Milan Vanecek | Academy of Sciences of the Czech Republic (original) (raw)

Papers by Milan Vanecek

AIP Conference Proceedings

Vacuum, Aug 1, 2009

The design, fabrication and test of piezoresistive sensors based on nanocrystalline diamond (NCD)... more The design, fabrication and test of piezoresistive sensors based on nanocrystalline diamond (NCD) films are reported. The CoventorWare FEM calculations of the mechanical stress and geometrical deformations of a 3-D structure are used for a proper localization of the piezoresistor on the carrying substrate. The boron-doped piezoresistive sensing element was realized using a directed patterned growth of NCD film on SiO 2 /Si by microwave plasma-enhanced chemical vapour deposition (CVD). The gauge factor of borondoped NCD films was investigated in the range from room temperature up to 200 C and from 0 to 5 N of the applied force. These NCD piezoresistive sensor elements are compared with a Silicon-on-Insulator (SOI) based piezoresistive sensor and their high-temperature applications are discussed.

Thin Solid Films, Nov 1, 2010

Thin blend polymer films made of poly(3-hexylthiophene-2,5-diyl) (electron donor) and fullerene d... more Thin blend polymer films made of poly(3-hexylthiophene-2,5-diyl) (electron donor) and fullerene derivatives as electron acceptors ([6,6]-thienylC61 butyric acid methyl ester and [6,6]-thienylC71 butyric acid methyl ester) are prepared by the spin-coating technique on indium tin oxide covered glass substrates. Time-resolved photo-induced changes of surface potentials are detected by Kelvin force microscopy (KFM). Changes of surface potentials by 10-150 mV reveal different quality and kinetics of charge generation in the two blends in short (minutes) and long (hours) time periods. This is attributed to a combination of electron accumulation, trapping, and organic material degradation under ambient conditions. As KFM characterizes the blend films directly without metal contact layer, it reveals differences in the opto-electronic behavior of the blends, which are not detected by common photovoltaic cell characterization.

Physica Status Solidi a Applications and Materials Science, 2009

Fourier transform photocurrent spectroscopy is used for a fast and sensitive quality assessment o... more Fourier transform photocurrent spectroscopy is used for a fast and sensitive quality assessment of photovoltaic thin film materials and solar cells. A Fourier transform infrared (FTIR) spectrometer is utilized with a solar cell or photoconductive layer as an external detector. Experimental conditions and procedures to obtain the spectral dependence of the optical absorption coefficient or absorptance from the normalized FTIR photocurrent signal are described and the results are compared to the standard measurements of transmittance, reflectance and the constant photocurrent method data. Resulting optical data are discussed in terms of homogeneity of optical properties, defects concentration and disorder in the active layer of thin microcrystalline silicon solar cells.

physica status solidi (a), 2015

ABSTRACT We evaluate the potential of three-dimensional (3D) thin-film silicon solar cells in the... more ABSTRACT We evaluate the potential of three-dimensional (3D) thin-film silicon solar cells in the superstrate configuration deposited on hexagonally ordered arrays of ZnO nanocolumns (NCs). These nanostructures are prepared by hydrothermal growth, which is an effective and versatile method to obtain ZnO NCs of high optical and electrical quality at low temperature. For the periods P investigated, varied between 0.9 and 1.4 μm, 3D solar cells based on hydrogenated amorphous silicon (a-Si:H) exhibit a photocurrent (JSC) boost in the red wavelength range as compared to flat cells; this JSC gain (by more than 1.5 mA cm−2 for P = 0.9 μm) is explained mostly by the increased effective optical thickness of the absorber layer grown on the vertical walls of the NCs. Combining this 3D concept with randomly textured interfaces, rigorous 3D optical simulations based on the finite element method predict that photocurrents significantly higher than those obtained with state-of-the-art substrates (up to 20 mA cm−2) are within reach, if the experimental obstacles specific for 3D design are overcome.

Characteristic features in photocurrent (PC) and electron paramagnetic resonance (EPR) spectra ar... more Characteristic features in photocurrent (PC) and electron paramagnetic resonance (EPR) spectra are discussed and attributed to main defects in the gap of optical-quality chemical vapor deposited diamond. A shoulder in the PC spectra with an onset at about 2.2 eV is attributed to the single-substitutional nitrogen defect (EPR P1 resonance at g=2.0024). A second feature in the PC spectra with an onset of about 1.3 eV is observed on ``as-grown'' samples with a hydrogen terminated surface. The defect level associated with this feature is hydrogen related, and this defect disappears after oxidation of the diamond sample surface. The EPR g=2.0028, which was also suggested to be H-related, is discussed.

A new spectroscopic approach to investigate the optical absorption of organic semiconductors is i... more A new spectroscopic approach to investigate the optical absorption of organic semiconductors is introduced. This research is focussed on polymer-fullerene blends in photoactive thin films for bulk heterojunction solar cells, which have gained an established place in the wide range of organic solar cell concepts. This is due to the promising efficiencies reported up till now and their vast increase at short timescale 1. Moreover, the prospect to produce them on large scale, at low cost and on flexible support are supplemental assets of this class of photovoltaics. Solar cells with a photoactive layer of MDMO-PPV:PCBM and with regioregular P3HT:PCBM have been investigated with the brand new technique of FTPS. FOURIER-TRANSFORM PHOTOCURRENT SPECTROSCOPY (FTPS)-In FTPS, the photocurrent spectrum of the sample (solar cell) is recorded by replacing the conventional detector of the spectrometer (KBr, DTGS) by the photoconductive sample. To accomplish this the light source is directed out of the spectrometer and the photocurrent signal coming from the sample is, after A/D conversion, Fourier analysed. The FTIR signal from the sample is normalized to the FTIR signal coming from a spectrally independent deuterated triglycine sulphate (DTGS) detector. In this way the absorption spectrum can be obtained at very short timescales and with very high sensitivity, revealing sub-bandgap absorption phenomena not to be uncovered with conventional spectroscopic methods. An extensive description of the FTPS principles can be found in ref. 2. The FTPS signal for each sample has been checked for its dependence on the light intensity, employing neutral density filters. All samples showed a linear dependence, a prerequisite for the correct interpretation of the measured spectra. All FTPS spectra cover absorption data ranging from the IR into the visual spectral region, leaving out a small part around the frequency of the HeNe laser of the Michaelson spectrometer. This drawback of the FTPS technique was circumvented by applying the Constant photocurrent method (CPM) on the films in order to fill in the missing part. EXPERIMENTAL RESULTS-Solar cells were constructed conform the standard procedure described in literature 3 , with this difference that an interfacial layer of LiF between the active layer and the Al electrode was omitted in the architecture. To exclude any influence on the absorption spectrum due atmospheric O 2 and H 2 O, all cells were sealed with a thin glass cover. In fig. 1, the FTPS spectrum of a solar cell with an active layer of MDMO-PPV:PCBM (1:4) is displayed. The spectra show the absorptance of the layers (absorptance A=1-T-R, where T is transmittance and R reflectance) over a dynamical range covering nine orders of magnitude. They are matched together at the saturation region around 2.5 eV. The band-to-band transitions of the constituting compounds are clearly observed. The absorption peak at 2.30 eV can be assigned to the π→π * transition in MDMO-PPV. The distinct peak at 1.77 eV is due to the orbitally forbidden HOMO-LUMO transition (h u →t 1u) in PCBM, analogous to the forbidden transition in C 60 at approximately the same energy. The symmetry breaking originates from a Herzberg-Teller distortion, characteristic for C 60 and methanofullerenes. The broad absorption band between 1.77 eV and 2.30 eV is due to the transition from HOMO and HOMO-1 to higher excited states 4. A remarkably broad fall-off in the optical absorption below 1.77 eV is present in all spectra, and ends in an exponential, Urbach like decay below 1.40 eV and extends to 1.2 eV. Below this energy supplemental sub-bandgap absorption features could be observed. INTERPRETATION-To account for the observed Urbach like tail, two possible explanations are proposed : 1) The absorption spectrum of conjugated polymers, including MDMO-PPV exhibit a Gaussian absorption profile, tailing of in an exponential decay at lower energies 5,6. The origin of the Urbach tail in organic semiconductors has been considered in literature and is explained in terms of absorption due to selftrapped excitons, a consequence of the strong exciton-phonon coupling present in these materials 7,8. Adding PCBM to the polymer matrix, which acts as a charge transfer centre, would enhance the chance of exciton dissociation significantly, prevent self-trapping and elevate the photocurrent production at the sub-band gap level. This mechanism could account for the broadening of the absorption spectrum below 1.77 eV and the shift of the exponential decay to start at 1.40 eV. 2) The creation of a charge transfer complex (CTC) should be considered. Wang et. al. proved that complexation between fullerenes and N-polyvinylcarbazol (PVK) is the crucial step in the creation of

MRS Proceedings, 1987

Recent Constant Photocurrent Method (CPM) results and a new model of a-Si:H density of states are... more Recent Constant Photocurrent Method (CPM) results and a new model of a-Si:H density of states are briefly reviewed. The small but systematic changes of subgap absorption (α) with the applied external stress have been found. When light-soaking is done under the applied stress the strong increase of α is observed. The μτ (mobility-lifetime) versus α duality is demonstrated. The increase of the dangling bond density, combined with the change of its charge state, is used for the explanation of duality. The light induced creation of dangling bond-impurity intimate pairs is speculated to be the driving force for the Fermi level shift.

MRS Proceedings, 1996

We measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurre... more We measure subgap absorption on n-type amorphous silicon using the “absolute” constant photocurrent method. We find that for typical monochromator probe beam intensities the measurement is not significantly influenced by lifetime changes. When the measurement is performed under light bias, an apparent increase in the defect absorption coefficient is observed, but no change in the photoexcitation threshold or spectral shape of the absorption band is seen. We show that this increase is likely due to a bias-light amplification of spectrally dependent lifetime changes. Our measurements suggest a larger electron capture cross section of positive valence band tail states compared to neutral dangling bonds.

Semiconductors and Semimetals, 2003