Overview and Outlook on Graphene and Carbon Nanotubes in Perovskite Photovoltaics from Single-Junction to Tandem Applications

Nanocarbon materials, such as graphene and carbon nanotubes (CNTs), have attracted considerable attention as the main or supplementary components in various optoelectronics boosting the device performance and improving the process conditions. Specifically, their application to perovskite solar cells, which are among the most promising photovoltaic devices acknowledged for eco-friendly energy generation, has significantly impacted the current standing of metal halide perovskite-based devices. The uniqueness of the nanocarbon applications can be attributed to their outstanding optical, electrical, chemical and mechanical properties, which conventional materials do not possess. This review overviews past and present reports on graphene- and CNT-incorporated perovskite solar cells. Versatile roles and various synthetic methodologies of the applied nanocarbons in perovskite solar cells, including the material growth methods and sources, and functions as transparent electrodes, charge-transporting layers, interfacial layers, additives and encapsulants, are categorized and graphically illustrated. The discussion expands from single-junction to tandem applications with silicon solar cells, where the nanocarbon materials also play an equally important yet divergent function. Applications of each graphene and CNTs to the silicon-perovskite tandem solar cells are interpreted in terms of what roles they play and how they solve the conventional problems. This review serves as the guideline for the photovoltaics researchers in advancing devices using nanocarbons.