TiO2 Mesocrystals Processed at Low Temperature as the Electron-Transport Material in Perovskite Solar Cells

Cube-like anatase TiO2 mesocrystals as effective electron-transporting materials toward high-performance perovskite solar cells

Electron-transporting materials (ETMs) with higher carrier mobility and a suitable band gap structure play a significant role in determining the photovoltaic performance of perovskite solar cells (PSCs). Herein, cube-like mesoporous single-crystal anatase TiO₂ (Meso-TiO₂) nanoparticles synthesized by using a facile hydrothermal method were utilized as an efficient ETM for PSCs. The superior semiconducting properties of the Meso-TiO₂ based ETM enabled the best power conversion efficiency (PCE) of 20.05% for a PSC. Moreover, the device retained 80% of its initial PCE after being stored in ambient conditions for 20 days under 25 ± 5% relative humidity. In contrast to the commercial TiO₂ ETM, the Meso-TiO₂ ETM based PSC showed a distinguished interface with better interfacial conditions and improved carrier extraction originating from the cube-like mesoporous single-crystal anatase TiO₂ ETM.

Synthesis of Cu3N and Cu3N-Cu2O multicomponent mesocrystals: non-classical crystallization and nanoscale Kirkendall effect

Mesocrystals are superstructures of crystallographically aligned nanoparticles and are a rapidly emerging class of crystalline materials displaying sophisticated morphologies and properties, beyond those originating from size and shape of nanoparticles alone. This study reports the first synthesis of Cu₃N mesocrystals employing structure-directing agents with a subtle tuning of the reaction parameters. Detailed structural characterizations carried out with a combination of transmission electron microscopy techniques (HRTEM, HAADF-STEM-EXDS) reveal that Cu₃N mesocrystals form by non-classical crystallization, and variations in their sizes and morphologies are traced back to distinct attachment scenarios of corresponding mesocrystal subunits. In the presence of oleylamine, the mesocrystal subunits in the early reaction stages prealign in a crystallographic fashion and afterwards grow into the final mesocrystals, while in the presence of hexadecylamine the subunits come into contact through misaligned attachment, and subsequently, to some degree, realign in crystallographic register. Upon prolonged heating both types of mesocrystals undergo chemical conversion processes resulting in structural and morphological changes. A two-step mechanism of chemical conversion is proposed, involving Cu₃N decomposition and anion exchange driven by the nanoscale Kirkendall effect, resulting first in multicomponent/heterostructured Cu₃N-Cu₂O mesocrystals, which subsequently convert into Cu₂O nanocages. It is anticipated that combining nanostructured Cu₃N and Cu₂O in a mesocrystalline and hollow morphology will provide a platform to expand their application potential.