In Situ Measurement of Evolving Excited-State Dynamics During Deposition and Processing of Organic Films by Single-Shot Transient Absorption.

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.

Single-shot transient absorption spectroscopy techniques and design principles.

Single-shot transient absorption (SSTA) spectroscopy is fundamentally identical to transient absorption (TA) spectroscopy but differs in its implementation to enable the measurement of sample response at a range of pump-probe time delays in a single laser shot. As in TA, a pump pulse in SSTA photoexcites a sample, inducing a change in the absorption of a probe pulse. Both commercial and home-built TA instruments typically execute serial measurements at a range of pump-probe time delays to yield transients that report on the dynamics of the photoexcited species, with the sample returning to the same relaxed state between each measurement. SSTA instruments acquire a range of pump-probe time delays simultaneously by somehow encoding the time delay into the profile of the probe beam. This dramatically reduces the time required for SSTA measurements, enabling the measurement of unstable systems undergoing irreversible processes that cannot be accurately characterized using typical TA instruments. The implementation of the encoded time delay must be appropriately designed and carefully calibrated to suit the targeted system and ensure accurate measurements. This review describes techniques used to encode the time delay and design principles for SSTA instruments. Strategies are presented to implement a broadband probe, account for spatial variations in pump and probe beam profiles that influence the intensity and noise of the spatially encoded signal, optimize detection, and correct for dynamic background signals. With these design principles in place, SSTA is capable of measuring an array of unstable and evolving systems that cannot be addressed using typical TA instruments.