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Charge transport optical characterization in semiconductor radiation detectors
Ridzoňová, Katarína ; Belas, Eduard (advisor) ; Toušek, Jiří (referee)
Measurements of DC photocurrent-voltage characteristic, photocurrents spectral response and laser induced transient current technique enable investigation of surface recombination centers, bulk trap levels and distribution of the inner electric field. In the presented work, the n- type planar CdZnTe detectors with quasi Ohmic contacts were studied by above mentioned methods. It has been shown that in the case of strongly absorbed light under the DC regime of illumination not only surface recombination influences the detector's transport properties. The effect of the space charge, created as a consequence of carriers trapped by the impurity levels, must be taken into account. Therefore some new theoretical models were created in order to describe measured photocurrent-voltage dependencies. Obtained data were fitted with the new theory and the mobility and surface recombination velocity for electrons were determined.
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Study of electric and photoelectric effects in organic semiconductors
Chomutová, Radka ; Toušek, Jiří (advisor) ; Pfleger, Jiří (referee)
This work studies various electrical properties of polymer layers of polyaniline, polythiophenes and poly(((benzothiadiazol)4,7-di(thien-2-yl)-5,6-difluoro-2,1,3-benzothiadiazol). Goal of this work is to determine electrical and photoelectrical properties as are electric conductivity, mobility of the free charge carriers, work function of the electrons and diffusion length of excitons. Conductivity was obtained through current and voltage contacts, mobility was measured by CELIV method (charge extraction by linearly increasing voltage). To determine work function a Kelvin probe was built and utilized. Diffusion length of excitons was determined from photovoltage spectrum formed by illuminating space charge region of the polymer.Space charge region on surface of the polymer can arise spontaneously or by laying suitable contact electrode. Powered by TCPDF (www.tcpdf.org)
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Some electrical and photoelectrical properties of organic semiconductors
Chomutová, Radka ; Toušek, Jiří (advisor) ; Křivka, Ivo (referee)
Title: Some electrical and photoelectrical properties of organics semiconductors Author: Radka Chomutová Department: Department of Macromolecular Physics Supervisor: doc. RNDr. Jiří Toušek, CSc., Department of Macromolecular Physics Abstract: The object of this study is to determine some electrical and photoelectrical properties of polymer materials and their importance considering their usage as a solar cell. Especially, thin layers of polythiophene (PT1,PT2) and poly-3-hexylthiophene (P3HT) are measured and determined. Studied parameters are Fermi level by Kelvin method, the concentration of free charge carriers, conductivity, exciton diffusion length and thickness of the space charge region by surface photovoltage method (SPV). Obtained values were then added to older measurements with similar focus for polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4- phenylenevinylene] (MEH-PPV). Keywords: sollar cell, diffusion lenght, surface photovoltage method, Kelvin probe, organic layer
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Photoluminescence of CdTe crystals
Procházka, Jan ; Hlídek, Pavel (advisor) ; Toušek, Jiří (referee) ; Oswald, Jiří (referee)
Title: Photoluminescence of CdTe crystals Author: Jan Procházka Department: Institute of Physics of Charles University in Prague Supervisor: Doc. RNDr. Pavel Hlídek, CSc. Abstract: Energy levels connected with defects in nominally undoped crystals CdTe, indium- doped crystals and chlorine-doped crystals were studied using low-temperature photoluminescence. The crystals are intended for X- and gamma- ray detectors operated at room temperature. An effect of annealing in cadmium or tellurium vapor on luminescence spectra was investigated. Some changes were interpreted by filling of vacancies not only by atoms coming from gaseous phase but also by impurities from defects like interstitials, precipitates, inclusions, grain boundaries etc. The luminescence bands assigned to defects important for compensation mechanism were examined, namely A-centers (complexes of vacancy in cadmium sublattice and impurity shallow donor) and complexes of two donors bound to a vacancy. It was shown, that temperature dependence of the luminescence bands results from more complicated processes than a simple thermal escape of bound excitons or thermal excitation of electrons (holes) from defects to bands. We observed expressive "selective pair luminescence" bands (SPL) on partially compensated In-doped samples during sub-gap...
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Research of organic layers for solar cells
Pečeňa, Rastislav ; Toušek, Jiří (advisor) ; Křivka, Ivo (referee)
Title: Research of organic layers for solar cells Author: Rastislav Pečeňa Department: Department of Macromolecular Physics Supervisor: doc. RNDr. Jiří Toušek, CSc., Department of Macromolecular Physics Abstract: The main object of this project is to determine some parameters of polymer layers PPV (poly(p-phenylene vinylene)), which are important for usage of solar cells. We will do that by measuring spectra of surface photovoltage, from which we can get particular parameters of material by their fitting: such as exciton diffusion length, thickness of space charge region (SCR), surface recombination velocity of excited electrons. The dependence shows where material is inducing the highest current density. Some measurements of organic materials enriched with nanoparticles will also take place. It is possible to find an approx. diameter of nanoparticles from spectral dependence of voltage. Keywords: solar cell, diffusion length, nanoparticles, organic layer
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User anotation of web resources using microformats
Toušek, Jiří ; Vojtáš, Peter (advisor) ; Dědek, Jan (referee)
This thesis explores the feasibility of using the principles of user-assisted annotation for semantic annotation of web resources using microformats. The main concept explored is the approach using the user assistance not to mine the data but to create a set of "annotation rules" that could then be used repeatedly to annotate the target data, without requiring further user assistance. The thesis considers multiple ways to represent these annotation rules. An important attribute of this approach is a chance these annotation rules could withstand changes in the data annotated as well as minor changes in the web resource structure. A prototype implementation is created as a part of this thesis on which these concepts are then verified.
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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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