Texture-etched ZnO Research Articles

The trade-off of light trapping between top and bottom cell in micromorph tandem solar cells with sputtering ZnO:Al glass substrate

A simulated and experimental investigation of the trade-off between light trapping and current matching in p-i-n structured a-Si:H/μc-Si:H tandem solar cells is presented, which aims to address the limited short circuit current density (Jsc) that results from the low long-wavelength light scattering of the fluorine-doped tin oxide (SnO2:F) substrates typically used. To this end, the mismatch of the Jsc between the top and bottom cells is reduced by utilizing a ZnO:Al substrate with optimized long-wavelength light scattering properties as the front contact, thereby improving the response of the bottom cell at the expense of the lower top cell's Jsc yet. A trade-off between the top and bottom cell's light response is subsequently found with SnO2 or ZnO:Al as a substrate, by introducing an n-type μc-SiOx intermediate reflector (IR) between the two component cells. An initial efficiency based on an approximate current matching of 11.90% is achieved for a-Si:H/μc-Si:H tandem solar cell by adopting a magnetron-sputtered and texture-etched ZnO:Al substrate and an optimized n-type μc-SiOx IR.

ZnO based Back Reflectors with a Wide Range of Surface Morphologies for Light Trapping in n-i-p Microcrystalline Silicon Solar Cells

Surface morphology of ZnO layers with as-deposited thickness between 550 to 1050nm was varied over wide range by changing etching time. The surface morphology of these ZnO layers was measured by atomic force microscopy (AFM) and statistically analyzed in terms of root mean square roughness and the diameter and depth of the craters. The texture-etched ZnO covered with a Ag and ZnO layers were applied into n-i-p μc-Si:H solar cells as back reflectors. The effects of the back reflector morphologies on solar cell performances were investigated. The link between the morphology, the scattering properties of ZnO layers, and the solar cell performance is discussed.

Transparent double-period electrode with effective light management for thin film solar cells

Aiming to significantly enhance solar cell efficiency through light management, we designed and fabricated transparent electrodes with a double-period structure. After demonstrating a significant increase of quantum efficiency in short wavelengths with a small-period self-textured ZnO : B, and in long wavelengths with large-period texture-etched ZnO : Al, respectively, we made the double-period transparent solar cell electrode by integrating self-textured ZnO : B and texture-etched ZnO : Al on μc-Si : H solar cells, using conventional physical and chemical vapor deposition techniques. Our investigation indicates that the double-period electrode can play an important role towards improving light absorption and trapping, thus effectively increasing the efficiency and reducing the cost of electricity from the solar cells.

Process monitoring of texture-etched high-rate ZnO:Al front contacts for silicon thin-film solar cells

In this study angular resolved scattering measurements are used to monitor the surface texture properties of sputter-etched ZnO:Al and to identify the influences of process parameter changes on the surface texture. Variations in pressure and temperature are possible drifts in the industrial fabrication that can influence the ZnO:Al properties. Therefore – with respect to industrial relevance – this study focuses on the influence of these parameters. Deposition parameter dependent trends in the angular resolved scattering are studied. Correlations between changes in the results of the angular resolved scattering measurements and the performance of a-Si:H/μc-Si:H tandem solar cells are shown. Additionally, it is demonstrated that the measurements can be used to optimize the surface texture of ZnO:Al for single-junction μc-Si:H solar cells by adjusting the deposition conditions according to the previously identified trends.

Variation of back reflector morphology in n–i–p microcrystalline silicon thin film solar cells using texture-etched ZnO

Surface morphology of ZnO layers was varied by changing etching time and statistically analyzed in terms of the root mean square roughness and the distribution of surface feature (crater) size. The texture-etched ZnO covered with Ag and ZnO layers was applied into n–i–p μc-Si:H solar cells as back reflectors. The effects of the back reflector morphologies on solar cell performances were investigated. The correlation between the back reflector morphology and the light trapping properties of solar cells was investigated, including the effect of crater size on the short circuit current density (JSC). We find that craters with the lateral size between 900nm and 2500nm are beneficial for light trapping and are suggested to be mainly responsible for the change in JSC in the solar cells. An improved light trapping results in an increase in JSC by 4.5mA/cm2 (25%) relative to a flat back reflector.

Gradient etching of silicon-based thin films for depth-resolved measurements: The example of Raman crystallinity

An etching procedure was applied to microcrystalline silicon (μc-Si:H) thin films in order to obtain a wedge-shaped profile for depth-resolved characterization. A microfluidic flow cell that merges deionized water with a potassium hydroxide solution (KOHaq) was utilized. The samples consisted of texture-etched ZnO:Al on a Corning Glass substrate, a microcrystalline p-doped layer serving as seed layer and the investigated intrinsic microcrystalline or amorphous silicon (a-Si:H). Along the etched profiles, microscopic Raman spectroscopy was used to estimate the crystalline volume fraction Xc for samples deposited with intentionally varied silane concentration to investigate the a-Si:H/μc-Si:H and μc-Si:H/a-Si:H transition.

Influence of rapid thermal annealing on surface texture-etched Al-doped ZnO films prepared by various magnetron sputtering methods

The influence of rapid thermal annealing (RTA) on surface texture formation as well as the light management obtainable by wet-chemically etching was investigated for transparent conducting Al-doped ZnO (AZO) thin films prepared by various types of magnetron sputtering deposition (MSD) with an oxide target. Texture-etched AZO films prepared by an r.f. (13.56MHz) power-superimposed d.c. magnetron sputtering deposition (rf+dc-MSD) exhibited a higher haze value than found in equivalent films prepared by d.c. MSD. The order that the RTA treatment and the etching were conducted considerably affected the obtainable surface texture. Conducting the etching after a heat treatment with RTA in air resulted in larger etch pits as well as higher haze values than were obtained in AZO films that were etched before the RTA. A high haze value generally above 70% in the range from visible to near infrared (at wavelengths up to 1200nm) was obtained in texture-etched AZO thin films that were prepared by rf+dc-MSD and etched after RTA at a temperature of 500°C for 3min.

Influence of Pressure and Plasma Potential on High Growth Rate Microcrystalline Silicon Grown by Very High Frequency Plasma Enhanced Chemical Vapour Deposition

Microcrystalline silicon (µc-Si) based single junction solar cells have been deposited by very high-frequency plasma-enhanced chemical vapor deposition (VHF-PECVD) using a showerhead cathode at high pressures in depletion conditions. The i-layers are made near the transition from amorphous to crystalline. An energy conversion efficiency of 9.9% is obtained with a single junction solar cell that is deposited on a texture-etched ZnO:Al front contact. The µc-Si i-layer is 1.5 µm thick, deposited at a rate of 0.5 nm/s. In order to control the material properties in the growth direction, the hydrogen dilution of silane in the gas phase is graded following different profiles with a parabolic shape. Materials with higher deposition rates were developed by increasing the RF power and the total gas flow such that the depletion condition is constant. At a deposition rate of 4.5 nm/s, a stabilized conversion efficiency of 6.7% is obtained for a single junction solar cell with a µc-Si i-layer of 1 µm. It is found that the defect density increases one order of magnitude upon the increase in deposition rate from 0.45 to 4.5 nm/s. This increase in defect density is partially attributed to the increased energy of the ion bombardment during the plasma deposition. We have introduced an additional method to limit the ion energy by controlling the DC self bias voltage using an external power source. In this way, the defect density in the µc-Si layers is decreased and the performance of the solar cells is further improved. It is observed that the performance of solar cells deposited at high rate improves under light soaking conditions at 50 °C, which we attribute to post deposition equilibration of a fast deposited transition material.

Influence on cell performance of bulk defect density in microcrystalline silicon grown by VHF PECVD

Microcrystalline silicon based single junction solar cells have been deposited on Asahi U-type SnO2:F and texture-etched ZnO by means of very high-frequency plasma-enhanced chemical vapor deposition (VHF PECVD) using a showerhead cathode at high pressures in depletion conditions. The defect densities in the intrinsic layers (in the solar cell configuration) have been estimated by means of Fourier-transform photocurrent spectroscopy. An Urbach tail parameter of 35meV and a defect density of smaller than 1015cm−3 show the high quality of the material. It was found that the cell performance of our cells is dominated by the bulk defect density of the i-layers: the fill factor and open circuit voltage of solar cells deposited at various conditions show a consistent decrease with increasing defect density. It was found that the bulk defect density increases one order of magnitude upon an increase in deposition rate from 0.45 to 4.5nm/s. We show that this increase in defect density can be reduced when the ion bombardment energy is reduced by artificially lowering the plasma potential in the deposition process using an external DC bias. On texture-etched ZnO:Al, at a deposition rate of 0.45nm/s, a stabilized conversion efficiency of 10.0% (4mm×4mm cell area) is obtained for a single junction solar cell with a μc-Si i-layer of 1.5μm thick. This belongs to the highest efficiencies reported for microcrystalline silicon solar cells in the superstrate (p–i–n) configuration. Upon increasing the deposition rate to 4.5nm/s, an efficiency of 6.4% (initial) was obtained. It is observed that the performance of solar cells that are deposited at high deposition rates improves under light soaking conditions at 50°C, even though these cells are deposited at the amorphous to crystalline transition regime.

High-efficiency µc-Si solar cells made by very high-frequency plasma-enhanced chemical vapor deposition

Microcrystalline silicon-based single-junction p–i–n solar cells have been fabricated by very high-frequency plasma enhanced chemical vapor deposition using a showerhead cathode at high pressures and under silane depletion conditions. The i-layers are made near the transition from amorphous to crystalline. It was found that, especially at high crystalline fractions, the open-circuit voltage and fill factor are very sensitive to the morphology of the substrate. At an i-layer deposition rate 0·45 nm/s, we have measured a stabilised efficiency of 10% (Voc = 0·52 V, FF = 0·74) for a cell made on texture-etched ZnO:Al. The performance is stable under light soaking. The defect density of the absorber layer is in the 1015 cm−3 range. In spite of the presence of oxygen contamination, good electrical properties and good infrared cell response are obtained. Copyright © 2006 John Wiley & Sons, Ltd.

Material study on reactively sputtered zinc oxide for thin film silicon solar cells

Aluminum doped zinc oxide (ZnO:Al, AZO) films were prepared by reactive mid frequency magnetron sputtering. We characterized the electrical and optical properties as well as the surface morphology obtained after wet chemical etching. The carrier mobility could be increased up to 42 cm 2/Vs and the transmission between 400 and 1100 nm was enhanced by the reduction of aluminum content in the targets. The working point of the reactive sputtering process strongly influences the etching behavior and was used to optimize the light scattering properties of the ZnO:Al films after wet chemical etching. Finally, the texture-etched ZnO:Al films were successfully applied as substrates for silicon thin film solar cells.

Upscaling of texture-etched zinc oxide substrates for silicon thin film solar cells

Large area (320×400 mm 2) glass/ZnO-films were prepared by high-rate d.c. magnetron sputtering from ceramic targets and compared to lab-type r.f.- and m.f.-sputtered ZnO. The very uniform and initially smooth films exhibit excellent electrical and optical properties (resistivity ≤5×10 −4 Ωcm, transmission >80% for visible light and 1500-nm thick films). Upon etching in diluted hydrochloric acid they develop a surface texture. Independent of sputter technique (d.c. or r.f.) and substrate size, higher substrate temperatures and lower sputter gas pressures have a similar influence on the film structure and lead to more robust and etch-resistant films. Showing excellent light scattering properties, amorphous silicon pin solar cells prepared on these large area glass/ZnO samples exhibit initial efficiencies up to 9.2%, proving the viability of sputtered and texture-etched ZnO as TCO-substrate for industrial solar module production.