Pore Size Dependent Hysteresis Elimination in Perovskite Solar Cells Based on Highly Porous TiO2 Films with Widely Tunable Pores of 15–34 nm

Pore size and porosity of the porous materials play an important role in catalysis, dye-sensitized solar cells and mesoscopic perovskite solar cells (PSC), etc. Increasing pore size and porosity of mesoporous TiO 2 is crucial for facilitating pore-filling of perovskite, charge extraction on TiO 2 /CH 3 NH 3 PbI 3 interface and thus cell performance enhancement. Highly porous TiO 2 films (TFs) with a large pore size that extends the limit of particle size have been achieved through a novel TiO 2 paste using copolymer P123 as a pore-adjusting agent and 2-butoxyethyl acetate as a solvent. A highly porous structure with the pore size of 34.2 nm and porosity of 73.5% has been obtained, the porosity of which is the largest that has ever been reported in the screen-printed TiO 2 thick films. The pore size and porosity of TFs can be successively adjusted in a certain range by tuning the P123 content in the pastes. As particle size and surface area of TFs are kept almost constant, the specific investigation on the effect of varied pore size on the performance of bilayer-structured PSCs becomes possible. The hysteresis phenomenon, the notorious problem of PSCs, is found to depend greatly on pore size and porosity of TFs, that is, pore-filling of perovskite. The suppressing effect of highly porous TFs on hysteresis by avoiding charges accumulation on the interface due to enhanced interfacial contact is proved by the invariable photocurrent response after prebias treatment. A hysteresis-free solar cell with an efficiency of 15.47% was achieved by depositing a 242 nm-thick perovskite capping layer upon 350 nm-thick TF with a pore size of 34.2 nm. This method developed for the preparation of highly porous TFs provides a new way to fabricate hysteresis-free PSCs and is widely applicable for the fabrication of other mesoporous metal oxide films with large pore sizes.