Long-wavelength emission and excimer formation for PCBM in solutions and polymer matrices

The present study explores the excited-state relaxation of PCBM (phenyl-C61-butyric acid methyl ester) fullerene derivatives in various organic environments, with a focus on the influence of molecular aggregation. A comparative analysis was performed on PCBM solutions in acetonitrile and toluene, as well as in solid matrices - polystyrene (PS) and poly(methyl methacrylate) (PMMA). A comprehensive study of linear optical properties has been performed, including absorption spectra, steady-state photoluminescence (PL) spectra, kinetics of PL decay, and PL lifetimes. The study shows that, depending on the environment and the degree of aggregation, PCBM exhibits distinct emission features arising from monomer species as well as from different types of aggregates, mainly from complexes such as excimers within the long-wavelength region of 650–850 nm. It has been shown that monomeric forms of PCBM exhibit short lifetimes in acetonitrile solution, whereas excimer complexes in toluene solution and polymer matrices are characterized by longer lifetimes. The results presented herein contribute to the understanding of the nature of long-wavelength luminescence in PCBM and can be used in the development of organic photovoltaic and optoelectronic devices.