[Show abstract][Hide abstract] 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 · 5.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The fabrication process of high performance quadruple junction thin film silicon solar cells is described and the application of the solar cells in an integrated photoelectrochemical water splitting device is demonstrated. It is shown that the performance of solar cells can be adjusted by varying the process parameters and the thickness of the absorber layers of the individual sub cells and by integrating microcrystalline silicon oxide as intermediate reflecting layers. Thereby current matching of the sub cells was improved and a high open-circuit voltage of 2.8 V was achieved. Furthermore, the solar cell stability against light-induced degradation was investigated. Efficiencies of 13.2% (initial) and 12.6% (after 1000 h of light-soaking) were achieved. Bias-free water splitting with a solar-to-hydrogen efficiency of 7.8% was demonstrated in an integrated photovoltaic–electrochemical device using the developed quadruple junction photocathode. Finally, it is shown that in the case of quadruple junction solar cells the light-induced degradation has a lower effect on the photovoltaic–electrochemical efficiency as on the pho-tovoltaic efficiency.
Solar Energy Materials and Solar Cells 08/2015; DOI:10.1016/j.solmat.2015.07.033 · 5.34 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An all earth-abundant and precious metal-free photocathode based on a low-temperature fabricated amorphous silicon tandem junction is demonstrated to be an efficient device for solar water splitting. With a particular designed Al/Ni layer stack as photocathode/electrolyte contact an onset potential for cathodic photocurrent of 1.7 V vs. RHE and a saturation photocurrent density of 7.2 mA/cm2 were achieved. For a high-cost alternative with a Ag/Pt layer stack an even higher photocathode performance is demonstrated. Above all we present an approach for a dedicated photovoltaic and electrochemical development for solar water splitting.
Chemical Physics Letters 08/2015; DOI:10.1016/j.cplett.2015.08.018/10.1016 · 1.99 Impact Factor
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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 12/2014; 558(1):012001. DOI:10.1088/1742-6596/558/1/012001
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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.
[Show abstract][Hide abstract] 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 · 5.34 Impact Factor
[Show abstract][Hide abstract] 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 · 16.15 Impact Factor
[Show abstract][Hide abstract] 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
[Show abstract][Hide abstract] 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