The effects of the inclusion of reduced graphene oxide (RGO) in TiO 2 layers on performance of perovskite solar cells were systematically investigated. For this purpose, a wet chemical approach was examined to embed graphene oxide (GO) across the thickness of compact TiO 2 (cTiO 2 ) and mesoporous TiO 2 (mTiO 2 ) layers, which was followed by a thermally driven in situ conversion from GO to RGO. The presence of RGO at loadings of 0.15 wt % in the cTiO 2 layer and of 0.015 wt % in the mTiO 2 layer led to a power conversion efficiency (PCE) from 6.6% to 9.3% by 40% increase.In the last few years, solar cells based on organicinorganic hybrid halide perovskites (e.g., methylammonium lead halides CH 3 NH 3 PbX 3 , where X = halogen) have witnessed tremendous growth and continued to attract a huge interest.1 A typical cell architecture consists of a perovskite light-absorbing layer sandwiched between a hole-transporting material (HTM) 2,3 and electron-transporting compact TiO 2 (cTiO 2 ) layers. Mesoporous TiO 2 (mTiO 2 ) scaffolds are frequently integrated as the electron conduction pathway especially when a typical triiodide perovskite without other halides (CH 3 NH 3 PbI 3 ) is used, because the carrier diffusion length of CH 3 NH 3 PbI 3 is too short (ca. 100 nm) 4 to collect the carrier generated in the perovskite layer 5 effectively at the electrode. Nevertheless, the relatively slow electron diffusion through TiO 2 layers limits the charge conduction in the solar cells when combined with HTM with high hole conductivity such as doped 2,2¤,7,7¤-tetrakis(N,N-bis(p-methoxyphenyl)amino)-9,9¤-spirobifluorene (spiro-OMeTAD).6 For the improvement of device performances, maximizing the charge-transport properties in TiO 2 layers is one of the leading challenges.To address this challenge, employing carbon-based materials is a promising option. Graphene is an ordered network of hexagonally-linked carbon atoms arranged with periodicity that aids rapid transport of electrons.7 Reduced graphene oxide (RGO) possesses some defects but is functionally similar to graphene.8 It demonstrates graphene-like properties including transportation of charges are amenable to functionalization, and more importantly, RGO is less expensive and easier to process than graphene.9 RGO has proven compatibility with metals, metal oxides, and organic materials. 10 The integration of RGO with photoactive materials by adapting simple synthetic strategies offers immense advantages to cost-effective processing and device development.A recent report investigating the role of graphene in perovskite solar cells based on cTiO 2 and mesoporous Al 2 O 3 layers suggests that graphene incorporation in cTiO 2 layer lowers interfacial resistance between the cTiO 2 layer and FTO (fluorinedoped tin oxide) promoting charge transport.11 In addition, there are a few reports on the enhanced efficiencies of dye-sensitized solar cells by using graphenenanostructured TiO 2 composites as photoanode materials.12 On the basis of these reports and the observation involving...

In perovskite solar cells (PSCs), electron selective layers (ESLs) assume a crucial role in blocking holes and transporting electrons for high performances. To achieve low-temperature solution processing of PSCs, organic n-type materials such as fullerene-based molecules have recently been employed to replace a typical TiO 2 ESL that requires high temperature (>450 • C) for fabrication. Herein, soluble C 60 -9-methylanthracene mono-adduct (C 60 (9MA)) is used as a thermal precursor to the C 60 ESL in planar PSCs.The best performing PSC device shows a power conversion efficiency of 15.0% under the simulated AM 1.5G one sun illumination, which is superior to that of the device with compact TiO 2 as an ESL (12.9%). Remarkably, a fill factor of the C 60 -based PSC (0.723) is enhanced compared to that of the TiO 2 -based one (0.671) owing to the low charge-transfer resistance at the interface of C 60 -perovskite. These results suggest that the thermal precursor approach to pristine fullerene films is a promising approach for fabricating ESL in high-performance PSCs with relatively low fabrication temperature (<140 • C).