Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第19007号 / 工博第4049号 / 新制||工||1623 / 31958 / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 伊藤 紳三郎, 教授 赤木 和夫, 教授 辻井 敬亘 / 学位規則第4条第1項該当

This thesis explores ways to use ion implantation and nanosecond pulsed laser melting, both energetic beam techniques, to engineer defects in silicon. These defects are chosen to facilitate the use of silicon in optoelectronic applications for which its indirect bandgap is not ideal. Chapter 2 develops a kinetic model for the use of point defects as luminescence centers for light-emitting diodes and demonstrates an experimental procedure capable of high-throughput screening of the electroluminescent properties of such defects. Chapter 3 discusses the dramatic change in optical absorption observed in silicon highly supersaturated (i.e., hyperdoped) with the chalcogens sulfur, selenium, and tellurium and reports the first measurements of the optical absorption of such materials for photon energies greater than the bandgap of silicon. Chapter 3 examines the use of silicon hyperdoped with chalcogens in light detectors and concludes that while these devices display strong internal gain that is coupled to a particular type of surface defect, hyperdoping with chalcogens does not lead directly to measurable sub-bandgap photoconductivity. Chapter 4 considers the potential for Silicon to serve as the active material in an intermediate-band solar cell and reports experimental progress on two proposed approaches for hyperdoping silicon for this application. The main results of this chapter are the use of native-oxide etching to control the surface evaporation rate of sulfur from silicon and the first synthesis of monocrystalline silicon hyperdoped with gold. / Engineering and Applied Sciences

Recently, ZnO has garnered widespread attention in the semiconductor community for its large set of useful properties, which include a wide bandgap and its resulting optical transparency, a large exciton binding energy, a significant piezoelectric response, and good electrical conductivity. In many ways, it shares many properties with a widely used and technologically important semiconductor GaN, which is widely used for blue LEDs and lasers. However, ZnO cannot substitute for GaN in most optoelectronic applications, because it cannot be doped p-type. On the other hand, unlike many traditional, covalently bonded semiconductors like GaN, ZnO can be easily formed aqueous solutions at close to room temperature and pressure in the form of large crystals or a variety of nanostructures, making possible applications that are normally very difficult with traditional semiconductors. In this light, we aimed to take advantage of aqueous solution-based, ZnO growth techniques and incorporated ZnO structures novel optoelectronic and photonic structures. By controlling the morphology of ZnO, we studied the effects of nanowire-based \(ZnO/Cu_2O\) solar cells. Carrier collection was increased using a nanowire-based device architecture. The main result, however, was the time evolution of the performance of these devices due to the movement of ionized defects in the material. The effects of geometry on the ageing characteristics were studied, which showed that the carrier collection could be increased further with ageing in a nanowire \(Cu_2O\) solar cell. The aging behavior was substantially different between nanowire and planar solar cells, which implies that future design of nanostructured solar cells must long term aging effects. In addition to solar cells, we explored the possibilities of using aqueous solution growth of ZnO to fabricated whispering gallery mode optical cavities and waveguides for enhancing extraction from a single photon source. In both applications, we used templated growth of ZnO to fabricate geometrically (near) perfect rods and disks for these photonics applications. Finally, since epitaxy is important in the process of optimizing device performance and fabrication, we showed the ability to grow ZnO epitaxially on single crystalline plates of Au, expanding the options of epitaxial substrates to include a metal. / Engineering and Applied Sciences

Tin(II) sulfide (SnS) is an earth-abundant, inexpensive, and non-toxic absorber material for thin film solar cells. SnS films are deposited by atomic layer deposition (ALD) through the reaction of a tin precursor, bis(N,N'-diisopropylacetamidinato)tin(II), and hydrogen sulfide. The SnS films demonstrate excellent surface morphology, crystal structure, phase purity, stoichiometry, elemental purity, and optical and electrical properties. / Engineering and Applied Sciences

The widespread adoption of hybrid organic-inorganic solar cells has been delayed by low performance. Improving performance requires a firm understanding of how to optimize both material composition and processing parameters. In this thesis, we examine processing parameters that include solution composition, annealing temperature, and the rates of spin casting and evaporative coating. We also find that the optimal weight ratio for the active layer of a ZnO:P3HT solar cell is 40 wt. % ZnO. / text

This dissertation focuses on characterizing the nanoscale and surface averaged electrical properties of transparent conducting oxide (TCO) electrodes such as indium tin oxide (ITO) and transparent metal-oxide (MO) electron selective interlayers (ESLs), such as zinc oxide (ZnO), the ability of these materials to rapidly extract photogenerated charges from organic semiconductors (OSCs) used in organic photovoltaic (OPV) cells, and evaluating their impact on the power conversion efficiency (PCE) of OPV devices. In Chapter 1, we will introduce the fundamental principles regarding the need for low cost power generation, the benefits of OPV technologies, as well as the key principles that govern the operation of OPV devices and the key innovations that have advanced this technology. In Chapter 2 of this dissertation, we demonstrate an innovative application of conductive probe atomic force microscopy (CAFM) to map the nanoscale electrical heterogeneity at the interface between an electrode, such as ITO, and an OSC such as the p-type OSC copper phthalocyanine (CuPc).(MacDonald et al. (2012) ACS Nano, 6, p. 9623) In this work we collected arrays of J-V curves, using a CAFM probe as the top contact of CuPc/ITO systems, to map the local J-V responses. By comparing J-V responses to known models for charge transport, we were able to determine if the local rate-limiting step for charge transport is through the OSC (ohmic) or the CuPc/ITO interface (nonohmic). These results strongly correlate with device PCE, as demonstrated through the controlled addition of insulating alkylphosphonic acid self-assembled monolayers (SAMs) at the ITO/CuPc interface. Subsequent chapters focus on the electrical property characterization of RF-magnetron sputtered ZnO (sp-ZnO) ESL films on ITO substrates. We have shown that the energetic alignment of ESLs and the organic semiconducting (OSC) active materials plays a critical role in determining the PCE of OPV devices and the appearance of, or lack thereof, UV light soaking sensitivity. For ZnO and fullerene interfaces, we have shown that either minimizing the oxygen partial pressure during ZnO deposition or exposure of ZnO to UV light minimizes the energetic offset at this interface and maximizes device PCE. We have used a combination of device testing, device modeling, and impedance spectroscopy to fully characterize the effects that energetic alignment has on the charge carrier transport and charge carrier distribution within the OPV device. This work can be found in Chapter 3 of this dissertation and is in preparation for publication. We have also shown that the local properties of sp-ZnO films varies as a function of the underlying ITO crystal face. We show that the local ITO crystal face determines the local nucleation and growth of the sp-ZnO films. We demonstrate that this effects the morphology, the chemical resistance to etching as well as the surface electrical properties of the sp-ZnO films. This is likely due to differences in the surface mobility of sputtered Zn and O atoms on these crystal faces during film nucleation. This affects the nanoscale distribution of electrical and chemical properties. As a result we demonstrate that the PCE, and UV sensitivity of the J-V response of OPVs using sp-ZnO ESLs are strongly impacted by the distribution of ITO crystal faces at the surface of the substrate. This work can be found in Chapter 4 of this dissertation and is in preparation for publication. These studies have contributed to a detailed understanding of the role that electrical heterogeneity, insulating barriers and energetic alignment at the MO/OSC interface play in OPV PCE.

Selectively-transparent and conducting photonic crystals (STCPCs) made of alternating layers of sputtered indium-tin oxide (ITO) and spin-coated silica (SiO2) nanoparticle films have potential applications in micromorph solar cells and building integrated photovoltaics (BIPVs). In this work, theoretical calculations have been performed to show performance enhancement of the micromorph solar cell upon integration of the STCPC an intermediate reflector. Thin semi-transparent hydrogenated amorphous silicon (a-Si:H) solar cells with STCPC rear contacts are demonstrated in proof-of-concept devices. A 10% efficiency increase in a 135nm thick a-Si:H cell on an STCPC reflector with Bragg peak at 620nm was observed, while the transmitted solar irradiance and illuminance are determined to be 295W/m2 and 3480 lux, respectively. The STCPC with proper Bragg peak positioning can boost the a-Si:H cell performance while transmitting photons that can be used as heat and lighting sources in building integrated photovoltaic applications.

The polymer light-emitting electrochemical cell (LEC) is an alternative method for producing electroluminescence (EL) from conjugated luminescent polymers. The in situ electrochemical doping process that leads to a dynamic p-n junction makes the devices highly insensitive to device thickness and relatively insensitive to electrode materials. These characteristics make an extremely large planar configuration accessible for observing the cross-section of the device and watching it turn on dynamically. By cooling the device to freeze ionic motion, the junction can be stabilized and photovoltaic (PV) characteristics investigated. In the planar configuration, the p-n junction was found to make up a small fraction of the inter-electrode spacing. Enabled by the insensitivity to electrode materials, small metallic particles embedded in the LEC film led to a large number of p-n junctions that could be turned on in series and parallel. This alleviates the issue of low specific emitting area suffered by planar devices and leads to improved EL effciency as well as a high open circuit voltage (Voc) when operated as a PV cell. The bulk homojunction fabrication process has been optimized by segregating the metallic particles to eliminate large aggregates. A new technique to achieve highly uniform EL from large planar LECs is also presented here. By the evaporation of a thin gold or silver film on top of an LEC, independent islands form that act as doping initiation sites across the device width. A bulk homojunction is turned on in the top layer of the LEC with a high applied bias. Island diameters and spacings are several orders of magnitude smaller than the particles in previously-reported bulk homojunction devices. Both island and particle devices had their interelectrode spacings scaled down by at least a factor of 10. The successful scaling is a promising result for the possibility of a sandwich configuration bulk homojunction device. In the case of silver island devices, cooling a 50-micron wide device after turn-on resulted in a PV cell with an open circuit voltage of 8.3 V, several times the band gap of the luminescent polymer used. / Thesis (Master, Physics, Engineering Physics and Astronomy) -- Queen's University, 2008-09-12 12:21:12.949

The arrangement of soft materials through solution processing techniques is a topic of profound importance for next generation solar cells; the resulting morphology has a major influence on construction, performance and lifetime. This thesis investigates the connections between the soft matter physics of colloidal systems and solid state dye sensitised (SSDS) and bulk heterojunction (BHJ) solar cells. A study of aqueous titanium dioxide nanoparticulate suspensions was carried out in order to observe how suspension structure can be controlled by altering the inter-colloid potential via pH-induced electrostatic charging. Measurements were performed at volume fractions between 0.025% and 8.2% with the solution pH set to 3.1, 3.5 or 4.5 before mixing. Suspensions with a volume fraction above 4% formed self-supporting gels regardless of the set pre-mix pH. These gels displayed shear thinning behaviour with a power law exponent of 0.8, a yield stress of 11(1) Pa and rheological response consistent with an aggregated fractal network. At lower volume fractions, suspensions exhibited consolidation interpreted as the collapse of a gel of fractal clusters with a fractal dimension of 2.36. The velocity of the suspension/supernatant interface exhibited delayed sedimentation behaviour, as well as further fractal-based power law scalings with volume fraction. Lower volume fraction suspensions were explored using dynamic light scattering. Limited aggregation of ‘stable’ suspensions was observed when compared to primary aggregate radii measured from electron microscopy images. To connect suspension structure and cell manufacture, the behaviour of more concentrated suspensions was observed during the drying of thin films, a process which forms an essential part of a SSDS solar cell. Lowering the pH of the suspension after mixing from 4 to 3 resulted in an ordering of observed crack domains. An increase in film delamination was also observed. Rates of mass loss during drying followed the expected three phase process, although there was an unexpected increase in rate during the initial phase (where rate is usually constant in time). Dynamic light scattering was found to be a useful but demanding technique for studying cluster formation in titanium dioxide suspensions. A non-linear fitting technique utilising the method of moments was thoroughly explored using computer simulated datasets. The algorithm reduced the systematic error in fitted parameters for moderately polydisperse (0:2 < < 0:4) datasets as compared to the commonly applied linear algorithm. The fitting algorithm was also robust to bad initial estimates of parameters. Finally, test solar cells have been built using blends of titanium dioxide and poly-3-hexylthiophene. Device performance was reduced with blend standing time after mixing but could be improved by remixing the blend before spin coating, implicating a reversible process (e.g. aggregation of titanium dioxide or crystallisation of P3HT) in the loss of performance. Addition of a titanium dioxide hole blocking layer before spin coating reduced cell performance. Combining the above studies and these device designs provides a future platform for continuation of this work in the context of real devices.

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%.