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Dive into the research topics where P. Fojtı́k is active.

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Featured researches published by P. Fojtı́k.


Solar Energy Materials and Solar Cells | 2003

Basic features of transport in microcrystalline silicon

J. Kočka; A. Fejfar; H. Stuchlíková; J. Stuchlík; P. Fojtı́k; T. Mates; Bohuslav Rezek; K. Luterová; Vladimir Švrček; I. Pelant

Charge transport in microcrystalline silicon is strongly influenced by its heterogeneous microstructure composed of crystalline grains and amorphous tissue. An even bigger effect on transport is their arrangement in grain aggregates or possibly columns, separated by grain boundaries, causing transport anisotropy and/or depth profile of transport properties. We review special experimental methods developed to study the resulting transport features: local electronic studies by combined atomic force microscopy, anisotropy of conductivity and diffusion length and also their thickness dependence. A simple model based on the concept of changes of transport path for description of the observed phenomena is reviewed and its consequences for charge collection in microcrystalline based solar cells are discussed.


Journal of Non-crystalline Solids | 2002

Model of transport in microcrystalline silicon

J. Kočka; H. Stuchlíková; J. Stuchlík; Bohuslav Rezek; T. Mates; Vladimir Švrček; P. Fojtı́k; I. Pelant; A. Fejfar

Abstract Large complexity of microstructure in hydrogenated microcrystalline silicon and existence of at least two different sizes of crystallites is demonstrated by combined atomic force microscope topography/local current map. We correlate activation energy and prefactor of the simplest transport property – dark conductivity, measured parallel to the substrate – with the crystallinity and roughness in wide range of microcrystalline silicon samples. This allowed us to formulate a simple model of transport based on the idea that, contrary to small grains, the formation of their aggregates (large grains/columns) dramatically changes the mechanism of transport from band like to hopping.


Journal of Applied Physics | 2001

Transport anisotropy in microcrystalline silicon studied by measurement of ambipolar diffusion length

Vladimir Švrček; I. Pelant; J. Kočka; P. Fojtı́k; Bohuslav Rezek; H. Stuchlíková; A. Fejfar; J. Stuchlík; A. Poruba; J. Toušek

We have studied charge transport anisotropy in microcrystalline silicon (μc-Si:H) by comparing diffusion length measured parallel to the substrate by steady stage photocarrier grating and perpendicular to the substrate by surface photovoltage method (SPV). We have developed a SPV evaluation procedure which allowed us to exclude the effect of light scattering at the naturally rough surface of the μc-Si:H. The procedure allows us to deduce not only the diffusion length, but also the depth of the depletion layer at the surface and recombination coefficients at both top and bottom interfaces of the film. With growing μc-Si:H film thickness the size of the crystallites increases, leading to higher roughness and thus also light scattering. At the same time density of grain boundaries decreases, resulting in an increase of the diffusion length and of the surface depletion layer depth. For all samples the diffusion length perpendicular to the substrate was several times higher than the diffusion length parallel to it, clearly confirming previous indication of the transport anisotropy resulting from the measurements of coplanar and sandwich conductivity.


Journal of Non-crystalline Solids | 2002

Role of grains in protocrystalline silicon layers grown at very low substrate temperatures and studied by atomic force microscopy

T. Mates; A. Fejfar; Ivo Drbohlav; Bohuslav Rezek; P. Fojtı́k; K. Luterová; J. Kočka; Christian Koch; M.B. Schubert; M. Ito; Kazuyoshi Ro; H. Uyama

Abstract We have investigated the role of the sample thickness and silane dilution on the structure and electronic properties of protocrystalline silicon thin films deposited at very low substrate temperatures (∼80 ° C ) . Coincidence of the maxima in surface roughness and ambipolar diffusion length ( ≳100 nm ) with formation of the network of interconnected crystalline grain aggregates was observed. While the presence of the isolated grain aggregates improves the photoconductive properties before the percolation threshold is reached, further increase in crystallinity may have opposite effect due to detrimental role of increasing concentration of the defective grain boundaries.


Applied Physics Letters | 2001

Amorphous/microcrystalline silicon superlattices—the chance to control isotropy and other transport properties

J. Kočka; J. Stuchlík; H. Stuchlíková; Vladimir Švrček; P. Fojtı́k; T. Mates; K. Luterová; A. Fejfar

Preparation of amorphous silicon/microcrystalline silicon superlattices allowed us a systematic study of transition from isotropic amorphous silicon to microcrystalline silicon with anisotropic (columnar) microstructure. The fact that just a few nm of amorphous interlayers are sufficient to interrupt columnar growth of crystallites is reflected in a clearly demonstrated isotropy of transport properties of all superlattice samples. Values of dark conductivity and diffusion length as well as grain size vary with changing crystallinity and so we can tailor the properties of the resulting material by adjusting thicknesses of amorphous and microcrystalline layers repeated to achieve a total desired thickness. Properly selected design of superlattice can lead to transport properties more suitable for solar cells than with pure microcrystalline silicon.


Philosophical Magazine Part B | 2002

Rapid crystallization of amorphous silicon at room temperature

P. Fojtı́k; K. Dohnalová; T. Mates; J. Stuchlík; I. Gregora; Jindřich Chval; A. Fejfar; J. Kočka; I. Pelant

Abstract A way in which thin films of hydrogenated amorphous silicon (a-Si: H) can be instantaneously crystallized at room temperature is reported. The metal-induced solid-phase crystallization (MISPC) method with nickel surface coverage is used. In comparison with previous reports on the MISPC of a-Si: H, the crystallization temperature is reduced by more than 350°C. This is achieved by introducing two novel technological steps: firstly, we use hydrogen-rich a-Si: H films (hydrogen content between 20 and 45at.% H) and, secondly, we apply a high transverse electric field. Polycrystalline silicon islands as large as 3 mm across appear instantaneously after having reached a threshold electric field of about 105Vcm−1. We report macroscopic visualization of the crystallization process as well as microscopic investigation (micro-Raman measurements and scanning electron microphotography) of the crystallized films. We have found that appropriate patterning of the nickel electrode helps to increase homogeneity of the resulting polycrystalline silicon.


Solar Energy Materials and Solar Cells | 2001

Charge transport in microcrystalline Si – the specific features

J. Kočka; A. Fejfar; P. Fojtı́k; K. Luterová; I. Pelant; Bohuslav Rezek; H. Stuchlíková; J. Stuchlík; Vladimir Švrček

Abstract The microcrystalline hydrogenated silicon has one clear advantage – stability against light-induced changes – but rather complicated growth and microstructure. As a result there are many problems and specific features of transport in microcrystalline silicon. These features – like inhomogeneity, anisotropy, influence of substrate and thickness dependence are discussed in detail.


Philosophical Magazine Part B | 2000

Visible photoluminescence and electroluminescence in wide-bandgap hydrogenated amorphous silicon

K. Luterová; I. Pelant; P. Fojtı́k; M. Nikl; I. Gregora; J. Kočka; J. Dian; Jan Valenta; P. Malý; J. Kudrna; J. Štěpánek; A. Poruba; P. Horváth

Abstract The work describes basic photoluminescence (PL) and electroluminescence (EL) properties of wide-bandgap (2.0 eV or greater) hydrogenated amorphous silicon (a-Si: H). Thin films of wide-bandgap (high-hydrogen-content) a-Si: H were prepared by microwave electron cyclotron resonance plasma-enhanced chemical vapour deposition from SiH4 under high dilution with He. The films exhibit spectrally broad (full width at half-maximum, 0.4 eV or greater) visible PL at room temperature, peaked at about 1.5eV. On the basis of measurements of the PL temperature dependence, PL dynamics, infrared absorption spectra, picosecond pump-and-probe experiments and hydrogen thermal desorption spectroscopy the dominant microscopic mechanism of visible PL has been revealed to be the radiative de-excitation of oligosilane (-(SiH2)2-) units or of a specific defect in their close vicinity. EL has been investigated in sandwich p+‒i‒n+ and p+‒p‒n‒n+ structures with (Cr‒Ni)/indium tin oxide contacts. The as-grown structures exhibit good rectifying properties, low conductivity and no EL. After being subjected to a ‘forming’ procedure (application of a sufficiently high voltage), the current flowing through the structures increases abruptly by several orders of magnitude, up to about 10 mA, and the structures begin to emit weak EL at room temperature. The EL occurs in reverse bias only and its external quantum efficiency is about 10−5%. It is concluded that the forming procedure leads to partial crystallization of a-Si: H between the contacts and impact lattice ionization can participate in the light emission.


Journal of Non-crystalline Solids | 2002

Importance of the transport isotropy in μc-Si:H thin films for solar cells deposited at low substrate temperatures

Vladimir Švrček; A. Fejfar; P. Fojtı́k; T. Mates; A. Poruba; H. Stuchlíková; I. Pelant; J. Kočka; Y Nasuno; Michio Kondo; Akihisa Matsuda

The influence of the substrate temperature during μc-Si:H deposition on the material structure and optoelectronic properties was explored in the range from 150 to 350 °C. The low temperature material is especially interesting with regard to the suppressed oxygen-related donor formation and high efficiency of the resulting solar cells. Surprisingly, in this material, conductivity is the same parallel and perpendicular to the substrate (as measured by dc and ac techniques). The same is true for the ambipolar diffusion length which was measured by steady-state photocarrier grating (SSPG) (L∥) and by surface photovoltage (SPV) (L⊥) methods. Finally, the relevance of the SPV method extended to the measurement in complete thin film solar cell structures is demonstrated.


Surface Science | 2003

Photon emission from polycrystalline Ag induced by scanning tunneling microscopy: comparison of different tip materials

P. Fojtı́k; Karen Perronet; I. Pelant; Jindřich Chval; Fabrice Charra

Abstract Photon emission from polycrystalline silver induced by scanning tunneling microscopy is studied for three different tip materials (Au, PtIr and W). Photon emission intensity curves as a function of the tip voltage are observed to be almost identical for platinum–iridium alloy and gold tips (and more than 10 times enhanced as compared with the tungsten tip). An evolution in topography and photon map for different applied voltages is investigated along with the study of the spatial distribution of photon emission in dependence upon the surface local differential height. It turns out that no clear correlation between a local curvature and enhancement of light emission can be found. Simultaneous measurements of tunneling current and photon intensity as a function of vertical tip displacement confirm the earlier observation, namely, that similar apparent barrier heights exist for both elastic and inelastic tunneling channels. The role of the tip material as well as its shape is discussed.

Collaboration


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I. Pelant

Academy of Sciences of the Czech Republic

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J. Kočka

Academy of Sciences of the Czech Republic

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A. Fejfar

Academy of Sciences of the Czech Republic

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K. Luterová

Academy of Sciences of the Czech Republic

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H. Stuchlíková

Academy of Sciences of the Czech Republic

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T. Mates

Academy of Sciences of the Czech Republic

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J. Stuchlík

Academy of Sciences of the Czech Republic

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Vladimir Švrček

National Institute of Advanced Industrial Science and Technology

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Bohuslav Rezek

Czech Technical University in Prague

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A. Poruba

Brno University of Technology

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