The presence of inhomogeneities in semiconductor materials used to fabricate solar cell devices may result in spatial non uniformities in the device properties which may affect current generation in these devices. Besides, current reducing defects such as inclusions, local shunts and optical blockages may be introduced during the various device manufacturing processes which may adversely affect the performance and overall efficiency of solar cells. Diagnostic techniques are therefore needed to identify these defects so as to improve the production technology. This thesis presents the Light Beam Induced Current (LBIC) technique for mapping spatial non uniformities in solar cell devices. The LBIC is a non destructive characterisation technique that uses a focused light beam to raster scan a solar cell surface as the photo-generated current is recorded as a function of position to generate a photo-response map. The technique was used to obtain photoresponse maps for a mc-Si, Back contact Back junction (BC-BJ) silicon solar cell and the InGaP/InGaAs/Ge concentrating triple junction (CTJ) solar cell from which various local current reducing defects were mapped. A reflection signal detector was incorporated into the LBIC measurement system to enable us distinguish between optical blockages on the cell surface and current reducing defects within the solar cell devices. By dynamically biasing the solar cell devices, the electrical activity of the identified defects was investigated and also point-by-point current-voltage (I-V) characteristics were obtained. An interval division algorithm was applied to the measured point-by-point I-V characteristics to extract device and performance parameters from which device and performance parameter uniformity of the devices were mapped. Dark and full cell solar illumination I-V characteristics were also measured to extract device parameters. Analysis of extracted parameters revealed differences between extracted dark and illuminated device parameters which was attributed to departure from the superposition principle due to non-linearity of the semiconductor device equations with respect to carrier concentration. An investigation into the effect of illumination intensity on the I-V parameters of a spot illuminated BC-BJ Si solar cell showed a linear increase and a logarithmic increase of the short circuit current and open circuit voltage respectively with intensity while the series resistance decreased with intensity, which was attributed to increase in conductivity of the active layer. The ideality factor and saturation current were observed to increase while the shunt resistance initially increased before decreasing at higher intensity levels. Under monochromatic illumination, the photo-response of the BC-BJ Si cell was higher at 785nm than at 445nm due to low absorption coefficient of Si for longer wavelength radiations, resulting in carrier generation within the bulk, where there is a higher probability of carriers being collected at the p-n junction before they recombine. Under solar illumination, as the spectral content was altered using long pass colour filters with cut off wavelengths of 610nm and 1000nm, the performance parameters were observed to decrease and this was mainly due to decrease in intensity. For the CTJ solar cell, however, blocking of radiations below 610nm resulted in current mismatch that severely degraded the short circuit current (Isc). The current mismatch affected the extracted device and performance parameters. With a 1000nm long pass filter, a dark I-V was obtained since only the bottom Ge subcell was activated.

Light Beam Induced Current (LBIC) measurement is a non-destructive technique used to perform localized characterization of solar cells using a light beam as a probe. The technique allows the determination of local photo response of a cell, the electrical parameters and defects that occur in the individual solar cell. The semiconductor materials used to create solar cells are not always defect free and these defects reduce the electrical performance of the device. It is therefore important to use a system that will allow the characterization and extract the solar cell parameters as can be done using the LBIC system. By analysing these parameters and cell defects, further studies can be done to enhance the cell’s lifetime and hence its efficiency. Light beam induced current (LBIC) is a technique that focuses light onto a solar cell device and thus creating a photo-generated current that can be measured in the external circuit for analyses. By scanning this beam probe across a solar cell while measuring the current-voltage characteristics, a map of various parameters can be obtained. This thesis presents the design of the LBIC system, the software interfacing of the data acquisition system and local photo-response within different solar cell technologies. In addition, this thesis represent two curve fitting algorithms namely: the Gradient Descent Optimisation and the Differential Evolution used for the extraction of solar cell device parameters. The algorithms are based on the one-diode solar cell model and make use of the light generated current-voltage (I-V) data obtained from the LBIC system. Different solar cell technologies namely; single crystalline (c-Si) and multicrystalline silicon (mc-Si) was used for analysis. LBIC maps and I-V characteristics of both technologies was obtained. The LBIC maps shows performance degrading defects present in the bulk and the surface of the solar cells as a function of spatial distribution. These localised defects acts as trapping mechanism for the charge carriers and therefore limits recombination within the solar cell and thus decreasing the performance of the solar cell device. The resulting I-V characteristics obtained from the LBIC system were used to determine the performance parameters using the two algorithms. The resultant effect of these parameters on the performance of the solar cells was observed. The overall results showed that LBIC is a useful tool for identifying and characterising defects in solar cells.

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This research presents the development of a transmittance- absorptance parameter for a solar pond. Such a parameter represents, directly, the fraction of the incident solar radiation which is absorbed at the bottom of the solar pond. It can be used to represent pond performance through an equation analogous to the Hottel-Whillier-Bliss Equation for a flat-plate solar collector. The above parameter is called the transmittance - absorptance product and is an energy-weighted quantity. Monthly values of the proposed parameter are developed from an hour-by-hour simulation. The simulation utilizes hourly values of spectral solar radiation reaching the earth's surface which are computed from a state-of-the-art algorithm that has been slightly modified to better estimate diffuse spectral radiation at large solar zenith angles; the modification is also presented. Thermal conductive losses through the water layers and the surrounding earth together with evaporative and convective losses are usually the only loss mechanisms considered for a solar pond. Under clear skies and to a lesser extent under cloudy skies, a longwave radiation heat loss also occurs from the pond surface. The estimation of radiative loss from any terrestrial surface requires detailed computations and atmospheric data. The procedure has been greatly simplified through a correlation which yields spectral atmospheric emissivity from the amounts of absorbing gases present in the atmosphere. It is recommended for further study that the performance of a solar pond be estimated using the proposed transmittance-absorptance product to compute the solar energy absorbed in the pond, and that the longwave radiative loss from the pond be included in the analysis. A comparison with data obtained from an existing solar pond is recommended to validate the results obtained in this study. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

The techniques of computer modeling and simulations are used to develop a design procedure for greenhouse solar heating systems. In this study a flexible computer program was written based on mathematical models that describe the various subsystems of the solar heating system that uses the greenhouse as the solar collector. Extensive simulation runs were carried out for predicting system thermal performance, and subsequently correlations were established between dimensionless variables and long term system performance. The combined greenhouse thermal environment - thermal storage model along with the empirical relationships and the values of constants approximated in the simulation yielded reasonably accurate computed results compared to observed data. The computer model was then applied to predict the system behaviour using long-term average climatological data as forcing functions. A parametric study was made to investigate the effects of various factors pertinent to greenhouse construction and thermal energy storage characteristics on system performance. The key performance indices were defined in terms of the 'total solar contribution' and the 'solar heating fraction'. Correlations were developed between monthly solar load ratio and total solar contribution, and between total solar contribution and solar heating fraction. The result is a simplified design method that covers a number of alternative design options. It requires users to obtain monthly average climatological data and determine the solar heating fraction in a sequence of computational steps. A crop photosynthesis model was used to compute the net photosynthetic rate of a greenhouse tomato canopy; the result may be used to compare crop performance under different aerial environments in greenhouses equipped with a solar heating system. This research program had attempted to generate technical information for a number of design alternatives, and as design optimization of greenhouse solar heating is subject to three major criteria of evaluation: thermal performance, crop yield and cost, recommendations were put forward for future work on economic analysis as the final step required for selecting the most cost effective solution for a given design problem. / Graduate and Postdoctoral Studies / Graduate

Several models were developed for the analysis of metal-semiconductor solar cells. The models presented are: (i) a limit model to obtain an idea of what the maximum conversion efficiency of metal-semiconductor solar cells is followed by; (ii) a model suitable for the prediction of the performance of metal/single-crystal silicon solar cells; and (iii) a general model for calculating the efficiency of solar cells fabricated from materials other than Si such as GaAs. Extensive use of numerical methods were required to arrive at solutions to the equations presented in the latter two models. The operation of the models is demonstrated using n-and p-type Si and GaAs with Au being taken as the barrier metal. Calculations are presented showing the effect on solar energy conversion efficiency of surface recombination velocity, barrier height, minority-carrier lifetime, barrier metal thickness, collecting grid configuration, and cell thickness. A comparison of practical and computed data for the Au/n-GaAs system yields good agreement. Based on the results of the calculations, it is shown that metal-semiconductor solar cells provide solar energy conversion of medium efficiency and improvements in efficiency depend on the development of high barrier-height systems. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A new "circuit" model is developed to study non-linear effects in holography. The model links classical signal detection theory to holography, for it clearly shows that the reconstructed images in a thin hologram can be computed from mathematical formulas obtained for the output of non-linear detectors. In preparation for hologram analysis, the results for the (time-) autocorrelation of electrical ʋth-law devices in response to signal plus noise are extended to memoryless non-linear detectors with arbitrary characteristics. Mathematical parallels are next established between holography and the non-linear detection of signals, and these are incorporated in the model. The ready-made formulas for electrical detectors apply directly to give formulas for the (space-)autocorrelation of holograms of diffuse objects. The autocorrelation function predicts distorted multi-pie images, their relative positions, orientations, widths, and strengths. The multiplicity of images is due to the generation of harmonics; the background halo-like noise components added to the faithful images are mainly due to the multiplicity of the first harmonic itself. The analysis is further expanded to include a still more general case in which the reflected light from the object is depolarized. A decrease in signal-to-noise ratio (decrease in fringe visibility); a loss of information, and an augmentation of non-linear distortion could be expected. A simple way of reducing these effects is suggested. Finally, holography with more than one reference beam is studied. An exact formulation is given for double-reference-beam holograms. For multiple-reference-beam holograms, only approximate closed forms are presented. It is shown that the addition of extra reference beams during the recording step of the hologram may amplify the faithful images without increasing their background noise significantly. Experiments confirm the theoretical expectations. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

A simple empirical model to calculate solar spectral diffuse and global irradiance under cloudless skies was investigated. This formulation takes into account absorption of radiation by molecules such as O3, H2O and the uniformly-mixed absorbing gases CO2 and O2. Attenuation by Rayleigh-scattering and aerosol extinction are included. Aerosol attenuation is calculated through Angstrom's turbidity parameters α and β. The diffuse radiation is assumed to be composed of three parts: (1) Rayleigh-scattered diffuse irradiance; (2) aerosol-scattered diffuse irradiance; and (3) irradiance arising out of multiple reflections between the atmosphere and the ground. The global irradiance is the sum of these three components of diffuse irradiance plus the direct irradiance. The input parameters include an extraterrestrial spectrum, zenith angle θ, turbidity coefficient β, wavelength exponent α, ground albedo p , water vapor content and ozone content. The model is shown to yield very good results up to air mass two when compared to accurate theoretical calculations. No comparisons with measured spectra are presented because of a lack of accurate specifications of the input parameters. Results are presented to show the effect of variation of certain of the input parameters. / Applied Science, Faculty of / Mechanical Engineering, Department of / Graduate

Orientadores: Maria Angela Fagnani / Tese (doutorado) - Universidade Estadual de Campinas, Faculdade de Engenharia Agricola / Made available in DSpace on 2018-08-07T22:58:20Z (GMT). No. of bitstreams: 1 Leite_CiceroAlexandre_D.pdf: 5359491 bytes, checksum: 85d9823bcfcf799676c4fee678ed8746 (MD5) Previous issue date: 2006 / Doutorado / Agua e Solo / Doutor em Engenharia Agrícola