Electrospun Composite Nanofiber Transparent Conductor Layer for Solar Cells
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Electrospun Composite Nanofiber Transparent Conductor Layer for Solar Cells Justin Ritchie1, Joël Mertens1, Heejae Yang1, Peyman Servati2, Frank K. Ko1 1
Dept. of Materials Engineering, University of British Columbia, 309-6350 Stores Road Vancouver, B.C. V6T 1Z4, Canada 2 Dept. of Electrical Engineering, University of British Columbia, 5500 - 2332 Main Mall Vancouver B.C. V6T 1Z4, Canada ABSTRACT Developing a durable and scalable transparent conductor (TC) as an electrode with high optical transmission and low sheet resistance is a significant opportunity for enabling next generation solar cell devices. High performance fibrous composite materials based on a carrier polymer with embedded functional nanostructures have the potential to serve as a TC with high surface area that can be deposited by the novel and scalable process of electrospinning. This work presents the development of a fibrous TC, where polyacrylonitrile (PAN) is used as a carrier polymer for multi-walled carbon nanotubes (MWCNT) to create electroactive nanofibers 200-500nm in diameter. Once carbonized, thin layers of this material have a low sheet resistance and high optical transmission. It is shown that in a two stage carbonization process, the second stage temperature of above 700C is the primary factor in establishing a highly conductive material and single layers of nanofibers are typically destabilized at high temperatures. A high performance TC has been developed through optimizing carbonization rates and temperatures to allow for single nanofiber layers fabricated by electrospinning MWCNT/PAN solutions onto quartz. These TCs have been optimized for concentrations of MWCNTs less than 20% volume fraction with well above 90% transmissivity and sheet resistances of between .5-1kohm/square. The required MWCNT loading is well below that for TCs based on random networks of MWCNTs. INTRODUCTION As the thin film photovoltaics (PV) industry develops, a scalable alternative to In2O3:Sn (ITO) must be developed. Depending on ITO as the primary transparent conductor for solar cell architectures will risk cost increases due to indium scarcity and limits the development of flexible solar cells because of ITO’s brittleness. Some estimates have assumed a ten year supply of relatively abundant indium remaining [1]. However, the global economic recession beginning in 2008 has delayed consumption rates of indium and reduced demand as reflected in recent prices of indium which has dropped well below its all-time high set in 2006-2007. Previous studies have calculated that the cost of manufacturing flexible ITO-coated flexible plastic substrates range from $7.90/g to $13.98/g which is based on a cost of ITO from Aldrich of $2.50/g. Currently the bulk cost of ITO from Aldrich is $3.03/g which is a 17.4% increase since the referenced calculation from 2009 [2]. A continual challenge is present by relying on a nonrenewable resource. ITO is on an increasing cost curve over the long-term thus heightening the importance of a scalable and durable ITO replacement to bolst
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