Investigations have been done as of why dust particles appear in a circular pattern on the backside of solar cells produced in sputtering chambers at Midsummer AB. An experimental approach was conducted, where solar cells were produced at standard conditions and their backside studied by material analytical methods. The solar cells dust particles were analyzed by energy-dispersive x-ray spectroscopy and x-ray diffraction, deducing that they consisted of iron selenide (Fe0.89Se). Furthermore, the dust particles appear due to formation of a thin iron selenide film that cracks and delaminate upon cooling from process temperature to room temperature. Iron selenide film thickness was found by energy-dispersive x-ray spectroscopy to occur in a pattern with radial symmetry with respect to the cell center, correlating with the film delamination pattern. The reason to the film formation was due to selenium reacting with the substrate steel at high temperatures (>400 ◦C) in deposition chambers having a selenium environment. The film delamination occurs at a critical film thickness at which stresses in the film is high enough for the film to yield and fracture. It was concluded that iron selenide film formation or delamination must be minimized in order to control dust particle formation. These two phenomena can be mitigated by protective substrate films, change of substrate material, selenium environment optimization or temperature profile optimization and should be researched further to find the most effective and viable solution.

<p>In this thesis we present a ⁷⁷Se NMR study of the iron selenide based high-temperature superconductor K_xFe_2−ySe₂ (T_c = 33 K). We observe NMR lineshapes as narrow as ∼ 4.5 kHz with an applied field along the crystal c-axis, and find no evidence for the co-existence of magnetic order with superconductivity. With an applied field along the ab plane, however, the lineshape splits into two peaks of equal intensities at all temperatures, suggesting that the tetragonal fourfold symmetry of the average structure may be locally lowered by vacancy superstructure. Knight shift data indicate that spin susceptibility decreases progressively with temperature, similar to other iron arsenide high-T_c systems. In the nuclear spin-lattice relaxation rate 1/T₁ we observe no Hebel-Slichter coherence peak, nor any enhancement in low frequency antiferromagnetic spin fluctuations in 1/T₁T. We also report on the effects of sulphur (S) substitution on the selenium sites in this system by conducting ⁷⁷Se NMR measurements on K_xFe_2−ySe_2−zS_z (z = 0.8, 1.6). We find that both spin susceptibility and low frequency spin fluctuations are suppressed with increasing S content along with T_c.</p> / Master of Science (MSc)

xxiv, 265 p. : ill. (some col.) / Stabilizing mechanisms and design considerations for generating misfit layer compounds with a variety of different structural motifs were explored using designed precursors consisting of elemental layers. Layer order in the precursor film and the behavior of binary reaction couples was used to avoid undesirable reaction intermediates. Electron diffraction patterns of CuCr2 Se4 were inconsistent with prior reports that this compound has the spinel structure and were more consistent with a hexagonal R 3 ̄ structure. STEM imaging also suggests CuCr2 Se4 prepared using the compositionally modulated kinetic trapping approach is a new polymorph of the spinel structure. Electrical and magnetic properties were consistent with prior literature reports. Magnetic susceptibility measurements show pronounced hard and easy axes of magnetization not previously documented, which are consistent with a hexagonal crystal symmetry. The [{(PbSe)m }0.99 (WSe2 )n ] r and [{(PbSe)m }1.00 (MoSe2 )n ] r systems were investigated by STEM, XRD and density functional theory (DFT) modeling. No crystallographic registration between MSe and TSe 2 layers was observed and the diffraction observed in the hk 0 and hkl directions, where h = k = 0, can be described by diffraction from discrete layers of finite thickness. A distortion of the MX structure for m > 4 was documented. The distortion in MSe layers was largest for m = 2 and independent of TSe2 thickness. A novel family of compounds, [{(FeSe)m }1+y (NbSe 2 )n ]r , were synthesized inspired by a geological precedent. Single FeSe and NbSe2 layer thicknesses ((0.571 ± 0.005) nm and (0.653 ± 0.002) nm respectively) are consistent with literature values for the binary compounds. STEM-HAADF images of the [{(FeSe) 5 }1+y (NbSe2 )5 ]r revealed a multilayer structure with two distinct structural subunits. STEM-EELS analysis of the film showed no intermixing between the Nb and Fe regions within the limit of the measurement. Another family of misfit layer compounds, [{(NbSe2 )m }1+y (CuCr2Se4)n]r, designed to test requirements for a stable misfit layer compound, were successfully synthesized. STEM analysis of the [{(NbSe2 )5 }1+y (CuCr2 Se4 ) 1 ]r compound showed a well segregated film with two distinct subunit structures. Thicknesses for individual layers of NbSe2 or CuCr2 Se4 ((0.648 ± 0.004) nm and (1.76 ± 0.01) nm respectively) are consistent with prior literature reports of the individual binary compounds. This dissertation includes previously published and unpublished co-authored material. / Committee in charge: Dr. Mark C. Lonergan, Chair; Dr. David C. Johnson, Advisor; Dr. James Hutchison, Member; Dr. Catherine Page, Member; Dr. Stephen Gregory, Outside Member; Dr. Ian M. Anderson, Honorary Member

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