J. Müller

Modified Thornton model for magnetron sputtered zinc oxide: film structure and etching behaviour

ZnO:Al films were prepared on glass substrates with different sputter techniques from ceramic ZnO:Al2O3 target as well as metallic Zn:Al targets using a wide range of deposition parameters. Independent of the sputter technique, sputter pressure and substrate temperature were found to have a major influence on the electrical and structural properties of the ZnO:Al films. With an increasing deposition pressure, we observed a strong decrease in the carrier mobility and also an increase of the etching rate. The surface morphology obtained after etching of RF sputtered ZnO:Al systematically changes from crater-like to hill-like surface appearance with increasing pressure. The correlation of sputter parameters, film growth and structural properties is discussed in terms of a modified Thornton model.

Microcrystalline silicon for large area thin film solar cells

Abstract We present a comprehensive study of microcrystalline silicon (μc-Si:H) solar cells prepared by plasma-enhanced chemical vapour deposition (PECVD) at 13.56 MHz excitation frequency. In the first step the cell development was performed in a small area PECVD reactor showing the relationship between the deposition process parameters and the resulting solar cell performance. Focus was on the influence of deposition pressure, electrode distance and the application of a pulsed plasma on high rate deposition of solar cells. Subsequent up-scaling to a substrate area of 30×30 cm 2 confirmed the suitability of the process for large area reactors. The influence of i-layer deposition parameters on solar cell performance was studied directly in p–i–n cells prepared on textured ZnO. Solar cell efficiencies up to 9% were achieved at deposition rates of 5–6 A/s for the i-layer using high plasma powers. Applied as bottom cell in a-Si:H/μc-Si:H tandem cells a stable cell efficiency of 11.2% could be obtained. The excellent homogeneity was proven by the realization of first modules with an aperture area of 689 cm 2 and an active area initial efficiency of 10.3% (stable: 8.9%) using an established base technology for laser patterning and back contact sputtering at RWE Solar GmbH.

New materials and deposition techniques for highly efficient silicon thin film solar cells

This paper reviews recent efforts to provide the scientific and technological basis for cost-effective and highly efficient thin film solar modules based on amorphous (a-Si:H) and microcrystalline (pc-Si:H) silicon. Textured ZnO:Al films prepared by sputtering and wet chemical etching were applied to design optimised light-trapping schemes. Necessary prerequisite was the detailed knowledge of the relationship between film growth, structural properties and surface morphology obtained after etching. High rate deposition using plasma enhanced chemical vapour deposition at 13.56 MHz plasma excitation frequency was developed for pc-Si:H solar cells yielding efficiencies of 8.1% and 7.5% at deposition rates of 5 and 9 Angstrom/s, respectively. These pc-Si: H solar cells were successfully up-scaled to a substrate area of 30 x 30 cm(2) and applied in a-Si:H/muc-Si:H tandem cells showing initial test cell efficiencies up to 11.9%

Development of highly efficient thin film silicon solar cells on texture-etched zinc oxide-coated glass substrates

Abstract ZnO films prepared by magnetron sputtering on glass substrates and textured by post-deposition chemical etching are applied as substrates for p–i–n solar cells. Using both rf and dc sputtering, similar surface textures can be achieved upon etching. Excellent light trapping is demonstrated by high quantum efficiencies at long wavelengths for microcrystalline silicon solar cells. Applying an optimized microcrystalline/amorphous p-layer design, stacked solar cells with amorphous silicon top cells yield similarly high stabilized efficiencies on ZnO as on state-of-the-art SnO 2 (9.2% for a-Si/a-Si). The efficiencies are significantly higher than on SnO 2 -coated float glass as used for module production.

State-of-the-art mid-frequency sputtered ZnO films for thin film silicon solar cells and modules

This article reports on the use of ZnO films in silicon thin film p-i-n solar cells and modules. It summarizes the status in the final phase of a joint research project aiming at the development of high-quality ZnO/glass substrates feasible for an industrial solar module production. The samples were prepared by reactive mid-frequency (mf) sputtering on large area (60×100 cm 2 ) glass sheets using low-cost metallic Zn:Al targets. These ZnO films exhibit resistivities down to 2.6×10 -4 Ω cm and high optical transmittance. Variation of the sputter pressure leads to films with significantly differing etching behavior in diluted acids. ZnO layers prepared in the high pressure regime develop strongly textured light scattering surfaces after etching, which is necessary to obtain highly efficient solar cells. Initial efficiencies of small area amorphous silicon (a-Si:H) cells on texture-etched ZnO-films prepared by mf-sputtering on 60×100 cm 2 soda-lime glass (3 mm thick) range from 8 to 9% (highest efficiency 9.2%, i-layer thickness 350 nm). First 0.6 m 2 modules on ZnO prove the principal applicability of the films for an industrial manufacturing process.

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

Abstract Large area (320×400 mm2) 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.

Electronic properties of microcrystalline silicon investigated by electron spin resonance and transport measurements

The electron spin resonance and transport properties of microcrystalline silicon were investigated. Identification and location of the resonance states in the material are discussed. We analyse the properties of the conduction electron resonance and the relation of the corresponding states with electronic transport. Conduction band tail states within the crystalline regions of the material are proposed as a requirement for the interpretation of the experimental results.

Electronic states in hydrogenated microcrystalline silicon

Abstract Electronic states in the mixed-phase material microcrystalline silicon prepared by plasma-enhanced chemical vapour deposition were investigated by continuous-wave and time-resolved electron spin resonance techniques in thermal equilibrium and under illumination. Samples prepared with various plasma-excitation frequencies v ex and various process gas mixtures (which leads to differences in the crystalline volume fractions and grain sizes) and samples with different p- and n-type doping levels were studied. Three main electron spin resonance contributions were found and attributed to dangling bonds in different structural environments in the material and to conduction electrons. The g values of the dangling bonds are shifted with respect to the g value of the dangling bond in amorphous silicon. The dangling-bond spin density remains largely unchanged over a wide range of plasma excitation frequencies but increases at the highest v ex and increases also at high silane gas concentrations when amorpho...

Amorphous and Microcrystalline Silicon Based Solar Cells and Modules on Textured Zinc Oxide Coated Glass Substrates

This paper addresses scientific and technological efforts to develop highly efficient silicon thin film solar modules on glass substrates. We present a comprehensive study of μc-Si:H p-i-n single junction and a-Si:H/μc-Si:H stacked solar cells prepared by plasma-enhanced chemical vapour deposition (PECVD) at 13.56 MHz excitation frequency. In the first step cell development was performed in a small area PECVD reactor showing the relationship between deposition process and resulting solar cell performance. Subsequent up-scaling to a substrate area of 30×30 cm 2 confirmed the scalability to large area reactors. Moreover, we developed textured ZnO:Al films by sputtering and post deposition wet chemical etching as front contact TCO-material with excellent light scattering properties. A-Si:H/μc-Si:H tandem cells developed on this textured ZnO yielded stable efficiencies up to 11.2 % for a cell area of 1 cm 2 . First solar modules were prepared in our recently installed process technology, which includes PECVD, sputtering, texture etching and laser scribing on substrate sizes up to 30x30 cm 2 . Initial module efficiencies of 10.8 % and 10.1 % were achieved for aperture areas of 64 cm 2 and 676 cm 2 , respectively.

High rate deposition of microcrystalline silicon solar cells using 13.56 MHz PECVD

In this paper, the authors present microcrystalline silicon (/spl mu/c-Si:H) p-i-n solar cells prepared at high deposition rates using plasma-enhanced chemical vapour deposition (PECVD) at 13.56 MHz excitation frequency. They studied the deposition regime of high RF-power P/sub RF/ (40-100 W for a 150 cm/sup 2/ electrode) and high deposition pressure p/sub dep/ (1-11 Torr) at different silane concentrations and substrate temperatures. In this regime, the prepared i-layers were amorphous or microcrystalline depending on the deposition parameters. The shift between the two growth regimes was achieved by a variation of either deposition pressure, plasma power or silane concentration. The best /spl mu/c-Si:H solar cells were prepared close to the transition to amorphous growth. A high deposition pressure was a prerequisite for obtaining high quality material at a high growth rate. The best solar cell efficiency achieved was 8.0% at 5 /spl Aring//s for a /spl mu/c-Si:H single junction solar cell.