Microsecond Bias Polarity Switching Reveals Hidden Charge Dynamics at Halide Perovskite Interfaces
We present a time-domain technique based on rapid bias polarity switching (BiPS) to probe charge transport and near-surface defects in halide perovskite single crystals. The method exploits interfacial extraction barriers, which cause carrier accumulation and subsequent release after bias reversal. BiPS combines surface sensitivity (200 nm-2 $μ$m) with millimeter-scale reach, enabling reconstruction of internal field profiles, detection of bulk space charge down to $10^9$ cm$^{-3}$, and resolution of microsecond-millisecond trap dynamics. In our setup the surface-state detection limit is $10^9$ cm$^{-2}$, and could be further improved by optimized illumination and readout. Applied to melt-grown CsPbBr$_3$ (Cr/Cr) and solution-grown MAPbBr$_3$ (Cr/SnO$_2$/Cr), BiPS reveals interfacial barriers that drive hole accumulation and defect filling. CsPbBr$_3$ shows long-lived space charge ($\sim 3\times 10^{11}$ cm$^{-3}$) and $\sim 250\times$ faster hole extraction than MAPbBr$_3$, while trap analysis yields capture times of 1-100 $μ$s, detrapping times of 20 $μ$s-3 ms, and activation energies of 300-500 meV. BiPS thus provides direct access to buried interfacial processes, disentangles electronic and ionic contributions, and offers a robust platform for evaluating contacts and guiding defect engineering in perovskite optoelectronic devices.