Molecular-Ink Route to Thin Film CZTSSe Solar Cells

Wednesday, October 19, 2011: 12:30 PM
L100 J (Minneapolis Convention Center)
Hugh W. Hillhouse, School of Chemical Engineering, Purdue University, West Lafayette, IN and Wooseok Ki, Chemical Engineering, University of Washington, Seattle, WA

Given the terawatt scale of future energy needs, the most promising future photovoltaic materials should be Earth abundant with their primary mineral resources distributed across several geographic regions and their supply chains robust to reduce concerns of price volatility. In addition, the process of forming the solar cell should be scalable, low-cost, and not utilize dangerous or toxic materials. The strongest initial candidate appears to be Cu2ZnSnS4 (CZTS), but there are other potential material systems such as FeS2, CuO, and PbS that are just beginning to be re-examined with solution phase processing. Most solution-phase CZTS thin film solar cells synthesized to date were obtained from deposition of metals (Cu, Zn, & Sn) or metal precursors first, followed by sulfurization with H2S. More recently, two approaches that form the chacogenide directly (CZTS) have yield substantially higher efficiency devices from solution processing (even higher than the best vacuum deposited absorbers). Both new approaches incorporate Se to form Cu2ZnSn(Sx,Se1-x)4 (CZTSSe). Power conversion efficiencies have reached 7.2% for a nanocrystal-ink route and 9.6% for a route that uses molecular precursors dissolved in hydrazine, which is toxic and carcinogenic. Here we report a new, facile, and scalable chemical route to Earth abundant element thin film solar cells by coating a solution of highly soluble, inexpensive, and commercially available precursors in an environmentally friendly non-toxic solvent to form device quality films. Air-stable CZTS photovoltaic devices with 4.1% total area power conversion efficiency are obtained. We have generalized the chemical route and have used it to also fabricate PbS devices that are 1.5% efficient. The importance of this new route is that it: (1) avoids toxic solvents, (2) has a metal yield of 100% (as compared to 70-90% from nanocrystal-ink routes), (3) has no liquid wastes, and (4) yields good electronic quality absorber material.

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