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<p>A new type of flexible silicon solar cell has been fabricated by interconnecting solar cell die on a flexible substrate. The fabrication process is described in this thesis. The solar cell die were diced using two methods. One method was to dice a solar cell completely through. The other method was to dice a solar cell from its back partially and then cleave through. To study the effects of different dicing methods on the performance of solar cell die, storage delay time measurement was employed to determine the lifetime of excess electrons in the p region of the two types of solar cell die. A laser beam induced current (LBIC) scanning technique was employed to study the electrical performance of the two types of solar cell die. The carrier diffusion lengths of two types of solar cell die were also determined by traveling light spot diffusion length measurement.</p><p>The theoretical response of a solar cell was compared to experimental results at various incident light angles. OpticLab software was used to model the incident light angle and lens spacing dependence of solar cell performance.</p> / Thesis / Master of Applied Science (MASc)

Perovskite solar cells have attracted much attention both in research and industrial domains. An unprecedented progress in development of hybrid perovskite solar cells (HPSCs) has been seen in past few years. The power conversion efficiencies of HPSCs has been improved from 3.8% to 24.2% in less than a decade, rivaling that of silicon solar cells which currently dominate the solar cell market. Hybrid perovskite materials have exceptional opto-electrical properties and can be processed using cost-effective solution-based methods. In contrast, fabrication of silicon solar cells requires high-vacuum, high-temperature, and energy intensive processes. The combination of excellent opto-electrical properties and cost-effective manufacturing makes hybrid perovskite a winning candidate for solar cells. As power conversion efficiencies of HPSCs improves beyond that of the established solar cell technology and their long-term stability increases, one of the crucial hurdles in the path to commercialization remaining to be adequately addressed is the cost-effective scalable fabrication. Spin-coating is the prevailing method for fabrication of HPSCs in laboratories. However, this technique is limited to small areas and results in excessive material waste. Two types of scalable manufacturing methods have been successfully demonstrated to fabricate HPSCs: (i) meniscus-assisted coating such as doctor-blade coating and slot-die coating; and (ii) dispersed deposition based on the coalescence of individual droplets, such as inkjet printing and spray coating. Electrospray printing belongs to the second category with advantages of high material utilization rate and patterning capability along with the scalability and roll-to-roll compatibility. In Chapter 3 of this dissertation, electrospray printing process is described for manufacturing of HPSCs in ambient conditions below 150 C. All three functional layers were printed using electrospray printing including perovskite layer, electron transport layer, and hole transport layer. Strategies for successful electrospray printing of HPSCs include formulation of the precursor inks with solvents of low vapor pressures, judicial choice of droplet flight time, and tailoring the wetting property of the substrate to suppress coffee ring effects. Implementation of these strategies leads to pin-hole free, low surface roughness, and uniform perovskite layer, hole transport layer and electron transport layer. The power conversion efficiency of the all electrospray printed device reached up to 15.0%, which is among the highest to date for fully printed HPSCs. The most efficient HPSCs rely on gold and organic hole-transport materials (HTMs) for achieving high performance. Gold is also chosen for its high stability. Unfortunately, the high price of gold and high-vacuum along with high-temperature processing requirements for gold film is not suitable for the large-scale fabrication of HPSCs. Carbon is a cheap alternative electrode material which is inert to hybrid perovskite layer. Due to the ambipolar transport property of hybrid perovskite, perovskite itself can act as a hole conductor, and the extra hole transport layer can be left out. Carbon films prepared by doctor-blade coating method have been reported as the top electrode in HPSCs. The efficiencies of these devices suffer from the poor interface between the doctor-blade coated carbon and the underlying perovskite layer. In Chapter 4, electrospray printing was applied for the fabrication of carbon films and by optimizing the working distance during electrospray printing, the interface between carbon and the underlying perovskite layer was greatly improved compared to the doctor-blade coated carbon film. The resulting HPSCs based on the electrospray printed carbon electrode achieved higher efficiency than that based on doctor-blade method and remarkably, this performance is close to that of gold based devices. In Chapter 5, preliminary results are provided on the laser annealing of hybrid perovskite films to further advance their scalable manufacturing. All layers of HPSCs require thermal annealing at temperature over 150 C for about half an hour or longer. The time-consuming conventional thermal annealing complicates the fabrication process and is not suitable for continuous production. High temperature over150 C is also not compatible with flexible substrates such as PET. Laser annealing is a promising method for overcoming these issues. It has several other advantages including compatibility with continuous roll-to-roll printing, minimal influence on non-radiated surrounding area, and rapid processing. Laser annealing can be integrated with the electrospray process to realize the continuous fabrication of hybrid perovskite film. Rapid laser annealing process with optimized power density and scanning pattern is demonstrated here for annealing perovskite films. The resulting hybrid perovskite film is highly-crystalline and pin-hole free, similar to that obtained from conventional thermal annealing. / Doctor of Philosophy / Hybrid perovskite solar cell (HPSC) is a promising low-cost and high efficiency photovoltaic technology. One of the big challenges for it to be commercially competitive is scalable fabrication method. This dissertation focuses on developing electrospray printing technology for HPSCs. This is a scalable method with high material usage rate that naturally lead to large scale fabrication of HPSCs. Electrospray printing parameter space was systematically studied and optimized to synthesize high-quality perovskite films and other functional layers including hole transport layer and electron transport layer. All electrospray printed high-efficiency perovskite solar cell devices were successfully demonstrated under the ambient condition and low temperature. Another achievement of this thesis is the electrospray printing of carbon film to replace the costly gold electrode in perovskite solar cells. Laser annealing technique is demonstrated for HPSCs, which is compatible with continuous fabrication and integrates easily with electrospray printing.

京都大学 / 0048 / 新制・課程博士 / 博士(エネルギー科学) / 甲第21884号 / エネ博第385号 / 新制||エネ||75(附属図書館) / 京都大学大学院エネルギー科学研究科エネルギー基礎科学専攻 / (主査)教授 佐川 尚, 教授 萩原 理加, 教授 野平 俊之 / 学位規則第4条第1項該当 / Doctor of Energy Science / Kyoto University / DGAM

Photovoltaic technologies use light from the sun to create electricity, using a wide range of materials and mechanisms. The generation of clean, renewable energy using this technology must become price competitive with conventional power generation if it is to succeed on a large scale. The field of photovoltaics can be split into many sub-groups, however the overall aim of each is to reduce the cost per watt of the produced electricity. One such solar cell which has potential to reduce the cost significantly is the dye sensitised solar cell (DSC), which utilises cheap materials and processing methods. The reduction in cost of the generated electricity is largely dependent on two parameters. Firstly, the efficiency that the solar cell can convert light into electricity and secondly, the cost to deposit the solar cell. This thesis aims to address both factors, specifically looking at altering the transparent conducting oxide (TCO) and substrate in the solar cell. One method to improve the overall conversion efficiency of the device is to implement the DSC as the top cell in a tandem structure, with a bottom infra-red absorbing solar cell. The top solar cell in such a structure must not needlessly absorb photons which the bottom solar cell can utilise, which can be the case in solar cells utilising standard transparent contacts such as fluorine-doped tin oxide. In this work, transparent conducting oxides with high mobility such as titanium-doped indium oxide (ITiO) have been used to successfully increase the amount of photons through a DSC, available for a bottom infra-red sensitive solar cell such as Cu(In,Ga)Se2 (CIGS). Although electrically and optically of very high quality, the production of DSCs on this material is difficult due to the heat and chemical instability of the film, as well as the poor adhesion of TiO2 on the ITiO surface. Deposition of a interfacial SnO2 layer and a post-deposition annealing treatment in vacuum aided the deposition process, and transparent DSCs of 7.4% have been fabricated. The deposition of a high quality TCO utilising cheap materials is another method to improve the cost/watt ratio. Aluminium-doped zinc oxide (AZO) is a TCO which offers very high optical and electronic quality, whilst avoiding the high cost of indium based TCOs. The chemical and thermal instability of AZO films though present a problem due to the processing steps used in DSC fabrication. Such films etch very easily in slightly acidic environments, and are susceptible to a loss of conductivity upon annealing in air, so some steps have to be taken to fabricate intact devices. In this work, thick layers of SnO2 have been used to reduce the amount of etching on the surface of the film, whilst careful control of the deposition parameters can produce AZO films of high stability. High efficiency devices close to 9% have been fabricated using these stacked layers. Finally, transferring solar cells from rigid to flexible substrates offers cost advantages, since the price of the glass substrate is a significant part of the final cost of the cell. Also, the savings associated with roll to roll deposition of solar cells is large since the production doesn't rely on a batch process, using heavy glass substrates, but a fast, continuous process. This work has explored using the high temperature stable polymer, polyimide, commonly used in CIGS and CdTe solar cells. AZO thin films have been deposited on 7.5um thick polyimide foils, and DSCs of efficiency over 4% have been fabricated on the substrates, using standard processing methods.

Polyfluorenes are polymers with outstanding properties: They are semi-conducting, relatively rigid, quite stable chemically and thermally, easily substituted and therefore potentially soluble in numerous solvents and more importantly, they exhibit blue electro- and photoluminescence. For all these reasons, these polymers are the subjects of numerous academic and industrial researches. The first subject of this work deal with the design, the synthesis and the characterisation of polyfluorenes end-capped with perylene dicarboximide derivatives. These perylene moieties are able to interact by energy transfer under specific conditions of illumination, proximity and orientation. Their observation by single molecule spectroscopy permitted therefore to gain valuable information concerning the three-dimensional folding of single polyfluorene chains. To complete this study, the synthesis and characterisation of a perylene end-capped trimer of fluorene was performed. This structure being monodisperse, a finer analysis of the energy-transfer occurring between both perylene dyes could be accomplished, which confirmed the structural hypothesis made for the polymer. During these studies, it has been observed that, in addition to the energy transfer occurring between both perylene derivatives, another energy transfer occurs between the polyfluorene backbone and the perylene derivatives upon excitation of the first. This led to the idea of the synthesis of a polyfluorene bearing perylenes dicarboximide as side chains. This perylene-rich polyfluorene has been used to build a photovoltaic cell efficient in the wavelengths of both polyfluorene absorption and perylene carboximide absorption. Another subject of this work was the design, synthesis and characterisation of polyfluorenes bearing bulky phenoxy groups as side-chains. These polymers, due to their lower tendency toward aggregation, exhibited a better stability of their emission colour upon annealing. Similarly, a series of homo- and copolymers of fluorene bearing bulky and hole accepting triphenylamine substituants was synthesised and characterised. In addition to their improved colour stability in comparison with dialkylpolyfluorenes, the LEDs build with these materials exhibited a very low turn on voltage.

Dye Sensitized Solar Cells (DSSC) possesses huge potential in solar energy utilisation and immense research has been carried out in order to improve its performance. There are several aspects that affect the solar cell’s performance, such as the photon collection efficiency of the cell, the reflectivity of the semiconductor, the transparency and conductivity of the transparent conductive oxide layer, and the photon-electron conversion efficiency. In this research, a pre-patterned substrate was used as a base to fabricate DSSC for improving the photon collection efficiency of DSSC. The pre-patterned substrate was prepared using maskless dry etching technique, resulting in micro-size features on the substrates and giving a 1% reduction on reflectance. The effect of Aluminium doped ZnO sputtered as the Transparent Conductive Oxide layer (TCO) in comparison with a typical DSSC fabricated on Tin doped Indium Oxide glass (ITO) was also studied. The research was carried out in two parts: substrate texturing of glass fabrication with Al:ZnO deposition, and DSSC cell assembly. The first half was carried out in the nanofabrication laboratory at University of Canterbury, New Zealand, and the second half was in National Nano Device Laboratory, Taiwan. The characteristics of both the substrates and the cells were measured using spectrophotometer with integrating sphere and solar cell simulation system. Decrease in reflectance of the Al:ZnO coated substrate at infrared region from 20% to 10 % was achieved. Due to the high resistivity of Al:ZnO and the problem of incapability in TiO2 coating, DSSC cells fabricated with these substrates have efficiencies around 2%, which is lower than the typical DSSC cells fabricated with ITO glass. Future adjustments on the substrate etching process and the cell assembly are needed for optimizing the results. The relatively high resistivity of Al:ZnO also needs to be lower for better DSSC cell performance.

Doctor of Philosophy / Department of Chemistry / Viktor Chikan / Semiconducting nanoparticles have emerged in the past few decades as an interesting material with great potential in various interdisciplinary applications such as light-emitting devices, solar cells and field-effect transistors, mostly notably for their size-dependent electronic structure and properties. Manipulation of their electronic-optical characters through defects control is one of the most important approaches towards realization of these applications. This thesis focuses on understanding the role of defects, including their impact on carrier density and conductivity at both room and elevated temperature, their impact on growth kinetics of colloidal nanoparticles and new opportunities for dopant control. To achieve these goals, colloidal CdSe quantum dots are doped with gallium atoms and important changes in electronic and optical properties of the material are reported, which shows a significant impact on the growth kinetics of quantum dots, and reveals clues about the mechanism of the gallium dopant incorporation into the CdSe. It is shown that the gallium doping significantly impacts the conductivity of CdSe thin film made of the quantum dots as well as the photoluminescence and chemical reactivity of the quantum dots, in agreement with the expected n-type character. P3HT/CdSe hybrid cells are constructed with Ga-, In- and Sn-doped CdSe QDs, demonstrating high conductivity and stronger electronic coupling which leads to enhanced charge separation and transport efficiency, both essential for hybrid inorganic-organic solar cells. This work also demonstrates a novel heating method that can drastically improve size distribution control of colloidal nanoparticle synthesis. Sub-2-nm ultra-small CdSe QDs are prepared with the induction (magnetic) heating and show excellent agreement of its emission profile compared with natural sunlight. The impact of extreme high heating rate on the development of more accurate nucleation and growth theories are also discussed. Finally, this study also investigates the stabilization of charges from intrinsic defects by looking for altered blinking behaviors of CdSe nanorods (NRs) under different polar environments. TMOS-PTMOS gradient films are prepared with infusion withdrawal dip-coating technique. Although no significant differences are observed of the fluorescence statistics of these NRs, permanent bleaching induced by exciting laser light is discovered, which significantly lowers raw blinking spot count and increases the “off” time of these fluorophores.