One of the most critical processes in CdTe device production is the activation process induced by cadmium chloride (CdCl2). In this thesis, the CdCl2 treatment has been optimized using both wet and thermal evaporation methods for close-spaced sublimated (CSS) devices. Maximum cell efficiencies of η=7.24% and η=9.37% respectively have been measured without the use of copper in the back contact. A clear link has been established between treatment conditions, electrical measurements and microstructure, where parameters such as the dwell annealing temperature for evaporated CdCl2 and the concentration of the solution for the wet treatment are varied. It has been shown that a certain concentration of chlorine is necessary to remove high densities of planar defects present in the as-deposited material. The CSS CdTe is deposited in a dual layer structure with smaller grains at the CdS interface and with larger grains developing towards the surface. The defects are initially removed in the smaller grains at the CdS interface. When the temperature and concentration increase, more grains recrystallize with the total removal of stacking faults. At a critical temperature and Cl concentration, the entire CdTe film recrystallizes into large grains with no stacking faults. The CdS grains and the interface with the CdTe also changes with sulphur migration into the CdTe. The results indicate that the recrystallization actually initiates at the CdS/CdTe junction. This has been observed clearly for both sputtered and electrodeposited CdTe. The recrystallization process gradually propagates towards the surface as the concentration of the CdCl2 solution in methanol is increased. This observation is not intuitive because the solution is initially in contact with the outer surface of the CdTe. Finally, the use of different chlorine containing compounds has been used as an alternative to CdCl2 and to further understand the role of chlorine in the process. All the samples treated with Cl containing compounds have shown the elimination of the dual layer structure and the recrystallization of the small grains at the interface. Tellurium tetrachloride (TeCl4) and zinc chloride (ZnCl2) have shown the most promising increase in conversion efficiency. The maximum efficiencies measured using these two solutions were 4.58% and 5.05% respectively. TeCl4 has shown an encouraging open circuit voltage of 594 mV, while the open circuit voltage using ZnCl2 was 494 mV. However, TeCl4 has shunting issues and low current density (17.9 mA/cm2), whereas ZnCl2 has the promising current density of 20.8 mA/cm2. This work has shown that alternatives to CdCl2 treatment exist, however further work is required to optimize the performance of these treatments to enable them to be competitive. Advanced materials characterization techniques are essential to understand and then enhance photovoltaic cell and module performance. New and improved tools are being developed to deliver fast, accurate and non-destructive characterization. One of these tools is coherence correlation interferometry (CCI) which has been developed by Taylor Hobson Ltd. This is a particular variant of scanning white light interferometry used for surface metrology with a high vertical resolution. In this thesis, it has been shown that the capability of the CCI can be extended to perform accurate thin film thickness measurements using the Helix Complex Field (HFC) function. The main attraction of this technique for thin film PV applications is that it allows surface metrology and thin film thickness measurements to be obtained simultaneously from the same area of the sample in the same system. The results obtained from CCI on a variety of materials, used in thin film PV, correlate very well the results obtained from other techniques such as ellipsometry, electron microscopy and atomic force microscopy. The CCI has also been used in the optimization of a new one-step interconnect process (OSI) for thin film PV module interconnects.

This thesis ‘’An Investigation of Tin Chalcogenide Precursors and Thin Film Materials for Applications in Energy Harvesting Devices’’ encompasses a range of research areas. The report can be divided into two categories: The first is the design of novel heavy tin chalcogenide complexes and compounds that demonstrate the recent advances in main group chemistry and act as potential precursor candidates for CVD processes. The second category follows on from the previous, and focuses on materials deposited and their successive development, characterisation and optimisation for device applications. Subsequently, an array of metal chalcogenide thin films have been deposited and characterised within this project. By designing of a number of the tin chalcogenide precursors and precursor solutions it has been possible to selectively deposit thin films of Sn, α-SnS and cubic-SnS polymorphs, SnS2, SnSe, and SnTe via a low-cost deposition route known as aerosol assisted chemical vapour deposition (AA-CVD). It is proposed that the processes developed in this PhD can be adapted to deposit a wider spectrum of metal chalcogenide materials using cost effective techniques. Even though there is a wide scope of the possible applications for the aforementioned materials, the study has only been extended towards the characterisation of the optoelectronic properties of phase pure α-SnS and cubic-SnS samples, and SnS2 thin films deposited onto FTO, Mo and graphene substrates. The optimum deposition parameters for the application of these materials has been defined. In collaboration with a research group at the Institut de Recerca de Energia de Catalunya (iREC), Barcelona, Spain, an extended study of the photovoltaic properties of the α-SnS and Cubic-SnS samples is also presented, from which a series of SnS based thin film photovoltaic devices have been fabricated and characterised. This study present some of the few reports explicitly comparing the PV properties of the two α-SnS and Cubic-SnS polymorphs.

xv, 132 p. : ill. A print copy of this title is available from the UO Libraries, under the call number: SCIENCE TK7871.15.F5 H325 2007 / The effects of sulfur alloying on the electronic properties of CuIn(SeS) 2 and CuInGa(SeS) 2 materials has been investigated using sophisticated junction capacitance techniques including drive-level capacitance profiling and transient photocapacitance and photocurrent spectroscopies. CISSe and CIGSSe materials are used as absorber layers in thin-film photovoltaic devices. By characterizing the electronic properties of these materials we hope to understand how these materials can be improved to make thin-film devices with better conversion efficiencies. Sulfur widens the bandgap of these materials by moving the valence band to lower energies and the conduction band to higher energies. This significantly affects the electronic structure of these devices by increasing the activation energies of dominant acceptor levels and lowering room temperature free hole carrier densities. Using optical spectroscopies we observe a large, broad defect that also changes its apparent energetic depth with sulfur alloying. The occupation of this defect was controlled both optically and thermally, and showed a striking temperature dependence. This temperature dependence was measured by recording the relative defect signal, the ratio of the TPC signal in the defect regime to the above bandgap regime, as a function of temperature. As the temperature of the measurement was decreased, steps in the relative defect signal were observed, indicating the turning off of the thermal pathway that emptied trapped charge from the defect. Remarkably, such steps were seen at the same temperature in CISSe and CIGSSe devices with similar sulfur content. In addition, no steps were seen in CMS devices. This points to a defect state specific to the incorporation of sulfur in the absorber material. We hope that a better understanding of the electronic structure of these materials will assist in the creation of improved wide-bandgap thin-film photovoltaic devices. / Adviser: J. David Cohen

No description available.

Solar irradiation is a vast and plentiful source of energy. The use of photovoltaic (PV) devices to convert solar energy directly to electrical energy is an elegant way of sustainable power generation which can be distributed or in large PV plants based on the need. Solar cells are the small building blocks of photovoltaics and when connected together they form PV modules. Thin film solar cells require significantly less energy and raw materials to be produced, as compared to the dominant Si wafer technologies. CIGS thin film solar cells are considered to be the most promising thin film alternative due to its proven high efficiency. Most thin film PV modules utilise monolithic integration, whereby thin film patterning steps are included between film deposition steps, to create interconnection of individual cells within the layered structure. The state of the art is that CIGS thin film modules are made using one laser patterning step (P1) and two mechanical patterning steps (P2 and P3). Here we present work which successfully demonstrates the replacement of mechanical patterning by laser patterning methods. The use of laser ablation promises such advantages as increased active cell area and reduced maintenance and downtime required for regular replacement of mechanical tools. The laser tool can also be used to transform CIGS into a conducting compound along a patterned line. We have shown that this process can be performed after all semiconductor layers are deposited using a technique we call laser micro-welding. By performing patterning at the end of the process flow P2 and P3 patterning could be performed simultaneously. Such solutions will further reduce manufacturing times and may offer increased control of semiconductor interfaces. While showing promising performance on par with reference processes there are still open questions of importance for these novel techniques, particularly that of long term stability. Thin film modules are inherently sensitive to moisture and require reliable encapsulation. Before the techniques introduced here can be seen industrially they must have achieved proven stability. In this work we present a proof of existence of stable micro-welded interconnections. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 731

The increasing energy demand, combined with the use of harmful non-renewable energy sources calls for the search of alternative methods to cover our energy need.Renewable energy can be harvested in different ways, through the movement of wind and water, biomass, or directly from the rays of the sun, as in the case of photovoltaic (PV) devices. Whilst crystalline silicon (c-Si) is the most common absorber used for solar cells, other technologies are emerging. Solar cells with copper indium gallium diselenide (CIGS) as an absorber have the possibility of being flexible, which is an advantage due to the many more application possibilities that appear compared to the rigid and heavy c-Si solar cells. CIGS solar cells have some long-term stability issues, especially regarding ingression of atmospheric species through the front contact layer. This calls for further research in the front contact of the CIGS solar cell, exploring alternative materials to prevent degradation. The front contact of a solar cell must be both optically transparent and conduct electricity. Transparent conductive oxides (TCO) are materials characterized by the ability to conduct electricity, while also possessing a certain degree of optical transparency. The combination of conductivity and transparency makes TCOs ideal as front contacts in solar cells. A very common TCO for front contacts in CIGS solar cells is aluminum-doped zinc oxide (AZO) due to its low cost, good electrical conductivity and optical transparency. Because of its low resistance to degradation in humid environments more robust TCO alternatives, such as indium-doped tin oxide (ITO), are being investigated. Indium-doped tin oxide possesses similar electrical and optical properties as AZO, but better stability in humid environments.The ITO was deposited through RF magnetron sputtering, on a glass substrate to be able to measure optical properties. Initially, experiments focusing on oxygen content in the deposition atmosphere were done, together with a reproducibility experiment. This gave useful information about sputtering parameters and stability of the deposition. Thereon, an experiment was done varying three parameters: oxygen content in deposition atmosphere, sputtering power and temperature of substrate. A statistical software was used to analyze the data, identifying the effects of the changing parameters. The best performing samples were made with an oxygen content of 0,4-0,6 vol%. A high sensibility for oxygen in the system was also observed, as a result of the initial reproducibility experiments. This led to the introduction of a sacrificial deposition step after the machine had been shut down. Optimal substrate temperature was around 150°Cand it was not possible to go higher due to sensibility of the underlying solar cell layers.A lower threshold for the film thickness, located somewhere between 125 and 175 nm, was observed. Films with thickness below this threshold experienced a large resistivityincrease. Further depositions with higher oxygen content are advised to see if the properties of the films further improve.

<p dir="ltr">Transitioning to a carbon-neutral future will require a broad portfolio of green energy generation and storage solutions. With the abundant availability of solar radiation across the Earth’s surface, energy generation from photovoltaics (PVs) will be an important part of this green energy portfolio. While silicon-based solar cells currently dominate the PV market, temperatures exceeding 1000 °C are needed for purification of silicon, and batch processing of silicon wafers limits how rapidly Si-based PV can be deployed. Furthermore, silicon’s indirect band gap necessitates absorber layers to exceed 100 µm thick, limiting its applications to rigid substrates.</p><p dir="ltr">Solution processed thin-film solar cells may allow for the realization of continuous, high-throughput manufacturing of PV modules. Thin-film absorber materials have direct band gaps, allowing them to absorb light more efficiently, and thus, they can be as thin as a few hundred nanometers and can be deposited on flexible substrates. Solution deposition of these absorber materials utilizing molecular precursor-based inks could be done in a roll-to-roll format, drastically increasing the throughput of PV manufacturing, and reducing installation costs. In this dissertation, solution processed synthesis and the characterization of two emerging direct band gap absorber materials consisting of earth abundant elements is discussed: the enargite phase of Cu₃AsS₄ and the distorted perovskite phase of BaZrS₃.</p><p dir="ltr">The enargite phase of Cu₃AsS₄ (ENG) is an emerging PV material with a 1.42 eV band gap, making it an ideal single-junction absorber material for photovoltaic applications. Unfortunately, ENG-based PV devices have historically been shown to have low power conversion efficiencies, potentially due to defects in the material. A combined computational and experimental study was completed where DFT-based calculations from collaborators were used inform synthesis strategies to improve the defect properties of ENG utilizing new synthesis techniques, including silver alloying, to reduce the density of harmful defects.</p><p dir="ltr">Chalcogenide perovskites are viewed as a stable alternative to halide perovskites, with BaZrS₃ being the most widely studied. With a band gap of 1.8 eV, BaZrS₃ could be an excellent wide-bandgap partner for a silicon-based tandem solar cell.Historically, sputtering, and solid-state approaches have been used to synthesize chalcogenide perovskites, but these methods require synthesis temperatures exceeding 800 °C, making them incompatible with the glass substrates and rear-contact layers required to create a PV device. In this dissertation, these high synthesis temperatures are bypassed through the development of a solution-processed deposition technique.A unique chemistry was developed to create fully soluble molecular precursor inks consisting of alkaline earth metal dithiocarboxylates and transition metal dithiocarbamates for direct-to-substrate synthesis of BaZrS₃ and BaHfS₃ at temperatures below 600 °C.</p><p dir="ltr">However, many challenges must be overcome before chalcogenide perovskites can be used for the creation of photovoltaic devices including oxide and Ruddlesden-Popper secondary phases, isolated grain growth, and deep level defects. Nevertheless, the development of a moderate temperature solution-based synthesis route makes chalcogenide perovskite research accessible to labs which do not have high temperature furnaces or sputtering equipment, further increasing research interest in this quickly developing absorber material.</p>

Inexhaustible solar energy, which provides a clean, economic and green energy, seems to be an alternative solution, for current and future energy demands. Harvesting solar energy presents a challenge in using eco-friendly, earth abundant and inexpensive materials. Although present CdTe and Cu (In, Ga)Se2 (CIGS) technologies, provide light-to-electricity comparable to silicon technology, toxicity of Cd and scarcity of In limits the widespread utilization. Future tera-watt level module capacity would then be feasible by the low-cost technologies. The chalcogenide thin film technology would therefore provide the exceptional utilization in the large-area module monolithic integrations benefitting from the low material consumption owing to the direct band gap. The current thesis presents the results obtained from the quest of other thin film materials and their utilization to an unconventional Cd-free buffer layer. The films suitability for the future applications was assessed through photovoltaics device studies in a comparative manner. Chapter-1 deals with the motivation for the solar energy and the importance of thin film photovoltaics. Alternative materials which are abundantly available would help to reach the future tera watt level production, where the conventional silicon technology alone cannot satisfy the global energy demand. The utilization of non-conventional thin film based solar cells and their working principles were elucidated. The histories of the copper based alternative materials were introduced. Chapter-2 deals with the versatile thin film growth technique that has been designed fabricated and installed further which can handle the growth of the absorber and the top TCO layers with insitu sulphurisation. The methodology of the absorber deposition was discussed in detail. The experimental details for the co-sputtering of CuInAl alloy were presented. A novel selenization method, assisted by the combination of inert gases was developed for the annealing of CuInAl alloyed precursor films. Chapter-3 deals with the presentation of the results obtained on buffer and window layers. Chemical Bath deposition technique was employed for the growth and optimization of the conventional CdS and non-toxic buffer ZnS buffer layers. A) Cadmium sulphide thin films suitable for the utilization of high efficiency solar cells were optimized. Optimization of the buffer involved the effects of cadmium precursors, ammonia concentration and buffer capsule effect. A green route was presented so as to consume the precursors to the maximum extent possible. B) The alternative non-toxic buffer Zinc Sulphide (ZnS) thin films were successfully grown using the above optimized conditions. Moreover the window layer was also optimized for better device partner. Zinc Oxide was used as a n-type partner for the p-type CIS films. The ZnO films were grown by the RF-sputtering from the single cathode exhibited good crystallinity with Zincite structure (hexagonal ZnS, a= 3.249A0 and c= 5.205A0). All the grown films showed high resistivity. Al: ZnO thin films were optimized in two methods 1) by dc co-sputtering from the elemental cathodes, Zinc and Aluminum, 2) dc-sputtering from the single 2% Al-doped ZnO cathode. Low resistivity Al:ZnO thin films were deposited in both the cases. Effect of Aluminum doping into ZnO crystal lattice upon the optical and electrical properties were discussed. Chapter-4 deals with the synthesis of various absorber materials, characterizations and some properties. Briefly the A) Optimization of the CuIn1-xAlxSe2 phase with better adhesion and better crystallinity. Aluminum doping into the crystal lattice of CuInSe2 aided the wide band gap tuning of CIAS thin films. Morphological investigations were carried out for the different set of thin films before and after selenization. Effects of copper and Aluminum concentrations on the lattice parameter of the selenized thin films were addressed. The present chapter deals with the A) electrical properties of CIAS films and its heterojunction partners. Resistivity measurements and effects of Cu/In ratio and the effect of Al doping were described in detail. The CIAS/ZnO heterostructure, CIAS/Al:ZnO heterostructure junction properties as a function of different sun illuminations were discussed. B) The alternative earth abundant, eco-friendly, non-toxic elements Cu2ZnSnS4, absorber thin films synthesis and characterizations. Photo conductive photo measurements showed CZTS a potential candidate for near infra-red photodectection. C) Cu2CoSnS4 (CCTS) nanostructures and quantum dots were synthesized via simple chemical routes. CCTS quantum dots were tuned to exhibit the red edge effect and cold white phosphors. D) Cu3BiS3 nano rods were synthesized and characterized structurally and optically. The transport properties of Cu3BiS3 nanorods were tailored for showing the metallic to semiconducting transitions. Chapter-5 Discusses the A) Efforts made in understanding the CIAS based solar cells through interfaces such as CIAS/ZnO, Mo/CIAS, CIAS/CdS/i-ZnO/Al:ZnO and improving the open circuit voltage VOC upon a rotating substrate, involving the inline and in situ processes, for fabricating the cell/ module were discussed. The device statistics for various set of cells were analyzed. B) Solar cells of CTS absorber with the non-toxic buffer ZnS were fabricated and device properties were analyzed. C) CCTS quantum dots embedded in the polymer matrix were utilized for making the inverted hybrid solar devices in combination of ITO/AZnO bilayered contact replacing the acidic PEDOT: PSS. D) The solar cells made of CCTS hollow spheres by spin coating the absorber in the configuration SLG/Mo/CCTS/CdS/ iZno-AZnO/Ni-Al-Al showed a lower efficiency of 0.02%. Chapter-6 concludes with the summary of present investigations and the scope for future work.