Organic-inorganic hybrid cathode interlayer materials for efficient organic solar cells

Organic solar cells (OSCs) have witnessed great progress over the past few years. A diversity of breakthroughs, such as record power conversion efficiencies of single junction or tandem OSCs, high performing flexible devices and large area modulus, have been successfully realized to date. These achievements depend on comprehensive materials innovation, including light harvesting materials, interlayer materials and novel electrodes. The cathode interlayer (CIL), usually a thin layer between the photoactive layer and the cathode, is responsible for electron extraction and hole blocking, thus playing crucial roles in the overall performance of OSCs. CIL materials, which can be solution processed at low temperature and show high conductivity, are vital for the convenient fabrication of flexible and large area OSC devices. However, traditional CIL materials such as inorganic metal oxides and pure organic semiconductors often suffer from different disadvantages, such as high temperature treatment for the former and film thickness-sensitive performance for the latter. Therefore, intensive research efforts have been made among the community on organic-inorganic hybrid CIL materials for the purpose of combining their advantages while reducing their drawbacks. To summarize, these hybrid CILs can be categorized as (i) organic modification of traditional inorganic CILs, (ii) simple organic-inorganic hybrid coordination complexes and (iii) hybrid cluster based CILs. Notably, class (i) can enable metal oxide based CILs to be processed at lower temperature, display more thickness-insensitive performance and improved stability. Moreover, classes (ii) and (iii) can be expressed with definite structures and are suitable for establishing a clear structure-performance relationship, which should be of significant potential for deeper understanding of the mechanism of action of CIL materials in OSCs. In particular, metal oxide cluster based CILs with both metal oxide cores and designable surface groups according to their composition show great promise for bridging the gap between inorganic and organic materials, hence providing new excellent CIL materials and corresponding OSCs. This review focuses on the research progress of hybrid CILs in recent years and is intended to inspire more innovative creation on this topic, which undoubtedly will provide great benefits to the development of OSCs.