1. Nichole Squair and Robert J. LeSuer Department of Chemistry and Physics, Chicago State University, Chicago, IL 60628 We report on the use of Scanning Electrochemical Microscopy (SECM) to investigate electron transfer across the dye-stained TiO 2 /electrolyte interface in dye sensitized solar cells (DSC). The substrate used in these experiments is a TiO 2 nanoparticle suspension coated onto a transparent tin oxide glass slide and stained with an anthocyanin dye. SECM approach curves with a 5 mm diameter Pt ultramicroelectrode and ferrocene as the redox mediator display negative feedback behavior in the absence of illumination. When the substrate is exposed to light, the approach curves display a shielding effect, which is indicative of ferrocenium being generated at the substrate. Current-time amperometric curves with various tip/substrate gaps can also be used to study the heterogeneous redox process. Fe(III)(CN) 6 3- generated at an illuminated substrate is detected at a UME tip placed at various distances from the surface. Theoretical simulations using slow charge transfer kinetics at the substrate qualitatively describe the current-time-distance behavior, however rigorous agreement between theory and experiment may require consideration of diffusion through the TiO 2 film. Abstract Conclusions  i-t amperometric curves appear to be most useful in probing photochemistry at anthocyanin- coated plates  Heterogeneous electron-transfer rate very slow  Response influence by diffusion through nanostructure?  Quantitative cyclic voltammetry and approach curves require higher efficiency DSC system and optimal redox mediator.  Future experiments include  Use of non-aqueous electrolytes.  Influence of redox mediator charge on electron-transfer process.  Influence of anthocyanin functionalization on heterogeneous et process.  i-t amperometric curves appear to be most useful in probing photochemistry at anthocyanin- coated plates  Heterogeneous electron-transfer rate very slow  Response influence by diffusion through nanostructure?  Quantitative cyclic voltammetry and approach curves require higher efficiency DSC system and optimal redox mediator.  Future experiments include  Use of non-aqueous electrolytes.  Influence of redox mediator charge on electron-transfer process.  Influence of anthocyanin functionalization on heterogeneous et process. Using scanning electrochemical microscopy to investigate electron-transfer processes in dye sensitized solar cells. ~74% of anthocyanin pigment from raspberries is cyanidin-3-sophoroside Spanos, Wrolstad. J. Assoc. Off. Anal. Chem. 1987, 70, 1036.  Incentives for organic dyes  Amenable to systematic functionalization  Wealth of knowledge regarding photosensitive compounds  Cost  Methodology development  Easy-to-fabricate DSC  Cherepy et al. J. Phys. Chem. B. 1997, 101, 9342.  Incentives for organic dyes  Amenable to systematic functionalization  Wealth of knowledge regarding photosensitive compounds  Cost  Methodology development  Easy-to-fabricate DSC  Cherepy et al. J. Phys. Chem. B. 1997, 101, 9342. Methods Scanning Electrochemical Microscopy Dye Sensitized Solar Cell SECM investigation of a DSC Fe(II)  Fe(III) at UME Influence of substrate illumination difficult to observe with SECM-LSV Two formulations used for TiO 2 film preparation  20% ethanolic suspension, stirred at RT for at least 24 h.  67% acidic aqueous suspension (with surfactant), stirred at RT for at least 24 h.  Both suspensions result in thick, cracked surfaces after doctor blading/sintering  Defects smaller and denser in ethanolic preparation  Greater surface roughness with ethanolic preparation  20% ethanolic suspension, stirred at RT for at least 24 h.  67% acidic aqueous suspension (with surfactant), stirred at RT for at least 24 h.  Both suspensions result in thick, cracked surfaces after doctor blading/sintering  Defects smaller and denser in ethanolic preparation  Greater surface roughness with ethanolic preparation AqueousEthanolic Sample concentration profile showing linear diffusion of Fe(III) from substrate and consumption at UME tip  Detection of Fe(III)(CN) 6 3- generated at DSC illuminated for 5 s.  Approximate distances based on fit of approach curve to insulator feedback  Theory for kinetic control of Fe(III)(CN) 6 3- generated at DSC from 1 mM Fe(II)(CN) 6 4- in 0.1 M KNO 3.  k eff = 1.5x10 -5 cm s -1, D 0 = 7.1x10 -6 cm 2 s -1 ; 5  m radius Pt tip  Flattening of experimental i-t curves may be due to diffusion through TiO 2 film.  Detection of Fe(III)(CN) 6 3- generated at DSC illuminated for 5 s.  Approximate distances based on fit of approach curve to insulator feedback  Theory for kinetic control of Fe(III)(CN) 6 3- generated at DSC from 1 mM Fe(II)(CN) 6 4- in 0.1 M KNO 3.  k eff = 1.5x10 -5 cm s -1, D 0 = 7.1x10 -6 cm 2 s -1 ; 5  m radius Pt tip  Flattening of experimental i-t curves may be due to diffusion through TiO 2 film. Current response at UME following 5 s illumination of anthocyanin/TiO 2 film i-t amperometric curves  5 mM ferrocenemethanol in 0.1 M KCl  5  m radius Pt UME tip (RG ~5)  LSV obtained at tip/substrate gap of ~ 1  m  Quantitative analysis of approach curves complicated by large substrate (linear diffusion)  5 mM ferrocenemethanol in 0.1 M KCl  5  m radius Pt UME tip (RG ~5)  LSV obtained at tip/substrate gap of ~ 1  m  Quantitative analysis of approach curves complicated by large substrate (linear diffusion)  Ultramicroelectrode (UME) brought close to (~ 1 um) a large electrode  Negative feedback  Blocking effect dominates current  Substrate does not regenerate redox mediator  Positive feedback  Substrate regenerates redox mediator completely  Mixed feedback  Can measure reaction kinetics at substrate  Ultramicroelectrode (UME) brought close to (~ 1 um) a large electrode  Negative feedback  Blocking effect dominates current  Substrate does not regenerate redox mediator  Positive feedback  Substrate regenerates redox mediator completely  Mixed feedback  Can measure reaction kinetics at substrate Fe(III)  Fe(II) at UME Illumination decreases [Fe(II)] near surface.Fe(III) production at surface is slow