Concentration-Dependent Precursor Engineering with Butylammonium Acetate for Inverted Triple-Cation Perovskite Solar Cells

Butylammonium acetate (BAAc) was investigated as a concentration-dependent precursor additive for inverted triple-cation perovskite solar cells based on FA0.80Cs0.07MA0.13PbI2.64Br0.39. BAAc loadings ranging from 0 to 7 mol% were systematically compared to determine how moderate and excessive additive concentrations influence film formation, defect behavior, and device operation. At 3 mol% BAAc, the mean equivalent-circle grain diameter increased from 0.508 to 0.719 μm, and the median increased from 0.495 to 0.688 μm, while cross-sectional SEM confirmed a comparable absorber thickness of 500 ± 20 nm across the series. The PbI2-to-perovskite diffraction peak height ratio decreased from 0.2691 to 0.0427, and the intensity-weighted photoluminescence lifetime increased from 230.2 to 350.0 ns. Light-intensity-dependent open-circuit voltage, impedance spectroscopy, thermal admittance spectroscopy, and space-charge-limited current measurements consistently indicated reduced trap-assisted recombination and transport loss at this concentration. In the EIS analysis, BAAc-3 showed the lowest transport resistance (222.17 ± 9.52 Ω) and the highest recombination resistance (8.211 ± 0.061 kΩ); all principal resistance parameters had relative standard errors below 10%, although systematic high-frequency residuals limit quantitative interpretation of the transport CPE. The champion BAAc-3 device reached 23.37% efficiency, compared with 20.42% for the control, and the 50-device mean increased from 19.63 ± 0.45% to 22.77 ± 0.30%. At 7 mol%, the morphological and electrical trends reversed and the champion efficiency decreased to 19.34%, which defines an over-treatment boundary. The 30-day dry–dark storage and 120 min maximum-power-point tests provide comparative, short-duration stability evidence. The concentration dependence is consistent with a literature-supported working model of precursor coordination and ammonium–halide interactions, without constituting direct spectroscopic identification of a specific complex.