A recent research shows that the pore filling of spiro-OMeTAD (2,2(,7,7(-tetrakis-(N,N-di-pmethoxyphenylamine) 9,9(-spirobifluorene) in mesoporous TiO2 films is quantified for the first time using XPS depth profiling and UV–Vis absorption spectroscopy. We have developed procedures to measure pore filling quantitatively using XPS depth profiling and absorption spectroscopy.We found that solution infiltration of spiro-OMeTAD in mesoporous TiO2 films effectively penetrates the entire depth of the film. The filling fraction is as high as 60–70% for films with thickness< 3 mm; however, as the film thickness increases, the filling fraction goes down, which may be the cause of poor efficiency of thick ss-DSCs.We have used a lower spin coating speed and higher spiro-OMeTAD solution concentration to increase the filling fraction and consequently the efficiency of ss-DSCs. Development of >10% efficient ss-DSCs will most likely require >5-mm-thick TiO2 films, and such significant increase in thickness will require both reduction in recombination and improvement in pore filling.
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-mm-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 than the solution concentration because the excess solution on top of the film can act as a reservoir 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.
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