Department of Physics

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.

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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.

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.

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.

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.

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.

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.

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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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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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.

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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.

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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

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We report investigation of effect of conduction band edge on the dye injection and transport by preparation of (Ti,Sn)O2 solid mixtures in ratios of 80:20 and 90:10 as possible applications in dye sensitized solar cells. SEM micrographs showed highly porous with nanometer sized particles of around 6 - 10μm diameter. X-ray diffraction patterns showed strong TiO2 anatase peaks with crystal orientation directions (101) being the strongest in both the solid mixtures and in pure TiO2. XPS studies have shown an apparent chemical shift for Ti 2p and O1s core level spectra with an energy difference between the unmodified and the solid mixture being 0.65eV. Initial I-V studies have shown high Voc but low short circuit photocurrent, showing a possible unfavorable band edge shift between the semiconductor and the dye LUMO level.

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Temperature-dependent electrical characterization of a highly structured TiO2/In(OH)x Sy /Pb(OH)x Sy /PEDOT:PSS eta solar cell has been carried out. The transport mechanism in this type of solar cell has been investigated. A schematic energy band diagram which explains the photoelectrical properties of the device has been proposed. The solar cell has been characterized in the temperature range 200–320 K at illumination intensities between 0.05 mW/cm2 and 100 mW/cm2. The diode ideality factor A under illumination has been found to vary between 1.2 and 1.6, whereas in the dark 6.9 ≤ A ≤ 10.1. The device has been found to undergo a thermally activated recombination under illumination, while tunnelling enhanced recombination has been established to dominate the current in the dark. The solar cell efficiency shows a logarithmic dependence on illumination in the whole temperature range investigated, achieving its maximum at an illumination of ∼45 mW/cm2. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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