Terahertz Time-Domain Spectroscopy as a Defect Fingerprinting Tool for Halide Perovskite Solar Cells: Toward a...

Organic-inorganic hybrid perovskites (OHPs) deliver certified single-junction power conversion efficiencies (PCEs) of up to 27.3% and National Laboratory of the Rockies (NLR)-certified perovskite-silicon tandem values of 34.85%, yet a substantial gap with the Shockley-Queisser (S-Q) limit persists. Grain-boundary (GB) defects are one principal contributor to this gap, driving non-radiative recombination, ion migration, and degradation alongside bulk, interfacial, contact-related, phase-related, and environmental loss channels. Rational passivation demands a non-contact tool capable of identifying and quantifying specific defect species in device-relevant thin films, a capability that conventional probes deliver only in part. This overview assesses the extent to which terahertz time-domain spectroscopy (THz-TDS, 0.2-2.5 THz) fulfills this role. Across five OHP compositions-MAPbI₃, MAPbBr₃, FAPbI₃, and FAPb(Br,I)₃ fabricated by sequential vacuum evaporation (SVE), together with solution-processed γ-CsPbI₃-the THz spectral window captures both intrinsic phonon modes and GB-localized molecular defect vibrations, enabling species-resolved characterization at room temperature. Notably, the oscillator strength of the SVE-specific 1.58 THz absorption in MAPbI₃ scales linearly with XPS-quantified CH₃NH₂ defect concentration, establishing a calibrated, contact-free proxy for defect concentration rather than an absolute defect count; the observable is the defect-induced perturbation of the Pb-X lattice, not the defect population itself. Building on these findings, we propose a three-pillar framework for THz-guided defect engineering: (I) quantitative defect measurement via oscillator-strength analysis, (II) material-specific fingerprint identification from a systematically constructed THz library, and (III) fingerprint-guided defect elimination with real-time feedback-together defining a closed-loop quality-control cycle that connects spectroscopic diagnosis to passivation strategy and, ultimately, to enhanced solar cell efficiency. Throughout, we distinguish capabilities demonstrated to date from extensions that remain proposals, and we define the measurement requirements needed before the framework can be transferred to inline manufacturing control.