Understanding the Anomalous J–V Curves in Carbon‐Based Perovskite Solar Cells as a Structural Transition Induced by Ion Diffusion

The investigation of hole transport layer‐free mesoporous carbon perovskite solar cells by analyzing current–voltage (J–V) curves under different scan rates, light intensities, and temperatures is presented. A distinctive bump in the curves is identified, previously reported in the literature. The voltage at the inflection point of this transition shows a linear correlation with the scan rate, directly yielding a characteristic relaxation time. Increasing temperature demonstrates a reduction in the magnitude and characteristic time of the bump. It also indicates an activation energy of 0.8 eV, suggesting a diffusion mechanism. Importantly, the intensity of the illumination has no influence on the overall behavior, indicating that the phenomenon is not a photovoltaic processes. It is proposed that the bump originates from transition between a metastable state at high scan rates and a stable one reached after relaxation. To accurately replicate the measured J–V curves, a novel lumped circuit model is introduced and validated. A schematic microstructural model depicts the reversible reduction in photocurrent as a decline in charge transfer capacity caused by the diffusion of large ions at the mesoporous titanuim dioxyde interface. Ultimately, this study also suggests a plausible reason for the well‐known hysteresis commonly observed with perovskite solar cells. The bump in the current–voltage curve in perovskite solar cells results from a relaxation process with a well‐defined characteristic time. This transition originates from an ionic diffusion, independent of the basic photovoltaic process. The macroscopic behavior can be reproduced using a simple lumped circuit model; a simplified microstructural model helps understanding the underlying mechanism.