One prerequisite for the commercialization of perovskite solar cells is long-term operational stability. Inorganic perovskite solar cells exhibit a high thermal stability and efficiencies of up to 21.75 %, which corresponds to 75 % of the radiative limit for the band gap of 1.72 eV [1]. However, most inorganic solar cells show a substantial hysteresis between the forward and reverse J-V scan. This hysteresis has been shown to be caused by mobile ions in combination with non-radiative recombination [2]–[5]. In addition, mobile ions have been shown to reduce the stabilized power output (SPO) of perovskite solar cells by accumulating at the interfaces with the charge transport layers (CTLs) [2]–[5]. This leads to charge accumulation, increasing the probability of non-radiative recombination and therefore decreasing the SPO [2]–[5].

This study is the first to investigate the density, mobility and type of mobile ions in state-of-the-art inorganic DMAI-CsPbI₃ and CsPbI₂Br perovskite solar cells and to quantify their influence on the stabilized performance parameters. First, the mobile ion density is quantified using bias-assisted charge extraction measurements. Second, the ionic losses are quantified by measuring J-V curves with a wide range of scan speeds as presented in a recent report [6]. Third, the effective ion mobility is quantified and the dominant type of ions is determined by revealing their activation energy using temperature-dependent measurements.

As a result, the studied inorganic perovskite solar cells have 1 order of magnitude higher ion densities and 1-2 orders of magnitude lower ion mobilities as compared to triple-cation (3Cat) perovskite solar cells. Even though the measured ion density is very similar in DMAI-CsPbI₃ and CsPbI₂Br solar cells, the ionic losses in DMAI-CsPbI₃ solar cells are much lower than in CsPbI₂Br solar cells and even lower than in 3Cat solar cells. It is shown that an effective interface passivation in DMAI-CsPbI₃ solar cells can decrease the non-radiative recombination at the interface and consequently decrease the ionic losses.

Temperature-dependent measurements revealed iodide ion vacancies as the dominant type of mobile ions in all measured solar cells. This was determined by quantifying activation energies for mobile ions of 0.63 eV in CsPbI₂Br solar cells, 0.78 eV in DMAI-CsPbI₃ solar cells and 0.86 eV in 3Cat perovskite solar cells and comparing them to literature values and reports of direct measurements of mobile ions in CsPbI₂Br solar cells.

Overall, this study shows a significantly higher ion density and lower effective ion mobility in inorganic perovskite solar cells as compared to organic-inorganic perovskite solar cells. Furthermore, the efficiency of DMAI-CsPbI₃ and CsPbI₂Br solar cells is limited by mobile ions, which accumulate at the perovskite/CTL interfaces and aggravate non-radiative recombination. Effectively passivated interfaces in DMAI-CsPbI₃ are less affected by mobile ions, as shown by lower ionic losses, lower voltage loss, lower hysteresis and increased stability as compared to CsPbI₂Br solar cells.