Low-temperature solution-processed efficient electron-transporting layers based on BF4- - capped TiO2 nanorods for high-performance planar perovskite solar cells

To fabricate cost-effective solar cells, some prerequisites such as low-energy consumption, simplified technological processes, and high efficiency must be achieved. Low-temperature solution-processed planar perovskite solar cells (PSCs) are desirable for achieving these aims. Although TiO2 is the most widely used material in preparing electron-transporting layers (ETLs) of PSCs, it usually requires a high-temperature sintering process to enhance electron mobility, which not only consumes considerable energy but is also unsuitable for polymer-based flexible substrates. Here, we propose a novel strategy involving the use of a sophisticated solvothermal method with the aid of oleic acid (OA) ligands to synthesize high aspect ratio and well crystallized TiO2 nanorods. Then, the ligand-exchange treatment is applied to substitute the insulating OA ligands with BF4- ions. With this protocol, low-temperature solution-processed high-quality TiO2 ETLs can be produced with high electrical conductivity and fast electron extracting and transporting rates. Finally, the PSCs with BF4--capped TiO2 nanorod ETLs achieves a high-power conversion efficiency of 18.76% and good reproducibility. This work offers a feasible method to achieve high-efficiency low-temperature solution-processed planar PSCs.