Hollow rice grain-shaped TiO₂ nanostructures for high-efficiency and large-area perovskite solar cells

Hollow rice grain-shaped titanium dioxide (TiO₂) nanostructures (NSs) with the radius of 200–300 nm are fabricated by electrospinning titanium isopropoxide solution and further calcination. The resulting hollow rice grain-shaped TiO₂ NSs are highly porous, which are beneficial to the infiltration of perovskites and provide a large contact area, as the building blocks to construct the mesoporous TiO₂ layer for the large-area (the active area is 1 cm²) CH₃NH₃PbI₃ based perovskite solar cells (PSCs). By varying the spin coating speed (2000 rpm, 4000 rpm, and 8000 rpm, respectively), the performance of PSCs changes with different TiO₂ NSs distribution densities. The optimized PSC employing the 4000 rpm spin coating speed exhibits a photovoltaic conversion efficiency (PCE) of 14.2% with the short circuit current density (J_SC) of 21.6 mA cm⁻², open circuit voltage (V_OC) of 1.07 V and fill factor (FF) of 0.61, which is superior to the plain structure based control group with the PCE of 9.6%. Furthermore, the PSC possesses a reproducible PCE value with weak hysteresis in its current density-voltage (J-V) curves. Moreover, photoluminescence (PL) measurements and finite-difference time-domain (FDTD) optical simulations reveal the enhanced fast charge carrier extraction/transport and light absorption in the proposed system, which makes electrospun hollow rice grain-shaped TiO₂ NSs a promising electron transportation material for high-efficiency and large-area photovoltaic devices.