Understanding of morphology evolution in local aggregates and neighboring regions for organic photovoltaics

Abstract

Fluorescence intensity and its ratio mapping combined with time-dependent optical microscopy and atomic force microscopy (AFM) were used to understand morphology evolution of local aggregates and neighboring regions for organic solar cells. Three solvents with different boiling points including chlorobenzene (CB), 1,3-dichlorobenzene (1,3-DCB) and 1,2-dichlorobenzene (1,2-DCB) were used to engineer morphology. These solvents affected morphology evolution factors such as solvent evaporation rate, formation (e.g., growth rate, size and/or quantity) of (6,6)-phenyl-C61-butric-acid methyl ester (PCBM)-rich aggregates, and packing/ordering of poly(3-hexylthiophene) (P3HT). Three local regions (1, 2 and 3) including microscale aggregates and their surrounding areas were identified to explore the mechanism of morphology evolution. Region 1 was the PCBM-rich aggregates; region 2 was the PCBM-deficient area; and region 3 was the area composed of a relatively normal P3HT/PCBM composite beyond region 2 for each solvent. The intensity of fluorescence spectra decreased as region 1 > region 2 > region 3 in thermally annealed (140 °C, 20 min) P3HT/PCBM blend film from each solvent. The highest fluorescence intensity in region 1 was probably caused by the relatively poor phase separation where both PCBM and P3HT formed large isolated domains. The higher fluorescence intensity ratio (720 nm/650 nm) suggested a larger relative amount of PCBM molecules, supported by similar morphologies in fluorescence intensity ratio mapping compared to those in optical images. The fluorescence intensity ratio was with the order of region 1 > region 3 > region 2 in both CB and 1,3-DCB based films, but with region 1 > region 2 > region 3 for the 1,2-DCB based film. The order of effective area taken up by aggregates was CB > 1,3-DCB > 1,2-DCB in annealed (140 °C, 10 min) bulk blend films. The final solar cell performance agreed with morphology results. This work is imperative with regards to revealing the mechanism of morphology evolution in local aggregates and surrounding regions for organic photovoltaic films.