Tobias Roschek

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%

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.

Structure of PECVD Si:H films for solar cell applications

Abstract The structure of undoped SiːH films and solar cells deposited under different hydrogen concentration and substrate temperatures were studied. The characterization techniques used were XRD, Raman spectroscopy, TEM, optical absorption, and hydrogen effusion. The high concentration films were amorphous in the as-deposited state but crystallized upon annealing at 700°C. Middle and low concentration films were nanocrystalline (nc) and remained nc up to 800°C annealing. A theoretical explanation is given for the stability of these films. Such films, on glass substrates, had optical absorption spectra close to those of amorphous material. The solar cell samples, showed some nc morphology in all-concentration states.

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.

Recent Progress in Up-Scaling of Amorphous and Micromorph Thin Film Silicon Solar Cells to 1.4 m 2 Modules

In this paper an overview of our developments towards industrialization of thin film silicon PV modules is presented. Amorphous silicon p-i-n solar cells have been developed in medium size single-chamber R&D KAI-M PECVD reactors. High initial efficiencies of 10.6 % and stabilized of 8.6 % could be achieved for a 1 cm 2 a-Si:H p-i-n solar cell of 0.20 m thick i-layer deposited on TCO from Asahi U type (SnO 2 ). On our in-house developed LPCVD ZnO we could further improve the stabilized a-Si:H p-i-n efficiency to a similar level of 8.5 %. Incorporating such cells in commercial available front TCO of lower quality still leads to high initial mini-module aperture efficiencies (10 × 10 cm 2 ) of 9.1% and stabilized ones of 7.46% (independently measured by ESTI JRC-Ispra). Transferring the processes from the KAI-M to the industrial size 1.1×1.25 m 2 KAI-1200 R&D reactors resulted in a-Si:H modules of 110.6 W using commercial TCO, respectively 112.4 W when applying in-house developed LPCVD front ZnO. Both initial module performances have been independently measured by ESTI laboratories of JRC Ispra. A typical temperature coefficient for the module power of -0.22 %/°C (relative loss) has been deduced from temperature dependent I-V characteristics at ESTI laboratories of JRC Ispra. Finally, micromorph mini-modules of 10 % initial aperture efficiency have been fabricated.

Large area deposition of intrinsic microcrystalline silicon for thin film solar cells

This paper addresses the development of intrinsic /spl mu/c-Si:H films on 30/spl times/30 cm/sup 2/ substrate size as an intermediate step towards industry-size substrates (typically -1 m/sup 2/) by PECVD using 13.56 MHz excitation frequency. The authors succeeded in preparing high quality /spl mu/c-Si:H i-layers with good homogeneity over 27/spl times/27 cm/sup 2/. The corresponding deposition rates were 4-11 /spl Aring//s. The excellent material quality of these intrinsic /spl mu/c-Si:H films was proven by small area (1 cm/sup 2/) p-i-n solar cells with 8.1 and 6.6% efficiency at deposition rates of 5 and 10 /spl Aring//s, respectively. The doped layers of these cells were prepared in a small area PECVD reactor.

Structure of Si:H Films Fabricated by Plasma-Enhanced Cvd using Hydrogen Diluted Plasma

The structure of undoped Si:H films deposited at a high rate of 6-9 A/s in an RF (13.56 MHz) plasma from hydrogen-silane gas mixtures at various substrate temperatures was studied using TEM (with in-situ annealing), XRD, Raman spectroscopy, optical absorption and hydrogen effusion. It is found that under our conditions the amorphous to crystalline transition occurs in a relatively narrow range of parameters, influenced mainly by hydrogen dilution and to a lesser degree by the substrate temperature. In the crystalline range the material is found to be nanocrystalline (average grain size 20 nm) and the crystals are essentially stable up to 800 o C annealing. The crystal structure of a mixed amorphousnanocrystalline phase of samples deposited near the edge of crystallinity is also found to be rather stable. Nanocrystalline Si films deposited under these latter deposition conditions reveal in hydrogen effusion a relatively compact material and show high solar cell efficiencies (6-8%) when incorporated as i-layers in pin solar cells.