Ultrathin flexible crystalline silicon: Microsystems enabled photovoltaics

Summary form only given. Reducing the thickness of crystalline silicon wafers has evolved in the solar industry. As of 2010, most of the silicon solar cell companies were working with 6 inch wafers with thicknesses between 180 and 200 μm. In addition, a significant portion of the crystalline silicon material is lost during sawing. The effective material usage is equivalent to a wafer with a thickness of 310-475 μm depending on the thickness of the cut wire. Although there is a strong cost driver to use thinner wafers, handling wafers thinner than 180 μm is challenging while maintaining adequate yield. We present an approach to create ultrathin (<;20 μm) and highly flexible crystalline silicon sheets on inexpensive substrates. As a demonstration, our group created silicon sheets capable of bending radii as small as 2 mm. Using Microsystem tools, we created a suspended sub millimeter honeycomb segmented silicon structure anchored to the wafer by small tethers. This structure is created in a standard thickness wafer enabling compatibility with common tools. The procedure allows all the high temperature steps necessary to create a solar cell to be done while the cells are on the wafer. In the transfer process, the cells attach to an adhesive flexible substrate which, when pulled away from the wafer, causes the tethers to break and releases the honeycomb PV cell structure. We have previously demonstrated that small and ultrathin crystalline silicon die of this scale can be converted into highly efficient solar cells with efficiencies up to 14.9% in thicknesses of 14 μm. Given the highly flexible nature of the silicon sheet demonstrations; achieving high-efficiency (>;15%), highly-flexible PV modules should be possible with this approach.