Laser-based Techniques and Applications - LATA
TPCI/NHRF
Diploma theses


Eleni Markou (2012)

The use of thin films in photovoltaic technology promises the decrease of the production costs and weight of solar cells, as a result of the reduction in material used. Moreover, film microscribing is the key factor for the transition of small individual solar cells, into their monolithic integration, in large area photovoltaic panels.

In this diploma thesis the production and micro processing of zinc oxide (ZnO) thin films is presented, because ZnO is the transparent, n-type semi conductive window of the photovoltaic cell. The growth of the ZnO thin film is achieved by pulsed laser deposition (PLD), on glass substrates under low pressure oxygen gas conditions. It is mentioned that in chalcopyrite-based (CIGS) solar cells configurations, which have presented the highest laboratory efficiencies until today, pure zinc oxide (ZnO) or doped with, e.g. Indium or Aluminum (ZnO:In), (ZnO:Al) , are used as windows.

First, thin films of zinc oxide (ZnO) were produced, by using either metallic Zn targets or ceramic targets In:ZnO(2%w), Al:ZnO(1.5%w), in the Laser Laboratory (LATA) of the Institute of Theoretical and Physical Chemistry at the National Hellenic Research Foundation (NHRF). For the ablation of each target, a pulsed Nd:YAG laser at 355 nm and frequency 10 Hz was used.

In order to improve the optical properties of thin ZnO films, with the purpose to further increasing the efficiency of solar cells, we investigated the gold nanoparticles deposition (Au). Several tests for different deposition conditions on glass substrates were carried out. The best conditions according to requirements (small nanoparticles, high reflectivity) were identified, and these were then applied to deposit Au nanoclusters on the ZnO/glass substrates.

All thin films were analyzed in terms of their optical and morphological properties. Specifically, the transmittance and reflectivity of all thin films were recorded, with the aid of the spectrometer (UV- VIS) of the Institute of Theoretical and Physical Chemistry at the NHRF. We also have estimated the thickness of several films theoretically. Furthermore, the morphology, topography of thin films’ surface and their roughness were investigated with the aid of an optical Microscope of NHRF and an Atomic Force Microscope of the Department of Manufacturing of the School of Mechanical Engineering at NTUA.

Moreover, optical measurements on industrial ZnO samples delivered by the photovoltaic production company “Heliosphera” were performed in order to compare them with the thin films that we deposited in the LATA laboratory. From the optical measurements, the thickness of the commercial ZnO was theoretically estimated by applying the Manifacier method: it was found that the thickness was very close to that given by the company.

Finally, the laser micro processing of the industrial ZnO sample of the “Heliosphera” company was performed. For their laser scribing, an experimental setup was developed, which consisted of a pulsed Nd:YAG laser (at 355 nm, repetition rate 10 Hz and energy 0.7- 2.0 mJ/p), various optical components (prisms, apertures, lenses) for shaping and focusing of the laser beam and a moving table where the thin films were placed. During the experiments, the effect of the energy laser, the focusing distance, the optical apertures and the velocity of the moving table on the quality and the width of the produced micro-channels, were systematically investigated. They resulted into a fully ablated ZnO thin film with a scribe channel with a width of 25-35 μm.





Vasilis Chountalas (2012)

 Thin film photovoltaic technology promises to reduce manufacturing costs of solar cell modules, in part due to reduced material usage and monolithic
series connection of cells. The latter requires 
relatively simple automation compared to the lay-out and soldering used today in wafer- based technologies.

The objective of the present work is to grow the absorber CIGSe and CIGTe thin films on soda lime glass substrates using electron beam evaporation
and to study the structural and
morphological properties of the resultant films, both as-deposited and after annealed at various temperatures.
After laser scribing, the morphology of channels was studied by Atomic Force Microscopy. Further, the LIPS technique was used to optimize focal
length for laser scribing as well as to investigate the stability of the ratios of chalcopyrite elements for various annealing temperatures.





Panagiota  Koralli (2010)

The present work deals with laser micro-machining (laser scribing) of Molybdenum (Mo) thin films (thickness 100- 390 nm) on microscope glass, see Fig.1, laser scribe P1. Such Mo thin films find application today as back electrodes in chalcopyrite-based solar cells. The growth of the Mo thin films took place in a vacuum chamber by Pulsed Laser Deposition (PLD). An excimer laser at 248 nm (KrF) was employed to ablate a Mo foil.


Fig.1: Contact film 2 corresponds to the Molybdenum thin film. 

The micro-machining of the Mo thin films was performed by the third harmonic of a Nd:YAG laser (355 nm). The repetition rate of the laser was 2 Hz and the energy density ~1.8 J/cm2. Various configurations for the beam shaping have been tested, consisting of a diaphragm, a telescope for beam collimating and a focusing lens on a movable table with micrometer resolution. The Mo thin films were placed on a XY translator, driven by stepper motors. The laser pulse energy was varied systematically and the effect on the scribing width was investigated.

A crucial question was how to set the laser beam focus (ablation area) on the thin film  in a controllable way, because this would define the scribe width as well as the number of laser pulses needed to fully ablate the Mo film. Finally, it was achieved by using a laser-induced plasma setup (LIPS). An optical fiber was placed close to the ablation region, the monochromator was set at ~569 nm (a Mo strong emission line) and the signal was observed on a fast oscilloscope. The laser pulse energy was reduced so that the plasma emission could be seen only when the focus was on the film surface. Under these conditions, only one laser shot was necessary to completely ablate the Mo thin film.




Fig 2: AFM picture of a ca 200 nm Mo thin film indicating the vertical cut profile.

The least scribe width that was achieved with this simple arrangement above was 70
μm (for film thickness ~390 nm) which is a value that coincides well with those typical obtained from literature data.
An indication of the quality of the cut is given by Atomic Force Microscopy diagnostics that was used to scan the scribe region.




Lia Georgiou (2009)

The present work deals with the study of the effects of the PLD parameters on the electric and morphological properties of metal oxide thin films, and their sensing properties.

 Initially, the basic properties and uses of the metal oxides with emphasis to the electrical properties of thin films are given. It follows the description of the common techniques for depositing thin films. The PLD setup and the parameters that effect the growth of thin films are described in detail.

 The next section of this diploma thesis is concerning with the use of gas sensors. The basic parameters of a sensor are also referred. The operation principle of gas sensors based on metal oxides is described. Next is the experimental section, where one can find details describing the growth of thin films and the procedure of electrical and morphological characterization and dynamic response test that is followed in the lab. Finally, dynamic resistivity response measurements in presence of hydrogen, carbon monoxide, or methane current in ambient air, show that sensors based on the CuxO thin-films examined can be straightforwardly implemented.
 
 
Keywords

 

Zinc Oxide (Zn), Nickel Oxide (NiO), Copper Oxides (CuxO), thin films, pulsed laser deposition (PLD), Van der Pauw, atomic force microscopy (AFM), gas sensors, semiconducting metal oxides, Methane (CH4), Carbon Monoxide (CO), Hydrogen (H2)

 
 
NOTE

The films have been grown in the LATA lab and the gases resistivity response measurements in the School of Electrical Engineering Department/National Technical University of Athens.




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