Axel Metz

Fixed charge density in silicon nitride films on crystalline silicon surfaces under illumination

A novel method is applied to determine the fixed positive charge density Q/sub f/ in plasma-enhanced chemical vapor deposited silicon nitride (SiN/sub x/) films on crystalline silicon surfaces. In this method, both surfaces of the SiN/sub x/-passivated silicon wafers are charged using a corona chamber and the deposited charge density is measured by means of a Kelvin probe. Subsequently, the carrier lifetime is measured and Q/sub f/ is determined from the dependence of the measured carrier lifetime on the corona charge density. This measurement technique allows us for the first time to determine the crucial parameter Q/sub f/ under illumination. In contrast to previous studies, a very high Q/sub f/ of about 2.3 /spl times/ 10/sup 12/ cm/sup -2/ is found, which is in good correspondence with CV measurements carried out in the dark. Finally, we show that the injection level dependence of the surface recombination velocity is mainly due to recombination in the space charge region at the Si/SiN/sub x/ interface.

Rear-Surface Passivation Technology for Crystalline Silicon Solar Cells: A Versatile Process for Mass Production

Over the past few years, significant progress has been made in integrating cell structure improvements on the cell front side into mass production, such as, e.g., selective emitters. With these improvements, the large-area aluminum back-surface field clearly limits the efficiency of typical industrial cells. Dielectric passivation of the cell rear side provides a means for significant improvement. However, it needs to be adapted to different wafer materials and cell structures in order to obtain economic efficiency, allowing for its implementation in mass production. In this paper, we report on our cost-effective passivated emitter and rear cell (PERC) technology that easily adapts to various wafer materials, such as multicrystalline, quasi-monocrystalline, and Czochralski-grown monocrystalline material. It is suitable for wafer thicknesses down to 120 µm and all base resistivities in the range from 1 to 3.5 Ω·cm. Additionally, we investigate the compatibility with homogeneous and selective emitters on the cell front side. For commercially available Czochralski wafers, we present an efficiency gain of more than 1.0% absolute in cell efficiency with a peak cell efficiency of up to 20.2%. The usability of our PERC solar cells in modules is demonstrated with a 289-W module containing 60 PERC cells. To emphasize the efficacy of high-performance cells in modules, a simple cell-to-module factor calculation is presented.

Record efficiencies above 21% for MIS-contacted diffused junction silicon solar cells

High-efficiency solar cells obtained by a simple cost-effective manufacturing process are required for a drastic reduction of the costs of solar electricity. In this paper, an improved and yet simple processing sequence for highly efficient MIS-contacted diffused n/sup +/p junction (MIS-n/sup +/p) silicon solar cells is presented. The process is characterised by: (i) formation of a metal-insulator-semiconductor (MIS) contact on an n/sup +/-diffused emitter; (ii) aluminium metallisation for front and rear electrodes; and (iii) low-temperature surface passivation by PECVD silicon nitride. For MIS-n/sup +/p solar cells with the front grid defined by Al evaporation through a shadow mask, efficiencies of up to 20.6% have been obtained. Furthermore, mask-free metallised cells with a mechanically grooved front surface have been fabricated. These cells have reached a confirmed efficiency of 21.1%, the highest value to date reported for MIS-n/sup +/p silicon solar cells.

18.5% efficient first-generation MIS inversion-layer silicon solar cells

In this paper, progress in the development of high-efficiency metal-insulator-semiconductor inversion-layer (MIS-IL) silicon solar cells at ISFH is presented. We fabricated MIS-IL solar cells showing independently confirmed energy conversion efficiencies of up to 18.5%. This represents the highest value reported to date for MIS-IL silicon cells. The increase in cell efficiency has been possible by improvements along several lines: (i) reduced perimeter recombination losses, (ii) a reduced contact resistance of the MIS front grid, and (iii) reduced rear surface recombination losses. The cells are characterised in detail and design modifications for further improvements towards 20% efficiency are presented.

Mechanically grooved high-efficiency silicon solar cells with self-aligned metallisation

Mechanically grooved MIS-contacted p-n junction silicon solar cells are presented. These cells take advantage of a well-defined surface structure for the mask-free formation of the front grid by means of oblique vacuum evaporation. Independently confirmed one-sun efficiencies of up to 18.6% are achieved, demonstrating the great potential of the chosen cell structure for a simplified fabrication of high-efficiency silicon solar cells.

Next generation of industrial silicon solar cells with efficiencies above 20

In order to make PV an economical source of energy, cell efficiencies have to be drastically increased. A new generation of high-efficiency crystalline silicon solar cells based on the OECO (obliquely evaporated contact) technology was introduced by us, using only few, simple and environmentally benign fabrication steps. In this paper progress of the OECO solar cells is presented. This includes the superior performance of the cells under realistic illumination conditions below 1 sun and at temperatures ranging from -40/spl deg/C to +80/spl deg/C. For the industrial size of 10/spl times/10 cm/sup 2/ record efficiencies up to 20% were achieved both on FZ-Si and on Ga-doped Cz-Si. This is the highest efficiency reported for large-area industrially feasible silicon solar cells. The custom-made construction of a 1 MW/year pilot line presently installed is outlined, demonstrating that economic and environmentally benign mass production of 20% efficient silicon solar cells is no vision any more.

Hydrogen passivation of defects in EFG ribbon silicon

Abstract To enhance the bulk lifetime of multicrystalline silicon material, gettering of impurities and hydrogen passivation of defects are investigated. In edge-defined film-fed grown (EFG) ribbon silicon, an aluminium-enhanced hydrogenation of defects by silicon nitride has been reported. On thin wafers, the formation of a full area aluminium back surface field will lead to wafer bending due to different thermal expansion coefficients of aluminium and silicon. To circumvent this problem, remote plasma-enhanced chemical vapour deposited (PECVD) silicon nitride (SiN x ) as passivation scheme for the front and rear surface is proposed. In this work, the bulk passivation by hydrogenation is investigated using two different hydrogen passivation techniques: (i) passivation in a remote hydrogen plasma and (ii) passivation due to a post-deposition anneal of remote PECVD-SiN x in a lamp-heated conveyor belt furnace. Measurements of the bulk lifetime show that the lifetime improvement due to remote hydrogen plasma passivation degrades under illumination with white light. In contrast, the hydrogen passivation by a post-deposition SiN x anneal is only effective if a phosphorous-doped emitter is present below the SiN x layer during the hydrogenation. This lifetime improvement is stable under illumination.

Advanced processing steps for high efficiency solar cells based on EFG material

In the past few years the quality of Edge-defined Film-fed Growth (EFG) material has strongly improved and can now compete with most standard multicrystalline materials. The maximum conversion efficiency of solar cells based on high quality EFG material is at the moment mostly limited by the applied solar cell processing steps. The state-of-the-art high efficiency process at the University of Konstanz (UKN) in combination with some additional processing steps is presented. The latter include hydrogen passivation of bulk defects, texturisation of the front surface by remote SF6 plasma (most samples shown here were textured at IMEC), surface passivation using a silicon oxide / silicon nitride stack and the application of Laser Fired Contacts (LFC). Single additional processing steps are investigated as well as various combinations of additional processing steps.

Thin multicrystalline silicon solar cells with silicon nitride front and rear surface passivation

State-of-the-art multicrystalline silicon (mc-Si) material with minority carrier diffusion lengths exceeding the wafer thickness is commercially available today. It is expected that the diffusion length to wafer thickness ratio will be increasing further due to improved material quality and due to the trend towards thinner wafers to reduce material costs. In order to fully exploit the material quality, a solar cell process that includes excellent rear surface passivation is needed. In this paper we first discuss loss mechanism due to the bulk resistivity of thin wafers, optical losses and losses due to rear surface recombination. Solar cell results for thin mc-Si solar cells with silicon nitride front and rear surface passivation are presented. Experimental results demonstrate that due to the excellent rear surface passivation of our plasma SiN/sub x/ films, the presented solar cell process is capable of improving the solar cell performance with decreasing cell thickness.