SBIR Phase I: Subwavelength Structures for Enhanced Absorption in Thin Silicon Films

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This Small Business Innovation Research (SBIR) Phase I project will develop thin film (less than 50 microns) crystalline silicon (Si) photovoltaic (PV) cells. At present, Si wafers account for approximately 50 per cent of the completed PV module cost. Weak near-infrared absorption requires 250-micron thick Si films to absorb available optical radiation. Preliminary calculations indicate that appropriately designed subwavelength structures can achieve comparable absorption in films 20-50 microns thick. These subwavelength structures can now be fabricated using recent advances in Si reactive ion etching (DRIE) technology. Preliminary tests with DRIE techniques have demonstrated aspect ratios (depth to linewidth) greater than 30 for grating periods less than 1 micron. Phases I will explore a technique of microstructured thin films lift-off from a conventional Si substrate. The lift-off film is bonded to a low-cost substrate for subsequent solar cell processing. Original wafers are re-used following a simple planarization step. Several lift-off steps from the same Si wafer will offset added processing costs and permit savings in Si material costs.The underlying technology is expected to find applications in homo- and hetero-epitaxial growth on nanostructured Si substrates. Commercial applications are anticipated in this new approach to thin film Si solar cell technology.