Current advances in perovskite oxides supported on graphene-based materials as interfacial layers of perovskite solar cells

Perovskite solar cells (PSCs) are emerging efficient photovoltaic devices, with record-high power conversion efficiencies (PCE) of more than 25.5%. However, PSCs exhibit some drawbacks, such as poor stability upon exposure to moisture or humidity, ultraviolet (UV) radiation and heat, which in turn limits the device lifetime and performance. In addition, the introduction of perovskite films comes with associated toxicity, which is a major environmental concern. Furthermore, the application of titanium dioxide (TiO2) as an electron transport layer (ETL) and 2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene (spiro-OMeTAD) as a hole transport layer (HTL), causes device instability. The wide-bandgap characteristic of TiO2 introduces charge carrier recombination in the ETL, which, in turn, impairs device performance. This is, over and above, the high cost of spiro-OMeTAD, coupled with its multi-step synthetic preparation method. To address the aforementioned shortcomings, approaches, such as modifying the interfacial architecture, have been explored by introducing versatile materials between the charge-collecting electrode and the perovskite active layers. In this regard, perovskite oxides are more appealing due to their wide bandgap and high electron mobility. However, perovskite oxides have limitations due to their agglomeration, which causes short-circuits and leakage current, in addition to their poor charge separation efficiency, surface hydrophilicity and weak visible-light absorption. As a result, nanocomposites of perovskite oxides with carbon-based materials, particularly graphene and its derivatives, have attracted significant research attention due to their exceptional optoelectronic properties, superior stability, and non-toxicity of graphene-based materials. Therefore, this review discusses the recent trends in graphene-based materials, their composites with perovskite oxides, effective ETLs or HTLs of PSCs and the subsequent improvement of photovoltaic performance. In addition, a summary of synthetic routes for perovskite oxides/graphene nanocomposites is presented. This review will foster the advancement of the fabrication of PSCs with improved PCE and stability.