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%.

The generation of new nanoscale fabrication techniques is both novel and necessary for the generation of new devices and new materials. Graphene, a heavily studied and versatile material, provides new avenues to generate these techniques. Graphene’s 2-dimensional form remains both robust and uncommonly manipulable. In this project we show that graphene can be combined with the yeast cell, Saccharomyces cerevisiae, arguably the most studied and utilized organism on the planet, to generate these new techniques and devices. Graphene oxide will be used to encapsulate yeast cells and we report on the development of a method to electrically read the behaviour of these yeast cells. The advantage of an encapsulation process for a cell sensor is the ability to create a system that can electrically show both changes in ion flow into and out of the cell and mechanical changes in the cell surface. Since the graphene sheets are mechanically linked to the surface of the cell, stresses imparted to the sheets by changes in the cell wall or cell size would also be detectable. The development process for the encapsulation will be refined to eradicate excess gold on the yeast cells as well as to minimize the amount of stray, unattached graphene in the samples. The graphene oxide encapsulation process will also be shown to generate a robust substrate for material synthesis. With regards to cell sensing applications, sources of noise will be examined and refinements to the device setup and testing apparatus explored in order to magnify the relevant electrical signal. The spherical topography of an encapsulated yeast cell will be shown to be an advantageous substrate for material growth. Zinc oxide, as a sample material being investigated for its own applications for photovoltaics, will be grown on these substrates. The spherical nature of the encapsulated cell allows for radial material growth and a larger photo-active area resulting in a device with increased efficiency over a planar complement. The zinc oxide nanorods are grown via an electrochemical growth process which also reduces the graphene oxide sheets to electrochemically reduced graphene. XRD analysis confirms that the material synthesized is infact zinc oxide. The nanorods synthesized are 200nm to 400nm in width and 1µm in length. The increase efficiency of the non-planar device and the effectiveness of the encapsulated cell as a growth substrate indicate encapsulated cells as a research avenue with significant potential.

Nanometer-scale silicon shows extraordinary electronic and optical properties that are not available for bulk silicon, and many investigations toward applications in optoelectronic devices are being pursued. Silicon nanoparticle films made from solution are a promising candidate for low-cost solar cells. However, controlling the properties of silicon nanoparticles is quite a challenge, in particular shape and size distribution, which effect device performance. At present, none of the solar cells made from silicon nanoparticle films have an efficiency exceeding the efficiency of those based on crystalline silicon. To address the challenge of controlling silicon nanoparticle properties, both theoretical and experimental investigations are needed. In this thesis, we investigate silicon nanoparticle properties via quantum mechanical modeling of silicon nanoparticles and synthesis of silicon nanoparticle films via colloidal grinding. Silicon nanoparticles with shapes including cubic, rectangular, ellipsoidal and flat disk are modeled using semi-empirical methods and configuration interaction. Their electronic properties with different surface passivation were also studied. The results showed that silicon nanoparticles with hydrogen passivation have higher HOMOLUMO gaps, and also the HOMO-LUMO gap depends on the size and the shape of the particle. In contrast, silicon nanoparticles with oxygen passivation have a lower HOMO-LUMO gap. Raman spectroscopy calculation of silicon nanoparticles show peak shift and asymmetric broadening similar to what has been observed in experiment. Silicon nanoparticle synthesis via colloidal grinding was demonstrated as a straightforward and inexpensive approach for thin film solar cells. Data analysis of silicon particles via SEM images demonstrated that colloidal grinding is effective in reducing the Si particle size to sub-micron in a short grinding time. Further increases in grinding time, followed by filtration demonstrated a narrowing of the Si particle size and size-distribution to an average size of 70 nm. Raman spectroscopy and EDS data demonstrated that the Si nanoparticles contain oxygen due to exposure to air during grinding. I-V characterization of the milled Si nanoparticles showed an ohmic behaviour with low current at low biases then Schottky diode behaviour or a symmetric curve at large biases. / Graduate / 0794 / 0544 / zahraalbu@hotmail.com

Nanometer-scale silicon shows extraordinary electronic and optical properties that are not available for bulk silicon, and many investigations toward applications in optoelectronic devices are being pursued. Silicon nanoparticle films made from solution are a promising candidate for low-cost solar cells. However, controlling the properties of silicon nanoparticles is quite a challenge, in particular shape and size distribution, which effect device performance. At present, none of the solar cells made from silicon nanoparticle films have an efficiency exceeding the efficiency of those based on crystalline silicon. To address the challenge of controlling silicon nanoparticle properties, both theoretical and experimental investigations are needed. In this thesis, we investigate silicon nanoparticle properties via quantum mechanical modeling of silicon nanoparticles and synthesis of silicon nanoparticle films via colloidal grinding. Silicon nanoparticles with shapes including cubic, rectangular, ellipsoidal and flat disk are modeled using semi-empirical methods and configuration interaction. Their electronic properties with different surface passivation were also studied. The results showed that silicon nanoparticles with hydrogen passivation have higher HOMOLUMO gaps, and also the HOMO-LUMO gap depends on the size and the shape of the particle. In contrast, silicon nanoparticles with oxygen passivation have a lower HOMO-LUMO gap. Raman spectroscopy calculation of silicon nanoparticles show peak shift and asymmetric broadening similar to what has been observed in experiment. Silicon nanoparticle synthesis via colloidal grinding was demonstrated as a straightforward and inexpensive approach for thin film solar cells. Data analysis of silicon particles via SEM images demonstrated that colloidal grinding is effective in reducing the Si particle size to sub-micron in a short grinding time. Further increases in grinding time, followed by filtration demonstrated a narrowing of the Si particle size and size-distribution to an average size of 70 nm. Raman spectroscopy and EDS data demonstrated that the Si nanoparticles contain oxygen due to exposure to air during grinding. I-V characterization of the milled Si nanoparticles showed an ohmic behaviour with low current at low biases then Schottky diode behaviour or a symmetric curve at large biases. / Graduate / 0794 / 0544 / zahraalbu@hotmail.com

The focus of this thesis is on the optimization and fabrication of p-i-n amorphous silicon (a-Si:H) solar cells both on glass and transparent plastic substrates. These solar cells are specifically fabricated on transparent substrates to facilitate the integration of thin film batteries with these solar cells. To comply with plastic substrates, different silicon layers are optimized at the low processing temperature of 135 C. In the first part of the optimization process, the structural, electronic, and optical properties of boron- and phosphorous-doped, hydrogenated nanocrystalline silicon (nc-Si:H) thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) at the substrate temperature of 135 C are elaborated. Additionally, in this part, the deposition of protocrystalline silicon (pc-Si) films on glass substrates are investigated. In the device integration and fabrication part of this thesis, the optimization process is continued by fabricating single junction devices with different hydrogen dilution ratios for the cell absorber layer. The optimum device performance is achieved with an absorber layer right at the transition from amorphous to microcrystalline silicon. To further improve the performance of the fabricated solar cells, amorphous silicon carbide buffer layers are introduced between the nc-Si p-layer and the undoped pc-Si absorber layer. Single junction p-p'-i-n solar cells are fabricated and characterized both on glass and plastic substrates. Our measurements show conversion efficiencies of 7.0% and 6.07% for the cells fabricated on glass and plastic substrates, respectively. In the last part of this research, the light trapping enhancement in amorphous silicon solar cells using Distributed Bragg Reflectors (DBRs) are experimentally demonstrated. Reflectance characteristics of DBR test structures, consisting of amorphous silicon (a-Si) / amorphous silicon nitride (SiN) film stacks are analysed and compared with those of conventional ZnO/Al back reflectors. DBR optical measurements show that the average total reflectance over the wavelength region of 600-800 nm is improved by 28% for DBR back structures. Accordingly, single junction amorphous silicon solar cells with DBR and Al back reflectors are fabricated both on glass and plastic substrates. Our results show that the short-circuit current density and consequently the conversion efficiency is enhanced by 10% for the cells fabricated on textured transparent conductive oxide substrates. In addition, these DBR back structures are designed and employed to improve the efficiency of semi-transparent solar cells. In this application, the optimized DBR structures are designed to be optically transparent for the part of the visible range and highly reflective for the red and infra-red part of the spectrum. Using these DBR structures, the efficiency of the optimum semi-transparent solar cell is enhanced by 5%.

The commercially available solar cells suffer from low conversion efficiency due to the thermalization and transmission losses arising from the mismatch between the band gap of the semiconductor materials and the solar spectrum. Advanced device architectures based on nanomaterial have been proposed and being successfully used to enhance the efficiency of the solar cells. Quantum dots (QDs) and nanowires (NWs) are the nanosclae structures that have been exploited for the development of the third generation solar cell devices and nanowire based solar cells, respectively. The optical and electrical properties of these materials can be tuned by their size and geometry; hence they have great potential for the production of highly efficient solar cell. Application of QDs and NWs with enhanced optoelectronic properties and development of low-cost fabrication processes render a new generation of economic highly efficient PV devices. The most significant contribution of this PhD study is the development of simple and cost effective methods for fabrication of nanowires and quantum dots for advanced solar cell architectures. In advanced silicon nanowires (SiNWs) array cell, SiNWs have been widely synthesised by the well-known vapor-liquid-solid method. Electron beam lithography and deep reactive ion etching have also been employed for fabrication of SiNWs. Due to the high price and complexity of these methods, simple and cost effective approaches are needed for the fabrication of SiNWs. In another approach, to enhance the cell efficiency, organic dyes and polymers have been widely used as luminescent centers and host mediums in the luminescent down shifting (LDS) layers. However, due to the narrow absorption band of the dyes and degradation of the polymers by moisture and heat, these materials are not promising candidates to use as LDS. Highly efficient luminescent materials and transparent host materials with stable mechanical properties are demanded for luminescent down shifting applications. In this project, simple fabrication processes were developed to produce SiNWs and QDs for application in advanced cell architectures. The SiNWs array were successfully fabricated, characterized and deployed in new cell architectures with radial p-n junction geometry. The luminescence down shifting of layers containing QDs in oxide and glass mediums was verified. The silica coated quantum dots which are suitable for luminescence down shifting, were also fabricated and characterized for deployment in new design architectures. Silicon nanowires were fabricated using two simplified methods. In the first approach, a maskless reactive ion etching process was developed to form upright ordered arrays of the SiNWs without relying on the complicated nano-scale lithography or masking methods. The fabricated structures were comprehensively characterized. Light trapping and photoluminescence properties of the medium were verified. In the second approach, combination of the nanosphere lithography and etching techniques were utilized for wire formation. This method provides a better control on the wire diameters and geometries in a very simple and cost effective way. The fabricated silicon nanowires were used for formation of the radial p-n junction array cells. The functionality of the new cell structures were confirmed through experimental and simulation results. Quantum dots are promising candidates as luminescent centers due to their tunable optical properties. Oxide/glass matrices are also preferred as the host medium for QDs because of their robust mechanical properties and their compatibility with standard silicon processing technology. Besides, the oxide layers are transparent mediums with good passivation and anti-reflection coating properties. They can also be used to encapsulate the cell. In this work, ordered arrays of QDs were incorporated in an oxide layer to form a luminescent down shifting layer. This design benefits from the enhanced absorption of a periodic QD structure in a transparent oxide. The down shifting properties of the layer after deployment on a crystalline silicon solar cell were examined. For this purpose, crystalline silicon solar cells were fabricated to use as test platform for down shifting. In order to examine the down-shifting effect, different approaches for formation of a luminescence down shifting layer were developed. The LDS layer consist of cadmium selenide- zinc sulfide (CdSe/ZnS) quantum dots in oxide and glass layers to act as luminescent centers and transparent host medium, respectively. The structural and optical properties of the fabricated layers were studied. The concept of spectral engineering was proved by the deployment of the layer on the solar cell. To further benefit from the LDS technique, quantum efficiency of the QDs and optical properties of the layer must be improved. Demand for the high quantum efficiency material with desired geometry leaded us to synthesis quantum dots coated with a layer of grown oxide. As the luminescence quantum efficiency of the QDs is correlated to the surface defects, one advantage of having oxide on the outer shell of the QDs, is to passivate the surface non-radiative recombination centers and produce QDs with high luminescent quantum yield. In addition, nanoparticles with desired size can be obtained only by changing the thickness of the oxide shell. This method also simplifies the fabrication of QD arrays for luminescence down shifting application, since it is easier to form ordered arrays from larger particles. QD superlattices in an oxide medium can be fabricated on a large area by a simple spin-coating or dip coating methods. The photonic crystal properties of the proposed structure can greatly increase the absorption in the QDs layer and enhance the effect of down shifting.

Silicon nanowires are a nanostructure consisting of elongated crystals of silicon. Like many nanostructures, silicon nanowires have properties that change with size. In particular, silicon nanowires have a band-gap that is tuneable with the diameter of the nanowire. They tend to absorb a large portion of the light incident upon them and they form a highly textured surface when grown on an otherwise flat substrate. These properties indicate silicon nanowires are good candidates for use in solar cells. Nanostructured silicon, in the form of nanocrystalline silicon, has been used to produce thin film solar cells. Solar cells produced using silicon nanowires could combine the properties of the silicon nanowires with the low material costs and good stability of nanocrystalline based solar cells. This thesis describes the process of optimisation of silicon nanowire growth on a plasma enhanced chemical vapour deposition system. This optimised growth of silicon nanowires is then used to demonstrate a prototype solar cell using silicon nanowires and amorphous silicon. Several steps had to be accomplished to reach this goal. The growth of silicon nanowires was optimised through a number of steps to produce a high density film covering a substrate. Developments were made to the standard deposition technique and it was found that by using pulsed plasma enhanced chemical vapour deposition the density of nanowire growth could be improved. Of a range of catalysts trialled, gold and tin were found to be the most effective catalysts for the growth of silicon nanowires. A range of substrates was investigated and the nanowires were found to grow with high density on transparent conductive oxide coated glass substrates, which would allow light to reach the nanowires when they were used as part of a solar cell. The silicon nanowires were combined with doped and intrinsic amorphous silicon layers with the aim to create thin film photovoltaic devices. Several device designs using silicon nanowires were investigated. The variant that showed the highest efficiency used doped silicon nanowires as a p-layer which was coated with intrinsic and n-type amorphous silicon. By the characterisation and optimisation of the silicon nanowires, a prototype silicon nanowire solar cell was produced. The analysis of these prototype thin film devices, and the nanowires themselves, indicated that silicon nanowires are a promising material for photovoltaic applications.

Polymer solar cells are promising in that they are inexpensive to produce, and due to their mechanical flexibility have the potential for use in applications not possible for more traditional types of solar cells. The performance of polymer solar cells depends strongly on the distribution of electron donor and acceptor material in the active layer. Understanding the connection between morphology and performance as well as how to control the morphology, is therefore of great importance. Furthermore, improving the lifetime of polymer solar cells has become at least as important as improving the efficiency.   In this thesis, the relation between morphology and solar cell performance is studied, and the material stability for blend films of the thiophene-quinoxaline copolymer TQ1 and the fullerene derivatives PCBM and PC70BM. Atomic force microscopy (AFM) and scanning transmission X-ray microscopy (STXM) are used to investigate the lateral morphology, secondary ion mass spectrometry (SIMS) to measure the vertical morphology and near-edge X-ray absorption fine structure (NEXAFS) spectroscopy to determine the surface composition. Lateral phase-separated domains are observed whose size is correlated to the solar cell performance, while the observed TQ1 surface enrichment does not affect the performance. Changes to the unoccupied molecular orbitals as a result of illumination in ambient air are observed by NEXAFS spectroscopy for PCBM, but not for TQ1. The NEXAFS spectrum of PCBM in a blend with TQ1 changes more than that of pristine PCBM. Solar cells in which the active layer has been illuminated in air prior to the deposition of the top electrode exhibit greatly reduced electrical performance. The valence band and absorption spectrum of TQ1 is affected by illumination in air, but the effects are not large enough to account for losses in solar cell performance, which are mainly attributed to PCBM degradation at the active layer surface. / The performance of polymer solar cells depends strongly on the distribution of electron donor and acceptor material in the active layer. Understanding the connection between morphology and performance as well as how to control the morphology, is therefore of great importance. Furthermore, improving the lifetime has become at least as important as improving the efficiency for polymer solar cells to become a viable technology.   In this work, the relation between morphology and solar cell performance is studied as well as the material stability for polymer:fullerene blend films. A combination of microscopic and spectroscopic methods is used to investigate the lateral and vertical morphology as well as the surface composition. Lateral phase-separated domains are observed whose size is correlated to the solar cell performance, while the observed surface enrichment of polymer does not affect the performance. Changes to the unoccupied molecular states as a result of illumination in ambient air are observed for the fullerene, but not for the polymer, and fullerenes in a blend change more than pristine fullerenes. Solar cells in which the active layer has been illuminated exhibit greatly reduced electrical performance, mainly attributed to fullerene degradation at the active layer surface. / <p>Paper 2 ingick som manuskript i avhandlingen. Nu publicerad. </p>