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A highly NIR-absorptive organic dye forms monomers when kinetically trapped in a polymer matrix, but upon thermal annealing, aggregation causes changes in excited-state processes, which are assayed via in situ transient absorption.

 

This study seeks to illuminate the aggregation and excited state dynamics of intercalated phthalocyanine:fullerene thin films during deposition viain situUV-vis spectroscopy and a spatially encoded transient absorption (TA) spectroscopy. 

In situ linear absorption and single-shot transient absorption are employed to understand how morphological changes during thermal annealing impact the electronic structure and excited-state dynamics of a polymer: fullerene blend. 

A significant advantage of organic semiconductors over many of their inorganic counterparts is solution processability. However, solution processing commonly yields heterogeneous films with properties that are highly sensitive to the conditions and parameters of casting and processing. Measuring the key properties of these materials in situ, during film production, can provide new insight into the mechanism of these processing steps and how they lead to the emergence of the final organic film properties. The excited-state dynamics is often of import in photovoltaic, electronic, and light-emitting devices. This review focuses on single-shot transient absorption, which measures a transient spectrum in a single shot, enabling the rapid measurement of unstable chemical systems such as organic films during their casting and processing. We review the principles of instrument design and provide examples of the utility of this spectroscopy for measuring organic films during their production. 

This study aimed to investigate the effects of thermal annealing on a film of squaraine (SQ) molecules in a polymethyl methacrylate (PMMA) matrix. Molecular aggregation is inferred from in situ absorption measurements, and excited state dynamics are measured using a spatially encoded transient absorption (TA) spectroscopy. TA spectra were well-replicated using a kinetic model that evolves as a function of annealing time and extent of aggregation. While linear absorbance spectra indicate that the SQ molecules are primarily uncoupled or weakly-coupled when initially deposited in a PMMA matrix, the kinetic model shows that some pi-stacked aggregates are already present. Excitons are funnelled by energy transfer to these aggregates in just a few picoseconds. The amount of pi-stacked aggregates increases during thermal annealing, further increasing the population of excitons that end up in these aggregates. 

The duration of transient absorption spectroscopy measurements typically limits the types of systems for which the excited state dynamics can be measured. We present a single-shot transient absorption (SSTA) instrument with a spatially encoded 60 ps time delay range and a 100 nm spectral range that is capable of acquiring a transient spectrum in 20 s. We describe methods to spatially overlap the flat-top pump and probe beams at the sample plane, calibrate the spatially encoded time delay, and correct for non-uniform excitation density. SSTA measurements of organic materials in solution and film demonstrate this technique.