Terahertz spectroscopy for probing free carriers in perovskite solar cells

Metal-halide perovskites have rapidly emerged as one of the most promising material platforms for next-generation optoelectronic devices, owing to their exceptional optical absorption, long carrier lifetimes, high defect tolerance, and solution-processable fabrication. Over the past decade, these advantages have propelled perovskite solar cells from laboratory curiosities to serious photovoltaic contenders, with single-junction efficiencies approaching the Shockley-Queisser limit and all-perovskite tandem architectures surpassing a power conversion efficiency of 30%.

Buried interface bottleneck

As the field transitions from rapid efficiency gains to performance optimization near theoretical limits, further progress is increasingly dictated not by bulk material quality but by non-radiative recombination and charge-transport losses at buried interfaces. This trend is evident in recent advances in all-perovskite tandem solar cells and large-area modules, where interfacial recombination control and buried-interface passivation have become central to achieving high efficiency and operational stability. However, these buried interfaces remain difficult to interrogate directly using conventional characterization techniques, which often provide only indirect insight into charge-transport processes. In this context, a recent study by Tan’s group provides a decisive demonstration of how terahertz (THz) spectroscopy can be used not merely to observe carrier dynamics but also to directly test whether interfacial passivation genuinely improves free-carrier transport.

Probing free carriers

THz time-domain and optical-pump-THz-probe (OPTP) spectroscopy provide a contact-free approach for selectively probing the dynamics of mobile photogenerated charge carriers on ultrafast timescales.5 This capability is particularly valuable in metal-halide perovskites, where performance-limiting processes, such as defect-mediated recombination and interfacial transport barriers, are often intertwined and difficult to disentangle using conventional optical or electrical probes. Because THz photons possess energies in the meV range, far below the perovskite band gap, the THz probe couples predominantly to intraband carrier motion rather than interband optical transitions. As a result, the measured THz response is intrinsically sensitive to the conductivity of delocalized, mobile free carriers while remaining largely insensitive to bound excitons or carriers immobilized in deep trap states.

Upon optical excitation, the pump-induced change in the transmitted THz signal, expressed as the relative change in THz electric field (ΔE/E) or, equivalently, the normalized change in transmittance (ΔT/T), can be quantitatively related to the photoinduced conductivity Δσ(ω, t) using the thin-film approximation, which is valid when the film thickness is much smaller than the THz wavelength (Figure 1A). In the transmission geometry, this relationship directly links an experimentally accessible observable to the intrinsic photoconductivity of the perovskite layer, without the need for electrical contacts or charge injection. Note that the THz photoconductivity signal predominantly reflects carriers that remain mobile on ultrafast timescales.