IPV

In charge: Dr. W. Beyer, IPV, w.beyer@fz-juelich.de

Aims and Objectives

The program involves basic research of semiconductor layers and interfaces as well as the technological development of solar cell structures based on amorphous and microcrystalline silicon and silicon alloys. Basic research includes the preparation of amorphous, microcrystalline and polycrystalline silicon films as well as surface structured transparent conductive oxide films, the improvement of the optical and electronic properties, the stability of amorphous and microcrystalline silicon material and the investigation of interfaces. Technology includes the development of solar cells and large area modules with high efficiencies as well as sensors.

Significant Results in 2003

Deposition Process, Growth and Material

Microcrystalline silicon was grown with hot wire CVD at 250°C under variation of the wire-substrate distance, the wire temperature and wire geometry. For layers with good electronic properties the deposition rate could be increased by a factor of 4.

With in-situ ellipsometry we achieve a semi-quantitative determination of the crystalline volume fraction during growth of microcrystalline silicon.

Microstructure investigations using effusion of implanted helium gave for amorphous germanium similar as for amorphous silicon a high concentration of isolated voids when standard deposition conditions, i.e. plasma deposition at a substrate temperature of 200°C, were applied. Measurements of (amorphous) silicon-germanium alloys, on the other hand, showed the presence of an interconnected void structure.

The influence of the Al doping concentration on the electrical and optical film properties of radio frequency sputtered ZnO:Al was investigated. The wavelength regime of very high transparency could be extended to the near infrared range. Scattering at ionized impurities was identified as the major mechanism limiting the electron mobility in these films.

Conductive zinc stannate (ZTO) films with very high transparency were prepared by magnetron sputtering. These ZTO films exhibited electron mobilities up to 32 cm₂/Vs although only an amorphous structure could be detected with XRD.

In thin film silicon technology, etching processes are of importance both for device processing as well as for deposition chamber cleaning. A standard process gas is sulfurhexafluoride which, however, is noted in the Kyoto Protocol as environmentally adverse due to its high global warming potential. For its replacement, the application of nitrogentrifluoride, in particular in dilution with rare gases was investigated. High etching rates were achieved and, at high loading factor, a gas utilization near 100% was observed. These properties make nitrogentrifluoride, despite its higher price, promising for replacement of sulfurhexafluoride.

Device Simulation

Analytic solutions of the semiconductor equations explain the most relevant relationships between material and solar cell properties of microcrystalline silicon based diodes. This was shown by a comparison with numerical simulations.

Solar Cells

Deposition of microcrystalline silicon with VHF-PECVD at high pressure and high power yield solar cells with high efficiency of 8.7% at deposition rates of 1.4 nm/s.

The highest open circuit voltages at high short circuit current are obtained for solar cells deposited near the transition from microcrystalline to amorphous growth. We achieved here the worldwide highest open circuit voltage of nearly 600 mV at AM 1.5 using HW deposited solar cells. Investigations of microstructure show for the active microcrystalline layer a very homogeneously distributed amorphous phase with a volume fraction exceeding 50%. Charge carriers generated in the amorphous phase are efficiently transferred to the crystalline phase where they use the better transport properties. In these cells, a weak Staebler-Wronski degradation is observed which is restricted to the disordered phase.

Solar cells with microcrystalline silicon show in- and meta-stabilities under treatment in water and air. While compact material is stable under atmospheric conditions, solar cells with highly crystalline, porous structure show dramatic changes of their properties.

On a-Si:C:H, silicon layers grow with amorphous structure under conditions which otherwise yield highly crystalline material. Such solar cells have a high open circuit voltage of 988 mV.

For a-Si/a-SiGe solar cells with optimized Si buffer layers and a-SiGe layer thickness of only 100 nm, a stable efficiency of 9.4% was achieved.

Solar Module Development

A comprehensive solar module characterization equipment consisting of flasher, DC-sun simulator, climatic exposure test cabinet and filter-wheel set-up for the characterization of the spectral response was installed. The adjustment of the light spectrum of the flasher enables the determination of the characteristics of amorphous, microcrystalline and tandem solar modules with an error less than 2 % with respect to the DC-sun simulator.

A stable efficiency of 10.2 % was achieved for a small area solar module (aperture area: 64 cm₂) based on amorphous and microcrystalline Si.

We investigated the role of the total gas flow during microcrystalline silicon solar cell deposition. Based on these results we developed a process which requires only very low hydrogen gas flows and thus promises cost reductions.

Sensors

An interferometer sensor was realized detecting the intensity profile of a standing wave. It consists of two transparent nip diodes with an absorber layer thickness of only 40 nm and a phase shifter. Based on an optimised multi-layer stack design, two sinusoidal photocurrents with a phase shift of 87° were obtained.