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The goal of this research was to understand the role of the glass substrate and molybdenum (Mo) back contact on the performance of Copper Indium Gallium diselenide (CIGS) / Cadmium Sulphide (CdS) based photovoltaic devices, and to improve the performance of these devices. The CIGS absorber layer was fabricated in a 2 stage process. In this process the metal precursors were deposited at 275oC followed by a high temperature selenization step. The advantage of the 2 stage process is that it is manufacturing friendly. The first step in fabrication of solar cells is to clean the substrate which is necessary to obtain good device performance. A variety of environmentally friendly solvents were evaluated, to determine the optimal cleaning agent. At elevated temperatures of processing sodium tends to diffuse out of Soda lime glass (SLG) and enter the semiconductor. The presence of this sodium during CIGS fabrication is necessary to obtain high efficiency CIGS based solar cells. A silicon nitride barrier layer was sputtered onto the SLG substrates, and this substrate was used to make complete devices. The CIGS absorber layer was deposited by the Type I recipe in two different vacuum systems.These devices were compared with standard devices the Si3N4 barrier layer, to understand the role of sodium on the devices fabricated from both of the systems. Furthermore, the influence of molybdenum processing parameters, such as thickness and rate of sputtering, on device performance were studied. The Voc of devices fabricated using the Type I process was limited to 460mV. In order to improve the Voc's a new absorber recipe (Type IV) was developed. Voc's of upto 490mV, Jsc's of upto 37.4mA/cm² and FF of 64%, were obtained. This improvement in performance was due to incorporation of gallium in the space charge region. Techniques such as I-V measurements, spectral response, SEM and EDS measurements were used to characterize the devices.

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.

Cadmium Telluride (CdTe) is a leading thin film photovoltaic (PV) material due to its near ideal bandgap of 1.45 eV and its high optical absorption coefficient. The typical CdTe thin film solar cell is of the superstrate configuration where a window layer (CdS), the absorber (CdTe) and a back contact are deposited onto glass coated with a transparent electrode. Substrate CdTe solar cells where the above listed films are deposited in reverse are not common. In this study substrate CdTe solar cells are fabricated on flexible foils. The properties of the Molybdenum back contact, Zinc Telluride (ZnTe) interlayer and CdTe absorber on the flexible foils were studied and characterized using X-Ray Diffraction (XRD), and Scanning Electron Microscopy (SEM). Substrate curvature and film flaking was observed during the fabrication as a result of differences in thermal expansion coefficients between the substrate and the deposited films, and also due to impurity diffusion from the foil into the film stack. In order to overcome this problem diffusion barriers where used to eliminate contamination. Silicon dioxide (SiO2), silicon nitride (Si3N4) and molybdenum nitride (MoxNy) were used as such barriers. Electrical characterization of completed devices was carried out by Current-Voltage (J-V), Capacitance-Voltage (C-V) and Spectral Response (SR) measurements. Roll-over was observed in the first quadrant of J-V curves indicating the existence of a back barrier due to a Schottky back contact. The formation of non-rectifying contact to p-CdTe thin-film is one of the major and critical challenges associated with the fabrication of efficient and stable solar cells. Several materials (ZnTe, Cu, Cu2Te, and Te) were studied as potential candidates for the formation of an effective back contact.

Cadmium telluride (CdTe) is a leading thin film photovoltaic (PV) material due to its near ideal band gap of 1.45 eV, its high optical absorption coefficient and availability of various device fabrication methods. Superstrate CdTe solar cells fabricated on glass have to-date exhibited efficiencies of 16.5%. Work on substrate devices has been limited due to difficulties associated with the formation of an ohmic back contact with CdTe. The most promising approach used to-date is based on the use of an interlayer between the CdTe and a metal electrode, an approach that is believed to yield a pseudo-ohmic contact. This research investigates the use of ZnTe and Sb2Te3 as the interlayer, in the development of efficient back contacts. Excellent adhesion and minimum stress are also required of a CdTe thin film solar cell device on a flexible stainless steel (SS) foil substrate. Foil substrate curvature, flaking, delamination and adhesion as a result of compressive strain due to the coefficient of thermal expansion (CTE) mismatch between the flexible SS foil substrate and the solar cell films have been studied. A potential problem with the use of a SS foil as the substrate is the diffusion of iron (Fe), chromium (Cr) and other elemental impurities into the layers of the solar cell device structure during high temperature processing. A diffusion barrier limiting the out diffusion of these substrate elements is being investigated in this study. Silicon nitride (Si3N4) films deposited on SS foils are being investigated as the barrier layer, to reduce or inhibit the diffusion of substrate impurities into the solar cell. Thin film CdTe solar cells have been fabricated and characterized by AFM, XRD, SEM, ASTM D3359-08 tape test, current-voltage (I-V) and spectral measurements. My individual contributions to this work include the Molybdenum (Mo) development, the adhesion studies, the silicon nitride (Si3N4) barrier studies, and EDS and SEM lines measurements and analysis of substrate out-diffused impurities. The rest of my colleagues focused on the development of CdTe, CdS, ZnTe, the CdCl2 heat treatment, and other back contact interlayer materials.

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We demonstrate the use of embedded CdS nanowires in improving spectral transmission loss and the low mechanical and electrical robustness of planar CdS window layer and thus enhancing the quantum efficiency and the reliability of the CdS-CdTe solar cells. CdS nanowire window layer enables light transmission gain at 300nm-550nm. A nearly ideal spectral response of quantum efficiency at a wide spectrum range provides an evidence for improving light transmission in the window layer and enhancing absorption and carrier generation in absorber. Nanowire CdS/CdTe solar cells with Cu/graphite/silver paste as back contacts, on SnO2/ITO-soda lime glass substrates, yield the highest efficiency of 12% in nanostructured CdS-CdTe solar cells. Reliability is improved by approximately 3 times over the cells with the traditional planar CdS counterpart. Junction transport mechanisms are delineated for advancing the basic understanding of device physics at the interface. Our results prove the efficacy of this nanowire approach for enhancing the quantum efficiency and the reliability in window-absorber type solar cells (CdS-CdTe, CdS-CIGS and CdS-CZTSSe etc) and other optoelectronic devices. We further introduce MoO3-x as a transparent, low barrier back contact. We design nanowire CdS-CdTe solar cells on flexible foils of metals in a superstrate device structure, which makes low-cost roll-to-roll manufacturing process feasible and greatly reduces the complexity of fabrication. The MoO3 layer reduces the valence band offset relative to the CdTe, and creates improved cell performance. Annealing as-deposited MoO3 in N2 reduces series resistance from 9.98 Ω/cm2 to 7.72 Ω/cm2, and hence efficiency of the nanowire solar cell is improved from 9.9% to 11%, which efficiency comparable to efficiency of planar counterparts. When the nanowire solar cell is illuminated from MoO3-x /Au side, it yields an efficiency of 8.7%. This reduction in efficiency is attributed to decrease in Jsc from 25.5mA/cm2 to 21mA/cm2 due to light transmission loss in the MoO3-x /Au electrode. Even though these nanowire solar cells, when illuminated from back side exhibit better performance than that of nanopillar CdS-CdTe solar cells, further development of transparent back contacts of CdTe could enable a low-cost roll-to-roll fabrication process for the superstrate structure-nanowire solar cells on Al foil substrate.