Showing papers by "Miro Zeman published in 2014"

Efficient Water‐Splitting Device Based on a Bismuth Vanadate Photoanode and Thin‐Film Silicon Solar Cells

Abstract: A hybrid photovoltaic/photoelectrochemical (PV/PEC) water-splitting device with a benchmark solar-to-hydrogen conversion efficiency of 5.2 % under simulated air mass (AM) 1.5 illumination is reported. This cell consists of a gradient-doped tungsten–bismuth vanadate (W:BiVO_4) photoanode and a thin-film silicon solar cell. The improvement with respect to an earlier cell that also used gradient-doped W:BiVO4 has been achieved by simultaneously introducing a textured substrate to enhance light trapping in the BiVO4 photoanode and further optimization of the W gradient doping profile in the photoanode. Various PV cells have been studied in combination with this BiVO_4 photoanode, such as an amorphous silicon (a-Si:H) single junction, an a-Si:H/a-Si:H double junction, and an a-Si:H/nanocrystalline silicon (nc-Si:H) micromorph junction. The highest conversion efficiency, which is also the record efficiency for metal oxide based water-splitting devices, is reached for a tandem system consisting of the optimized W:BiVO_4 photoanode and the micromorph (a-Si:H/nc-Si:H) cell. This record efficiency is attributed to the increased performance of the BiVO_4 photoanode, which is the limiting factor in this hybrid PEC/PV device, as well as better spectral matching between BiVO_4 and the nc-Si:H cell.

Experimental Demonstration of 4n2 Classical Absorption Limit in Nanotextured Ultrathin Solar Cells with Dielectric Omnidirectional Back Reflector

Abstract: The experimental demonstration of the 4n2 classical absorption limit in solar cells has been elusive for the last 30 years. Especially the assumptions on front and internal rear reflectance in a slab of absorbing material are not easily fulfilled unless an appropriate light-trapping scheme is applied. We propose an advanced metal-free light-trapping scheme for crystalline silicon wafers. For different bulk thicknesses, at the front side of the wafers we applied a nanotexture known as black-silicon. At the rear side, we implemented a random pyramidal texture coated with a distributed Bragg reflector. Such a dielectric back reflector was designed to exhibit a maximized omnidirectional internal rear reflectance in the region of weak absorption of crystalline silicon. Integrating the measured absorptance spectra of our wafers with the reference solar photon flux between 400 and 1200 nm, we could calculate the so-called implied photogenerated current densities. For wafers thinner than 35 μm, we achieved more t...

Influence of transparent conductive oxides on passivation of a-Si:H/c-Si heterojunctions as studied by atomic layer deposited Al-doped ZnO

Abstract: In silicon heterojunction solar cells, the main opportunities for efficiency gain lie in improvements of the front-contact layers. Therefore, the effect of transparent conductive oxides (TCOs) on the a-Si:H passivation performance has been investigated for Al-doped zinc oxide (ZnO:Al) layers made by atomic layer deposition (ALD). It is shown that the ALD process, as opposed to sputtering, does not impair the chemical passivation. However, the field-effect passivation is reduced by the ZnO:Al. The resulting decrease in low injection-level lifetime can be tuned by changing the ZnO:Al doping level (carrier density = 7 × 1019–7 × 1020 cm−3), which is explained by a change in the TCO workfunction. Additionally, it is shown that a ~10–15 nm ALD ZnO:Al layer is sufficient to mitigate damage to the a-Si:H by subsequent sputtering, which is correlated to ALD film closure at this thickness.

Quadruple-junction thin-film silicon-based solar cells with high open-circuit voltage

Abstract: We have fabricated a-SiOx:H/a-Si:H/nc-Si:H/nc-Si:H quadruple-junction thin-film silicon-based solar cells (4J TFSSCs) to obtain high spectral utilization and high voltages. By processing the solar cells on micro-textured superstrates, extremely high open-circuit voltages for photovoltaic technology based on thin-film silicon alloys up to 2.91 V have been achieved. Optical simulations of quadruple-junction solar cells using an advanced in-house model are a crucial tool to effectively tackle the challenging task of current matching among the individual sub-cells in such devices. After optimizing the optical design of the device and the absorber thicknesses, an energy conversion efficiency of 11.4% has been achieved. The open-circuit voltage, short-circuit current density, and fill factor were 2.82 V, 5.49 mA/cm2, and 73.9%, respectively. Based on this demonstration, strategies for further development of highly efficient 4J TFSSCs are proposed.

Full-wave optoelectrical modeling of optimized flattened light-scattering substrate for high efficiency thin-film silicon solar cells

Abstract: The flattened light-scattering substrate (FLiSS) is formed by a combination of two materials with a high refractive index mismatch, and it has a flat surface. A specific realization of this concept is a flattened two-dimensional grating. When applied as a substrate for thin-film silicon solar cells in the nip configuration, it is capable to reflect light with a high fraction of diffused component. Furthermore, the FLiSS is an ideal substrate for growing high-quality microcrystalline silicon (µc-Si:H), used as bottom cell absorber layer in most of multijunction solar cell architectures. FLiSS is a three-dimensional structure; therefore, a full-wave analysis of the electromagnetic field is necessary for its optimal implementation. Using finite element method, different shapes, materials, and geometrical parameters were investigated to obtain an optimized FLiSS. The application of the optimized FLiSS in µc-Si:H single junction nip cell (1-µm-thick i-layer) resulted in a 27.4-mA/cm2 implied photocurrent density. The absorptance of µc-Si:H absorber exceeded the theoretical Yablonovitch limit for wavelengths larger than 750 nm. Double and triple junction nip solar cells on optimal FLiSS and with thin absorber layers were simulated. Results were in line with state-of-the-art optical performance typical of solar cells with rough interfaces. After the optical optimization, a study of electrical performance was carried out by simulating current–voltage characteristics of nip solar cells on optimized FLiSS. Potential conversion efficiencies of 11.6%, 14.2%, and 16.0% for single, double, and triple junction solar cells with flat interfaces, respectively, were achieved. Copyright © 2012 John Wiley & Sons, Ltd.

Optimization of amorphous silicon double junction solar cells for an efficient photoelectrochemical water splitting device based on a bismuth vanadate photoanode

Abstract: A photoelectrochemical water splitting device (PEC-WSD) was designed and fabricated based on cobalt-phosphate-catalysed and tungsten-gradient-doped bismuth vanadate (W:BiVO4) as the photoanode. A simple and cheap hydrogenated amorphous silicon (a-Si:H) double junction solar cell has been used to provide additional bias. The advantage of using thin film silicon (TF-Si) based solar cells is that this photovoltaic (PV) technology meets the crucial requirements for the PV component in PEC-WSDs based on W:BiVO4 photoanodes. TF-Si PV devices are stable in aqueous solutions, are manufactured by simple and cheap fabrication processes and their spectral response, voltage and current density show an excellent match with the photoanode. This paper is mainly focused on the optimization of the TF-Si solar cell with respect to the remaining solar spectrum transmitted through the W:BiVO4 photoanode. The current matching between the top and bottom cells is studied and optimized by varying the thickness of the a-Si:H top cell. We support the experimental optimization of the current balance between the two sub-cells with simulations of the PV devices. In addition, the impact of the light induced degradation of the a-Si:H double junction, the so-called Staebler–Wronski Effect (SWE), on the performance of the PEC-WSD has been studied. The light soaking experiments on the a-Si:H/a-Si:H double junctions over 1000 hours show that the efficiency of a stand-alone a-Si:H/a-Si:H double junction cell is significantly reduced due to the SWE. Nevertheless, the SWE has a significantly smaller effect on the performance of the PEC-WSD.

Optical Enhancement of Silicon Heterojunction Solar Cells With Hydrogenated Amorphous Silicon Carbide Emitter

Abstract: In this paper, the electrical and optical properties of p-type hydrogenated amorphous silicon carbide (a-SiC:H) are compared with p-type hydrogenated amorphous silicon (a-Si:H) widely used as emitter material of silicon heterojunction solar cells. The difference in solar-cell performance of the two emitters shows that p-type a-SiC:H emitter is able to enhance the short-circuit current density (J sc ) by reducing the parasitic absorption loss and reflection loss without degrading the electrical performance of devices. The application of the p-type a-SiC:H emitter can lead to a J sc increase of about 1 mA/cm 2 , compared with the p-type a-Si:H emitter. Our silicon heterojunction solar cell with p-type a-SiC:H emitter shows an active-area efficiency of 20.8% and the short-circuit current density of 40.3 mA/cm 2 .

The Nature and the Kinetics of Light-Induced Defect Creation in Hydrogenated Amorphous Silicon Films and Solar Cells

Abstract: The nature and the kinetics of light-induced defect creation in hydrogenated amorphous silicon (a-Si:H) films and so- lar cells are investigated by means of Doppler broadening positron annihilation spectroscopy, Fourier transform photocurrent spec- troscopy, and J-V characterization. There is a strong correlation between the open volume deficiencies in a-Si:H and the Staebler- Wronski effect (SWE). The carrier generation and recombination profiles in the absorber layer are spatially correlated, and the re- combination due to defects in the top and bottom parts of the absorber layer is different. Furthermore, the various defect distri- butions in the bandgap have different defect creation kinetics. It is demonstrated that the SWE defect formation kinetics in a solar cell are very complex and can impossibly be described by one time scaling ∼ t β as is often claimed.

Enhancing the driving field for plasmonic nanoparticles in thin-film solar cells

Abstract: The scattering cross-section of a plasmonic nanoparticle is proportional to the intensity of the electric field that drives the plasmon resonance. In this work we determine the driving field pattern throughout a complete thin-film silicon solar cell. Our simulations reveal that by tuning of the thicknesses of silicon and transparent conductive oxide layers the driving field intensity experienced by an embedded plasmonic nanoparticle can be enhanced up to a factor of 14. This new insight opens the route towards more efficient plasmonic light trapping in thin-film solar cells.

Effect of substrate morphology slope distributions on light scattering, nc-Si:H film growth, and solar cell performance.

Abstract: Thin-film silicon solar cells are often deposited on textured ZnO substrates. The solar-cell performance is strongly correlated to the substrate morphology, as this morphology determines light scattering, defective-region formation, and crystalline growth of hydrogenated nanocrystalline silicon (nc-Si:H). Our objective is to gain deeper insight in these correlations using the slope distribution, rms roughness (srms) and correlation length (lc) of textured substrates. A wide range of surface morphologies was obtained by Ar plasma treatment and wet etching of textured and flat-as-deposited ZnO substrates. The srms, lc and slope distribution were deduced from AFM scans. Especially, the slope distribution of substrates was represented in an efficient way that light scattering and film growth direction can be more directly estimated at the same time. We observed that besides a high sigma(rms), a high slope angle is beneficial to obtain high haze and scattering of light at larger angles, resulting in higher short-circuit current density of nc-Si:H solar cells. However, a high slope angle can also promote the creation of defective regions in nc-Si:H films grown on the substrate. It is also found that the crystalline fraction of nc-Si:H solar cells has a stronger correlation with the slope distributions than with srms of substrates. In this study, we successfully correlate all these observations with the solar-cell performance by using the slope distribution of substrates.

The Optical Spectra of a-Si:H and a-SiC:H Thin Films Measured by the Absolute Photothermal Deflection Spectroscopy (PDS)

Abstract: The new absolute PDS setup allows to measure simultaneously the absolute values of the optical transmittance T, reflectance R and absorptance A spectra in the spectral range 280 2000 nm with the typical spectral resolution 10 nm in ultraviolet and visible spectral range and 20 nm in the near infrared region. The PDS setup provides the dynamic detection range in the optical absorptance up to 4 orders of magnitude using non-toxic liquid perfluorohexane Fluorinert FC72. Here we demonstrate the usability of this setup on a series of intrinsic as well as doped a-Si:H and a-SiC:H thin films deposited on glass substrates by radio frequency (RF) plasma enhanced chemical vapor deposition (CVD) from hydrogen, silane and methane under various conditions. The increase of the Tauc gap with increasing carbon concentration in intrinsic a-SiC:H was observed. The defect-induced localized states in the energy gap were observed in doped a-Si:H as well as undoped a-SiC:H below the Urbach absorption edge.

Accelerated performance degradation of CIGS solar cell determined by in-situ monitoring

Abstract: An hybrid degradation setup, in which humidity, temperature and illumination are used in order to accelerate degradation of CIGS, has been developed. This setup consists of a climate chamber, which can vary the temperature and humidity. Furthermore, an area of 80x80 cm2 is illuminated, which allows both the study of light induced behvaior and the in-situ measurement of the IV curve of the cell during the test, so the degradation behavior can be observed in time. The IV output is automatically logged and characteristic parameters like efficiency, currents, voltages, ideality factor and resistance can be extracted. The 40x40 cm2 in the center of the illuminated area was calibrated BAA according to IEC norm 60904-9. Twelve cells or minimodules can be degraded and measured in-situ at the same time. The continuous in-situ IV measurements allowed us to do various studies, including the determination of the temperature dependency and of the impact of light and dark exposure on the performance of CIGS solar cells. Furthermore, the impact of different Na and K concentrations in the CIGS absorber layer on the ininial as well as long term performance of CIGS solar cells was studied.

Influence of deposition pressure and selenisation on damp heat degradation of the Cu(In,Ga)Se2 back contact molybdenum

Abstract: The impact of the molybdenum (Mo) microstructure and selenisation on degradation caused by 105hour exposure to standard 'damp heat' has been investigated. Degradation effects were already observed without magnification after several hours of exposure. The degradation resulted in large volume expansion due to the formation of a thick non-conductive MoOx layer consisting of various oxides and suboxides on top of the metallic molybdenum. This MoOx layer showed cracks and the appearance of needle-like structures.The degradation effect was the most severe for layers with the highest sputter pressure and thus the most porous microstructure. This can be attributed to higher mobility of the degrading species in the intergranular material (like MoOx) than in metallic Mo. The effect of selenisation was observed in the visual and optical characteristics - the dense selenised molybdenum layer retained the highest reflectance. Likely, the presence of MoSe2 prevented rapid oxidation of the Mo. cop. 2014 Elsevier B.V.

Optical and Electrical Simulation of μc-Si:H Solar Cells: Effect of Substrate Morphology and Crystalline Fraction

Abstract: Hydrogenated microcrystalline silicon (μc-Si:H) is an important material for high-efficiency multijunction solar cells. Due to its complex microstructural properties, it is difficult to describe the electronic behavior clearly. In this study, we measure opto-electronic properties including the mobility gap of μc-Si:H films in solar cells, as well as physical properties such as the crystalline fraction profile. The height distribution function of the ZnO substrates is obtained by AFM scans, which is used for optical simulation. All the parameters that we obtained from measurements were used as input parameters of a model in the ASA simulator. We obtained a good fit between measurements and simulations.

Determination of defect density of state distribution of amorphous silicon solar cells by temperature derivative capacitance-frequency measurement

Abstract: In this contribution, we demonstrate the application temperature dependent capacitance-frequency measurements (C-f) to n-i-p hydrogenated amorphous silicon (a-Si:H) solar cells that are forward-biased. By using a forward bias, the C-f measurement can detect the density of defect states in a particular energy range of the interface region. For this contribution, we have carried out this measurement method on n-i-p a-Si:H solar cells of which the intrinsic layer has been exposed to a H2-plasma before p-type layer deposition. After this treatment, the open-circuit voltage and fill factor increased significantly, as well as the blue response of the solar cells as is concluded from external quantum efficiency. For single junction, n-i-p a-Si:H solar cells initial efficiency increased from 6.34% to 8.41%. This performance enhancement is believed to be mainly due to a reduction of the defect density in the i-p interface region after the H2-plasma treatment. These results are confirmed by the C-f measurements. After H2-plasma treatment, the defect density in the intrinsic layer near the i-p interface region is lower and peaks at an energy level deeper in the band gap. These C-f measurements therefore enable us to monitor changes in the defect density in the interface region as a result of a hydrogen plasma. The lower defect density at the i-p interface as detected by the C-f measurements is supported by dark current-voltage measurements, which indicate a lower carrier recombination rate.

Optimized nano-textured interfaces for thin-film silicon solar cells: identifying the limit of randomly textured interfaces

Abstract: Thin-lm solar cells contain nano-textured interfaces that scatter the incident light, leading to increased absorption and hence increased current densities in the solar cell. In this manuscript we systematically study optimised random nano-textured morphologies for three dierent cases: amorphous hydrogenated silicon solar cells (a-Si:H, bandgap 1.7 eV), nano-crystalline silicon solar cells (nc-Si:H, bandgap 1.1 eV) and tandem solar cells consisting of an a-Si:H and a nc-Si:H junction. For the optimisation we use the Perlin texture algorithm, the scalar scattering theory, and a semi-coherent optical device simulator.

Structural analyses of seeded thin film microcrystalline silicon solar cell

Abstract: This contribution investigates the effect of seeding the growth of thin film microcrystalline silicon (µc-Si : H) deposited by radio frequency plasma-enhanced chemical vapor deposition on the material properties of µc-Si : H film and the device performance of p-i-n and n-i-p µc-Si : H solar cells. By means of Raman measurement, x-ray diffraction (XRD) and transmission electron microscopy (TEM), we investigate the structure of seeded µc-Si : H. In particular, the effect of seed layers on the crystallinity development is investigated. Measurements of the depth profile of the crystalline mass fraction using Raman spectroscopy show that seed layers lead to a more rapid and uniform crystallinity development in growth direction. The amorphous incubation layer is suppressed and crystallization begins directly from onset of film growth without evolving through the intermediate growth phases. From TEM analyses, we observe that crystal sizes are not affected by seed layers. Horizontal cracks are however observed to dominate the early growth of µc-Si : H in p-i-n solar cell and this is reduced upon seeding. For the n-i-p cells, these cracks are not affected by seeding. XRD results also indicate that the use of seed layers does not affect the crystal sizes but affects the direction of preferential orientation. Solar cell external parameters show that seeding of p-i-n solar cells leads mainly to increase in short-circuit current density, Jsc with a slight drop in open-circuit voltage, Voc. For the n-i-p cells, a reverse effect is observed. In this case, the Voc increases and the Jsc decreases. Copyright © 2012 John Wiley & Sons, Ltd.

Quantum confinement and band offsets in amorphous silicon quantum wells

Abstract: Quantum wells (QWs) are nanostructures consisting of alternating layers of a low and high band-gap semiconductor. The band gap of QWs can be tuned by changing the thickness of the low band-gap layer, due to quantum confinement effects. Although this principle is well established for crystalline materials, there is still controversy for QWs fabricated from amorphous materials: How strong are the confinement effects in amorphous QWs, where, because of the disorder, the carriers are localized to start with? We prepare an atomistic model of QWs based on a-Si:H to gain insight into this problem. The electronic structure of our atomistic QWs model is described with first-principles density functional theory, allowing us to study the confinement effects directly. We find that the quantum confinement effect is rather weak, compared to experimental results on a similar system.

The influence of atmospheric species on the degradation of aluminum doped zinc oxide and Cu(In,Ga)Se2 solar cells

Abstract: Aluminum doped zinc oxide (ZnO:Al) layers were exposed to the atmospheric gases carbondioxide (CO2), oxygen (O2), nitrogen (N2) and air as well as liquid H2O purged with these gases, in order to investigate the chemical degradation behavior of these layers. The samples were analyzed by electrical, compositional and optical measurements before, during and after exposure to these conditions in order to follow the degradation behavior of these layers in time. We have shown that ZnO:Al layers degraded in the presence of a mixture of H2O and CO2. Individually, CO2does not impact the degradation at all during the tested period, while the individual impact of H2O is small. However, when CO2 is also present, the concentration of OH increases greatly in the bulk and even more at the air/ZnO:Al and the ZnO:Al/glass interfaces. Carbon based species are then also present, indicating that Zn5(OH)6(CO3)2 is also formed at the grain boundaries. The degradation of ZnO:Al was accompanied by the occurrence of holes in the ZnO:Al layer near the ZnO:Al/glass interface. The impact of gaseous O2 as well as water purged with N2 and O2 on ZnO:Al degradation is very small. Complete Cu(In,Ga)Se2 solar cells were also exposed to unpurged liquid H2O and H2O purged with CO2, O2, N2 and air. The samples exposed to H2O purged with air and CO2 showed a rapid decrease in efficiency after approximately 180 hours of exposure. This efficiency decrease is mainly driven by a very rapid decrease in current density and an increase in series resistance.

Optical modeling of an efficient water splitting device based on bismuth vanadate photoanode and micromorph silicon solar cells

Abstract: We demonstrate a photoelectrochemical/photovoltaic (PEC/PV) device based on a combined configuration with a bismuth vanadate (BiVO 4 ) photoanode in the front and a thin film solar cell as power source in the rear. An optical model of this PEC/PV configuration is built by our in-house developed Advance Semiconductor Analysis (ASA) software. Micromorph (a-Si:H/nc-Si:H) cells are investigated in the optical model in reference to the photoanode filtered solar spectrum. The optimum micromorph cell with 400 nm a-Si:H i-layer is confirmed to have the best performance when combined with BiVO 4 photoanode. A solar-to-hydrogen efficiency of 5.2%, the highest conversion efficiency of metal oxide materials ever reported is achieved.