Progress in Producing Large Area Flexible Dye Sensitized Solar Cells
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Progress in Producing Large Area Flexible Dye Sensitized Solar Cells Krishna C. Mandal, Michael Choi, Caleb Noblitt and R. David Rauh EIC Laboratories, Inc., 111 Downey Street, Norwood, MA 02062-2612, USA ABSTRACT Dye sensitized nanocrystalline TiO2 solar cells have been reported with over 11% efficiency and are extremely promising as very low cost and lightweight photovoltaic sources. However, most reports are for cells of low area fabricated on glass, which withstands processing temperatures of ~450oC. In this paper, we describe the fabrication and performance of cells made on flexible ITO-coated polyethylene terephthalate (PET) substrates with 6” x 3” dimensions. To improve the efficiency in the cells, we enhanced the ITO current collection efficiency with metallization fingers. The fingers resulted in a >10 fold increase in short-circuit current under normal solar illumination compared to cells without metallization. Further improvements were realized by passivating the metallization fingers at the metal/polymer electrolyte interface. INTRODUCTION Dye sensitized solar cells (DSSC) are usually prepared by first depositing colloidal TiO2 onto a transparent conductive oxide coated glass substrate. The colloidal layer is then sintered at ~450oC to form a porous pure anatase layer 5-10 µm thick. Next, the layer is exposed to a solution of a sensitizing dye carrying -COO- functional groups, which are believed to bind to the Ti+4 surface sites. The Ru(II) complexes usually employed as sensitizing dyes apparently undergo electron transfer to the Ti(IV) sites on nearly the optical time scale, leaving the oxidized sensitizer, a process which has been argued to protect the dye against slower irreversible photodegrative pathways. These sensitized electrodes may be used as the photoanode in regenerative photoelectrochemical (PEC) cells. The regenerative redox couple is typically I-/I3- in a liquid nonaqueous electrolyte. Grätzel and co-workers were the first to have reported >11% solar conversion efficiencies for small area devices [1]. At its current state of development, at least in the published literature, the DSSC has relied mostly on the use of glass substrate materials. Glass is used because it is optically transparent and it can withstand the high temperature annealing currently required to achieve high photoactivity of the nanoporous TiO2. However, lightweight polymer substrates are clearly advantageous for portability, adaptability and ease of handling. Furthermore, the use of liquid electrolytes is unacceptable due to sealing difficulties and inhomogeneous electrolyte distribution, especially if a flexible version of the DSSC is to be realized [2]. It is the purpose of this study to evaluate DSSCs in a flexible, large-area cell configuration. To achieve this, we have employed a commercial ITO-coated polymer as the substrate rather than glass. The electrolyte is based on a plasticized polyvinylidene difluoride (PVDF) composition derived from a material used in flexible Li-ion batteries. Cells were nominally
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