Hysteresis Index: A Figure without Merit for Quantifying Hysteresis in Perovskite Solar Cells

Not long after the breakthrough publications on lead halide perovskite-based solar cells, researchers noticed something peculiar about these devices: the current-voltage scans, the most common technique to determine a solar cell's efficiency, yielded different results depending on the scan direction. In the very first swathe of publications, researchers neglected discussing this observation, opting to simply publish the scan that yielded the higher efficiency. However, this was untenable as this scan-direction dependence clearly indicated that the absorber system was undergoing a transient change due to the external bias, and as such, the scanned measurements were yielding non-steady-state values. Unger et al.(1) and Snaith et al.(2) proposed two approaches to remedy this issue, both of which sought to take the unknown, internal changes of the material into account by devising steady-state measurements. This was done by either significantly slowing down the scan rate, such that the system was allowed to reach steady state at each point of the scan, or by measuring the stabilized or steady-state photocurrent at the maximum power point voltage. The advantage of the former technique is that performance parameters such as the short-circuit current (Jsc), open-circuit voltage (Voc), fill factor (FF), and series resistance can be determined with high fidelity. The latter has the advantages of being much faster to perform, allowing for rapid estimation of the performance of a multitude of cells, and enabling approximation of the solar cell performance under actual operating conditions. The community largely acknowledged and debated the phenomenon of hysteresis, but only slowly took up either of the two recommendations.