Photon Energy-Dependent Hysteresis Effects in Lead Halide Perovskite Materials

Lead halide perovskites have a range of spectacular properties and interesting phenomena and are a serious candidate for the next generation of photovoltaics with high efficiencies and low fabrication costs. An interesting phenomenon is the anomalous hysteresis often seen in current-voltage scans, which complicates accurate performance measurements but has also been explored to obtain a more comprehensive understanding of the device physics. Herein, we demonstrate a wavelength and illumination intensity dependency of the hysteresis in state-of-the-art perovskite solar cells with 18% power conversion efficiency (PCE), which gives new insights into ion migration. The perovskite devices show lower hysteresis under illumination with near band edge (red) wavelengths compared to more energetic (blue) excitation. This can be rationalized with thermalization-assisted ion movement or thermalization-assisted vacancy generation. These explanations are supported by the dependency of the photovoltage decay with illumination time and excitation wavelength, as well as by impedance spectroscopy. The suggested mechanism is that high-energy photons create hot charge carriers that either through thermalization can create additional vacancies or by release of more energetic phonons play a role in overcoming the activation energy for ion movement. The excitation wavelength dependency of the hysteresis presented here gives valuable insights into the photophysics of the lead halide perovskite solar cells.