Vladimir Smirnov

Forschungszentrum Jülich, Jülich, North Rhine-Westphalia, Germany

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Publications (42)65.08 Total impact

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    ABSTRACT: Direct solar-to-hydrogen conversion via water splitting was demonstrated in an integrated photovoltaic–electrochemical (PV–EC) device using a hydrogenated amorphous silicon thin film tandem junction (a-Si:H/a-Si:H) solar cell as photocathode. The solar cell was adapted to provide sufficient photovoltage to drive both the hydrogen and oxygen evolution reactions. The best results, in terms of photoelectrochemical stability and performance, were obtained with an Ag/Pt layer stack as H2 evolving photocathode back contact and with a RuO2 counter electrode for O2 evolution. Under irradiation by simulated sunlight (AM 1.5 spectrum with 100 mW/cm2), we achieved 6.8% solar-to-hydrogen efficiency at 0 V applied bias in a two-electrode set-up. This sets a fresh benchmark for integrated thin film silicon tandem based photoelectrochemical devices. In addition, the photovoltage at constant current (−3 mA/cm2) was measured over a prolonged period of time and revealed an excellent chemical stability (operation over 50 h) of the photocathode. Furthermore, we present an empirical serial circuit model of the PV–EC device, in which the corresponding photovoltaic and electrochemical components are decoupled. This allows for a detailed comparison between the solar cell and the PV–EC cell characteristics, from which the relevant loss processes in the overall system could be identified. The model was further used to compare calculated and measured photocurrent–voltage characteristics of the investigated PV–EC device which showed excellent agreement.
    Solar Energy Materials and Solar Cells 09/2015; 140. DOI:10.1016/j.solmat.2015.04.013 · 5.03 Impact Factor
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    ABSTRACT: Reducing the optical losses and increasing the reflection while maintaining the function of doped layers at the back contact in solar cells are important issues for many photovoltaic applications. One approach is to use doped microcrystalline silicon oxide (μc-SiOx:H) with lower optical absorption in the spectral range of interest (300 nm to 1100 nm). To investigate the advantages, we applied the μc-SiOx:H n-layers to a-Si:H single junction solar cells. We report on the comparison between amorphous silicon (a-Si:H) single junction solar cells with either μc-SiOx:H n-layers or non-alloyed silicon n-layers. The origin of the improved performance of a-Si:H single junction solar cells with the μc-SiOx:H n-layer is identified by distinguishing the contributions because of the increased transparency and the reduced refractive index of the μc-SiOx:H material. The solar cell parameters of a-Si:H solar cells with both types of n-layers were compared in the initial state and after 1000 h of light soaking in a series of solar cells with various absorber layer thicknesses. The measurement procedure for the determination of the solar cell performance is described in detail, and the measurement accuracy is evaluated and discussed. For an a-Si:H single junction solar cell with a μc-SiOx:H n-layer, a stabilized efficiency of 10.3% after 1000 h light soaking is demonstrated. Copyright © 2015 John Wiley & Sons, Ltd.
    Progress in Photovoltaics Research and Applications 05/2015; DOI:10.1002/pip.2629 · 9.70 Impact Factor
  • physica status solidi (RRL) - Rapid Research Letters 04/2015; 9(4). DOI:10.1002/pssr.201570620 · 2.34 Impact Factor
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    ABSTRACT: We present a nanoimprint based approach to achieve efficient light management for solar cells on low temperature transparent polymer films. These films are particularly low-priced, though sensitive to temperature, and therefore limiting the range of deposition temperatures of subsequent solar cell layers. By using nanoimprint technology, we successfully replicated optimized light trapping textures of etched high temperature ZnO:Al on a low temperature PET film without deterioration of optical properties of the substrate. The imprint-textured PET substrates show excellent light scattering properties and lead to significantly improved incoupling and trapping of light in the solar cell, resulting in a current density of 12.9 mA/cm2, similar to that on a glass substrate. An overall efficiency of 6.9% was achieved for a flexible thin-film silicon solar cell on low cost PET substrate. (© 2015 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)
    physica status solidi (RRL) - Rapid Research Letters 03/2015; 9999(9999). DOI:10.1002/pssr.201510040 · 2.34 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 12/2014; 44:192–202. DOI:10.1016/j.egypro.2013.12.027
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    ABSTRACT: Thin film silicon tandem junction solar cells based on amorphous silicon (a-Si:H) and microcrystalline silicon (µc-Si:H) were developed with focus on high open-circuit voltages for the application as photocathodes in integrated photoelectrochemical cells for water electrolysis. By adjusting various parameters in the plasma enhanced chemical vapor deposition process of the individual µc-Si:H single junction solar cells, we showed that a-Si:H/µc-Si:H tandem junction solar cells exhibit open-circuit voltages over 1.5 V with solar energy conversion efficiencies of 11% at a total silicon layer thickness below 1 µm. Our approach included thickness reduction, controlled SiH4 profiling, and incorporation of intrinsic interface buffer layers. The applicability of the tandem devices as photocathodes was evaluated in a photoelectrochemical cell. The a-Si:H/µc-Si:H based photocathodes exhibit a photocurrent onset potential of 1.3 V vs. RHE and a short-circuit photocurrent of 10.0 mA/cm2. The presented approach may provide an efficient and low-cost pathway to solar hydrogen production.
    Journal of Materials Research 11/2014; 29(22):2605-2614. DOI:10.1557/jmr.2014.308 · 1.82 Impact Factor
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    ABSTRACT: We experimentally investigate the light-trapping effect of plasmonic reflection grating back contacts in prototype hydrogenated amorphous silicon thin-film solar cells in substrate configuration. These back contacts consist of periodically arranged Ag nanostructures on flat Ag reflectors. We vary the period, unit cell, and width of the nanostructures to identify design strategies for optimized light trapping. First, a general correlation between the reduction of the period of the nanostructures down to 550 nm and an increase of the absorptance, as well as external quantum efficiency is found for various unit cells formed by nanostructures. Second, increasing the width of the nanostructures from 200 to 350 nm, an enhanced light-trapping effect of the thin-film solar cells is found independent of the period. As a result, we identify a design for improved light trapping for the given solar cell parameters within the considered variations. It consists of thin-film solar cells applying a combination of a period of 600 nm and a structure width of 350 nm. The implementation of back contacts with this configuration yields enhanced power conversion efficiency as compared to reference solar cells processed on conventionally used randomly textured substrates. In detail, the enhancement of the short-circuit current density from initially 14.7 to initially 15.6 mA/cm2 improves the power conversion efficiency from 9.1 to 9.3%.
    Journal of Photonics for Energy 11/2014; 5(1):057004. DOI:10.1117/1.JPE.5.057004 · 1.38 Impact Factor
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    ABSTRACT: In this study amorphous silicon tandem solar cells are successfully utilized as photoelectrodes in a photoelectrochemical cell for water electrolysis. The tandem cells are modified with various amounts of platinum and are combined with a ruthenium oxide counter electrode. In a two-electrode arrangement this system is capable of splitting water without external bias with a short-circuit current of 4.50 mA cm−2. On the assumption that no faradaic losses occur, a solar-to-hydrogen efficiency of 5.54 % is achieved. In order to identify the relevant loss processes, additional three-electrode measurements were performed for each involved half-cell.
    ChemPhysChem 10/2014; DOI:10.1002/cphc.201402552 · 3.36 Impact Factor
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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. DOI:10.1016/j.solmat.2013.12.024 · 5.03 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 08/2014; 4(11). DOI:10.1002/aenm.201301871 · 14.39 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 07/2014; 92(7/8):905-908. DOI:10.1139/cjp-2013-0610 · 0.93 Impact Factor
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    ABSTRACT: The electronic properties of undoped microcrystalline silicon oxide films have been investigated by transient photocurrent (TPC) density of states (DOS) spectroscopy, supported by dark conductivity, steady-state photoconductivity, and constant-photocurrent measurements (CPM). Film compositions span the range from amorphous to microcrystalline and contain up to 10% oxygen content, yielding optical bandgap values E-04 (the photon energy at which the absorption depth equals one micrometre) between 1.85 and 2.11 eV. Carrier transport is consistent with multiple-trapping in a localised DOS, which depends upon film structure and oxygen content. TPC measurements indicate that both conduction band-tail energy and deep defect density increase with increasing oxygen content, accompanied by a reduction in majority carrier mobility-lifetime product. CPM measurements on amorphous films show a broadening of the Urbach tail with increasing oxygen content. Significantly higher oxygen incorporation without seriously compromising electronic quality appears possible in microcrystalline films. This suggests potential application as solar cell absorber layers offering increased optical bandgap and open-circuit voltage.
    Canadian Journal of Physics 07/2014; 92(7/8):753-757. DOI:10.1139/cjp-2013-0618 · 0.93 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; DOI:10.1155/2014/249317 · 2.66 Impact Factor
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    ABSTRACT: In this study, we experimentally investigate the light-trapping effect of plasmonic reflection grating back contacts in prototype hydrogenated amorphous silicon thin-film solar cells in substrate configuration. The plasmonic reflection grating back contacts consist of periodically arranged Ag nanostructures on flat Ag reflectors. By varying the geometrical parameters of these back contacts, design strategies for optimized light trapping are identified. First, a general correlation between a reduction of the period of the plasmonic reflection grating back contact and an increase of the absorptance as well as external quantum efficiency is found for various unit cells of the nanostructures i.e. square unit cell, hexagonal unit cell and face-centered unit cell. Second, the width of the nanostructures is varied. With increasing width, an enhanced light-trapping effect of the thin-film solar cells is found independent of the period. As a result, an optimized design for improved light trapping in the studied thin-film solar cells is a combination of a period of 600 nm and a structure width of 350 nm. Solar cells fabricated on plasmonic reflection grating back contacts with this optimized configuration yield enhanced power conversion efficiencies as compared to reference solar cells processed on state-ofthe- art randomly textured substrates. In detail, the power conversion efficiency is enhanced by around 0.2 % from 9.1 % to 9.3 %. This increase is largely due to the enhancement of the short-circuit current density of around 7 % from 14.7 mA/cm2 to 15.6 mA/cm2.
    Proceedings of SPIE; 04/2014
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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; 92(92):763-767. DOI:10.1139/cjp-2013-0629 · 0.93 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; 92(92):768-773. DOI:10.1139/cjp-2013-0630 · 0.93 Impact Factor
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    International Journal of Photoenergy 01/2014; 2014:1-10. DOI:10.1155/2014/176965 · 2.66 Impact Factor
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    ABSTRACT: Amorphous silicon (a-Si:H) solar cells in p-i-n configuration were developed at a low deposition temperature of 140 °C, suitable for application on transparent flexible plastic substrates. Deteriorated electronic properties of the p-layer with decreasing temperature were identified as the main reason for reduced solar cell performance. Optimization of the p-layer properties resulted in an efficiency of 8.2 % for a solar cell fabricated entirely at 140 °C. As a parallel application scenario, a-Si:H/a-Si:H tandem solar cells are designed for application in integrated photoelectrochemical water splitting modules. Here we benefit from the increased open circuit voltages with values around 1.9 V which provides ample margin for possible overpotential losses in water splitting modules.
    Conference Record of the IEEE Photovoltaic Specialists Conference 01/2014; DOI:10.1109/PVSC.2014.6925579
  • Steve Reynolds, Oleksandr Astakhov, Vladimir Smirnov
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    ABSTRACT: Electron irradiation of silicon thin films creates localised states, which degrade their opto-electronic properties. We present a series of transient photocurrent spectroscopy (TPC) measurements on electron-irradiated amorphous and microcrystalline silicon films, annealed at progressively increasing temperatures. This has enabled localised states associated with both dangling bonds and conduction band tails to be examined over a wide energy range. Trends in the evolution of the DOS following electron irradiation followed by isochronal annealing steps indicate reductions in the deep defect density, which correlate with spin density. We also find a steepening of the conduction band tail slope in amorphous silicon on annealing. Both defect density and tail slope may be restored close to as-prepared material values. Earlier CPM data are re-examined, and a similar trend in the valence band tail slope is indicated. Computer simulations predict that following e-irradiation changes in deep defect density primarily control solar cell performance, and will tend to obscure effects related to band tails.
    Journal of Physics Conference Series 01/2014; 558(1):012001. DOI:10.1088/1742-6596/558/1/012001
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    ABSTRACT: This paper reports on the development of phosphorous doped microcrystalline silicon oxide (mu c-SiOx:H) films as an emitter window layer in flat p-type silicon heterojunction (SHJ) solar cells featuring intrinsic a-SiOx:H buffer layers. We investigated the material properties of n-type mu c-SiOx:H films grown at various input gas ratios and correlated the results of SHJ solar cells utilizing varying oxygen content and thickness of the emitter layer to the corresponding film properties. A maximum efficiency of 19.0% was achieved. The excellent short circuit current of 35.8 mA/cm(2) for flat cells was attributed to the low optical losses in the emitter window. (C) 2013 The Japan Society of Applied Physics
    Japanese Journal of Applied Physics 12/2013; 52(12R):122304. DOI:10.7567/JJAP.52.122304 · 1.06 Impact Factor