First-hand information for everyone
This paper reports a large computer study that asks whether mobile ions inside perovskite solar cells hurt their steady‑state performance. The authors ran paired simulations of 601 device designs (1202 total). In each pair one device allowed ions to move under the electric field and the other had the same materials but with ions fixed in place. Their main finding is that, for devices that are otherwise efficient, the presence of mobile ions changes steady‑state performance only a little.
To reach this conclusion the team used a drift‑diffusion device model (a physics model that follows how electrons, holes and ions move under electric fields) implemented in the IonMonger software. They combined that with a Design of Experiments, or factorial, approach to vary 32 model parameters across ranges taken from experiments. The study covered both device architectures (n‑i‑p and p‑i‑n) and both organic and inorganic contact layers. Key parameters they varied include ion density (10^17 to 10^19 cm^-3), perovskite thickness (300–500 nm), activation energy for ion migration (0.3–0.5 eV), dielectric constants, carrier lifetimes, and interfacial recombination speeds.
The paired‑device setup compares a “DEI” case, with dynamic electric‑field‑driven ions that can redistribute, to an “SUI” case, where ions are uniformly distributed and immobile. The authors evaluated devices under a steady‑state protocol equivalent to a very slow voltage scan (10 µV/s) so the ions could reach their equilibrium positions at each voltage. Using the factorial design let them decompose which material or interface properties amplify or reduce the impact of ionic redistribution.
The simulations also reproduce effects seen in experiments. Mobile ions can screen the internal electric field and reduce current, especially if carrier mobilities are low or recombination lifetimes are short. On the other hand, ions that pile up at interfaces can sometimes lower surface recombination and raise the open‑circuit photovoltage by tens of millivolts. The net result in these simulations is that, for well‑designed high‑efficiency devices, the steady‑state power conversion efficiency and voltages are not strongly altered by ion motion.
There are important caveats. These results apply to steady‑state operating conditions where ions have time to settle. They do not directly predict transient behaviour such as fast scan hysteresis or time‑dependent degradation. The conclusions also depend on the ranges and assumptions chosen for the model (for example the study limited interfacial recombination speeds to ranges expected for good devices and chose a minimum ion density intended to represent operated cells). Finally, computational models are only as good as the input data, so experimental confirmation under carefully controlled preconditioning and slow‑measurement protocols is still important.