Time-resolved optical spectroscopy: A versatile, complementary tool for advancing cutting-edge materials technologies
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MATERIAL MATTERS
Time-resolved optical spectroscopy: A versatile, complementary tool for advancing cutting-edge materials technologies By Christopher Grieco
W
ith the rapid expansion of technology comes an increasing demand for new and improved functional materials. Developing materials for a wide variety of applications, such as solar cells, transistors, light-emitting devices (LEDs), fuel cells, energy storage, and drug delivery is crucial for advancing our everyday life. One overlooked tool for measurements of materials is time-resolved optical spectroscopy, which is mainly used by physical chemists today. Time-resolved optical spectroscopy offers materials researchers and engineers powerful methods for analyzing materials in action. A stimulus, which is often (but not limited to) a pulse of light, initiates a dynamical process in a material that is tracked over time by meas-uring its interaction either with or through the emission of light (Figure 1). But what do spectroscopists actually do, and how can they contribute to materials science? A spectroscopist studies how molecules and materials interact with light. Measuring their specific interactions yields information about the material. Often, a spectroscopist is interested in solving fundamental problems; however, time-resolved optical spectroscopy
can also be applied to measure materials functionality over time. For example, it can be used to watch electrons migrate in a solar cell or energy flow within an LED before it produces light. It can be used to watch chemical bonds break or form, or monitor structural organization of polymers in thin films. Such information is particularly useful for designing and testing a new material and can help solve contemporary problems in materials science and engineering. For instance, transient absorption spectroscopy (TAS) is a time-resolved method that is used to monitor excited species that form and decay in a material following light absorption. A particularly appealing application of TAS is to study charge generation and recombination in solar-energy-conversion materials, such as organic semiconducting polymers and inorganic perovskites. TAS measurements of charge-carrier recombination were used in combination with x-ray scattering to explain how the chemical structure in conjugated block-copolymer materials can be tailored to optimize thin-film nanomorphology and enhance their performance in solar cells.1 Figure 2 illustrates how more charges survive in the blockcopolymer material compared to a polymer mixture, because the latter is able to undergo nanoscale self-assembly. This stimulus process limits phase separation, enabling light-generated, bound charges to transfer and separate at the interfaces between the polymer domains. light Other exciting opportuniphotodetector ties include multidimensional sample spectroscopy techniques, such Figure 1. Simplified illustration of a time-resolved optical as two-dimensional infrared spectroscopy experiment. A stimulus induces a change in (2D IR) spectroscopy, which the
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