Pore Filling of Spiro‐OMeTAD in Solid‐State Dye‐Sensitized Solar Cells Determined Via Optical Reflectometry

A simple strategy is presented to determine the pore‐filling fraction of the hole‐conductor 2,2‐7,7‐tetrakis‐N,N‐di‐pmethoxyphenylamine‐9,9‐spirobifluorene (spiro‐OMeTAD) into mesoporous photoanodes in solid‐state dye‐sensitized solar cells (ss‐DSCs). Based on refractive index determination by the film's reflectance spectra and using effective medium approximations the volume fractions of the constituent materials can be extracted, hence the pore‐filling fraction quantified. This non‐destructive method can be used with complete films and does not require detailed model assumptions. Pore‐filling fractions of up to 80% are estimated for optimized solid‐state DSC photoanodes, which is higher than that previously estimated by indirect methods. Additionally, transport and recombination lifetimes as a function of the pore‐filling fraction are determined using photovoltage and photocurrent decay measurements. While extended electron lifetimes are observed with increasing pore‐filling fractions, no trend is found in the transport kinetics. The data suggest that a pore‐filling fraction of greater than 60% is necessary to achieve optimized performance in ss‐DSCs. This degree of pore‐filling is even achieved in 5 μm thick mesoporous photoanodes. It is concluded that pore‐filling is not a limiting factor in the fabrication of “thick” ss‐DSCs with spiro‐OMeTAD as the hole‐conductor.