TiO2 nanorod arrays hydrothermally grown on MgO-coated compact TiO2 for efficient perovskite solar cells

Abstract

The morphology of electron transport layers has a significant impact on the device architecture and electronic processes of mesoscopic perovskite solar cells (PSCs). In this study, ultrathin MgO is coated on the surface of compact TiO₂ (c-TiO₂). The MgO-coated c-TiO₂ is first used as seeds to hydrothermally grow one-dimensional (1D) TiO₂ nanorod (NR) arrays for PSC devices. Rutile nanorod arrays are fabricated via a facile solvothermal method using tetrabutyl titanate (TBT) as the Ti precursor. The microstructures and morphologies, including nanorod diameter, length, and areal density, of the TiO₂ NR arrays are varied by controlling the concentration of TBT from 0.3 M to 0.7 M. Furthermore, the profound effects of the MgO modification and titania nanorod morphology on the pore-filling of perovskite CH₃NH₃PbI₃, charge separation and recombination at the perovskite/titania nanorod interface are investigated. Our results reveal that the Ti precursor concentration strongly affects the open-circuit voltage (V_OC), short-circuit current density (J_SC), and fill factor (FF) of the 1D TiO₂ NR array-based device. Under optimized conditions with MgO coating and at 0.4 M TBT, our champion cell with 1D TiO₂ NRs demonstrates a power conversion efficiency (PCE) of 17.03% with J_SC = 22.01 mA cm⁻², V_OC = 1.06 V, and FF = 0.73. Under the same fabrication conditions, MgO modification enhances the average PCE to 16.24% for the PSCs with the MgO coating from 13.38% for the PSCs without the MgO coating. The devices show an approximately 18% improvement in performance, which mainly results from the open-circuit voltage and fill factor enhancements. Moreover, advantageously, the MgO modification is found to reduce the current density–voltage (J–V) hysteresis with respect to the scan direction and improve the UV stability of the non-encapsulated cells. Therefore, this study presents a promising approach to fabricate efficient and stable one-dimensional TiO₂ nanorod array-based perovskite solar cells.