Back Amorphous-Crystalline silicon Heterojunction (BACH) solar cells which combine the benefits of back contact and heterojunction silicon solar cells have been fabricated at the University of Toronto. p- and n-type amorphous silicon deposited at low temperature (~<200 oC) by DC Saddle-Field PECVD system forms interdigitated hetero-emitter and base contacts on the rear-side. A photolithography approach using thermal oxide for electrical isolation demonstrates the proof-of-concept. Three methods for fabricating simplified and advanced BACH cells were explored. The best performing 1 cm2 cell showed an AM1.5G conversion efficiency of 8.11%, VOC = 0.536 V, JSC = 20.1 mA/cm2 and FF = 75.5%. The BACH cell performance is limited by poor surface passivation and un-optimized cell design. With completely low temperature processing, highly passivated front and rear surfaces, and independent optimization of front-side optical antireflective features and rear-side electrical junctions and contacts, the BACH cell has the potential of becoming highly cost competitive.

Back Amorphous-Crystalline silicon Heterojunction (BACH) solar cells which combine the benefits of back contact and heterojunction silicon solar cells have been fabricated at the University of Toronto. p- and n-type amorphous silicon deposited at low temperature (~<200 oC) by DC Saddle-Field PECVD system forms interdigitated hetero-emitter and base contacts on the rear-side. A photolithography approach using thermal oxide for electrical isolation demonstrates the proof-of-concept. Three methods for fabricating simplified and advanced BACH cells were explored. The best performing 1 cm2 cell showed an AM1.5G conversion efficiency of 8.11%, VOC = 0.536 V, JSC = 20.1 mA/cm2 and FF = 75.5%. The BACH cell performance is limited by poor surface passivation and un-optimized cell design. With completely low temperature processing, highly passivated front and rear surfaces, and independent optimization of front-side optical antireflective features and rear-side electrical junctions and contacts, the BACH cell has the potential of becoming highly cost competitive.

To address the commonly poor short wavelength response of the conventional solar cell structure which consists of a highly doped thin emitter layer on top of a thicker and less doped base, the novel concept of the Buried-Windowed-Emitter is introduced. This new solar cell structure makes use of a high quality semiconductor layer on top of the traditionally made highly doped emitter and greatly enhances the spectral response of the solar cell by giving the superficially generated carriers a higher chance of collection at the junction. In the proposed BWE structure the emitter is windowed in order to electrically connect the top layer to the base for current collection. The efficacy of the proposed novel device is proven by computer aided device simulations using the available device simulation tools such as MEDICI. The results of simulation show that the proposed novel Buried-Windowed-Emitter solar cell will not only improve the short wavelength spectral response of the overall cell as expected, but also will boost the spectral efficiency for all the wavelengths. Another exciting conclusion from the results of the computer simulation of the BWE solar cell is that the minority carrier lifetime in the top layer does not need to be very high for a superb performance and values as low as 1µs can still boost the short circuit current of the cell to values close to the theoretical limit of the photo-current collectable by a silicon solar cell. This is indeed a good news for manufacturability of this device as it should be practically feasible to achieve epitaxial films with minority carrier lifetime in this range. In order to increase the understanding about the rather complex structure of the proposed Buried-Windowed-Emitter solar cell, an analytical circuit level model, similar to the case of the standard solar cell, is developed for the proposed device. The developed analytical model helps to understand the importance of the main design parameters such as the dimensions of the pattern of the windowed emitter. On the path to fabricate the proposed BWE solar cell, great deal of work is done on the development of a low temperature (<300°C) epitaxial silicon technology using the benefits of Plasma Enhanced Chemical Vapor Deposition (PECVD). Highly doped epitaxial silicon layers of up to around 1µm thickness are achieved with sheet resistivity as low as 7Ω/sq which is much lower than what is reposted in the literature in similar deposition conditions. Intrinsic, phosphorous doped n-type and boron doped p-type epitaxial films have been developed on silicon substrates. Measurement of reflection spectra of the deposited epitaxial films is proposed as a fast, non destructive and process-integrate-able method to assess the crystalline quality of the epitaxial films. Effects of higher temperature post deposition annealing have been studied on the develop epitaxial films A full technology is developed for the fabrication of the proposed novel solar cells. Photo-masks are designed to create 10 different architectures for the design of the windowed emitter in the BWE cell. All the steps taken in the successful fabrication of the novel BWE cells are presented in detail and the relevant findings are discussed and proposed as future research topics. Three kinds of cells are fabricated using the developed technology to separately study the effects of partial coverage of the windowed emitter, the optical performance of the developed epitaxial silicon films and the performance and manufacturability of the novel BWE solar cell The results show that the concept of windowed-emitter by itself (even without the top layer) is capable of enhancing the performance of the solar cell when compared to a standard design. It also promises high conversion efficiency for the BWE solar cell in case a high quality top layer can be deposited on top of the windowed emitter. The results further reveal the lower than expected quality of the low temperature epitaxial films despite the indication of their full crystallinity through other analyses. Use of the epitaxial films as the emitter of the solar cell is proposed as a direct and effective method of studying the photovoltaic performance of the low temperature epitaxial films. Further development of the epitaxial technology will lead to feasibility of a BWE solar cell with very high photovoltaic performance.

In this project, a new type of InP solar cell was investigated. The main idea is that light is converted to electrical current in p-i-n photodiodes formed in thin InP semiconductor nanowires epitaxially grown on an InP substrate. Two different types of samples were investigated. In the first sample type (series C03), the substrate was used as a common p-type electrode, whereas a short p-segment was included in all nanowires for the second sample type (B07). Current – voltage (I-V) characteristics with and without illumination were measured, as well as spectrally resolved photocurrents with and without bias. The main conclusion is that the p-i-n devices showed good rectifying behavior with an onset in photocurrent that agrees with the corresponding energy band gap of InP. An interesting observation was that in series B07 (with included p-segments) the photocurrent was determined by the band gap of hexagonal Wurtzite crystal structure, whereas series C03 (without p-segments) displayed a photocurrent dominated by the InP substrate which has a Zincblende crystal structure. We found that the overall short-circuit current was ten as large for the latter sample, stressing the importance of the substrate as a source of photocurrent.

This project employs the biological compounds hemin, melanin, and retinoic acid as photoactive dyes in dye-sensitized solar cells (DSSCs). These dyes are environmentally and economically superior to the standard ruthenium-based dyes currently used in DSSCs because they are nontoxic and widely available. Characterization by linear sweep voltammetry yielded averaged maximum overall conversion efficiency values of 0.059% for retinoic acid, 0.023% for melanin, and 0.015% for hemin. Absorption spectra of hemin and retinoic acid suggest that they would complement each other well when used in tandem in one cell because hemin has a secondary maximum absorption peak at 613nm and retinoic acid has maximum absorption at 352nm. Cells made with hemin or melanin performed better with the use of lower temperatures to seal the cells, and hemin cells performed exceptionally well with exclusion of the sealing procedure. These biologically-derived cells have the potential to advance the development of inexpensive and safer solar energy sources, which promise to serve as clean energy sources in the near future.

Single crystalline silicon spheres have been used as alternative material for solar cell fabrication. This innovative technology has several advantages over traditional wafer technology. However, the material, process flow and characterization techniques are very different from the planar technology due to the spherical geometry. In material characterization, microwave photoconductivity decay is used to measure carrier lifetime. This technique is analyzed theoretically by mathematical treatment in this thesis. Furthermore, the carrier lifetime is measured in order to investigate rapid thermal grown oxide quality in the role of surface passivation of silicon sphere. A traditional way of making spherical cells is to create a p-n junction by high temperature diffusion of phosphorous dopants into p-type silicon spheres. To further reduce the fabrication cost, a low temperature epitaxial film highly doped with phosphorous is deposited on the sphere surface to form an emitter layer using Plasma Enhanced Chemical Vapour Deposition (PECVD). The process flow of device fabrication is very different from silicon wafer thus a new set of process steps are derived for silicon spheres. Two main device structures, omission of insulating layer and silicon nitride as insulating layer between emitter film and substrate, are proposed. The deposition parameters, pressure, power, and deposition time are optimized for spherical geometry. The quality of the junction is evaluated by its current-voltage characteristic and capacitance-voltage characteristic. The results are also compared to similar device structures in planar technology. To examine the photovoltaic performance, illuminated current-voltage measurement is taken to provide information on short circuit current, open circuit voltage and fill factor. Furthermore, spectral response of quantum efficiency is investigated to assess the ability of carrier collection for a spectrum of wavelength. Limitations on spherical diode performance are concluded from the measurement results.

We present a novel thylakoid based bio-solar cell capable of generating a photoelectric current of    0.7 µA/cm2. We have introduced an electro conductive polymer, PEDOT-S, to the thylakoid membrane. PEDOT-S intervenes in the photosynthesis, captures electrons from the electron transport chain and transfers them directly across the thylakoid membrane, thus generating a current. The incorporation of the electro conductive polymer into the thylakoid membrane is therefore vital for the function of the bio-solar cell. A liposomal model system based on liposomes formed by oleic acid was used to develop and study the incorporation of PEDOT-S to fatty acid membranes. The liposomes allow for a more controllable and easily manipulated system compared to the thylakoid membrane. In the model system, PEDOT-S could successfully be incorporated to the membrane, and the developed methods were applied to the real system of thylakoid membranes. We found that a bio-compatible electrolyte and redox couple was required for this system to function. The final thylakoid based bio-solar cell was evaluated according to performance and reproducibility. We found that this bio-solar system can generate a low but reproducible current.

Patterning of multi crystalline silicon Solar cell is prepared with photolithography etching. Electroless plating is used to get metallization of Nickel contacts. SEM analysis of Nickel deposition and measurement of contact resistance for series and shunt resistance is done. To increase the fill factor, the screen printed electrodes are subjected to different firing temperatures there by increasing the efficiency of solar cell. Nickel-silicide formation at the interface between the Silicon and Nickel enhances stability and reduces the contact resistance, resulting in higher energy conversion efficiency.

Single crystalline silicon spheres have been used as alternative material for solar cell fabrication. This innovative technology has several advantages over traditional wafer technology. However, the material, process flow and characterization techniques are very different from the planar technology due to the spherical geometry. In material characterization, microwave photoconductivity decay is used to measure carrier lifetime. This technique is analyzed theoretically by mathematical treatment in this thesis. Furthermore, the carrier lifetime is measured in order to investigate rapid thermal grown oxide quality in the role of surface passivation of silicon sphere. A traditional way of making spherical cells is to create a p-n junction by high temperature diffusion of phosphorous dopants into p-type silicon spheres. To further reduce the fabrication cost, a low temperature epitaxial film highly doped with phosphorous is deposited on the sphere surface to form an emitter layer using Plasma Enhanced Chemical Vapour Deposition (PECVD). The process flow of device fabrication is very different from silicon wafer thus a new set of process steps are derived for silicon spheres. Two main device structures, omission of insulating layer and silicon nitride as insulating layer between emitter film and substrate, are proposed. The deposition parameters, pressure, power, and deposition time are optimized for spherical geometry. The quality of the junction is evaluated by its current-voltage characteristic and capacitance-voltage characteristic. The results are also compared to similar device structures in planar technology. To examine the photovoltaic performance, illuminated current-voltage measurement is taken to provide information on short circuit current, open circuit voltage and fill factor. Furthermore, spectral response of quantum efficiency is investigated to assess the ability of carrier collection for a spectrum of wavelength. Limitations on spherical diode performance are concluded from the measurement results.

In the quantum dot implementation of an intermediate band solar cell presented in this thesis, the offset of the intermediate band with respect to the conduction band is approximated by the ground state energy of a single electron in a single quantum dot heterojunction. The ground state energy is calculated with the radial Schrodinger equation with a Hamiltonian whose potential is composed from the step-like conduction band offset of the quantum dot heterojunction and the 1/r electrostatic potential of the hydrogenic impurity. The position of the intermediate band is tuned by adjusting the radius of the quantum dots. By assuming that the centrally located impurities are ionized, the location of the Fermi energy is guaranteed to be within the intermediate band. An intermediate band solar cell contains three bands: a conduction band, a valence band; and an intermediate band. The addition of an intermediate band augments the photogeneration of carriers. These additional carriers allow for an increased theoretical efficiency as compared to a conventional homojunction solar cell. The challenges in implementing an intermediate band solar cell involve centering the intermediate band at an energy level matched to the solar spectrum and aligning the Fermi energy within the intermediate band. The latter is necessary to ensure both a supply of electrons capable of photon induced transition to the conduction band as well as a large population of holes that allow photon induced electrons to transition from the valence band to the intermediate band. This thesis presents a novel material system, InPAs quantum dots enveloped in AlGaAs barriers grown on GaAs substrates, with which to implement an optimized QD-IBSC. This novel material system is selected based upon a refined set of design rules that include a requirement that the quantum dot/barrier pair offer a negligible valence band offset. With such a design rule the existence of hole levels is avoided, thus reducing bandgap narrowing at the valence band edge and the existence of minibands below the intermediate band.