Low-Frequency Carrier Kinetics in Perovskite Solar Cells.

Hybrid organic-inorganic halide perovskite solar cells have emerged as leading candidates for third-generation photovoltaic technology. Despite the rapid improvement in power conversion efficiency (PCE) for perovskite solar cells in recent years, the low-frequency carrier kinetics that underlie practical roadblocks such as hysteresis and degradation remain relatively poorly understood. In an effort to bridge this knowledge gap, we perform here correlated low-frequency noise (LFN) and impedance spectroscopy (IS) characterization that elucidates carrier kinetics in operating perovskite solar cells. Specifically, we focus on planar cell geometries with a SnO2 electron transport layer and two different hole transport layers-namely, poly(triarylamine) (PTAA) and spiro-OMeTAD. PTAA and spiro-OMeTAD cells with moderate PCEs of 5-12% possess a Lorentzian feature at ∼200 Hz in LFN measurements that corresponds to a crossover from electrode to dielectric polarization. In comparison, spiro-OMeTAD cells with high PCEs (>15%) show 4 orders of magnitude lower LFN amplitude and are accompanied by a cyclostationary process. Through a systematic study of more than a dozen solar cells, we establish a correlation with noise amplitude, PCE, and fill factor. Overall, this work establishes correlated LFN and IS as an effective methodology for quantifying low-frequency carrier kinetics in perovskite solar cells, thereby providing new physical insights that can rationally guide ongoing efforts to improve device performance, reproducibility, and stability.