Mechanisms for Spontaneous Generation of Interlayers in Organic Solar Cells

The structure of bulk heterojunction organic photovoltaic devices generally includes interlayers, thin films positioned between the active layer and one or both of the electrodes, intended to enhance the performance and/or stability. Interlayers can consist of organic or inorganic materials. They play different roles, such as enhancing adhesion and reducing energetic barriers, and are deposited using techniques such as spin coating and thermal deposition. Here we focus on interlayers that do not require a distinct deposition process but rather spontaneously form at the film surface or the active layer/electrode interface. This review paper identifies the underlying mechanisms that induce the interlayer self-generation, and classifies the relevant studies based on (reported or realized) driving forces for interlayer formation. By doing so, this perspective not only offers directive tools for the design of interlayers at surfaces and buried interfaces, but also provides insights on thermodynamic and/or kinetic aspects of processes that occur spontaneously during device fabrication and/or testing but have been overlooked until now.