Degradation in Field-aged Crystalline Silicon Photovoltaic Modules and Diagnosis using Electroluminescence Imaging

Degradation phenomena observed in field-aged crystalline silicon photovoltaic modules include EVA browning, delamination between the glass-encapsulant and the cell-encapsulant interfaces, degradation of the anti-reflective coating, corrosion of busbars and contacts, cracks, humidity ingress, etc. The type and severity of the defects observed vary significantly between cells, modules and installations as affected by a number of both internal and external parameters. This study presents mild to severe degradation effects observed in crystalline silicon PV modules operating outdoors for different periods of time and investigated through non-destructive testing techniques including I-V characterisation, UV fluorescence, IR thermography and Electroluminescence (EL) Imaging. The identification and diagnosis of defects and further correlation to the electrical degradation of the module is achieved through the complementary contribution of these techniques. Severe electrical degradation and mismatch between the cells are identified through IR thermography and EL imaging. Diagnosis of rather uniformly degraded modules is enhanced through EL Imaging by which shunts, higher resistance regions, cracks, broken metallization are identified, while the module may appear to operate reliably. Signs of early degradation are further diagnosed through UV fluorescence and EL Imaging, allowing to monitor the evolution of defects and evaluate module reliability

CdTe solar cells: growth phenomena and device performance

A systematic study is presented on the control of CdTe and CdS layers during their growth, with the understanding gained being implemented in the production of solar cells with enhanced performance. In particular the growth mechanisms for close space sublimation (CSS) — grown CdTe were evaluated as a function of processing gas (N2, 02 and H2) and nitrogen pressure. Films were shown to form via the Volmer-Weber growth mode with films deposited under nitrogen showing well defined crystal facets. Inclusion of oxygen in the deposition ambient produced islands of a rounded morphology, reduced size and increased number density, whilst hydrogen was shown to increase the island number density and the level of substrate coverage. Growth mechanisms were deduced from the morphologies observed at different stages of growth by ex-situ AFM and SEM and by comparison with growth literature, especially the work of P. Barna. Nucleation density, step flow and impurity incorporation are all invoked in the discussion.Factors influencing the cell performance were evaluated with the aid of a optical beam induced current (OBIC) and external quantum efficiency (EQE) system built as part of this work and having the capacity to measure EQE or OBIC maps with a resolution of 12.5pm. The system was used to evaluate the photovoltaic response of CdTe/CdS devices as a function of wavelength with the impact of the nitric-phosphoric acid (NP) etch on the back surface, the uniformity of CdTe/CdS devices deposited by different methods and the effect of absorber layer thickness of PV uniformity being assessed. The performance of CdTe/CdS devices was evaluated as a function of variables that could be influenced by growth of the CdTe and CdS layers. The use of lower substrate temperature and the incorporation oxygen in CdS increased V„ from 0.51 to 0.65V is discussed. Oxygen in the CdTe was also shown to influence the junction position and hence efficiency, while oxygen in the CdS layer was also shown to be vital for the formation of hetero-junctions. The CdTe grain size was shown to be significantly increased for deposition under higher nitrogen pressures (Grain diameter = [0.027P + 0.9]gm, where P is the pressure in Torr), with the average grain diameters being 0.94pm at 2Torr and 5.63pm at 200Torr. Device performance was improved as a result with the peak device efficiency being increased from 2.1% at 2Torr to 14.1% at 100Torr. The series resistance was shown to be minimised for larger grain size, owing to the reduced contribution of grain boundaries. Suggestions for the fabrication of high efficiency solar cells are given with reference to the efficiency limiting factor

Characterization Of Microstructural And Chemical Features In Cu-in-ga-se-s-based Thin-film Solar Cells

Thin-film solar cells are potentially low-cost devices to convert sunlight into electricity. Improvements in the conversion efficiencies of these cells reduce material utilization cost and make it commercially viable. Solar cells from the Thin-Film Physics Group, ETH Zurich, Switzerland and the Florida Solar Energy Center (FSEC), UCF were characterized for defects and other microstructural features within the thin-film structure and at the interfaces using transmission electron microscopy (TEM). The present thesis aims to provide a feedback to these groups on their deposition processes to understand the correlations between processing, resulting microstructures, and the conversion efficiencies of these devices. Also, an optical equipment measuring photocurrents from a solar cell was developed for the identification of defect-prone regions of a thin-film solar cell. The focused ion beam (FIB) technique was used to prepare TEM samples. Bright-field TEM along with scanning transmission electron microscopy (STEM) and energy dispersive X-ray spectroscopy (EDS) including elemental distribution line scans and maps were extensively used for characterizing the absorber layer and interfaces both above and below the absorber layer. Energy-filtered transmission electron microscopy (EFTEM) was applied in cases where EDS results were inconclusive due to the overlap of X-ray energies of certain elements, especially molybdenum and sulfur. Samples from ETH Zurich were characterized for changes in the CIGS (Cu(In,Ga)Se2) microstructure due to sodium incorporation from soda-lime glass or from a post-deposition treatment with NaF as a function of CIGS deposition temperature. The CIGS-CdS interface becomes smoother and the small columnar CIGS grains close to the Mo back contact disappear with increasing CIGS deposition temperature. At 773 K the two sodium incorporation routes result in large differences in the microstructures with a significantly larger grain size for the samples after post-deposition Na incorporation. Porosity was observed in the absorber layer close to the back contact in the samples from FSEC. The reason for porosity could be materials evaporation in the gallium beam of the FIB or a processing effect. The porosity certainly indicates heterogeneities of the composition of the absorber layer near the back contact. A Mo-Se rich layer (possibly MoSe2) was formed at the interface between CIGS/CIGSS and Mo improving the quality of the junction. Other chemical heterogeneities include un-sulfurized Cu-Ga deposits, residual Se from the selenization/ sulfurization chamber in CIGS2 and the formation of Cu-rich regions which are attributed to decomposition effects in the Ga beam of the FIB. Wavy absorber surfaces were observed for some of the cells with occasional discontinuities in the metal grids. The 50 nm thick CdS layer, however, remained continuous in all the samples under investigation. For a sample with a transparent back contact, a 10 nm Mo layer was deposited on ITO (indium tin oxide) before deposition of the CIGS2 (Cu(In,Ga)S2) layer. EFTEM maps indicate that a MoS2 layer does not form for such a Mo/MoS2-ITO back contact. Instead, absorber layer material diffuses through the thin Mo layer onto the ITO forming two layers of CIGS2 on either side of Mo with different compositions. Furthermore, an optical beam induced current (OBIC) system with micron level resolution was successfully developed and preliminary photocurrent maps were acquired to microscopically identify regions within a thin-film solar cell with undesirable microstructural features. Such a system, when fully operational, will provide the means for the identification of special regions from where samples for TEM analysis can be obtained using the FIB technique to study specifically the defects responsible for local variations in solar cell properties

Local optical and electrical characteristics of optoelectronic devices

Konverze solární energie a miniaturizace polovodičových součástek a s tím spojená životnost, spolehlivost a účinnost zařízení jsou základní premisy této práce. Práce je zaměřena na studium a nedestruktivní diagnostiku optoelektronických součástek, především solárních článků. Ty jsou výhodné pro studium především proto, že mají přístupný pn přechod blízko povrchu a obsahují značné množství nehomogenit. Vzhledem k rozměrům nehomogenit bylo ještě donedávna obtížné zkoumat jejich lokální fyzikální (tj. elektrické a optické) charakteristiky, které by umožnily lépe pochopit jejich chování. Vybudování vlastního měřicí pracoviště, které splňuje specifické požadavky pro oblast měření lokálního optického vyzařování a lokálně indukovaného proudu, umožnilo dosáhnout lokalizaci a detekci nehomogenit s rozlišením přibližně 100 nm. Jádrem práce je charakterizace nedokonalostí s využitím nedestruktivních technik, a to nejen z makroskopického hlediska, ale především v mikroskopickém měřítku s využitím sondové mikroskopie. Nedílnou součást práce tedy tvoří studium problematiky charakterizačních technik pro optoelektronické součástky, studium mikroskopických technik, především sondových a problematika zpracování naměřených dat. Pro účely mikroskopické charakterizace je použit mikroskop se skenující sondou v blízkém optickém poli, který kromě morfologie povrchu umožňuje zkoumat také lokální optické, optoelektrické a elektrooptické vlastnosti struktur ve vysokém prostorovém rozlišení. Z makroskopického hlediska jsou v rámci práce zkoumány vzorky s využitím techniky lokálně indukovaného proudu, voltampérových charakteristik vzorků, emise ze závěrně polarizovaných vzorků ale i jejich teplotních závislostí. Společným využitím těchto technik je možné lokalizovat defekty a nehomogenity struktury, které byly následně podrobeny kompozitní analýze a dále zobrazeny s využitím elektronové mikroskopie. Mezi konkrétní výstupy práce patří specifikace možností využití nedestruktivních charakterizačních technik pro studium optoelektronických součástek a zvláště pak pro klasifikaci jejich defektů. Dále jsou formou metodiky popsány experimentální charakterizační techniky a postupy charakterizace defektů. Klíčovým výstupem je katalog objevených typů defektů, ve kterém jsou ukázány konkrétní defekty vzorků a jejich lokální vlastnosti v mikroskopickém měřítku společně s popisem jejich vlivu na celý vzorek.Solar energy conversion, miniaturization of semiconductor devices and associated lifetime, reliability and efficiency of devices are the basic premise of this work. This work is focused on the study of optoelectronic devices especially solar cells and its nondestructive diagnostic. Solar cells are advantageous for study mainly because the pn junction is located near the surface and contains a lot of inhomogeneities. It has been difficult until recently to investigate their local physical (electrical and optical) parameters due to the size of inhomogeneities. Behavior of inhomogeneities can be well understood with knowledge of its local properties. Establishment of measurement workplace, that satisfies requirements for measurement of local emission and optically induced current measurement, allows us detection and localization of inhomogeneities with spatial resolution more or less 100 nm. The core of thesis is characterization of imperfection using nondestructive techniques in the macroscopic region but primarily in microscopic region using scanning probe microscopy. Integral parts of the work are characterization techniques for photoelectrical devices, microscopic techniques and data processing. Scanning near-field optical microscope is used for the purpose of microscopic characterization such as topography, local optical, photoelectrical and electrooptical properties of structures in high spatial resolution. Locally induced current technique, current voltage characteristics, emission from reversed bias pn junction measurement including its thermal dependence are used for samples investigation in macroscopical region. It is possible to localize defects and structure inhomogeneity using mentioned techniques. Localised defects are consequently analyzed for composition and measured using electron microscopy. Specific outputs of work are classification of photoelectric devices defects and specification of nondestructive characterization techniques used for defect detection. Experimental characterization techniques are described together with defects measurement procedures. The key output is the catalog of serious defects which was detected. Particular defects of samples are shown including describe of its properties and physical meaning.

Surfaces and interfaces of low dimensional III-V semiconductor devices

The demand for fast and energy efficient (opto-)electronic applications needs high mobility semiconductor materials, such as InAs with a very high electron mobility and GaSb with a very high hole mobility. Beyond the material itself, also an innovative device geometry is needed, for example, the gate-all-around geometry that provides higher efficiency and electrostatic control for computational units. Vertically or laterally grown nanowires and nanosheets are excellent candidates for realizing such beneficial device geometries. The logic operations and charge transport could be realized in different device architectures, such as the concepts of tunnelFETs instead of classical FETs or new neuromorphic hardware instead of complementary metal-oxide-semiconductor (CMOS).With both the excellent functional properties of III-V materials and the flexibility of nanostructuring into 1D nanowires and 2D nanosheets, III-V semiconductors could be the stars for next-generation applications. For example, lateral grown InxGa1−xAs nanowires have a high spin-orbit coupling and moderate bandgap promising for quantum computing devices. GaSb nanowires are excellent high-speed p-channels for III-V CMOS, and InAs/InP nanowires have an energy barrier in the axial direction which can be used for photovoltaic and sensor applications. Due to the high surface-to-bulk ratio of nanowires and nanosheets, their surface condition becomes the key to the device performance. In this work, III-V nanowire and nanosheet devices are studied with an emphasis on surfaces and interfaces, using a wide range of characterization methods. The dissertation explores the fabrication of novel nano-devices and the characterization of their surface chemistry, topography, electronic properties, electrical transport and interaction with photons. The characterization techniques include scanning tunneling microscopy/spectroscopy (STM/S) for atomic level topography and electronic properties. Development of a Scanning gate microscopy (SGM) system with additional single-mode focused lasers for simultaniously probing influence of static and optical fields. Synchrotron based X-ray techniques, mainly X-ray photoelectron spectroscopy (XPS) is used for evaluating surface chemistry. Surface treatment processes, e.g., ultra-high vacuum (UHV) annealing, digital etchants, atomic hydrogen cleaning, and atomic layer deposition (ALD), are applied and the resulting surface chemistry, structure and electronic properties measured. Beyond studying the surface properties, we also investigate the device efficiency and performance down to the nanometer scale. Therefore, we perform measurements to monitor the device while the local gate and/or a focused light interact with the device.In conclusion, in this thesis the surfaces and interfaces of low-dimensional materials for future device applications are studied using many different characterization methods. It is the hope that the thesis will assist in the progress toward novel devices and improve the energy efficiency and performance of devices. Both the method development and the results give relevant contributions opening for future quantum technologies and (opto)electronics

Photovoltaics with Silicon Nanoparticles

Photovoltaik mit Siliziumnanopartikeln Bislang besteht der Lichtabsorber nahezu aller installierter photovoltaischer (PV) Module aus ein- oder multikristallinem Silicium (c-Si oder mc-Si). Wegen ihrer geringeren Kosten pro installierter Leistung existiert jedoch seit einigen Jahren ein Trend hin zu Dünnschichtzellen. Das Fernziel der dieser Arbeit zugrundeliegenden Untersuchungen besteht deswegen darin, druckbare PV Zellen zu realisieren, deren Dünnfilm-Absorberschicht aus Si Nanopartikeln (NP) hergestellt wird. Si bietet sich an, da es ungiftig ist und eine nahezu unbegrenzt verfügbare Ressource darstellt. Die Partikelform wurde gewählt, weil NP dispergiert und somit materialeffizient verdruckt werden können. In einem ersten Schritt wurden in dieser Arbeit c-Si Substrate mit hoch dotierten Si NP beschichtet und mittels Laser-Temperns eine dotierte Substratschicht erzeugt. Für das Tempern standen ein infraroter (IR, Wellenlänge λ = 808nm) Dauerstrahllaser und ein gepulster, ultravioletter (UV, λ = 248nm) Laser zur Verfügung. Für beide Laser konnte durch elektrische (Vierpunkt-Leitfähigkeit) und analytische (SIMS / ECV) Messungen nachgewiesen werden, dass Dopanden in das Substrat eingebracht werden. Während der IR-Laser Dotiertiefen von ca. 100µm erzeugte, betrugen sie für den UV-Laser lediglich ca. 200nm. Durch Anwenden dieser Technik auf dotierte Substrate mit komplementär dotierten NP, konnten pn-Übergange geschaffen werden, die einen PV Effekt zeigen. Die dünnere Emitterschicht (≈Dotiertiefe) ist ein wichtiger Grund dafür, dass die maximalen Konversionseffizienzen η der UV-Laser behandelten Proben (η ≈ 6%) deutlich über denen der IR-Laser behandelten Proben (η ≈ 2%) liegen. Als Hauptgrund für die für c-Si PV vglw. niedrigen Konversionseffizienzen werden chemische und strukturelle Defekte vermutet. Ihre Reduktion lässt eine deutliche Effizienzsteigerung erwarten. Wegen der zu großen Dotiertiefe wurde der IR-Laser für weitere Experimente ausgeschlossen. Für die Produktion von PV Zellen kann die UV-Laser NP Dotiermethode weiterhin interessant sein, um das Aluminium (Al) aus dem Standardrückseitendotierprozess zu verdrängen: Bei einer Reduktion der Substratdicke zukünftiger Generationen von PV Zellen würde das Al wegen des von Si abweichenden, temperaturabhängigen Ausdehnungskoeffizienten zu einer Substratverbiegung führen. Mit der präsentierten Technik konnte eine Rückseitendotierung von mc-Si Solarzell-Halbzeugen realisiert werden; bislang ist aber nicht klar, ob die Technik wirklich zu einer ähnlichen oder sogar höheren Effizienz als der Standardprozess führen kann. Diese Methode könnte bereits heute für die PV-Industrie interessant sein, weil sie eine Verbiegung verhindern sollte und ein unabhängiges Bearbeiten von Vorder- und Rückseite ermöglicht. Nachdem es möglich war, Si Substrate umzudotieren, wurden in einem zweiten Schritt grundlegende Versuche durchgeführt, um pn-Übergänge nur aus Si NP herzustellen. So wurden pn-Proben einerseits durch Spark-Plasma-Sintern (SPS) von p- auf n-typ NP-Schüttungen und andererseits durch sukzessive Schleuderbeschichtung von p- auf n-typ Dispersionen und deren Lasertemperung hergestellt. Während die SPS-Versuche primär das Ziel verfolgten, einen direkteren Einblick in den nur aus NPn erzeugten pn-Übergang zu ermöglichen, stellt die Schleuderbeschichtungsmethode eine deutlich anwendungsnahere Methode dar. In beiden Fällen wurde eine elektromotorische Kraft gemessen, wobei nicht abschließend geklärt werden konnte, ob deren Ursprung die PV ist. Dass es aber gelang, leitfähige Schichten auf isolierenden Substraten zu realisieren, stellt einen ersten Schritt in Richtung druckbarer Si PV aus NP dar.Today, the light absorber of almost all installed photovoltaic (PV) modules is made of single or multi crystalline silicon (c-Si or mc-Si). However, due to the reduced costs per installed power, a trend exists towards thin film cells since a few years. Thus, the future's aim of this work's investigations is to realize printable PV cells with a thin film absorber made from Si nanoparticles (NP) solely. Si is chosen, because it is non-toxic and an abundant resource. The particle form is selected, because NP can be dispersed, thus making them printable, resulting in a high material-efficiency for the future application. As a first step, in this work c-Si substrates were coated with highly doped Si NP and via laser annealing, a doped substrate layer was created. For the annealing, a continuous wave, infra red (IR, wave length λ = 808nm) and a pulsed, ultra violet (UV, λ = 248nm) laser were available. For both lasers, electrical (four point conductance) and analytical (SIMS / ECV) measurements proved a successful incorporation of dopants into the substrate. While the IR laser created doping depths of approx. 100µm, for the UV laser they were determined to be approx. 200nm. By applying this method on doped substrates and complementary doped NP, pn-junctions could be realized that exhibit a PV effect. The thin emitter layer (≈doping depth) is one important reason for the higher maximum conversion efficiencies η of the UV laser annealed samples (η ≈ 6%) when compared to the IR laser annealed ones (η ≈ 2%). Chemical and structural defects are supposed to be the major reason for the low conversion efficiencies when compared with c-Si PV. By reducing them, a considerable efficiency increase is expected. Due to the too big doping depth, the IR laser was excluded from further experiments. For the production of PV cells, the UV laser NP doping method may also be interesting to replace the aluminium (Al) from the standard back surface doping process: A reduction of the substrate thickness of future PV generations, would lead to a bending of the substrate due to the different thermal expansion coefficients of Si and Al. Using the presented technique, a back surface doping could be realized on semi-finished mc-Si PV cells; however, so far it is not certain, whether this technique can really create similar or even higher efficiencies as the standard process. Nevertheless, this method can be interesting for the PV industry already today, because a substrate bending should be prevented and an independent handling of the front and the back side is possible. After the feasibility of Si substrates doping, fundamental experiments were conducted to create pn-junctions form Si NP solely, in a second step. Thus, pn-samples were created by spark-plasma sintering (SPS) of p- on n-type NP material on the one hand and by successive spin coating of p- and n-type dispersions and their laser annealing on the other hand. While the SPS experiments primary aimed to get a directer insight of the pn-junction created only from NP, the spin coating method represents a much more application oriented way. In both cases, an electromotive force was measured, whereas it is not finally clarified, whether its origin really is PV. However, the success to realize conductive layers on insulating substrates is interpreted as a first step towards the future's aim of printable Si PV from NP

Elastic Mid-Infrared Light Scattering: a Basis for Microscopy of Large-Scale Electrically Active Defects in Semiconducting Materials

A method of the mid-IR-laser microscopy has been proposed for the investigation of the large-scale electrically and recombination active defects in semiconductors and non-destructive inspection of semiconductor materials and structures in the industries of microelectronics and photovoltaics. The basis for this development was laid with a wide cycle of the investigations on the low-angle mid-IR-light scattering in semiconductors. The essence of the technical idea was to apply the dark-field method for spatial filtering of the scattered light in the scanning mid-IR-laser microscope. This approach enabled the visualization of large-scale electrically active defects which are the regions enriched with ionized electrically active centers. The photoexcitation of excess carriers within a small volume located in the probe mid-IR-laser beam enabled the visualization of the large-scale recombination-active defects like those revealed in the optical or electron beam induced current methods. Both these methods of the scanning mid-IR-laser microscopy are now introduced in detail in the present paper as well as a summary of techniques used in the standard method of the lowangle mid-IR-light scattering itself. Besides the techniques for direct observations, methods for analyses of the defect composition associated with the mid-IR-laser microscopy are also discussed in the paper.Comment: 44 pages, 13 figures. A good oldi