Chemically Robust Phthalocyanines: Photosensitizer and Electron Shuttle in Solid State Dye Sensitized Solar Cells
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Chemically Robust Phthalocyanines: Photosensitizer and Electron Shuttle in Solid State Dye Sensitized Solar Cells Patrick J. Dwyer1, Rory J. Vander Valk1, and Stephen P. Kelty1 1
Center for Computational Research, Department of Chemistry and Biochemistry, Seton Hall University, South Orange, New Jersey 07079, U.S.A ABSTRACT A completely solid state dye sensitized solar cell (DSSSC) is proposed in which chemically robust phthalocyanine (Pc) sensitizers, F16ZnPc and F40ZnPc, are sandwiched between n-TiO2 and p-NiO. While the energy conversion efficiencies of conventional Grätzel cells are continually increasing, the DSSSC design effectively solves the long term stability issues of the volatile liquid electrolyte. Through analysis of the electronic structure of the Pc|semiconductor systems, the free energy associated with hole injection into the valence band of NiO upon photoexcitation of the sensitizer and electron injection into the conduction band of TiO2 from the reduced form of the sensitizer as well as the competing charge recombination processes are calculated. Thermodynamically, the charge injection processes are found to be favored over the undesired charge recombination processes. These findings suggest promising energy conversion for the NiO|Pc|TiO2 DSSSC. INTRODUCTION Since their discovery in 1991 by Grätzel [1], dye-sensitizer solar cells (DSSCs) have attracted extensive research attention and have become one of the most promising renewable energy sources [2, 3]. In a conventional Gratzel cell, photoexcitation of the adsorbed sensitizer is followed by an ultrafast (fs) electron injection from the sensitizer into the conduction band of an n-type semiconductor substrate, typically TiO2. Extensive experimental and theoretical efforts to understand and tune the properties of n-type DSSCs over the past two decades have led to conversion efficiencies as high as 13% [4]. More recently, interest in p-type DSSCs has grown as well [5], driven particularly in regard to tandem photo cells that incorporate both (anodic) ntype and (cathodic) p-type electrodes [6]. The principles of operation in a p-type DSSCs are much like that of n-type cells except photoexcitation of the sensitizer results in the injection of holes into the valence band of the semiconductor. Although the efficiency of both n-type and p-type Gratzel cells is continually increasing, current cell design typically contain organic solvents and highly corrosive redox mediators. This leads to long term problems in sensitizer desorption, electrode stability, and electrolyte leakage. Recent efforts have been focused on removing the liquid electrolyte to form completely solid state dye sensitized solar cells (DSSSCs) [7, 8]. The DSSSC was first conceptually realized as early as 1995 by Tennakone et al. [9]. In this first study the natural flower pigment (cyaniding) was employed as a sensitizer on the n-type semiconductor TiO2 and p-type semiconductor CuI. Photoexcitation of the dye results in electron injection into the CB of TiO2 leaving the oxidized form of
