Efficient Bulk Heterojunction CH₃NH₃PbI₃–TiO₂ Solar Cells with TiO₂ Nanoparticles at Grain Boundaries of Perovskite by Multi-Cycle-Coating Strategy

A novel bulk heterojunction (BHJ) perovskite solar cell (PSC), where the perovskite grains act as donor and the TiO₂ nanoparticles act as acceptor, is reported. This efficient BHJ PSC was simply solution processed from a mixed precursor of CH₃NH₃PbI₃ (MAPbI₃) and TiO₂ nanoparticles. With dissolution and recrystallization by multi-cycle-coating, a unique composite structure ranging from a MAPbI₃–TiO₂-dominated layer on the substrate side to a pure perovskite layer on the top side is formed, which is beneficial for the blocking of possible contact between TiO₂ and the hole transport material at the interface. Scanning electron microscopy clearly shows that TiO₂ nanoparticles accumulate along the grain boundaries (GBs) of perovskite. The TiO₂ nanoparticles at the GBs quickly extract and reserve photogenerated electrons before they transport into the perovskite phase, as described in the multitrapping model, retarding the electron–hole recombination and reducing the energy loss, resulting in increased <i>V</i>_OC and fill factor. Moreover, the pinning effect of the TiO₂ nanoparticles at the GBs from the strong bindings between TiO₂ and MAPbI₃ suppresses massive ion migration along the GBs, leading to improved operational stability and diminished hysteresis. Photoluminescence (PL) quenching and PL decay confirm the efficient exciton dissociation on the heterointerface. Electrochemical impedance spectroscopy and open-circuit photovoltage decay measurements show the reduced recombination loss and improved carrier lifetime of the BHJ PSCs. This novel strategy of device design effectively combines the benefits of both planar and mesostructured architectures whilst avoiding their shortcomings, eventually leading to a high PCE of 17.42% under 1 Sun illumination. The newly proposed approach also provides a new way to fabricate a TiO₂-containing perovskite active layer at a low temperature