In this work, a multi-electron transporting layer (ETL) for efficient perovskite solar cells is investigated. The multi-ETL consists of five conditions including SnO₂, SnO₂/SnO_x, TiO₂, TiO₂/SnO₂, and TiO₂/SnO₂/SnO_x. The best performance of PSC devices is found in the SnO₂/SnO_x double-layer and exhibits a power conversion efficiency equal to 18.39% higher than the device with a TiO₂ single-layer of 14.57%. This enhancement in efficiency can be attributed to a decrease in charge transport resistance (R_ct) and an increase in charge recombination resistance (R_rec). In addition, R_ct and R_rec can be used to explain the comparable power conversion efficiency (PCE) between a PSC with a SnO₂/SnO_x double-layer and a PSC with a triple-layer, which is due to the compensation effect of R_ct and R_rec parameters. Therefore, R_ct and R_rec are good parameters to explain the efficiency enhancement in PSC. Thus, the R_ct and R_rec from the electrochemical impedance spectroscopy (EIS) technique is an easy and alternative way to obtain information to understand and characterize the multi-ETL on PSC.

This study explores the use of mid-infrared (MIR) free-electron laser (FEL) irradiation as a tool for tailoring the surface properties of electrochemically synthesized TiO₂—graphene quantum dots (QDs). The QDs, prepared in colloidal form via a cost-effective electrochemical method in a KCl—citric acid medium, were exposed to MIR wavelengths (5.76, 8.02, and 9.10 µm) at the Kyoto University FEL facility. Post-irradiation measurements revealed a pronounced inversion of zeta potential by 40–50 mV and approximately 10% reduction in hydrodynamic size, indicating double-layer contraction and ionic redistribution at the QD—solvent interface. Photoluminescence spectra showed enhanced emission for GQDs and TiO₂/GQD composites, while Tauc analysis revealed modest bandgap blue shifts (0.04–0.08 eV), both consistent with trap-state passivation and sharper band edges. TEM confirmed intact crystalline structures, verifying that FEL-induced modifications were confined to surface chemistry rather than bulk lattice damage. Taken together, these results demonstrate that MIR FEL irradiation provides a resonance-driven, non-contact method to reorganize ions, suppress defect states, and improve the optoelectronic quality of QDs. This approach offers a scalable post-synthetic pathway for enhancing electron transport layers in perovskite solar cells and highlights the broader potential of photonic infrastructure for advanced nanomaterial processing and interface engineering in optoelectronic and energy applications.