High Efficiency Mesoscopic Solar Cells Using CsPbI₃ Perovskite Quantum Dots Enabled by Chemical Interface Engineering

All-inorganic α-CsPbI₃ perovskite quantum dots (QDs) are attracting great interest as solar cell absorbers due to their appealing light harvesting properties and enhanced stability due to the absence of volatile organic constituents. Moreover, ex situ synthesized QDs significantly reduce the variability of the perovskite layer deposition process. However, the incorporation of α-CsPbI₃ QDs into mesoporous TiO₂ (m-TiO₂) is highly challenging, but these constitute the best performing electron transport materials in state-of-the-art perovskite solar cells. Herein, the m-TiO₂ surface is engineered using an electron-rich cesium-ion containing methyl acetate solution. As one effect of this treatment, the solid-liquid interfacial tension at the TiO₂ surface is reduced and the wettability is improved, facilitating the migration of the QDs into m-TiO₂. As a second effect, Cs⁺ ions passivate the QD surface and promote the charge transfer at the m-TiO₂/QD interface, leading to an enhancement of the electron injection rate by a factor of 3. In combination with an ethanol-environment smoothing route that significantly reduces the surface roughness of the m-TiO₂/QD layer, optimized devices exhibit highly reproducible power conversion efficiencies exceeding 13%. The best cell with an efficiency of 14.32% (reverse scan) reaches a short-circuit current density of 17.77 mA cm^-2, which is an outstanding value for QD-based perovskite solar cells.