Pore-Filling of Spiro-OMeTAD in Solid-State Dye Sensitized Solar Cells: Quantification, Mechanism, and Consequences for Device Performance
In this paper, the pore filling of spiro-OMeTAD (2,2',7,7'-tetrakis-(N,N-di-p-methoxyphenylamine)9,9'-spirobifluorene) in mesoporous TiO2 film is quantified for the first time using XPS depth profiling and UV-Vis absorption spectroscopy. It is shown that spiro-OMeTAD can penetrate the entire depth of the film, and its concentration is constant throughout the film. We determine that in a 2.5-mu m-thick film, the volume of the pores is 60-65% filled. The pores become less filled when thicker films are used. Such filling fraction is much higher then the solution concentration because the excess solution on top of the film can act as a reservior during the spin coating process. Lastly, we demonstrate that by using a lower spin coating speed and higher spiro-OMeTAD solution concentration, we can increase the filling fraction and consequently the efficiency of the device.