ENERGIX-Stort program energi

The crystalline solar cell market is today completely dominated by diamond sawn high efficiency single crystal cells with the multi-crystalline (mc) cells no longer being competitive in terms of efficiency and the gap in productivity/production cost between multi- and single-crystal wafers/cells closing. As such, studies of mc materials have become less topical. Nevertheless, the DiaMApp project brought forward a fundamental understanding of the effects of crystal orientation and defects on wafer cracking behaviour, which is transferrable to both single crystals of silicon and possibly other materials. Some of the main findings were -Speed and scratching directions are less important than diamond shape when it comes to amount of sawing damages at a <100> surface -Diamonds wear much faster than expected -Dry cutting will, as expected, increase the friction coefficient, but will not affect the sawing damage significantly. This is an important point, since dry in-cut is currently proposed as an alternative to achieve thinner wafers in industry. -Microcracks expand mainly along <111>-directions -How the <111> planes are oriented relative to the sample surface will determine microcrack length and amount of chipping -The number and the alignment of crack planes relative to the surface orientation and the scratching direction in a sample will determine the complexity of the resulting crack- and chipping pattern and thereby determine the material removal mechanism. In addition, new recipes for both surface damage removal and optimized texturing of mc wafers, taking into account all crystal orientations, have been developed. A combinatory technique that quantifies the effect of the etching procedure on different crystal directions in the mc wafers based on LAUE diffraction and white light interferometry has also been developed. This is fundamental knowledge which is applicable also for other systems. An optimised damage removal solution hydroflouric (HF), (HNO3) and acetic aid (CH3COOH) solution was demonstrated to give best resuts when used for for 5 minutes at room temperature. Damage removal was subsequently followed by a chemical polishing step where wafers were treated in a 5% KOH solution containing Isopropyl alcohol (IPA) and sodium hypochloride (NaOCl) at 85C for 10 minutes prior to further texturing. A texturing solution based on KOH with higher concentrations of NaOCl was demonstrated to give the best even, overall texture of wafers. In order to make the project more up to date also for mono-crystalline wafers, ALD deposition of passivating/highly conducting optical TiOx layers on high efficiency N-type mono wafers was also carried, showing promise as a future deposition technique for these wafers.

1) It is expected that industry will have a better fundamental basis for understanding the factors that effect wafer breakage and hence have the opportunity to adjust processes accordingly 2) A combinatory method to use Laue scanning and White Light Inferometry to determine crystal orientations and relative heights on a surface has been developed that can be used for different materials applications in addition to silicon wafers. 3) Different precursors for ALD deposition of TiOx conducting/passivation layers on mono-crystalline wafers have been tested and will be useful for precursor selection industrially.

In the project DiaMApp we will solve fundamental questions about the mechanisms operating during diamond wire sawing of high performance multicrystalline silicon and how the surface can be treated to achieve black silicon for high efficiency solar cell applications. Multicrystalline silicon is the technology with the highest potential for enabling widespread deployment of photovoltaic energy generation through low cost and high productivity. Introducing diamond sawing for multicrystalline silicon wafering is, however, essential to meet the ever-increasing demand on lower cost and higher efficiency. To achieve this, two main challenges need to be solved; 1) yield and 2) surface texturation and passivation. There is no established understanding of the fundamental mechanisms operating during diamond sawing today. This project aims to explain the mechanisms operating during scratching, such as phase transformations, initiation and propagation of microcracks and chipping, and how they will vary with crystallographic- and mechanical parameters. Furthermore, this understanding on microscopical scale needs to be translated into parameters that can be feed into a global model for a multi wire saw to be able to optimize the process for speed and reduced breakage. The as-cut wafer surface will be treated to remove saw damages and to create a surface texture which is optimized for light absorption (black silicon) and the subsequently passivation which is absolutely crucial to meet today's quality demand for high efficiency solar cell concepts. We aim to develop an etching procedure tailored for diamond sawn high performance multicrystalline silicon wafer surfaces. Lastly we will do a proof of concept test where diamond sawn multicrystalline wafers with DiaMApp-surface are processed in a high efficiency solar cell line. The project contains 3 WPs: WP1: Wafer sawing WP2: Texturisation WP4: Proof of concept The project will include 1 PhD and several project/master students