From spacer engineering to defect dominance: contrasting A ₂ PbBr ₄ and A ₄ AgBiBr ₈ layered perovskites

. Increasing spacer length systematically increases the interlayer spacing and strongly affects thin-film morphology, particularly for the longer alkyl chains. We combine UV-vis/PL with excitonic analysis (Elliott model) and modulated surface photovoltage spectroscopy (SPV) to disentangle excitonic, band-to-band, and defect-assisted transitions. The Pb-based series exhibits strong excitonic signatures and pronounced spacer-length dependence in charge separation: excitonic-regime SPV is prominent for BA/PEA and strongly quenched for HA/OA, consistent with increased electronic insulation across the organic barrier. In contrast, the Ag-Bi double perovskites show orders-of-magnitude weaker SPV and clear sub-bandgap SPV features that are nearly spacer-independent; defect-related transitions are resolved at 1.38 eV and 2.29 ± 0.06 eV. Time-resolved microwave conductivity further corroborates suppressed long-range transport for longer spacers and highlights fundamentally inferior carrier generation/transport in the defect-rich double perovskites. Overall, we find that spacer engineering can tune transport and spectral onsets when the inorganic layer is relatively defect-tolerant (Pb-based), but becomes secondary when deep defects in the inorganic framework dominate the photophysics (AgBi-based).