One-dimensional vertical nanostructures, nanowire arrays, are investigated for applications in photovoltaics. Specifically, III-V core-shell p-i-n nanowire arrays are grown by molecular beam epitaxy on silicon substrates, using the self-assisted vapour-liquid-solid growth method. GaAs1-xPx nanowires are grown with an optimized composition to maximize the potential efficiency of a GaAsP nanowire-on-silicon tandem solar cell under AM1.5G illumination. Photovoltaic devices are fabricated and assessed by optical and electrical characterization techniques, to identify areas for refinement of device design and processing. Combining the unique properties of nanowire arrays, the quality and tunability of III-V materials, and the economics and infrastructure of silicon-based device fabrication, this work examines a novel approach to affordable renewable energy. Methods of substrate removal via etching are investigated for optical characterization of nanowire arrays, and an improved technique for electrical characterization of ITO contacts is explored. The first nanowire-on-silicon tandem device utilizing a radial p-n junction nanowire structure is reported, achieving an open circuit voltage of 1.2 V, a short circuit current density of 7.6 mA/cm2, a fill factor of 40%, and an efficiency of 3.5%. Finally, projects for future improvements to the work described herein are suggested. / Thesis / Master of Applied Science (MASc)

A hole-collecting interlayer layer for organic solar cells, NiO, processed by atomic layer deposition (ALD) was studied. ALD-NiO film offered a novel alternative to efficient hole-collecting interlayers in conventional single-junction organic solar cells. Next, surface modifications with aliphatic amine group containing polymers for use as electron-collecting interlayers were studied. Physisorption of the polymers was found to lead to large reduction of the work function of conducting materials. This approach provides an efficient way to provide air-stable low-work function electrodes for organic solar cells. Highly efficient inverted organic solar cells were demonstrated by using the polymer surface modified electrodes. Lastly, charge recombination layers of the inverted tandem organic solar cells were studied. Efficient charge recombination layers were realized by using the ALD and the polymer surface modification. The charge recombination layer processed by ALD provided enhanced electrical and barrier properties. Furthermore, the polymer surface modification on the charge recombination layers showed large work function contrast, leading to improved inverted tandem organic solar cells. The inverted tandem organic solar cells with the new charge recombination layer showed fill factor over 70% and power conversion efficiency over 8%.

Un grupo de investigadores del Instituto Tecnológico de Israel ha observado que el uso de foto ánodos de Fe2O3 produce una mejora importante en el rendimiento de las celdas fotoelectroquímicas. Las mejoras están basadas en el uso de láminas muy delgadas de Fe2O3 que mejoran la eficiencia en el transporte de carga. La elección de Fe2O3 está basada en su estabilidad en agua, su alta eficiencia de absorción de luz, su no toxicidad y su bajo coste. Las celdas fotoelectroquímicas son importantes para la electrólisis del agua y la producción de hidrógeno. / A research group at the Israel Institute of Technology has outlined the use of Fe2O3 photoanodes to enhance the efficiency of photoelectrochemical cells. The improvement is based on the design of extremely thinsheets of Fe2O3, which allow for a more efficient transport of charge carriers, reducing the amount of them that are lost due to recombination. Fe2O3 was chosen as a photoanode material due to its high stability in water, its high light absorption efficiency, the fact that it is non toxic and its low cost. Photoelectrochemical cells are extremely important in order to electrolyse water, hence producing hydrogen,a green fuel.

In this project, samples with a metal/semiconductor/metal structure were fabricated and investigated with the potential application as the interconnecting layer of a tandem solar cell in mind. Degenerately doped p-Si and n-Si were used as bottom (metal like) contacts, as Si represents one of the most common materials for the bottom cell of tandem devices. A transparent, wide bandgap semiconductor in the form of SnO₂ was investigated for the intermediate layer as it is a common choice for the selective back contact of top cells based on perovskites. However, atomic layer deposition (ALD) was used as an alternative to the typical solution based application of the SnO₂ layer. The top layer was simply chosen as a triple layer metal contact stack (Ni-Al-Ni) to provide for good contact with the SnO₂.The goal of the project was to study the electrical properties of the samples through I-V measurements and how the I-V characteristic depends on the oxide’s thickness under the possible influence of the contact areas. Three different thicknesses of the SnO2 layer were used for the p-Si sample: 50, 200 and 400 Å. For the n-Si samplesonly one thickness (400 Å) was studied. Using the diode equation, four parameterswere calculated (Jo, Rsh, Rs and n) for different measurements combing different contact configurations. The latter included measurements between the front and the back of the samples and measurements between contacts on the front with and/orwithout SnO2 layer. From the results, it was concluded that as the thickness of SnO₂ increases, the saturation current (Jo) decreases while both shunt resistance (Rsh) andseries resistance (Rs) increase. The ideality factor (n) neither depends significantly on effective area, nor on SnO2 thickness. The p-Si and n-Si samples show similar behavior in the case of 400 Å SnO2 thickness. The contact areas only appreciatively affect Jo, but it is not clear what lies behind this dependence. In all cases, the top contacts obtained major wear during measurements, reducing the number of trustworthy measurements that could be used on the smaller areas. The resistivity through the oxide layer was calculated to ρSnO₂ = 247±96 MΩ cm, which is higher than for SnO₂ deposited by other techniques, and too high for tandem cell application. Schottky barriers formed at the interfaces will typically limit the charge transport further.

A key to solar cells with lower environmental impact is higher efficiency and reduced material usages. Bifacial solar cells may have a higher efficiency as light can enter from two directions and tandem solar cells may use a larger part of the incoming solar spectrum, increasing the efficiency. However, both these applications require transparent and conducting contacts. This thesis aimed to investigate how suitable the transparent conductive oxides aluminum doped zinc oxide (AZO) and indium doped tin oxide (ITO) are as contacts in bifacial CIGS cells or CIGS/Silicon tandem cells. The contacts must remain stable when CIGS is deposited on top of it, meaning that they have to endure first 500°C and then 600°C in combination with copper, indium, gallium and selenium vapours. A thin layer of AZO topped with ITO and pure ITO films of different thicknesses were deposited by RF- and DC-sputtering, varying the oxygen flow. Opto-electrical characterization showed that the transparency in the infrared was balanced against high conductivity due to a shift in the plasmon peak's position. No great difference was seen between pure ITO samples and AZO/ITO samples, so only the first where further processed. The ITO films were annealed to 500°C in the CIGS deposition chamber, exposed to selenium vapour. The films' sheet resistances dropped drastically, which was mainly attributed to activation of tin donors. ITO produced with low oxygen flows also appeared more crystalline according to x-ray diffraction measurements. Photon absorption in the ITO was used to estimate the current loss in bifacial and tandem applications and graphs with current loss and sheet resistance can be used to select an ITO deposition process. Commercial ITO was exposed to 100s of the CIGS deposition process but only during selenium and gallium vapour. A layer of gallium selenide could be identified on the surface, but the ITO appeared to remain stable. Sodium fluoride pre deposition treatment lowered the samples absorption for all wavelengths compared to non-treated samples.