Ion transport and defect states within the band gap of lead halide perovskites studied via the moving grating technique

Abstract Ion migration is a distinctive feature and the primary cause of instability in metal halide perovskites (MHPs) used in photovoltaics. Among the various methods developed to study the electrical transport properties of MHPs, the Moving Grating Technique (MGT) has emerged as a valuable tool to obtain insights into ion mobility. In this technique, a photoconductor is illuminated with a moving interference pattern, inducing a direct current that reaches a maximum when the grating velocity matches the moving velocity of the charge carriers. In MHPs, it has been found that the induced current exhibits a low-velocity peak related to ion movement, alongside the free carrier’s peak. In this work, we use the MGT to study ion migration in perovskite thin films, supporting our findings with a detailed theoretical model incorporating the material’s density of states and recombination mechanisms. Our MGT measurements on FA 0.8 Cs 0.2 Pb(I 0.9 Br 0.1 ) 3 reveal three characteristic peaks: a high-frequency positive peak for free carriers, a broad negative peak attributed to ions, and a novel positive low-frequency peak. The detailed model successfully reproduces the novel positive low-frequency peak by including a donor defect state near the conduction band edge. On the other hand, measurements and simulations on standard MAPbI 3 thin films show the typical high-frequency free carrier peak and a broad negative ion peak, but lack the low-frequency positive peak, supporting the conclusion that this peak is linked to the specific donor defect state in the FA 0.8 Cs 0.2 Pb(I 0.9 Br 0.1 ) 3 material.