Vladimir Smirnov

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (28)33.12 Total impact

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    ABSTRACT: We summarize an extensive study on the impact of absorber layer defect density on the performance of amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon solar cells. To study the effects of the absorber layer defect density we subjected set of a-Si:H and μc-Si:H cells to a 2 MeV electron bombardment. Subsequently the cells were stepwise annealed to vary the defect density. The cells have varying thicknesses and are illuminated from either the p- or n-side. For reference we subjected i-layers to the same treatment as the cells. The procedure enabled the reversible increase of the i-layer defect density (Ns) with two orders of magnitude according to electron spin resonance measurements (ESR) performed on reference samples. The large variation of Ns induces substantial changes in the current-voltage characteristics (J-V) and the external quantum efficiency spectra (EQE). These changes in device characteristics provide a solid reference for analysis and device simulations. It was found that performance of a-Si:H cells degraded weakly upon Ns increase up to 10^17 cm -3 and dropped steeply as defect density was increased further. In contrast, performance of μc-Si:H cells showed continuous reduction as Ns raised. By comparing p- and n-side illuminated cells we found that, for Ns above 10^17 cm-3, the p-side illuminated a-Si:H cells outperformed the n-side illuminated ones, however, the difference was barely visible at Ns below 10^17 cm-3. On the contrary, the device performance of n-side illuminated μc-Si:H cells was much more affected by the increase in defect density, as compared to the p-side illuminated cells. EQE results evidenced a significant asymmetry in collection of electrons and holes in μc-Si:H devices, where carrier collection was limited by holes as defect density was increased. Based on the experimental data we speculate that the improvement of absorber material in terms of as-deposited defect density is not of primary importance for the performance of a-Si:H cells, whereas in μc-Si:H based solar cells, the reduction of the absorber layer defect density below the state-of-the-art levels, seems to improve the cell performance.
    Solar Energy Materials and Solar Cells 10/2014; 129:17-31. · 5.03 Impact Factor
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    ABSTRACT: Hydrogenated amorphous silicon thin film tandem solar cells (a-Si:H/a-Si:H) have been developed with focus on high open-circuit voltages for the direct application as photocathodes in photoelectrochemical water splitting devices. By temperature variation during deposition of the intrinsic a-Si:H absorber layers the band gap energy of a-Si:H absorber layers, correlating with the hydrogen content of the material, can be adjusted and combined in a way, that a- Si:H/a-Si:H tandem solar cells provide open-circuit voltages up to 1.87 V. The applicability of the tandem solar cells as photocathodes was investigated in a photoelectrochemical cell (PEC) measurement set-up. With platinum as a catalyst, the a-Si:H/a-Si:H based photocathodes exhibit a high photocurrent onset potential of 1.76 V vs. the reversible hydrogen electrode (RHE) and a photocurrent of 5.3 mA/cm2 at 0 V vs. RHE (under halogen lamp illumination). Our results provide evidence that a direct application of thin film silicon based photocathodes fulfill the main thermodynamic requirements to generate hydrogen. Furthermore, the presented approach may provide an efficient and low-cost route to solar hydrogen production.
    International Journal of Photoenergy 06/2014; · 2.66 Impact Factor
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    ABSTRACT: Solution-based semiconductors give rise to the next generation of thin-film electronics. Solution-based silicon as a starting material is of particular interest because of its favorable properties, which are already vastly used in conventional electronics. Here, the application of a silicon precursor based on neopentasilane for the preparation of thin-film solar cells is reported for the first time, and, for the first time, a performance similar to conventional fabrication methods is demonstrated. Because three different functional layers, n-type contact layer, intrinsic absorber, and p-type contact layer, have to be stacked on top of each other, such a device is a very demanding benchmark test of performance of solution-based semiconductors. Complete amorphous silicon n-i-p solar cells with an efficiency of 3.5% are demonstrated, which significantly exceeds previously reported values.
    Advanced Energy Materials 03/2014; · 14.39 Impact Factor
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    ABSTRACT: Metastability effects because of atmospheric exposure, high purity gasses, and deionized water in hydrogenated microcrystalline silicon thin films with different crystalline volume fractions were studied using well accepted steady-state characterization methods of dark conductivity, steady-state photoconductivity, steady-state photocarrier grating (SSPG) and dual beam photoconductivity (DBP) methods. A standard measurement procedure has been established before using the steady state methods, in which a steady state condition of dark conductivity was established by monitoring the time dependence of dark conductivity. Samples deposited on smooth glass and rough glass substrates exhibit similar reversible and irreversible changes in the properties of microcrystalline silicon film. A reliable correlation of reversible and irreversible changes indicate that dark conductivity and photoconductivity values increase, sub-bandgap absorption spectrum obtained from DBP method decrease and correspondingly minority carrier diffusion lengths obtained from the SSPG method increase in the metastable state in various amount for microcrystalline films with crystalline volume fraction, ICRS > 0.30. Amorphous silicon and microcrystalline silicon films with ICRS < 0.30 do not show detectable metastable changes as samples exposed to atmospheric condition as well as high purity oxygen gas and deionized water.
    Canadian Journal of Physics 02/2014; · 0.90 Impact Factor
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    ABSTRACT: Metastability effects in hydrogenated microcrystalline silicon thin films due to air, high purity nitrogen, helium, argon, and oxygen were investigated using temperature-dependent dark conductivity, photoconductivity, and steady-state photocarrier grating methods. It was found that short-term air, nitrogen, and inert gases caused a small reversible increase of �Dark and �photo within a factor of two, but they did not affect the minority carrier ��-products significantly. These changes are partially reduced by vacuum treatment and completely reduced after heat treatment at 430 K. However, oxygen gas treatment at 80 °C resulted in more than an order of magnitude increase in both �Dark and �photo and an increase in the diffusion length, LD, by 50% from that of the annealed-state value in highly crystalline samples, while no significant metastability is detected in amorphous and low crystalline silicon thin films. A following heat treatment partially recovers both �Dark and �photo to their annealed-state values, while LD decreases only slightly. Such increase in the LD values could be due to a decrease in the density of recombination centers for holes below the Fermi level, which may be related to passivation of defects by oxygen on the surface of crystalline grains.
    Canadian Journal of Physics 02/2014; · 0.90 Impact Factor
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    ABSTRACT: Theoretically predicted values of the open circuit voltage (VOC) for a-Si:H or μc-Si:H based solar cells are substantially higher than the values achieved in of state-of-the-art devices. Fundamentally, open circuit voltage is determined by generation-recombination kinetics, where recombination is often controlled by the defect density in the absorber layer of a solar cell. The latter aspect is the focus of the paper. The relation between the VOC and the bulk recombination in the absorber layer is addressed in experiment by varying the defect density. The absorber layer defect density (spin density, NS, monitored with ESR) in a-Si:H and μc-Si:H solar cells was varied over two orders of magnitude using a 2 MeV electron bombardment and successive stepwise annealing. The results of the electron bombardment experiment are analyzed with respect to the illumination intensity dependency of the VOC, measured for the same set of a-Si:H and μc-Si:H solar cells.Wefind that the VOC of a-Si:H solar cells is not limited by defects in the bulk of the absorber layer, even at relatively high defect density up to 3–5 × 1016 cm-3 and, therefore, other limiting mechanisms have to be identified to improve voltage in these devices. In contrast, μc-Si:H solar cells show nearly classical VOC–NS relation. The bulk defect density in μc-Si:H absorber layer is thus likely the key limiting factor for VOC in these devices at present status of material quality (NS of 3–7 × 1015 cm-3). Further optimization of μc-Si:H in terms of bulk defect density is highly relevant for VOC improvement in solar cells.
    Canadian Journal of Physics 01/2014; 92(7/8):905-908. · 0.90 Impact Factor
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    Steve Reynolds, Aad Gordijn, Vladimir Smirnov
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    ABSTRACT: Microcrystalline silicon thin film solar cells exhibit optimal PV efficiency when the absorber layer contains similar proportions of crystalline and amorphous phases. When the crystalline fraction is reduced below 30%, efficiency falls very steeply, from around 8% to as low as 2%, and does not recover until fully amorphous growth conditions are established. We demonstrate that an electrical model, comprising two parallel-connected diodes scaled to reflect material composition, qualitatively predicts the features observed in the PV parameters. However the scale of the reduction in fill-factor is not reproduced. As an alternative approach, a homogeneous transport model is proposed in which carrier mobilities are scaled in accordance with values determined by the time-of-flight experiment. This model predicts a large reduction in fill-factor for low-crystallinity absorbers more in keeping with measurement. A novel carrier transport landscape is proposed to account for mobility variations.
    Energy Procedia 01/2014; 44:192–202.
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    ABSTRACT: deposited by very high frequency plasma enhanced chemical vapour deposition on smooth glass substrates were investigated using temperature-dependent dark conductivity, steady state photoconductivity, and sub-bandgap absorption measurements obtained using the dual beam photoconductivity (DBP) method. No significant changes in dark conductivity and photoconductivity were detected even after long-term air exposure of samples in room ambient as well as after oxygen exposure when samples were characterized in oxygen ambient. However, characterization of the oxygen-exposed state in high vacuum caused an increase in dark conductivity and photoconductivity as well as a significant decrease in the sub-bandgap absorption coefficient spectra in the low energy region in samples with ICRS � 0.40. These changes are partially irreversible for samples ICRS � 0.80 and mostly reversible for compact materials with significant amorphous fraction. No detectable metastable changes occurred in microcrystalline silicon samples with IC RS � 0.40 as well as in pure amorphous silicon.
    Canadian Journal of Physics 11/2013; · 0.90 Impact Factor
  • Proceedings of the 28st European Photovoltaic Solar Energy Conference; 01/2013
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    ABSTRACT: In this work, we investigate the light trapping of thin-film silicon solar cells which apply plasmonic Ag back contacts with non-ordered Ag nanostructures. The preparation, characterization and three-dimensional electromagnetic simulations of these back contacts with various distributions of non-ordered Ag nanostructures are presented. The measured reflectance spectra of the Ag back contacts with non-ordered nanostructures in air are well reproduced in reflectance spectra derived from the three-dimensional electromagnetic simulations of isolated nanostructures on Ag back contacts. The light–matter interaction of these nanostructures is given by localized surface plasmons and, thus, the measured diffuse reflectance of the back contacts is attributed to plasmon-induced light scattering. A significant plasmonic light-trapping effect in n-i-p substrate-type μc-Si:H thin-film solar cell prototypes which apply a Ag back contact with non-ordered nanostructures is identified when compared with flat reference solar cells.
    Materials Science and Engineering: B. 10/2012;
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    ABSTRACT: The focussed beam of a low-power helium–neon laser is used to study accelerated light-induced degradation (Staebler–Wronski effect) and high steady-state photocarrier generation rates in amorphous and microcrystalline silicon thin-film solar cells, at up to 13 MW m− 2 irradiance. Even at these high power densities, COMSOL® simulations indicate that heat diffusion into the substrate, aided by spreading conduction via the Ag back-contact, restricts the temperature rise to less than 14 °C. Short-circuit current may be measured directly, and the I–V characteristic estimated by taking into account shunting by the inactive part of the cell. The improved resistance to degradation of microcrystalline silicon cells is shown to persist to high irradiance. Computer simulations of an amorphous silicon solar cell are presented that are consistent with measured un-degraded and degraded properties, and offer insight into prevailing defect creation processes and carrier recombination mechanisms.
    Journal of Non-Crystalline Solids 09/2012; 358(17):2202–2205. · 1.72 Impact Factor
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    ABSTRACT: Metastability effects in amorphous and microcrystalline silicon thin films induced by exposure to atmospheric gases and water are investigated. A simple procedure is described which allows studying such effects in a reproducible and reliable manner on a short time scale. The method is applied to thin film silicon materials with different structure composition ranging from amorphous to highly crystalline. It is shown that the materials can be brought back into a well defined state even after pro-longed and repeated degradation cycles.
    Japanese Journal of Applied Physics 07/2012; · 1.07 Impact Factor
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    ABSTRACT: Metastability effects in microcrystalline silicon (μc-Si:H) thin films have been investigated using dark con-ductivity, σ D , photoconductivity, σ ph , and sub-bandgap absorption methods. Nitrogen and inert gasses can cause reversible aging effect in conductivities but not in the sub-bandgap absorption. However, DI water and O 2 gas treatment result in both reversible and nonreversible effects in conductivities as well as in the sub-bandgap absorption. Only oxygen affected the dark conductivity reversibly in amorphous silicon, a-Si: H, films, other results were unaffected from the aging and annealing processes applied. © 2012 Published by Elsevier B.V.
    Journal of Non-Crystalline Solids 01/2012; · 1.72 Impact Factor
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    ABSTRACT: Metastability effects in amorphous and microcrystalline silicon thin films induced by exposure to atmospheric gases and water are investigated. A simple procedure is described which allows studying such effects in a reproducible and reliable manner on a short time scale. The method is applied to thin film silicon materials with different structure composition ranging from amorphous to highly crystalline. It is shown that the materials can be brought back into a well defined state even after pro-longed and repeated degradation cycles.
    Japanese Journal of Applied Physics 01/2012; 51(51, page 070210). · 1.07 Impact Factor
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    ABSTRACT: N-type hydrogenated microcrystalline silicon oxide (μc-SiOx:H) layers were used as window layers in n-side illuminated microcrystalline silicon n–i–p solar cells. Optical, electrical and structural properties of μc-SiOx:H films were investigated by Photothermal Deflection Spectroscopy, conductivity and Raman scattering measurements. μc-SiOx:H layers were prepared over a range of carbon dioxide (CO2) flow and film thickness, and the effects on the solar cell performance were investigated. By optimising the μc-SiOx:H window layer properties, an improved short-circuit current density of 23.4 mA/cm2 is achieved, leading to an efficiency of 8.0% for 1μm thick absorber layer and Ag back contact. The correlation between cell performance and μc-SiOx:H layer properties is discussed. The results are compared to the performance of solar cells prepared with alternative optimised window layers. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2010; 7(3‐4):1053 - 1056.
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    ABSTRACT: Steady-state photoconductivity measurements have been carried out on thin-film silicon pin structures of i-layer thickness typically 4 μm, where crystalline composition has been varied by adjustment of the silane concentration in the process gas. In amorphous and low-crystallinity cells, strongly-absorbed light incident from the p-side at photon fluxes in excess of 1014 cm-2 s-1 produces strongly sub-linear intensity dependence, ‘S’ shaped reverse current-voltage curves and amplification of a second weakly-absorbed beam, termed photogating. These effects are linked to the formation of space charge and attendant low-field region close to the p-i interface, as confirmed by computer simulation. More crystalline devices exhibit little or no such behaviour. At lower intensities of strongly-absorbed light there is a markedly steeper increase in reverse current vs. voltage in low-crystalline when compared to amorphous cells, particularly with light incident from the n-side. This suggests the mobility-lifetime product for holes is much larger in the former case, consistent with the higher hole mobilities reported in time of flight studies. Thus the prospect of composition-dependent changes in mobility as well as defect density should be borne in mind when developing materials for application in microcrystalline silicon solar cells. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
    physica status solidi (c) 03/2010; 7(3‐4):505 - 508.
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    ABSTRACT: Electronic transport and paramagnetic defects detected by Electron Spin Resonance (ESR) in both intrinsic and - type silicon oxide prepared by PECVD were investigated. The structure and alloy composition of the material were varied all the way from microcrystalline silicon (µc-Si:H) to amorphous silicon oxide (a-SiOX:H). The transition- phase-mixture material is called “microcrystalline silicon oxide” (µc-SiOX:H). In undoped samples we find a strong reduction of the dark conductivity from 10-3 to 10-12 S/cm and an increase of the spin density from1017 to 3×1019 cm-3 as the crystallinity decreases from 80% to 0%. The varia- tion of the dark conductivity in phosphorous doped samples was even higher from 101 to 10-12 S/cm. ESR spectra of in- trinsic material consist of a single featureless line with g- values in the range of 2.0043…2.005 depending on the structure and alloying. The spectra of -type material exhibit a broader range of g-values of 1.998…2.0043 due to strong variations of th
    physica status solidi (c) 02/2010; C7(3-4):1053-1056.
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    ABSTRACT: The development of p-type and front ZnO layers, being window layers in n-i-p solar cells, is considered in this work. Electrical and optical properties of these layers were investigated on glass substrates, subject to thickness and doping variations. Subsequently, the effects of p-layer thickness and doping and front ZnO thickness on solar cell performance were studied. The optimal condi- tions for concerned layers are obtained, and discussed in terms of the built-in voltage, potential barriers and total reflection.
    physica status solidi (c) 01/2010; 7:1069-1072.

Publication Stats

7 Citations
33.12 Total Impact Points

Institutions

  • 2009–2014
    • Forschungszentrum Jülich
      • Photovoltaics (IEK-5)
      Jülich, North Rhine-Westphalia, Germany
    • University of Dundee
      Dundee, Scotland, United Kingdom
  • 2010
    • Zhengzhou University
      • School of Physical Engineering
      Cheng, Henan Sheng, China
  • 2002–2003
    • Abertay University
      Dundee, Scotland, United Kingdom