A New View of Microcrystalline Silicon: The Role of Plasma Processing in Achieving a Dense and Stable Absorber Material for Photovoltaic Applications

To further lower production costs and increase conversion efficiency of thin-film silicon solar modules, challenges are the deposition of high-quality microcrystalline silicon (μc-Si:H) at an increased rate and on textured substrates that guarantee efficient light trapping. A qualitative model that explains how plasma processes act on the properties of μc-Si:H and on the related solar cell performance is presented, evidencing the growth of two different material phases. The first phase, which gives signature for bulk defect density, can be obtained at high quality over a wide range of plasma process parameters and dominates cell performance on flat substrates. The second phase, which consists of nanoporous 2D regions, typically appears when the material is grown on substrates with inappropriate roughness, and alters or even dominates the electrical performance of the device. The formation of this second material phase is shown to be highly sensitive to deposition conditions and substrate geometry, especially at high deposition rates. This porous material phase is more prone to the incorporation of contaminants present in the plasma during film deposition and is reported to lead to solar cells with instabilities with respect to humidity exposure and post-deposition oxidation. It is demonstrated how defective zones influence can be mitigated by the choice of suitable plasma processes and silicon sub-oxide doped layers, for reaching high efficiency stable thin film silicon solar cells

Křemíkové sluneční články: experimentální studium a modelování základních materiálových parametrů

Tato práce se zabývá vývojem fotoelektrických charakterizačních metod pro potřeby výzkumu v oblasti tenkovrstvých solárních článků na bázi křemíku. Abychom získali relevantní výsledky je nutné aplikovat fotovodivostní spektroskopii a měření voltampérových křivek, přímo na reálné struktury, které však mohou být mnohovrstvé, vícepřechodové a s nanostrukturovanými rozhraními. Jak analytické tak numerické optické modely jsou použity pro studium absorpce světla a výpočet absorpčního koeficientu křemíkových vrstev v oblasti pod absorpční hranou. Podle sklonu absorpční hrany a absorpce na dně zakázaného pásu pak lze posuzovat uspořádanost a množství poruch v materiálu. Na základě studia elektrické interakce dvou částí dvoupřechodového solárního článku byly vyvinuty metody pro měření fotovodivostních spekter a voltampérových křivek přímo těchto jednotlivých částí bez nutnosti jejich přímého kontaktování. Použitelnost Fourierovské fotovodivostní spektroskopie jakožto robustní metody pro měření fotovodivosti amorfního křemíku je zde podrobně analyzována. Detailně je řešena otázka frekvenční závislosti a je provedeno srovnání s fototermální deflekční spektroskopií.This work concerns with today's challenges of photoelectrical characterization methods in the research and development of thin film silicon solar cells. Relevant results are obtained only when photocurrent spectroscopy and measurement of current-voltage characteristics, are applied on the real structures that can however be multi-layered, multi-junction devices with nanostructured interfaces. Analytical and numerical optical models comprising light scattering are used for analysis of light absorption and for evaluation of optical absorption coefficient of silicon layers in sub-gap region. The slope of absorption edge and residual absorption in mid-gap indicate material disorder and defect density. Based on the investigation of electrical interaction between sub-cells in the dual-junction solar cell we developed new methods of evaluation of photocurrent spectra and current-voltage characteristics individually for each sub-cell with no need to contact them directly. Usability of Fourier Transform Photocurrent Spectroscopy as a robust method for photocurrent spectroscopy of amorphous silicon is thoroughly analyzed here. The issues of frequency dependence are addressed in detail and comparison with photothermal deflection spectroscopy is made.Faculty of Mathematics and PhysicsMatematicko-fyzikální fakult

Silicon solar cells: methods for experimental study and evaluation of material parameters in advanced structures

This work concerns with today's challenges of photoelectrical characterization methods in the research and development of thin film silicon solar cells. Relevant results are obtained only when photocurrent spectroscopy and measurement of current-voltage characteristics, are applied on the real structures that can however be multi-layered, multi-junction devices with nanostructured interfaces. Analytical and numerical optical models comprising light scattering are used for analysis of light absorption and for evaluation of optical absorption coefficient of silicon layers in sub-gap region. The slope of absorption edge and residual absorption in mid-gap indicate material disorder and defect density. Based on the investigation of electrical interaction between sub-cells in the dual-junction solar cell we developed new methods of evaluation of photocurrent spectra and current-voltage characteristics individually for each sub-cell with no need to contact them directly. Usability of Fourier Transform Photocurrent Spectroscopy as a robust method for photocurrent spectroscopy of amorphous silicon is thoroughly analyzed here. The issues of frequency dependence are addressed in detail and comparison with photothermal deflection spectroscopy is made

Photovoltaic silicon solar cells: study of materials and solar structures by the method of Fourier photoconductive spectroscopy

The use of Fourier transform infrared spectrometer for the measurement of Fourier Transform Photocurrent Spectroscopy (FTPS) has been recently reported and it is basis of this thesis. The main task of the thesis is extension of the FTPS to Surface Photovoltage (SPV) method. This method gives an unique possibility to evaluation of effective diffusion length L and effective surface recombination velocity (SRV) for thin or non-symmetrical semiconductor structures that can't be measured by standard techniques. These parameters play key role in good performance of solar cells absorbers. The experiments were carried out on c-Si material with surface passivated by silicon nitride. Sample is illuminated from the passivated side and the SPV signal is detected between the back capacitive contact and non-illuminated ohmic contact. One-dimensional continuity equation is solved to fit the measured spectrum. We have to ensure linearity of measured signal versus incident light intensity and account for frequency dependence. One measurement takes 30 seconds - that enables arbitrary variations. The influence of surface barrier height on SRV was assumed and additional constant illumination (light bias) was used to investigate its behavior and new relations were found. Beside SPV new elegant technique for interference..

Methodology of not destructive determination of boron concentration profile in Si wafers

A new method of determination of diffusion profiles in fast, not destructive and contactless (with help of infra-red reflexion) way was designed, theoretically analysed, realized either in computational and in experimental field and finally successfully tested

Methodology of not destructive determination of phosphorus concentration profile in Si wafers

A new method of determination of diffusion profiles in fast, not destructive and contactless (with help of infra-red reflexion) way was designed, theoretically analysed, realized either in computational and in experimental field and finally successfully tested

Silicon solar cells: methods for experimental study and evaluation of material parameters in advanced structures

This work concerns with today's challenges of photoelectrical characterization methods in the research and development of thin film silicon solar cells. Relevant results are obtained only when photocurrent spectroscopy and measurement of current-voltage characteristics, are applied on the real structures that can however be multi-layered, multi-junction devices with nanostructured interfaces. Analytical and numerical optical models comprising light scattering are used for analysis of light absorption and for evaluation of optical absorption coefficient of silicon layers in sub-gap region. The slope of absorption edge and residual absorption in mid-gap indicate material disorder and defect density. Based on the investigation of electrical interaction between sub-cells in the dual-junction solar cell we developed new methods of evaluation of photocurrent spectra and current-voltage characteristics individually for each sub-cell with no need to contact them directly. Usability of Fourier Transform Photocurrent Spectroscopy as a robust method for photocurrent spectroscopy of amorphous silicon is thoroughly analyzed here. The issues of frequency dependence are addressed in detail and comparison with photothermal deflection spectroscopy is made

Photovoltaic silicon solar cells: study of materials and solar structures by the method of Fourier photoconductive spectroscopy

The use of Fourier transform infrared spectrometer for the measurement of Fourier Transform Photocurrent Spectroscopy (FTPS) has been recently reported and it is basis of this thesis. The main task of the thesis is extension of the FTPS to Surface Photovoltage (SPV) method. This method gives an unique possibility to evaluation of effective diffusion length L and effective surface recombination velocity (SRV) for thin or non-symmetrical semiconductor structures that can't be measured by standard techniques. These parameters play key role in good performance of solar cells absorbers. The experiments were carried out on c-Si material with surface passivated by silicon nitride. Sample is illuminated from the passivated side and the SPV signal is detected between the back capacitive contact and non-illuminated ohmic contact. One-dimensional continuity equation is solved to fit the measured spectrum. We have to ensure linearity of measured signal versus incident light intensity and account for frequency dependence. One measurement takes 30 seconds - that enables arbitrary variations. The influence of surface barrier height on SRV was assumed and additional constant illumination (light bias) was used to investigate its behavior and new relations were found. Beside SPV new elegant technique for interference..