Halide Ion Migration and its Role at the Interfaces in Perovskite Solar Cells

Abstract

Lead-halide perovskite solar cells (PSCs) based on unusual semiconductors made with ions, have shown impressive improvement in photovoltaic performance in few years exceeding nowadays 25 % power conversion efficiency. However, PSCs suffer from a lack of stability and show significant hysteresis in current-voltage curves, which are impeding commercialization. We confirmed the importance of halide ion migration in the hysteresis effect which has direct consequences on device efficiency. Using impedance spectroscopy, in addition to the geometrical capacitance found at high frequency for fresh samples without bias, we observed a second capacitance at low frequency after ageing or under bias. This second capacitance is interpreted as a charge accumulation layer at interfaces, which can be promoted by the presence of grain boundaries. Through glow-discharge optical emission spectroscopy elemental depth profiles, we found that under dark conditions, iodide ions diffuse through the electron transport layer versus ageing time. These ions interact chemically with the front-end electrode after four weeks and form silver iodide.

Experimental evidence of ion migration in aged inorganic perovskite solar cells using non-destructive RBS depth profiling

“…The equilibrium charged vacancy concentration is predicted to exceed 0.4 % at room temperature 66 . The J-V hysteresis is also measured under dark conditions at room temperature both in hybrid and inorganic perovskite solar cells, which suggests ionic conduction to take place in dark 67 .…”

“…Another notable observation is the mobility of Cs + in the various device architecture studies in this work (Tables S1, S2, S3). It has been suggested that if amount of Cs + is higher than 15%, the α-CsPbI 3 phase converts to δ-CsPbI 3 phase, which lead to degradation 36,37 and a J-V hysteresis in the IHPs 67 . When compared to a mobile organic counterpart, e.g., MA + present in organic-inorganic hybrid halide perovskites, the elemental peak of Cs + in our work is smooth and the slight changes in concentration confirms its less mobile nature than MA + and FA + 46,61 .…”

“…Furthermore, some indirect techniques i.e. Kelvin probe force microscope (KPFM) 45 , photothermal induced resonance (PTR) 46 and impedance spectroscopy (IS) 47,48 are also utilized to study ion migration in conventional PSCs. Most of aforementioned techniques are indirect and investigate ion migration without elemental sensitivity.…”

“…20,[42][43][44] Theoretical studies, i.e., density-functional theory (DFT), have suggested that the small activation energy of the elements leads to the migration of vacancies and interstitials in the PSCs. 42,43 Furthermore, some indirect techniques, i.e., Kelvin probe force microscopy (KPFM), 45 photothermalinduced resonance (PTR) 46 and impedance spectroscopy (IS) 47,48 have also been utilized to study ion migration in conventional PSCs. Most of the aforementioned techniques are indirect and investigate ion migration without any elemental sensitivity.…”

Evaluating the Capacitive Response in Metal Halide Perovskite Solar Cells

“…As the current‐voltage ( J‐V ) curves and capacitive responses of PSCs are connected, the analysis of the capacitive responses will be helpful to understand the nature of charge distribution within the device and also the kinetics of the charge transport/migration processes. Capacitance spectroscopy has been employed to investigate the degradation mechanism in PSCs, [20,21] as well as provides the knowledge of ionic migration and origin of the anomalous J‐V hysteresis observed in PSCs [22] . Such information is useful for the quantitative evaluation of electrical parameters of PSCs which have a prominent role in the device performance [23–26] …”