The Original Design Principles of the Y-Series Nonfullerene Acceptors, from Y1 to Y6

Since 2019, the power conversion efficiencies of organic photovoltaics have risen sharply from ∼13% to ∼19% because of the newly invented Y-series nonfullerene acceptors (Y-NFAs; mainly Y1 to Y6). However, comprehensive insights into the design principles behind these molecules have not been fully elucidated or explained in the literature. In this Perspective, I share our original insights into the understanding, prediction, and design principles of Y1 to Y6 and offer a brief history behind the discoveries of the Y-NFAs.