Tunable electronic properties of TiO₂ nanocrystals by in situ dopamine functionalization for planar perovskite solar cells

A high-quality TiO₂ electron transport layer (ETL) should possess a good energy-level alignment, perfect electron mobility, and excellent charge extraction efficiency. Nevertheless, the poor conductivity of amorphous TiO₂ film from low-temperature preparation is an inherent problem while the high-crystallined TiO₂ after high-temperature sintering (>450 °C) limits its application in the low-cost fabrication and flexible substrates. Here, we develop a low-temperature approach to in situ functionalize TiO₂ nanocrystals with dopamine molecules (DA-TiO₂ NCs) in the synthesis of the nanocrystals through a non-aqueous sol-gel method. We systematically investigate the tunable electronic properties of TiO₂ NCs with the variable DA modification and confirm that DA-TiO₂ can tailor the energy level alignment as well as increase the charge-extraction capacity, substantially reducing charge accumulation between the TiO₂ and perovskite layer. The in situ anchored amino groups (-NH₃⁺) not only preferentially boost the in-plane growth of perovskite grains but also induce the red-shift absorption of perovskite via the strong electrostatic interaction with Br⁻ ions. Based on the DA-TiO₂ NCs ETLs, the power conversion efficiency of the fully ambient-processed planar FA_xMA_1-xPbBr_yI_3-y PSCs is optimized to 19.45%.