Low-Cost, Large-Area Nanocrystalline TiO 2 -Polymer Solar Cells on Flexible Plastics
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Low-Cost, Large-Area Nanocrystalline TiO2 -Polymer Solar Cells on Flexible Plastics Krishna C. Mandal, Anton Smirnov, D. Peramunage, and R. David Rauh, EIC Laboratories, Inc., Norwood, MA. 02062-2612, USA ABSTRACT This paper describes our recent research on the development of fully flexible and low-cost dye-sensitized nanocrystalline TiO2 solar cells (DSSC). At EIC Laboratories, we have developed various large-area solid-state versions, which include both ionically, and hole-conducting allpolymer DSSCs made by continuous coating and lamination process. Various critical steps involve ∼10 micron thick nanocrystalline TiO2 film deposition on conducting PET plastics by various methods such as spraying, spin coating, and screen printing, a low temperature sintering (∼150°C) process, and polymer laminated electrolyte have been demonstrated as valuable active components. The photovoltaic (PV) performance presented in this paper show the fabricated solar cells of ∼1.2 cm2 area with reproducible AM1.5 (Air Mass 1.5) efficiencies of 4.7%. A prototype solar cell with a new hole conducting star polymer with AM 1.5 efficiency ∼2.12% has been thoroughly characterized and discussed. INTRODUCTION Dye sensitized solar cells (DSSC), first described by Grätzel [1], have generated intense interest because of their prospects for order-of-magnitude cost reductions over current photovoltaic technologies. Much research and development on these devices still focuses on using glass substrates due to the possibility of high temperature sintering of the nanostructured TiO2 photocatalytic films (~450oC), a process which is linked to maximizing device efficiency. However, there is also a parallel interest in minimizing the weight and manufacturing costs of these cells by constructing them on flexible polymer substrates. We report here results on constructing DSSCs using an all-polymer laminate construction on commercially available poly(ethyleneterephthalate) substrates coated with transparent conductive indium tin oxide (ITO). The devices incorporate high conductivity plasticized polymer electrolytes, which have been employed in high energy density Li-ion batteries. While we find that indeed there are reductions in efficiency using lower temperature processing of the nanocrystalline films, we can still achieve broadband efficiencies ∼5% using attenuated solar simulated radiation. Modified DSSCs with an all-solid state construction, in which the electrolyte is replaced by an organic hole conductor, were also demonstrated. EXPERIMENTAL DETAILS Commercially available plastic (PET) substrates coated with conductive layer of ITO (Neovac Corp., ~60 Ω/sq and ~80% transparent) were washed in a mild liquid detergent (Liquinox), rinsed with DI-water, washed with isopropanol and finally allowed to dry in a class 100 clean room environment. Substrates were masked with Kapton tape to protect the outer 1/8 inch of the ITO surface in order to provide areas for electrical contact. The TiO2 slurry was prepared by incremental addition of 20 ml DI-water,
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