Excited state dynamics of organic semiconductors measured with shot-to-shot correction of scatter and photoluminescence

Molecular aggregates and crystallites that emerge during the casting of semiconducting organic films from solution are highly sensitive to deposition conditions. Their structure determines the electronic properties, and ultimately the utility, of these materials as semiconductors. We report the development of a single-shot transient absorption (SSTA) spectrometer to measure the evolving excited state dynamics of organic molecules and polymers in situ during their casting into films from solution. The formation of molecular aggregates and crystallites changes the electronic structure, excited state dynamics, and concomitantly, the photoluminescence (PL) yield and structural heterogeneity. When these latter contributions are dynamic, they hinder accurate SSTA measurements of excited state dynamics. In this work, an additional optical chopper in the probe beam path is shown to correct for dynamic scatter and PL from the pump pulse. The importance of the correction provided by this optical chopping scheme to the measurement of photoluminescent and/or scattering samples is demonstrated using SSTA measurements of a sulforhodamine solution with high PL, a drop-cast sulforhodamine film with less PL, and a film of a prototypical organic semiconductor, poly(3-hexylthiophene-2,5-diyl).