Melt-infiltration of spiro-OMeTAD and thermal instability of solid-state dye-sensitized solar cells

A method for achieving complete pore-filling in solid-state dye-sensitized solar cells termed melt-infiltration is presented: after the customary solution-processed deposition of spiro-OMeTAD, the device is heated above the glass transition temperature of spiro-OMeTAD to soften the material and allow capillary action to pull additional spiro-OMeTAD from the overlayer reservoir into the pores. The pore-filling fraction increases from 60–65% to 90–100% as a result of melt-infiltration. The organic D–π–A dye used in this study is found to withstand the thermal treatment without performance loss, unlike ruthenium-based dyes. Through our experiments, we find that the 4-tert-butylpyridine (tBP) additive, commonly used in dye-sensitized solar cells, evaporates from the device during heat treatment at temperatures as low as 85 °C. This significantly impacts device performance, potentially excluding its use in commercial applications, and demonstrates the need for a more thermally stable tBP alternative. Melt-infiltration is expected to be a viable method for achieving complete pore-filling in systems where volatile additives are not required for operation.