Analysis of Fluctuation Processes of Solar Cells

Práce rozebírá problematiku nedestruktivní diagnostiky křemíkových solárních článků. Ačkoliv současné výrobní postupy a technologie jsou na velmi vysoké úrovni, další rozvoj v oblasti fotovoltaiky se nyní jeví limitovaný i v důsledku neschopnosti nalezení lokálních a objemových defektů, včetně jejich interpretace. Předmětem výzkumu je tedy nedestruktivní studium procesů s vlivem na provoz, životnost a spolehlivost vzorků. Za tímto účelem byly použity hlavně šumové analytické metody ve spojení se sledováním optické aktivity defektů, měřením kapacitních charakteristik a studiem transportu náboje. Tyto metody umožňují sledovat objemové nedokonalosti, defekty krystalické mříže, lokálně namáhané oblasti a v neposlední řadě i průrazné mechanizmy s možným vyústěním až v destrukci vzorků. Na základě získaných poznatků a pochopení probíhajících procesů mohou být nalezeny doporučení pro výrobce a způsoby úpravy výrobních postupů. Významná část práce je zaměřena na náhodný n-stavový impulsní šum známý jako mikroplazmatický šum. Tento šum vzniká v důsledku průrazů ve velmi malých oblastech pn přechodu a indikuje významné snížení životnosti, popřípadě destrukci pn přechodu jako takového. Tyto prostorově lokalizované oblasti byly studovány samostatně s použitím optických i elektrických metod a podařilo se získat mnoho zajímavých informací. Neméně významná část práce byla věnována i modelování šumových a fluktuačních procesů objemových nedokonalostí, kde se podařilo navrhnout jejich fyzikální interpretaci a analytický popis.The thesis deals issue of the silicon solar cells non-destructive testing. The manufacturing technology of solar cells currently features a very high level of perfection. Its further development appears to be limited by amongst other issues imperfect diagnostic methods. The objective of presented research consists in non-destructive studies of processes that influence specimen life and reliability. To this end, I will employ mainly noise based analytical methods in connection with observation of defect optical activities, capacitance measurement etc. These methods are closely related to some specimen bulk imperfections, crystal-lattice defect induced traps, local-stress-subjected regions and, finally, breakdowns, which might bring about specimen destruction. Based on a detailed study and understanding of transport processes, regions in which noise is generated can be identified and appropriate technological measures can be proposed and adopted. Presented research focuses, first of all, on the real solar cell structures, which are inhomogeneous in their nature and are difficult to diagnose. The significant part of this study is attend to the random n-level (in most case just two-level) impulse noise, usually referred to as microplasma noise. This noise is a consequence of local breakdowns in micro-sized regions and brings about reduction of lifetime or destruction of the pn junction. The micro-sized regions have been studied separately by electrical and optical methods and defect properties have been put forward. Nevertheless, no less significant part of the thesis is devoted to the fluctuation modeling of the bulk imperfections in the semi-analytical form.

Effect of the Different Crystallinity of Ionic Liquid Based Solid Polymer Electrolyte on the Performance of Amperometric Gas Sensor

Solid polymer electrolytes (SPE) based on ionic liquid, poly-(vinylidene fluoride) and solvent N-methyl-pyrrolidone represent an effective component in electrochemical sensors. The advantage lies in their composition, which offers an opportunity to prepare SPE layers with a different porosity and microstructure. The study shows how the SPEs of different crystallinities affect the performance of an amperometric gas sensor from the point of view of current response (sensitivity), limit of detection and current fluctuations. The morphology of SPE has an impact not only on its conductivity but also on sensor sensitivity due to the morphology of the interface SPE/working electrode (WE)

Crack resistance of bismuth ferrite films obtained on a flexible substrate

Ultrathin BiOx and FeOx layers were obtained by Atomic Layer Deposition (ALD) on the surface of a flexible Kapton substrate (poly (4,4’-oxydiphenylene-pyromellitimide)) at a temperature of 250 °C. The layer thickness was 50 - 100 nm. Surface morphology, electrical polarization, and mechanical properties were investigated by Atomic Force Microscope, Piezoelectric Force Microscopy and Force Modulation Microscopy. Chemical analysis was performed by X-ray Photoelectron Spectroscopy, where the formation of Bi2O3 and Fe2O3 phases, as well as intermediate phases in the Bi-Fe-O system, was observed. With a small increase in the Bi content of the film, the BFO / Kapton structure becomes more crack resistant. Modification of the Kapton surface with bismuth and iron oxides showed that such a composition exhibits multiferroic behavior

Structure Tuning and Electrical Properties of Mixed PVDF and Nylon Nanofibers

The paper specifies the electrostatic spinning process of specific polymeric materials, such as polyvinylidene fluoride (PVDF), polyamide-6 (PA6, Nylon-6) and their combination PVDF/PA6. By combining nanofibers from two different materials during the spinning process, new structures with different mechanical, chemical, and physical properties can be created. The materials and their combinations were subjected to several measurements: scanning electron microscopy (SEM) to capture topography; contact angle of the liquid wettability on the sample surface; Raman spectroscopy; X-ray photoelectron spectroscopy (XPS); and Fourier-transform infrared spectroscopy ({FT-IR}) to describe properties and their changes at the chemical level. Crystallization events were determined by differential scanning calorimetry (DSC). Furthermore, the contact angle of the wettability of the liquid on the surface was measured for the materials, and the permittivity was measured to observe the dielectric properties. The advantage of the addition of co-polymers was to control the properties of PVDF samples and understand the reasons for the changed functionality. The innovation point of this work is the complex analysis of PVDF modification caused by mixing with nylon PA6. Here we empathize that the application of nylon during the spin influences the properties and structure (polarization, crystallization) of PVDF

Comprehensive study of solar cell structure defects by means of noise and light emission analysis

This paper discusses the issue of silicon solar cells localized defects from metrological and physical points of view. Structure imperfections represent the real problem because of solar cells long-term degradation and conversion efficiency decreasing. To this aim we pay our attention to research relating to the defect light emission and correlation with rectangular microplasma fluctuation. A sensitive CCD camera has been used for mapping of surface photon emission. The operation point of the samples has been set to reverse bias mode, and different electric field intensity was applied. We managed to get interesting information using a combination of optical investigation and electrical noise measurement in time and spectral domain. It will be revealed that a direct correlation between noise and photon emission exists and the results related to several defect spots are presented in detail in this paper

Impact of local defects on photon emission, electric current fluctuation and reliability of silicon solar cells studied by electro-optical methods

This paper, for the first time, investigates localized defects of silicon solar cells. These imperfections represent real problem because of solar cell long-term degradation and decreasing conversion efficiency. To solve this issue, this paper does systematic research about optical investigation of local defect spots and correlation with rectangular microplasma fluctuation. Sensitive CCD camera has been used for mapping of surface photon emission. The operation point of the samples has been set to reverse bias mode and the different electric field intensity was applied. It turns out, that some solar cells exhibit an imperfection in the bulk and close to the edges. Nevertheless, we confine ourselves to bulk defects of potential barrier. We managed to get interesting information using combination of optical investigations and electrical noise measurement in the time and spectral domain. It will be revealed that a direct correlation between noise and photon emission exists and the results related to several defect spots are presented in detail in this paper

The effect of thermal treatment on ac/dc conductivity and current fluctuations of PVDF/NMP/[EMIM] [TFSI] solid polymer electrolyte

The experimental study deals with the investigation of the effect of diverse crystallinity of imidazolium ionic-liquid-based SPE on conductivity and current fluctuations. The experimental study was carried out on samples consisting of [EMIM][TFSI] as ionic liquid, PVDF as a polymer matrix and NMP as a solvent. After the deposition, the particular sample was kept at an appropriate temperature for a specific time in order to achieve different crystalline forms of the polymer in the solvent, since the solvent evaporation rate controls crystallization. The ac/dc conductivities of SPEs were investigated across a range of temperatures using broadband dielectric spectroscopy in terms of electrical conductivity. In SPE samples of the higher solvent evaporation rate, the real parts of conductivity spectra exhibit a sharper transition during sample cooling and an increase of overall conductivity, which is implied by a growing fraction of the amorphous phase in the polymer matrix in which the ionic liquid is immobilized. The conductivity master curves illustrate that the changing of SPEs morphology is reflected in the low frequency regions governed by the electrode polarization effect. The dc conductivity of SPEs exhibits Vogel–Fulcher–Tammann temperature dependence and increases with the intensity of thermal treatment. Spectral densities of current fluctuations showed that flicker noise, thermal noise and shot noise seems to be major noise sources in all samples. The increase of electrolyte conductivity causes a decrease in bulk resistance and partially a decrease in charge transfer resistance, while also resulting in an increase in shot noise. However, the change of electrode material results in a more significant change of spectral density of current fluctuations than the modification of the preparation condition of the solid polymer electrolyte. Thus, the contact noise is considered to contribute to overall current fluctuations across the samples

Comprehensive characterization of PVDF nanofibers at macro- and nanolevel

This study is focused on the characterization and investigation of polyvinylidene fluoride (PVDF) nanofibers from the point of view of macro- and nanometer level. The fibers were produced using electrostatic spinning process in air. Two types of fibers were produced since the collector speed (300 rpm and 2000 rpm) differed as the only one processing parameter. Differences in fiber’s properties were studied by scanning electron microscopy (SEM) with cross-sections observation utilizing focused ion beam (FIB). The phase composition was determined by Fourier-transform infrared spectroscopy (FTIR) and Raman spectroscopy. The crystallinity was determined by differential scanning calorimetry (DSC), and chemical analysis of fiber’s surfaces and bonding states were studied using X-ray photoelectron spectroscopy (XPS). Other methods, such as atomic force microscopy (AFM) and piezoelectric force microscopy (PFM), were employed to describe morphology and piezoelectric response of single fiber, respectively. Moreover, the wetting behavior (hydrophobicity or hydrophilicity) was also studied. It was found that collector speed significantly affects fibers alignment and wettability (directionally ordered fibers produced at 2000 rpm almost super-hydrophobic in comparison with disordered fibers spun at 300 rpm with hydrophilic behavior) as properties at macrolevel. However, it was confirmed that these differences at the macrolevel are closely connected and originate from nanolevel attributes. The study of single individual fibers revealed some protrusions on the fiber’s surface, and fibers spun at 300 rpm had a core-shell design, while fibers spun at 2000 rpm were hollow