A research team from Sungkyunkwan University (SKKU), led by professors Taeyeon Kim and Doo-Hyun Ko from the Department of Chemistry, has published a detailed review in the journal ACS Nano. The review, titled "Carrier Dynamics in Nonfullerene Acceptor Organic Photovoltaics through Ultrafast Spectroscopy," explores the behavior of organic optoelectronic devices—materials that can be as light, flexible, and transparent as plastic. These devices are used in solar cells and other electronic applications, and recent advances in nonfullerene acceptor materials have significantly improved their ability to convert light into electricity. Understanding the microscopic processes that occur when light is absorbed and electricity is generated has been a long-standing challenge. Different research groups have proposed conflicting interpretations of these processes, making it difficult to compare results. To address this, the SKKU team compiled research on ultrafast spectroscopy, a technique that studies how light interacts with matter on extremely short timescales—measured in femtoseconds (one quadrillionth of a second). The review outlines three key methods—transient absorption (TA), pump–push–probe (PPP), and two-dimensional electronic spectroscopy (2DES)—each capable of observing different stages of the process, but each with its own limitations. The team also worked to standardize the terminology used to describe intermediate states in the energy conversion process, which had previously varied widely across studies. They established clear criteria for identifying these states from experimental data, showing that some conflicting results may stem from differences in experimental conditions, such as the wavelength of light used or the way materials are processed, rather than from fundamental physical differences. The review highlights that the molecular structure of nonfullerene acceptors, their nanoscale crystallinity, and the electric properties of the molecules help reduce energy barriers at the interface and prevent the loss of electrical charge, offering design principles for more efficient and stable materials. Looking ahead, the study emphasizes the importance of operando measurements—those taken while the device is in operation—as a critical area for future research. This review serves as a roadmap for engineers and scientists aiming to develop better materials, which could lead to more efficient, durable, and commercially viable solar energy solutions.