New Silicon Wafer Production Method to Make Solar Cells Cheaper

An etched silicon wafer. (Credit: Flickr @ Rob Bulmahn http://www.flickr.com/photos/rbulmahn/)
An etched silicon wafer (Credit: Flickr @ Rob Bulmahn http://www.flickr.com/photos/rbulmahn/)

A team of researchers from the Nanoengineering Research Centre (CRNE) and the Department of Electronic Engineering at the Universitat Politècnica de Catalunya—BarcelonaTech (UPC) has found a way to manufacture crystalline silicon materials cheaper and faster. The results of their research have recently been published online in Applied Physics Letters. This thin crystalline silicon wafer production method can be used to fabricate the latest generation of solar cells.

Thin crystalline silicon wafers measuring around 10 µm (micres) are costly but also very sought after in the field of microelectronics, especially in view of the growing demand for 3D circuit integration with microchips. Silicon wafers also have potential photovoltaic applications in the medium term in the conversion of sunlight to electricity and the production of more affordable, more flexible and lighter solar cells.

In recent years, techniques have been developed to obtain increasingly thinner crystalline silicon wafers from monocrystalline cylindrical ingots. Layers cut from the ingots using a multithreaded saw impregnated with abrasive material have a minimum thickness of around 150 µm. Obtaining wafers that are any thinner is more complicated, as existing methods only allow such wafers to be obtained one at a time. Furthermore, 50% of the silicon is lost in the process.

The technology developed by the research team—David Hernández, Trifon Trifonov and Moisés Garín, led by Professor Ramon Alcubilla – enables a large number of crystalline layers, controlled for thickness, to be produced from a single crystalline silicon wafer in just a single step. The outcome is a kind of crystalline silicon “millefeuille” produced more efficiently, more rapidly and more affordably than by existing methods.

The methodology developed by the scientists is based on making small pores in the material and applying a high temperature during the manufacturing process. Multiple separate crystalline silicon wafers are obtained by carefully controlling the pore profiles. Precise control over diameter controls both the number of layers and their thickness. The millefeuille silicon layers are then separated by exfoliation.

The resulting number of silicon layers is determined by the thickness of the layers themselves and the initial thickness of the wafer. The CRnE researchers have succeeded in creating up to 10 thin wafers (5-7 µm thick) from a single 300 µm thick wafer.

The demand for thin and ultra-thin crystalline silicon wafers responds to the application possibilities offered by 3D circuit integration of micro-electromechanical systems (MEMS) with conventional microchips and also to the latest generation of photovoltaic technology. Wafer cutting for solar cell production, for example, has been steadily improving.

Thickness has been reduced (350 µm in the 1990s to 180 µm currently) while efficiency has been enhanced, resulting in reduced manufacturing costs; nonetheless, greater reductions are likely to be difficult to achieve. It has been shown that, despite lesser thickness, the wafers retain a high capacity to absorb solar energy and convert it into electricity.

Hernández, D., Trifonov, T., Garín, M., & Alcubilla, R. (2013). “Silicon millefeuille”: From a silicon wafer to multiple thin crystalline films in a single step Applied Physics Letters, 102 (17) DOI: 10.1063/1.4803009

The above story is based on or reprinted from materials provided by Polytechnic University of Catalonia.

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