Direct Experimental Evidence of Halide Ionic Migration under Bias in CH 3 NH 3 PbI 3−x Cl x ‑Based Perovskite Solar Cells Using GD-OES Analysis

International audienceIn recent decades, the development of organic−inorganic hybrid perovskite solar cells (PSCs) has been increasing very quickly due to their high initial efficiency and low-cost process. However, key points such as crystal growth mechanisms, current−voltage hysteresis, and instability remain still unexplained or misunderstood. Among several possibilities, ionic migration in PSCs has been suggested to explain the hysteresis effect. However, direct experimental evidence of ionic migration under operation or measurement conditions of PSCs is still missing. This work shows directly the ionic migration of halogen components (I − and Cl −) of a CH 3 NH 3 PbI 3−x Cl x perovskite film under an applied bias using glow discharge optical emission spectrometry (GD-OES). Furthermore, no migration of lead and nitrogen ions is observed on a polarization time scale less than 2 min. The ratio of fixed to mobile iodide ions is deduced from the evolution of the GD-OES profile lines as a function of the applied bias. The average length of iodide and chloride ion migration is deduced from the experimental results. H ybrid organic−inorganic perovskite materials (HOIPs) have recently emerged as an exciting topic of research in chemistry and materials science for their attractive optical−electrical properties. HOIPs have found potential applications in optoelectronics such as photovoltaics, photodetectors, light-emitting diodes, and lasers. 1−3 It has been discovered that these materials are able to exhibit remarkable performance in terms of photocurrent generation. 4 Key features of these hybrid perovskites include direct bandgap, large exciton diffusion length, and strong optical absorption. 5−7 However, despite their processing advantages (low-cost solution process) and outstanding solar to electrical energy conversion properties approaching the performance of standard inorganic materials such as silicon, HOIP materials suffer from several drawbacks. The stability of HOIP materials under operational conditions (bias, light, environment, ...) is one of the biggest challenges to be addressed before commercializa-tion. 8 HOIPs are mixed ionic−electronic conductors where ions as well as electron/hole carriers can migrate in the material under electric fields. 9 It has been suggested that ionic migration in HOIPs could impact optoelectronic performance and affect device operation and long-term stability. The J−V hysteresis behavior in perovskite solar cells (PSCs) was first reported by Snaith et al. 10 and Unger et al. 11 in mesoporous structured perovskites and by Xiao et al. 12 for planar heterojunction structures. Various mechanisms have been proposed to explain the origins of the J−V hysteresis such as filaments, giant dielectric constant, unbalance between hole and electron mobility, trapping or ferroelectricity effects, and ionic migration effects. 13−19 The general formula for HOIPs materials is ABX 3 , where A is an organic cation, B is a divalent metal, and X is a halide anion. Methylammonium lead iodide (MAPbI 3) materials have been extensively studied for photovoltaic applications. Possible mobile ions in MAPbI 3 crystals include MA + ions, Pb 2+ ions, I − ions, vacancies, and other defects such as hydrogen-related impurities (H + , H 0 , and H −). 20 Considering the activation energy of ion migration, 21−24 it is reasonable to expect that both the MA + ions and I − ions are mobile in the MAPbI 3 films, while the Pb 2+ ions are almost immobile. However, whereas th