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Transport properties of perovskites
Raja, Marek ; Belas, Eduard (advisor) ; Holovský, Jakub (referee)
This work studies charge transport in halide perovskites made of methylammonium lead tribromide CH3NH3PbBr3. By finding and using bipolar pulsation parameters, we describe the transport properties of both holes and electrons. The shapes of the measured current waveforms with the L-TCT method are simulated by the Monte Carlo simulations. Theoretical models of charge density distribution are based on a drift-diffusion equation with consideration of the infinite and finite lifetime of a charge carrier caused by a shallow and deep trap. Theobtained values ofdrift mobility, electric field profile, transit time, and surfacerecombination rate are obtained by Monte Carlo simulation. We have successfully shown the effect of pulsing with unipolarand bipolar biases. By finding thepulsation parameters at which the sample does not polarize, we calculated the hole mobility around 13 cm2 V-1 s-1 . We arrived at the ambiguity of determining the effect of the expanding deep trap region and the effect of space charge formation. Thus, we found multiple possible models to describe the measured current waveforms. This work confirms the high sensitivity of perovskites to the method and history of measurement.
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Silicon solar cells: methods for experimental study and evaluation of material parameters in advanced structures
Holovský, Jakub ; Vaněček, Milan (advisor) ; Sládek, Petr (referee) ; Hlídek, Pavel (referee)
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.
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Photovoltaic silicon solar cells: study of materials and solar structures by the method of Fourier photoconductive spectroscopy
Holovský, Jakub ; Toušek, Jiří (advisor) ; Hlídek, Pavel (referee)
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...
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Role of Urbach Energy in Photovoltaics
Vlk, Aleš ; Abelová, Lucie ; Hájková, Zdeňka ; Remeš, Zdeněk ; Holovský, Jakub ; Ledinský, Martin
Organic-inorganic halide perovskites provide new opportunities for improvement of optoelectronic device performance, especially the efficiency of solar cells. To evaluate the quality of a new material many parameters has to be taken into account. Here, we discuss one of the often overlooked semiconductor’s parameters, Urbach energy, which is an easily accessible measure of material disorder. Moreover, we present its importance on the example of organic-inorganic halide perovskites.
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Silicon solar cells: methods for experimental study and evaluation of material parameters in advanced structures
Holovský, Jakub ; Vaněček, Milan (advisor) ; Sládek, Petr (referee) ; Hlídek, Pavel (referee)
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.
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Photovoltaic silicon solar cells: study of materials and solar structures by the method of Fourier photoconductive spectroscopy
Holovský, Jakub ; Toušek, Jiří (advisor) ; Hlídek, Pavel (referee)
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...
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