Project: Silicon Wafers for Solar Photovoltaics by Powder Sintering

Monocrystalline and multicrystalline silicon are going to continue to play the major role as active semiconductors for solar photovoltaic (PV) applications. This market is increasing each year by more than 40%. The spin-off S'TILE was created in February 2007 from Poitiers University and CNRS with private investors to develop a new method of producing silicon material for PV. S'tile activity is COly focused on wafer preparation. As a matter of fact, the production of conventional multi-crystalline wafers show strong limitations associated with the silicon melting, the ingot crystallization and the wafer sawing. These steps involve a tremendous amount of material and energy consumption ; this is the cause of the high costs of wafer manufacturing which is now considered as the bottleneck for the development of solar photovoltaic. In addition, wafer areas cannot be increased beyond current dimensions 156 x 156 mm2 due to excessive breakage during sawing and handling after sawing. Ribbon technologies try to overcome these limitations by preparing crystalline wafers directly from the silicon melt. Although the material consumption is strongly decreased in ribbon technology, the low crystal pulling rate doesn't allow for significant production yields. Compaction and Sintering of Silicon Powder Beds is a new process first introduced by Poitiers University and now under development in S'TILE. The objective of the company is to optimize the process by making R&D on the different steps and then to transfer the technology into industry. In contrast with conventional production methods, the process largely simplifies the technology, reduces silicon material losses and energy consumption. It also offers a very promising opportunity for increasing area cells together with a great variety of shapes, size, forms and surface morphology of wafers, which can be determined by the geometry of the moulds used to press the silicon powders. The process includes two important operations of sintering by hot pressing to solidify and make denser the material and thermal recrystallization to grow the grains for better electrical properties. S'tile was laureate of the 2007 national prize for the creation of innovative companies. The grant is presently used in a 2 year project started in first term 2007 to develop the sintering process and design a new hot press for 156 mm wafers. Up to now recrystallization has been studied in cooperation with Lisbon University and ISE Fraunhofer Freiburg. It can be done different techniques ; generally the wafer is partially or locally fused by exposing the surface to strong energy irradiation. Resistive heaters, lamp or laser irradiation techniques have been widely used in the semiconductor industry. S'tile developed a full surface technique using furnace radiative heaters. Lisbon University and ISE Freiburg are currently applying Zone Melting Recrystallization (ZMR) on silicon deposited layers for solar photovoltaic. In all cases preliminary experiments have shown that crystal growth occurred with a microstructure, carrier mobility and lifetimes significantly improved. In order to optimize these crystallization techniques according to the sintered material, complementary studies are needed. The first objective of the present project is to make research on both sintering and recrystallization in order to achieve physical and electrical properties compatible with solar photovoltaic. An important condition to obtain a good crystallization is to clean and prepare the wafer surface by appropriate techniques. Physico-chemical techniques such as polishing or plasma etching will be studied and compared during this phase. During the second phase of development, doping and electrical contact will be optimized and solar cells will be done and tested. According with the results of the first phase a new equipment of recrystallization will be designed, build and tested. Some problems must be solved for this step such as minimization of wafer deformation, impurity contamination and increase of grain dimension without creating structural defects. The last phase of the project will be devoted to pre-production to demonstrate the process feasibility by producing wafers and solar cells and their characterizations. A pilot line will be installed at S'tile including equipments of hot pressing, recrystallization and a new equipment of surface cleaning with all the facilities for clean handling of Si material and realize a pre-production of wafers. S'tile intends to launch its own production within 2 years from the end of the project._x000D_The consortium is composed of S'tile company and Lisbon University both having large expertise and back-grounds in semiconductor for PV. Cooperations with other research centers very active in the field are included as sub-contractors in the project such as Fraunhofer ISE Freiburg, INSA de Lyon, CEA/INES, and Ecole des MINES/St Etienne._x000D_

Acronym	SiPoSi (Reference Number: 4336)
Duration	02/06/2008 - 07/06/2010
Project Topic	The aim of SiPoSi project is to develop a new cost effective route for the production of silicon wafers for solar photovoltaics with an innovative process using hot-pressing directly from silicon powders and by partiallly fusing the wafer for a better recrystallization.
Project Results (after finalisation)	SiPoSi project was devoted to the development of a new route for the production of silicon wafers for solar photovoltaic energy with an innovative and cost effective process. The technology is based on two CO processes one of making wafers by using sintering of silicon powders and the other of partial melting of the wafer using zone melting recrystallisation. To demonstrate the ability of this new tecnology to be transferred into industry , one of the CO demonstration was the preparation of solar cells on top of the wafer following different by deposition and surface treatments. A very important effort was concentrated on the recrystallization process itself and on the development and design of an adapted equipement. The different phases and tasks have been focused on material and processes optimizations and on the preparation of test solar cells for demonstration ; a task was devoted on the design of a pilot line for pre-industrialization of the technology. The program was completed last October 2010. The wafer preparation used a new sintering process developed at S’Tile-Poitiers. The following recrystallization step compared different ways of Zone Melting recrystallization (ZMR) processes. One used an equipement developed at Fraunhofer ISE Freiburg and the other at Lisbonne University. S'Tile at Poitiers made some experiments on furnace and laser recrystallization. Each technology was tested and has given different and partially interesting results. The Lisbonne ZMR and the furnace heater S'Tile could recrystallize the wafer producing large crystals, very comparable to multicrystalline conventional multicrystalline wafers. Test solar cell were obtained with 8.6% solar efficiencies. However defective zones on the wafers with uncomplete recrystallization were observed ; defects were shown to be due to contaminations during sintering ; by improving the conditions of sintering, the observed defectivity after ZMR at Lisbonne was significantly decreased. It allowed to determine the best best adapted characterstics of sintered material for optimum ZMR processing. Then, by using the surface ZMR available at Fraunhofer ISE Freiburg, a serie of test cells with a conversion efficiency of 9.2% was optained on different 2x2 cm2 area cells. As the cell technology used didn't include the complete optical confinement process (texturization, back mirror), including this stpes, this was considered to correspond to an efficiency of 11 % that is the most interesting result obtained using thin crystalline film technology on foreign substrates.
Network	Eurostars
Call	Eurostars Cut-Off 1

Project partner

Number	Name	Role	Country
2	Fundacao da Faculdade de Ciencias da Universidade de Lisboa	Partner	Portugal
2	S'Tile	Coordinator	France