Electrochemical impedance spectroscopy was used to analyze 10 μm thickness TiO₂ films containing 250 nm TiO₂ nanoparticles (NPs) with anatase crystalline phase deposited on FTO substrates. The TiO₂ films were sensitized with CdSe quantum dots (QDs) (4.5 nm) and decorated with Au nanoparticles (NPs) (7 nm) and poly-3-octylthiophene (P3OT) in three different configurations, namely TiO₂/QDs, TiO₂/Au/QDs, and TiO₂/Au/QDs/P3OT. The characteristic absorption bands for each component, TiO₂, P3OT, CdSe QDs, and Au NPs, are centered at 338 nm (3.67 eV), 459 nm (2.7 eV), 552 nm (2.24 eV), and 517 nm (2.40 eV), respectively. The UV-Vis spectrum of the composite film TiO₂/Au/QDs/P3OT shows clear absorption peaks from 338 to 700 nm. Equivalent circuits were proposed for the different configuration interfaces to fit experimental impedances. The results suggest that the introduction of Au NPs into the TiO₂/Au/QDs films strongly reduces resistance to the electron flux from QDs to TiO₂ and increases the overall charge transport. Meanwhile, P3OT in the TiO₂/Au/QD/P3OT films promotes hole transport and increases the capacitance with the electrolyte, reducing the electron leakage and improving the fill-factor and the final light energy conversion efficiency.

High defect concentrations at the interfaces are the basis of charge extraction losses and instability in perovskite solar cells. Surface engineering with organic cations is a common practice to solve this issue. However, the full implications of the counteranions of these cations for device functioning are often neglected. In this work, we used 4-fluorophenethylammonium cation with varying halide counteranions for the modification of both interfaces in methylammonium-free Pb-based n-i-p devices, observing significant differences among iodide, bromide, and chloride. The cation treatment of the buried and top interfaces resulted in improved surface quality of the perovskite films and largely improved carrier dynamics with reduced nonradiative recombination. Consequently, the optimal interface-modified methylammonium-free perovskite solar cells surpassed 20% efficiency and demonstrated remarkable operational stability. Our findings underscore the potential of comprehensive surface engineering strategies in advancing the perovskite film and device quality, thereby facilitating their broader and more successful applications.