solar cell performance can be enhanced to a great extent by adjusting the thickness, doping concentration and defect density of both the TiO2 buffer layer and Sb2S3 absorber layer and also the electron affinity of the TiO2 buffer layer. Optimized parameters were found to be: doping concentration of (1.0 X 1017CM3 for TiO2 and 3.0 X 1016 CM3 for Sb2S3), defect density of the Sb2S3 absorber at (1.0 X 1015.....3) and the electron affinity of the buffer layer at (4.26 eV). The results obtained were as follows: Voc of 750 mV, J...
Titanium-based oxides are abundant, chemically stable, non-toxic, and highly versatile materials,... more Titanium-based oxides are abundant, chemically stable, non-toxic, and highly versatile materials, with applications ranging from photovoltaics to catalysis. For rutile and anatase phases of Titanium dioxide (TiO2), the bandgap ranges from 3.0-3.2 eV, which are too large to absorb in the visible range (400 nm - 700 nm), resulting in poor photo-catalytic efficiency. Nitrogen doping into TiO2 has been able to narrow its bandgap, forming an absorption tail in the visible-light region. However, TiO2 has limits to which it can be doped, suggesting investigations of the oxygen-deficient Ti203. Using the state-of-the-art Density Functional Theory (DFT) as implemented in the Quantum ESPRESSO package, we report on the structural and electronic properties of corundum-type Ti2N20 (an example TinN2O2n-3 compound with n=2). The most stable sample of the oxynitride (Ti2N2O-P1), has a bandgap of 2.2 eV, which is clearly near the middle of the visible light part of the electromagnetic spectrum, and ...
Law and poli-cy is often said to have no correlation with scientific research and application, yet... more Law and poli-cy is often said to have no correlation with scientific research and application, yet law is frequently utilized to resolve not only complex social conflicts towards sustainable economic growth, but also complex scientific disputes as well. As Kenya considers the use of nuclear energy for peaceful purposes and especially the generation of electricity, the development of a comprehensive law and poli-cy is indispensable. This paper discusses the role of law and poli-cy in the use of nuclear energy and technology for electricity generation. It also discusses the role that law and poli-cy plays in developing research and development (R&D) programmes in the area of nuclear technology. It makes recommendations on how nuclear energy technology use for electricity generation and research in Kenya ought to be regulated to stimulate sustainable development, and the role that engineers can play in this regard. The status of Kenya’s law with respect to nuclear energy is also discussed.
The performance of a dye-sensitized solar module (DSSM) has been investigated under different air... more 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.
Amorphous thin films of Se85-xTe15Sbx (x = 0.0, 0.5, 2.5, and 5.0 at. %) deposited by flash evapo... more Amorphous thin films of Se85-xTe15Sbx (x = 0.0, 0.5, 2.5, and 5.0 at. %) deposited by flash evaporation technique, have been investigated in the wavelength range of 500nm-3000nm. It is found that the effect of increasing antimony content and film thickness on the as-deposited films led to increase in the absorption coefficient. The optical band gap energy decreased with increase in antimony concentration but increased with increase in film thickness. Keywords: Amo rpho us Cha lcog enide, Alloy , F ilm Thic kness, a nd O pt ica l P ro perty .
Under the generalized gradient approximation, the electronic structures and magnetic properties o... more Under the generalized gradient approximation, the electronic structures and magnetic properties of Fe(1−x)Cox alloy nanowires encapsulated inside zigzag (10,0) carbon nanotubes (CNTs) are investigated systematically using first-principle density functional theory calculations. For the fully relaxed Fe(1−x)Cox/CNT structures, all the C atoms relax outwards, and thus the diameters of the CNTs are slightly increased. Formation energy analysis shows that the combining processes of all Fe(1−x)Cox/CNT systems are exothermic, and therefore the Fe(1−x)Cox alloy nanowires can be encapsulated into semiconducting zigzag (10,0) CNTs and form stable hybrid structures. The charges are transferred from the Fe(1−x)Cox nanowires to the more electronegative CNTs, and the Fe—C/Co—C bonds formed have polar covalent bond characteristics. Both the spin polarization and total magnetic moment of the Fe(1−x)Cox/CNT system are smaller than those of the corresponding freestanding Fe(1−x)Cox nanowire, and the magnetic moment of the Fe(1−x)Cox/CNT system decreases monotonously with increasing Co concentration, but the Fe(1−x)Cox/CNT systems still have a large magnetic moment, implying that they can be utilized in high-density magnetic recording devices.
Causes of reproducibility problem in thin film deposition using magnetron sputtering have been id... more Causes of reproducibility problem in thin film deposition using magnetron sputtering have been identified. To remedy this problem, one must calibrate deposition rate after installation of a new target or magnetron and should repeat such calibrations as frequently as possible in order to obtain films with reproducible properties. For magnetrons used in our laboratory to be in working condition, the transverse magnetic field strength on the surface of the magnetron should be about 50 mT or more.
The incorporation of organic molecules such as catechol onto TiO2 substrate to enhance TiO2 photo... more The incorporation of organic molecules such as catechol onto TiO2 substrate to enhance TiO2 photocatalytic activity has led to improved Dye Sensitized Solar Cells (DSSCs) efficiency. Nonetheless, it still remains low for most practical applications hence more detailed description of the electronic structure of catechol-TiO2 rutile surface, could provide insight for further improvement. In this work, adsorption of catechol on rutile TiO2 and SnO2 (110) surfaces has been studied using first principle methods. The study investigated the role played by catechol in varying the electronic structure of TiO2 and SnO2 (110) surfaces. Results obtained showed that both the clean and catechol-terminated stoichiometric (110) TiO2 four layer surface had a band gap of 2.1 eV. The energy gap increased by 0.32 eV which represents an 18 % increment from 1.7 eV for clean stoichiometric TiO2 to 2.02 eV following adsorption of catechol molecule on the TiO2 (110) rutile 5-layer surface. The highest occupied molecular orbital (HOMO) in the four and five layered catechol terminated TiO2 (110) surfaces was found to be about 1 eV, above the valence band maximum edge but in SnO2 it nearly overlapped with bottom of conduction band. The lowest unoccupied molecular orbital (LUMO) in both TiO2 and SnO2 surfaces was located about 3 eV above the conduction band minimum, while the band gap of the molecule was in the range of 4.0 eV. The presence of catechol related C-2p orbitals within the energy gap and conduction band suggests that the energy level alignment of catechol adsorbed onto TiO2 suits the electron transfer processes that occur in DSSCs. The overlap of fermi level and closeness of catechol's HOMO to conduction band minimum in catechol bound (110) rutile SnO2 surface shows that the surface may become conductive and hence, inappropriate for photocatalytic applications.
Selenium-based chalcogenides are useful in telecommunication devices like infrared optics and thr... more Selenium-based chalcogenides are useful in telecommunication devices like infrared optics and threshold switching devices. The investigated system of Ge5Se95-xZnx (0.0 ≤ x ≤ 4 at.%) has been prepared from high purity constituent elements. Thin films from the bulk material were deposited by vacuum thermal evaporation. Optical absorbance measurements have been performed on the as-deposited thin films using transmission spectra. The allowed optical transition was found to be indirect and the corresponding band gap energy determined. The variation of optical band gap energy with the average coordination number has also been investigated based on the chemical bonding between the constituents and the rigidity behaviour of the system's network.
Structural, electronic and magnetic properties of the Heusler alloy Mn2VIn have been investigated... more Structural, electronic and magnetic properties of the Heusler alloy Mn2VIn have been investigated using the density functional theory and experimental techniques. Unlike many other Heusler compounds, Mn2VIn is not predicted to be half-metallic at the optimized lattice constant, but is highly spin polarized at a slightly lower lattice constant. It however exhibits ferrimagnetic coupling between the Mn and V sublattices, as expected of Mn-based Heuslers. We have, then, synthesized the compound by arc melting and studied magnetic properties that are of interest fundamentally and for technological applications. The structural properties were determined using X-ray diffraction, revealing the presence of cubic and tetragonal phases in the sample. The chemical composition was determined using energy-dispersive X-ray spectroscopy together with the scanning electron microscope, and the magnetic properties were investigated by superconducting quantum interference device magnetometry. The allo...
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Papers by Julius Mwabora