Electronic Characterisation of Earth-Abundant Sulphides for Solar Photovoltaics

This thesis explores the electronic characterisation of materials for use as absorber layers within photovoltaic solar cells: an attractive solution to the energy crisis. Primarily, XPS was used to characterise CuSbS₂, Cu₃BiS₃, SnS, and Cu₂ZnSnS₄. All of these materials can be classified as earth-abundant: an important factor when considering materials that are both readily available and environmentally friendly. To varying degrees, all of these materials are established as potential absorber layers, with reports of successful, albeit low-efficiency devices. Also, the literature is often scant with regards to more fundamental characterisation of these materials, specifically in terms of how the underlying electronic structure affects properties that are pertinent to solar cells. Where XPS is utilised, it often lacks rigour, and is obfuscated by the complexities of the spectra. Work is then presented with two aims. First, to use a combination of high-quality XPS measurements and density of states calculations, in order to give insight into the electronic structure of the materials. Second, to present the full potential of XPS, as applied to solar absorbers, and how this technique complements others that are used for characterisation. Without exception, each material demonstrated natural band positions that make them unsuitable for use with established solar cell technologies. That is, a low IP (4.71–5.28 eV), resulting in a large CBO (0.5–0.85 eV) with CdS, for example. This offers some explanation to the poor efficiencies. These properties were found to be a consequence of the bonding nature of the valence and conduction bands, differing to the conventional absorber materials because of the influence of lone-pair electrons or other second-cation states. The power of XPS was demonstrated for application to absorber materials. The presence, effects, and formation pathways of contamination of the materials were elucidated using XPS, showing how the presence of such can act adversely to the performance of cells, especially with regards to oxidation, and also how these factors have been overlooked in the past. When coupled with other, complementary techniques, it has the ability to aid in the phase identification of a grown material, and also to help determine the presence of unwanted phases, which cause detriment to the device. For these applications, the methods, fitting procedures and analysis considerations detailed in this thesis should be followed. Research interest in these materials should be maintained based on the findings, with new approaches to cell design being aided by the characterisation methods developed here

Self-Compensation in Transparent Conducting F-Doped SnO2

The factors limiting the conductivity of fluorine-doped tin dioxide (FTO) produced via atmospheric pressure chemical vapor deposition are investigated. Modeling of the transport properties indicates that the measured Hall effect mobilities are far below the theoretical ionized impurity scattering limit. Significant compensation of donors by acceptors is present with a compensation ratio of 0.5, indicating that for every two donors there is approximately one acceptor. Hybrid density functional theory calculations of defect and impurity formation energies indicate the most probable acceptor-type defects. The fluorine interstitial defect has the lowest formation energy in the degenerate regime of FTO. Fluorine interstitials act as singly charged acceptors at the high Fermi levels corresponding to degenerately n-type films. X-ray photoemission spectroscopy of the fluorine impurities is consistent with the presence of substitutional F O donors and interstitial F i in a roughly 2:1 ratio in agreement with the compensation ratio indicated by the transport modeling. Quantitative analysis through Hall effect, X-ray photoemission spectroscopy, and calibrated secondary ion mass spectrometry further supports the presence of compensating fluorine-related defects

Core Levels, Band Alignments, and Valence-Band States in CuSbS2 for Solar Cell Applications

The earth-abundant material CuSbS2 (CAS) has shown good optical properties as a photovoltaic solar absorber material, but has seen relatively poor solar cell performance. To investigate the reason for this anomaly, the core levels of the constituent elements, surface contaminants, ionization potential, and valence-band spectra are studied by X-ray photoemission spectroscopy. The ionization potential and electron affinity for this material (4.98 and 3.43 eV) are lower than those for other common absorbers, including CuInxGa(1–x)Se2 (CIGS). Experimentally corroborated density functional theory (DFT) calculations show that the valence band maximum is raised by the lone pair electrons from the antimony cations contributing additional states when compared with indium or gallium cations in CIGS. The resulting conduction band misalignment with CdS is a reason for the poor performance of cells incorporating a CAS/CdS heterojunction, supporting the idea that using a cell design analogous to CIGS is unhelpful. These findings underline the critical importance of considering the electronic structure when selecting cell architectures that optimize open-circuit voltages and cell efficiencies

Band Alignments, Band Gap, Core Levels, and Valence Band States in Cu₃BiS₃ for Photovoltaics

The earth-abundant semiconductor Cu₃BiS₃ (CBS) exhibits promising photovoltaic properties and is often considered analogous to the solar absorbers copper indium gallium diselenide (CIGS) and copper zinc tin sulfide (CZTS) despite few device reports. The extent to which this is justifiable is explored via a thorough X-ray photoemission spectroscopy (XPS) analysis: spanning core levels, ionization potential, work function, surface contamination, cleaning, band alignment, and valence-band density of states. The XPS analysis overcomes and addresses the shortcomings of prior XPS studies of this material. Temperature-dependent absorption spectra determine a 1.2 eV direct band gap at room temperature; the widely reported 1.4–1.5 eV band gap is attributed to weak transitions from the low density of states of the topmost valence band previously being undetected. Density functional theory HSE06 + SOC calculations determine the band structure, optical transitions, and well-fitted absorption and Raman spectra. Valence band XPS spectra and model calculations find the CBS bonding to be superficially similar to CIGS and CZTS, but the Bi^{3+} cations (and formally occupied Bi 6s orbital) have fundamental impacts: giving a low ionization potential (4.98 eV), suggesting that the CdS window layer favored for CIGS and CZTS gives detrimental band alignment and should be rejected in favor of a better aligned material in order for CBS devices to progress

Kuusi ehdotusta elokuvaksi tai televisiosarjaksi

Tämän opinnäytetyön tavoite on tuottaa elokuvatuotantoyhtiö Sons of Lumièrelle kuusi elokuva- ja televisiosarjaehdotusta, joiden pohjalta voidaan tehdä päätös käsikirjoittamisvaiheen aloittamiseksi. Ehdotukset pohjautuvat suomalaisiin lasten- ja nuortenkirjoihin. Luetut kirjat on analysoitu ja teorian sekä haastattelujen pohjalta on luotu lukemisen raamit, joihin ehdotukset nojaavat. Tavoitteena on tutkia elokuva-adaptaation tekemistä sekä tuottaa tietoa siitä, miten erilaiset elokuvatuotantoyhtiöt ideoivat ja millainen tämä prosessi on. Sons of Lumièren tarve tuottaa lasten- tai nuortenelokuva ja/tai televisiosarja pohjautuu haluun tuottaa liiketaloudellista voittoa, sekä tehdä laadukasta lasten- ja nuorten audiovisuaalista tuotantoa. Erityisesti suosituista suomalaisista lastenkirjoista sovitetut lastenelokuvat ovat löytäneet yleisönsä Suomessa. Menetelminä käytetään kartoitusta, haastatteluja sekä kirjoittajan luomia lukemisen raameja, jotka pohjautuvat teoriaan. Tätä opinnäytetyötä varten haastateltiin kolmea suomalaista elokuvatuottajaa. Kysymykset liittyivät ideointiin, ideointimenetelmiin ja ideointiprosessiin. Elokuva-alan ammattilaiset voivat hyödyntää tätä opinnäytetyötä yhteen kokoavana tekstinä, jonka avulla elokuva-alan ammattilaiset voivat lähteä tekemään elokuva-adaptaatiota ja pohtia yleisesti elokuvien ideointiprosessia.This thesis proposes six feature films or television series for production company Sons of Lumière. The proposals are based on Finnish children and youth books that have been analyzed using theory-based approach with a focus on the frames of reading. The aims of the thesis are threefold: (a) to make a film adaptation, (b) to document how different production companies create ideas, and (c) to analyze the process through which ideas are created. The aim of the production company is to produce high-quality films for child and youth audience that are also financially profitable. Earlier Finnish children and youth movies have been successful in reaching their audiences in Finland. The methods used in this thesis include mapping, interviews and the frames for reading. Three Finnish movie producers have been interviewed on the process to develop ideas, brainstorming and the methods of creating ideas. The thesis advances the ability of film professionals to make film adaptations and to reflect on the general information about brainstorming and the process when developing ideas